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-side Reynolds numbers were in the range of 7.5×103–5.2×104 for water flow. A constant annular mass flow rate was chosen to obtain the highest possible Reynolds number of 1.1×104. Typically, the heating water inlet temperature was 68.1±0.1∘C. All six variants with inward-facing, raised dimples on the inner tube increased the values of heat transfer coefficient significantly above those for the smooth tube. Heat transfer enhancement ranged from 25% to 137% at constant Reynolds number, and from 15% to 84% at constant pumping power. At a constant Reynolds number, the relative J factor (ratio of heat transfer coefficient to friction factor, relative to smooth tube values), had values from 0.93 to 1.16, with four dimpled tube configurations having values larger than unity. Despite the extremely simple design, this outperforms almost all heat transfer enhancements recommended in the literature. A correlation based on the results of the present work appears to be sufficiently accurate for predicting heat transfer coefficients and friction factors for the design of dimpled-tube heat exchangers.