Effect of pressure ratio and relative humidity on the moist air transonic non-equilibrium condensation shock: Experiments and numerical investigation

In Compressed Air Energy Storage (CAES) systems, compressors are frequently subjected to complex operating conditions, such as fluctuating humidity and varying back-pressure, Because the interstage flow channels of a compressor are aerodynamically similar to a curved, rotating Laval nozzle, the precipitation and persistent presence of the liquid phase in the airflow are unavoidable under such operating conditions, which significantly impacts compressor performance. This complex environment easily induces non-equilibrium condensation (NEC) phenomena, which not only drastically alter the internal flow field structure but may also trigger intense shock wave oscillations. This study combines experimental measurements with numerical simulations to investigate the coupled interaction between flow-field evolution and phase-change dynamics under different nozzle pressure ratios (NPR = 1.8, 1.6, 1.4) and relative humidity levels (RH = 30%, 50%, 70%). The results show that NPR primarily governs the effective expansion process. Increasing NPR intensifies flow expansion, driving the shock wave downstream toward the outlet and modifying the spatial development of the condensation region. Moreover, NPR plays a critical role in maintaining the structural integrity of the nucleation zone. In contrast, RH influences the pressure field mainly through latent heat release. Under low-RH conditions, a pronounced condensation shock forms, characterized by a steep pressure gradient and stronger flow disturbance. At high RH, continuous and substantial latent heat release suppresses excessive flow acceleration, producing a thermal-blocking effect. Specifically, relative thermal efficiency peaks at 75.33% under 50% RH, where a spatially distributed phase transition softens the condensation shock gradient. Conversely, excessive latent heat at 70% RH triggers severe thermal choking, plummeting the efficiency to 16.47%. Furthermore, NPR = 1.6 induces an abrupt aerodynamic compression immediately following the phase-change zone, creating a local efficiency minimum at 56.59%. Overall, the findings clarify how NPR and RH jointly determine the location and strength of nucleation-induced condensation shocks, providing practical guidance for compressor operation and stability control under variable CAES working conditions.