Numerical Study Of Natural Convection Heat Transfer Based On Energy Gradient Method

Natural convection heat transfer is widely used not only in our daily life but also inengineering practice. Actually, the physical mechanism of flow instability in natural convectionis always the focus of this study. Till now, as the application of natural convection in industry,national defense, energy, environmental protection and daily life is much more extended thanbefore, lots of researchers contribute generous work into the investigation of natural convectionheat transfer, and obtain some satisfactory results. The aim of natural convection investigation isto explore the physical mechanism of flow instability. Based on this, we can realize our socialgoal to save resources and protect the environment by our subjective will. The physicalmechanism of flow instability in natural convection is still unknown neither through theexperimental study nor via the numerical simulations. Though some artificial methods areobtained to control natural convection heat transfer, the efficiency of natural convection heattransfer is not expected.This thesis applies energy gradient method into the study of natural convection in aninclined cavity, a rectangular cavity fixed with a thin fin and ribbed radiator respectively, inorder to reveal the physical mechanism of flow instability. The main research content of naturalconvection in an inclined cavity summarized as follows:(1) Energy gradient method is appliedto reveal the physical mechanism of flow instability.(2) The effect of the flow time, length of thecavity and the inclination angle of the cavity on flow instability respectively. It is observed thatthe intensity of flow instability increases with the increase of flow time, the position of flowinstability migrate to the left side with the increase of the cavity length, and the offset position offlow instability is determined by the critical inclination angle. The main research content ofnatural convection in a rectangular cavity fixed with a thin fin summarized as follows:(1)Energy gradient method is applied to reveal the physical mechanism of flow instability.(2) Theinner relationship between flow instability and heat transfer rate.(3) Analysis of the effect of finnumber, fin position and fin length on flow instability and heat transfer rate.(4) Analysis of theeffect of Ra on heat transfer and the relationship between Ra and logarithmic function ofK max.It is observed that the effect of fin number and fin position on heat transfer is negligible, and thecavity poses a more efficient heat transfer rate when the fin is fixed on the middle height of thecavity. The main research content of ribbed radiator summarized as follows:(1) Energy gradientmethod is applied to reveal the physical mechanism of flow instability.(2) Analysis of the effectof different fin height, fin space, fin structure on flow instability and heat transfer rate. It is foundthat the flow character of the base flow is commonly controlled by the fin height and the fin space, which affect the heat transfer rate of the cavity. While the effect of the fin structure onheat is negligible.Energy gradient method can reveal the physical mechanism of flow instability and can betreat as the theoretical principle to control heat transfer. The main aim of the study in this thesisis to validate the accuracy of application of energy gradient method in natural convection. Then,we can master some methods to control natural convection heat transfer and supply somemethods to control heat transfer in the fields of industry, national defense, the daily life and soon. At last, we can control heat transfer by our subjective will in order to protect resources andconserve energy.