Investigation Into Burning Rate,Heat Feedback Mechanisms And Radiation Properties Of Pool Fire In Cross Flow

Burning behaviors of pool fire is a classic topic in the area of fire research and combustion science.Previous researchers conducted considerable amount of theoretical analyzes and experimental investigations into pool fires' burning behaviors.However,researches focusing on the impact of some special scale and boundary conditions on pool fires are still limited:(1)burning rate evolution of optical thin pool fire in cross flow air speed.Previous studies on pool fires in cross flows mainly focus on small scale(conduction-and convection-controlled)and large scale(total radiation-controlled)pool fires,while the data for optical thin pool fires,whose sizes are at the transition phase between the convection-controlled and the radiation-controlled,were still lacking under cross flow conditions;(2)burning rate evolution of pool fires with various lip heights in cross flows.Previous investigations mainly focus on pool fires with very small lip heights in cross flows and pool fires with various pool fires in still air.There is still no study on the coupled effect of lip height and cross flow air speed on burning rate;(3)radiation properties of pool fire in cross flows.Current investigations into the radiation properties of pool fire were mainly conducted in still air,leaving the radiation properties of pool fire in cross flows barely studied.So,it requires further investigations into the burning behaviors of pool fire in cross flows.Both experimental investigations and theoretical analyzes were conducted to study the evolution of burning rate,heat feedback mechanisms and radiation properties of pool fires in cross flows.With a series of liquid pools and porous burners,the burning rate evolution of optical thin pool fire were investigated firstly,then the coupled effect of lip height and cross flow air speed.At last,radiation properties of propane pool fire in cross flows were studied.Major findings include:(1)The burning rate evolution of optical thin pool fires in cross flows was quantified.Wind tunnel experiments were conducted with five square n-heptane with side length of 25-70 cm.It was found that there were several transitions for the burning rate with increase in cross flow air speed in this range for different size pool fires.For relative smaller pools(25 and 35 cm),the burning rate increased and then decreased.The first increase trend results from the enhanced forced convection led by cross flow.As the cross flow air speed further increased,the flame residence time decreased,which caused the flame to detach from pool leading edge,allowing the cross flow to cool the fuel surface directly.For larger pools,the burning rate first increased then decreased and finally increased again.With relative weak cross flow,the mixing of air and fuel vapor was enhanced,along with the attenuated effect of radiation heat feedback blockage caused by the vapor cone near the fuel surface.As the cross flow air speed further increased,the radiation heat feedback declined with the tilted flame,resulting in the decrease of burning rate.In relative strong cross flow,the burning of pool fire transited to forced convection controlled,leading to a final increase of burning rate.The corresponding cross flow air speeds for these transitions correlated well with buoyancy induced velocity scaled based on Froude number,indicating that the heat transfer mechanisms reached balance at the turning points.(2)The coupled effect of cross flow and lip height on pool fires' burning rate was investigated experimentally.N-heptane pool fires with side length of 5-20 cm and lip height of 0.25-2 times of pool side length were employed.It was found out that for pool fires in still air,the burning rate showed non-monotonic evolution trend.The heat transfer mechanisms were then analyzed.As the cross wind was introduced,the burning rate showed different evolution trend with pool scale and lip height.For small pools(D=5 and 10 cm),the burning rate increased monotonically with cross flow air speed when h*was small,but first decreased then increased with large h*.For larger pools(D=15 and 20 cm),the burning rate first increased,then decreased,followed by a final increasing trend with small h*;while for larger h*,the burning rate first decreased and then increased with increasing cross flow air speed.These different trends were discussed in terms of the considerable change in the distance from the flame to the fuel surface,which affected the conduction,convection and radiation feedbacks to the fuel.It was observed that the flame was "pushed down"into the fuel-containing vessels by relatively strong cross flows,which further changed the heat feedback.A burning rate correlation was proposed to characterize the combined effects of cross flow air speed and lip height.(3)The evolution of radiation properties of modeled pool fires(propane porous burners)in cross flows were studied experimentally.Bench scale experiments were conducted to quantify the evolution of radiation fraction of pool fires using square porous burner of different sizes(8,10,15 and 20 cm),cross flow air speeds(0-?5 m/s)and heat release rates(HRR,9.24-27.72 kW),employing propane as the fuel.It was found that both radiation heat flux and radiation fraction decrease with increasing cross flow air speed from?0.5 m/s up to?5 m/s.Combined with conclusion of previous researchers that radiation fraction is proportional to flame volume(also interpret as flame surface area),a triangular approximation of flame projection area was proposed,following by a quantification of flame surface area with factors including pool scale,flame height and flame base drag length.A new correlation related to the flame surface area exposed to the radiometer was proposed to interpret the changing radiation fraction of pool fires of different size and heat release rate(HRR)in cross flows.The proposed function correlates the experimental data well under relative strong cross flow conditions(Fr>1).