Local heat transfer distribution on a flat plate impinged by a swirling jet generated by a twisted tape

An experimental investigation is conducted to study the local heat transfer distribution on a flat surface normally impinged by a swirling air jet. Twisted tapes of twist ratios equal to 2, 3.2, 4.5 and 7.5 (corresponding swirl numbers S = 0.79, 0.49, 0.35, 0.21) are inserted in a circular tube to generate swirling effect. Experiments are carried out for Reynolds number varying from 500 to 3000 for jet to plate spacing varying from 1 to 4. The local heat transfer characteristics are estimated using thermal images obtained by thermal infrared imaging technique. The jet flow profile on the target plate is evaluated by the flow visualisation carried out using lamp black technique. The heat transfer rate is found to initially increase with the increase in twist ratio (or decrease in swirl number) from 2 to 4.5 and thereafter it reduces with the increase in the twist ratio from 4.5 to 7.5. The heat transfer rate is maximum for a twist ratio of 4.5 and minimum for a twist ratio of 7.5. The jet to plate spacing also shows strong influence on the heat transfer rate. With the increase in jet to plate spacing, the heat transfer rate decreases. The maximum heat transfer rate is obtained at z/d = 1 for the different twist ratios and Reynolds number.