In the tryout of liquid-propellant rocket engines (LPREs), the parameters that govern working processes in the LPRE systems (the pressure, the flow velocity, the gas and liquid temperature, the turbopump speed, etc.) exhibit low-and high-frequency oscillations. High-frequency oscillations in a combustion chamber, which are potentially dangerous to the LPR operational reliability and integrity, are the least understood. The most important tool in the study and development of measures aimed at their elimination in the flight of liquidpropellant launch vehicles is a mathematical simulation of high-frequency processes in a combustion chamber.This paper overviews recent publications and analyzes the state of the art in the numerical study of highfrequency dynamic processes in LPRE combustion chambers with the aim to assess the possibility of using the available numerical methods to simulate the above-mentioned processes in the problem of theoretical prediction of LPRE high-frequency stability and the combustion chamber pressure and flow rate oscillation amplitudes. Consideration is given to the currently adopted mechanisms of high-amplitude oscillations in the LPRE systems involving the dynamic interaction of physical and chemical processes in the mixing and combustion zone in conditions of periodical heat removal under the action of acoustic oscillations and turbulence in the flow and combustion of the propellant components and combustion products.The analysis conducted shows that the methods of mathematical simulation of high-frequency acoustic oscillations in an LPRE can be divided into three basic groups: methods for the calculation of the acoustic oscillation parameters in cylindrical chambers based on analytical mathematical models of a relatively low order with the use of the Bessel functions, methods for the study of thermoacoustic phenomena using approaches of computational fluid dynamics, and hybrid methods, in which combustion dynamics is calculated separately from the combustion product acoustic oscillation parameters. The main results obtained in the framework of the abovementioned groups are overviewed. The advantages and drawbacks of the numerical study of combustion product thermoacoustic oscillations in LPRE chambers are analyzed.
Space propulsion systems ensure multiple startups and shutdowns of the main liquid-propellant rocket engines in microgravity conditions for spacecraft preset motions and reorientation control. During the passive flight of a space stage (after its main engine shutdown), the liquid propellant in the tanks continues moving by inertia in microgravity and moves as far away from the propellant management device as possible. In this case, the pressurization gas is displaced to the propellant management device, which creates the potential danger of the gas entering the engine inlet in quantities unacceptable for multiple reliable engine restarts. In this regard, the determination of the parameters of fluid movement in propellant tanks under microgravity conditions is a pertinent problem to be solved in the designing of liquid-propellant propulsion systems. This paper presents an approach to the theoretical calculation of the parameters of motion of the gas–liquid system in the propellant tanks of today’s space stages in microgravity conditions. The approach is based on the use of the finite element method, the Volume of Fluid method, and up-to-date computer tools for finite-element analysis (Computer Aided Engineering - CAE systems). A mathematical simulation of the spatial motion of the liquid propellant and the formation of free gas inclusions in passive flight was performed, and the motion parameters and shape of the free liquid surface in the tank and the location of gas inclusions were determined. The liquid motion in a model spherical tank in microgravity conditions was simulated numerically with and without account for the hot zone near the tank head. The motion parameters of the gas-liquid interface in a model cylindrical tank found using the proposed approach are in satisfactory agreement with experimental data. The proposed approach will significantly reduce the extent of experimental testing of space stages under development.
The space propulsion systems ensure se veral start-ups and shutdowns of main liquid-propellant rocket engines under microgravity conditions for the spacecraft program movements and reorientation control. During the passive flight of the space stage (after its main engine shutdown), the liquid propellant in the tanks continues to move by inertia in microgravity away from the propellant management device as much as possible. In this case, the pressurization gas is displaced to the propellant management device, which creates the potential danger of gas entering the engine inlet in quantities unacceptable for the reliable engine restart. In this regard, determining the parameters of fluid movement in propellant tanks in microgravity conditions is an urgent problem that needs to be solved in the design period of liquid propulsion systems. We have developed an approach to the theoretical computation of the parameters of the motion of the ‘gas — fluid’ system in the propellant tanks of modern space stages in microgravity conditions. The approach is based on the use of the finite element method, the Volume of Fluid method and modern computer tools for finite-element analysis (Computer Aided Engineering — CAE systems). For the passive leg of the launch vehicle space flight, we performed mathematical modeling of the spatial movement of liquid propellant and forming free gas inclusions and determined the parameters of movement and shape of the free surface of the liquid in the tank as well as the location of gas inclusions. The numerical simulation of the fluid movement in an experimental sample of a spherical shape tank was performed with regard to the movement conditions in the SE Yuzhnoye Design Bureau ‘Drop tower’ for studying space object s in microgravity. The motion parameters of the ‘gas — fluid’ interface obtained as a result of mathematical modeling are in satisfactory agreement with the experimental data obtained. The use of the developed approach will significantly reduce the amount of experimental testing of the designed space stages.
A mathematical model describing the nonlinear dynamical interaction of a launch vehicle (LV) and its marching liquid-propellant engine in the active phase of the LV flight is developed on the basis of the finiteelement discretization of the "propulsion-LV structure" self-oscillating system using three-dimensional and onedimensional finite elements. An approach to the computation of the liquid-propellant launch vehicle selfoscillation parameters self-oscillations of the liquid launch vehicle under POGO instability is developed. In the proposed approach, the rocket structure is considered as a complex multiply connected dissipative system "LV structure-liquid propellant in tanks" and is schematized by three-dimensional finite elements, which allows investigating the spatial vibrations of the LV structure and the liquid propellant in the tanks. Modeling of the low-frequency dynamics of the rocket engine pumps is performed on the basis of the theory of cavitation selfoscillations in pumping systems developed at the Institute of Technical Mechanics of the National Academy of Sciences of Ukraine and the State Space Agency of Ukraine (ITM of NASU and SSAU). The most significant nonlinearities in the numerical solution of the non-linear problem of liquid-propellant rocket POGO oscillations, namely, the nonlinearity of the dependence of the cavitation volume and the cavitation time constant on the pump operational parameters and the nonlinearity of the dependence of the LV structure oscillation decrements on the LV structure vibration amplitudes, were taken into account in the model of the system low-frequency dynamics. Numerical modeling of POGO self-oscillations of a two-staged LV with a total mass of 165 tons and with a mass of 130 tons of the propellant in the first stage tanks is carried out. For the computation case of the resonant interaction of the LV structure and the liquid-propellant rocket engine (LPRE), the limiting cycle parameters of the dynamic "LPRE-LV structure" system are determined. It is shown that in the case of LV POGO selfoscillations the structural elements vibrate and the pressures and the flow rates in the liquid-propellant rocket engine oscillate at a frequency of 15.9 Hz, which is close to the natural frequency of the second mode of the structural longitudinal oscillations. The scientific software developed may be used in the theoretical determination of the POGO self-oscillation parameters of prospective liquid-propellant rockets (including rockets whose structure has a complex spatial configuration) with respect to elastic longitudinal and transverse oscillations of the LV structure and in assessing dynamic loads on LV structures.
The most critical operating conditions of solid rocket motors (SRMs) are often due to the development of dynamic processes characterized by excess values of operating parameters. Pressure surges and a sharp increase in the combustion product temperature may impair the strength of the combustion chamber structure, cause its failure, and lead to critical conditions of the motor operation, up to extinguishing the propellant combustion in the motor. It is shown that both in steady and in unsteady operating conditions of an SRM, dynamic processes in its combustion chamber feature a complex interrelation of a large number of processes in the gas-dynamic space of the combustion chamber: physical, chemical, and thermodynamic (heat and mass exchange) processes. It is found that current studies of SRM operation instability are aimed at identifying mechanisms of combustion chamber pressure oscillations, which are usually due to combustion product vortex formation in the chamber space and acoustic feedback resulting from collisions of vortices with the SRM’s combustion chamber components or nozzle. Other lines of investigation are the analysis of SRM resonant damping and the establishment of a relationship between aluminum droplet combustion and SRM internal instability. It is noted that accelerations and vibrations of mixed-propellant combustion surfaces may greatly affect the combustion rate and the agglomeration, on-surface confinement, and burn-up of metal additives, which, in its turn, governs the combustion chamber acoustics. It is pointed out that the interaction of SRM combustion chamber pressure oscillations and the response of the SRM structure observed in flight tests of some rockets should be taken into account in predicting the stability of SRM dynamic processes. This interaction may call into question the sufficiency of SRM static tests and subsequent conclusions on the magnitude of its dynamic effect on the rocket structure.
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