The effects of heat transfer evaluation methods on Nusselt number for mini-channel tube bundles

Heat transfer performance of tube bundles, which are widely used in practical applications, has been extensively investigated. Most previous experimental and numerical works for tube bundles have been performed with tube diameter in a range of 10 to 51mm and a Reynolds number of 8,000≤Re≤30,000. Recently, tube bundles with small diameter have garnered interest, since a mini-channel tube provides greater compactness. The present work investigates the applicability of previous correlations available in the open literature to tube bundles with small diameter of 1.5mm and a Reynolds number of 3,000≤Re≤7,000. Experiment results and a commercial CFD package were used to analyze the thermal-hydraulic performance of the tube bundles. The average convective heat transfer coefficients of tube bundles in most previous works were evaluated based on the difference between the bulk fluid temperature and the average surface temperature of the tubes. However, the tubular heat exchanger design process makes use of the concepts of overall thermal resistance and the LMTD, which is defined from the temperature difference between two working fluids. This paper examines the variation of the shell-side convective heat transfer coefficients of tube bundles caused by the two different analysis methods. The comparison showed that the average Nusselt number evaluated based on the LMTD method is 22.6% smaller than that based on the surface average temperature. This shows that it is necessary to consider the discrepancy in the heat transfer coefficient definition for proper design of heat exchangers. Also, the Zukauskas correlation developed for larger diameter tubes and higher Reynolds number are still in good agreement with experimental data for the present small diameter tube bundle within a discrepancy of 4.7%.