THE ISOLATED BUBBLE REGIME IN POOL NUCLEATE BOILING

We consider an isolated bubble boiling regime in which vapour bubbles are intermittently produced at a prearranged set of nucleation sites o n an upward facing overheated wall plane. In this boiling regime, the bubbles depart from the wall and move as separate entities. Heat trans fer properties specific to this regime cannot be described without bub ble detachment size, and we apply our previously developed dynamic the ory of vapour bubble growth and detachment to determine this size. Bub ble growth is presumed to be thermally controlled. Two limiting cases of bubble evolution are considered: the one in which buoyancy prevails in promoting bubble detachment and the one in which surface tension p revails. We prove termination of the isolated regime of pool nucleate boiling to result from one of the four possible causes, depending on r elevant parameters values. The first cause consists in the fact that t he upward flow of rising bubbles hampers the downward liquid flow, and under certain conditions, prevents the liquid from coming to the wail in an amount that would be sufficient to compensate for vapour remova l from the wall. The second cause is due to the lateral coalescence of growing bubbles that are attached to their corresponding nucleation s ites, with ensuing generation of larger bubbles and extended vapour pa tches near the wall. The other two causes involve longitudinal coalesc ence either: (1) immediately in the wall vicinity, accompanied by the establishment of the multiple bubble boiling regime or (2) in the bulk , with the formation of vapour columns. The longitudinal coalescence i n the bulk is shown to be the most important cause. The critical wall temperature and the heat flux density associated with isolated bubble regime termination are found to be functions of the physical and opera ting parameters and are discussed in detail. Copyright (C) 1996 Elsevi er Science Ltd.