EXPERIMENTAL STUDY OF FULLY DEVELOPED TURBULENT FLOW IN INTERNALLY FINNED TUBES

A steady, hydrodynamically and thermally fully developed turbulent water flow in internally finned tubes with constant-heat-flux boundary condition is investigated experimentally. In this experiment, 70 degrees C hot water passes the inner tube while 20 degrees C cold water flows through the annulus passage. In order to eliminate thermal contact resistance, the circular tube having a length of 1400 mm with inner fins is directly milled from a cylinder. Apart from a smooth tube, three internal-finned-tube heat exchangers having 4, 6, and 8 longitudinal trapezoidal fins are tested. For Re = 5000, the tested tubes with 4, 6, and 8 fins can improve thermal performance by approximately 35.4, 44.6, and 67.2%, respectively, but merely increase friction factor by 1.92, 4.8, and 6.73%, respectively, compared to a smooth tube. The increases with respect to to a smooth tube in both Nu and f become smaller at a higher Re for N = 4, 6, and 8. As pressure drop, increases, the Q of the tested finned tubes first grows rapidly, and this growth curve gradually becomes flatter at a higher 4P. Consequently, the heattransfer performance will become worse as 4P > 3.5 kPa for the proposed heat exchangers. Additionally, the performance evaluation criterion (PEC) is proposed to assess the performances of the proposed internally finned tubes. It is shown that the heat exchanger with 8 fins has the best thermo-hydraulic performance. Finally, correlating equations for Nu and f with respect to Re are presented for a range of 3000 <= Re <= 7250 and N = 0, 4, 6, and 8.