Numerical Investigation of Thermo-Flow Characteristics of Tubes with Transverse Micro-Fins

The article presents the results of numerical studies of heat transfer and pressure drops in a channel with transverse micro-fins. The main aim of the study was to prepare the thermal and flow characteristics of such a channel for a variable longitudinal spacing of micro-fins. For the tested pipe with an internal diameter of D = 12 mm, the absolute height of the micro-fins was e = 0.243 mm, which is 2% of its diameter. The tests were carried out for turbulent flow in the range of Reynolds numbers of 5000–250,000 with the variable spacing of micro-ribs in the range of L = 0.28–13.52 mm, which corresponds to their dimensionless longitudinal distance, L / D = 0.023–1.126. For the studied geometries, the characteristics of the friction factor, ft ( Re ), and the Nusselt number, Nu ( Re ), are shown in the graphs. The highest values of Nu were observed for a spacing of L / D = 0.092 in the range of Re = 5000–60,000, while the lowest were observed for a geometry of L / D = 0.035 for Re = 60,000–250,000. The friction factors, however, were the highest for the two geometries L / D = 0.161 and L / D = 0.229 over the entire range of the tested Re numbers. A large discrepancy was observed between the friction factors calculated from the Colebrook–White equation (for irregular relative roughness depicted in the Moody diagram) and those obtained from simulations (for pipes with the same roughness height but regular geometry created by micro-fins). An analysis of the heat transfer efficiency of the tested geometries was also presented, taking into account the criterion of equal pumping power, i.e., the PEC (performance evaluation criteria) coefficient. The highest values of the PEC coefficient, up to 1.25–1.28, were obtained for micro-fin spacings of L / D = 0.069 and L / D = 0.092 in the Re number range of 20.000–30.000.