The study developed a wet steam model incorporating non-equilibrium phase change of water vapor using the Eulerian–Eulerian method and vapor condensation theory. It comparatively investigated the steady-state performance and condensation behavior in microchannels with single and double throttle orifices, hydrostatic dry gas seals (H-DGS) for steam. The analysis examined their ability to suppress condensation under varying face clearances and steam temperatures. The results indicate that the tandem-orifice configuration facilitates a two-stage pressure drop, which moderates the local steam expansion rate at the throttle hole throat and reduces the peak supersaturation level. This mitigates the intensity of non-equilibrium condensation by distributing the nucleation zone at the throttle hole throat and suppressing rapid droplet growth. Optimal mitigation is observed with an orifice diameter dt = 0.6–0.8 mm and spacing L1 = 9.1 mm, where flow through the throttle section dominates (t 16 μm). Under these conditions, the design prevents supersonic acceleration at the throat, lowers the local supercooling, and thus inhibits condensation onset—thereby extending the stable operating range. Consequently, the critical phase transition clearance increases from 16–20 μm (single orifice) to 26 μm at Tin = 430 K, and the critical temperature Tphase is reduced by 10 K at t = 20 μm. When the face clearance is small (t 10 μm) and thus governs the flow resistance, the advantage diminishes as the throttling effect shifts away from the orifices. Performance outcomes, including an approximately 18.5% lower throat velocity, 16.5% lower gas consumption, and 14% higher gas film stiffness, follow from these fluid-dynamic mechanisms, at tandem-orifice H-DGS with L1 = 13.5 mm, dt = 0.8 mm, and t = 14 mm. Experimental validation on a dedicated test platform confirms the predicted behavior, supporting the use of multi-stage throttling for condensation control in steam H-DGS design.
Zhang et al. (Mon,) studied this question.