Heat transfer enhancement in dimpled tubes

Heat transfer enhancement was investigated in a coaxial-pipe heat exchanger using dimples as the heat transfer modification on the inner tube. Tube-si de Reynolds numbers were in the range of 7.5 x 10(3)-5.2 x 10(4) for water flow. A constant annular mass flow rate was chosen to obtain the highest po ssible Reynolds number of 1.1 x 10(4). Typically, the heating water inlet t emperature was 68.1 +/- 0.1 degreesC. All six variants with inward-facing, raised dimples on the inner tube incre ased the values of heat transfer coefficient significantly above those for the smooth tube. Heat transfer enhancement ranged from 25% to 137% at const ant Reynolds number, and from 15% to 84% at constant pumping power. At a co nstant Reynolds number, the relative J factor (ratio of heat transfer coeff icient to friction factor, relative to smooth tube values), had values from 0.93 to 1.16, with four dimpled tube configurations having values larger t han unity. Despite the extremely simple design, this outperforms almost all heat transfer enhancements recommended in the literature. A correlation ba sed on the results of the present work appears to be sufficiently accurate for predicting heat transfer coefficients and friction factors for the desi gn of dimpled-tube heat exchangers. (C) 2001 Elsevier Science Ltd. All righ ts reserved.