The Spray And Atomization Process And Its Effects On Combustion Performance Of Pintle Injector

Pintle injector has great application potential in liquid rocket engines owe to its advantages in simple structure,inherent combustion stability and deep-throttling ability.However,the understandings on the spatial distribution and spray characteristics of the spray from pintle injector are not intensive,and only limited knowledge have been obtained on the relationship between spray characteristics and combustion performance.In this study,experimental research and theoretical analysis are conducted to investigate the spray characteristics of pintle injector as well as the influence on the combustion process.Backlit photography and Phase Doppler Anemometry(PDA)are used to study the distribution characteristics of pintle injector in three indexes-spray angle,spray boundary and volume rate distribution.The spray of pintle injector is shaped as a quasiaxisymmetric cone with rough boundary,and its distribution can be characterized by a longitudinal plane across the axis of symmetry.Under specific operating conditions,there exists a “hollow region” in the upstream of the spray field.It disappears at certain distance when the spray flows downstream,generating a hollow-solid cone spray pattern.On other conditions when there is no “hollow region” in the upstream,a solid cone spray pattern is formed.Whether the hollow region exists or not and at which scale depend on the momentum ratio and total momentum.By simplifying the working process of pintle injector as the interaction between planar film and jet,a theoretical analysis has been conducted and this work can support the experimental phenomena about the spatial distribution of the spray of pintle injector.On the basis of the investigation on the distribution characteristics,the spray characteristics at two-dimensional longitudinal plane is obtained with PDA.The droplet size and droplet velocity is dominated by the change of radial position.To specify,the droplet diameter indicates a “N” trend in hollow-solid spray,while a slow drop in solid spray.Both axial and radial velocity show a increase-decrease trend along radial direction.Nevertheless,the axial velocity decrease in most region,and the increase trend only exists in the spray core.The radial velocity,however,mainly increases with in the radial direction,and goes through a slight drop before leveling off.The spatial distribution of the spray affects the spray characteristics by spray density and hollow region.From inside to outside,the spray field of the pintle injector is divided into three regions based on the volume rate distribution: spray core,mainstream and outer spray.The spray density directly influence the spray characteristics in the mainstream and outer spray.The disappearance of hollow region,which is brought by the interaction of neighboring spray element,acts on the spray core,and have effects on the value and variation tendency of SMD and axial velocity.A series of test specimen has been test to study the influence of annular gap,radial orifice diameter,radial orifice shape,layout of orifices and pintle head wall on the spray distribution.Dimensionless influence factor is used to estimate the effect results and scale.It indicates that the most influential factors are orifice diameter,annular gap and orifice shape.Furthermore,experiments are conducted to research the effects of orifice shape on the spray characteristics.It is found that when the orifice shape is changed from circular to rectangular,the atomization characteristics change little.However,when the heightto-width ratio is changed by changing the orifice number,the droplet size distributes more uniformly.Meanwhile,the impacts zone of liquid film expands both inwards and outwards,and the axial velocity in the D-V contour distributes from “single peak” to“double peaks”.Therefore,the optimization of the pintle injector structure can be achieved by choosing orifice shape.The combustion process and performance of liquid/liquid pintle engine are investigated by unilateral windowing test.Liquid oxygen and special fuel are used as propellants.Under quasi-stable condition,the combustion field can be divided into head combustion zone,spray zone,interaction combustion zone,outer combustion zone and joint combustion zone.Compared to gas/liquid pintle engine,the outer combustion zone of liquid/liquid pintle engine is longer in axial direction,and the interaction combustion zone are jointed to outer combustion zone,rather than merging in the combustion chamber center.Compared with the cold spray,the hot spray has a wider radial range,and it is concentrated near the radial orifices and pintle head wall,with no clear physical boundaries.Total momentum ratio(TMR)is the key factor of the spray and combustion characteristics of pintle injector.With the increase of TMR,the radial range of the cold spray expands,the spray angle increases,and the scale of the hollow region also increases.The augment of TMR also leads to a increase in the value and variation range of Sauter Mean Diameter(SMD).For hot spray,the optimized TMR is about 1.Either too high or two low TMR can do harm to the improve of combustion efficiency.Too low TMR can leads to ignition difficulties,while too high TMR compresses the head combustion zone and confines the development of interaction combustion to the central combustion chamber.