Thermal and friction characteristics of a circular tube fitted with perforated hollow circular cylinder inserts

The objective of the present work is to measure the heat transfer and pressure drop in a circular tube integrated with the perforated hollow circular cylinder (PHCC) inserts with air as the working medium. The experiments are performed with PHCC inserts of three different diameter ratios (DR = 0.5, 0.65, and 0.8) and three perforation index (P1 = 8%, 16%, and 24%) in a circular tube turbulent flow regime, where the Reynolds number is varied from 6000 to 27,000. A fixed value of pitch ratio (PR = 2) is considered in all the experiments. The experimental results demonstrated that the Nusselt number (Nu) and friction factor (f) are increased with the decrease of pitch ratio. The results also reveal that the heat transfer rate in the tube fitted with PHCC inserts is significantly increased with the corresponding marginal increase in pressure drop. In the range of the present work, heat transfer rate and friction factor are obtained to be around 150-230% and 160-450% higher than those of the plain tube values, respectively. Thermal enhancement factor (TEF) in the present study is in the range of 1.21-1.47 with a maximum value for DR = 0.65 and P1 = 8%, respectively. In addition, the empirical correlations for the Nusselt number and friction factor are also developed, based on the experimental data. (C) 2017 Elsevier Ltd. All rights reserved.