An experimental analysis of fluctuating temperature measurements using hot-wires at different overheats

Abstract

Abstract. Adjacent parallel hot-wire anemometers at different temperatures have sometimes been used to measure fluctuating temperatures in turbulent flows. This work presents an extensive experimental comparison of temperatures measured with a parallel-wire probe to temperatures simultaneously measured with a standard cold-wire probe. The results show the parallel-wire probe to work well in low intensity flows with temperature signals which are not too small. However, the parallel-wire probe temperature measurements are not accurate for high turbulence intensities or for small temperature signals, and in general the cold-wire system is probably to be preferred. MotivationsThe cold-wire temperature transducer, often used for the measurement of turbulent temperature fluctuations, has several inherent limitations. One is its small diameter and resultant fragility. Another is its comparatively low frequency response, for all but very small diameter sensors. In principle, both of these difficulties could be overcome by using a pair of constant-temperature hot wires to measure temperature and velocity. Hot wires are generally five to ten times larger in diameter, which makes them far more rugged, and their usable frequency responses may easily reach 10 kHz. Even higher bandwidths may be obtained from hot wires if loss of spatial resolution is not important.The output voltage of a hot wire varies with the wire temperature. If two sensors at different constant temperatures are situated parallel to each other, so that they each experience the same flow speed and temperature, then their two differing output voltages may be used to solve the voltage response equations for the flow speed and temperature. This is the principle of the parallel-wire probe. The sensors of such probes are generally confined to relatively low overheats so as to maintain a high temperature sensitivity.The idea of a parallel-wire probe is not new, and the use of varied temperature sensitivities to separate temperature and velocity signals is almost as old as the hot-wire itself. Corrsin suggested the concept in the forties (Corrsin 1949) and, in the following years, he made several applications of it. In 1949 and 1950, Corrsin and Uberoi carefully tested the combined temperature and velocity response of a hot-wire, and, by using several different values of the wire temperature, they obtained measurements of temperature-velocity correlations in a heated jet. A similar principle was employed by Mills et al. (1957) to determine the temperature fluctuation spectrum in nonisothermal grid turbulence. The technique of mixing probe sensitivities has been employed with checkered success many times in the intervening years, and dozens of papers have been written on the subject of hotwire temperature response (an exhaustive survey is given by Freymuth 1982). In addition, the dual-overheat probe has been 'reinvented' at least once (Sakao 1973).A recent application of the mixed sensitivity probe was made by Blair and Bennett (1984). They employed an array of con...

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