Numerical study of nucleate boiling. Diffuse interface simulations of steam bubble departures from superheated wall
Examensarbete för masterexamen
Applied mechanics (MPAME), MSc
Nucleate boiling is a type of boiling which is characterised by the nucleation, growth and rise of vapour bubbles at the superheated surface. Because latent heat is typ ically much larger than sensible heat, the heat transfer associated with nucleate boiling is much larger than that of other forms of liquid cooling. Despite its im portance, the effect of the wall and fluid properties on heat transfer is not yet fully understood, due to the several mechanisms involved and the many governing param eters. In a field where experiments are cumbersome and hard to realise, numerical simulations are of out most importance to increase our understanding. Here we in vestigate the effect of contact angle θw, wall superheat temperature ∆T and number of nucleation sites Nbub on the departure of boiling steam bubbles in quiescent liquid water by means of a novel Diffuse Interface (DI) algorithm for weakly compressible flows. More specifically we estimate the time-averaged Nusselt number, quantifying the non-dimensional wall heat flux. The maximum time averaged Nusselt number was found for ∆T = 2.5 K, θw = 45° and Nbub = 1, which was the lowest super heat temperature, smallest contact angle and lowest number of nucleation sites that were tested. However these results are to be taken with reservations since the simu lated bubbles were of several orders of magnitude larger than physical bubble nuclei, probably underestimating the effect of phase change on heat transfer. More work is required to obtain an algorithm that is stable enough to simulate a wider range of parameters.
boiling , phase change , diffuse interface , bubble , heat transfer , nucleate boiling , contact angle