Enhancement of heat transfer using twisted tape insert in a plain tube

The use of insert within a tube is one of the techniques enhancing the convective heat transfer coefficient at the expense of hydraulic loss penalty. This mutual effect which is a strong function of insert geometry and flow conditions is to be analyzed in detail to improve the overall system performance. This study presents a numerical investigation for analyzing the thermo-hydraulic characteristics of twisted tape insert in a plain tube (with a 13-mm-diameter and 1200-mm-long) which is widely used in refrigeration applications where the compactness of heating/cooling element is significant. The effect of twist ratio on the thermo-hydraulic performance was conducted for the Reynolds number range of 5,025？14,871 where most of refrigeration applications are operated in. The verification of the simulation results was compared to Dittus-Boelter empirical equation (for the plain tube case) and good agreement was obtained. Results indicate that the variation of the twist ratio between 6？9 increases the friction factor and Nusselt number 197？245%, 180？220%, 163？189% and 156？179%, and 32？54%, 28？46%, 28？33% and 28？33% relative to the plain tube case, respectively for the specified operating conditions. The performance evaluation criteria, which are measure of enhancing heat transfer against hydrodynamic forces, changed between 0.90？1.02, 0.89？1.00, 0.91？0.96 and 0.92？0.96 based on the twist ratio rise. The maximum enhancement in the performance evaluation criteria was obtained at the lowest Reynolds number and twist ratio. The performance evaluation criteria lower with increasing Reynolds number but can be increased by increasing the twist ratio