Heat transfer to NaK flowing through unbaffled rod bundles

Abstract

The paper presents the results of an extensive experimental study of heat transfer to NaK flowing through unbaffled rod bundles (or banks). In-line flow results are reported on nine different test rods located in the center of a 19-rod bundle. They were obtained under conditions of fully established velocity and temperature profiles, and also under entrance-region conditions. The results on the first five test rods were in fairly good agreement with theoretical predictions and with earlier results obtained on mercury; but those on the last four fell low in the molecular-conduction (low-Péclet-number) regime. The lower the flow rate the lower the coefficients fell, indicating the presence of an interface resistance which was quite sensitive to the flow rate. It was concluded that the difficulty was due to less-than-perfect wetting of the heating surface, coupled with the presence of inert seal gas at the heating surface. The oxygen concentration in the NaK was at all times less than 25 ppm, which was far below the saturation level corresponding to the coldest part of the circulation loop. Results were also obtained for 90°-cross and 45°-oblique flows through rod bundles, for the condition where only the test rod was heated and for that where all the rods were heated. Measurements were made at several rod locations in the bank, and both rod-average and circumferentially local heat-transfer coefficients were obtained. The 90°-cross-flow results were in good agreement with previously published results on mercury, and with predictions based upon the theoretical correlation of Hsu [21]. In the low Péclet region, the all-rods-heated coefficients were appreciably higher than those for the condition where only the test rod was heated, but the difference decreased as the flow rate increased. The 45°-oblique-flow coefficients, falling lower than Hsu's [26, 27] theoretical prediction, were 75 percent of those for the 90°-cross-flow case. The circumferential variation of the local normalized heat-transfer coefficient was found to be independent of flow rate, and the same for both 90°- and 45°-flow directions. Also, for both types of flow, the circumferential surface temperature profile was that of a simple cosine function. On the basis of the results obtained in the study, supported by previous theoretical and experimental results, semi-empirical equations are proposed for predicting heat-transfer coefficients for both 90°- and 45°-cross flows for the condition where only the test rod is heated, and for that where all rods are heated.