Single-phase forced convection heat transfer and pressure drop in circular tubes in the laminar and transitional flow regimes

Forced convection experiments are challenging to perform in smooth horizontal circular tubes, because very low heat fluxes are required to ensure that the buoyancy effects do not enhance the heat transfer coefficients. Previous work in the transitional flow regime mainly focused on mixed convection and limited work has been conducted for forced convection heat transfer. It was therefore the purpose of this study to experimentally investigate the heat transfer and pressure drop characteristics during specifically forced convection conditions.1 Vertical upward and downward flows through a smooth circular test section were considered. Experiments were conducted using water (Prandtl numbers between 3.5 and 8.1) at Reynolds numbers of 400-6000 with constant heat fluxes varying from 1 to 8 kW/m(2). Although the flow direction and heat flux had no influence in the laminar flow regime, it was found that the fully developed laminar forced convection Nusselt numbers were not constant at 4.36, but were a function of Reynolds number for Reynolds numbers higher than 1000. Therefore, a revised laminar Nusselt number correlation for smooth circular tubes was developed. The fully developed laminar forced convection friction factors were, as expected, equal to 64/Re. For both the heat transfer and pressure drop characteristics, transition occurred at the same mass flow rates for all the heat fluxes, including isothermal flow, but the critical Reynolds numbers increased with an increase in heat flux. The width of the transitional flow regime in the fully developed region remained constant for all heat fluxes.