Numerical investigation of boiling regime transition mechanism by a Lattice-Boltzmann model

A numerical study has been performed to investigate the hydrodynamic aspect s of the pool boilingon horizontal-, vertical- and downward-facing surfaces . The FlowLab code, which is based on a Lattice-Boltzmann (LB) model of two -phase flows, is employed. Macroscopic properties, such as surface tension (a) and contact angle (beta), are implemented through the fluid-fluid (G(si gma)) and fluid-solid (G(t)) interaction potentials. The model is found to express a linear relation between the macroscopic properties (sigma, beta) and microscopic parameters (G(sigma), G(t)). The simulation results on bubb le departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of two-phase flow regime transi tion mechanism are investigated for different surface-coolant configuration s. Results of the LB simulation clearly demonstrate that not only the bubbl e nucleation site density (related, e.g. to the heater surface condition an d heat fluxes), but also the surface position have a profound effect on the flow regime (pool boiling) characteristics. The results of the LB simulati on of hydrodynamics of two-phase flow on the horizontal surface provide the pictures quite similar to the experimental observation for saturated pool boiling. Two mechanisms of flow (boiling) regime transition on the vertical surface are predicted for the local bubble coalescence at bubble generatio n site and the downstream bubble coalescence. On the downward-facing surfac es, friction between bubbles and the surface wall is found to significantly enlarge the bubble size prior the bubble slip upwards. This behavior is re sponsible for the earlier bubble coalescence, and therefore, lowers the max imum heat removal rate, in a similar regime of nucleate boiling on a downwa rd-facing surface. (C) 2001 Elsevier Science B.V. All rights reserved.