Acoustically generated temperature gradients in short plates

Abstract

Thermoacoustic heat transport and its applications, thermoacoustic engines, have been discussed in a number of articles over the past several years. Lacking from these articles is a thorough, quantitative experimental investigation of the basic theory underlying thermoacoustic heat transport. A logical starting point for such a study is to investigate the simplest class of thermoacoustic engine—a stack of short plates referred to as a ThermoAcoustic Couple (TAC). The utility of this choice is that the theory can be reduced to its simplest form for analysis of the results. The results of measurements of thermoacoustically generated temperature gradients in TACs subjected to acoustic standing waves are reported. The value of the temperature gradient, which results from an acoustically generated entropy flow in the gas in thermal contact with the TAC, is a function of the peak acoustic pressure amplitude, the mean gas pressure, the Prandtl number of the gas, the configuration of the TAC, and its position in the standing wave. Measurements were made with a computer-controlled apparatus for drive ratios (the ratio of the acoustic pressure amplitude to the mean pressure of the gas) from approximately 0.1%–2.0%, in argon and helium having mean pressures from approximately 100–368 kPa, for three different TACs as a function of their positions in the standing wave. The results are compared with predictions based on a theory by Wheatley et al. [J. Acoust. Soc. Am. 74, 153–170 (1983)]. The measurements agree well with theory for drive ratios less than approximately 0.4%. However, the agreement diminishes at higher drive ratios, where two regions of behavior are observed. Agreement is, in general, best in the vicinity of acoustic particle velocity nodes at all drive ratios investigated.