Heat Transfer and Friction Characteristics for Turbulent Flow of Dimpled Tubes

The hydrodynamic and heat transfer characteristics of dimpled tubes with different arrangements were investigated in this paper. The dimples on the tube are outward-facing, raised, with aligned and staggered arrangements. The experiments were carried out in counter-current flow patterns with cold air along the tube inside and hot water around the outside of the tube. The Reynolds numbers of air flow were in the range of 1.6.10(4) to 5.4.10(4). By comparing the performances among three tubes (smooth tube, dimpled tube with aligned arrangement, and dimpled tube with staggered arrangement), we could find that the dimpled tubes enhanced the Nusselt number by about 18.6 similar to 22.7 %, while the friction factors of the dimpled tubes increased by 18.6 similar to 25.9 % for the aligned arrangement and by 14.3 similar to 29.8 % for the staggered arrangement. The test results also showed that different dimple arrangements have very little effect, which is always within 5 %. By the integrated performance evaluation of (Nu(a)/Nu(s))/(f(a)/f(s))(1/3), the maximum of the heat transfer enhancement with the same friction penalty, about 19 %, could be achieved by optimization of the dimpled-tube design.