Single-phase mixed convective heat transfer and pressure drop in the laminar and transitional flow regimes in smooth inclined tubes heated at a constant heat flux

Abstract

Heat transfer and pressure drop experiments were conducted in the laminar and transitional flow regimes at different inclination Grashof numbers that generated different levels of buoyancy. A wide range of Grashof numbers were covered by varying both the inclination angle and heat flux. The experiments were conducted on an experimental set-up on which flow could occur through a smooth tube positioned at different inclination angles, from vertical downward (−90°) flow to vertical upward (+90°) flow. The test section had an internal diameter of 5.1 mm, length of 4.6 m and a square-edged inlet was used. The experiments were conducted at different inclination angles between Reynolds numbers of 1000 and 6000 at heat fluxes of 4–8 kW/m2. The test fluid was water and the Prandtl numbers varied from 3 to 7. It was found that an increase in the inclination angle from horizontal flow (0°) to vertical (±90°) flow, decreased the buoyancy effects which led to decreased laminar heat transfer coefficients and friction factors for both upward and downward flows. The onset of buoyancy effects was significant near the vertical inclinations and caused a rapid increase in the laminar heat transfer coefficients and friction factors when the inclination angles moved from vertical to horizontal orientations. An inclined tube Grashof number which is a function of inclination angle was defined and used to express the laminar Nusselt numbers as a forced convection part plus an enhancement component owing to mixed convection. The laminar friction factors were expressed as a function of forced convection/isothermal part multiplied by the mixed convection part. Furthermore, it was found that the critical Reynolds numbers at which transition started increased as the inclination angles increased from horizontal to vertical, while the end of transition were inclination angle independent. This caused the width of the transitional flow regime to decrease, as well as the transition gradients to increase, with increasing inclination angle at different heat fluxes. It was also found that the flow directions (upward and downward) had a negligible effect on the heat transfer coefficients and friction factors in the entire transition and quasi-turbulent regions.