The effects of the evaluation method on the average heat transfer coefficient for a mini-channel tube bundle

Heat transfer performance of tube bundles, which are widely used in practical applications, was investigated. Experimental and numerical studies for tube bundles were performed with a tube diameter of 1.5 mm and a shell-side Reynolds number of 3,000 ⩽ Re ⩽ 7,000. The average convective heat transfer coefficients of the tube bundles in most previous investigations were based on the difference between the bulk fluid temperature and the average surface temperature of the tubes. However, the tubular heat exchanger design process uses the concepts of overall thermal resistance and the log-mean-temperature-difference (LMTD), which is defined as 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 evaluation 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 temperature method. This shows that it is necessary to consider the discrepancy in the heat transfer coefficient definition for proper design of heat exchangers. The Zukauskas correlation developed for larger diameter tubes and higher Reynolds numbers are still in good agreement with experimental data for the present small diameter tube bundle within a discrepancy of 4.7%.