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This manuscript describes an experimental and numerical investigation of transcritical thermoacoustic instability in a standing-wave setup using the refrigerant octafluoropropane (R-218) as the working fluid. Thermoacoustic instability is excited by two microtube heat exchangers separated by a vacuum-jacketed microtube stack. R-218 is allowed to flow axially through the microtubes into a closed resonator while heating and cooling fluids flow radially over the microtubes to create a temperature gradient. The fluid achieved pressure amplitudes up to 669 kPa (97 psi) at a temperature difference ΔT=Thot−Tcold of 150 K and a base pressure, P0, of 1.3 times the critical pressure (3.43 MPa). The high pressure amplitudes obtained are attributed to the strong density variations near the critical point of the working fluid. The thermoacoustic response was characterized in a set of parametric studies in which ΔT, base pressure, and resonator length were varied. A modeling approach based on linearized Navier–Stokes equations reproduces the experimental results with fair agreement. This work demonstrates promising application of transcritical working fluids to thermoacoustic engines as devices for energy extraction and waste heat removal.

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