Coupled Lagrangian impingement spray model for doublet impinging injectors under liquid rocket engine operating conditions

To predict the effect of the liquid rocket engine combustion chamber conditions on the impingement spray, the conventional uncoupled spray model for impinging injectors is extended by considering the coupling of the jet impingement process and the ambient gas field. The new coupled model consists of the plain-orifice sub-model, the jet-jet impingement sub-model and the droplet collision sub-model. The parameters of the child droplet are determined with the jet-jet impingement sub-model using correlations about the liquid jet parameters and the chamber conditions.The overall model is benchmarked under various impingement angles, jet momentum and offcenter ratios. Agreement with the published experimental data validates the ability of the model to predict the key spray characteristics, such as the mass flux and mixture ratio distributions in quiescent air. Besides, impinging sprays under changing ambient pressure and non-uniform gas flow are investigated to explore the effect of liquid rocket engine chamber conditions. First, a transient impingement spray during engine start-up phase is simulated with prescribed pressure profile. The minimum average droplet diameter is achieved when the orifices work in cavitation state, and is about 30% smaller than the steady single phase state. Second, the effect of non-uniform gas flow produces off-center impingement and the rotated spray fan by 38°. The proposed model suggests more reasonable impingement spray characteristics than the uncoupled one and can be used as the first step in the complex simulation of coupling impingement spray and combustion in liquid rocket engines.

Periodic atomization characteristics of an impinging jet injector element modulated by Klystron effect

An experimental study on the Klystron effect of periodic injection modulated by pressure drop fluctuations on subsequent atomization is conducted. Time-resolved atomization backlit images and atomization Mie scatter images are captured by using the high speed camera. It is found that periodicity of forced atomization relies on pressure drop fluctuation amplitude and phase differences between atomization and pressure drop fluctuations relate to fluctuation frequencies. This feature of periodic atomization induced by Klystron effect corresponds to periodicities and high amplitudes of pressure fluctuations in unstable combustion chambers and chaos and low amplitudes of pressure fluctuations in stable combustions chambers. Drastically periodic varying of gross surface area of droplets with time was shown in Mie scatter images. The importance of periodic impinging jet atomization modulated by pressure drop fluctuations for acoustic liquid propellant combustion instabilities is illustrated.

Experimental of combustion instability in NTO/MMH impinging combustion chambers

This paper presents an experimental study into dynamics of chamber pressure and heat release rate during self-excited spinning and standing azimuthal mode in NTO/MMH (nitrogen tetroxide/monomethylhydrazine) impinging combustion chambers.Nine cases including two combustion chamber configurations were conducted.The operating conditions of all unstable cases were located in the instability region according to Hewitt empirical correlation.The results show that chamber pressure oscillations keep pace with the corresponding OH*chemiluminescence intensity over the whole combustion region in the spinning and standing modes.It is indicated that the Rayleigh index is positive over the whole combustion area in all the unstable cases.A significant supersonic flame front structure of the first-order spinning mode was found in a cylindrical chamber,which means that a detonation wave could exist in the cylindrical chamber without a center body.The pressure and heat release rate oscillations at the pressure node are nonnegligible although their amplitudes are lower than those at the pressure antinode in the first-order standing mode with an annular chamber.Besides,the dominant frequency of pressure and heat release rate oscillations at the pressure node is twice as high as that at the pressure antinode.

Break-up Characteristics of Gelled Propellant Simulants with Various Gelling Agent Contents

Gel propulsion systems have many advantages with respect to high performance, the energy management of liquid propulsion systems, storability, high density impulse, and low leakage of liquid propellants. The atomization process provides sufficient contact surface area between the gelled fuel and oxidizer jets. It is important to study how injection characteristics of gelled propellants are related with break-up and spray distribution. The break-up and mixing processes are very important in achieving maximum efficiency and necessitate the careful study of combustion instability. Gelled propellants are non-Newtonian fluids in which the viscosity is a function of the shear rate, and they have a high dynamic shear viscosity which depends on the amount of gelling agent contents. The present study has focused on the break-up process, wave development of ligament and liquid sheets formed by impinging jets with various gelling agent contents. Especially, the break-up processes of the impinging jets at the initial conditions are studied. The break-up process of like-on-like doublet impinging jets are experimentally characterized using non-Newtonian liquids which are mixed by ionized water 98.5 wt%, Carbopol 941 0.5wt% or 1.0wt%, and NaOH(concentration 10%) 1.0wt%. For the like-on-like doublet injector, the generation of a liquid sheet at the impinging point of two jets was observed. The spray shape with elliptical pattern is distributed in a perpendicular direction to the momentum vectors of the jets. Gelled propellant simulants with high viscosity jets are more stable and produce less pronounced surface waves than low viscosity jets. Generally, the break-up length decreased due to the increasing Reynolds number. However, surface waves and atomized droplets increased. Gelled propellant simulants from like-on-like doublet impinging jets have the spray shape of closed rim patterns at low pressure. Also, the rim patterns of spray have no disturbances on the spray sheet. As the injection pressure increased, rimless patterns which were composed of ligament sheets and small droplets emerged due to the effect of the aerodynamic action. Periodic wave-like structures observed from the near impingement point and atomized droplets were observed at a location further downstream.