FIGURE 1. The thermoacoustic Stirling heat engine, constructed from steel pipe.

From the following article:

A thermoacoustic Stirling heat engine

S. Backhaus and G. W. Swift

Nature 399, 335-338(27 May 1999)

doi:10.1038/20624

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a, Scale drawing of the engine. The variable acoustic load consists of an adjustable valve leading to a 2.2-l tank20. The valve setting controls the acoustic power flowing into the load &Wdot; load, and hence Th. Piezoresistive pressure sensors 'P' allow measurement of20 the acoustic power flowing past their midpoint, &Wdot;2mic. b,Details of the torus-shaped section. The regenerator is a stack of 120-mesh stainless-steel screen whose hydraulic radius is approx42 microm. This is smaller than the average thermal penetration depth of the helium (300 microm), ensuring the good thermal contact between the helium gas and the screen necessary for thermodynamic reversibility. The two cold heat exchangers are of shell-and-tube construction, with the oscillating helium in the tubes and flowing cooling water in the shell. The hot heat exchanger is electrically heated Ni–Cr ribbon wound in a zig-zag fashion on an alumina frame. The buffer tube provides thermal insulation between the hot heat exchanger and ambient temperature while transmitting the acoustic power out of the hot zone. Its internal surface is slightly tapered and polished to reduce boundary-layer streaming, which would otherwise convect heat from the hot heat exchanger16. Thermal insulation around the hot components is not shown. The two tapered channels of the jet pump are 51 mm long and 34 mm high, and their short, tapered dimensions are adjustable, typically set in the range 1-2 mm. The lower edges of the channels are rounded with a 0.8-mm radius, to reduce Kin to nearly zero (see text). The taper increases the cross-sectional area at the upper end, thereby decreasing the velocity there and preventing the upper end from developing an equal and opposite

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(see text). The temperature of the helium is measured at locations labelled 'T' using 1.6-mm-diameter type-K thermocouples. c, Impedance model of the engine. The dashed lines enclose the regenerator; its volumetric-velocity source (Th/Tc - 1)U1c is due to the temperature gradient in the regenerator and causes the acoustic power gain. The resonator and variable acoustic load are modelled as a parallel combination of Rres with a reactive impedance Xres.

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