Study On Combustion Mechanism And Flame Structure Of Horizontal Jet Spray At Sub-atmospheric Pressure

Liquid fog combustion is widely used in industry,military logistics,emergency support,and other fields.On the plateau,the influence of the low-pressure environment on the horizontal spray flame structure and its changing mechanism has not been clear.The study of spray combustion under a low-pressure environment can not only enrich the theoretical system of two-phase combustion but also have important guiding significance for the engineering application of plateau heating technology.In this paper,a horizontal jet fuel burner is taken as the research object,and the combustion mechanism and flame structure change law of horizontal jet diesel spray under sub-atmospheric pressure are studied by combining theoretical derivation,experimental study,and numerical calculation.Firstly,without considering the atomization and evaporation process of fuel,the analytical equation of flame trajectory of forced swirl for horizontal jet spray combustion is derived.The change law of the flame trajectory and uplifted characteristics with atmospheric pressure is analyzed theoretically and verified by experiments.The results show that the flame trajectory coincides with each other when the rated air volume flow rate is maintained at different atmospheric pressures.With the same equivalence ratio,the slope of the flame trajectory is proportional to the atmospheric pressure.If the flame trajectories coincide with each other at any two atmospheric pressures,the equivalence ratio is equal to the inverse ratio of air density.Secondly,the horizontal jet spray combustion experiment system in the low-pressure chamber is constructed.The multivariate correlation analysis was carried out employing uniform test and the primary and secondary factors of the influence of different parameters on the change of flame structure were obtained.The probability distribution contours of continuous flame images were obtained by thermocouple acquisition of flame temperature and MATLAB programming calculation.Rated air volume flow rate and constant equivalence ratio were respectively maintained at 0.05-0.10 MPa atmospheric pressure,and the flame morphology and temperature characteristics of horizontal spray were investigated.The characteristic slopes k and k_f were used to characterize the uplifted characteristics of the whole horizontal spray flame and the flame fluctuation region.The dimensionless horizontal projection length of flame fluctuation is used as the evaluation factor of flame stability.When the equivalence ratio remains unchanged,the flame stability under sub-atmospheric pressure becomes worse,indicating that the diameter of the combustion cylinder and the strength of the swirl should be increased moderately at high altitude.The dimensionless three-region theory of horizontal jet spray flame（recirculation zone,continuous flame zone,and buoyancy plume flame zone）is proposed to describe the temperature characteristic distribution of flame trajectory under sub-atmospheric pressure.Based on the discrete phase（DPM）random trajectory model,large eddy simulation（LES）and probability density function（PDF）are used to calculate the horizontal spray flame with rated air volume flow rate and constant equivalent ratio respectively.The combustion characteristics under different atmospheric pressure were revealed from the micro perspective of the burning time,burning path,and burning path rate of multi-droplet.The maximum droplet burning time and path control the horizontal spray flame length under different atmospheric pressure;The combustion mechanism of the flame is analyzed from the perspective of component change.The longer the average droplet group burning path L_d-ave,the less ideal the combustion effect,and the L_d-ave should be controlled below 85mm.The density distribution can better reveal the morphologic mechanism of the flame region of buoyancy plume.In order to deeply explore the influence of equivalence ratio on horizontal spray flame at different sub-atmospheric pressures,the experimental study was carried out with 0.05Mpa and0.08Mpa as the quantitative atmospheric pressure,and the evolution law of flame under different equivalence ratio and atmospheric pressure was revealed.The results showed that the influence of atmospheric pressure on the entrainment coefficient in the horizontal spray flame was weaker than that of the equivalence ratio.The change of flame stability with the decrease of atmospheric pressure is mainly reflected in the decrease of flame temperature in the backflow area.At 0.05Mpa atmospheric pressure,if the flame trajectory length is equal to that of 0.08MPa,the ratio of equivalence ratio is 1.15.Finally,in order to explore the key theoretical and technical problems such as too long flame,low temperature and unstable combustion in the application of active service diesel burners on the plateau,the design of the burners on the plateau is more scientifically guided from the theoretical perspective,the characteristics of flame shape and temperature distribution under different spray cone angles,oil injection pressure,secondary air ratio,and power at atmospheric pressure of 0.05MPa were investigated.The fluctuation and stability mechanism of flame shape affected by different operating parameters at extremely low atmospheric pressure is revealed.The semi-empirical mathematical equations of global explicit flame trajectories including atmospheric pressure,equivalence ratio,fuel consumption,and operating parameters are established.