Rarefied flow expansion in linear aerospikes

Abstract

The formation of the kinetic boundary layer (KBL) on diverging surfaces is studied experimentally and computationally. The diverging surfaces are chosen to follow profiles commonly used for linear aerospikes, in order to study the KBL formation in an ideal and two-dimensional flow expansion bounded only on one side. Experiments with different operating conditions at low Re number (from 132 to 2826) and in the transition regime (Kn number from 6.4 to 0.42) are used to evaluate the thickness of the KBL. The ambient pressure, the ratio between the stagnation and surface temperature, the roughness and the shape of the surface are parametrically varied maintaining the same pressure ratio between the total and the ambient pressures. Simulations are validated and used to study the influence of gas-surface interaction on formation of the KBL and to quantify the non-equilibrium state of the flow field. It is shown that the topography of the surface does not influence the growth of the KBL, but the flow can be tailored by modifying the shape of the surface. Using the experimental and computational data, a phenomenological model is developed to predict growth of the KBL. The thickness of the KBL is proportional to the length along the surface and to the mean free path. Furthermore, the ratio between the mean free path on the surface and the thickness of the KBL is found to be an invariant for the diverging surfaces modified through linear corrections. In the aerospike system, the KBL reduces the thrust by 35% and the proposed corrected geometry increases the real thrust by 20%.