Experimental demonstration of high-amplitude thermoacoustic instabilities under transcritical temperature conditions in a standing-wave device

Several experiments have been carried out at Purdue University’s Maurice J. Zucrow labs to investigate the thermoacoustic response of the refrigerant octafluoropropane (R-218) in its transcritical thermal conditions as the working fluid in a canonical standing-wave thermoacoustic device. Thermoacoustic instabilities were initiated by a microtube heat exchanger that allowed hot fluid and cold fluid to flow radially over either end of the microtube bundle while the working fluid flowed axially through the microtubes. Parametric studies were conducted in which the bulk pressure, temperature difference, resonator length, and resonator diameter were varied and the effects of each of these parameters on the thermoacoustic response are discussed. Additionally, the impact of viscous losses due to flow turning are assessed. The fluid achieved pressure amplitudes as high as 690 kPa with a bulk pressure of 1.3 times the fluid’s critical pressure (3.43 MPa) and a temperature difference, ΔT = Thot −Tcold, of 150 K. To the author’s knowledge, this is the highest ever thermoacoustic pressure amplitude achieved in a non-reacting flow at this ΔT range and is attributed to the strong density variations near the critical point of the working fluid. Finally, the feasibility for energy extraction is assessed in a set of tests which characterized the ability of the working fluid to pump itself through a recirculation line with check valves. This set of tests showed that the working fluid was able to create self-sustained circulation by inducing a pressure differential across the check valves with the thermoacoustic response. This circulation was induced while still maintaining a significant pressure amplitude, demonstrating promising results as a feasible method for energy extraction and waste heat removal.