The Downwash And Detachment Study Of Jet Fire Diffusion Flame In Cross-flow

Burning behavior of jet flame has been one of the classic scientific problems of fire and combustion.The physical behavior of jet fires can be described as:gaseous fuel was discharged from the burner with a certain amount of momentum and then mixed with the air,ignited and relied on the heat from the flames and the supply of fuel to maintain a stable combustion.Jet fires often appear in the oil pipeline and oil tank leakage,because of its full suction,high combustion efficiency,with a high flame temperature,flame radiation and flame impact on the equipment and the surrounding environment can cause serious damage,and thus often become an important factor in the induction of leakage accidents.Combustion stability(lift-off stability,critical lift-off)of jet flame is the most important aspect of jet fire combustion;it will determine the distance of flame impact,thermal radiation intensity and other external environment damage to the size of the parameters.At present,the research on jet flame is mainly concentrated in the case of static and no wind,and the research on the cross-flow wind is relatively less.In the general case(such as in the outdoors,on the hillside),there usually exists the cross-flow wind.In aspect of cross-flow wind effect on burning behavior of jet flame,predecessors mainly through the wind tunnel test preliminarily revealed the appearance of the flow field,the reverse vortex,and the change of the flame downwash length with the fixed flow rate with the cross-flow wind from the macroscopy.While in aspect of the further analysis of cross-flow wind effect on the jet flame under the critical downwash and downwash length,the critical detachment and its scale effects,lack of relevant research is obvious currently.Based on the above factors,this paper aims to combine the theoretical analysis to fill this block in this field.This paper conducted experiments including flame critical downwash and downwash length,critical detachment behavior in cross-flow wind.In aspect of flame critical downwash and downwash length,in this work,the flame downwash length evolution of non-premixed jets for different pipe nozzle diameters at various fuel discharge velocities and cross-flow air speeds as well as the critical condition under which the flame downwash occurs have been quantified for the first time.Pipe nozzles with inner diameters of 8 mm,10 mm,13 mm and 15 mm were employed in the experiments,using propane as the fuel and with fuel jet velocities ranging between 0.38 and 2.42 m/s.The experimental results showed that the flame downwash length increased with increasing cross-flow air speed for a given fuel jet exit velocity.It was also found that,with increasing cross-flow air speed,the downwash length increased more rapidly for the higher fuel jet exit velocity than that for the lower fuel jet velocity.The critical cross-flow air speed,when flame downwash occurs,was found to be little dependent on the nozzle diameter but increase with the fuel jet exit velocity following a linear relationship.A new correlation for flame downwash length was proposed based on physically the coupling effects due to the competition of the fuel jet momentum to the cross-flow momentum and the total fuel mass supply.The proposed correlation was shown to well characterize the flame downwash length in terms of nozzle inner diameters,dimensionless fuel mass flow rates and the jet momentum ratio.The findings of this work provide basic knowledge and have potential practical applications for burner design and flare implementation,which allow predictions to be made regarding the possible threat and the establishment of the necessary safety length for the stack to prevent the damage for reducing the potential risk.In aspect of critical detachment behavior in cross-flow wind,in this paper,critical detachment at the windward and leeward sides as well as the difference in critical air speeds of the cross-flow for flame detachments at the windward and leeward sides of the nozzle exit have been quantified for the first time.Experiments were conducted in a wind tunnel employing jet nozzles with inner diameters of 3 mm,5 mm,8 mm,and 10 mm and an outer diameter of 15 mm;and with inner diameters 13 mm and 15 mm and an outer diameter of 20 mm using propane as the fuel.The result showed that the critical air speeds of the cross-flow for flame detachment at the windward and leeward sides,as well as its difference decreased as the fuel jet velocity increased.The critical air speed for flame detachment at the windward side was weakly dependent on nozzle diameter,while that at the leeward side as well as its difference increased with the increase in nozzle diameter.An analysis was conducted to characterize this difference based on the Damkohler number related to local strain rate,defined as the ratio between the mixing time and characteristic reaction time.A satisfactory correlation was obtained for the difference in critical air speeds as a function of the fuel jet velocity and nozzle dimensions(inner diameter and wall thickness).