Mixed regime of film boiling over a horizontal cylinder in an upward flow of saturated liquid

摘要

In this article, liquid-vapour interface evolution, heat transfer, vapour wake dynamics, and its interaction with the liquid wake are studied numerically for film boiling over a horizontal cylinder in an upward flow of saturated liquid. Computations have been performed using a numerical framework developed for phase change problems on unstructured grids in which the liquid-vapour interface is captured using a coupled level set and volume of fluid method. The problem is predominantly studied in the mixed regime where both buoyancy and inertia are comparable, and affect the interface evolution and heat transfer. The relative importance of buoyancy and inertia is governed by the Froude number (Fr). The simulations are performed for conditions resulting in low values of Fr, with the liquid flow Reynolds number (Re-l) varied in the range 10-170. The onset of the mixed regime is observed at quite a low magnitude of Fr. With an increase in Fr, the periodic nature of bubble release is gradually lost and vapour detachment from the vapour wake becomes increasingly random and transient. Additionally, the effect of several other parameters such as wall superheat, cylinder diameter, surface tension, vapour Prandtl number, and system pressure is discussed, and it is observed that the interface dynamics and heat transfer result from an interplay of these parameters. The mutual interaction of the liquid and vapour wakes is then presented. While the liquid flow expectedly affects the vapour wake dynamics, the rising vapour itself affects the liquid wake characteristics. Finally, a comprehensive correlation for the Nusselt number in the mixed regime of flow film boiling is presented and validated against some of the available empirical data in this regime. Published by AIP Publishing.