Roughness induced forced convective laminar-transitional micropipe flow: Energy and exergy analysis

Abstract

Variable fluid property continuity, Navier-Stokes and energy equations are solved for roughness induced forced convective laminar-transitional flow in a micropipe. Influences of Reynolds number, heat flux and surface roughness, on the momentum-energy transport mechanisms and second-law of thermodynamics, are investigated for the ranges of Re = 1-2,000, Q = 5-100 W/m(2) and epsilon = 1-50 mu m. Numerical investigations put forward that surface roughness accelerates transition with flatter velocity profiles and increased intermittency values (gamma); such that a high roughness of epsilon = 50 mu m resulted in transitional character at Re (tra) = 450 with gamma = 0.136. Normalized friction coefficient (C(f)*) values showed augmentation with Re, as the evaluated C(f)* are 1.006, 1.028 and 1.088 for Re = 100, 500 and 1,500, respectively, at epsilon = 1 mu m, the corresponding values rise to C(f)* = 1.021, 1.116 and 1.350 at epsilon = 50 mu m. Heat transfer rates are also recorded to rise with Re and epsilon; moreover the growing influence of epsilon on Nusselt number with Re is determined by the Nu (epsilon=50 mu m)/Nu (epsilon=1 mu m) ratios of 1.086, 1.168 and 1.259 at Re = 500, 1,000 and 1,500. Thermal volumetric entropy generation ((S) over bar'''(Delta T)) values decrease with Re and epsilon in heating; however the contrary is recorded for frictional volumetric entropy generation ((S) over bar'''(Delta P) data, where the augmentations in (S) over bar'''(Delta P) are more considerable when compared with the decrease rates of (S) over bar'''(Delta T).