New friction factor and heat transfer coefficient equations for laminar forced convection of liquid with variable properties in straight tubes

Abstract

When there are large temperature differences between the wall and the fluid, the friction factors and heat transfer coefficients of forced laminar convection with variable fluid properties are different from the theoretical results deduced with constant property assumptions. However, most existing equations for variable properties are obtained by regression analysis of experimental data with a specific kind of fluid, and cannot reflect the property-temperature sensitivities at different fluid temperatures and for different kinds of liquid. In this paper, the governing equations of forced laminar convection of ethanol and water are numerically solved using CFD method and the results are used to verify the deduced equations. Compared with dynamic viscosity, the variations of density, thermal conductivity and specific heat capacity in the cross section can be neglected. A new explicit equation of friction factor and a new explicit equation of heat transfer coefficient for forced liquid laminar convection with variable properties heated in straight tubes are obtained. The deduced equations show good predictions of friction factors and Nusselt numbers for different kinds of liquid. Based on the equations, a dimensionless parameter is derived to predict the heat flux effects and viscosity-temperature sensitivities on friction factors and heat transfer coefficients.