The Unsteady Atomization Mechanism And Combustion Characteristics Of Gas-Liquid Swirl Coaxial Injector

Liquid-centered swirl coaxial injector is widely used in nontoxic cryogenic bipropellant liquid rocket engine.However,understanding on the atomization mechanism and combustion characteristics of this injector are still needed.Self-pulsation could occurs in liquid-centered swirl coaxial injector,and it may excites combustion instability.In the present study,the atomization mechanism,spray characteristics,self-pulsation characteristics,self-pulsation mechanism and combustion characteristics of liquid-centered swirl coaxial injector are investigated theoretically and experimentally.First,linear instability of conical liquid film with coaxial gas flow were conducted.The dispersion relation of conical liquid film with coaxial gas flow was derived based on the spray model.In the spray model,the spray cone angle,film thinning and coaxial gas flow were considered.A prediction model of the spray characteristics of pressure swirl injector was proposed by combining the inviscid theory and empirical film thickness relation.Effects of pressure drop,back pressure,orifice length,injector diameter and coaxial gas flow on the growth of surface wave were investigated.The conical film can be either dominated by sinuous surface wave or both sinuous and varicose surface waves depending on the operation condition of injector.The wave velocity of sinuous surface wave is slightly smaller than film velocity while that of varicose surface wave is exactly equal to film velocity.The back pressure improves surface wave while the orifice length suppresses it.Surface wave is mainly related with the relative velocity between gas and liquid flow.Second,self-pulsation characteristics and self-pulsation mechanism were investigated.The effects of recess length,gas core diameter and operation condition on the inner flow pattern,spray pattern,self-pulsation frequency and self-pulsation strength were analyzed.A self-pulsation mechanism was proposed and validated with the results of literature.The results show that the flow pattern in the recess chamber of a liquid-centered swirl coaxial injector can be classified into three types,namely,outer,critical,and inner mixing flow.Once the critical flow approaches,self-pulsation is about to occur.When the flow pattern moves away from critical mixing flow,self-pulsation is suppressed.When the flow pattern moves the opposite way,self-pulsation is intensified.The self-pulsated spray with a big gas core looks like a Christmas tree,while the self-pulsated spray with small gas core looks like a string of pearl.Self-pulsation is strongly related to Kelvin–Helmholtz instability,and Kelvin–Helmholtz-like instability should be a necessary condition.A possible mechanism is that self-pulsation occurs only when the Kelvin–Helmholtz vortex is strong enough to overcome the inertia of liquid.The critical mixing flow may strengthen the Kelvin–Helmholtz vortex,which is how the recess length and injection condition affect the self-pulsation.Third,unsteady spray characteristics of liquid-centered swirl coaxial injector were investigated.Steady spray characteristics of pressure swirl injector and liquid-centered swirl coaxial injector were investigated.Then effects of self-pulsation and recess length on the spray characteristics were investigated.The results show that from the point of spatial distribution,self-pulsation is good for the spray,it uniformizes the mass flux along radius and increases the spray angle.However,when self-pulsation occurs,the SMD increases at all the measuring points.Namely,from the point of atomization performance,self-pulsation has negative effects.When the gas to liquid ratio is small,the recess length effects the atomization of liquid-centered swirl coaxial injector by exciting self-pulsation.The global sauter mean diameter(GSMD)increases with the increase of recess length.When the gas to liquid ratio is large enough,self-pulsation is suppressed.Recess length has little effect on the atomization.Finally,the effects of recess on the combustion characteristics of liquid-centered swirl coaxial injector were also investigated.Experiments of gas generator with and without recess were conducted within a large range of operation condition.Effects of recess on the combustion performance and combustion stability were analyzed.The results indicate that the combustion varies from stable to low frequency instability with the increase of gas to liquid mass ratio.The flame cannot stabilized at the tip of pressure swirl injector,and a lifted triple flame occurs at some distance away from the injector tip.When the triple flame oscillates in the recirculation zone,low frequency combustion instability occurs.Although recess can improve the combustion performance,it worsen the combustion stability,because it excites medium frequency combustion instability.The medium frequency combustion instability can occurs in a larger range of operation condition than the low frequency combustion instability.Low frequency combustion instability is not always caused by the coupling of supply system and combustion process,combustion itself can also excites low frequency combustion instability.When the mass flow rate of gas is large enough,the oscillation in the supply system with medium frequency can also affects combustion and produces medium frequency combustion instability.