Effect of Variation of the Aspect Ratio of Rectangular Twisted Tapes Inserted in a Circular Pipe on the Thermal Performance

Abstract

This study aims to determine the effect of the aspect ratio of the cross section of a rectangular twisted tape on the laminar heat transfer characteristics inside an isothermally heated circular pipe. Detailed three-dimensional simulations reveal facts that would be difficult to estimate otherwise. A novel study of the evolution of the axial and secondary velocity fields as well as the temperature field within the pipe is presented to understand the flow physics behind the heat transfer enhancement. The three-dimensional simulation results are used to accurately calculate the portion of the total energy of the flow that is converted to secondary kinetic energy. In addition to the Nusselt number and friction factor, three parameters, viz. relative increase in heat transfer coefficient, relative increase in pressure drop, and thermo-hydraulic performance factor are used to quantify the effect of the twisted tape. While both the heat transfer and the pressure drop increase with a decrease in the twist ratio (TR), the thermo-hydraulic performance factor (THPF) is found to become almost independent of TR as the aspect ratio (AR) decreases to low values. It is inferred from the present results that although a twisted tape with a small pitch and large aspect ratio enhances heat transfer performance, a lower aspect ratio may provide a better option from a power requirement perspective. Hence, this work shows, for the first time, that a square-like cross section of the twisted tape could be advantageous.