Study On The Spray And Combustion Characteristics Of LOX/GCH₄ Pintle Rocket Engine

Owing to its advantages of simple structure,environmental friendliness,deep-throttling ability and inherent combustion stability,the LOX/GCH₄ pintle injector with discrete radial orifices has become the important development direction of the reusable variable thrust liquid rocket engine technology.However,the understanding of spray and combustion characteristics of such injector is not intensive.In this dissertation,experiment researches and numerical simulations were conducted to systematically study the pintle injection element and the LOX/GCH₄ pintle engine,aiming to reveal the influence of working conditions on the breakup-atomization process and combustion performance of the gas-liquid pintle injector.As the research object,the pintle injection element using air and water as the simulants was designed.The influence of the local momentum ratio on the spray process is analyzed by adjusting the circular-orifice diameter and the air flow.Research methods include Malvern particle analyzer,background light imaging and two-phase flow large eddy simulation of spray.The backlight imaging results and Sauter mean diameter distribution(D_SMD)of the far-field spray show that as the local momentum ratio decreases,the limiting effect of the axial gas film on the radial liquid jet is strengthened,and the gas-liquid mixing becomes poorer.The radial distribution of D_SMDgradually changes from an inverted"V"shape to an inclined"N"shape,and D_SMD does not decrease significantly due to more obvious droplet enrichment.The non-dimensional boundary band model of spray windward side based on the local momentum ratio was obtained for circular-orifice jet.It is found that when the local momentum ratio is about 1,the spray distribution range basically overlaps with the gas film action range with uniform liquid mist.The establishment process of the gas-liquid flow field in near-orifice area was obtained by large eddy simulation,and the primary breakup mechanism of the liquid jet dominated by Rayleigh-Taylor（R-T）unstable surface wave was revealed for the gas-liquid pintle injector.Continuous high-speed photography spray images show that the high-frequency spray oscillation mode in near-orifice area is a traveling wave structure,which is produced by the R-T unstable surface wave before the continuous liquid jet fractures.Through the conventional parameter measurements and the open-window fire tests with a single-camera scheme,the spray distribution and flame shape inside the LOX/GCH₄ pintle engine prototype were observed simultaneously for the first time.Pure methane is used for ensuring successful ignition when starting the engine,and then the nitrogen-premixing is turned on to switch the combustion state to avoid overheating.The spray and flame structures,and their dynamic characteristics were investigated during the whole work process.Combining the optical observation results with time history curves of pressure and flow,it is found that the hysteresis in the establishment of GCH₄ mass flow rate causes the combustor with oxygen-rich environment to experience ignition delay,intermittent combustion and flame enhancement before entering the stable combustion state.The flames respectively present“C”-shaped and right-leaned“V”-shaped distribution in the stable combustion state before and after nitrogen addition,and both of them have low-frequency dominant oscillation modes.In addition,the difference of the flow coefficient of radial orifice in the hot tests and cold tests is analyzed,and the calculation method for the flow coefficient of low temperature propellant is proposed.A numerical simulation platform for the spray and combustion process of LOX/GCH₄ pintle engine was established,and then the influence of LOX injection diameter was studied.It is found that as the LOX injection diameter increases,the droplet evaporation rate decreases significantly and the combustor pressure gradually moves away from the target value until the combustion terminates.Besides,the combustor pressure oscillation amplitude gradually increases,and low-frequency combustion instability occurs when the LOX injection diameter exceeds 60μm.The main oscillation frequencies are concentrated around 40Hz,80Hz and its high-order resonance,and 120Hz,which are respectively caused by the transverse swinging process of spray and flame,the unsteady oscillation of LOX evaporation rate,and the convection phenomenon of high-temperature gas in the combustor axial direction.Through comprehensive considersion of test and simulation results,the influence of operating conditions on the overall engine performance was analyzed.The stable combustion without nitrogen addition is in a rich-fuel environment.Along with the increase of the local momentum ratio,the combustion efficiency first shows a downward trend.The excessive liquid oxygen momentum leads to further separation of the oxidant and fuel distribution regions,which inhibits the diffusion combustion.As the local momentum ratio increase further,two stable small recirculation zones are gradually formed near the pintle tip,and the combustion reaction is facilitated in the combustor head zone.However,the combustion efficiency will no longer change significantly when the local momentum ratio reaches about 7.The local momentum ratio is at a low level with nitrogen addition.Before the LOX/GCH₄ mixing ratio is close to the optimal equivalent ratio（i.e.3.2）,the combustion efficiency is positively and negatively correlated with the nitrogen addition level and the LOX/GCH₄ mixing ratio,respectively.And a higher combustion efficiency means a greater probability of flameout after opening the pneumatic valve of nitrogen.Increasing the local momentum ratio by adjusting the skipping distance,orifice number and shape,injection pressure drop and other parameters can finitely accelerate LOX evaporation and combustion reaction.For an appropriate local momentum ratio,the distribution range of LOX spray basically overlaps with the premixed-gas film action range,which is conducive to well mixing and rapid reaction of the propellants.In addition,as the propellant mass flow rate is smaller under lower operating conditions,the influence of mixing ratio and local momentum ratio on combustion performance is weaker.On the basis of the existing simulation model,the inlet condition change and the transverse velocity disturbance were respectively simulated by loading different user-defined functions.Then the dynamic response characteristics of the combustor pressure oscillation and combustion flow field were studied.When the propellant component content and the local momentum ratio remain unchanged,a linear downward regulation of the combustor pressure for variable-condition adjustment can be achieved with stable combustion flow field and slight combustor pressure oscillation.Because the combustion efficiency is calculated on a basis of the real-time injected mass flow rate of all the propellants,the delay phenomenon resulted from the process of LOX evaporation causes the combustion efficiency to always be too large.The transverse velocity disturbance can effectively excite the first-order transverse oscillation mode acoustic response in the combustor with the disturbance frequency.The LOX evaporation and GOX/GCH₄ mixing reaction is enhanced with more uniform combustor temperature,and there is a tendency for diffusion flame to transform into premixed flame.With the increase of the disturbance amplitude at the same disturbance frequency,the growth rate of the phase difference between combustor pressure oscillation and heat release of reaction oscillation is slow first and then fast.The phase difference is close to 0°when the disturbance amplitude is about 60m/s,and the most violent oscillation of the combustor pressure appears.The phase difference under the same disturbance amplitude is determined by the relative size between the disturbance frequency and the natural frequency of the first-order transverse oscillation mode.The closer the disturbance frequency and the natural frequency are,the more intense the combustor pressure oscillation is.