Hot-wire Anemometry in Low-Density Flows

Abstract

The present paper deals with the use of hot-wire anemometry in obtaining quantitative flow data in the low-density, high-temperature regime. In conventional usage, high fluid temperatures result in decreased sensitivity due to structural limitations of the hot-wire probe. For intermittent testing, however, the wire supports remain at a low temperature, allowing the wire to be operated below the recovery temperature with high sensitivity. Since it is generally quite difficult to obtain a probe in a low-density tunnel over a wide range of flow conditions, the wire heat balance is formulated, and solutions to the resulting boundary-value problem are obtained using numerical techniques. The coefficients in the equations (e.g., thermal conductivity, emissivity, accommodation coefficient, etc.) are all determined experimentally by static calibration. This permits a dynamic calibration curve to be generated outside of the wind tunnel for each probe. Nomenclature D = wire diameter E = output voltage I = current, amps k = thermal conductivity L ••= wire length M ••= Mach number p — static pressure Qe = Joule heating rate, Btu/sec q = heat-transfer rate; Btu/ft2-sec Rw = wire resistance, ft R = average wire resistance, ft T = temperature T --= average wire temperature TE := support temperature v = velocity x --= coordinate along wire a. — accommodation coefficient e = emissivity X = mean free path 77 = coordinate normal to flat plate p = density £ = coordinate along flat plate from leading edge Subscripts