Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!ihnp4!zehntel!hplabs!sri-unix!Scheuer.Wbst@XEROX.ARPA From: Scheuer.Wbst@XEROX.ARPA Newsgroups: net.physics Subject: Re: Hot Wire Anemometers Message-ID: <12149@sri-arpa.UUCP> Date: Mon, 6-Aug-84 17:23:00 EDT Article-I.D.: sri-arpa.12149 Posted: Mon Aug 6 17:23:00 1984 Date-Received: Wed, 8-Aug-84 08:37:57 EDT Lines: 51 The following description of the how wire anemometer is taken from Holman, J.P., Experimental Methods for Engineers (2nd Ed.), McGraw-Hill Book Co., 1966, pp. 206-209. "The hot-wire anemometer is a device that is most often used in research applications to study varying flow conditions. A fine wire is heated electrically and placed in the flow stream. the heat transfer rate from the wire has been shown to be (ref. King, L.V.,"On the Convection of Heat from Small Cylinders in a Stream of Fluid, with Applications to Hot-wire Anemometry", Phil. Trans. Roy. Soc. London, vol. 214, no. 14, p373, 1914.) q=(a + b(rho*u)^.5)*[T(w)-T(oo)] Btu/(hr)(ft^2) where T(w) = wire temperature T(oo) = fluid temperature rho = fluid density u = fluid velocity a,b = constants that are obtained by a calibration of the device (and those letters at the end are some archaic units, or something like that.) The heat transfer rate must also be given by q = i^2 * R(w) = i^2 * R(0)[1 + alpha*(T(w)-T(0))] where i = electric current R(0) = resistance of wire at reference temperature T(0) alpha = temperature coefficient of resistance For measurement purposes the hot wire is connected to a bridge circuit shown in the book (too complicated to reproduce here). The current is determined by measuring the voltage drop across a standard resistor and the wire resistance is determined from the bridge circuit....Time constants of the order of 1 msec may be obtained with 0.0001 in. diam. platinum or tungsten wires operating in air. A modification of the hot-wire method consists of a small cylinder that is coated with a thin metallic film a few microns thick. This film then serves as the variable resistance and is extremely sensitive to fluctuations in the fluid velocity. Such hot-film probes have been used for measurements involving frequencies as high as 50,000 hz. The calibration of hot-wire probes is quite complicated, and the interested reader is referred to the discussion by Kovasznay for more information. [ref. Kovasznay, L.S.G., "Hot-wire Method", in Physical Measurements in Gas Dynamics and Combustion," p. 219, Princeton University Press, Princeton, N.J., 1954.]" Mark