Heat transfer from transverse tubes adjacent to a thermoacoustic stack

The goal of this work was to test the usefulness of the time-average steady-flow equivalent (TASFE) approximation for the analysis of heat exchangers for thermoacoustic engines. The TASFE approximation assumes that the heat transfer in an acoustic standing wave with zero mean flow is equivalent to averaging the corresponding steady-flow correlation over a sinusoidal distribution of gas speeds. The experiment tested three simple heat-exchanger configurations located immediately adjacent to the hot end of a thermoacoustic stack. Two configurations consisted of bare, parallel, water-carrying tubes oriented transverse to the mean acoustic flow. A third configuration consisted of a single layer of woven copper screen soldered to transverse tubes. Experiments with pure helium and with a mixture of helium and argon found qualitative, and sometimes quantitative, agreement with expectations of the TASFE approximation at both large and small acoustic amplitudes. In addition, large heat transfer with zero temperature difference between the heat exchanger and the stack was observed and interpreted.