Heat transfer enhancement during condensation in smooth tubes with helical wire inserts

This study investigates the heat transfer characteristics of a horizontal tube-in-tube heat exchanger with a helical wire inserted in the inner tube. The influence of the pitch (or helix angle) of the wire on the heat transfer performance and pressure drop during condensation (having all other geometric parameters the same) was investigated experimentally. Tests were conducted for condensing refrigerants R22, R134a, and R407C at an average saturation temperature of 40C, with mass fluxes ranging from 300-800 kg/m2s and with vapor qualities ranging from 0.85-0.95 at condenser inlet to 0.05-0.15 at condenser outlet. Measurements were made for three helical wire-inserted tubes with different pitches of 5, 7.77, and 11 mm. The local and average heat transfer coefficients were compared not only with the measured data of a smooth tube, but also with the results of micro-fin tubes. The tube with a helical wire pitch of 5 mm inserts was found to have the highest enhancement factor, which can be elucidated by the extension of the annular flow regime. Heat transfer coefficient correlations for helical wire inserts were deduced, and they predicted the experimental data to within 20%.