LAMINAR FLOW HEAT TRANSFER IN INTERNALLY FINNED TUBES WITH TWISTED-TAPE INSERTS
Heat transfer and pressure drop measurements for laminar flow of oil were made on smooth and internally finned aluminium tubes with twisted-tape inserts. Two oils, SAE 10 and SAE 20 were chosen as heat transfer media, giving a Prandtl number range of 114 to 490 and Reynolds numbers based on the hydraulic diameter, between 50 and 2000. Cases for both cooling and heating the oil at a uniform wall temperature were investigated. A general expression for the Nusselt number in terms of the tape twist ratio, Reynolds number, Prandtl number and viscosity ratio is presented. Heat transfer coefficients in finned tubes with tape inserts were found to be as much as four times the smooth tube values when cooling at the same pumping power and three times in the case of heating. Preliminary numerical calculations are found to be in agreement with experimental results for a smooth tube with a flat tape insert at a constant wall temperature.