Experimental study of refrigerant condensation heat transfer and pressure drop in horizontal tubes with full and partial coiled wire inserts (CWIs)

Abstract

The condensation heat transfer coefficient (HTC) and pressure loss in the horizontal smooth and wire-inserted tubes were investigated experimentally. Three different coiled wire inserts (CWIs) of the full length of 1000 mm and partial lengths of 100 mm and 200 mm with 0.8 mm wire thickness and 9.5 mm pitch were studied. The main condenser was a heat exchanger of double pipe type with an 8 mm inner diameter for the refrigerant and an active length of 1 m cooled by water circulating in the annulus. The experiments were conducted for different refrigerant mass velocities within 90–400 kg/m2s and low vapor mass fraction (x < 0.3). The findings revealed that using 1000 mm (full-length), 200 mm, and 100 mm length CWIs improved the HTC by 95%, 40%, and 18%, respectively, over the smooth tube. The HTC per unit pressure drop was also evaluated to assess the performance of full and partial CWIs. The results showed that the HTC per unit pressure loss decreases for all tubes as the mass velocity increases. In addition, the performance of the tubes with CWIs approached that of the smooth tube at higher mass velocities. The ratios of HTC per unit pressure drop at higher mass fluxes for full-length, 200 mm, and 100 mm length CWIs to that of the smooth tube were about 0.4, 0.78, and 0.9, respectively. Although the pressure drop increased almost linearly with the partial CWI length, the HTC increased sharply for the small inserts and gradually for the larger insert lengths. The results obtained for the condensation HTC in smooth and full wire-inserted tubes were also compared with the estimations of well-known correlations in the literature. Furthermore, three new correlations were developed to predict condensation HTC in smooth tubes and tubes with full and partial CWIs. It was shown that the new correlations are more accurate than the available ones.