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°C hot water passes the inner tube while 20°C cold water flows through the annulus passage. In order to eliminate contact thermal 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 exchanger having 4, 6, and 8 longitudinal trapezoidal fins are tested. For Re=5000, the tested tubes with four, six and eight fins can improve thermal performance by approximately 35.4%, 44.6% and 67.2%, 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, Q of the tested finned tubes first grows rapidly, and this growth curve gradually becomes flatter at a higher ΔP. Consequently, the heat transfer performance will become worse as ΔP>3.5 kPa for the proposed heat exchangers. Finally, correlating equations of Nu and f with respect to Re are presented for 3000 ≤ Re ≤ 7250.