Study Of Shapes And Combustion Characteristics Of Jet Diffusion Flames Under Crossflow

The dynamic behavior of the jet flame under the action of external flow field has always been an important object of basic research on combustion and fire science.The air flow field may have an important effect on the shapes and combustion characteristics of jet diffusion flames.The research on the jet flame in crossflow is of great significance for improving the combustion efficiency of industrial boilers,reducing pollutant emissions from oil fields and chemical plants,reducing the false alarm rates and missed alarm rates of early fire detection,and so on.In this paper,firstly,from the perspective of phenomenological,the momentum equations in the horizontal and vertical directions were established and solved by considering the coupling effect of buoyancy,shear force and inertial force on the turbulent jet diffusion flame.The tilt angle formula was obtained and further simplified based on the Richardson number of the flame.And a global flame tilt angle model with three different control modes of buoyancy-transition-momentum was established.Secondly,a small scale wind tunnel platform was independently developed.Experiments of the shapes and combustion characteristics of jet diffusion propane flames with nozzle diameter of 3 mm(momentum-controlled)and 8 mm(buoyancy-controlled/transitional)were conducted in this wind tunnel.Reynolds number(Re)of the fuel jet was in the range of 310?3305,Froude number(Fr)0.351?756.Wind speed was in the range of 0.5?4.0 m/s,and jet-crossflow momentum flux ratio(RM)0.077?13.188.The heat release rate was in the range of 0.698?2.790 kW.The morphological parameters such as flame length and tilt angle were determined by image analysis.The flame temperature and soot volume fraction were inversed based on an improved colorimetric temperature measurement method.The critical condition of flame smoke point was determined by the relationship between the luminous flame length and the stoichiometric flame length.Based on the results,there were three typical flame shapes with increasing Rm.At lower Rm,the flame existed below the exit plane of the nozzle and concentrated in the low-pressure region on the leeward side of the nozzle,called as down-wash phenomenon.With increasing RM,flames presented a three-zone structure:the flame attached to the leeward side of the nozzle,the long symmetric tail of the flame,and the junction that connects these two zones.With continuously increasing RM,the down-wash zone became small and eventually disappeared,and the flame presented a two-zone structure.The flame length increased slightly and then decreased as the crossflow velocity increased or Rm decreased.The relationship between the dimensionless flame length and Fr was LF=18.8Fr0.239.Based on the experimental results in this paper,the critical crossflow velocity and Rm value of the transition point of flame length under crossflow condition were given.For a given nozzle diameter,the critical Rm value tended to be fixed.According to the experimental results and theoretical analysis of the flame tilt angle,three dominated regimes of the flame tilt angle were identified and the applicable ranges of the global flame tilt angle formulas were determined combining RM and Fr:crossflow-/buoyancy-dominated(RM<0.01,Fr<0.1),transitional(0.01<RM<10,0.1<Fr<103),and jet-/momentum-dominated regime(RM>10,103<Fr<105).With increasing crossflow velocity or decreasing Rm value,the flame temperature and soot volume fraction decreased,and flame radiative fraction first increased slightly and then decreased for cases with nozzele diameter of 8 mm,and decreased for cases with nozzele diameter of 3 mm.Finally,the critical crossflow velocity and Rm value at smoke point were given.As the fuel flow increased(Fr increased),the critical crossflow velocity and Rm value increased.The relationship between the critical Rm value and Fr was RM =0.170Fr0.345.The relationship between the dimensionless flame length at smoke point and Fr was LF,sp=13.8Fr0.279.Furthermore,at smoke point,flames with nozzele diameter of 8 mm had a typical three-zone structure,while for 3 mm,they had a typical two-zone structure.In experiments,when the fuel flow rate was greater than a certain value,the ratio of the dimensionless flame length at smoke point to the maximum dimensionless flame length tended to be fixed,and the ratio of 3 mm was larger than that of 8 mm.As the fuel flow increased(Fr increased),the flame radiative fraction at smoke point increased for cases with nozzele diameter of 8 mm,while for 3 mm,it first increased and then decreased;furthermore,the former is much larger than the latter.The different trends for the cases with different nozzles were mainly resulted from the different flame flow field and soot trajectories caused by different Rm values at smoke point.The relationshiop between the ratio of the flame radiative fraction to the dimensionless flame length and the Rm value was(?)for the cases with nozzle diameter of 8 mm,(?)the cases with nozzle diameter of 3 mm.