Heat transfer and friction characteristics of laminar flow through a circular tube with small pipe inserts

A novel type of pipe insert was developed to improve convection heat transfer in the present work. Heat transfer performance and pressure drop was numerically studied. Pipe inserts with different dimensionless spacer length (S/D = 3.33, S/D = 5, S/D = 6.67, S/D = 8.33, S/D = 10, S/D = 11.67) were investigated at the Reynolds number range of 100-1750. Liquid water was used as the working fluid. The results showed that the maximal Nusselt number was enhanced by 3.4-10.3 times as that of the smooth tube. The friction factor resulted in an increase of 5.6-13.5 times. Performance Evaluation Criterion (PEC) values were approximately 1.91-4.33. The Nusselt number increased with the decrease in dimensionless spacer length. A small spacer length resulted in a high heat transfer coefficient. However, it also brought a high flow resistance and eventually deteriorated the heat transfer performance. Therefore, a suitable dimensionless spacer length S/D = 6.67 was recommended in this paper. The effect of pipe shape (R/D = 3.33, R/D = 5, R/D = 6.67) on the thermal characteristic was also investigated in this study. It indicated that the pipe shape had a little impact on the heat transfer performance in the laminar regime. Compared with other inserts, pipe inserts can transfer more heat for the same pumping power for their structure, because their special structure make it possible for the fluid to flow from the central region to the wall region, which reconstructs the velocity profile and temperature profile in the tube and results in a high heat transfer performance.