Numerical study of the effects of surface roughness on the mixed convection heat transfer of a laminar flow inside a horizontal curved dimpled tube

Abstract

The effects of surface roughness on the hydro-thermal performance of laminar mixed convection heat transfer of water have been investigated numerically. The geometry was a semicircular curved horizontal dimples tube with a fixed dimensionless radius of curvature (2R/D = 6.62) and dimensionless roughness height of e/D = 0.1. Under low values of Re (100,200,400) and Gr numbers (20,000.100000,400,000), a constant heat flux was imposed on the tube walls. The three-dimensional governing equations were discretized, using a finite volume method. Dimensionless temperature, convective heat transfer coefficient, axial dimensionless velocity contours, secondary flow vectors, and surface friction coefficient are presented and discussed. The results showed that for identical conditions, the dimpled tube has better thermal performance compared to a smooth tube, especially in low Re values. So that in the low Re values, the dimpled tube has 58.2% higher heat transfer coefficient in comparison with the smooth ones. However, this predominance in thermal characteristics is mitigated by increasing Re number. Additionally, it has been shown that the skin friction coefficient of a curved dimpled tube is less than that of a smooth one in low Reynolds values. Using Colburn j factor and friction coefficient (f), and presenting j/f versus Re shows that the dimpled tube is more efficient at Re < 200.