Mixed convection with flow reversal in the entrance region of inclined tubes

Abstract

PurposeTo determine the axial evolution of the hydrodynamic and the thermal fields for mixed convection in inclined tubes and to investigate the presence of flow reversal.Design/methodology/approachThe elliptical, coupled, steady state, three‐dimensional governing partial differential equations for heated ascending laminar mixed convection in an inclined isothermal tube were solved numerically using a finite volume staggered grid approach.FindingsThe axial evolution of the velocity profiles and fluid temperatures show that upstream diffusion has an important effect near the inlet of the heating region. As a result, both the wall shear stress and the Nusselt number are affected upstream of the heating zone. Flow reversal occurs of GF≥9 × 105. The shape and size of the region with negative velocities depends strongly on the value of the Grashof number. The effect of the Grashof number on the axial evolution of the wall shear stress and the Nusselt number is shown to be very important in the region of developing flow.Research limitations/implicationsThe results have been calculated for one Reynolds number (Re=100), a single fluid (air) and one tube inclination (45°).Practical implicationsFurther results of this type can be mapped and would be useful for heat exchanger design.Originality/valueThis is the first time that flow reversal has been calculated numerically for inclined tubes. Most previous studies concern horizontal or vertical tubes and use axially parabolic equations which are easier to solve but can not calculate the flow field in the region with backflow.