Investigation Of Fluid Flow,heat Transfer And Variable Property Effects In Helical Tubes

Helical tubes are widely used in nuclear engineering and petroleum industries due to advantages of compactness and high heat transfer coefficients.Helical coil steam generators are used in the High Temperature Reactor-Pebble Module(HTR-PM)designed by Tsinghua University.Due to the large temperature differences between the hot and cold fluids in the steam generators,variable property effects on the flow and heat transfer processes should be considered.Analysis of the flow and heat transfer in helical tubes and the variable property effects are important for improving the design accuracy of helical coil steam generators.Based on the boundary layer theory,new friction factor equation for turbulent flow in rough helical tubes is developed by using order-of-magnitude analysis.The constants in the equation are determined by regression analysis of experimental data.The equation can predict the friction factor in rough helical tubes precisely.The friction factors in rough helical tubes can be divied into transitionaly rough regime and fully rough regime,which is dominated by the roughnesses.Rougness effects on friction factors increase with the increasing of the curvature ratio and Reynolds number.The relationship between the reciropal of dynamic viscosity and temperature is approximated by first order Taylor seriers.New explicit friction factor and Nusselt number equations are obtained for laminar convection of liquids with variable properties in straight tubes.New friction factor and Nusselt number equations are also obtained for laminar convection with variable properties in helical tubes through analyzing the relationship of property variation effects in straight tubes and that in helical tubes.The equations can be used for both heating and cooling conditions and large temperature differences between the wall and the fluid.Based on the derived equations,a dimensionless number is defined to evaluate the property variation effects.Based on the classical turbulent boundary layer theory,viscosity variation effects on turbulent flow are simplified to the influences on the critical point between the linear and the logarithmic distribution regions of the dimensionless velocity and temperature.New friction factor and Nusselt number equations are obtained for turbulent convection of liquids with variable properties in straight tubes.The new equations show accurate predictions of experimental data for both heating and cooling conditions and for different kinds of liquids.Based on the derived equations,new friction factor and Nusselt number equations for turbulent convection with variable properties in helical tubes are obtained by modifying the property variation effects.Turbulent convection of supercritical water in helical tubes with different boundary conditions are simulated to investigate the effects of buoyancy forces and centrifugal forces on the flow and heat transfer characteristics.Compared with constant property water flow,centrifugal forces near the wall decrease due to lower density of supercritical water,which results in stronger secondary flow intensities in the cross section.Due to large buoyancy forces,the secondary flow intensities near the bottom wall region are higher than that near the top wall region.When the bulk temperature is near the pseudocritical temperature,the temperature distributions in the cross section are flattened and the non-uniformity of the wall temperature distribution decrease due to the higher effective thermal conductivities.