Non-equilibrium Phase Change Research Articles

Closure Conditions for Computation of Thermal Nonequilibrium in Two-Phase Flows

Computational systems for describing multiphase process and system behavior must have constitutive relations which adequately describe physical processes. It is shown that nonequilibrium phase change for steady flow is an initial value, history dependent process which requires an accurate description of both the quantity of interfacial area and the interfacial heat transfer for proper description. In addition, proper selection of the initial condition for onset of nonequilibrium must be specified if the resultant constitutive relations are to prove accurate. Examples are given of application to both post-dryout heat transfer and flashing of initially subcooled fluids.

Dissipative model of film boiling crisis

A dissipative model of the film boiling crisis based on the variational hypothesis of nonequilibrium phase change is presented. Transfer systems—characteristic for film and transition boiling of a liquid droplet on a plane horizontal and isothermal heating surface—were constructed. The value of the minimum film boiling temperature T p, min was calculated from the criterion of equality of local potentials for two competitive transfer systems. The curves p = p( T p, min ) for hydrodynamic and thermodynamic models of the film boiling crisis for water have been determined and compared with the results achieved for the dissipative model.

Toward a unified approach for thermal nonequilibrium in gas-liquid systems

A unified viewpoint is described which can be utilized to determine nonequilibrium liquid-vapor phase change in both post-dryout and decompressive flashing systems. It is shown that nonequilibrium phase exchange is in general a first order relaxation process and, as such, is a path dependent initial value problem rather than a local phenomenon. In all cases three quantities must be adequately specified: the initial value or inception criteria, the interfacial area density and the interfacial heat transfer rates.

Condensation heat transfer: Comments on non-equilibrium temperature profiles and the engineering calculation of mass transfer

Two condensation heat transfer topics of relevance to nuclear reactor safety are critically reviewed. (1) The theory of non-equilibrium phase change developed by Bornhorst and Hatsopoulos is examined, and an anomaly in the vapor phase temperature profile noted. The source of the anomaly is clarified, and it is suggested that the theory is of doubtful validity. (2) The advisability of using simple engineering methods to calculate mass transfer phenomena during condensation from steam-air mixtures is discussed. A review of a suitable method is presented, with emphasis on the calculation of condensation rate, aerosol particle deposition rate and absorption of soluble gaseous species.

Two−phase equation of state and free−energy model for dynamic phase change in materials

This paper describes the free−energy approach to calculating the dynamic phase change induced by shock waves. A thermodynamic model is developed for the nonequilibrium phase change assuming that the rate of transition is determined by the difference in the Gibbs free energy among all the phases present and that the transition proceeds toward the phase of the lowest free energy. A two−phase equation of state is derived for the Hugoniot states with the pressure and temperature as independent variables. In order to demonstrate the validity of the theory, the analysis of experimental results is made by applying the free−energy model. It is found that in highly ompressible solids. e.g., bulk modulus 280 kbar and 14% change in density at the transition pressure for δ−plutonium, there is no sharply defined transition point. Instead, the transition takes place gradually at the rate increasing with pressure up to the transition point where the process completes very rapidly. DP/DT taken at the transition point is a variable input in the present scheme. The slope of the phase boundary at the transition pressure is determined from DP/DT. The importance of the phase boundary for the shock−loading and shock−unloading processes is explained.

Kinetic theory of condensation and evaporation. II

The macroscopic temperature and pressure jumps that occur at a condensate surface during a nonequilibrium phase change are calculated using a variational principle on the linearized Boltzmann equation. When evaluated for the BGK collisional model, the macroscopic jumps have values within 1% of a recent numerical calculation. Furthermore, the microscopic density and temperature jumps are estimated using an ansatz for the distribution function of molecules impinging on the surface. The jumps obtained in this way are correct to within 5% of the numerical values when evaluated for the BGK model. Also a simple procedure is described by which the jumps occuring at a surface for which the mass accommodation (condensation) coefficient σ≠1 may be calculated from those for which σ=1. In addition the connection of this kinetic theory with the thermodynamics of irreversible processes is made and the elements of the appropriate Onsager matrix are expressed in terms of the macroscopic jumps for arbitrary values of σ.

589. 高速二相流に関する研究,(II)

A series of two-phase flashing-flow experiments with converging-diverging nozzle were performed. The throat diameter of the test nozzle was 10 mm ??, and the nozzle length 200mm. The maximum pressure of the reservoir was 30kg/cmcm2G, and the quality of the steam was in the range 0.0-0.51 (calculated from volume., weight-and energy-balance of the reservoir, on the assumption of thermodynamical equilibrium). The reservoir pressure, discharge flow rate and the distributions of pressure, fluid temperature and weight velocity along the test nozzle axis were measured. The experimental results can be summarized as follows.(1) The observed discharge flow rate is smaller than the value calculated with saturated flow model, except in the range of quality below 0.01.(2) The fluid temperature in the test nozzle was about 50°C lower than the saturated temperature, except in the regions near the inlet and outlet.These results indicate that non-equilibrium phase change occurred in the convergingdiverging nozzle of interest.