Mélanie Legrand

Priya Jaitly

Adeline Feri

Christophe D'enfert
email: christophe.denfert@pasteur.fr

Kaustuv Sanyal
email: sanyal@jncasr.ac.in

Candida albicans : An

Keywords:

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Mélanie Legrand, 1 Priya Jaitly, 2 Adeline Feri, 1,3,4 Christophe d'Enfert, 1, * and Kaustuv Sanyal 2, * Saccharomyces cerevisiae and Schizosaccharomyces pombe have served as uncontested unicellular model organisms, as major discoveries made in the field of genome biology using yeast genetics have proved to be relevant from yeast to humans. The yeast Candida albicans has attracted much attention because of its ability to switch between a harmless commensal and a dreaded human pathogen. C. albicans bears unique features regarding its life cycle, genome structure, and dynamics, and their links to cell biology and adaptation to environmental challenges. Examples include a unique reproduction cycle with haploid, diploid, and tetraploid forms; a distinctive organisation of chromosome hallmarks; a highly dynamic genome, with extensive karyotypic variations, including aneuploidies, isochromosome formation, and loss-ofheterozygosity; and distinctive links between the response to DNA alterations and cell morphology. These features have made C. albicans emerge as a new and attractive unicellular model to study genome biology and dynamics in eukaryotes.

Candida albicans: A Model for Studying Genome Biology

Maintenance of genome integrity and the accuracy of DNA replication are at the core of cell function, survival, and propagation. Thus, deciphering the molecular mechanisms that underlie genome biology is of crucial importance, especially as they have relevance in numerous human diseases such as cancer but also because they underlie species evolution and adaptation. For more than 50 years, Saccharomyces cerevisiae (see Glossary) and Schizosaccharomyces pombe have served as uncontested yeast models for molecular understanding of the processes underlying eukaryotic genome biology [START_REF] Botstein | Yeast: an experimental organism for modern biology[END_REF][START_REF] Cavanaugh | Big lessons from little yeast: budding and fission yeast centrosome structure, duplication, and function[END_REF][START_REF] Fair | The power of fission: yeast as a tool for understanding complex splicing[END_REF][START_REF] Hinnebusch | YeastBook: an encyclopedia of the reference eukaryotic cell[END_REF][START_REF] Noma | The yeast genomes in three dimensions: mechanisms and functions[END_REF][START_REF] Nurse | Nobel Lecture: Cyclin dependent kinases and cell cycle control[END_REF]. Major discoveries in this field have benefited from the 'awesome power of yeast genetics' and proven to be relevant across eukaryotes. In addition, the ascomycetous yeast Candida albicansa distantly related cousin of S. cerevisiae (Box 1) -has attracted considerable interest because of its dominant importance as a human pathogen. While normally a commensal of humans, C. albicans is also responsible for superficial infectionsthrush, oropharyngeal candidiasis, vaginal candidiasisin healthy individuals as well as disseminated infections in hospitalised patients that receive broad-spectrum antibiotic treatment and have debilitated immunity [START_REF] Brown | Hidden killers: human fungal infections[END_REF]. The investigation of putative C. albicans virulence factors, in particular the ability to alternate between yeast and filamentous forms; the exploration of C. albicans interactions with the host; and the search for new antifungal targets have been accompanied by the development of a molecular toolkit that allows gene function to be accurately characterised in this species (Box 1) [START_REF] Ene | The cryptic sexual strategies of human fungal pathogens[END_REF][START_REF] Erwig | Interactions of fungal pathogens with phagocytes[END_REF][START_REF] Gow | Candida albicans morphogenesis and host defence: discriminating invasion from colonization[END_REF][START_REF] Lohse | Development and regulation of singleand multi-species Candida albicans biofilms[END_REF][START_REF] Netea | Immune defence against Candida fungal infections[END_REF][START_REF] Noble | Candida albicans cell-type switching and functional plasticity in the mammalian host[END_REF][START_REF] Perfect | The antifungal pipeline: a reality check[END_REF][START_REF] Shapiro | Regulatory circuitry governing fungal development, drug resistance, and disease[END_REF][START_REF] Sudbery | Growth of Candida albicans hyphae[END_REF]. Notably, this toolkit has enabled exploring other aspects in depth, especially the genome biology of C. albicans. A number of features distinguish C. albicans from other yeast speciesa unique reproduction cycle where haploid, diploid, and tetraploid forms are observed; a distinctive organisation of chromosome hallmarks; a highly dynamic genome, with extensive karyotypic Highlights C. albicans is a major human pathogen, with features that distinguish it from other yeast species.

Rather than meiosis, ploidy reduction upon mating occurs by a parasexual process of concerted chromosome loss.

Centromeres have unique organisation and are epigenetically regulated. Neither centromeres nor the DNA replication origins share any common defining DNA sequence.

Loss-of-heterozygosity events are frequent and widespread in the genome of C. albicans. variations, including chromosomal loss and gain, as well as rearrangements, isochromosome formation, segmental duplication, and loss-of-heterozygosity (LOH); and distinctive links between the response to DNA alterations and cell morphology (Figure 1, Key Figure). In this review, we highlight these unique features that have helped C. albicans emerge as a new unicellular model to study genome biology and its evolution.

Candida albicans: A Yeast Species with a Cryptic Reproduction Cycle

Sequencing of the C. albicans genome has revealed that this diploid species, long thought to be devoid of sex [START_REF] Bougnoux | Mating is rare within as well as between clades of the human pathogen Candida albicans[END_REF][START_REF] Graser | Molecular markers reveal that population structure of the human pathogen Candida albicans exhibits both clonality and recombination[END_REF][START_REF] Jones | The diploid genome sequence of Candida albicans[END_REF][START_REF] Riggsby | DNA content, kinetic complexity, and the ploidy question in Candida albicans[END_REF] is actually equipped with the majority of the genetic circuit required for sexual reproduction. Similar to mating type (MAT) loci in S. cerevisiae, mating type-like (MTL; Figure 2A) regions encoding the transcriptional regulators that control expression of mating specific genes [START_REF] Hull | Identification of a mating type-like locus in the asexual pathogenic yeast Candida albicans[END_REF] and elements of a functional pheromone response pathway exist in the C. albicans genome [START_REF] Tzung | Genomic evidence for a complete sexual cycle in Candida albicans[END_REF]. While many of the meiotic regulators from S. cerevisiae have counterparts in C. albicans, the latter lacks key meiotic components including IME1, the master switch for entry into the meiotic pathway in S. cerevisiae, and SPO13, which in S. cerevisiae is essential for proper execution of meiosis I. Existence of functional analogues of these genes in C. albicans is difficult to rule out as extensive sequence divergence may cause difficulty in their in silico identification. While IME1 is essential for meiosis in a number of yeast species, strikingly, this gene is absent from many sexual species in the CTG clade, suggesting a possible cladespecific rewiring of the meiotic cycle (Box 1) [START_REF] Butler | Evolution of pathogenicity and sexual reproduction in eight Candida genomes[END_REF]. In addition, the highly conserved pheromone response pathways were detected in nonmating species [START_REF] Butler | Evolution of pathogenicity and sexual reproduction in eight Candida genomes[END_REF]. The apparent plasticity of mating and meiosis pathways in Candida species reinforces the fact that knowledge of gene products involved in sexual reproduction is insufficient to accurately predict the reproductive behaviour of an organism.

Hints from in silico data have fuelled the quest for evidence of mating and sexual reproduction in C. albicans. While engineered C. albicans strains homozygous for MTL loci are able to mate and form tetraploids both in laboratory conditions and in a mammalian host [START_REF] Hull | Evidence for mating of the asexual yeast Candida albicans in a mammalian host[END_REF][START_REF] Magee | Induction of mating in Candida albicans by construction of MTLa and MTLalpha strains[END_REF], meiosis remains to be demonstrated in C. albicans. Tetraploid cells revert to diploidy by undergoing a parasexual process of concerted chromosome loss [START_REF] Bennett | Completion of a parasexual cycle in Candida albicans by induced chromosome loss in tetraploid strains[END_REF][START_REF] Dumitru | In vivo and in vitro anaerobic mating in Candida albicans[END_REF][START_REF] Lachke | Skin facilitates Candida albicans mating[END_REF][START_REF] Miller | White-opaque switching in Candida albicans is controlled by mating-type locus homeodomain proteins and allows efficient mating[END_REF]. The combination of mating and subsequent concerted chromosome loss that allows C. albicans to alternate between diploid and tetraploid (but also haploid and diploid) has been referred to as a parasexual cycle (Figure 2B). The apparent lack of conventional meiosis in C. albicans suggests that the function of meiotic genes may have diverged [START_REF] Tzung | Genomic evidence for a complete sexual cycle in Candida albicans[END_REF][START_REF] Butler | Evolution of pathogenicity and sexual reproduction in eight Candida genomes[END_REF][START_REF] Banerjee | UME6, a novel filament-specific regulator of Candida albicans hyphal extension and virulence[END_REF][START_REF] Chen | CaNdt80 is involved in drug resistance in Candida albicans by regulating CDR1[END_REF]. A classic example is Ndt80, the meiosisspecific transcription factor and a key modulator of progression of meiosis and sporulation in S. cerevisiae, that functionally diverged to participate in the biofilm pathway in C. albicans [START_REF] Nobile | A recently evolved transcriptional network controls biofilm development in Candida albicans[END_REF][START_REF] Nocedal | Gene regulatory network plasticity predates a switch in function of a conserved transcription regulator[END_REF]. Similarly, Ume6, a key transcriptional regulator of early meiosis-specific genes in S. cerevisiae, has been rewired towards autophagy and hyphal growth regulation in C. albicans [START_REF] Banerjee | UME6, a novel filament-specific regulator of Candida albicans hyphal extension and virulence[END_REF][START_REF] Bartholomew | Ume6 transcription factor is part of a signaling cascade that regulates autophagy[END_REF]. The latest example of such rewiring is Rme1; the function of which diverged from preventing meiosis by repressing IME1 in S. cerevisiae to regulating chlamydospore formation in C. albicans (Hernandez-Cervantes et al., personal communication).

Although we cannot rule out the possibility of C. albicans undergoing meiosis in conditions that have not been explored thus far, the hypothetical absence of meiosis does not prevent C. albicans from generating genetic and phenotypic diversity necessary for this opportunistic human pathogen to adapt to new environments. In addition to extensive shuffling of parental chromosomes resulting in new combinations of homologues, completion of the parasexual cycle often gives rise to aneuploid strains and is accompanied by recombination events between homologous chromosomes [START_REF] Forche | The parasexual cycle in Candida albicans provides an alternative pathway to meiosis for the formation of recombinant strains[END_REF][START_REF] Hickman | Parasexual ploidy reduction drives population heterogeneity through random and transient aneuploidy in Candida albicans[END_REF]. Unexpectedly, recombination events during the nonmeiotic parasexual cycle are dependent on the DNA double-strand break (DSB) inducing Spo11; the meiosis-specific endonuclease that initiates meiotic recombination in S. cerevisiae [START_REF] Forche | The parasexual cycle in Candida albicans provides an alternative pathway to meiosis for the formation of recombinant strains[END_REF].

The majority of C. albicans isolates are found in the diploid state and diploidy is considered the preferred ploidy state of C. albicans. Nevertheless, nondiploid isolates have also been reported [START_REF] Hickman | The 'obligate diploid' Candida albicans forms mating-competent haploids[END_REF]. Changes in ploidy including haploidy, tetraploidy, or aneuploidy (primarily monosomy or trisomy) are thought to provide C. albicans with a rapid response to changing environments within the host [START_REF] Berman | Ploidy plasticity: a rapid and reversible strategy for adaptation to stress[END_REF]. Deviations from diploidy seem to harbour a fitness cost in the long term, as experimental evolution experiments using clinical and laboratory haploid, diploid, and polyploid C. albicans strains in complete medium and under nutrient-limited conditions have revealed that the stabilised genome nearly always reaches diploidy, a phenomenon termed as ploidy drive [39].

Large genomic changes, similar to the ones observed in products of in vitro parasexual genome reduction or long-term evolution experiments, are well tolerated by C. albicans and have been associated with acquisition of new phenotypic traits, such as drug resistance. Although population genetics approaches have recently confirmed the predominance of clonal reproduction in the C. albicans population [START_REF] Bougnoux | Mating is rare within as well as between clades of the human pathogen Candida albicans[END_REF][START_REF] Schmid | Last hope for the doomed? Thoughts on the importance of a parasexual cycle for the yeast Candida albicans[END_REF][START_REF] Zhang | Selective advantages of a parasexual cycle for the yeast Candida albicans[END_REF], the work by Ropars and colleagues on genomes of 182 C. albicans isolates from diverse origins revealed the occurrence of gene flow in this population [START_REF] Ropars | Gene flow contributes to diversification of the major fungal pathogen Candida albicans[END_REF] (Figure 2C). These lines of evidence highlight the fact that parasexuality (or possibly sexuality) also occurs in nature and significantly contributes to C. albicans genetic and phenotypic diversities.

The Candida albicans Genome: An Organisational View

The essential elements of a eukaryotic chromosomenamely centromeres, DNA replication origins, and telomereshave been identified in C. albicans (Figure 3). The centromere (CEN) is an essential chromosomal element that facilitates sister chromatid separation via kinetochore formation during mitosis. The CEN DNA sequences of C. albicans are 3-5 kb long and are all unique; devoid of any common sequence motif or repeat [START_REF] Sanyal | Centromeric DNA sequences in the pathogenic yeast Candida albicans are all different and unique[END_REF] (Figure 3A). The absence of a CEN-specific DNA sequence and the inability of the exogenously introduced CEN DNA to function as a native CEN suggest epigenetic regulation of CEN identity in C. albicans [START_REF] Baum | Formation of functional centromeric chromatin is specified epigenetically in Candida albicans[END_REF]. However, upon CEN deletion, neocentromeres are formed efficiently in C. albicans, mostly proximal and rarely distal to the native CEN [START_REF] Thakur | Efficient neocentromere formation is suppressed by gene conversion to maintain centromere function at native physical chromosomal loci in Candida albicans[END_REF][START_REF] Ketel | Neocentromeres form efficiently at multiple possible loci in Candida albicans[END_REF][START_REF] Burrack | Neocentromeres provide chromosome segregation accuracy and centromere clustering to multiple loci along a Candida albicans chromosome[END_REF]. Similar to the native CENs, neocentromeres also cluster in 3D with other functional CENs [START_REF] Burrack | Neocentromeres provide chromosome segregation accuracy and centromere clustering to multiple loci along a Candida albicans chromosome[END_REF]. Gene conversion (GC) at the CENs can interchange the deleted CEN with the native CEN [START_REF] Thakur | Efficient neocentromere formation is suppressed by gene conversion to maintain centromere function at native physical chromosomal loci in Candida albicans[END_REF] and possibly explains the low frequency of SNPs across C. albicans CENs. Besides CEN clustering, structural integrity of the kinetochore is required for CEN function in C. albicans. Depletion of an essential kinetochore protein disrupts the integrity of the kinetochore architecture [START_REF] Thakur | A coordinated interdependent protein circuitry stabilizes the kinetochore ensemble to protect CENP-A in the human pathogenic yeast Candida albicans[END_REF][START_REF] Roy | CaMtw1, a member of the evolutionarily conserved Mis12 kinetochore protein family, is required for efficient inner kinetochore assembly in the pathogenic yeast Candida albicans[END_REF] and results in delocalisation and degradation of CENP-A [START_REF] Thakur | A coordinated interdependent protein circuitry stabilizes the kinetochore ensemble to protect CENP-A in the human pathogenic yeast Candida albicans[END_REF] that forms centromeric chromatin.

DNA replication initiates from multiple discrete genetic locithe DNA replication origins (ORI). Based on the location, ORIs in C. albicans are of two types [START_REF] Tsai | Origin replication complex binding, nucleosome depletion patterns, and a primary sequence motif can predict origins of replication in a genome with epigenetic centromeres[END_REF]: (i) arm ORIs, which are located on the chromosomal arms; and (ii) centromeric ORIs, which are present on [START_REF] Koren | Epigenetically-inherited centromere and neocentromere DNA replicates earliest in S-phase[END_REF] or close to the CENs [START_REF] Mitra | Rad51-Rad52 mediated maintenance of centromeric chromatin in Candida albicans[END_REF] (Figure 3B). While a subset of arm ORIs are defined by a 15-bp AC-rich consensus motif and a nucleosome-depleted pattern, centromeric ORIs are defined by epigenetic mechanisms [START_REF] Tsai | Origin replication complex binding, nucleosome depletion patterns, and a primary sequence motif can predict origins of replication in a genome with epigenetic centromeres[END_REF] and replicate earliest in the genome [START_REF] Koren | Epigenetically-inherited centromere and neocentromere DNA replicates earliest in S-phase[END_REF]. The centromeric ORIs together with the homologous recombination (HR) proteins, Rad51 and Rad52, play a key role in loading CENP-A onto the CENs [START_REF] Mitra | Rad51-Rad52 mediated maintenance of centromeric chromatin in Candida albicans[END_REF]. While replicating CEN DNA, the moving replication forks from CEN-proximal ORIs stall at CEN due to the presence of the kinetochore acting as a physical barrier [START_REF] Mitra | Rad51-Rad52 mediated maintenance of centromeric chromatin in Candida albicans[END_REF]. The fork stalling accumulates single-stranded DNA that attracts HR proteins Rad51 Parasexual cycle: combination of mating and subsequent concerted chromosome loss that allows C. albicans to alternate between diploid and tetraploid (but also haploid and diploid). Ploidy drive: process that brings an organism to its base ploidy level. Saccharomyces cerevisiae: commonly known as baker's yeast or brewer's yeast, S. cerevisiae is a single-celled eukaryote that operates in a manner similar to a human cell and therefore is used as an important model organism in genetics and molecular biology. Spo11: endonuclease that initiates meiotic recombination by catalysing the formation of double-strand breaks in DNA. Telomeres: regions of repetitive nucleotide sequences located at the termini of a eukaryotic chromosome to ensure chromosome end replication and protection from degradation or end-to-end chromosome fusion. and Rad52, which are shown to interact with CENP-A in C. albicans [START_REF] Mitra | Rad51-Rad52 mediated maintenance of centromeric chromatin in Candida albicans[END_REF]. As a consequence, CENP-A is deposited onto the CENs. Consistent with this, in a CEN-deleted strain, the neocentromere becomes the earliest replicating region [START_REF] Koren | Epigenetically-inherited centromere and neocentromere DNA replicates earliest in S-phase[END_REF].

A telomere, at the termini of a eukaryotic chromosome, ensures chromosome end replication and protects the chromosome from degradation or end-to-end chromosome fusion. C. albicans telomeres are unique in containing tandem copies of unusually long 23-bp repeating units [START_REF] Mceachern | Unusually large telomeric repeats in the yeast Candida albicans[END_REF] (Figure 3C). However, they are assembled into heterochromatin via the classical Sir2-mediated pathway [START_REF] Mceachern | Unusually large telomeric repeats in the yeast Candida albicans[END_REF][START_REF] Freire-Beneitez | Candida albicans repetitive elements display epigenetic diversity and plasticity[END_REF]. The subtelomeric regions of C. albicans consist of the telomere-associated (TLO) family of genes, which encode for the subunits of the mediator complex; a crucial component for transcription initiation [START_REF] Zhang | The Tlo proteins are stoichiometric components of Candida albicans mediator anchored via the Med3 subunit[END_REF]. There are 15 TLO genes (including one pseudogene) in C. albicans but other non-C. albicans species have either one or two TLO genes [START_REF] Jackson | Comparative genomics of the fungal pathogens Candida dubliniensis and Candida albicans[END_REF]. In addition, overexpression of TLO genes in C. albicans influences many growth-and virulence-related properties [START_REF] Dunn | Functional diversification accompanies gene family expansion of MED2 homologs in Candida albicans[END_REF]. The expansion in the number of TLO genes could thus explain the ability of C. albicans to adapt in various host niches.

A special feature of the C. albicans genome is the major repeat sequence (MRS). The MRS is a long tract (10-100 kb) of repetitive DNA that is present on all chromosomes, except chromosome 3. Structurally, an MRS is composed of three subunits: the repetitive RPS Box 1. Candida albicans, an Alternative Yeast Model with an Extended and Adapted Molecular Genetics Toolkit Because C. albicans decodes the CUG codon as serine instead of leucine and is predominantly a diploid, numerous tools have been adapted for genetic engineering of this species, allowing most molecular approaches to be developed in this species from insertional mutagenesis to gene tagging and two-hybrid screens as well as the production of mutant collections. The recent identification of stable haploids allows new approaches to be developed. CRISPR-cas9-based gene editing allows one-step generation of mutants in the diploid background, speeding up their construction [START_REF] Santos | The genetic code of the fungal CTG clade[END_REF][100][101][102][103][104][105][106][107][108][109][110][111][112]. C. albicans undergoes a unique haploid-diploid-tetraploid life cycle. A phenotypic switch from white to opaque form due to homozygosis of the MTL locus is the initial step of this cycle. Opaque cells of opposite mating-type then fuse together to form tetraploids. These tetraploids undergo a nonmeiotic parasexual cycle to return to the diploid state. Diploid subunit flanked by nonrepetitive elements RB2 and HOK (Figure 3D). Chromosome 3 contains only the RB2 element without the RPS or HOK unit [START_REF] Chibana | The enigma of the major repeat sequence of Candida albicans[END_REF]. Surprisingly, the MRS, being a repetitive region, is not assembled into classical heterochromatin but carries marks of both euchromatin and heterochromatin [START_REF] Freire-Beneitez | Candida albicans repetitive elements display epigenetic diversity and plasticity[END_REF]. The MRS covers about 3% of the total genome, yet its function remains elusive, except that it is considered to be a hotspot for genome rearrangements in C. albicans [START_REF] Lephart | Effect of the major repeat sequence on chromosome loss in Candida albicans[END_REF]. The MRS is a preferred site for chromosomal translocations [START_REF] Chibana | The enigma of the major repeat sequence of Candida albicans[END_REF], and the expansion and contraction of its RPS region give rise to chromosome length polymorphism [START_REF] Chibana | Fine-resolution physical mapping of genomic diversity in Candida albicans[END_REF]. Furthermore, the presence of the MRS affects the frequency of nondisjunction, whereby a homologue bearing a larger MRS is more likely to be lost at the time of chromosome segregation [START_REF] Lephart | Effect of the major repeat sequence on chromosome loss in Candida albicans[END_REF]. Thus, the MRS serves as an important means of generating karyotypic diversity in C. albicans and needs to be studied in greater detail for its function and origin.

Candida albicans, a Heterozygous Diploid with a Dynamic Genome

Although genome variation has so far been explored primarily either in haploid or homozygous diploid genomes, it is now being tackled in diploid organisms having heterozygosity, with a hope of better understanding the genomics of adaptation in various environmental or host niches. Population genomics studies have revealed a number of aspects regarding heterozygosity in the C. albicans genome [START_REF] Jones | The diploid genome sequence of Candida albicans[END_REF][START_REF] Ropars | Gene flow contributes to diversification of the major fungal pathogen Candida albicans[END_REF][START_REF] Hirakawa | Genetic and phenotypic intra-species variation in Candida albicans[END_REF][START_REF] Muzzey | Assembly of a phased diploid Candida albicans genome facilitates allele-specific measurements and provides a simple model for repeat and indel structure[END_REF][START_REF] Muzzey | Extensive and coordinated control of allele-specific expression by both transcription and translation in Candida albicans[END_REF] (Figure 4A). Natural heterozygosity is observed across the C. albicans genome with an average rate of one SNP every 237-283 bases. By comparing a large number of strains, a higher level of heterozygosity is found to correlate with faster growth rates. These observations likely reflect the loss of alleles that influence fitness in strains that have undergone partial or complete homozygosis. Genome sequencing data have identified about 3600 open reading frames (ORFs) with high-confidence SNPs leading to changes in the amino acid sequence. In addition, SNPs found in regulatory regions, or even in ORFs, can also affect transcription levels and/or translation efficiency between the alleles [START_REF] Muzzey | Extensive and coordinated control of allele-specific expression by both transcription and translation in Candida albicans[END_REF]. Thus, extensive allelic differences may function to increase genetic and phenotypic diversity in an organism devoid of a true sexual cycle and contribute to the acquisition of new phenotypic traits such as drug resistance.

Population genomics studies in C. albicans have revealed that genome heterozygosity can vary from 48% to 89% -heterozygous and homozygous regions being defined as such based on the number of heterozygous SNPs within 5-kb windows [START_REF] Jones | The diploid genome sequence of Candida albicans[END_REF][START_REF] Butler | Evolution of pathogenicity and sexual reproduction in eight Candida genomes[END_REF][START_REF] Ropars | Gene flow contributes to diversification of the major fungal pathogen Candida albicans[END_REF][START_REF] Hirakawa | Genetic and phenotypic intra-species variation in Candida albicans[END_REF]. The levels of heterozygosity are primarily influenced by large LOH events encompassing whole chromosomes or extending from a specific chromosomal locus to the telomere. These LOH events have been shown to be pervasive in C. albicans isolates and can be detected on all chromosomes. LOH can involve an entire chromosome upon chromosome loss due to chromosome nondisjunction during mitosis. Depletion of the centromeric histone H3, Cse4/CENP-A at centromeres has been reported in response to changes in ploidy and the environment, and is associated with chromosome instability [START_REF] Brimacombe | Chromatin rewiring mediates programmed evolvability via aneuploidy[END_REF]. The genome of C. albicans contains two homologous histone H2A-encoding genes, HTA1 on chromosome 3 encoding Hta1p and HTA2 on chromosome 1 C. albicans can become haploid or vice versa by chromosome loss and autodiploidisation, respectively. Several host factors or environmental stress can lead to karyotypic variations in C. albicans. The genome of C. albicans is remarkably plastic and can tolerate segmental aneuploidies, whole chromosome aneuploidies, and loss-of-heterozygosity events. Accumulation of such DNA alterations can be associated to polarised growth and drug resistance in C. albicans. Several chromosomal elements influence the organisation and stability of the C. albicans genome. These include centromeres (CEN), which are all unique and different in C. albicans; replication origins, which are of two types, arm origins and centromeric origins; telomeres, which are unusually long and associated with a family of telomere-associated genes (TLO); and the major repeat sequence (MRS), a feature unique to C. albicans. If left unrepaired, a DSB may result in chromosome truncation or chromosome loss, characterized by LOH events that span an entire chromosome or an arm of it. Several tools have been developed to study LOH events in C. albicans (Box 2). They have in particular revealed that DNA DSBs are predominantly repaired by GC but other repair events such as BIR/MCO and GC with crossover (CO) can also be observed at a significant frequency [START_REF] Feri | Analysis of repair mechanisms following an induced double-strand break uncovers recessive deleterious alleles in the Candida albicans diploid genome[END_REF]. They have also revealed that the frequency at which LOH events arise and the nature of these LOH are influenced by environmental parameters (see below).

A link between LOH, aneuploidy, and the elevation of antifungal resistance by studying drugresistant C. albicans isolates has been precisely established (Figure 4B). Gain-of-function alleles of genes involved in the resistance to azole antifungals have been shown to be codominant with wild-type alleles and therefore high levels of resistance cannot be achieved in the presence of the wild-type allele. It is only upon homozygosis of the gain-of-function allele, as a result of LOH, that high levels of azole resistance can be achieved [START_REF] Coste | Genotypic evolution of azole resistance mechanisms in sequential Candida albicans isolates[END_REF][START_REF] Ford | The transcription factor Mrr1p controls expression of the MDR1 efflux pump and mediates multidrug resistance in Candida albicans[END_REF][69]. In addition, the appearance of aneuploidy and, in particular, the formation of an isochromosome composed of the two left arms of chromosome 5 is often associated with the acquisition of azole resistance [START_REF] Selmecki | Aneuploidy and isochromosome formation in drug-resistant Candida albicans[END_REF]. Increased copy number of ERG11 and TAC1, both located on the left arm of chromosome 5, accounts for the majority of drug resistance associated with the chromosome 5 isochromosome [START_REF] Selmecki | An isochromosome confers drug resistance in vivo by amplification of two genes, ERG11 and TAC1[END_REF]. Large-scale genome changes have been characterized in S. cerevisiae as a means of adaptation in response to stress [START_REF] Chang | Dynamic large-scale chromosomal rearrangements fuel rapid adaptation in yeast populations[END_REF][START_REF] Yona | Chromosomal duplication is a transient evolutionary solution to stress[END_REF]. Similarly, in vitro exposure to oxidative stress, elevated temperature, and antifungal drugs [START_REF] Forche | Stress alters rates and types of loss of heterozygosity in Candida albicans[END_REF][START_REF] Harrison | A tetraploid intermediate precedes aneuploid formation in yeasts exposed to fluconazole[END_REF], as well as passaging through an animal model of infection result in increased genomic rearrangements in C. albicans [START_REF] Ene | Global analysis of mutations driving microevolution of a heterozygous diploid fungal pathogen[END_REF][START_REF] Forche | Rapid phenotypic and genotypic diversification after exposure to the oral host niche in Candida albicans[END_REF]. The large-scale changes described above provide C. albicans with the ability to rapidly generate genetic and phenotypic diversity within the host environment.

Although LOH events are frequent and widespread in the genome of C. albicans, several studies have observed that, for some chromosomes, one haplotype or even a part of it is never found in the homozygous state in the C. albicans laboratory strain SC5314. This homozygosis bias has been observed in haploids, in parasexual derivatives, and in rad52 mutant derivatives [START_REF] Forche | The parasexual cycle in Candida albicans provides an alternative pathway to meiosis for the formation of recombinant strains[END_REF][START_REF] Hickman | The 'obligate diploid' Candida albicans forms mating-competent haploids[END_REF][START_REF] Andaluz | Rad52 function prevents chromosome loss and truncation in Candida albicans[END_REF]. These observations suggest that recessive lethal and deleterious alleles can be found in the heterozygous state in the genome of C. albicans. Several groups, including ours, have identified such recessive lethal or deleterious alleles on chromosome 3A and chromosome 4B [START_REF] Feri | Analysis of repair mechanisms following an induced double-strand break uncovers recessive deleterious alleles in the Candida albicans diploid genome[END_REF][START_REF] Ciudad | Phenotypic consequences of a spontaneous loss of heterozygosity in a common laboratory strain of Candida albicans[END_REF]. Overall, consistent with clonal reproduction, C. albicans strains harbour state with heterozygosity at the MTL locus. Homozygosis at the MTL locus allows occasional white-opaque phenotypic switching and more efficient mating between opaque cells. Transition from tetraploidy to diploidy or diploidy to haploidy is independent of meiosis and involves random concerted chromosome loss with the intermediate aneuploidy progeny cells.

Haploids are shown in blue, diploids in red, and tetraploids in green. (C) Genome sequencing of 182 C. albicans isolates confirms a predominantly clonal population structure and reveals, for the first time, footprints of admixtures in two genetic clusters (green arrows), demonstrating the occurrence of (para)sexuality in the C. albicans natural environment. LOH (BIR/ MCO and GC) is a major driver of intraclade evolution and major LOH events can result in the emergence of clusters with altered virulence/niche restriction, possibly due to pseudogenization [START_REF] Ropars | Gene flow contributes to diversification of the major fungal pathogen Candida albicans[END_REF]. Abbreviations: BIR, break-induced replication; GC, gene conversion; LOH, loss of heterozygosity; MCO, mitotic crossover.

recessive lethal and deleterious alleles that constrain the outcome of LOH events. Although homozygosity of some alleles has been linked to fitness advantage in a specific host niche, overall genome-wide heterozygosity remains prevalent in the C. albicans population. For these reasons, the fate of cells having undergone LOH still needs to be addressed in C. albicans.

Specificities of DNA Repair Mechanisms in Candida albicans

DNA DSBs, where both strands of the double helix are severed, are the most serious forms of DNA damage. Indeed, failure to repair a DSB leads to loss of the CEN-lacking chromosome fragment, while improper repair of a DSB can lead to gross chromosomal rearrangements such as translocations, inversions, and deletions. Two major pathways of DSB repair are known: HR and nonhomologous end joining (NHEJ). Characterisation of deletion mutants of genes involved in HR (RAD52, RAD51, RAD59, RAD54, and RDH54) have shown that HR plays a crucial role in DNA damage repair in C. albicans [START_REF] Andaluz | Rad52 function prevents chromosome loss and truncation in Candida albicans[END_REF][START_REF] Bellido | Genetic interactions among homologous recombination mutants in Candida albicans[END_REF][START_REF] Garcia-Prieto | Role of the homologous recombination genes RAD51 and RAD59 in the resistance of Candida albicans to UV light, radiomimetic and anti-tumor compounds and oxidizing agents[END_REF][START_REF] Legrand | Role of DNA mismatch repair and double-strand break repair in genome stability and antifungal drug resistance in Candida albicans[END_REF][START_REF] Legrand | The contribution of the S-phase checkpoint genes MEC1 and SGS1 to genome stability maintenance in Candida albicans[END_REF]. In contrast, the characterisation of mutants impaired for NHEJ has suggested that this process is not efficient in C. albicans [START_REF] Andaluz | Rad52 function prevents chromosome loss and truncation in Candida albicans[END_REF][START_REF] Legrand | Role of DNA mismatch repair and double-strand break repair in genome stability and antifungal drug resistance in Candida albicans[END_REF]. Recent results on the mode of repair of CRISPR-cas9-induced DSBs are in agreement with this observation but suggest that NHEJ could occur in C. albicans, albeit rarely. Indeed, it has been shown that HR is the predominant repair pathway of CRISPR-Cas9induced DSBs in S. cerevisiae and C. albicans, in contrast to other yeast species such as Candida glabrata and Naumovia castelli [START_REF] Vyas | New CRISPR mutagenesis strategies reveal variation in repair mechanisms among fungi[END_REF]. Moreover, while HR-defective S. cerevisiae mutants could still repair a CRISPR-Cas9-induced DSB in the absence of a repair template, likely via NHEJ, the same is not true in C. albicans, consistent with limited efficiency of NHEJ in this species [START_REF] Vyas | New CRISPR mutagenesis strategies reveal variation in repair mechanisms among fungi[END_REF]. Nevertheless, NHEJ is likely to occur in C. albicans as scars typical of this repair process have been reported at repaired CRISPR-Cas9-induced DSBs [START_REF] Ng | Dramatic improvement of CRISPR/ Cas9 editing in Candida albicans by increased single guide RNA expression[END_REF]. Notably, all of these studies have been performed in diploid isolates of C. albicans that are heterozygous at the MTL locus. In S. cerevisiae, NHEJ is downregulated in a/a diploid cells when compared with homozygous diploid or haploid cells only expressing MATa or MATa. This is shown to be accomplished by transcriptional repression of specific genes by the a1/a2 repressor [START_REF] Valencia | NEJ1 controls non-homologous end joining in Saccharomyces cerevisiae[END_REF]. Therefore, NHEJ studies in MTL homozygous diploid cells or haploid cells should address the absence or presence of NHEJ in C. albicans.

Several studies have shown that treatment of C. albicans with DNA-damaging agents triggers polarized growth [START_REF] Bachewich | Cell cycle arrest during S or M phase generates polarized growth via distinct signals in Candida albicans[END_REF][START_REF] Da Silva Dantas | Thioredoxin regulates multiple hydrogen peroxide-induced signaling pathways in Candida albicans[END_REF]. Mutants with altered expression of genes coding for proteins involved in DNA damage response and cell cycle regulation also display aberrant filamentous morphology [START_REF] Thakur | A coordinated interdependent protein circuitry stabilizes the kinetochore ensemble to protect CENP-A in the human pathogenic yeast Candida albicans[END_REF][START_REF] Roy | CaMtw1, a member of the evolutionarily conserved Mis12 kinetochore protein family, is required for efficient inner kinetochore assembly in the pathogenic yeast Candida albicans[END_REF][START_REF] Andaluz | Rad52 function prevents chromosome loss and truncation in Candida albicans[END_REF][START_REF] Bachewich | Cell cycle arrest during S or M phase generates polarized growth via distinct signals in Candida albicans[END_REF][START_REF] Bachewich | Cyclin Cln3p links G1 progression to hyphal and pseudohyphal development in Candida albicans[END_REF][START_REF] Bensen | The mitotic cyclins Clb2p and Clb4p affect morphogenesis in Candida albicans[END_REF][START_REF] Shi | Critical role of DNA checkpoints in mediating genotoxic-stress-induced filamentous growth in Candida albicans[END_REF][START_REF] Thakur | The essentiality of the fungusspecific Dam1 complex is correlated with a one-kinetochoreone-microtubule interaction present throughout the cell cycle, independent of the nature of a centromere[END_REF][START_REF] Loll-Krippleber | A study of the DNA damage checkpoint in Candida albicans: uncoupling of the functions of Rad53 in DNA repair, cell cycle regulation and genotoxic stressinduced polarized growth[END_REF] (Figure 5). Strikingly, these data suggest that genotoxic-stressinduced polarised growth involves but does not require the expression of hyphal-specific genes. It is possible that stress-induced polarised growth is different from standard hyphal growth. Another key aspect of DNA repair is the accessibility of the DNA lesion to the DNA repair machinery. Proteins involved in chromatin assembly and remodelling have been shown to be important for efficient DNA repair in C. albicans as they change chromatin structure and allow repair proteins to access damaged DNA through the acetylation of histone H4 [START_REF] Tscherner | The histone acetyltransferase Hat1 facilitates DNA damage repair and morphogenesis in Candida albicans[END_REF].

Similarly, involvement of HR in the kinetochore assembly has been demonstrated in C. albicans [START_REF] Mitra | Rad51-Rad52 mediated maintenance of centromeric chromatin in Candida albicans[END_REF]. In mammalian cells, acetylation of histone H4 has been shown to also play a critical role in directing changes both in chromatin organisation and in promoting recruitment of DSB repair proteins to sites of DNA damage [START_REF] Dhar | The tale of a tail: histone H4 acetylation and the repair of DNA breaks[END_REF]. Moreover, different chromatin signatures associated with HR or NHEJ repair have been recently defined [START_REF] Clouaire | Comprehensive mapping of histone modifications at DNA double-strand breaks deciphers repair pathway chromatin signatures[END_REF]. It has also been demonstrated that, apart from rapidly accumulating DNA damage, C. albicans cells lacking the histone acetyltransferase Hat1 also switch from yeast-like to polarized growth [START_REF] Tscherner | The histone acetyltransferase Hat1 facilitates DNA damage repair and morphogenesis in Candida albicans[END_REF]. Altogether, these data corroborate

Polarised growth

Accumula on of DNA damage that perturbations of cell-cycle progression, a direct consequence of DNA damage, induce filamentous growth in C. albicans, in a manner dependent on the DNA damage/replication checkpoint kinase Rad53.

Concluding Remarks

In this review, we have highlighted some of the distinctive features uncovered from recent studies on the genome biology of C. albicans that make this yeast a model complementary to S. cerevisiae and S. pombe; two model species that are at the two extremes of yeast evolution. C. albicans exhibits specificities with respect to: (i) its life cycle whereby haploid, diploid, and tetraploid alternate through means of a meiosis-independent, recombinogenic parasexual cycle; (ii) chromosomal landmarks, especially the CENs that have unique organisation and epigenetic properties; (iii) an unusual genome plasticity, that frequently generates aneuploidies and LOH events; (iv) an almost obligate usage of HR for DSB repair; and (v) a coupling of the DNA damage response to morphogenetic shifts. A unique combination of these attributes in one organism allows investigators to address a variety of questions in genome biology that are not generally studied using the conventional yeast models. For instance, as most studies on recombination in S. cerevisiae and S. pombe employ laboratory haploid or homozygous diploid strains, little is known about the impact of heterozygosity on the biology of yeasts. Results from studies in C. albicans clearly demonstrate that heterozygosity is often advantageous, yet allows the propagation of recessive deleterious or lethal alleles that are detrimental upon homozygosis.

Despite changes in epidemiology, C. albicans remains responsible for a majority of yeast infections and the treatment of these infections is still a challenge in specific cases, such as systemic or recurrent vulvovaginal candidiasis. One of the key questions is whether C. albicans genome plasticity and hallmarks indeed contribute to making this species such an important pathogen. There is no doubt that LOH, ploidy changes, and isochromosome formation contribute to the elevation of antifungal resistance and treatment failure. Moreover, exposure of C. albicans to a variety of stressesantifungals, oxidative stress, high temperaturehas been shown to promote recombination events that could help C. albicans permanently adapt to changes in environment. Residence of C. albicans in animals including in healthy humans, whether as a commensal or pathogen, is accompanied by mutation and recombination events, predominantly short-range LOH (76; Sitterlé et al., personal communication), that could not only reflect adaptation to the host but also help in repairing DNA alterations caused by constant exposure to DNA damaging agents (e.g., reactive oxygen species produced by immune cells).

A recent study reports that a combination of mutations and LOH events acquired during serial passaging in the gastrointestinal tract of mice, where C. albicans does not normally reside, allows C. albicans to shift from a poor gastrointestinal commensal and harmful bloodstreamborne pathogen to an efficient commensal and poor systemic pathogen [START_REF] Tso | Experimental evolution of a fungal pathogen into a gut symbiont[END_REF]. Therefore, the high genomic plasticity of C. albicans can bear long-term impact on its biology and gaining further insights into the genome biology of this species will certainly impact not only on our understanding of eukaryotic genome biology in general but also the mechanisms that make C. albicans such a successful opportunistic pathogen (see Outstanding Questions).

Figure 1 .(

 1 Figure 1.

Figure 2 .(

 2 Figure 2.

Figure 3 .

 3 Figure 3. Schematic of Essential Chromosomal Elements in Candida albicans. (A) A 3-5-kb long CENP-A-rich centromere (CEN) that lacks any common DNA sequence elements or pericentric repeats. The AT% of CEN regions is not significantly different from the rest of the genome and the CEN DNA sequence does not show any DNA methylation. The CEN function is dependent on the chromosomal context rather than DNA sequence, and thus epigenetically regulated. (B) DNA replication origins (ORIs), identified as ORC-bound regions, are categorised into arm ORIs and centromere ORIs. Like centromeres, origins do not show any strong common DNA sequence motif. (C) Telomere repeats in C. albicans are unusually long and subtelomeric regions have an unusually high number of TLO genes in this organism. The high copy number of TLO genes in C. albicans is expected to be the result of subtelomeric recombination, mediated positively by TLO recombination element and negatively by Sir2 [98]. (D) The MRS, which further consists of three sequence elements, namely, RB2, RPS, and HOK. The RB2 (6 kb) and HOK (8 kb) elements are nonrepetitive sequences that occur only once per MRS, flanking the RPS element. The repeated sequence (RPS) is a 2-kb-long repetitive sequence whose number can vary in an MRS. Each RPS unit carries an SfiI restriction enzyme site. SfiI mapping of the C. albicans genome served as a valuable tool to study chromosomal rearrangements before the whole genome sequence was available.

Figure 4 .

 4 Figure 4. Heterozygosity, LOH, Genome Dynamics, and Antifungal Resistance. (A) Heat map illustrating the density of heterozygous SNPs across chromosome 3 for seven isolates. Regions that have undergone LOH appear white and are most often extending towards the telomere, indicating that they are the result of either mitotic recombination, break-induced replication, or gene conversion with crossover. (B) Schematic view of the impact of environmental stresses on genome dynamics and adaptation in C. albicans. Abbreviation: LOH, loss-of-heterozygosity.

Figure 5 .

 5 Figure 5. DNA-Damage-Induced Morphogenesis in Candida albicans. Schematic of the link between accumulation of DNA damage induced by various genotoxic stresses and polarised growth in C. albicans.

In vivo passaging Mb In vitro stresses als Inducers of genome dynamics High temperatures Aneuploidies (Isochromosomes) Loss of heterozygosity Incr al resistance n through genome rearrangements Individuals with increased fitness

										Isolate 1
										Isolate 2
										Isolate 3
										Isolate 4
										Isolate 5
										Isolate 6
										Isolate 7
	0.0	0.2	0.4	0.5	0.7	0.9	1.1	1.3	1.4	1.6
	0	5	15	30	50	75	100	150
									Density

Acknowledgements M.L., C.D., and K.S. are supported by a grant from the Indo French Centre for the Promotion of Advanced Research (CEFIPRA, Project no. 5703-2). Work in the laboratory of C.D. has been supported by the French Government's Investissement d'Avenir program (Laboratoire d'Excellence Integrative Biology of Emerging Infectious Diseases, ANR-10-LABX-62-IBEID). A.F. was the recipient of a PhD. grant from INRA Jouy-en-Josas and Institut Pasteur. Work in the laboratory of K.S. is supported by grants from the Department of Biotechnology, Government of India (BT/PR13909/BRB/


M Hamdi

O Rouaud

D Tarlet

M Havet

Experimental investigation on convective heat transfer enhancement by EHD in wire to plate configuration

Keywords: Electrohydrodynamics, PIV, Convective heat transfer, Forced convection Dielectric barrier discharge IR: Infrared

The purpose of this study was to experimentally investigate the enhancement of forced convection heat transfer by corona wind in a channel scaled to potential future drying systems. An experimental rig was designed to investigate the thermal and dynamic effects of electro-hydrodynamic (EHD) induced air flow on the grounded bottom wall. The primary air flow, blown into the channel at controlled temperature and velocity, was disturbed by EHD secondary flow generated by a wire electrode. Using IR imaging, the heat transfer exchange at the wall was calculated in several configurations (i.e. electrodes location, wire-electrode to plate distance, applied voltage, primary air flow velocity), allowing the selection of optimal configurations according to the objective of minimum energy consumption or homogeneous transfer. Particle Image Velocimetry (PIV) characterization conducted on some configurations highlighted the vortices generated by the combination of the secondary and the primary air flows. Analysis of the flow patterns allowed to explain the enhancement of the local and average heat transfer coefficients. Higher enhancement could be obtained in the cross-wire configuration but led to heterogeneous treatment. For a more homogeneous one, the longitudinal wire configuration should be preferred thanks to the development of contrarotating vortices moving in the channel.

Introduction

One of the most promising techniques to enhance convective heat transfer in air is based on the use of the Electrohydrodynamic (EHD) induced air flow. The basic principle of EHD is the mechanism by which an electric field is used to influence the flow pattern. The effect can be carried out by corona discharge to induce flow motion (ionic wind) using either AC or DC high voltages.

Since EHD induced flows do not use any mechanical moving parts, they are of great interest and can be used in many thermal engineering applications. The advantages of using EHD technique include the reduction in size of the facility, improved control of thermal characteristics by monitoring the electrical field intensity, and simplicity in setting up due to the minima l requirements [START_REF] Omidvarborna | Effect of applied EHD on in-tube condensation of R-134a within an assembled experimental rig including a laboratory heat exchanger[END_REF]. Moreover, their fast response and simplicity make them more attractive as the electric energy is converted simply into kinetic energy and many studies have demonstrated the excellent potential of EHD as a technology for improving the energy efficiency of thermal systems. [START_REF] Ahmad | Compound effect of EHD and surface roughness in pool boiling and CHF with R-123[END_REF][START_REF] Huang | Heat transfer enhancement by needle-arrayed electrodes -An EHD integrated cooling system[END_REF][START_REF] Ould Ahmedou | Effect of Process Parameters on the EHD Airflow[END_REF][START_REF] Karayiannis | EHD boiling heat transfer enhancement of R123 and R11 on a tube bundle[END_REF][START_REF] Bardy | Transient exergetic efficiency and moisture loss analysis of forced convection drying with and without electrohydrodynamic enhancement[END_REF].

Several studies have been carried out concerning the use of EHD in process engineering. Large varieties of geometrical configurations, operating parameters and working fluid have been the subject of numerical and experimental investigations. EHD flows have been much investigated for their potential applications as flow actuators [START_REF] Benard | Detachment and attachment of an axisymmetric non-reactive jet with turbulent shear layer: Control by plasma actuator[END_REF] or electrohydrodynamic thrust [START_REF] Monrolin | Electrohydrodynamic thrust for in-atmosphere propulsion[END_REF], in heat exchangers [START_REF] Smith | Electrohydrodynamic augmentation of a reflux thermosyphon[END_REF] and cooling devices [START_REF] Kim | Ion wind generation and the application to cooling[END_REF], for electrosprays [START_REF] Gañán-Calvo | Current and droplet size in the electrospraying of liquids. Scaling laws[END_REF], collecting particles [START_REF] Zouzou | Effect of a filamentary discharge on the particle trajectory in a plane-to-plane DBD precipitator[END_REF] or corona-enhanced chemical reactions [START_REF] Chen | Electron Density and Energy Distributions in the Positive DC Corona: Interpretation for Corona-Enhanced Chemical Reactions[END_REF]. Some studies such as [START_REF] Léger | Control of low velocity airflow along a flat plate with a DC electrical discharge[END_REF] and [START_REF] Artana | Flow control with electrohydrodynamic actuators[END_REF] demonstrated the ability of corona discharges to reduce drag by modifying the boundary layer.

The use of EHD in the presence of a bulk flow to alter an external boundary layer has been an area of growing interest in the aerospace community while has received little attention for heat transfer enhancement. However, studies on this subject have shown that the increase in the heat transfer coefficient can be high depending on the configuration. For example the increase can reach 250% for a heated plate in air [START_REF] Velkoff | Low-velocity heat transfer to a flat plate in the presence of a corona discharge in air[END_REF], 270% in a wire / plate configuration [START_REF] Tada | Heat Transfer Enhancement in a Convective Field by Applying Ionic Wind[END_REF] and up to 830% for HCFC-123 boiling application [START_REF] Karayiannis | EHD boiling heat transfer enhancement of R123 and R11 on a tube bundle[END_REF]. A list of relevant studies using EHD in process engineering applications, dealing with heat transfer enhancement, is presented in Table 1. Many studies deal mainly with natural convection flow [START_REF] Wang | Experiment and simulation of natural convection heat transfer of transformer oil under electric field[END_REF][START_REF] Kasayapanand | Electrode arrangement effect on natural convection[END_REF][START_REF] Molki | The enhancement effect of corona discharge on natural convection heat transfer in triangular channels[END_REF][START_REF] Asadzadeh | Natural convective heat transfer of Fe 3O 4/ethylene glycol nanofluid in electric field[END_REF], and are rarely focused on the forced convection flow. Velkoff and Godfrey [START_REF] Velkoff | Low-velocity heat transfer to a flat plate in the presence of a corona discharge in air[END_REF] reported the first experiments on heat transfer enhancement by ionic winds in the presence of external flows. They demonstrated that the heat transfer was enhanced in the low-velocity regime; however, the ionic wind was swamped by the bulk flow. Thus, it was ineffective as the bulk velocity increases. A wide range of studies either experimental [START_REF] Go | Enhancement of external forced convection by ionic wind[END_REF][START_REF] Nakhla | Melting performance enhancement in latent heat storage module using solid extraction electrohydrodynamics (EHD)[END_REF][START_REF] Ohadi | Heat transfer enhancement of laminar and turbulent pipe flow via corona discharge[END_REF][START_REF] Chen | Influence of electrode configuration on the heat transfer performance of a LED heat source[END_REF], and numerical [START_REF] Ould Ahmedou | Effect of Process Parameters on the EHD Airflow[END_REF][START_REF] Deylami | Effects of EHD on the flow and heat transfer characteristics in a rectangular corrugated channel[END_REF][START_REF] Deylami | Numerical investigation of using various electrode arrangements for amplifying the EHD enhanced heat transfer in a smooth channel[END_REF][START_REF] Rafiei-Siahestalkhi | Numerical Investigation of Electrohydrodynamic Forced Convection Heat Transfer from a Circular Cylinder[END_REF] have been conducted

using EHD for heat transfer intensification in a channel. These studies have shown that the most effective EHD-induced flow can be achieved by lowering the bulk flow velocity.

Moreover, the EHD phenomenon in a channel depends on various parameters such as Reynolds number and applied voltage. The EHD is also a function of geometrical aspect such as the number of electrodes, the distance between electrodes, the wire radius, and the characteristics of the channel [START_REF] Ould Ahmedou | Assessment of the electrohydrodynamic drying process[END_REF]. It can be noticed that many questions regarding the optimization of the operating parameters remain unanswered while the control of heat transfer remains a key factor for many industrial applications.

During the past decade, EHD has undergone a growing interest in aerodynamic induced secondary flow in drying process. The literature points out that the EHD-induced flow has widely attracted the researchers' attention and they developed assisted-drying processes [START_REF] Yang | Electrohydrodynamic (EHD) drying of the Chinese wolfberry fruits[END_REF][START_REF] Dinani | Drying of Mushroom Slices Using Hot Air Combined with an Electrohydrodynamic (EHD) Drying System[END_REF][START_REF] Zhong | Numerical investigation of the multipin electrohydrodynamic dryer: Effect of cross-flow air stream[END_REF][START_REF] Lai | EHD-enhanced drying with multiple needle electrode[END_REF][START_REF] Polat | Determination of drying kinetics and quality parameters for drying apricot cubes with electrohydrodynamic, hot air and combined electrohydrodynamichot air drying methods[END_REF]. Compared to conventional hot air forced convective drying systems and freezedrying, EHD drying systems in a channel can improve the energy efficiency by reducing energy consumption. Increasing locally, the heat and mass transfer lead to the reduction of the high inlet velocity of the drying air. This contributes to a significant reduction of the energy consumption in the whole drying process.

As reported by Kudra and Martynenko [START_REF] Kudra | Energy Aspects in Electrohydrodynamic Drying[END_REF], EHD drying was recognized as an energy efficient and environmentally friendly technique: the drying time was reduced by about 15-40% with a drying rate multiplied by 1.3 to 4.52, depending on the operating conditions, type of material and amount of humidity in the sample. Ould Ahmedou et al [START_REF] Ould Ahmedou | Assessment of the electrohydrodynamic drying process[END_REF] have shown that the ionic wind generated by a corona discharge could lead to weight losses equivalent to those resulted from forced convection at 4.2 m.s -1 . They concluded that EHD drying process is a promising alternative to enhance heat and mass transfer. Lai [START_REF] Lai | A prototype of EHD-enhanced drying system[END_REF] was one of the first to demonstrate that EHD technology could be scaled up from laboratory bench to industrial size. The results revealed that the drying enhancement by corona wind in real applications depends on many factors such as corona polarity, electrode spacing and height, conveyor moving velocity, characteristics of material, etc. He concluded that an accurate tune of these factors is necessary to optimize the operating conditions of the system. Regarding the electrode configuration, one can notice from Table 1, that wire-plate and point-plate configuration are widely used. Lai and Wong [START_REF] Lai | EHD-Enhanced Drying with Needle Electrode[END_REF] have found that the wire / electrode configuration was more suitable in EHD drying when compared to the needle at smaller values of supplied voltage. Defraeye and Martynenko [START_REF] Defraeye | Electrohydrodynamic drying of multiple food products: Evaluating the potential of emitter-collector electrode configurations for upscaling[END_REF],

based on a numerical study, recommended the wire-to-mesh configuration for drying because the configuration provides a more uniform drying. It is however difficult to generalize these results as the electrodes configurations also depend on the drying process type (continuous or batch…)

and the majority of studies have been conducted with small-size prototypes.

Given the simple design and reduced energy consumption, drying systems based on EHD enhanced heat transfer demonstrate a promising potential for bulk and industrial drying applications, especially for heat-sensitive products [START_REF] Anukiruthika | Electrohydrodynamic drying of foods: Principle, applications, and prospects[END_REF]. Nevertheless, as already raised [START_REF] Lai | Electrohydrodynamically-enhanced forced convection in a horizontal channel with oscillatory flows[END_REF][START_REF] Defraeye | Future perspectives for electrohydrodynamic drying of biomaterials[END_REF],

the main obstacle is the up-scaling and optimization of the EHD process and its associated system. For this reason, a parametric analysis is crucial to determine how the governing parameters affect the heat transfer rate in EHD drying applications.

Therefore, this paper focuses on both thermal and flow structure characterization for EHD induced secondary flow inside a semi-industrial scale channel using wire to plate configuration. Combining both Infrared (IR) thermal imaging and Particle Imaging Velocimetry (PIV) measurement technics, the influence of geometrical and operating parameters involved in the process were evaluated, with the aim to optimize the heat transfer intensification ratio to energy consumption.

Local and average convective heat transfer coefficients were calculated from the surface plate temperature measured by the IR system. Using air velocities resulting from PIV post-processing, the vortex structures generated by the interaction of secondary flow and primary flow were related to the heat transfer intensification. The effect of the inlet air velocity, the influence of geometrical parameters such as arrangement of wire-electrodes (parallel or perpendicular to primary air flow), wire-electrode to plate distance (d) and applied voltage (V) were investigated and discussed. From an analysis of energy consumption in each configuration, optimal configurations were selected. Since the final application of this study is dedicated to drying process, the analysis was extended by taking into consideration other criteria such as the homogeneity of the heat transfer coefficient.

To our knowledge, this work is one of the first where both PIV and IR imaging were used in a semi-industrial scale channel to analyze heat transfer enhancement due to EHD secondary air flow.

Material and methods

Experimental Set-up

The experimental set-up consisted of a rectangular channel section (15 cm × 19 cm) as shown in Fig. 1. Air flow was introduced into the channel at a controlled humidity using an Air Handling Unit (Munters ML 180, France). The inlet air temperature was adjusted by a cooling unit (Hitema, Italy) and the primary air flow velocity U  was controlled by valves. A honeycomb filter was placed at the entrance of the channel to stabilize the air flow. The investigation area, i.e the plate, had dimensions of 15 cm × 15 cm and was mounted flush to the bottom wall of the channel 100 cm downstream from the inlet. Electrically heated, the surface of the plate was considered to evaluate the heat transfer coefficient. The heat was supplied by Joule effect using a DC power supply (EL302, 30V 2A). A stainless-steel wire electrode (Ø =0.2 mm) was placed in the flow channel over the plate. To induce the ionic wind, a positive corona discharge was applied to the wire using a high voltage supply (High voltage supplier, Spellman, USA). Fig. 1 also shows two configurations for wire electrode arrangement, parallel and cross-wire. For the parallel arrangement, the wire electrode was placed parallel to the primary flow direction, while the wire was oriented across the air flow in the cross-wire arrangement. For both arrangements, different values for inter-electrode gap, d, were considered. was also introduced into the channel through an opening at the upper wall to measure the mean velocity of primary flow at the start of the investigation area. Moreover, temperature at different locations in the channel and the plate were recorded using several K type thermocouples. All the above-mentioned instruments were connected to a data logger (AOIP Datalog S20, France) where the measurements were acquired and recorded for further processing.

Thermal instrumentation

The heating plate, shown in Fig. 1, consisted of a heating foil (17.02 Ω) insulated on both sides by a thin layer of Kapton (50 µm) and covered with a 30 µm aluminum layer on the upper face.

The surface was used as both the ground electrode and the heat transfer surface. This surface was painted matt black (Electrotube, France) in order to achieve an emissivity close to 1. The assembly was placed on a 4 cm thick polystyrene insulating plate. The temperature field of the plate was measured with an infrared camera (Silver 450M Cedip, France) in the range of 3-5 µm. The camera had an accuracy of ±0.01 °C with a spatial resolution of 0.428 mm/pixel. The acquisition frequency was chosen at 50 Hz with a maximum resolution of 1280×800. The control and data recording of the camera were provided by a specific software. The camera was installed on a vertical support and adjusted vertically at the correct focal distance. A rectangular box, made with thick black matt paper, was placed between the camera and the channel to avoid reflections. Temperature measurement was conducted through a circular opening of 12 cm diameter at the top wall of the channel. To avoid air escaping from the channel, which could have affected the heat transfer on the surface plate, the hole was covered with a plastic foil, transparent at the working wavelength, and positioned at an angle > 35° to avoid any reflections. The circular opening did not provide access to the temperature of the heating plate corners. The cross section shown in Fig. 2 illustrates the thermal instrumentation in the cross-wire arrangement. The flow seeding was carried out by incense particles of diameter of 0.8 μm [START_REF] Hamdi | Comparison of different tracers for PIV measurements in EHD airflow[END_REF]. To generate seeding a combustion box was designed and placed at the air intake channel where several incense sticks were burned simultaneously. As reported in [START_REF] Hamdi | Comparison of different tracers for PIV measurements in EHD airflow[END_REF], seeding the flow with incense smoke particles is suitable for EHD flows. The particles concentration in the channel was measured by an optical particle counter (Grimm: model G1.10, Austria) and was of the order of 10 5 particles.L -1 , giving a particles density suitable for PIV measurement. The values of the measurement parameters are shown in Table II.

Results and discussions

For the purpose of heat transfer optimization in the channel, energy balance must be performed. The heat flux generated by Joule effect in the heating plate is mainly evacuated by convection with air through the upper face. The remaining heat is evacuated by irradiation with the channel walls and conduction through the downwardly polystyrene. The local and mean heat transfer coefficient respectively designated by hloc and hm were calculated for different configurations with the change in the electric power consumed P (W) for EHD generation. The power P was defined from the current and the voltage (P = V I).

The EHD number "NEHD" introduced by [START_REF] Sadek | The influence of electric fields on convective heat and mass transfer from a horizontal surface under forced convection[END_REF] was defined by relation (1) which expresses the ratio of electrostatic forces to the inertial forces. This number is interesting to characterize the EHD flow configuration.

𝑁 𝐸𝐻𝐷 = 𝑈 𝑠 𝑈 ∞ = 𝐼𝐿 𝑤𝑖𝑟𝑒 𝜌𝑏𝐴 𝑝 𝑈 ∞ 2 (1)
The secondary flow velocity 𝑈 𝑠 was estimated from the experimentally measured electrical current value. Analytical relationship to calculate the velocity is given by equation ( 2). This relationship is described in the literature by [START_REF] Yamamoto | Electrohydrodynamics in an electrostatic precipitator[END_REF] and [START_REF] Lai | Oscillatory electrohydrodynamic gas flows[END_REF].

𝑈 𝑠 = [√ 𝐼𝐿 𝑤𝑖𝑟𝑒 𝜌𝑏𝐴 𝑝 ] (2)
Experimental investigations for varying arrangement of wire-plate electrode under different values of the mean primary flow velocity U  and the applied voltage V have been conducted. From Table III, it can be noticed that the maximum applied voltage was limited due to the electrical arcing which mainly depends on the electrode gap. In the case of cross wire configuration and primary flow velocity of 0.3 m.s -1 (Re = 3525), Fig. 4 shows the corresponding current voltage characteristic curve for three different electrode gaps. Increasing the electrode gap (from d = 2 cm to d = 6 cm), led to the increase of the corresponding threshold voltage (from Vi = 7.8 kV to Vi =12 kV). As the electrode gap increased, the slope of the increase on current/voltage decreased and the value of the voltage limit increased. As a result, the corona discharge application range increased.

Temperatures and heat transfer enhancement

Fig. 5 shows the temperature distribution on the plate given by infrared thermal camera images of both cross wire (5a-c) and longitudinal wire (5e-g) configurations for varying electrode gap and for a primary air flow velocity U  = 0. Because the power supplied to the plate may be slightly different between each experiments, as well as air temperature, it was not possible to evaluate the heat transfer enhancement directly from the temperature distribution. Only the determination of the mean heat transfer coefficient makes it possible the comparison of the different configurations.

Figure 7. Evolution of the mean heat transfer coefficient (h m ) as a function of power consumption in longitudinal wire configuration

As the cases of forced convection without EHD correspond to P = 0 W, Fig. 6 and Fig. 7 confirm first of all the benefit of EHD to enhance the heat transfer coefficient in all the studied configurations. Moreover, the mean heat transfer enhancement due to EHD was, as expected, significant at low primary air flow velocity in most cases. Whatever the interelectrode gap, the primary air flow velocity value of U  = 0.3 m/s provided a greater value of the mean heat transfer coefficient hm as compared to other velocities. At this low value of U  , inertial forces are reduced and electrostatic forces take over.

The influence of the electrode gap also clearly appears on these figures. At the lowest value of d, the primary air flow velocity had no influence on the heat transfer coefficient. Electrostatic forces were so high that they generated an impinging jet that blocked the primary air flow. It can also be noticed that the power consumption in all cases was slightly higher when the electrode gap was set to 4 cm. In one hand, the maximum voltage was limited to 9 kV for d = 2 cm due to potential arcing. On the other hand, the ions collected by the plate for d = 6 cm were lower than for d = 4 cm due to the increased wire-electrode distance.

From Fig. 6, the optimal configuration in the cross-wire configuration is selected for the primary air flow velocity of 0.3 m/s and an electrode gap set to 6 cm. It offered the largest heat transfer coefficient (hm=34.6 W.m -2 .K -1 ) corresponding to an intensification of the mean heat exchange of the order of 8 times greater than the case of forced convection without EHD.

Moreover, this maximum of heat transfer enhancement was obtained with a low value of the electric power (P=1.68 W). From Fig. 7, the optimal configuration in the longitudinal wire case is selected also for the primary air flow velocity of 0.3 m/s but for the electrode gap of 4 cm. This case allowed to reach a high heat transfer coefficient (hm=30.9 W.m -2 .K -1 ) which was 7 times greater than the case without EHD, with a low electric power (P = 1.66 W). It can be noticed that at the larger electrode gap (d = 6 cm), the heat transfer enhancement was relatively small and presented a peak value at the intermediate electric power. This is due to the fact that the origin of the jet generated by the ionic wind was far from the plate. The jet then expanded beyond this latter, was more spread out and had a lower cooling effect. All these results confirmed the need to investigate the modification of the flow pattern by the ionic wind.

Flow structure

In this section, the dynamic behavior of the flow structures involved in heat transfer intensification were investigated. Cross wire and longitudinal configurations with d = 4 cm and U  = 0.3 m/s were firstly analyzed and compared. For both configurations at V = 16 kV, the values of the heat transfer coefficients were similar: hm = 25 W.m -2 .K -1 .

Cross wire configuration

PIV measurement was performed in a plan positioned parallel to the primary air flow and perpendicular to the wire electrode located at distance Lx=1/2 Lp. Fig. 8-a Fig. 8 illustrates the mappings of the flow generated by EHD combined with primary air flow directed from right to left. In the investigation area, it is clearly shown that the bulk flow, strongly altered by electrostatic forces, has been modified. It has taken the shape of an air jet that impinges the plate on an area located underneath the wire and then expands in both sides.

Furthermore, the IR images previously shown in Fig. 5 demonstrates that intense cooling took place specifically in the area underneath the wire. It is obvious from Fig. 8a that the velocity component "Uz" had a major effect for cooling the plate. Consequently, the use of the velocity component "Uz" was considered as a relevant criterion of characterization that can be very appropriate to obtain a link with the heat transfer coefficient.

The effect of the primary flow velocity on the flow pattern neighboring the plate was investigated.

U  = 0.6 m./s, c) U  = 1 m./s
When analyzing the velocity field mapping of the Uz component, it can be seen that the maximum values of |Uz|, about 1 m/s, was localized in the air jet zone underneath the wire and decreased with the increase of the primary flow velocity as described by many authors [START_REF] Liu | The effect of electrode polarity on EHD enhancement of boiling heat transfer in a vertical tube[END_REF], [START_REF] Ould Ahmedou | Effect of Process Parameters on the EHD Airflow[END_REF].

For a primary flow velocity of 0.3 m/s, the air jet was clearly visible and directed from the wire to the plate. By increasing the primary flow velocity to 0.6 m/s, the air jet was driven downstream of the plate. A flow vortex could be clearly recognized at the leading edge of the plate in the investigation area (Fig. 9-b). By increasing again the primary flow velocity to 1 m/s as shown in Fig. 9-c, the primary flow became dominant making the air jet less relevant but still existing. As a result, the center of the vortex moved towards the jet location and blocked the upstream halfzone of the plate. A similar flow structure was obtained for the other electrode gaps set to d = 2 cm and d = 6 cm. Therefore, the evolution of the flow structures when varying the primary velocity for these cases can be explained in the same way.

For a deeper analysis, the velocity profiles of the Uz component were plotted along the line 1

shown in Fig. 10. This line parallel to the x-axis is located at distance z = 1 cm above the plate in such a way to be as much as possible close to the plate while keeping a threshold distance away from the surface plate to avoid laser reflections. To obtain the optimum electrode gap distance, the velocity profiles of Uz component were compared for the optimal primary flow velocity of U  = 0.3 m./s. Fig. 13 shows the velocity profiles of Uz component along line 1 for different electrode gaps. rise to the highest average heat transfer coefficient (Fig. 6c). These results show the necessity to take into account other aspects related to process application (homogeneous treatment vs local treatment) to conclude on both cases regarding the optimal heat transfer intensification case.

Longitudinal wire configuration

For the case of longitudinal wire configuration, PIV measurement was carried out basically in Fig. 14 shows the mean velocity vector field in xz and yz plane. As shown in both planes, the EHD-generated flow combined with the primary air flow formed an air jet that impinged the plate. The air jet was located along the plate, underneath the wire (Fig. 14-a) and formed two parietal jets developing on both sides of impinging zone (Fig. 14-b). As a result, both velocity components Uz and Uy could be considered as the most relevant components influencing the cooling of the plate.

Therefore, in longitudinal wire configuration, one can consider the velocity component along y axis, Uy, as a relevant means of characterization in addition to Uz. For a better exploitation of Uy and Uz, the line 2 displayed in Fig. 15 was used as reference for plotting the velocity profiles.

PIV measurement conducted in the yz plane, shown in Fig. 15, reveals that the |Uy| component presented relatively high values in an area very close to the plate on both sides the air jet.

Therefore, the Uy component of the spreading air jet might have a major influence on cooling effect, say heat transfer intensification, on both sides of the plate.

Conclusion

The present study is one of the first where both PIV and IR thermography were performed to investigate the heat transfer enhancement by ionic wind in a rectangular channel, which dimensions correspond to real size of potential future drying systems.

The experiments clearly demonstrated that such an approach is necessary to understand, in different wire-to plate electrode configurations, the results of heat transfer enhancement versus flow structure alteration.

Thermal characterization essentially made possible to identify and select optimal configurations according to different aspects like transfer enhancement, thermal homogeneity and energy consumption. The study of flow characterization by PIV figured out the relationship between the geometric and operating parameters with the heat transfer intensification distribution. It enabled the understanding of the mechanisms of secondary flows structures combined with the primary flow (air jet, vortex...). The following conclusions were derived:

In all cases, the highest heat transfer enhancement by EHD was obtained for the smaller primary air flow velocity (0.3 m/s). As expected, at low Reynolds number, primary air flow provided the most efficient use of the secondary air flow induced by EHD. This in fact corresponds to a large EHD number.

For the single wire-electrode arranged perpendicular to the primary air flow, the greatest increase in heat transfer occurred when the wire-electrode was set at the highest distance from the plate (d = 6 cm) where it provided the best intensification (hm/h0 = 8). For the minimum value of wire to plate distance, the heat transfer enhancement was localized over a very limited area beneath the wire. In this case, the generated secondary flow was similar to an impinging jet that caused a strong intensification on an area located beneath the wire. Such a configuration should be selected when the objective consists in getting local heat/mass transfer enhancement without any necessity to get homogeneous treatment.

For the case of single wire-electrode arranged parallel to primary air flow, the highest increase in heat transfer enhancement was achieved for wire to plate distance of 4 cm which provided the most efficient combination of primary and secondary air flow where the intensification was improved 7 times (hm/h0 = 7). In this case, the air jet was generated along the wire where the maximum velocity was similar to that achieved with the cross wire. It was shown that contrarotating vortices appeared and were developing as progressing in the channel. Such a configuration should be selected when the objective consists in getting an homogeneous treatment due the change in the vortices shape structures of the secondary flow along the plate.

Whatever the configuration, results indicated that such a deep analysis of heat transfer enhancement and flow structure is required for a correct design of future system (geometrical parameters and applied voltage) according to the objective. High local enhancement can be reached when decreasing the electrode gap but the enhancement is then limited by arcing phenomenon. A more homogeneous enhancement requires a larger inter-electrodes gap that lead to a larger jet with lower impacting velocities.

  AbbreviationsEHD:Electro Hydro Dynamic PIV: Particle Image Velocimetry

Figure 1 .

 1 Figure 1. Schematic diagram of the experimental setup and arrangement of electrodes

Figure 2 .Figure 3 .

 23 Figure 2. Cross-sectional view of the experimental set-up in cross wire-electrode configuration

Figure 4 .

 4 Figure 4. Current-Voltage characteristic curve in cross wire configuration for varying electrode gap (U  = 0,3 m.s -1 )

Figure 5 .

 5 Figure 5. Temperature distribution mapping of: (a,b,c) cross-wire configuration and (e,f,g) longitudinal wire configuration for U  = 0.3 m/s.

Fig. 6 and

 6 Fig.6and Fig.7displays the evolution of the mean heat transfer coefficient hm as a function of the power consumption in cross wire and longitudinal wire configurations, respectively, for three primary air flow velocities.

Figure 6 .

 6 Figure 6. Evolution of the mean heat transfer coefficient (h m ) as a function of power consumption in cross wire configuration

Figure 8 .

 8 Figure 8. Mapping of mean velocity field U (a) and streamlines velocity contour of U z component (b)

Fig. 9 Figure 9 .

 99 Figure 9. U z contour mapping and streamlines for varying primary flow velocities: a) U  = 0.3 m./s, b)

Figure 10 . 12 .

 1012 Figure 10. Position of the line 1 on the x z plan (cross-wire configuration, d = 4 cm)

Figure 11 .Figure 12 . 12 .

 111212 Figure 11. Velocity component U z , along the line 1 Figure 12. Surface temperature profiles T p

Figure 13 .

 13 Figure 13. U z component profiles, along line 1 for primary flow velocity of U  = 0.3 m./s at different electrode gaps.

  Figure. 14 Mean velocity vector and contour mapping for the optimal configuration in: a) in xz plane and b) yz plane

Figure . 15

 15 Figure .15 Mapping of the U y contours component and streamlines in xz plan

Figure. 16 Figure. 17 18 .

 161718 Figure. 16 Mapping of the U y contours component and streamlines on the yz plan at positions: (a) L x = 1/4 L p , (b) L x = 1/2 L p and (c) L x = 3/4 L p

Figure 18 .Figure 19 .

 1819 Figure 18. Velocity profiles of U z component, along a line located at z = 1 cm in xz plane

Table I

 I Some studies carried out on the use of EHD in processes specifically dedicated to heat transfer 90 enhancement, with varying electrodes configurations and working fluids. 91

	[9]	Experimental	HFE-7000	wire / inner tube wall
	Reference Type of research Fluid	Electrode shape	Application
	[17-19]	Experimental	Air	wire / plate
	[19,20]	Numerical	Air	wire / plate
	[16,21]	Experimental	Air	multi-wires / plate
	[21,22]	Numerical	Air	multi-wires /plate
	[3]	Experimental	Air	needle-array / plate
	[23-25]	Experimental	Air	wire / tube
	[26]	Experimental	Air	Multi-rod / plate	Convective
	[27,28] [29]	Numerical Experimental	Air Air	wire / cylinder wire / tube and 2 wires / tube	heat transfer
	[30]	Experimental	Air	needle / meshed plate
	[10]	Experimental	Air	wire / semi-cylindrical
				contour
	[31]	Experimental	Air	wire / triangular tube
	[32]	Experimental	Fe3O4 / ethylene glycol	wire / wire
	[19]	Experimental /	Oil	wire / plate
		Numerical
	[6,33-36]	Experimental	Air	wire / plate
	[37]	Numerical	Air	wire / plate
	[4]	Numerical	Air	multi-wires / plate
	[38]	Numerical	Air	wire / mesh and
				periodic wire / mesh	Drying
	[39,40]	Experimental	Air	single-pin, needle / plate
	[41-43]	Experimental	Air	multi-pin, multi-needle / plate
	[44]	Experimental	Air	needle / wire
	[45]	Experimental	Air	multi-pin/wire
	[46]	Experimental	HFC-134a	wire / inner tube wall
	[47]	Experimental/	HCFC-123	wire / inner tube wall
		Theoretical			Condensing
	[1]	Experimental	HFC-134a	wire / inner tube wall	systems
	[48]	Experimental	HFC-134a	rod / inner tube wall
	[49]	Experimental	HFC-245fa	rod / inner wall
	[50]	Experimental	HCFC-123	concentric wire / cylinder
	[51]	Experimental	HFC-134a	concentric wire / cylinder
	[52]	Experimental	HFE 7000	rod / inner tube wall
	[5]	Experimental	HCFC-123 and CFC-11	rod / tube wall	Boiling
	[53]	Experimental	N-pentane	rod / cylinder sheet	systems
	[2]	Experimental	HCFC-123	wire mesh / tube wall
	[54]	Numerical	liquid CCl4	wire / wire
	[55]	Experimental	NaCl solutions	pin / plate
	[56]	Experimental	HFC-134a	wire mesh / rectangular ribs
	[57,58]	Numerical	Water	wire / plate	Evaporating
	[59]	Experimental	HCFC-123 and HCFC-	plate multilayered /plate	systems
			141b

Table II .

 II PIV acquisition parameters

	Acquisition frequency	5 Hz
	Number of snapshots	300
	Size of the interrogation area	32×32
	Time between two laser pulses	300 µs
	Post-processing algorithm	Adaptive correlation

Table III

 III

	Table III. Experimental configurations
	Configuration	Primary air flow	Electrode gap	Voltage V (kV)
		Velocity U  (m/s)	d (cm)
		0.3
	Cross wire	0.6
		1.0

shows either cross wire and longitudinal wire configurations under varying primary flow velocity, electrode gap and applied voltage.

Acknowledgement

Acknowledgement to Mohammad Javad Javaherian (Ph.D. candidate at the Department of Aerospace & Ocean Engineering, Virginia Tech, USA) for providing language help with this manuscript. The authors gratefully acknowledge the support of the Pays de Loire program for outstanding research in the field of energetics (PERLE2).

Highlights

 Using EHD in thermal process leads to heat transfer enhancement and reduces energy consumption  EHD induced secondary flow disturbs the thermal boundary layer and increases heat transfer  Wire /plate electrode arrangement is efficient for combined secondary flow generation with primary flow in a channel  Longitudinal wire configuration improves the homogeneity of heat transfer compared to cross wire configuration  Combination of thermal and dynamic flow characterization is the main key for EHD process optimization