The Self-Pulsation And Its Effects On Spray Combustion Characteristics For Gas-Liquid Swirl Coaxial Injector

Gas-liquid swirl coaxial(GLSC)injector is widely used in nontoxic cryogenic bipropellant liquid rocket engine.However,GLSC injectors are known to experience self-pulsation easily under certain operating conditions with some special geometrical parameters.Self-pulsation may amplify combustion instability when pressure oscillation caused by self-pulsation and by combustion instability matches with each other.Up to now,self-pulsation characteristics and mechanisms have not been investigated clearly.Researches of the effects of self-pulsation on combustion are still insufficient.In the present study,the self-pulsation process,self-pulsation mechanism,the effects of critical operating conditions and geometrical parameters together with the combustion behavior for the liquid-centered converged and opened swirl coaxial injectors are investigated experimentally,numerically as wel as theoretically.First,the effects of critical geometrical parameters on self-pulsation were investigated.Experimental investigations were conducted based on the two kinds of standard injectors.The oscillation start process of self-pulsation under different injection conditions was analyzed.The effects of gas-liquid injection conditions and backpressure on self-pulsation pattern,amplitude-frequency characteristics and self-pulsation boundary were investigated.When the gas-liquid ratio is small,severe self-pulsation occurs at the beginning of injection for the steady atomization.Self-pulsation frequency is largely affected by the liquid injection,and is approximately linearly proportional to the liquid Reynolds number.Whether self-pulsation occurs or not and self-pulsation intensity essentially depend on the relative magnitude of liquid film angle and recess angle.Self-pulsation is the strongest when liquid film angle is around the recess angle.Backpressure does not only suppress self-pulsation.When the internal flow is in the the outer mixing behavior under ordinary pressure,self-pulsation is strengthened initially and then suppressed with increasing the backpressure.Second,the effects of critical geometrical parameters on self-pulsation were studied.The effects of recess length on the amplitude-frequency characteristics and boundary of self-pulsation for the the two kinds of injectors were compared and analyzed.The complete self-pulsation boundaries of different injectors were obtained for the first time.Aiming at the liquid-centered converged swirl coaxial injector,the effects of annulus width and post thickness on spray pattern,self-pulsation characteristics,internal flow dynamics and atomization qualities were explored.Results showed that annulus width and post thickness of inner injector together with the recess length are the most critical geometrical parameters deciding whether self-pulsation occurs or not and self-pulsation amplitude.The self-pulsation boundary is narrower and self-pulsation is weaker for the injectors without recess.For the injectors with larger recess length,the operating condition ranges with self-plsation is larger.Furthermore,with the increase of gas injection,there is a break in the self-pulsation boundry.Increasing the annulus width and post thickness leads to an accompanying decay of the pressure fluctuation in recess chamber,and then self-pulsation is supressed.The mass flow rate distribution of self-pulsated spray is double-peaked,while that of stable spray is single-peaked.The atomization qualities of self-pulsation spray are more susceptible to the injection conditions rather than the geometrical parameters.Subsequently,self-pulsation mechanisms of the GLSC injector were investigated in depth.Based on the experimental and numerical methods,the internal flow dynamics in recess chamber during self-pulsation were obtained,then the transfer mechanisms of the internal disturbance were analyzed.Meanwhile,a theoretical analysis model has been established to analyze the flow pattern by calculating the liquid film angle in recess chamber.The self-pulsation mechanisms were proposed ultimately.Accrding to the different mechanisms,self-pulsation can be classified into two types,including self-pulsation because of the periodic blocking action of the conical liquid sheet,and self-pulsation because of the periodic squeezing action of annular gas to the conicl liquid sheet.The former is caused by the periodic pressure oscillation in recess chamber because of the periodic blocking action of the conical liquid sheet.While the latter is attributed to the periodic squeezing of annular gas to the conicl liquid sheet,corresponding to self-pulsation occurring in the injectors with larger recess length after the break.Besides,when self-pulsation intensify is larger enough,pressure of the liquid manifold oscillates as well.Finally,visual combustion experiments were conducted to explore the effects of self-pulsation on combustion process.The side-on images of spray and flame were captured synchronously through the non-contact optical observation methods.Effects of recess length and injection conditions on flame oscillation were studied.The relationship among self-pulation,flame oscillation and pressure oscillation of combustor and LET-manifold were analyzed.The combustion process of the GLSC injector can be divided into three types,including steady combustion,combustion with spray and flame oscillation as well as unsteady combustion.Stable flame mainly distributes at the surface of the conical spray,the recirculation zone at the backward facing step of the injection faceplate and the impacting region of spray and combustor wall surface.The self-pulsated flame distributes azimuthally with almost an axisymmetric pattern and oscillates with periodic intensity variations in flame emission.For the injector with small recess length,flame attaches to the injection faceplate and obviously oscillates radially.However,flame transforms to longitudinal oscillation and detaches from the faceplate periodically when the recess length is large enough.When self-pulsation occurs,flame together with the pressure of the LET manifold oscillates at the same frequency.Besides,self-pulsation of flame and spray are found to be the strongest simultaneously when flow is around the critical mixing flow.Furthermore,when the self-pulsation frequency is coupled with the first-longitudinal-mode(1L)natural acoustic frequency of combustor,combustor pressure will be induced to oscillate at the same frequency.