Non-Equilibrium Phase Change And Shock Effect In Supersonic Steam Flow

Supersonic steam flow with spontaneous condensation is of considerable interest in several natural and industrial processes including cloud formation, flight of aircraft in humid conditions, supersonic steam flow in wind tunnel experiments, steam flows in turbines and chemical industry equipments. Recently, with the development of steam jet pumps using sources of low grade or waste heat, with the steam ejector showing a good application perspective in the modern nuclear power safety engineering and with the expansion in desalination industry based on the MED-TVC (multi-effect distillation with thermal vapor compression technique), the research for the special process of energy exchange and phase transition in the supersonic steam flow are more and more important and attractive.Due to the strong compressibility of steam, different from the subsonic flow, many special characteristics in the supersonic steam flow will occur. Especially, with the appearance of the compression shock or expansion shock, the supersonic steam flow shows strong multi-dimensional and boundary effects. As the supersonic steam expands rapidly, the steam temperature falls rapidly and very high supersaturation can be achieved. Eventually the steam acquires sufficient supercooling level for non-equilibrium spontaneous condensation and the subsequent release of latent heat thus results in a deceleration of the flow and a rise in pressure, known traditionally as the "condensation shock". The non-equilibrium steam flow state is very different from the isentropic supersonic flow, and the energy, momentum, mass exchanges occur rapidly between the gas and liquid phases.In the present study, the supersonic non-equilibrium steam flow with spontaneous condensation and condensation shock was numerically studied based on the computational fluid dynamics method. A conservative compressible numerical model coupling with non-isothermal classical nucleation model and droplet growth model modified and recommended by Young and Gyarmathy, in which the thermodynamic state and properties of steam are calculated by the Virial Equations, was developed and used to predict the spontaneous condensation phenomenon with homogenous nucleation in the supersonic steam flow. In order to capture the transient characteristics of the supersonic turbulent flow and the condensation shock effect, the turbulence model was constructed and the improved high resolution Roe-FDS scheme was used. The validation of the numerical results was accomplished with the experiment conducted by Moore and the results showed a good agreement between numerical simulation and the experiment data. Based on the simulation, the non-equilibrium thermodynamic phenomenon, the condensation shock, the aerodynamic shock and other special phenomena in the supersonic steam flow were studied. And the main research work was carried out as the following:1) The numerical simulation for the prediction of the non-equilibrium steam flow with spontaneous condensation is completed used the multi-dimensional model. The difference between the dry isentropic expansion and the non-equilibrium flow with spontaneous condensation has been investigated particularly. Based on the CFD simulation for the supersonic steam flow, the qualitatively description of the thermodynamic processes of spontaneous condensation, droplet growth, condensation shock and the other non-equilibrium phenomenon in the supersonic steam has been accomplished.2) The thermodynamic process of the occurrence and development of the nonequilibrium phase transition and condensation was numerically studied. The pressure characteristic, temperature characteristic and other non-equilibrium thermodynamics characteristic were studied and the non-equilibrium thermodynamic phenomenon such as condensation shock and the effects of the nozzle geometry on the unsteady flow were investigated. The research explored the effect of steam pressure, temperature, supercooling level, super-saturation ratio and expansion rate on the onset of the nucleation and the intensity of condensation shock in the supersonic steam flow.3) The analysis of the boundary effects in the supersonic steam flow was accomplished. The process of formation for the "X"-type condensation shock and the physical characteristics of the condensation shock have been revealed. Based on the mass generation rate, the condensation shock was divided into three zones, respectively named as the condensation shock developing region, the condensation shock intersecting region and the condensation shock fading region. The physical properties and thermodynamic characterizations for different regions have been summarized.4) The coupling calculations for the condensation shock and normal aerodynamic shock were carried out in the supersonic steam flow. The thermodynamic and aerodynamic properties of both condensation shock and normal aerodynamic shock were studied and the difference between the condensation shock and the aerodynamic shock was investigated. The study showed that the rapid response behavior of condensation shock is similar to but not totally same as the normal aerodynamic shock wave. And with the back pressure increases the normal aerodynamic shock moves toward the nozzle throat, and the shock has a complex influence on the homogeneous nucleation condensation and the condensation shock. If the normal aerodynamic shock passes through the location of the condensation shock, the supercooling level abruptly becomes negative and liquid droplets rapidly evaporate in response to the rapid pressure and temperature rise across the shock and the spontaneous condensation and condensation shock will be weakened or even not occur at all.