Natural Convection In A Vertical Heated Circular Tube

A vertical circular tube is widely used in heat exchanger as a carrier of heat transfer medium.For example,the vertical circular tube is applied in the energy receiver of solar power tower.When the solar radiation gathered shines on the wall of the circular tube,heat transfer occurs between the wall of and the fluid in the tube and nature convection of the fluid results.Accordingly,it is of fundamental and practical significance to investigate natural convection and heat transfer in the vertical circular tube owing to their application.In this study,a simple scaling analysis is employed to discuss the dynamics and heat transfer of natural convection in the vertical circular tube.Under consideration are two kinds of heating conditions:isothermal heating and constant flux heating.The scaling laws of the thickness and the velocity of the thermal boundary layer in unsteady and steady stages are obtained for two kinds of heating conditions.Additionally the volume flow rate and the Nusselt number in the steady stage are gained for isothermal heating.Numerical simulation is also performed to observe the structure of the temperature and the flow for different Rayleigh numbers and dimensionless curvatures.Numerical results are used to validate the obtained scaling laws,and the scaling prediction is consistent with numerical results.Additionally,numerical results show that for the dimensionless curvature of A=1 and the Rayleigh number of Ra=102,the thermal boundary layer is unclear with a vertical stratification of the temperature in the fullydeveloped stage;for the Rayleigh number of Ra=106,a vortex structure appears at the downstream of the circular tube.In fact,the vortex structure becomes large,the symmetry is broken and heat transfer is depressed with the increase of the dimensionless curvature.Although the dynamics and heat transfer of nature convection in the circular tube for different dimensionless parameters has been discussed and quantified using a simple scaling analysis and the scaling laws have also been validated by numerical results,the further analysis and the experimental validation should be necessary.