HIGH TEMPERATURE, LOW DENSITY BOUNDARY-LAYER CONTROL BY CRYOGENIC PUMPING

Abstract
Results are presented of a study of the flow in a hypersonic nozzle operated at low densities and high stagnation enthalpy levels with both natural and controlled boundary layers. The boundary-layer control was established by cryogenic pumping on the nozzle walls, using liquid hydrogen as the cryogen. A flow calibration procedure was evolved for this nozzle which included a fully frozen (in vibration) nitrogen flow model, large low density corrections to pitot and static pressure measurements, and a non-Sutherland viscosity variation. The regimes of isentropic flow in the nozzle were identified by this calibration, and flow conditions within these regimes are given. The boundary- layer control technique permitted the attainment of indicated Knudsen numbers one order of magnitude greater than those produced with no boundary-layer control. At the lowest levels of static density produced in the nozzle with boundary-layer control, evidence of an entropy-increasing process was found which was tentatively identified as a departure from rotational equilibrium in the flow.

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