Study On Unsteady Characteristics And Mechanisms Of Cryogenic Cavitation

Cavitation is the basic problem that causes the failure of cryogenic pumps in rocket engines.Deep understanding of the unsteady cavitation characteristics and mechanism of cryogenic fluids will contribute to the optimization of the performance of cryogenic pump systems for liquid rocket engines.Since the cryogenic cavitation has more significant thermal effects than water cavitation,the physical mechanisms are more complicated.At the same time,visual observation of the unsteady cavitation phenomenon is extremely difficult due to cryogenic temperature.Therefore,so far,the experimental data are scarce and the understanding of the unsteady characteristics and mechanisms of cryogenic cavitation is still insufficient.For these reasons,this article mainly carried out the following worrk:1、Based on the H-T bubble dynamics model,a cryogenic single-bubble dynamic model including compressibility,thermal effect,surface tension,and non-condensable gas effect was solved.Effects of pressure difference on bubble growth and collapse processes were analyzed.The cryogenic bubble dynamic model was validated by experimental data of water ultrasonic cavitation.Studies have shown that for the bubble growth process,the thermal effect prolongs the growth time,the bubble wall velocity firstly increases and then decreases,the stronger the thermal effect,the earlier the maximum bubble wall velocity is reached,and the smaller the value is;With larger initial pressure difference,the bubble wall velocity and growth rate will be bigger.For the bubble collapse process,the thermal effect suppresses the bubble oscillation and the maximum collapse pressure amplitude and prolongs the collapse time;The stronger the thermal effect,the earlier the maximum bubble wall velocity is reached and the smaller the value is.With stronger thermal effect,the collapse time is more sensitive with the initial pressure difference,and they have exponential relationship.2、Based on the homogeneous mixture model,a numerical simulation framework for the unsteady cryogenic cavitation was established considering the thermal effects and compressibility of gas and liquid.The coupling mechanism of thermal effects,vorticity and cryogenic cavitation was studied in depth.Based on the experimental data of Hord,the bubble number density of the Sauer-Schnerr cavitation model was modified to 108.The gas-liquid compressibility was coupled in the pressure-velocity coupling equation and the large eddy model(LES)was used to establish compressible numerical simulation framework for unsteady cryogenic cavitation.Simulations of ogive cavitation revealed that the shedding process in the cavitation zone can be divided into two modes:partially shedding mode(PSM)and fully shedding mode(FSM).In the PSM,vorticity transmission is mainly concentrated in the near wall region.The vortex size is suppressed by the thermal effect,forming small cavitation clouds and shedding from the primary cavity at a frequency of 2500 Hz.The size of primary cavity is slowly reduced.If thermal effect isn’t considered in simulation,vortex transmission is concentrated in the vapor-liquid interface and the tail of the cavitation zone.And the size of the vortex is close to the entire cavitation zone,therefore the cavitation zone will periodically and completely shed off from the wall.When the primary cavity in the PSM is reduced to the size of the vortex,the entire cavitation zone will be shed off by the vortex,then the FSM forms.3、Three-dimensional numerical simulation of unsteady liquid hydrogen cavitation on the surface of NACA0015 was performed.The frequency and pressure fluctuation characteristics and mechanism in the cavitation process were revealed.When σ/2a is greater than about 2.0,the number of St in liquid hydrogen cavitation is much smaller than that in water cavitation with no thermal effect at the same σ/α value.The thermal effect in liquid hydrogen cavitation has a strong inhibitory effect on the formation of large cavitation clouds,resulting in the generation of small cavitation clouds,the shedding mechanism of cavitation area is mainly affected by the combination of vortex and thermal effects.When σ/2α is reduced to about 2.0,due to the increase of vortex strength,the relative suppression effect of thermal effect on the vortex is reduced,the flow instability is dominated by the large vortex cavitation cloud like in the traditional fluid,and the St number is returned to the same level in water cavitation.4、Two sets of visualized experimental devices for liquid nitrogen cavitation in Venturi tubes were successfully constructed.The length and shedding frequency characteristics of the cavitation zone were analyzed,and the unsteady mechanism was analyzed from the perspective of pressure waves;A device for liquid nitrogen hydrofoil cavitation visualization experiments was successfully established.The device measures the pressure distribution on the surface of the hydrofoil,analyzes the length and temperature characteristics of the hydrofoil attached to the hydrofoil,and conducts a preliminary study on the gap vortex cavitation on the side of the hydrofoil.For venturi cavitation,the length of the cavitation zone is inversely proportional to the pressure ratio Pr,and there is a critical pressure ratio(Prc).When Pr<Prc the linear growth rate of the cavitation zone is larger.Numerical simulations and experimental studies show that Prc,becomes larger as the thermal effect intensity increases.The cavitation instabilities were analyzed using two Strouhal numbers Stc and Std based on the cavity length and the throat diameter of the venture respectively.For cloud cavitation,Stc is between 0.30-0.40,while for sheet cavitation,it is reduced to 0.04-0.08.In addition,in the cloud cavitation zone,Std increases linearly with increasing Pr and as the thermal effect intensity increases,the transition point from sheet cavitation to cloud cavitation is delayed and the corresponding transition pressure Prc becomes smaller.In addition,the pressure waves generated by the cavitation cloud collapse propagate in the cavitation zone to form the condensation front and dominate the detachment of the cloud cavitation zone.The new Strouhal number based on the propagation velocity of the condensing front is around 0.5.Wallis sound velocity can predict the propagation speed of the condensing front.For liquid nitrogen cavitation over NACA66 hydrofoil,the cavitation zone gradually develops downstream as the cavitation number decreases,but the overall cavitation zone does not exhibit periodic shedding,while for water cavitation,a periodic shedding phenomenon occurs once the length of the zone exceeds 50%of the chord length.With the decrease of cavitation number,the temperature drop in the cavitation zone becomes larger,and in the lower cavitation number area,the temperature drop amplitude is larger.The length of the gap vortex cavitation firstly becomes shorter and then longer as the cavitation number decreases,while the width becomes larger as the cavitation number decreases.