The effect of thermal boundary conditions on heat transfer and flow resistance characteristics of small-diameter tube bundle

Heat exchanger with small-diameter tube bundle plays a crucial role in various industrial applications due to their compact structure and high thermal-hydraulic efficiency. These tube bundles operate in various thermal conditions, potentially resulting in variations of their heat transfer and flow characteristics. However, there is limited literature available on this issue with inconformity between conclusions drawn by different researchers. The reason remains unclear and those laws for tube bundle have been not yet discussed. Based on this, the study investigates the effect of thermal boundary conditions on the ariside heat transfer and flow resistance characteristics of tube bundle in crossflow. Three thermal boundary conditions considered include uniform wall temperature (UWT), uniform heat flux (UHF) and uniform convective coefficient (UCC) boundary conditions. The commercial software platform ANSYS Workbench is used for simulations in present study. A three-dimensional periodic domain of a tube bundle composed of tubes with 1 mm outer diameter and 0.7 mm inner diameter is selected, then three thermal boundary conditions are set on the inner wall of tubes respectively. The fluid material is air, and two solid materials with various thermal effusivity are applied. Based on ample numerical simulations, the effect of the thermal boundary condition on the average Nusselt number (Nu) and pressure loss coefficient (f) are evaluated by considering two evaluation methods commonly used in literature, and a parameter analysis of thermal effusivity ratio and the tube wall thickness is presented. A sinificant difference is observed between results by these two evaluation methods, and the optimum one is to firstly compute the heat transfer coefficient of each tube row and then take their arithmetic mean value. Besides, the result shows that the pressure loss coefficient is independent of thermal boundary condition. Further analysis reveals that the effect of thermal boundary conditon depends on the thermal effusivity ratio and the mean Nusselt number definition of single tube, while one determines the amplitude and the latter desides the tendency. Last but not least, smaller tube wall thickness brings more signifcant thermal boundary condition effect, with a max deviation of 8.7 % between Nusselt number on UTW condition and that on UHF condition observed among all computational cases. In summary, this work clarifies the reason for inconformity between conclusions in previous studies and provides reference to properly evaluate the effect of thermal boundary condition in heat exchanger design.