Omar Saket

Junkang Wang

Nuno Amador-Mendez

Martina Morassi

Arup Kunti

Fabien Bayle

Stéphane Collin

Arnaud Jollivet

Andrey Babichev

Tanbir Sodhi

Jean-Christophe Harmand

François H Julien

Noelle Gogneau

Maria Tchernycheva
email: maria.tchernycheva@c2n.upsaclay.fr

Investigation of the effect of the doping order in GaN nanowire p-n junctions grown by molecularbeam epitaxy

Keywords: nanowires, GaN, doping, EBIC, PAMBE

We analyse the electrical and optical propreties of single GaN nanowire p-n junctions grown by plasma-assisted molecular-beam epitaxy using magnesium and silicon as doping sources. Different junction architectures having either a n-base or a p-base structure are compared using optical and electrical analyses. Electron-beam induced current microscopy of the nanowires shows that in the case of a n-base p-n junction the parasitic radial growth enhanced by the Mg doping leads to a mixed axial-radial behavior with strong wire-to-wire fluctuations of the junction position and shape. By reverting the doping order p-base p-n junctions with a purely axial well-defined structure and a low wire-to-wire dispersion are achieved. The good optical quality of the top n nanowire segment grown on a p-doped stem is preserved. A hole concentration in the p-doped segment exceeding 10 18 cm -3 was extracted from electron beam induced current mapping and photoluminescence analyses. This high concentration is reached without degrading the nanowire morphology.

Introduction

The discovery of the p-type doping of nitride semiconductors using Magnesium (Mg) impurities in 1989 [START_REF] Amano | P-Type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Beam Irradiation[END_REF][START_REF] Akasaki | Photoluminescence of Mg-doped p-type GaN and electroluminescence of GaN p-n junction LED[END_REF][START_REF] Nakamura | Thermal Annealing Effects on P-Type Mg-Doped GaN Films[END_REF] enabled novel optoelectronic applications, the main one being the developement of energy saving solid state lighting [START_REF] Feezell | Invention, development, and status of the blue light-emitting diode, the enabler of solid-state lighting[END_REF]. However, reaching high hole concentration in nitrides is challenging due to the limited solubility of Mg in GaN, to the high activation energy of the Mg acceptor and to compensation problems [START_REF] Wang | Calculation of shallow donor levels in GaN[END_REF][START_REF] Alves | Compensation mechanism in MOCVD and MBE grown GaN[END_REF][START_REF] Iida | Compensation effect of Mg-doped a-and cplane GaN films grown by metalorganic vapor phase epitaxy[END_REF]. The nanowire (NW) geometry is favorable for nitride p-doping thanks to a decreased formation energy of defects at the surface [START_REF] Pernot | Doping and Transport Wide Band Gap Semiconductor Nanowires 1[END_REF] and to the possibility to relax the compressive strain induced by Mg incorporation [START_REF] Kirste | Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements[END_REF][START_REF] Flynn | The dependence of Raman scattering on Mg concentration in Mg-doped GaN grown by MBE[END_REF]. In addition, the small NW footprint is also favorable for achieving dislocation-free growth on a variety of lattice-mismatched substrates like Si [START_REF] Tchernycheva | Growth of GaN free-standing nanowires by plasma-assisted molecular beam epitaxy: structural and optical characterization[END_REF] or nonconventional substrates [START_REF] Wölz | Epitaxial Growth of GaN Nanowires with High Structural Perfection on a Metallic TiN Film[END_REF][START_REF] Kumaresan | Self-induced growth of vertical GaN nanowires on silica[END_REF], which is particularly important for device applications. A large number of studies has been dedicated to the plasma-assisted molecular-beam epitaxy (PAMBE) growth of Mg-doped GaN NWs [START_REF] Furtmayr | Nucleation and growth of GaN nanorods on Si (111) surfaces by plasma-assisted molecular beam epitaxy -The influence of Si-and Mg-doping[END_REF][START_REF] Stoica | Doping of III-Nitride Nanowires Grown by Molecular Beam Epitaxy[END_REF][START_REF] Schuster | Doped GaN nanowires on diamond: Structural properties and charge carrier distribution[END_REF][START_REF] Limbach | The influence of Mg doping on the nucleation of self-induced GaN nanowires[END_REF][START_REF] Sutter | Morphology and Optical Properties of Mg Doped GaN Nanowires in Dependence of Growth Temperature[END_REF][START_REF] Zhao | Recent Advances on p-Type III-Nitride Nanowires by[END_REF]. It was shown that Mg doping incorporation may be inhomogeneous over the NW volume with a tendency to segregate at the NW lateral surfaces [START_REF] Fang | Si Donor Incorporation in GaN Nanowires[END_REF][START_REF] Siladie | Dopant radial inhomogeneity in Mg-doped GaN nanowires[END_REF]. Nevertheless, the NW morphology was confirmed to be favorable for Mg incorporation and a high hole concentration was reported not only in GaN NWs [START_REF] Fang | Axial pn junction and space charge limited current in single GaN nanowire[END_REF][START_REF] Kamimura | p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements[END_REF] but also in a more challenging case of Al-rich AlGaN alloys [START_REF] Sutter | Morphology and Optical Properties of Mg Doped GaN Nanowires in Dependence of Growth Temperature[END_REF][START_REF] Siladie | Mg and In Codoped p-type AlN Nanowires for pn Junction Realization[END_REF][START_REF] Zhao | Aluminum nitride nanowire light emitting diodes: Breaking the fundamental bottleneck of deep ultraviolet light sources[END_REF][START_REF] Zhao | Optical and electrical properties of Mgdoped AlN nanowires grown by molecular beam epitaxy[END_REF][START_REF] Wang | p-Type dopant incorporation and surface charge properties of catalyst-free GaN nanowires revealed by micro-Raman scattering and X-ray photoelectron spectroscopy[END_REF][START_REF] Tran | On the mechanism of highly efficient p-type conduction of Mg-doped ultra-wide-bandgap AlN nanostructures[END_REF].

Due to the NW nanoscale diameter, doping quantification in NWs is not straightforward. Hall measurements are indeed extremely challenging in NWs [START_REF] Storm | Spatially resolved Hall effect measurement in a single semiconductor nanowire[END_REF][START_REF] Hultin | Comparing Hall Effect and Field Effect Measurements on the Same Single Nanowire[END_REF][START_REF] Haas | Electron Interference in Hall Effect Measurements on GaAs/InAs Core/Shell Nanowires[END_REF]. The most popular way to estimate the NW doping concentration is based on contactfree optical methods, namely luminescence or Raman spectroscopy [START_REF] Gîrgel | Optical characterization of magnesium incorporation in p-GaN layers for core-shell nanorod light-emitting diodes[END_REF][START_REF] Hortelano | Cathodoluminescence study of Mg activation in nonpolar and semi-polar faces of undoped/Mg-doped GaN coreshell nanorods[END_REF][START_REF] Mohajerani | Evaluation of local free carrier concentrations in individual heavily-doped GaN:Si micro-rods by micro-Raman spectroscopy[END_REF][START_REF] Kumar | Spectroscopic signatures of native charge compensation in Mg doped GaN nanorods[END_REF][START_REF] Wang | Spatially Resolved Doping Concentration and Nonradiative Lifetime Profiles in Single Si-Doped InP Nanowires Using Photoluminescence Mapping[END_REF][START_REF] Chen | Determination of n-Type Doping Level in Single GaAs Nanowires by Cathodoluminescence[END_REF][START_REF] Tchoulfian | Thermoelectric and micro-Raman measurements of carrier density and mobility in heavily Si-doped GaN wires[END_REF]. However optical methods require a separate calibration, for example using photoelectrochemical measurements [START_REF] Kamimura | p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements[END_REF]. Scanning probe microscopy can also be used to assess doping: e.g. Kelvin probe force microscopy and the electron beam induced current microscopy (EBIC) were successfully applied to assess carrier concentrations in GaN NW p-n junctions [START_REF] Fang | Axial pn junction and space charge limited current in single GaN nanowire[END_REF][START_REF] Minj | Direct assessment of p-n junctions in single GaN nanowires by Kelvin probe force microscopy[END_REF].

It was reported by different groups, that high Mg doping is accompanied with a morphological degradation of the NWs, promoting in particular a shell formation [START_REF] Fang | Axial pn junction and space charge limited current in single GaN nanowire[END_REF][START_REF] Kamimura | p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements[END_REF]. Apart from a few exceptions [START_REF] Brubaker | Toward Discrete Axial p-n Junction Nanowire Light-Emitting Diodes Grown by Plasma-Assisted Molecular Beam Epitaxy[END_REF][START_REF] Sarwar | Tuning the polarizationinduced free hole density in nanowires graded from GaN to AlN[END_REF], most of the axial junction nitride NW devices use a p-n or a p-i-n junction with the p-segment grown on top [START_REF] Luna Bugallo | Visible-blind photodetector based on p-i-n junction GaN nanowire ensembles[END_REF][START_REF] Jacopin | Interplay of the photovoltaic and photoconductive operation modes in visible-blind photodetectors based on axial p-i-n junction GaN nanowires[END_REF][START_REF] Cuesta | Effect of Bias on the Response of GaN Axial p-n Junction Single-Nanowire Photodetectors[END_REF][START_REF] Guan | Nitride Nanowires for Light Emitting Diodes Light-Emitting Diodes[END_REF]. The parasitic p-doped shell formed during the top segment growth may be an issue for these devices since it changes the current injection / extraction path [START_REF] Ra | Understanding the p-Type GaN Nanocrystals on InGaN Nanowire Heterostructures[END_REF] and also makes difficult the doping quantification using electrical methods. Reverting the doping order can solve these problems. Memory effects are known to be reduced in the MBE growth of nitrides compared to the Metal-Organic Chemical Vapour Deposition (MOCVD) [START_REF] Smorchkova | Mg doping of GaN layers grown by plasma-assisted molecular-beam epitaxy[END_REF], so an abrupt doping profile can be obtained by growing an n-doped segment on top of a p-doped one. However, one may expect that if the morphology of the p-doped segment is degraded by the Mg incorporation, the material quality of the top segment may be compromised.

In the present work we investigate this open question by comparing the electrical and optical properties of n-base and p-base p-n junction GaN NWs. The NWs are grown by PAMBE on Si (111) substrate using Mg and Si as doping sources. The NW morphology and optical properties are first assessed on a series of homogeneously Mg-doped samples. Then individual NWs containing n-base and p-base p-n junctions are analyzed by cathodoluminescence (CL) and EBIC. For the n-base junction, the analyses reveal a parasitic radial shell resulting in a mixed axial and radial behaviour and a high wire-to-wire inhomogeneity. When the doping order is inverted, purely axial junctions are achieved characterized by a lower wire-to-wire dispersion. The analysis of the EBIC profiles provides an estimate of the hole concentration in excess of 10 18 cm -3 . For this high hole concentration, the NWs preserved a cylindrical morphology and exhibited good optical properties.

Nanowire growth

Self-assembled GaN NWs were grown on a conductive oxide-free Si(111) substrate by PAMBE, active nitrogen (N) being supplied by a radiofrequency plasma cell. Prior to the growth of GaN NWs, a 2.5 nm thick AlN buffer layer was deposited on the substrate [START_REF] Tchernycheva | Growth of GaN free-standing nanowires by plasmaassisted molecular beam epitaxy: structural and optical characterization[END_REF][START_REF] Largeau | N-Polar GaN Nanowires Seeded by Al Droplets on Si(111)[END_REF] to allow for a better control of the NW nucleation, density and orientation [START_REF] Li | GaN based nanorods for solid state lighting[END_REF][START_REF] Landré | Molecular beam epitaxy growth and optical properties of AlN nanowires[END_REF][START_REF] Brubaker | Effect of AlN buffer layer properties on the morphology and polarity of GaN nanowires grown by molecular beam epitaxy[END_REF][START_REF] Songmuang | From nucleation to growth of catalyst-free GaN nanowires on thin AlN buffer layer[END_REF][START_REF] Bertness | Nucleation conditions for catalyst-free GaN nanowires[END_REF]. The GaN NW growth was performed under N-rich conditions with an N/Ga ratio of about 1.36, a high growth rate of 0.7 monolayers/sec and at a fixed substrate temperature of 800 °C. The n-type and p-type doping of GaN NWs was achieved by using Si and Mg, respectively, the doping level being modulated by adjusting the dopant cell temperature.

Two sets of samples were investigated. First, a series of five homogeneously p-type doped GaN NW samples was grown during 3 h under identical condition except for the doping level. The Mg cell temperature T Mg was increased by increments of 50°C from 225°C to 425°C. In addition, an undoped reference NW sample and one homogeneously ntype doped NW sample using the Si cell temperature T Si of 1150°C were grown.

The resulting doped GaN NWs are vertically oriented with a predominantly hexagonal shape. Figure 1 displays crosssectional scanning electron microscopy (SEM) images of the undoped reference sample (panel a), p-doped GaN NWs with T Mg of 375 °C and 425°C (panels b and c) and n-doped NWs (panel d). The average NW diameter for the undoped reference sample is 40 nm and the length is 1.1 µm. For the Si doped sample the NW morphology and in particular the diameter remain similar to the undoped case, while the average NW length is reduced to 0.86 µm. In the literature, the Si doping was reported to delay the NW nucleation and to reduce the axial growth rate [START_REF] Furtmayr | Nucleation and growth of GaN nanorods on Si (111) surfaces by plasma-assisted molecular beam epitaxy -The influence of Si-and Mg-doping[END_REF], which is consistent with the observed length reduction. For very high doping levels, Si doping may lead to inverse tapering [START_REF] Furtmayr | Nucleation and growth of GaN nanorods on Si (111) surfaces by plasma-assisted molecular beam epitaxy -The influence of Si-and Mg-doping[END_REF][START_REF] Stoica | Doping of III-Nitride Nanowires Grown by Molecular Beam Epitaxy[END_REF] and even trumpet shape formation [START_REF] Bolshakov | Dopant-stimulated growth of GaN nanotube-like nanostructures on Si(111) by molecular beam epitaxy[END_REF]. In our study, this high doping level was not reached and a cylindrical NW shape with a diameter close to the one in undoped reference was preserved.

For the Mg doped NWs an increase of the diameter is observed, which increases up to 80 nm for the higher doping cell temperature, while the average NW length slightly decreases to 1 µm. These observations are consistent with previous publications, where the Mg doping was reported to speed up the NW nucleation, while leading to a decrease in the NW growth rate and to a strong increase of the NW diameter [START_REF] Furtmayr | Nucleation and growth of GaN nanorods on Si (111) surfaces by plasma-assisted molecular beam epitaxy -The influence of Si-and Mg-doping[END_REF]. It is important to note that up to a Mg temperature of 425 °C, the NWs preserve a cylindrical shape with only limited coalescence between adjacent NWs.

We note that it is difficult to precisely analyze the radial growth. It can be done by transmission electron microscopy (TEM) by introducing AlN marker layers during the NW growth [START_REF] Tchernycheva | Growth of GaN free-standing nanowires by plasmaassisted molecular beam epitaxy: structural and optical characterization[END_REF][START_REF] Tchernycheva | GaN/AlN free-standing nanowires grown by molecular beam epitaxy[END_REF][START_REF] Galopin | Morphology of self-catalyzed GaN nanowires and chronology of their formation by molecular beam epitaxy[END_REF], however the marker layers create potential barriers and cannot be applied in samples dedicated to electrical measurements.

After morphological and optical analyses of Mg-doped NWs (detailed in the next section), a second set of two samples containing p-n junctions with a different doping order was grown. The Si cell temperature was fixed at 1150°C. For the Mg cell temperature the highest value T Mg =425 °C was chosen. The "n-base" p-n junction consists of a bottom ndoped segment grown for 3 h and a top p-doped segment grown for 2 h. The "p-base" p-n junction consists of a bottom p-doped segment grown for 1.5 h and a top n-doped segment grown for 3 h. The Si(111) substrate was also changed from n-type to p-type to match the substrate doping type with the NW base doping type. The corresponding cross-sectional morphologies of the "n-base" and "p-base" p-n junctions are shown in Figure 1(e) and (f), respectively. Focusing first on the "n-base" p-n junction (figure 1(e)) one can notice a widening of the NWs in their upper part. Such sort of inverse tapering results from an enhancement of the radial growth induced by the Mg incorporation during the growth of the ppart [START_REF] Fang | Axial pn junction and space charge limited current in single GaN nanowire[END_REF][START_REF] Kamimura | p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements[END_REF][START_REF] Wang | p-Type dopant incorporation and surface charge properties of catalyst-free GaN nanowires revealed by micro-Raman scattering and X-ray photoelectron spectroscopy[END_REF][START_REF] Minj | Direct assessment of p-n junctions in single GaN nanowires by Kelvin probe force microscopy[END_REF]. For its "p-base" counterpart (figure 1(f)), although a mild coalescence can be found in the upper side, which is typical for long dense NWs, the shape of individual NWs has almost no inverse tapering.

Photoluminescence analyses of the nanowire ensembles

The optical properties of the p-doped GaN NW series and of the n-doped sample were investigated by low-temperature photoluminescence (PL) spectroscopy. The characterization was performed at 10 K by exciting the NW ensembles with a continuous-wave laser emitting at 266 nm. The laser was focused on the top surface of the NW ensembles to a spot of 3 µm diameter using an ultraviolet objective with a numerical aperture of 0.36. The PL spectra were analyzed in a 460 mm focal length grating spectrometer equipped with a liquid nitrogen cooled charge-coupled device (CCD) camera.

Figure 2(a) presents the PL spectra collected from the GaN NWs grown with different Mg cell temperatures. The signal was dispersed by a 600 grooves/mm grating, allowing for a spectral resolution of around 1 meV. For the sample grown with the lowest Mg cell temperature T Mg = 225 °C, the spectra present a dominant near band edge (NBE) excitonic emission with a peak energy located at 3.47 eV. Two additional lower intensity peaks around 3.45 eV and 3.42 eV were also observed. The former one at 3.45 eV is well-known for nominally undoped GaN NWs grown on Si substrates, which is usually assigned to be the excitonic recombination in presence of inversion domain boundaries [START_REF] Auzelle | Attribution of the 3.45 eV GaN nanowires luminescence to inversion domain boundaries[END_REF][START_REF] Pfüller | Nature of excitons bound to inversion domain boundaries: Origin of the 3.45-eV luminescence lines in spontaneously formed GaN nanowires on Si(111[END_REF]. The latter one at 3.42 eV can be associated with the recombination of excitons bound to plane stacking faults [START_REF] Lähnemann | Luminescence associated with stacking faults in GaN[END_REF]. The NBE emission exceeds by two orders of magnitude any residual signal in the 3.0-3.3 eV energy band, so for the lowest Mg cell temperature no optical features evidencing the p-doping could be observed. We note that the spectrum of the n-doped NW With the increase of the Mg cell temperature in p-doped NWs, the NBE emission peaks broaden and merge into one asymmetric peak starting from T Mg = 325°C. Moreover, a wide emission band peaked around 3.27 eV together with its phonon replicas starts to show up at T Mg = 275 °C, which is associated with the donor-acceptor pair (DAP) transitions. At larger T Mg , the DAP emission becomes preponderant while the NBE emission weakens. For T Mg = 425 °C, the DAP intensity exceeds the NBE by two orders of magnitude.

The observed modification of the spectral shape points to the increase of the Mg incorporation in the GaN NWs with increasing T Mg . It has been reported [START_REF] Kamimura | p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements[END_REF] that the relative intensity of the DAP emission can be used to estimate the pdoping. In the following, the integrated intensity of the emission from the Mg-related DAP transitions is referred to as I Mg and the integrated intensity of the rest of the NBE transitions is called I NBE .

By plotting the ratio of the I NBE by I Mg as a function of the Mg cell temperature, we find a quasi-linear relationship in semi-logarithmic scale between I NBE /I Mg and T Mg , as depicted in figure 2(b). The value of I NBE /I Mg decreases from 0.55 for T Mg = 225 °C down to 3.75×10 -4 for T Mg = 425 °C. Similar range of I NBE /I Mg was reported for GaN NWs in ref. [START_REF] Kamimura | p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements[END_REF], where the hole concentration in GaN NWs was quantified by using photoelectrochemical methods and correlated with the optical signatures.

Applying their calibration, the hole concentration in our NWs can be estimated as approx. 6.0×10 17 cm -3 for T Mg = 225 °C going up to approx. 1.2×10 18 cm -3 for T Mg = 425 °C. We note that this high hole concentration is achieved for morphologies preserving the NW shape while the morphological degradation happens for higher Mg fluxes. For the same hole concentration, the NW shape reported in previous studies is much more affected by radial growth and coalescence [START_REF] Kamimura | p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements[END_REF]. We speculate that the better NW morphology for high Mg-doping in our investigation may be due to the high growth rate of 0.7 monolayers/sec, which modifies the growth kinetics.

Cathodoluminescence analyses of the p-n junction individual nanowires

The optical properties of the p-n junction NWs were analyzed by CL. To enable spatially resolved measurements, the NWs were dispersed on a silicon substrate. The experiments were performed in an Attolight Chronos microscope, recording the luminescence with an Andor Newton 920 CCD camera (1024×256 pixels, pixel width 26 µm) installed on a Horiba iHR320 dispersive spectrometer with 150/500 grating. The spectral resolution for the GaN NBE is estimated to be 2.3 nm. The measurements were performed at 10 K with an acceleration voltage of 6 kV and impinging current of ~10 nA. The spectra are integrated over the NW length. In both spectra, DAP and NBE contributions are observed in agreement with the presence of n-doped and p-doped regions, however the relative intensity of the two peaks is different. For the n-base p-n junction NW, the intensity of the DAP transition (highlighted in red in figure 3) dominates the spectrum, while for the p-base junction NW the spectrum is dominated by the NBE transition (highlighted in green in figure 3). This change of the relative intensity points to the higher volume of the pdoped material in n-base p-n junction NWs. The full-width-athalf-maximum (FWHM) of the NBE peak also differs: it is equal to 10 meV for the n-base junction and to 5 meV for the p-base junction.

To analyze the localization of the two contributions, CL spectra in different regions were examined. Figure 3(c) shows the normalized CL spectra at different points along the NW axis from the bottom to the top for the n-base NW junction. The excitation positions are indicated on the corresponding SEM image in the inset. It is observed that a strong DAP emission is present everywhere along the wire axis. The DAP completely dominates over the NBE in the top p-doped segment and slowly decreases in the n-doped base segment. The NBE peak is broad with a FWHM close to 10 meV over the entire NW length. The observed presence of the DAP peak over the entire NW length may be understood if we suppose that a p-doped shell is present around the n-doped base. This hypothesis will be further supported by EBIC investigation discussed in the next section.

The same analyses were performed for the p-base junction NW as shown in figure 3(d). The DAP emission also dominates over the NBE in the bottom p-doped segment. However, in the top n-doped segment the GaN NBE emission becomes preponderant. The broadening of the NBE peak narrows towards the NW top going from 10 meV in the lower part to 5 meV in the top n-doped segment. These observations are consistent with the nominal structure of the p-base junction NW.

EBIC characterization of GaN n-base and p-base p-n junction nanowires

Hitachi SU8000 SEM microscope equipped with a Kleindiek micro probe-station composed of 4 mobile nano-probes was used for the NW analyses. All EBIC measurements were made using the acceleration voltage of 10 kV and the impinging current of 60 pA [START_REF] Piazza | Nanoscale analysis of electrical junctions in InGaP nanowires grown by template-assisted selective epitaxy[END_REF][START_REF] Saket | Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications[END_REF]. To perform single NW EBIC measurements, the sample is cleaved without disconnecting the NWs from their substrate and the cross section is exposed to the electron beam. The individual NWs are contacted on their top by a nano-probe, while the bottom contact is provided by the substrate connected to the ground. To measure the induced current, a low noise current preamplifier SR570 coupled with a Gatan Digiscan system is used. The EBIC map is constructed point by point by scanning the sample surface.

First the n-base p-n junction NWs were analyzed. Figure 4 presents the NW schematic (panel a)), examples of EBIC maps of different NWs (panels b) -e)) as well as an example of a single NW current-voltage (I-V) characteristic recorded with a blanked electron beam (panel f)). The EBIC maps reveal the junction activity as a bright contrast. The I-V curve presents a typical diode-like shape with the opening voltage around 3.5 V in agreement with the expected p-n junction structure of the NW. The leakage current remains low for reverse biases up to -5 V.

We note that the EBIC signal from the NW junctions is weak or absent without external bias (cf. figure S2 in Supplementary Material showing the EBIC maps under different external biases). This is attributed to the presence of a Schottky barrier between the NW and the nano-manipulator. Indeed, for a forward bias, a dark contrast is observed in the vicinity of the contact point (cf. figure S2 in Supplementary Material), proving the Schottky nature of the contact [START_REF] Saket | Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications[END_REF]. This Schottky barrier can be flattened by applying a negative bias thus revealing the p-n junction signal.

Figure 4(b), (c), (d) and (e) displays SEM images and the corresponding EBIC maps under -5 V external bias for four individual NWs. A strong difference is observed with respect to the nominal NW structure schematically illustrated in figure 4(a). The junction signal is markedly displaced toward the substrate with respect to the expected position of the junction according to the growth rate calibration. In addition, the EBIC signal shows an irregular shape, which does not correspond to the one expected for an axial junction NW. The signal position and shape strongly vary from wire to wire. In particular, the extension of the region producing the induced current varies from 400 nm up to 1 µm.

These observations deviating from the nominal NW structure can be explained by the parasitic radial growth, which is enhanced in presence of Mg dopants. During the growth of the Mg-doped upper segment, the lateral overgrowth leads to the lateral deposition of a p-doped shell around the n-doped NW base. This shell has an irregular shape [START_REF] Tchernycheva | Growth of GaN free-standing nanowires by plasmaassisted molecular beam epitaxy: structural and optical characterization[END_REF][START_REF] Van Treeck | Influence of the source arrangement on shell growth around GaN nanowires in molecular beam epitaxy[END_REF] which depends on the local environment of the NW due to shadow effects or eventual coalescence of the neighbouring NWs. As a result, a mixed axial and core-shell p-n junction is formed in the NWs, which explains the extended irregular EBIC signal and the difficulty to define the position of the junction. The hypothesis of a p-doped parasitic shell is coherent with the previously described CL results, which showed a strong DAP signal not only in the top p-doped part, but also in the bottom n-doped segment.

The growth analyses have suggested that the problem of the radial parasitic growth can be reduced by inverting the doping order. To test this hypothesis, p-base NW junctions were analyzed under the same experimental conditions. Figure 5 presents an example of a single NW EBIC map and an I-V curve for a representative p-base NW junction (other examples confirming good wire-to-wire reproducibility are shown in figure S3 of Supplementary Materials). The p-n junction signal in p-base NWs is given by a dark contrast in EBIC maps, which reflects the opposite direction of the built-in field and thus of the induced current compared to the n-base wires, as expected for the inverted doping order. The change of the doping order also results in the I-V curve inversion as illustrated in figure 5(c). We note that the absolute value of the current in the I-V curve is reduced compared to the n-base NW (cf. Fig. 4(f)) which may be related to the difference in the junction area between these two cases: a mixed axial and core/shell junction in the n-base NWs and a purely axial junciton in the p-base NWs.

The EBIC signal from the junction was observed under an external bias above 1 V (for this doping order, positive bias corresponds to the reversely polarized p-n junction). No signal was observed without an external bias, which may be attributed either to the Schottky contact with the nanomanipulator or to the potential barrier at the NW/substrate interface. (Indeed, for some NWs, an induced current was observed close to the substrate indicating that the carrier collection may be difficult through the p-GaN/p-Si interface.)

The EBIC signal of the junction in p-base NWs has a regular shape, which is reproducible between different NWs (cf. figure S3 in Supplementary Materials). The junction location can be approximated by the maximum of the induced current, its position is similar for different NWs and corresponds to the nominal one.

The regular shape of the EBIC signal in the p-base NW junction shows that contrary to the n-base NWs, it behaves as a regular axial junction. We cannot completely exclude the parasitic radial growth during the deposition of the top Sidoped segment, however it should be much weaker than for the case of a p-doped top segment. This is consistent with the cylindrical morphology of Si-doped NWs as described in the growth section. The EBIC does not reveal any electrical activity of this hypothetical n-doped shell, therefore, if this latter exists it should be depleted.

Thanks to the regular shape of the EBIC signal, p-base NW junctions can be used for doping estimation. The doping concentration can be assessed using the model proposed by Tchoulfian et al. [START_REF] Tchoulfian | Direct Imaging of p-n Junction in Core-Shell GaN[END_REF], in which the EBIC profile is analyzed to extract the depletion region width. By fitting the signal decay, the minority carrier's diffusion lengths can also be estimated. The analyses of a typical EBIC profile along the NW axis collected under reverse bias of 2 V are illustrated in figure 6(a). The EBIC profile can be divided into three parts: the diffusion current in the p region, the diffusion current in the n region and the drift current in the space charge region. The induced current is theoretically modelled as a convolution of the generation function with the probability of charge collection. This latter can be approximated to unity in the depletion layer, elsewhere the collection probability can be written as:

𝑄 𝑥 = 𝑒 & ' ( ) ,
(1) where 𝐿 -is the diffusion length of the corresponding minority carriers [START_REF] Saket | Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications[END_REF]. Based on this expression the minority carrier diffusion lengths are extracted by the exponential fitting of the decay of the current from either side of the depletion region edge. By analyzing different NWs, the electron diffusion length in the p-doped region is estimated to be 59 ± 6 nm, while the hole diffusion length in the n-doped segment is 49 ± 5 nm. Compared to the literature reports for n-base p-n junction NWs [START_REF] Fang | Axial pn junction and space charge limited current in single GaN nanowire[END_REF] (giving values 40 -77 nm for holes and 122 -170 nm for electrons), the hole diffusion length in our NWs is comparable, while the electron diffusion length is shorter.

The space charge region can be estimated as the part of the NW where the EBIC profile deviates from the exponential decay [START_REF] Tchoulfian | Direct Imaging of p-n Junction in Core-Shell GaN[END_REF]. By analyzing several NWs, the space charge region is extracted to be equal to 65 ± 6 nm. To estimate the electron and hole concentrations we need to measure the depletion region extension in the n-and p-doped parts of the junction, which requires a precise knowledge of the junction position. To localize the interface between the p-and n-doped segments, a common method is to analyze the secondary electron contrast which depends on the doping type [START_REF] Saket | Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications[END_REF]. However, in the present case the NWs are very thin and the contrast difference between the p-and n-doped parts is insufficient to localize the junction interface with precision. The junction position may be estimated by searching the maximum of the EBIC profile, however this method can be erroneous especially in the case when the space charge region is narrow and its extension is comparable with the beam/matter interaction volume. For these reasons, we should not rely on the EBIC peak position and the electron and hole concentrations cannot be separately extracted from the present EBIC experiments. However, we can estimate their minimal values and their respective dependence. The correlation between the electron and the hole concentrations is obtained by assuming the interface coordinate in different positions along the space charge region starting from x p (coordinate of the space charge region boundary with the p-doped segment) to x n (coordinate of the space charge region boundary with the n-doped segment). x n and x p are determined as positions at which the EBIC profile deviates from the exponential decrease in the p-and n-doped segments of the junction. For each assumed junction position, the carrier concentrations are calculated using the electrostatic model of the p-n junction as detailed in [START_REF] Saket | Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications[END_REF]. The computed electron and hole concentrations are plotted in figure 6(b).

Considering the error bars on the xn and xp values extracted from the experimental profile, for each electron concentration the lowest, the highest and the average hole concentrations are traced. The lowest limit for the hole (electron) concentration is determined as the horizontal (vertical) asymptote of the curves. These limits correspond to the hypothesis that the full space charge region is located in the p-doped (n-doped) segment. The lowest limit for the hole concentration corresponds to 10 18 cm -3 , while the actual concentration can be higher.

The optical estimation described in section 3 for the Mg cell temperature of 425 °C used for the p-n junction growth yielded hole concentration of 1.2×10 18 cm -3 . Therefore, the low boundary for the hole concentration obtained from EBIC analyses is compatible with the estimation based on the PL spectra. It is important to note that this high hole concentration is achieved without deterioration of the NW morphology.

Conclusions

In conclusion, the electrical and optical properties of n-base and p-base p-n junctions in PAMBE grown GaN NWs were investigated. For the n-base junctions, a parasitic radial shell accompanying the top p-doped segment growth results in a mixed axial and radial behaviour and a large wire-to-wire inhomogeneity. Abrupt purely axial p-n junctions with a good wire-to-wire reproducibility are demonstrated for an inverted doping order, which is favourable for axial NW devices such as photodetectors or LEDs. An active hole concentration in the 10 18 cm -3 range was achieved without degrading the wire morphology.

Figure 1 .

 1 Figure 1. Cross-sectional SEM images of GaN NWs grown with different doping profiles. (a) Undoped NWs; (b) Mg-doped NWs with T Mg =375°C; (c) Mg-doped NWs with T Mg =425°C; (d) Si doped NWs with T Si =1150°C; (e) n-base p-n junction NWs; (f) p-base p-n junction NWs.

Figure 2 .

 2 Figure 2. (a) -Low-temperature PL spectra in logarithmic scale of p-doped GaN NWs grown with different TMg. (b) -The integrated emission intensity ratio I NBE /I Mg in logarithmic scale as a function of T Mg .

Figure 3 displays

 3 CL spectra of individual p-n junction NWs with the n-base (panel (a)) and the p-base (panel (b)).

Figure 3 .

 3 Figure 3. CL spectra of (a) n-base and (b) p-base single NW p-n junctions at 10 K. Normalized CL spectra for different excitation points along the NW axis for (c) n-base and (d) p-base NW junction. Color bands highlight the NBE (green) and the DAP (red) spectral ranges. The insets show SEM images acquired simultaneously with CL spectra showing the excitation positions corresponding to spectra in (c) and (d).

Figure 4 .

 4 Figure 4. (a) -Schematic of an n-base NW p-n junction showing the expected position of the junction. (b), (c), (d) and (e) -SEM images and the corresponding EBIC maps of four different NWs. (f) -Single NW I-V curve recorded with a blanked beam.

Figure 5 .

 5 Figure 5. (a) -Schematic of a p-base NW junction. (b) -SEM image of an individual NW and the corresponding EBIC map under +2 V. (c) -Single NW I-V curve recorded with a blanked beam.

Figure 6 .

 6 Figure 6. (a) -EBIC profile along the NW axis under reverse (+2 V) bias and the exponential fit of the current decay (inset shows a close-up on the induced current peak); (b) -Extracted hole concentration as a function of the electron concentration. The dashed line shows the lowest limit of the hole concentration.

Acknowledgements

This work was financially supported by EU H2020 ERC project "NanoHarvest" (grant no. 639052), the ITN Marie Curie project INDEED (grant no. 722176), by the Labex GANEX (grant no. ANR-11-LABX-0014), by the Region Ilede-France in the framework of C'Nano IdF and by the European Union (FEDER 2007(FEDER -2013)).


Brady Manescau

Hui-Ying Wang
email: wang@ensma.fr

Bruno Coudour

Jean Pierre Garo

Influence of Heat Tightness of an Enclosure Fire on Ignition Risk of Unburnt Gases in a Connected Exhaust System -An Experimental Study

Keywords: fire growth, ignition risk, compartment fire, liquid fuel, heat tightness, extraction duct, dilution duct, depression

An experimental study was performed from a reduced scale enclosure with a length/height and width of 2 m. A dodecane pan of 40 cm in diameter is placed at the center of the enclosure floor. An external ventilation system provides an air supply rate with an Air Change Per Hour (ACPH) ranging from 3 to 5. Influence of the intermediate levels of thermal insulation of enclosure on ignition risk in a connected exhaust system is experimentally evaluated. The results show that thermal insulation of an enclosure leads to faster fire growth, implying more important peak in heat release rate, and thus more dangerous fire in regards to the ignition risk. Heat tightness of enclosure enhances the mass loss rate of liquid fuel, but reduces the air supply rate from the admission duct due to decrease of the depression level in the compartment. As a consequence, the fire becomes quickly very-under-ventilated, and a large vaporized fuel is converted into the unburnt gases such as hydrocarbons, CO and H2. In the early stages of a fire, a hotter unburnt fuel layer with a concentration above the Lower Flammability Limit (LFL) is formed in the extraction duct connected to a mechanically ventilated enclosure fire. With a long time delay in a range of 16 to 21 min in the current study, the energy released per mass of oxygen consumed allows to raise the smoke temperature above 350°C. Occurrence of flame extinction in vitiated air enclosure makes a sudden increase of the depression level inside enclosure due to cooling effects. This results in a sudden supply of fresh air from dilution duct, providing a sufficient amount of oxygen to trigger ignition of a fuel rich mixture near the extraction duct. It is found that ignition of unburnt volatiles at entrance of the extraction duct occurs more easily when the compartment is more heat-tight with a reduction by about 30% in the ignition delay.

INTRODUCTION

Explosions or gas deflagrations caused by re-ignition have gained increasing attention typically when a compartment containing a fire has a very limited fresh air supply during its extinction phase. Intensive research has been focused on the phenomenological analysis of backdraught in an enclosure after flame exhaust if there is a sudden supply of fresh air, e.g. due to a window or door opening or breaking, possibly due to the fire [START_REF] Gojkovic | Initial backdraft experiments[END_REF][START_REF] Horvat | Numerical simulation of backdraft phenomena[END_REF]. The experimental study of Lassus et al. [START_REF] Lassus | Influence of ventilation on ignition risk of unburnt gases in the extraction duct of under ventilated compartment fire[END_REF] showed that even in a highly confined setting, an extraction duct is an important pathway for the onset of re-ignition of a fuel rich mixture if there is a sudden supply of fresh air from an interconnecting ventilation system. Fundamental research [START_REF] Prétrel | Pressure variations induced by a pool fire in wellconfined and force-ventilated compartment[END_REF][START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF][START_REF] Wahlqvist | Validation of FDS for large-scale well-confined mechanically ventilated fire scenarios with emphasis on predicting ventilation system behavior[END_REF][START_REF] Hu | A comparison between observed and simulated flame structures in poorly ventilated compartment fires[END_REF][START_REF] Takeda | Oscillatory phenomenon and inverse temperature profile appearing in compartment fires[END_REF] has been also carried on the phenomena of periodic oscillations and temporary flame quenching in a highly confined compartment equipped with mechanically ventilation system. The oscillation is caused by phenomenon of ghosting flame where the flame moves away from the pool and towards the air inlet [START_REF] Wahlqvist | Validation of FDS for large-scale well-confined mechanically ventilated fire scenarios with emphasis on predicting ventilation system behavior[END_REF]. The low-frequency oscillating flames with a drastic change in flame behavior were experimentally observed in a vitiated air enclosure [START_REF] Hu | A comparison between observed and simulated flame structures in poorly ventilated compartment fires[END_REF][START_REF] Takeda | Oscillatory phenomenon and inverse temperature profile appearing in compartment fires[END_REF]. They postulated that issues involved for such oscillatory behavior are evaporation rate of condensed fuel [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF], phenomena leading to a reverse flow in the supply ventilation system [START_REF] Prétrel | Pressure variations induced by a pool fire in wellconfined and force-ventilated compartment[END_REF] and successive combustion of unburnt gases [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF][START_REF] Takeda | Oscillatory phenomenon and inverse temperature profile appearing in compartment fires[END_REF].

As a condensed fuel, liquid pool fires usually generate a significant hazard in an enclosure due to large amount of unburnt volatile gases. The liquids could originate e.g. from leaks of transformers generators or other machinery. Knowing the liquid fire burning rate is the starting point of any fire safety analysis. Some empirical correlations [START_REF] Peatross | Ventilation effects on compartment fire characterization[END_REF][START_REF] Utiskul | Theoretical and experimental study on full-developed compartment fires[END_REF][START_REF] Tewarson | The influence of oxygen concentration on fuel parameters for fire modelling[END_REF] for mass loss rate of a liquid fuel is limited to the large enclosure where the heat fluxes from hot wall can be negligible. A global approach [START_REF] Orloff | Froude modelling of pool fires[END_REF][START_REF] Hamins | A global model for predicting the burning rates of liquid pool fires[END_REF][START_REF] Beaulieu | Effect of oxygen on flame heat flux in horizontal and vertical orientations[END_REF] for the mass burning flux of liquid fire is based on the measured radiative heat feedback to the condensed fuel surface or flame temperature. The burning rate of liquid pool can be significantly different in highly confined spaces, essentially attributed to the influence of air ventilation, heat radiation from hot walls and gas layer [START_REF] Vilfayeau | Numerical simulation of under-ventilated liquid-fueled compartment fires with flame extinction and thermally-driven fuel evaporation[END_REF]. The existing empirical correlations [START_REF] Peatross | Ventilation effects on compartment fire characterization[END_REF][START_REF] Utiskul | Theoretical and experimental study on full-developed compartment fires[END_REF][START_REF] Tewarson | The influence of oxygen concentration on fuel parameters for fire modelling[END_REF][START_REF] Orloff | Froude modelling of pool fires[END_REF][START_REF] Hamins | A global model for predicting the burning rates of liquid pool fires[END_REF][START_REF] Beaulieu | Effect of oxygen on flame heat flux in horizontal and vertical orientations[END_REF] cannot take into account the factors such as oscillating flames and hot walls which affect significantly burning rate of the liquid pool fires in a highly confined enclosure. The quantitative agreement between experimental data and fully predictive simulation is again fair concerning the burning rate of liquid fuel in an enclosure [START_REF] Sikanen | Predicting the heat release rates of liquid pool fires in mechanically ventilated compartments[END_REF]. Only using the mass loss rate collected experimentally, numerical simulation manages to correctly predict the thermal behaviour [START_REF] Merci | Numerical simulations of full-scale enclosure fires in a small compartment with natural roof ventilation[END_REF], visibility [START_REF] Wegrzynski | Experimental and numerical evaluation of the influence of the soot yield on the visibility in smoke in CFD analysis[END_REF] and pressure [START_REF] Hostikka | Fire-induced pressure and smoke spreading in mechanically ventilated buildings with air-tight envelopes[END_REF] for the mechanically ventilated compartments.

In order to contain the potential release of radioactive material and avoid dispersion to the outside, nuclear installations are built with increasingly air/ heat-tight envelopes [START_REF] Prétrel | Pressure variations induced by a pool fire in wellconfined and force-ventilated compartment[END_REF]. Thermal insulation of enclosure is as such particularly relevant not only for Nuclear Power Plants [START_REF] Prétrel | Pressure variations induced by a pool fire in wellconfined and force-ventilated compartment[END_REF], but also probably for an increasingly popular low-energy consumption type of building [START_REF] Cabeza | Experimental study on the performance of insulation materials in Mediterranean construction[END_REF]. Heat-tightness in buildings envelopes has high potential in energy demand reduction and consequently in energy savings [START_REF] Forneau | Comparison of fire hazards in passive and conventional houses[END_REF]. Air or heat-tight envelopes can sufficiently increase the peak overpressure and temperature from fast-growing fires to cause structural damage [START_REF] Hostikka | Fire-induced pressure and smoke spreading in mechanically ventilated buildings with air-tight envelopes[END_REF]. The increased pressure and temperature were identified [START_REF] Hostikka | Fire-induced pressure and smoke spreading in mechanically ventilated buildings with air-tight envelopes[END_REF][START_REF] Forneau | Comparison of fire hazards in passive and conventional houses[END_REF] as one of the consequences of better energy efficiency, resulting in a reverse flow in the supply ventilation system [START_REF] Prétrel | Pressure variations induced by a pool fire in wellconfined and force-ventilated compartment[END_REF]. The effect of heat permeability of enclosure has an importance on the fire behavior, notably on the thermal stratification of unburnt fuel gases [START_REF] Sinai | Comments on the role of leakages in field modelling of under-ventilated compartment fires[END_REF]. The more heat-tight is a compartment, the more heat feedback is received to the surface of burning liquid fuel [START_REF] Nasr | Heat feedback to the fuel surface of a pool fire in an enclosure[END_REF]. The re-ignition related risks may become more significant when the enclosure is more heat-tight due to a large formation of unburnt fuel. In this work, the investigation aims to identify experimentally the impact of thermal insulation of an enclosure on both the burning rate of liquid pool and ignition delay at entrance of the extraction duct.

EXPERIMENTAL SET-UP

The schematic experimental facility with a 2 m in length/height and width is shown in Figure 1(a,b,c). An external ventilation system consists of two intake ducts and an extraction duct. The intake was placed closer to the floor at a height of 0.3 m and the exhaust was placed near the ceiling at a height of 1.7 m. The extraction duct has a length of 1.1 m, and the dilution duct has a length of 2.5 m. Ducts have a square section of 0.2x0.2 m 2 . 1a) 3D view of the experimental facility 1b) side (x, z) view for the measurement locations 1c) measurement positions from the top (x, y) view Figure 1. Mechanically ventilated enclosure fire connected to an extraction duct Theoretically, the total flow loss in the duct from wall friction and duct fittings are assigned as dimensionless loss numbers to the ducts, as :

D L f K  (1)
where D and L are the duct diameter and length, respectively. The parameter, f, can be estimated from an approximation [START_REF] Zigrang | A review of explicit friction factor equations[END_REF]. In the current work, all ducts are made of smooth galvanized steel, and the ventilation ducts can be considered as non-rough. This allows to neglect the regular pressure losses, and consider only the singular head losses. The air flow rates at the intake and dilution ducts are regulated as a function of the depression level by means of an adjustment of rectangular flaps located upstream of these ducts, which can generate an aeraulic resistances.

The pressure difference between the inside and outside is adjusted only in non-fire conditions. The experimental facility is not perfectly airtight mainly due to passage of the thermocouples through the wall and door gap. A leak of area in the order of 12-14 cm 2 [START_REF] Magnognou | Risk assessment of unburnt gas Ignition in an exhaust system connected to a confined and mechanically ventilated enclosure fire[END_REF] was estimated from conservation of the volume flow rate at the admission and extraction ducts as a function of the depression level.

A mechanical extraction is carried out by using System-Air Euro S7440, providing a maximum volume flow rate of 720 m 3 /h, and pressure difference of Pa 186 P max   . This is a centrifugal fan with single-phase variable speed to select the desired airflow rate [START_REF] Magnognou | Risk assessment of unburnt gas Ignition in an exhaust system connected to a confined and mechanically ventilated enclosure fire[END_REF]. As shown in Fig. 1a, the fan is placed at the outlet of a duct orthogonal to the dilution duct at a height of 2.6 m. The fan characteristic curve (i.e., delivered flow rate as a function of pressure) is shown in Fig. 2. Before activating the fire, a period of 10 minute was included for establishing a depression below atmospheric pressure inside the enclosure. The depression level depends on the Air Change Per Hour (ACPH) of the enclosure with a range from 3 to 5, corresponding to a volume flow rate of 24 and 40 m 3 /h, respectively. The air inlet velocity in non-fire conditions varies from 0.1667 m/s (3 ACPH) to 0.2778 m/s (5 ACPH). After activating the fire, velocity, temperature and pressure inside the enclosure as well as the ventilation ducts depend on the fire dynamic. In our previous work [START_REF] Magnognou | Risk assessment of unburnt gas Ignition in an exhaust system connected to a confined and mechanically ventilated enclosure fire[END_REF], a circular heptane or dodecane pan with a diameter varying from 23 to 40 cm was employed. A series of fire experiment were conducted by combining the three parameters such as pool size, ACPH and fuel type for investigating the extinction/ignition phenomena. In this work, a stainless-steel circular pan with a diameter of 40 cm was chosen. The liquid fuel pan was placed in the middle of the enclosure, slightly elevated at a height of 0.3 m. Dodecane is typically used as the fuel load, and occurrence of an auto-ignition was identified experimentally. The fuel mass was continuously monitored using a load cell, installed under the pan. The performed measurements include pressure, temperature and chemical species concentrations of gases filling the enclosure. A good repeatability was obtained with a difference below 5% regarding the temperature measurements and the mass loss rate during each fire tests. As shown in Fig. 1b,c), the gas temperature was analyzed from different vertical profiles inside the enclosure, at the extraction duct and at the dilution one. Each of the vertical profiles comprises eight thermocouples positioned at regular intervals of 0.2 m over the height of the enclosure.

Two situations are taken into account for the intermediate levels of thermal insulation of enclosure. In the first situation, walls of the enclosure are made of 20-cm-thick cellular concrete with a thermal conductivity of  =1 W/m/K, a density of  =860 kg/m 3 , and a specific heat of Cp=0.88 kJ/kg/K. There is a relatively strong conductive heat loss via the walls during fire tests, and such case is named as 'heat loss'. In the second situation, the 20-cm-thick cellular concrete walls and ceiling are recovered by a 3.5-cm-thick insulating material as Promatech H with the following properties :  = 0.175 W/m.K,  =400 kg/m 3 and Cp=0.92 kJ/kg.K. In such case, the conductive heat loss via the walls becomes rather weak, and the enclosure is considered as 'heat tightness' or 'thermal insulation'. The reduction in the volume with an insulating material is low than 8%, and affects somewhat the amount of oxygen initially present in the enclosure. But the different wall thicknesses do not affect the oxygen supply rate by mechanical ventilation, and thus, the occurrence of auto-ignition of unburnt gas near the extraction duct.

Since experiments in full scale are expensive, the numbers of tests are limited. With reducedscale enclosure, more tests may be carried out but the scale reduction must respect several scaling laws adapted to the studied problem. Fire compartment tests have been conducted at a reduced geometric scale, maintaining a constant Froude number, Fr=u 2 /gL where L and u denote the characteristic length and velocity, respectively. Reynolds number is large enough to ensure the turbulence of the flow. The Froude number is then made constant by L u . From dimensionless variable of the energy equation, the source term contains the Zukoski number [START_REF] Zukoski | Fluid dynamics aspects of room fires[END_REF], defined as

g ) T C ( L Fr Q 2 / 1 p 2 / 5  
. In order to obtain a similitude between a full scale and a reduced scale compartments, a constant Froude number (Fr) must be maintained, and the ratio (

L / Q 5 2 
) between heat release rate ( Q  ) and characteristic length (L) should be preserved. Based on this concept, our reduced scale experimental device with a volume of 8 m 3 corresponds to a full scale 100 m 3 compartment with a HRR below 1 MW.

RESULTS AND DISCUSSION

Histories of the relative pressure (P0-P) for 3 and 5 ACPH are illustrated in Figure 3(a,b). Here, P0 denotes the atmospheric pressure, and P the enclosure one. The fire-induced pressure can be said to follow three main stages : 1) activating the fire causes an overpressure inside the compartment; 2) with some time delay, the fully developed fire becomes ventilation controlled due to lack of oxygen with appearance of pressure oscillations; 3) occurrence of flame extinction in vitiated air enclosure makes a sudden increase of the depression level inside enclosure due to cooling effects. There is a tendency to enhance the fluctuations of the pressure with an increase of ACPH from 3 to 5. With thermal insulation, decreasing the thermal inertia (   p C ) of the envelope by 70% led to a reduction in the depression peak by about 35%. During the fully developed fire, oscillations of the pressure become less sensitive to the intermediate levels of thermal insulation with an increase of ACPH to 5 (cf. Fig. 3b). On the other hand, the strong oscillation of the pressure has been identified as a potential risk for the dynamic confinement in order to prevent from hazardous gas leaks. 3a) at 3 ACPH 3b) at 5 ACPH Figure 3. Impact of the intermediate levels of thermal insulation on the pressure inside enclosure

The inlet velocity of fresh air supplied from the admission duct for 3 and 5 ACPH is presented in Fig. 4(a,b). After activating the fire, an overpressure inside the compartment causes a reverse flow in the supply admission duct. During the fully developed fire, there is a tendency to enhance the fluctuations of the inlet flow velocity with an increase of ACPH from 3 to 5. At 3 ACPH, the lower depression level with thermal insulation (cf. Fig. 3a) around 750 s leads to also a reverse flow in the supply admission duct. Finally, occurrence of flame extinction makes a strong supply of fresh air from the admission duct. With thermal insulation at 5 ACPH, the peak of inlet air velocity at the admission duct is sensitively reduced by a factor of 25%. Influence of the intermediate levels of thermal insulation of enclosure on the mass loss rate at 3 and 5 ACPH is illustrated in Fig. 5. With heat loss, a rise of ACPH from 3 to 5 contributes to an increase of the liquid vaporization rate due to enhanced convective mass transfer over the liquid surface. As an illustration (TL4, cf. Fig. 1c), the temperature profiles along the height at different instants are shown in Figure 6(a,b,c). It can be seen that with thermal insulation of enclosure, the hot smoke layer (T>200°C) descends rapidly to the level of liquid surface. As the upper smoke layer is established at 900 s, the hot gas temperature with heat tightness is about 2.5 times of that with heat loss. This results in a significant increase of the mass loss rate due to the enhanced radiative heat feedback from a hot gas layer. When the radiative heat transfer becomes dominant over the liquid surface, the liquid vaporization rate is practically insensitive to an increase of ACPH. In the initial growth phase, burning is fuel-controlled with a stable flame. This is followed by a fully developed, post-flashover phase with oscillations of the mass loss rate due to appearance of an unstable flame in a vitiated air enclosure [START_REF] Zavaleta | Cable tray fire tests in a confined and mechanically ventilated facility[END_REF][START_REF] Vilfayeau | Numerical simulation of under-ventilated liquid-fueled compartment fires with flame extinction and thermally-driven fuel evaporation[END_REF].

H ) Y Y ( m Q O 2 O , 2 O A       (3)
Here, mA  denotes the inlet air flow rate (kg/s), YO2 the oxygen mass fraction and o H  the energy released per kilogram of oxygen consumed (13100 kJ/kg).

Note that the boiling point of dodecane is about 220°C, and ignition of such liquid fuel needs a preheating process by using a torch. During preheating phase, heat flowing into the liquid interior over a pool size of 40 cm is uncontrollable. The difference in the early fire growth for the four fire scenarios is observed. By assuming a quadratic growth for the four fire scenarios, an empirical formula is adopted [START_REF]Standard growth curves[END_REF] as follows:

t a Q 2   (4)
The early fire growth factor, a, is defined as 10 3 kW/t1 2 . With heat loss, the growth time, t1, is about 160 s for 3 ACPH and 632 s for 5 ACPH, corresponding to a fast and slow fire growth, respectively. With thermal insulation, the growth time, t1, is about 320 s corresponding to a medium fire growth in spite of ACPH. The oscillatory phenomena of the HRR are attributed to the coupling process between the fuel evaporation rate (cf. Fig. 5) and the inlet air flow rate (cf. Fig. 4), both attached to the intermediate levels of thermal insulation of enclosure. With heat loss of enclosure (cf. Fig. 7a), the HRR curves show a gradual growing trend with a moderate peak of about 90 kW. The oxygen concentration initially present in the enclosure and the one supplied by mechanical ventilation entrain a sufficiently ventilated fire with an almost complete reaction. The effective HRR corresponds approximately to the theoretical one derived from the mass loss rate. It is observed that thermal insulation of enclosure leads to an ultra-fast fire growth (cf. Fig. 7b), thus reducing the delay leading to flashover which is clearly identified by a rapid increase of HRR from 40 to 140 kW. The ultra-fast growing fire in a very consistent manner becomes progressively oxygen limited, that is to say ventilation-controlled, and the theoretical HRR is higher than the effective one. The extinction due to unstable combustion in a vitiated air enclosure happened earlier when the wall envelope is more heat-tight. 7a) with heat loss 7b) with thermal insulation Figure 7. Impact of the intermediate levels of thermal insulation on the heat release rate for different ACPH As an illustration at 5 ACPH, the histories of oxygen concentration at entrance of the extraction duct and near the floor in the vicinity of fire source are shown in Fig. 8(a,b). It seems that during the initial growth phase, the oxygen concentration allows to entrain a sufficiently ventilated fire. For most fuels of interest for fire applications, the work of Huggett [START_REF] Huggett | Estimation of the rate of heat release by means of oxygen consumption[END_REF] indicates that combustion occurs when the local oxygen concentration is above a critical value, YO2,lim, evaluated from this expression :

  o m f p lim , 2 O H T T C Y    (5)
The combustion products have an average specific heat Cp of 1.2 kJ/(kg.K), and the adiabatic flame temperature, Tf, is about 1600 K [START_REF] Beyler | Flammability limits of premixed and diffusion flames[END_REF]. In a vitiated air enclosure at a temperature of approximately Tm=1000 K (cf. Fig. 17), the limiting oxygen mass fraction would evaluate to YO2,lim=0.05. This value is consistent with the visual identification of a visible flame extinction during each fire tests. The fire weakens earlier with thermal insulation and finally stops due to insufficient amount of oxygen even at 5 ACPH. After the fire extinction of the liquid fuel pan, the remaining flames as ghosting flame near the ceiling were not observed. Starting from 700 s, thermal insulation contributes to excessive combustion products, resulting in a decrease of oxygen concentration to about 2% in a fuel-rich hot upper layer near the extraction duct. Thermal insulation of enclosure induces a reduction in oxygen concentration by a factor of 60%. Occurrence of flame extinction makes a sudden increase of oxygen molar fraction to 7% at 900 s with heat tightness and 10% at 1250 s with heat loss due to a return of fresh air from dilution duct. This is followed later by a rise of the O2 concentration, mainly attributed to air supply thanks to mechanical ventilation. Evolution of the oxygen concentration near the floor (cf. Fig. 8b) follows globally the trend near the extraction duct. Thanks to air supply from the admission duct, the oxygen concentration near the floor can be maintained to about 10% at 750 s even with thermal insulation just before occurrence of flame extinction.

    13 . 0 O 65 . 0 CO 2 2    (6)
It seems that a molar fraction of oxygen above 10% is required to stabilize the fire behaviour with a linear increase of CO2. At the stage where the fire becomes oxygen limited, the CO2 growth rate decreases due to reduction in the chemical reactivity. Making the compartment more heat-tight does not change the trend of CO2 formation as a function of O2. The empirical correlation for CO production is derived using the theoretical analyses in conjunction with experimental data [START_REF] Lassus | Estimation of species concentration during a fire in a reduced scale room[END_REF][START_REF] Bryner | Carbon monoxide production in compartment fires: reduced-scale enclosure test facility[END_REF]. This correlation presents three fire regimes as a function of the oxygen concentration, as follows:

(a)                                  co , MOC 2 O MOC co , MOC 2 O MOC MOC co , MOC 2 O 2 O MOC , 2 O MOC 2 O CO X , 0 X ( X X 1 a b X X b ) X , X X ( a X X a ) X , X X ( 0 X (7)
Here XMOC represents the Minimum Oxygen Concentration for a nearly complete reaction of a fuel-air mixture, and XMOC,co the Minimum Oxygen Concentration for oxidation of CO to CO2. The CO production is zero when the oxygen concentration is above XMOC, corresponding to a well-ventilated fire. The range of oxygen concentration between XMOC,co and XMOC implies an underventilated fire with a significant CO production due to an incomplete oxidation reaction.

The oxygen concentration below XMOC,co results in a maximum CO production, implying a very under-ventilated fire. In Bryner's work [START_REF] Bryner | Carbon monoxide production in compartment fires: reduced-scale enclosure test facility[END_REF], oxygen concentration can reach up to zero by using a gas burner to simulate the fire. In the current enclosure fire using a liquid fuel pan, the minimum oxygen concentration is about 2% with thermal insulation (cf. Fig. 8).

The previous work [START_REF] Lassus | Estimation of species concentration during a fire in a reduced scale room[END_REF] proposes also the following relationships as :

      n mH C c CO (8)          n m 2 H C d H CO (9)
These correlations [START_REF] Takeda | Oscillatory phenomenon and inverse temperature profile appearing in compartment fires[END_REF][START_REF] Peatross | Ventilation effects on compartment fire characterization[END_REF] , where s denotes the stoichiometric coefficient. The fuel supply rate mF  , and air inflow rate mA  are controlled by the intermediate levels of thermal insulation via heat transfer and depression level in enclosure. The yields of CO and unburnt hydrocarbons, defined as the fraction of the fuel mass that is converted into these species, are commonly used as an indication for the level of underventilation in the enclosure [START_REF]The SPFE Handbook of Fire Protection Engineering[END_REF]. To the best knowledge of the authors, only the yields of CO and unburnt hydrocarbons are a function of the GER in well established upper layer of an enclosure fire [START_REF]The SPFE Handbook of Fire Protection Engineering[END_REF]. In the current enclosure fire, there is unlikely to be sufficient time for two-layer establishment due to the strong fluctuations of HRR (cf. Fig. 7). Moreover, identification of the ignition time delay controlled by the fire scenario depends on the time evolution of the local concentration of CO, H2 and unburnt hydrocarbons near the extraction duct.

As illustrated in Fig. 12(a,b), the empirically based correlation [START_REF] Hu | A comparison between observed and simulated flame structures in poorly ventilated compartment fires[END_REF] for CO production is evaluated as a function of oxygen concentration at entrance of the extraction duct. Both the measurement and correlation indicate that thermal insulation enhances the CO production due to large vaporized fuel. As shown in Fig. 13(a,b), the histories of carbon monoxide derived from correlation in transient mode are in a good agreement with the measured one within an error of 6% at 3 ACPH. However, a deviation of more than 40% from the measured CO concentration is found at 5 ACPH with heat loss. This is probably attributed to an increase of the depression level under heat loss conditions, which enhances smoke oscillations (turbulence effect) via leakage. Figs. 14 and15(a,b) shows the histories of the chemical species such as hydrocarbons and hydrogen concentrations from the measurement and correlations (8-9) at entrance of the extraction duct. The first period, corresponding to sufficiently ventilated conditions ) 7 . 0 0 (    [START_REF] Magnognou | Risk assessment of unburnt gas Ignition in an exhaust system connected to a confined and mechanically ventilated enclosure fire[END_REF] with low CO, H2 and CmHn productions, lasts till 500 s with heat loss, and only 100 s with heat tightness of enclosure. During the second period, we observe a gradual rise of unburnt fuel production with heat loss, and a rapid growth with heat-tightness. During the last period, corresponding to very under-ventilated conditions ) 2 . 1 (   [START_REF] Magnognou | Risk assessment of unburnt gas Ignition in an exhaust system connected to a confined and mechanically ventilated enclosure fire[END_REF], thermal insulation contributes to the most severe production of unburnt fuels, even for 5 ACPH. The growth rate of the unburnt volatiles becomes faster when the enclosure envelope is very heat-tight, particularly during the post-flashover stage due to lack of oxygen. Globally, the attempt to use an empirical methodology has been successful for the calculation of the unburnt fuels production from the measured oxygen concentration. The empirical correlations are predictive, and applicable for other fire scenarios [START_REF] Lassus | Estimation of species concentration during a fire in a reduced scale room[END_REF], allowing to assess qualitatively the ignition risk of a fuel rich mixture near the extraction duct if a good similitude is preserved. In the current enclosure, fire extinction of the liquid fuel pan occurs due to vitiated air with an oxygen concentration below 5% (cf. Fig. 8). This section aims to verify theoretically and experimentally Babrauskas's extinction/ignition criteria [START_REF] Babrauskas | Ignition Handbook[END_REF], derived primarily from the Calorimeter apparatus. Theoretically, an auto-inflammation risk takes place when the unburnt fuel concentration is above the Low Flammability Limit (LFL) at the point of Auto-Ignition Temperature (AIT) [START_REF] Babrauskas | Ignition Handbook[END_REF]. The LFLi of each fuel and the LFL of gas mixtures as a function of both the fuel molar fraction Xi and the gas temperature are determined as follows :

          0 0 0 i i T 1300 T T 1 ) T ( LFL ) T ( LFL and 1 i i i ) T ( LFL X 100 ) T ( LFL           (10)
The Upper Flammability Limit (UFL) can be also calculated as :

          0 0 0 i i T 1412 T T 1 ) T ( UFL ) T ( UFL and
1 i i i ) T ( UFL X 100 ) T ( UFL           (11)
The values of the LFLi and UFLi at T0=25°C as well as AIT for three fuel species [START_REF] Babrauskas | Ignition Handbook[END_REF] are summarized in Table 2. Note that even at a more developed stage with an enhanced evaporation rate, the concentration of the unburnt volatiles is never above the Upper Flammability Limit (UFL) with a value of about 20%. The molar fraction of unburnt volatiles in addition to the LFL of the gas mixtures near the extraction duct is presented in Figure 16(a-d). According to the visual identification, occurrence of auto-ignition inside the extraction duct was observed experimentally at 3 or 5 ACPH with heat tightness (cf. Figs. 16b,d) and only at 5 ACPH with heat loss (cf. Fig. 16c). An auto-ignition of a fuel rich mixture with presence of combustion products near the extraction duct represents very complex chemistry and physical processes, there is still a lack of knowledge for complete understanding of such random phenomenon. In regards to ignition risk in a connected exhaust system, the three characteristic stages can be described, as follows :

i) In the early stages of a fire, starting from 350 s with heat loss and 200 s with thermal insulation, a stratified hotter unburnt fuels layer with a concentration above the Lower Flammability Limit (LFL) is formed near the ceiling. It seems that the point of Auto-Ignition Temperature of about 220°C for the primary fuel as dodecane (cf. Tab.2) is not high enough for triggering an auto-ignition of a fuel rich mixture in addition to combustion products. It is noticeable that the auto-ignition temperature of a mixture is chemical composition-dependent (hydrocarbons, CO, H2, etc.), and difficult to be accurately quantified through bench-scale experiments.

ii) With a long time delay in a range of 16 to 21 min in the current study, the energy released per mass of oxygen consumed allows the smoke temperature to raise above 350°C, high enough for an auto-ignition of a mixture of the unburnt gases and vitiated air. However, during this stage, the large vaporized fuel due to enhanced heat transfer does not completely contribute to the heat release. Thus, a part of the fuel vapor accumulates with a concentration considerably higher than LFL, particularly with thermal insulation. Consequently, there is insufficient oxygen available near the extraction duct for triggering the ignition.

iii) Occurrence of flame extinction with an oxygen concentration below 5% makes a sudden increase of the depression level inside enclosure due to cooling effects. This induces a sudden supply of fresh air from dilution duct, characterized by a sharp increase of oxygen concentration to 10% (cf. Fig. 8a) at the extinction point of about 1250 s with heat loss and to 7% at 900 s with heat tightness. The supply of fresh air leads to a rapid dilution of unburnt fuel with a concentration close to a stoichiometric fuel-air mixture. This is ideal for triggering the ignition of a fuel rich mixture with a peak reaching a typical flame temperature higher than 550°C. With heat loss of enclosure (cf. Fig. 16c), occurrence of auto-ignition near the extraction duct was consistently observed at 5 ACPH with a longer time delay of about 1320 s after the extinction at 1250 s. Thermal insulation of enclosure (cf. Fig. 16d) contributes to a significant reduction in the ignition delay time to about 780 s due to fast-growing fires.

It is found that the delay for the transition from extinction of the fuel pan to ignition near the extraction duct is about 60-70 s. With heat loss of the enclosure at 3 ACPH (cf. Fig. 16a), after extinction at 1300 s, dilution of the unburnt fuel from the return of fresh air makes its concentration decrease below LFL during the transition delay. The temperature peak was not experimentally detected by the thermocouple near the extraction duct. This implies that an autoignition of the unburnt gases does not take place there, probably attributed to cooling of the smoke. Note that the response time of the thermocouples used in the experiment is close to a second by including thermal mass inertia and accumulation of soot particles. Besides, raw thermocouple data are corrected by heat transfer effects via convection and radiation. During auto-ignition of the unburnt gases, accurate measurement of flame temperature by using thermocouples is difficult by taking into account a multitude of potential errors, including surface reactions, radiation, convective gain or loss. The leakage of the experimental facility is also uncontrollable during each fire tests.

Globally, the compartment fire dynamics depend on the Global Equivalence Ratio (GER) associated with pool size, ACPH and fuel type. In a specific configuration with an admission duct placed closer to the floor and an exhaust near the ceiling, the GER above 1.5 represents a set of dangerous conditions for spontaneous ignition at entrance of the extraction duct (cf. Figs. 16a-d). The auto-ignition of unburnt fuels at the extraction duct occurs without needing a pilot flame as ghosting flame. To the knowledge of the authors, any attempt to draw a general conclusion for all configurations is discouraged. In order to obtain a good similitude between various scale compartments, a preservation of the ratio between heat release rate and characteristic length with a constant Froude number is needed. shows the histories of gas-phase temperature at a thermocouple tree in the center of the compartment at a height of 60 cm (TL5, cf. Fig. 1b,c) from the liquid surface. It is seen that thermal insulation of the wall envelope at 3 ACPH causes a strong decreasing trend of the temperature from 120 s. This is attributed to the flame moving from the fuel source due to lack of oxygen, expanding radially and horizontally towards the air inlet [START_REF] Wahlqvist | Validation of FDS for large-scale well-confined mechanically ventilated fire scenarios with emphasis on predicting ventilation system behavior[END_REF]. An inverse dependence of the gas temperature with time is observed. This is contrary to the trend at the extraction duct where the gas temperature rises with time due to energy accumulation. The auto-ignition near the extraction duct at 900 s (cf. Fig. 17a) makes an oscillation of the smoke layer temperature, and its impact on the gas temperature in the vicinity of fuel pan is characterized by a sharp temperature spike. An increase of ACPH to 5 (cf. Fig. 17b) helps maintain a relatively high temperature level for many minutes only with a slightly decreasing trend. As shown in Fig. 6c, thickness of the hot smoke layer (T>200°C) increases from 1.3 m with heat loss to 1.9 m with thermal insulation. Thus, impact of the temperature oscillation within the smoke layer due to ignition near the extraction duct on the centerline gas temperature seems less remarkable with heat loss. Finally, a sharp decreasing trend of the gas temperature from 20 min with heat loss and 16 min with heat tightness corresponds to the onset of extinction.

17a) at 3 ACPH 17b) at 5 ACPH Figure 17. Influence of heat permeability on the history of gas-phase temperature in the center of the compartment above the liquid pool fire

CONCLUSION

In summary, heat-tightness of enclosure can sensitively increase the liquid vaporization rate, and reduce the air supply flow rate due to a decrease of depression level inside enclosure. This implies that burning is quickly ventilation controlled with thermal insulation of enclosure, forming significant amount of unburnt volatiles. If the concentration of unburnt volatiles is higher than LFL, and the fire has been established for sufficiently long (in this case, more than 15 min), there is significant risk of ignition inside the extraction duct without needing a pilot flame like a ghosting one. Occurrence of an ignition of the hotter unburnt fuels at entrance of the extraction duct is attributed to a return of air from dilution duct at the stage of fire extinction which induces a sudden increase of depression level inside enclosure. An ignition can happen early near the extraction duct, as long as the enclosure remains sufficiently heat tight. Globally, the causes involved for occurrence of auto-ignition near the extraction duct are amount of accumulated unburnt volatiles above LFL, gas temperature above 350°C, oxygen concentration above 7% with heat tightness and 10% with heat loss, and phenomena leading to fire extinction with an oxygen concentration below 5%.

The intermediate levels of thermal insulation of enclosure appears a critical factor for time delay of occurrence of ignition via vaporization rate of condensed fuel and depression level inside enclosure. The work is continuing with the aim of improving the evaluation of auto-ignition temperature near the extraction duct by analyzing a more detailed chemical species. The future investigation aims also to identify the scale effect of enclosure on heat fluxes over the liquid surface via radiation contribution from flame, smoke layer and hot walls.

Figure 2 .

 2 Figure 2. Relationship between the stall pressure and the free volume flow rate of the fan

2 / 1 

 21 that comes from dimensionless variable of momentum conservation. Flow time scale is obtained from mass conservation by L ventilation flow rate is the inverse of 

  4a) at 3 ACPH 4b) at 5 ACPH Figure 4. Impact of the intermediate levels of thermal insulation on the inlet air velocity at the admission duct

Figure 5 .

 5 Figure 5. Impact of the intermediate levels of thermal insulation on the mass loss rate for different ACPH

AFigure 9 .

 9 Figure 9. Relationship between the oxygen concentration near the floor and the one at entrance of the extraction duct

Figure 10 .Figure 11 .

 1011 Figure 10. History of the molar fraction of carbon dioxide at the extraction duct for 3 ACPH

12b) with 5 ACPHFigure 12 .Figure 14 .Figure 15 .

 5121415 Figure 14. Effect of the intermediate levels of thermal insulation on hydrocarbons concentration in transient mode at entrance of the extraction duct

Fig. 17

 17 Fig.17(a,b) shows the histories of gas-phase temperature at a thermocouple tree in the center of the compartment at a height of 60 cm (TL5, cf. Fig.1b, c) from the liquid surface. It is seen that thermal insulation of the wall envelope at 3 ACPH causes a strong decreasing trend of the temperature from 120 s. This is attributed to the flame moving from the fuel source due to lack of oxygen, expanding radially and horizontally towards the air inlet[START_REF] Wahlqvist | Validation of FDS for large-scale well-confined mechanically ventilated fire scenarios with emphasis on predicting ventilation system behavior[END_REF]. An inverse dependence of the gas temperature with time is observed. This is contrary to the trend at the extraction duct where the gas temperature rises with time due to energy accumulation. The auto-ignition near the extraction duct at 900 s (cf. Fig.17a) makes an oscillation of the smoke layer temperature,

  Fig.17(a,b) shows the histories of gas-phase temperature at a thermocouple tree in the center of the compartment at a height of 60 cm (TL5, cf. Fig.1b, c) from the liquid surface. It is seen that thermal insulation of the wall envelope at 3 ACPH causes a strong decreasing trend of the temperature from 120 s. This is attributed to the flame moving from the fuel source due to lack of oxygen, expanding radially and horizontally towards the air inlet[START_REF] Wahlqvist | Validation of FDS for large-scale well-confined mechanically ventilated fire scenarios with emphasis on predicting ventilation system behavior[END_REF]. An inverse dependence of the gas temperature with time is observed. This is contrary to the trend at the extraction duct where the gas temperature rises with time due to energy accumulation. The auto-ignition near the extraction duct at 900 s (cf. Fig.17a) makes an oscillation of the smoke layer temperature,

Table 1 .

 1 Empirical constants used in the correlations for CO, CmHn and H2 concentrations

	Parameter	With heat loss	With heat tightness
		(	7 . 0				) 2	(	2				) 3
	XCMO	11.1%				11.1%
	XCMO, co	6.25%				6.25%
	a	1.98					7.45
	b	8					1.86
	c	1.33					1.2
	d	0.75					0.6

Table 2 .

 2 Values of the AIT and LFL/UFL at T0=25°C for three fuel species

	Parameter	CO	H2	Dodecane
	LFL (T0=25°C) 12.5%	4%	0.6%
	UFL (T0=25°C) 74%	75%	4.7%
	AIT (°C)	588	520	204