Effects Of Ullage Height On Burning Behaviors Of Pool Fires

Fuel-burning under boundary conditions is usually referred to as a pool fire.Pool fire is the most common type of fire accident in industrial production,storage,and transportation.Pool fire has the following hazardous characteristics,i.e.high combustion intensity,high flame temperature,strong heat radiation,and difficulty in extinguishing.Ullage height is the distance between the pool upper rim and the internal fuel surface.Classic studies about pool fires were usually conducted with minor ullage height.In this condition,the ullage height had little influence on the burning behaviors of pool fires and generally was neglected.However,the ullage height would increase as the fire developing and the fuel consumed.The burning behaviors of pool fires would significantly be changed when the ullage height increased to a certain value.The existence of ullage height,on the one hand,affects the mixing process of fuel vapor and the entrained air in the fire plume which results in changed flame shape and combustion mode.On the other hand,it affects the heat feedback mechanisms resulting in changed radiative and convective heat transfer between the flame base the fuel surface and changed conductive heat transfer between the pool sidewall and the fuel.The above two ullage height effects will significantly influence the burning behaviors.And,this will bring uncertainties to the fire risk assessment,emergency response,and fire control which are based on the classic pool fire model.In-depth analysis of the flame characteristics and burning behaviors under the influence of ullage height has extreme importance in the field of theoretical study and practical application(such as industrial fire safety monitoring and prevention and control).This paper uses a combination of experimental research,theoretical analysis,and numerical simulation to systematically investigate the evolutions of the flame characteristics,heat feedback mechanism,combustion emission,plume flow,and combustion mode of pool fires under the influence of ullage height.The main work is as follows.A series of experiments were conducted for pool fires with varying ullage heights.The effects of ullage height on the flame characteristics(flame height and flame pulsation frequency)of pool fires were investigated.During the experiment,the fuel level maintaining and supplying device was used to precisely control and keep the ullage height stable during each test.In all tests,ullage height was systematically changed from zero to the value that the flame self-extinguished to simulate a pool fire with any ullage height conditions.Based on the force analysis of the fire plume,an equivalent hydraulic diameter was developed to quantify the influence of edge height.The applicability of previous classic models of flame characteristics on those with different ullage heights were tested.After that,more universal models of flame height and pulsation frequency were established based on the proposed equivalent hydraulic diameter.The reliability of those models had been verified by a large amount of literature data.A series of experiments were conducted for measuring the heat transfer and combustion emission of pool fires under different ullage height conditions.The effects of ullage height on the heat feedback mechanisms,combustion emission,and plume centerline temperature of pool fires were investigated.A series of modular devices generated by precision lathe were applied to measure and analyze the radiative,conductive,and convective heat flux received by the fuel under different ullage height conditions.FPA(Fire Propagation Apparatus)was used to measure the combustion emission in real-time.The evolutions of CO,CO2,and smoke particle density fraction were analyzed.Combustion efficiency was also calculated based on the combustion emission.The applicability of previous classic plume temperature models on those with different ullage heights were also tested.It proved that previous classic models could not well predict those with significant ullage height conditions.Following that,based on physical discussion and non-dimensional analysis,a new virtual origin model considering the effects of ullage height was established.After that,more universal plume centerline temperature models were correlated and validated by reference data.A series of numerical simulations were conducted for pool fires with different ullage heights.The effects of ullage height on the plume flow and combustion mode of pool fires were investigated.Firstly,the numerical models were systematically validated by the time-averaged and the instantaneous data obtained from the experiments.According to the numerical results,the evolution of plume flow and plume flow patterns as ullage height increased were revealed and classified.Based on the characteristics of premixed and non-premixed combustion,effects of ullage height on the combustion mode were investigated.