Enhancement of heat transfer using elliptical twisted inner pipe with convergent conical ring turbulator for turbulent flow in double pipe heat exchanger

Abstract

This study focuses on evaluating the impact of a fully twisted inner pipe along with conical rings turbulator by conducting a numerical analysis of the double-pipe heat exchanger (DPHE), considering both parallel and counterflow configurations for heat transfer. As modification, a straight elliptical pipe compared with two other pipes with three twists and full twists along their lengths is also investigated to compute the heat transfer rate, pressure drop, and turbulence characteristics. Modelling has been performed using ANSYS Fluent, considering the RNG k-ε turbulence model. The regime of turbulent flow is studied within the range of Reynolds numbers from 5,000 to 26,000. Validation has been conducted comparing with the experimental studies, and reasonable agreement has been observed. The full twists effect generates a swirling motion, and conical rings are inserted inside the outer pipe as a passive turbulator to guide the flow towards the inner pipe, where the hot fluid passes. Comparing the results, the inner twisted pipe with the conical ring exhibits a significant improvement in the Nusselt number (Nu), reaching 445 in the counterflow direction with the six rings. The evaluation of entropy generation is described as a function of frictional and thermal contributions. According to the findings, turbulators slightly increase entropy development. Analysis of total entropy generation shows that, because of the high viscosity, frictional entropy generation is dominant. Also, when the vortex flow becomes stronger, the total entropy creation rate decreases down. The Performance Evaluation Criteria (PEC) is also greater than 1 for both parallel and counterflow flow, indicating that enhancement of the rate of heat transfer, outweighs the decrease in pressure drop. Particularly in counterflow directions the PEC is 2.3 which is impressive. Overall, full twists along the pipe lengths enhance the heat exchanger's performance, and full twists with six conical rings fortify most in both flow directions.