On the Flame Height Definition for Upward Flame Spread

Research journal paper published in Fire Safety Journal in 2007Flame height is defined by the experimentalists as the average position of the luminous flame and, consequently is not directly linked with a quantitative value of a physical parameter. To determine flame heights from both numerical and theoretical results, a more quantifiable criterion is needed to define flame heights and must be in agreement with the experiments to allow comparisons. For wall flames, steady wall flame experiments revealed that flame height may be defined by a threshold value on the wall heat flux. From steady wall flame measurements, three definitions of flame height from wall heat flux are retained: the first is based on the continuous flame while the two others are based on threshold values of 4kW/m^2 and 10kW/m^2. These definitions are applied to determine flame heights from a two-dimensional time-dependent CFD model used to describe flame spread along a slab of PMMA. Results show that the predicted flame heights are consistent with the available data of the literature. Defining flame height by threshold values on the wall heat flux of 4kW/m^2 and 10kW/m^2 allows to correlate the wall heat flux in term of (x-x_p)/(x_fl-x_p), which is the dimensionless characteristic length scale for upward flame spread. It is also found that the continuous flame is not a characteristic length for the heat transfer to the unburnt fuel and is not really appropriate to define flame height in upward flame spread

Développement d'un modèle diphasique dédié au calcul de l'interaction d'un brouillard d'eau et d'un feu en milieu compartimenté (application à la lutte incendie)

Un modèle diphasique multi-classes est développé pour décrire les interactions entre un feu de compartiment et un brouillard d'eau. Pour modéliser les transferts radiatifs dans les milieux gazeux chargés en particules, une équation de transfert radiatif multiphasique a été établie et validée pour de grosses particules opaques relevant de l'optique géométrique. Pour prendre en compte les régions solides internes à l'écoulement, la technique des régions bloquées est généralisée aux écoulements diphasiques réactifs turbulents. Les résultats numériques montrent que la diffusion du rayonnement thermique par les gouttes d'eau constitue un mécanisme important dans l'atténuation de l'énergie radiative par le nuage de gouttes. Deux régimes sont mis en évidence : un régime d'extinction et un régime d'accroissement du feu. Ce dernier se produit lorsque l'écoulement induit par le nuage de gouttes augmente l'entraînement d'air vers la zone de réaction et que les gouttes pénètrent difficilement dans la zone de flamme.AIX-MARSEILLE1-BU Sci.St Charles (130552104) / SudocSudocFranceF

Fire propagation through a multiphase media

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Predicting fire suppression efficiency using polydisperse water sprays

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Dynamic and thermal study of a pulsatile flow in presence of a heated rotating solid

International audienceDynamic and thermal study of a pulsatile flow in presence of a heated rotating solid. This work concerns the dynamic and the thermal study between a heated rotating solid and a pulsatile flow evolving periodically in a cylindrical cavity. The reference frequency of pulsatile flow was determined by the evolution regime to his stationary state. The frequency varied from the 1/3 reference frequency to 10 once this value. The periodicity is assured by the periodical inlet conditions. The numerical study concerns the Reynolds number less than 100 and the Taylor number less than 500, in order to avoid the presence of instabilities in the flow. Two numerical methods have been used; the finite-elements method and the finite-differences combined with ADI scheme. The fluid considered is the air with constant physiques proprieties. The influence of ratio conductivity, K, between solid conductivity and fluid conductivity, has taken from K = 10(-2) to K = 10(+3). The influence of the two periodical combined motions; rotation of the solid and axial flow, on the thermal transfer mechanism is studied. The results show that the velocity components are in phase with the rate flow but the pressure and the different thermal parameters are in difference of phase. We note also the oscillatory shape of different dynamical and thermal parameters. The frequency influence appears at the greater values. The Nusselt number is proportional to the frequencies. The results are in good agreement with previous numerical and experimental studies

Stress exercise haemodynamic performance and opening reserve of a stented bovine pericardial aortic valve bioprosthesis

Objectives: Despite unusual high rates of patient-prosthesis mismatch (PPM), excellent midterm clinical outcomes have been reported after surgical aortic valve replacement (SAVR) with the Avalus™ bioprosthetic valve (Medtronic). To elucidate this "PPM conundrum,"the Avalus valve haemodynamics were assessed during exercise testing. Methods: Of the 148 patients who had undergone SAVR with the Avalus valve at our institution, 30 were randomly selected among those in whom stress test was deemed feasible and underwent a resting transthoracic echocardiography immediately followed by exercise echocardiography. Severe PPM was defined as indexed effective orifice area (iEOA) ≤0.65 cm2 /m2 and moderate PPM as iEOA >0.65 and ≤0.85 cm2 /m2. Measured PPM was determined with the use of the measured iEOA at rest or stress, while the estimated PPM was based on the estimated iEOA, derived from the mean EOA reported for each valve size in the manufacturer chart. Results: Measured EOA significantly increased from rest to peak exercise in all PPM groups (p < .05) and the rates of moderate and severe measured PPM decreased from 40% and 20% to 27% and 0%, respectively. The patients with low-flow state (flow < 250 ml/s) had significantly lower measured rest EOA (p = .03). On the basis of the estimated iEOA, there was no severe PPM and 19 patients had moderate PPM (63.3%), with a significantly lower opening reserve than the patients without estimated PPM (p = .04). The estimated iEOA was more reliably correlated to the measured iEOA at maximal stress than the measured iEOA at rest, especially in patients with a low-flow state. Conclusions: This study supports the concept of an opening reserve of the Avalus valve to explain the PPM conundrum and promotes the use of exercise Doppler-echocardiography to complete the assessment of mismatch, especially in patients with a low-flow state. Published estimated EOA seems reliable to predict the haemodynamic performance of the Avalus valve, whether the flow conditions at rest

Universal scaling in wildfire fractal propagation

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