1st and 2nd Law Characteristics in a Micropipe: Integrated Effects of Surface Roughness, Heat Flux and Reynolds Number

Abstract

A computational study of the integrated effects of surface roughness, heat flux, and Reynolds number on the 1st and 2nd law characteristics of laminar-transitional flow in a micropipe is presented. Analyses are carried by solving the variable fluid property continuity, Navier–Stokes, and energy equations for the surface roughness, heat flux, and Reynolds number ranges of 1–50 μ m, 5–100 W/m2, and 1–2000, respectively. Computations put forward that surface roughness not only accelerates transition to lower Reynolds number but also augments heat transfer rates, such that the transitional Reynolds numbers and intermittency values are evaluated as ∼ 1650, ∼ 575, and ∼ 450 and 0.132, 0.117, and 0.136 for the surface roughness cases of 1, 20, and 50 μ m, respectively. Thermocritical Reynolds numbers are identified by determining the viscous dissipation rates, which characterize the heating/cooling behavior and the related Reynolds number range. Surface roughness comes out to have no role on entropy generation at low Reynolds numbers; moreover, entropy generation is found to be inversely proportional with mean temperature variation, where the trends become almost asymptotic at the lower limit of the investigated Reynolds number range. Being independent of surface roughness, heat flux, and Reynolds number, radial irreversibility distribution ratio is determined to be negligible at the pipe centerline, indicating that the frictional entropy is minor and the major portion of the total entropy generation is thermal based.