Natural convective boiling in vertical rectangular narrow channels

Abstract

Natural convective boiling in a confined narrow space is frequently encountered in engineering, but there is as yet very little information on heat transfer enhancement obtained from boiling in vertical narrow channels with symmetric heating. To understand the heat transfer mechanism of boiling in the channels, experiments were performed for saturated R-113 at heat flux from very low to critical in vertical narrow channels under atmospheric pressure. The channels consist of a pair of flat plates, either transparent or porcelain. The plates were covered with a plating electric heating film by a new technique. The effects of gap sizes, channel heights, and heat fluxes on heat transfer and critical heat flux (CHF) were studied systematically. The results based on observation and measurement illustrate that the smaller the gap size, the smaller the incipient heat flux. The boiling flow pattern depends on gap sizes and heat fluxes. The point of maximum wall superheat is not always located on the top of the boiling surface. Wall superheat decreases as the channel narrows at the lower heat fluxes, but the reverse holds at the higher heat fluxes. For a heating channel, there exists an optimum channel size for heat transfer enhancement A increase in channel height has the same effect on heat transfer behavior as a decrease in gap size. The CHF decreases with decreasing gap size or increasing channel height. The fact that the mass velocity in the channel falls off sharply upon heating might be related to the boiling crisis.