Nozzle-lip effects on argon expansions into the plume backflow

The structure of the flow of argon through a 2-cm-long, 4-mm-diam tube, around the tube lip, and into vacuum has been investigated using the Direct Simulation Monte Carlo technique. Two lip geometries have been compared, and the shape and thickness of the lip were found to have a significant influence on the flow field structure and the total flux into the backflow. The effects of the temperature and scattering characteristics of the front lip surface were investigated and found to affect the backflow only for certain lip shapes. The number density and temperature of the gas were also found to have a significant effect on the backflow due to changes in the collisional processes in the expansion flow, which affects the freezing of the parallel component of the random motion of the gas. HE expansion of gases out of a rocket nozzle or out- gassing orifice into a low-density background is an impor- tant problem with importance to spacecraft contamination, heat transfer, and infrared sensor interference. The expansion of gases around the nozzle lip and into the region upstream of the nozzle exit plane, usually referred to as the backflow region, is a particularly complex gas dynamic problem. It has been recognized for many years*~6 that the expansion of the subsonic and lower Mach number supersonic regions of the boundary layer around a nozzle lip can produce significant flux into the high-angle backflow region not predicted by the conventional Prandtl-Meyer uniform supersonic flow analysis. It has recently been recognized that nonequilibrium effects,7 due to the rapid rarefaction of the flow out of a nozzle exiting to vacuum or near vacuum, can affect the structure of the flow around a nozzle lip. Consequently, accurate modeling of the flowfield around a nozzle lip and into the high-angle backflow region cannot be obtained using standard equilibrium gas dynamic models. The Direct Simulation Monte Carlo (DSMC) modeling technique,8 which models a gas flow by following some representative number of molecules through simulated collisions, does account for nonequilibrium effects, and is essentially the only technique presently available for accurate prediction of these flows. The shape of the nozzle lip may play an important role in determining the flux into the backflow. Pipes et al. 9 observed some effects of nozzle-lip shape on the flowfield at the exit plane for carbon dioxide and nitrogen expansions from small nozzles, with a pronounced dependence of the effect on the level of condensation in the flow. Direct evidence of lip shape effects on flow into the backflow region was observed by Chirivella10 for pure nitrogen flows. Hueser et al. 11 used the DSMC technique to study the detailed flowfield around the nozzle lip for conditions simulating the inertial upper stage IUS motor at 282-km altitude. They found large differences between the flowfield predicted by the DSMC technique and that predicted by the equilibrium Method of Characteristi cs technique. They modeled a nozzle lip with finite thickness, but investigated only one lip shape and one lip thickness for one set of initial flow conditions. The results of a detailed DSMC theoretical analysis of the effects of lip shape and lip thickness on the flowfield around the lip of a nozzle with zero half angle (tube) will be presented in this paper. In addition, the effects of upstream conditions (gas density and temperature), front lip surface temperature, and front lip surface accommodation coefficient on the backflow structure have been investigated and will be de- scribed. As will be shown, the degree of translation nonequi- librium in the gas expansion, which is dependent on the gas density and temperature, can significantly affect the structure and degree of backflow. In addition, translation nonequi- librium processes interact with the different lip shapes and thicknesses in a nonsimple manner.