The Dynamics Of Spray Combustion In Variable Thrust Rocket Engines

The spray combustion in liquid rocket engines is an highly nonlinear dynamic process,the throttling in a variable thrust rocket engine makes it more complicated.However,comprehensive understanding on the dynamics is not enough.Moreover,the knowledge on the dynamics during throttling is rather rare.In the present study,the spray combustion dynamics of dual-manifold swirl injectors and pintle injectors for variable thrust rocket engine under fixed conditions and throttling conditions were investigated by using methods combined theory,numerical simulation and experiments.The responses of air core,spray pattern and size of droplets under fixed conditions,linear throttling conditions and oscillating conditions were obtained experimentally and numerically.The potential influences of the spray dynamics on the combustion were also discussed.The results indicates that the main motion of inner flow is the scaling of the air core.The variation of mass flow rate propagates inside the nozzle in long-wave manner.Therefore,the variations of film thickness and velocity at different axial positions are almost in-phase.As a result,thick and fast film is periodically emanated from the injector,and worsens the atomization.Under small throttling rate,the spray pattern and size of droplets could be treated as quasi-steady.The delay becomes important when the throttling rate getting bigger.Besides,the rapid variation of atomization during the beginning of throttling at larger throttling rate leads to a spatiotemporally non-uniform atomization field,which may trigger nonlinear phenomena such as anti-regulation and combustion instabilities.The response delay and anti-regulation were observed under throttling process.Characteristics of the two phenomena were summarized and the mechanisms were revealed.The combustion dynamics of the dual-manifold swirl injector were studied experimentally.Deep throttling capacity of the dual-manifold swirl injector was demonstrated.For oxygen adjusting,the response delay mainly results from the delay in the supply system because of the compressibility of gaseous oxygen as well as the cavity between the flow controller and the injector.As for fuel adjusting,the response delay results from a delay in combustion process,mainly due to the slow evaporation process.The anti-regulation mainly results from the interval between large amounts of heat absorption and large amounts of vapors released during the evaporation process of droplets.There is a competition between fuel adjusting and oxygen adjusting in affecting the anti-regulation.Fuel adjusting contributes to the anti-regulation,whereas oxygen adjusting suppresses the anti-regulation.In addition,the effects on the anti-regulation are positively related with the adjusting rate.Spatial distribution models of pintle injectors with different structures were built.The flow characteristics and breakup modes of the propellants were analyzed.Meanwhile,the spatial distribution of droplet size were obtained.The theoretical models predict the spray angle,spatial distribution of film and droplet size well.The spray angle and spatial distribution are dominated by momentum ratio,block factor,jet diameter/radial film thickness and axial film thickness.Axial flow blocks the radial flow,so nominal discharge coefficient of the orifice increases when momentum ratio increases.The pintle injector was simplified into a pintle injector element.The experiments shown that the pintle injector element forms a mantle-like spray.Then based on theory,numerical simulation and experiments,the spray of pintle injector was identified as a partially hollow petal-like cone.Under different operating conditions,the spray pattern can be classified as the close arch spray,the open sector sheet and fully developed sector spray.Similarly,the breakup of the spray can be classified as dripping-like breakup,perforated sheet breakup,impact breakup,turbulent jet dominated breakup and turbulent film breakup.The radial distributions of mean diameter of droplets are self-similar at different axial position.When local momentum ratios increases,the jet-influenced core region moves outwards,the mean diameter of droplet downstream the fan sheet decreases and mean diameter increases in the periphery of the spray.Through optical measurements,the distribution of spray and flame inside the combustion chamber of a pintle injector model engine were obtained.The effects of operating conditions on the spray and flame were revealed.In addition,the mechanism of self-pulsation of spray and flame was explored.Under fixed conditions,the spray mainly located in fan areas at the back side of the pintle tip.Combustion occurs mainly at three areas: the central combustion zone just downstream the pintle tip,the head combustion zone on the either side of the pintle and the zonary combustion zone located at the windward side of the spray.The head combustion zone supplies the heat for evaporation of fuel and is important for flame stabilization.The spray and flame are mainly affected by the momentum ratio.As momentum ratio increases,the spray is less deflected.As a result,the head combustion zone is compressed and becomes smaller.Meanwhile,the flame in the central combustion zone transfers from ‘single flame' to ‘double flame' when momentum ration increases.Under condition of low mixture ratio,the spray combustion may suffer from self-pulsation due to coupling of spray breakup,heat release and acoustic.