Analysis of fluid flow and heat transfer in an internally finned tube using a porous medium approach

Abstract

In this thesis, the fluid flow and heat transfer characteristics in tubes with various fins such as circular-sectored fins, axial internal fins, and helical internal fins are studied by modeling the finned tube as a porous medium under a steady laminar forced convection condition. For the circular-sectored finned tube, analytical solutions of both the velocity and the temperature distributions are obtained and validated by comparing these with the exact solution for velocity distributions and the numerical solutions for temperature profiles. The analytical solutions made possible by the use of the porous medium model are shown to predict the volume-averaged velocity and temperature distributions in the finned tube quite well. For the internally and helically finned tubes, the physical domain is divided into two regions separated by a circular arc of radius ri: one is the central circular region of the fluid extending to the tips of the fins and the other constitutes the remainder of the flow area. The latter region including the fins is modeled as a porous medium. Analytical models to predict thermal performance of the finned tubes are developed and validated by comparing these with the numerical solutions that are obtained from the conjugate heat transfer problem comprising both the fins and the fluid. The analytical solutions obtained by the porous medium model are shown to predict the averaged velocity and temperature distributions in the internally finned tube within 4.2% and 4.6%, respectively. And the analytical model predicts the friction factor and Nusselt numbers in the helically finned tube within 4.3% and 5.8%, respectively. In addition, the analytical solutions are helpful in identifying and studying the effects of variables of engineering importance. Based on the analytical solutions, parameters of engineering importance are identified to be the angle of the circular sector α, porosity ε, fin height ι, and effective conductivity ratio $k_{se}/k...