Effects of characteristic parameters on heat transfer enhancement of repeated ring-type ribs in circular tubes

Heat transfer enhancement of repeated ring-type ribs in circular tubes was experimentally investigated. Air, water and ethylene glycol-water solution (33.3% EG by vol.) were used as the working fluids. The rib height-to-tube inner diameter ratio (e/d) and rib pitch-to-tube inner diameter ratio (p/d) were arranged in the range of 0.025-0.069 and 0.29-5.8 respectively. The Reynolds number (Re) was in the range of 3601-26025 and the Prandtl number (Pr) was in the range of 0.7-15.6. Empirical equations for the Nusselt number (Nu) and Darcy friction factor (f) are proposed. The Nu value increases with the e/d value and it decreases with an increase of the p/d value. In addition, the Nu value increases with the Re value and it is proportional to the 0.45 power of the Pr value. The Nu enhancement index and mechanical energy consumption index were used to compare the heat transfer enhancing tubes to a smooth tube. At e/d⩽0.043, for achieving an effective heat transfer enhancement, the p/d value needs to be smaller than 4.35; at e/d⩾0.069, for avoiding a large pressure drop, the p/d value should be larger than 1.45.