Effects of outer tube length on saturated pool boiling heat transfer in a vertical annulus with closed bottoms

The mechanism of pool boiling heat transfer has been studied extensively in the past since it is closely related with the thermal design of more efficient heat exchangers. One of the effective means to increase heat transfer is to consider annuli. Some geometry has closed bottoms. It is well known from the literature that the confined boiling can result in heat transfer improvements up to 300%-800% at low heat fluxes, as compared with unconfined boiling. However, a deterioration of heat transfer appears at higher heat fluxes for confined than for unrestricted boiling. The cause for the deterioration was suggested as bubble coalescence at the upper regions of the annulus. To apply the vertical annulus with closed bottoms to the thermal design of a heat exchanger investigation of any possible ways to prevent the deterioration is needed in advance. Up to the author's knowledge, no previous results concerning the ways have been published yet. Therefore, the present study is aimed at the investigation of the way to improve heat transfer in the annulus with closed bottoms through changing the length of the outer tube