| Examining upward flame spread over thermally thin fuel is of great significance to evaluate the risk of combustibles and prevent fire hazard.In fire scenario,when being exposed to an hot source,the combustible materials begin to be deformed,bubbled and melt,which make it common that flame spreads over unflat fuel surface.Previous studies sugguest that flame spreading over unflat fuel surface behaves different,while it is comparatively less researched.Therefore,two representative sample materials of thermally thin A4 paper sheet and two-layer corrugated carboard were selected,and vertical flame spread over the flat surface and the wavy surface were investigated in detail.This paper experimentally and numerically investiagted the thickness of A4 paper sheet on upward flame spread.Two-sided upward spreading flames observed from the experiment and simulation experience the similar stages:flame growth,flame merg-ing and flame decaying.The experimental measurements and the simulation show that with the increasing sample thickness,the burning duration time increaes,the moving rate of flame base decrease,and the flame spread rate(FSR)decreases as well.In terms of flame contour,flame spread rate,the moving rate of flame base,dimensionless flame height and burning duration time,the model predictions are in good agreement with the experiment.The correlation of dimensionless flame standoff distance with the normal-ized distance is established.The relationships among flame standoff distance,flame net heat flux received by the fuel surface and local burning rate were also analyzed.It is found that a smaller flame standoff distance can cause a higher flame net heat flux to the fuel surface,which in turn induces a larger local burning rate.Moreover,profiles of the net heat flux that a higher net heat flux is imposed on the fuel surface near the flame base and decreases with the distance from the flame base,are similar.The result shows a-0.26 power dependency of the flame net heat flux on the dimensionless distance,which agrees reasonably with the-0.25 power predicted by the natural-convective boundary layer theory.Also,the effect of sample thickness on downward flame spread over flat surface of the paper sheet was studied.It is concluded that the flame spread rate and the downward moving rate of flame base decrease with the increasing sample thickness,while burning time increases.In the burned region,the flame standoff distance firstly increases and then decays.In addition,the flame radiative heat flux to the fuel surface occupied the dominant position during downward flame spread,and the correlation of flame radiative heat flux with the normalized distance was developed.Corrugated cardboard is assembled by the flat and wavy layers.It is selected to investigate upward flame spread over the flat surface and the wavy surface in natural convection by Direct Numerical Simulation(DNS).The model predictions demonstrate that the flame height and upward moving rate of flame base in the wavy side are larger than these in the flat surface.Flame standoff distance near the wavy surface is smaller,comparing with that near the flat surface,which suggests that the flame closely ap-proaches the wavy surface,and it in turn leads to a higher average temperature gradient near the wavy surface.Comparing with the local burning rate on the flat surface,the local burning rate on the wavy surface behaves differnet and are relatively larger while both them show a decaying trend on the whole.It is also found that the flame on the wavy surface readily splits into several small flamelets which promotes the moving of flame base,and thus the moving rate of flame base on the wavy surface is larger.Moreover,the effect of the corrugation size on flame spread was also examined.It is shown that the flame height in the wavy side for the larger C flute corrugated cardboard is less than that for the smaller B flute corrugated cardboard.A smaller flame stand-off distance and a larger local burning rate are obtained on the wavy surface for the B flute corrugated cardboard.To valid the model predictions,an experiment on upward flame spread over the surfaces of A4 paper sheet,C and B flute corrugated cardboards was performed.The experiment agrees well with the model predictions,and both them demonstrate that flame height,flame spread rate and the rate of flame base moving in the wavy side are larger than those in the flat side.Furthermore,the moving rate of flame base,and FSR in the flat side or in the wavy side for the C flute cardboard are less than those for the B flute corrugated cardboard.With the upward flame propagating over the wavy surface,the influence of layer thickness on flame spread was computationally studied through DNS.The results show that when the layer thickness is less tnan 0.390 mm,both FSR and the moving rate flame base decrease with the increasing layer thickness.Dimensionless flame stabdoff distance in the wavy side was quantitatively compared with that in the flat surface for different layer thickness,and the correlation of the flame standoff distance in the flat side or in the wavy side with the normalized disttance was established.With the end of flame meriging stage,the flame on the flat surface continues to spread until the flame extinction occurs.However,studies on the characteristics of single flame on the flat surface are limited after the end of flame meriging stage.Therefore,two key parameters of flame net heat flux and local burning rate were systematically investigated,and the power function of both them with the normalized distance were established.This paper experimentally and numerically investigated the effect of the sample thickness,corrugation size and layer thickness on vertical flame spread.The relation-ship of flame standoff distance,flame net heat flux and local burning rate were examined in detail,and the correlations of them with the normalized distance were established.Moreover,the characteristics of flame on the flat surface or on the wavy surface was quantitatively compared,and the difference between the flame on the flat surface and on the wavy surface was analyzed.This paper can provide a solid basis on which to guide the flame spread over unflat or wavy surface. |
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