Experimental And Numerical Studies Of Enhancing Heat Transfer In A Tube By Inserting Porous Media At The Core

Energy is one of the most important material bases for the social development and human civilization; however, it is also known that the problem of energy shortage is existed all around the world. As a fast developing country, China is facing a more significant energy constraint, which makes energy-saving one of the most urgent tasks for the country.The most important method to save energy is by designing and manufacturing all kinds of efficient heat exchange devices; this makes the development of heat transfer enhancement technology an effective way to solve the energy shortage problem. Traditionally, the technology of heat transfer enhancement is targeted to the surface of heat exchange, to increase heat transfer by destroying the heat boundary layer and increasing the disrupting of the fluid boundary layer near the wall. However, when this type of technology is effective in enhancing heat transfer, it also inevitably increases the flow resistance, which has a negative impact on the performance evaluation criteria (PEC).Different from the traditional technology that targets at the heat exchanging surface, Prof. W. Liu raised a new concept that targets at the fluid, which is enhancing heat transfer in the core flow along a tube. He believes that the core of fluid running in a confined space (i.e. a tube) is worthy to be good used for heat exchange augmentation. By homogenizing the temperature of the core flow, a huge temperature gradient is formed at the boundary layer near the exchange surface, which resulting in a significant heat transfer enhancement effect.In the application of this concept, metal porous media are considered to be an important component in the forced-convective heat transfer in a tube, for its excellent performance in temperature uniformity. This paper is developed base on both experimental and numerical studies on features of heat exchange and the air flow in a tube under the constant uniform heat flux condition, which are conducted with different types of porous inserts. Also, according to the method of field synergy of physical quantity, this paper presents an analysis of the relationships among various synergy angles in the flow. Results of the studies indicated that, outstanding integrated performance of heat transfer enhancement can be obtained by inserting porous media in the core region of a tube. These results prove the correctness of the concept of core flow heat transfer enhancement.This paper presented a numerical analysis of the convective heat transfer process when there are circular ring porous media inserted in the core of the flow. There are three types of insertion of circular ring porous media, which are:(1) insert in the core of a fully-developed laminar flow; (2) insert in the core of a developing laminar flow; (3) insert discontinuously along the stream in the core of a developing laminar flow.The results of numerical experiments show that, under the method of circular ring porous inserts, the porosityεof the porous media has a prominent impact on the integrated performance of heat transfer enhancement in a tube; the greater theεis, the better the heat transfer performance. Nevertheless, when a tube with inner diameter of 18mm and the porous insert has a fixed outer diameter Ro, the best value for the inner diameter Ri of the inserts is lmm. Further decrease of Ri has little positive or even negative impact on the integrated performance of heat transfer. Additionally, for type (3) insertion of circular ring porous media, in order to obtain the best integrated performance of heat transfer enhancement, the axial intervals Le of the circular ring porous inserts should be minimized to make porous medium arrangement more closely, thereby the attenuation of heat transfer coefficient h on the tube wall could be alleviated. When the Le is small enough, an axial incontinuous insertion can present a better integrated performance of heat transfer enhancement than a continuous insertion.Besides, for this research, an experimental system for the research of convective heat transfer of air in a tube is designed and manufactured, and the reliability of the experimental results is verified by comparing them with the empirical values of convective heat transfer in a smooth tube. Different experiments of the convective heat transfer are also conducted with three types of self-designed mash-pattern porous inserts with different porosities respectively. Furthermore, numerical simulations are also conducted as well as an analysis of the research problem based on the physical model of experimental conditions.Inserting a mesh-pattern porous medium at the core of tube is experimentally proved to be an effective way for heat transfer enhancement. The heat transfer rates of the tube with porous inserts are about 1.6-5.5 times larger than the smooth tube cases in a laminar flow when the Reynolds number ranges between 1000～19000. However, the flow resistance also increases at the same time. The experimental results of PEC indicates that the method has good integrated performance of heat transfer enhancement in laminar flow where the PEC values range from 1 to about 1.44. Meanwhile, the porosityεof the mesh-pattern porous media has significant impacts on both heat transfer and flow performance of tube; under different Reynolds number ranges, the porosity for the best heat exchange is different.By comparing with the experimental results, a numerical model is developed with a relatively high accuracy, which describes the convective heat transfer in a tube where there are mesh-pattern porous inserts in the core. The numerical results show that the application of porous inserts effectively makes the temperature profiles of fluid flow become more uniform at the core of tube, therefore formed a significant temperature gradient near the wall, which is a major reason for the enhancement of heat transfer. Moreover, the porous radius ratio Rrad should be large enough but less than 1 to obtain a good integrated performance by using porous inserts for enhancing heat transfer. As a result, heat transfer is enhanced greatly with an acceptable flow resistance increase.This paper presents the research of the heat transfer and flow performance with porous inserts from three aspects:theoretical analysis, numerical simulation and experimental research. The results proved the correctness of the core flow heat transfer enhancement concept, thereby provides references and guidance for designing new devices for heat transfer.