Effects of Heat Exchanger Tube Parameters on Nucleate Pool Boiling Heat Transfer

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Abstract
In an effort to determine the combined effects of major parameters of heat exchanger tubes on the nucleate pool boiling heat transfer in the scaled in-containment refueling water storage tank (IRWST) of advanced light water reactors (ALWRs), a total of 1966 data points for q″ versus ΔT have been obtained using various combinations of tube diameters, surface roughness, and tube orientations. The experimental results show that: (1) increased surface roughness increases the heat transfer coefficient for both horizontal and vertical tubes, and the effect of surface roughness is more pronounced for the vertical tubes compared to the horizontal tubes, (2) the two heat transfer mechanisms, i.e., increased heat transfer due to liquid agitation by bubbles generated and reduced heat transfer by the formation of large vapor slugs and bubble coalescence, are different in two regions of low heat flux (q″ ≤ 50 kW/m2 ) and high heat flux (q″ > 50 kW/m2 ) depending on the orientation of tubes and the degree of surface roughness, and (3) the heat transfer rate decreases as the tube diameter is increased for both horizontal and vertical tubes, but the effect of tube diameter on the nucleate pool boiling heat transfer for vertical tubes is greater than that for horizontal tubes. Two empirical heat transfer correlations for q″, one for horizontal tubes and the other for vertical tubes, are obtained in terms of surface roughness (ε) and tube diameter (D). In addition, a simple empirical correlation for nucleate pool boiling heat transfer coefficient (hb ) is obtained as a function of heat flux (q″) only.

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