Abstract
The fluid flow and heat and mass transfer in a supersonic separator are not understood well due to the complicated interaction of the supersonic flow, swirling flow, phase transition and shock waves. In the present study, we develop a wet steam model to investigate the flow structure inside a supersonic separator with the co-existence of non-equilibrium condensation and shock waves. A study of the effect of the inlet subcooling and inlet saturation on the condensation behaviour is conducted to evaluate the performance of the supersonic separation with a focus on the shock wave. The numerical result shows that the degree of supersaturation of the water vapour can reach a maximum value of 4.28 within the designed supersonic separator and generate a peak nucleation rate of approximately 1021 kg m−3 s−1. The occurrence of the shock wave changes the equilibrium thermodynamic state, which leads to the re-evaporation of the condensed droplet. Higher inlet subcooling and inlet saturation not only shift downstream the position of the shock wave, but also induce an earlier condensation and higher liquid fraction. For the present nozzle, when the inlet subcooling and inlet saturation are about 34 K and 0.28 respectively, the shock wave intersects the region of the intense nucleation process, the non-equilibrium condensation process is terminated due to the increase of the pressure and temperature downstream the shock wave. Stronger swirling flow results in non-uniform distribution of the static pressure and decreases the nucleation rate of water vapour. The high swirling flow with a maximum swirl velocity of 150 m/s weakens the liquid fraction by 25% compared to the no swirling flow. This indicates that it is important to balance the swirling flow and condensation process to achieve an efficient performance of the supersonic separator.
Highlights
The supersonic separator has been introduced for natural gas processing with a focus on the removal of water vapour[1, 2]
Zhao et al [11] numerically investigated the effect of the converging profile on non-equilibrium condensation processes in a supersonic nozzle, and the results showed that a slow converging section of the supersonic nozzle delayed the occurrence of the condensation position
Figure presents the static pressure and Mach numbers inside the supersonic separator considering non-equilibrium condensation, and the distributions of the condensation parameters are shown in Fig. 9 including the degree of supersaturation, nucleation rate, liquid mass generation rate and liquid fraction
Summary
The supersonic separator has been introduced for natural gas processing with a focus on the removal of water vapour[1, 2]. The coexistence of non-equilibrium condensation and a shock wave in supersonic flows is a major feature of the separator. The key working principle of the supersonic separation is nonequilibrium condensation of water vapour in supersonic flows. The supersonic separation benefits from the shock wave [3] when supersonic flows are transformed to subsonic flows, which recovers the pressure to maximum improve the energy efficiency [4]. Non-equilibrium condensation in a supersonic nozzle has been investigated for several decades without considering a shock wave [5,6,7]. Dykas & Wróblewski [8]
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