Analysis Of Natural Convection Heat Transfer Enhancement With High Ra On The Outer Surface Of A Horizontal Cylinder

Natural convection heat transfer on the outer surface of a horizontal cylinder is a common phenomenon of convective heat transfer.The fluid does not depend on any external energy input,and only depends on the temperature difference between the outer surface of the cylinder and the main stream to drive the flow from the bottom to the top near the outer surface of the cylinder and bring out the heat.At present,the research on natural convection heat transfer process of horizontal cylinder is mainly focused on the case of small Rayleigh number,while there is little research on the natural convection heat transfer process on the outer surface of horizontal cylinder with high Ra,such as containment,large storage tank,oil pipeline and other large-scale cylindrical surface.In order to improve the heat transfer effect of the natural convection heat transfer process with high Ra,this paper will carry out numerical simulation research on the smooth cylinder surface and the enhanced heat transfer cylinder surface with high Ra,and analyze the natural convection heat transfer law of the cylinder surface and the enhancement ability of the enhanced heat transfer structure.This paper will use commercial CFD numerical simulation software STAR CCM+14.02 developed a numerical simulation model for natural convection heat transfer on the outer surface of small and large cylinders,including basic governing equations,turbulence models and natural convection models.The applicability of the numerical model in a large range of Ra is verified by using the common empirical formula of natural convection and the experimental data of natural convection heat transfer with high Ra on the outer surface of horizontal cylinder.On this basis,the model is used to simulate the natural convection heat transfer process on a large smooth cylinder surface,and the distribution of temperature field,velocity field and convective heat transfer coefficient along the cylinder surface is analyzed.The research determined the main factors affecting the natural convection heat transfer effect,the factors are the thickness of the temperature boundary layer and the flow velocity of the fluid near the outer surface of the cylinder.This conclusion provide guidance for the design of heat transfer enhancement structure.Based on the calculation results of natural convection process on the surface of the smooth cylinder with high Ra,five kinds of heat transfer enhancement structures,including longitudinal rib,transverse rib,spiral rib,concave and convex structure,are designed in this paper.In this paper,the influence of the basic geometric parameters of each structure on the heat transfer enhancement effect and the main ways of heat transfer enhancement is analyzed by means of numerical simulation.It is found that the enhancement effect of all kinds of heat transfer structures is achieved by the destruction of the temperature boundary layer and the mixing of the fluid near the wall.In addition,the best design scheme and the maximum heat transfer effect of all kinds of enhanced heat transfer structures are determined,and the comprehensive comparison and evaluation of the heat transfer enhancement effects of each structure are carried out.The results show that the convective heat transfer coefficient on the outer surface of the cylinder can be increased by more than 36%,and the heat transfer coefficient of the spiral fin structure with the best heat transfer enhancement effect can be increased by 77%.Besides that,the convective heat transfer coefficient of the cylinder surface can be increased by more than 50% by the structure with rough surface,and the heat transfer coefficient of the concave surface with the best heat transfer enhancement effect can be increased by 83%.