EFFECT OF BUNDLE GEOMETRY ON BOILING HEAT TRANSFER COEFFICIENT

Abstract

Boiling heat transfer phenomenon in tube bundle is complex and considerably different from that on a single tube heated alone in a channel. In tube bundle, an enhancement in heat transfer coefficient on the tubes located above the bottom most tube is observed which is likely to be done to enhanced agitation of fluid around upper tubes on account of vapour bubbles rising up from lower tubes and impinging on the tube surface. This enhancement of heat transfer coefficient on upper tubes depends upon a large number of parameters such as heat flux, fluid flow velocity and tube bundle geometry. In the present work available experimental data on boiling heat transfer in tube bundles has been analyzed and correlations for the average boiling heat transfer coefficient of a tube bundle have been developed. The data is for saturated boiling heat transfer at atmospheric pressure for a number of bundle configurations using distilled water under pool as well as under cross flow with low velocities. From these data it is seen that the average heat transfer coefficient of a tube bundle is higher as compared to that of a single tube heated alone in a channel. Also it is found that the enhancement in bundle heat transfer coefficient increases with heat flux and mass flux. The effect of mass flux is negligible at high heat fluxes. The bundle heat transfer coefficient increases with number of tubes first and then decreases. I A modified Chen-type correlation, incorporating an enhanced value of single phase liquid convective heat transfer coefficient, Fh1 ;F being the enhancement factor and h1 the liquid phase convective heat transfer coefficient; and a microconvective component, h., representing the pool boiling mc contribution evaluated at effective wall superheat has been used to correlate the bundle heat transfer coefficient : h = Fh1 + hmic. Based on the experimental data, the following relation has been developed for the bundle enhancement factor F, in terms of boiling number Bo. pitch to tube diameter ratio p/d and number of tubes in a column N: F = (42.71 + 328.64 N - 49.25 N2)Bo 0.6703 e-0.0217(pfd) The same experimental data has been utilized to develop a correlation for bundle Nusselt number to predict average bundle heat transfer coefficient, given below : Nu = (4.9428 + 0.9858 N - 0.1317 N2) Bo0.2115 e-0.0373(p/(i) These correlations have been found to predict the values of heat transfer coefficient with standard deviations of 16.7% and 23.2% respectively. It can therefore be concluded that these correlations can predict the heat transfer coefficient reasonably well. (