Study On Flame Spread Behavior And Gas-Liquid Heat Transfer Mechanism Of Jet Fuel Under Longitudinally Forced Air Flows

The liquid fuel widely used in petroleum and chemical industries.With the rapid development of the national economy,the demand for liquid fuel is increasing,so the storage and safe use of the liquid fuel is crucial.The liquid fuel is flammable,so the leaking fuel is extremely susceptible to fire.The longitudinal air flow commonly exists at the scene of a fire accident.The longitudinal air flow can carry the extra oxygen to the combustion zone,making the combustion more intense.Under the longitudinal air flows,the flame is inclined and elongated,which may expand the heat radiation to surrounding objects.Therefore,it is necessary to conduct in-depth studies on the characteristics of flame spread over liquid fuels such as flame length,flame tilt angle,flame spread rate and flame radiation under longitudinal air flow conditions.In this paper,five pools with the same length of 100 cm but different widths of 4,8,12,16 and 20 cm are used to conduct the flame spread experiments.The longitudinal air flow speed ranges from-2.57 to +2.57 m/s.The electronic balance,CCD camera,micro thermocouple and infrared camera are used to collect experimental data.The specific research work of this article is as follows:The characteristics of flame spread over liquid fuels,namely the flame tilt angle,flame height,and flame spread rate are studied under the effects of longitudinal air flows.It is found that the length of flash flame is increased by the concurrent air flow,but decreased by the opposed air flow.Under the longitudinal air flows,the flame height is shorter than that under still air,regardless of pool width.As the wind speed increases,the decreasing rate of the flame height gradually decreases,and the flame height eventually approaches a critical value.Compared with the flame height under no wind conditions,the flame height under maximum wind conditions is reduced by about 60%.The non-dimensional parameter,the characteristic scale ratio of pool ?(28)d/ D is introduced into the Welker’s model to obtain a new-proposed model of flame tilt angle,which is in good agreement with the experimental data.Similarly,the characteristic scale ratio ?(28)d/ D is introduced into the Oka et al.’s to obtain a new-proposed model of flame height,which is also in good agreement with the experimental data.Under opposed air flows,the flame spread rate decreases with increasing wind speed.This is because the longitudinal air flow suppresses the gas thermal expansion and brings the combustion products to the tailing portion of the flame.Both factors greatly reduce the heat transfer to the flame front,so the fuel evaporation rate and the flame spread rate greatly decrease.Under concurrent air flow conditions,the flame moves close to the oil surface and transfers more heat to the oil surface.The fuel evaporation rate is accelerated to induce the larger flame spread rate.The gas-liquid hydrodynamic study of flame spread over liquid fuel under the effects of longitudinal air flow is carried out.Under the concurrent air flow conditions,with the increase of wind speed,the effect of liquid-phase buoyancy on the movement of subsurface flow increases;under opposed air flow conditions,by contrast,with the increase of wind speed,this effect decreases.The movement of subsurface flow of flame spread over liquid fuel is mainly driven by surface tension,while the effect of buoyancy is relatively small,regardless of the direction and magnitude of air flow speeds.Under concurrent air flow conditions,when the sum of the velocity of gas thermal expansion and the velocity of the gas flow inside the boundary layer is greater than the opposed velocity of buoyancy flow,no gas-phase recirculation cell is formed in front of the flame,the flame pulsation disappears,and the flame spreads in a uniform mode;on the contrary,when the sum of the velocity of gas thermal expansion and the velocity of the gas flow inside the boundary layer is smaller than the opposed velocity of buoyancy flow,the gasphase recirculation cell is formed in front of the flame,the flame spreads in the pulsating mode.For a given fuel layer thickness of 10 mm and pool width of 4 cm,the flame spread over jet fuel is separated into the pulsating flame spread regime and the uniform flame spread regime with the critical air flow speed of 1.07 m/s.Under opposed air flow conditions,the gas-phase recirculation cell is formed under all wind speeds,so the flame always spreads in the pulsating regime.The length of surface flow varies irregularly with wind speed under the concurrent flame spreads.This is because the flame inclines to the cold oil surface in front of flame front.Though there is a subsurface flow zone below the inclined flame,the infrared camera cannot obtain the actual length of subsurface flow from directly above the oil pool.In opposed flame spreads,for 10 mm thick fuel layer,the subsurface flow length increases with increasing wind speed;for 3 mm thick fuel layer,the subsurface flow length increases firstly and then decreases as the wind speed increases.This is because the oil surface will fluctuate significantly under the strong wind conditions.The fuel distributes unevenly and the subsurface flow is unable to develop freely in such conditions.The heat transfer model including the liquid-phase convective heat flux,flame radiation heat flux and convective heat dissipation at the bottom is established for flame spread over liquid fuels.It is found that the gas-liquid heat transfer model can quantitatively calculate the heat fluxes during the flame spread process and the calculated results follows well with the experimental data in all wind speeds and oil layer thicknesses,which verifies the correctness of the model.For opposed flame spreads or concurrent flame spread with wind speed u < 0.88 m/s,the main heat transfer is liquid phase heat convection which accounts for about 80% of the total heat flow.This conclusion is the same as the heat transfer model of flame spread under still air condit ions.Under the concurrent air flow speed u > 0.88 m/s,the heat transfer mechanism is coupled by the flame heat radiation and liquid phase heat convection.With the maximum air flow speed u = 2.57 m/s,the flame radiation heat transfer can account for 29% of the total heat flow.For the flame spread of the 10 mm thick oil layer,the heat dissipation of the bottom wall of the oil layer is negligible;for the flame spread of the 3 mm thick oil layer,there is significant heat dissipation of the substrate,and the heat flow loss of the substrate accounts for about 11.5% of the total heat flow.