Flat Plate Boundary-Layer Studies in a Partially Ionized Gas

Abstract

A combined experimental and numerical investigation was conducted on the flat plate boundary-layer flow of a partially ionized gas. Free molecule cylindrical Langmuir probes were used to measure charged particle density, electron temperature and plasma potential distributions in the boundary layer. The ambipolar diffusion flux to the plate surface was determined by an array of flush-mounted surface electrodes. A probe size-study was carried out and an empirical formula was obtained to make correction for the sheath-fringing field effect on ion current collection to small flush probes. Numerical integrations were made of the charged species conservation and electron energy equations in the quasi-neutral region of the boundary layer for a thin plasma sheath. Mechanisms leading to the cooling of the electrons were investigated. Numerical profiles of charged particle density, electron temperature and plasma potential compared favorably with experimental results. Ambipolar diffusion fluxes predicted theoretically agreed very well with the measured flush probe ion saturation current. The validity of employing such a flush-mounted surface electrode in determination of the free stream charged particle density was then established.