Spectral analysis of wall-pressure fluctuations in electronic expansion valves

Electronic expansion valves (EEVs) regulate refrigerant mass flow in air-conditioning and refrigeration systems through throttling, during which unsteady cavitating two-phase flows and wall-pressure fluctuations develop. A helical groove can be employed in the downstream pipe as a passive modification of the internal flow. However, how the helical groove modifies the jet motion associated with the wall-pressure fluctuations remains incompletely understood. Here, we investigate how a helical groove modifies the asymmetric jet motion associated with the dominant low-frequency wall-pressure fluctuation using volume-of-fluid-based multiphase large-eddy simulations of the configurations with and without the groove. The frequency ranges identified from measured acoustic spectra are used only as reference bands for the simulated wall-pressure analysis. The wall-pressure fluctuations are concentrated in the near-orifice impingement region, where the dominant low-frequency component is associated with the asymmetric lateral motion of the jet. At z/D=0.5, where z and D denote the streamwise coordinate measured from the orifice and the downstream pipe diameter, respectively, the groove reduces the rms jet-center displacement and asymmetric wall-pressure fluctuation by 12.5% and 18.4%, respectively, and their low-frequency band-integrated cross-spectral magnitude by 55.3%. The maximum magnitude-squared coherence below 1 kHz also decreases from 0.559 to 0.395 with the groove. Spectral proper orthogonal decomposition further shows that the leading streamwise- and radial-velocity eigenvalues decrease by 27.4% and 29.4%, respectively, while the normalized radial-velocity mode is redistributed from the outer part of the pipe toward the interior. A secondary high-frequency response is observed in connection with fragmented near-wall small-scale motions. These findings support the interpretation that the helical groove reduces the dominant low-frequency wall-pressure fluctuation by weakening the asymmetric jet motion toward the wall and provide a structural description of the associated changes in coherent motion.