Enhancement of the heat transfer characteristics in a helical tube with dimples under the action of a pulsating flow

The heat transfer performance of a spiral tube is enhanced by combining pulsating flow with a dimpled wall structure. Firstly, the influence of the layout parameters of the dimples on the steady-state heat transfer characteristics of the fluid in the spiral tube, over the Reynolds number range 7000 to 11000, was studied through numerical simulation. Then, the combined heat transfer enhancement effect of pulsating flow combined with the dimpled structure on the spiral tube was analyzed, and the mechanism of enhanced heat transfer was postulated. The results show that, under steady-state conditions, the overall heat transfer performance along both the flow direction and the circumferential direction of the dimpled structure with an aspect ratio of a/b > 1 is superior to that of the dimpled structure with a/b≤1. The comprehensive heat transfer enhancement effect reaches a maximum when the circumferential layout quantity n=3 and the helical spacing angle φ=π/3. The addition of pulsating flow further enhances the turbulence intensity of the fluid near the dimples, improves the coordination between the velocity and temperature fields, and, during more than half of the pulsation period, the average Nusselt number Nu is higher than the steady-state value. Within the scope of the study, the comprehensive enhanced heat transfer effect of the helical tube is optimized when the dimensionless pulsation amplitude A is 0.25 and the dimensionless frequency Wo is 13.26, with the values of the performance evaluation criterion (PEC) in the range 1.051 to 1.079. When Re=7000, the PEC value of the dimpled structure combined with pulsating flow increased by 1.35%-2.08% compared to the value for a single pulsating flow, and by 2.19%-4.76% compared to the value for a single dimpled structure.