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), a total of 2,317 data (i.e., 1,121 data points with horizontal tubes, 1,063 data points with vertical tubes, and 133 data points with 45° inclined tubes) for q" versus △T has been obtained using various combinations of tube diameters (D=9.7~25.4mm), surface roughness (ε=15.1-60.9nm), and tube orientations (θ=0, 45, and 90°).
The experimental results show that (1) increased surface roughness enhances heat transfer for both horizontal and vertical tubes, (2) the two heat transfer mechanisms, i.e., enhanced 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 fluxes (q"≤50kW/ ㎡) and high heat fluxes (q" gt 50kW/㎡) 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 ($h_b$) is obtained as a function of heat flux (q") only.
Introducing the above empirical correlations into the computer code developed at KAIST, characteristics of the PRHRS have been analyzed for various tube diameters and number of tubes. According to the result of computer code simulations, 19.05 mm tube diameter for the heat exchanger is found to be the most efficient for decay heat removal from the reactor core.