| The propellant conveying stability and mass flow rate control accuracy of powder propellant feeding system,which is a key component of powder engine,have a direct effect on combustion performance of powder enginer.At high pressure,the control accuracy of powder mass flow rate and powder conveying stability are influenced by particle pickup mechanism,powder fluidization and congvering characteristics.Meanwhile,the flow stability of gas solid two phase is relevant to the combustion oscillation of powder engine,and its conveying and coupling mechanism are still unknown.Therefore,carring out the researchs on the powder propellent conveying characteristics and its coupling mechanism with engine combustion oscillations has important academic significance on revealing the flow laws of gas solid two phase at high pressure,which is also benefit to improve the design of powder propellant feeding system.By using experimental tools mainly and introducing the corresponding pressure signal analysis methods,the powder conveying characteristics at high pressure and the coupling mechanisms between conveying stability and combustion oscillation were studied,and the main research work and conclusions are as follows:(1)Based on the the particle pickup velocity,the formular of incipient mass flow rate of powder propellant at high pressure is deduced,and an semi-empirical prediction model of particle incipient mass flow rate is also built too.The result shows that the particle incipient mass flow rate is in an approximate logarithmic relationship with initial pressure,while shows a "S" type growth rule with the increasing of powder loading rate;the particle incipient mass flow rate increases at first and then decreases with the increase of particle size,and increases with the increase of throttling orifice area.The calculation results of above semi-empirical model agrees well with the experimental values,and the prediction accuracy has a substantial increase compared with the theoretical model which is based on the gas solid two phase equilibrium flow.The semi-emirical prediction can provide more accurate propellent mass flow rate for ignition energy calculation of powder engine(2)The particle pickup and fluidization characteristics at high pressure were researched by using experimental and numerical methods.The result shows that particle pickup process is consists of three stages,namely,static-picking up stage,a large amount of particles output stage and a small amount of particles output stage.Particle size and powder loading rate have a large effect on the granular temperature distribution in throttling orifice plate and particle velocity flow field,and the "core-annulus" flow structure can easily form under a certain condition.Under the condition of low pressure,the bubblings and vortexes are mainly concentrating on the upper of powder box,and with a state of powder fluctuation.But there will forming a stable inclined molded surface for powder at high pressure.The powder fluidization performance increases first and then decreases with the increase of pressure,and reaches unanimity after a certain pressure.Larger and smaller particle sizes have an comparable effect on the fluidization performance,and are both better than the middle size particles.In addition,compared to the mean square error analysis of pressure signal,high order statistics analysis is more exquisite.While the wavelet transform analysis results show that the effective signal spectrum of powder fluidization at high pressure mainly focuses on 0-40 Hz,and the high frequency signal mainly represents the gas phase turbulent fluctuation,while low frequency signal represents gas-solid interaction and powder movement.(3)The gas-solid two phase choking conveying characteristics were studied through experiments.The results show that two phase velocity in pipeline nozzle can be increased,and the powder can from a spraying surface with certain angle,with the increase of initial pressure and nozzle diameter,the spraying angle increases.The conveying stability of nozzle downstream gas solid flow is better than the upstream at low pressure,and vice at high pressure.Meanwhile,the dimensionless number of gas solid conveying stability at high pressure does not change significantly with pressure,while the particle size increasing will enhance the stability of gas-solid conveying accordingly.In gas-solid choking state,the fluidization performance mainly has a gas turbulence effect on upstream flow of nozzle,and the influence degree drops first,then jumps,and drops again with the fluidization pressure increases.Large size particles can enhance gas-solid interaction,while middle size particles decrease gas turbulence effect;the relevance between fluidized chamber and nozzle downstream flow increases with particle size increases;the coupling strength between nozzle upstream and downstream increases first and then decreases with the increase of flow pressure.Meanwhile,big size particles will weaken the influence of the upstream to downstream,while the middle size particles enhance the flow correlations between upstream and downstream.At the same time,a judement for gas-solid choking flow is built basing on the coupling characteristcis of upstream and downstream flow.(4)In accordance with the formular of incrompressible fluid mass flow rate,a semi-empirical model of particle mass flow rate under the condition of gas-solid choking was built,which is proposed through fitting the solid phase apparent sonic velocity formular by considering particle density,heat conductivity coefficient and mass ratio of solid-gas.Of course,the equation of gas solid mixture density is corrected by numerical results.The above semi-empirical modal has a better prediction accurancy than the gas-solid two phase equilibrium flow model.(5)The influence of powder propellant conveying on combustion oscillation of powder engine under choking and non-choking states were studied through methods of mean square error,the wavelet change and coherence function analysis.The results show that when Al/AP powder engine conducts multiple pulse ignition tests under the gas-solid non-choking condition,the correlation between powder conveying and low frequency combustion oscillation is strong,and thermal decomposition characteristics of AP powder has a large effect on low frequency combustion oscillation;When powder engine conducts thrust regulation tests,the most relevant moment between powder conveying and combustion oscillation occurs in the transposition stage,and the combustion stability is the worst at this time;adding the turbulence ring to combustor will enhance the low frequency coupling relationship between fluidization conveying and combustion oscillation at low pressure and high pressure,while it has the strongest coupling correlation in all frequency domain during the transposition stage.Under gas-solid choking condition,there is almost no high frequency or low frequency coupling between powder fluidization conveying and the combustion pressure oscillation,which provides a good way for powder feeding system design. |
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