بررسی ویژگی های جریان و انتقال حرارت مواد متخلخل کامپوزیتی و تجزیه و تحلیل عملکرد آن توسط FSP و EDEP

In this paper, the heat transfer characteristics of porous material adopted in the receiver of a concentrated solar power (CSP) with different structure parameters are numerically investigated. The commercial software FLUENT and the user defined function program (UDF) are adopted to implement the simulation. The porous material geometry is represented by periodic structures formed with packed tetrakaidecahedron. The air flow and heat transfer characteristics under the boundary conditions of constant heat flux and constant wall temperature are studied. The field synergy principle (FSP) and the entransy dissipation extremum principle (EDEP) are used to analyze the flow and heat transfer performance of the composite porous material. From the numerical results the best composite of the porous material is obtained. The effects of different boundary conditions are revealed. It is also demonstrated that the FSP and the EDEP are inherently consistent.

The 21st century is the time when the science and technology are developing rapidly and the energy crisis and the environment pollution problems have been turning into the most important affairs. All countries pay their attentions to the new energy, like the wind energy, the solar energy, the water energy, the geothermic energy, and the tide energy. In the solar power generation, the main research region is the tower solar thermal power generation. In the tower power generation system, the key equipment of heat transfer is the receiver which receives the solar energy and transmits it to the heat transfer medium. In recent years, researchers have developed many highly efficient solar receivers [1], [2], [3] and [4]. Fig. 1 is the diagram of a tower concentrated solar power (CSP) system. The heat transfer medium in the tower solar receiver is air [5], [6], [7] and [8]. Fig. 2 is the diagram of the pressurized volumetric air receivers [3]. From Fig. 2 we can find that the main heat transfer component in the air receiver is the porous material (i.e., the inlet/outlet absorber in Fig. 2). The porous material has many unique advantages, such as large surface area, low density, light weight, sound insulation, and good penetrability, hence, is widely adopted as the inter-medium of absorbing solar energy [9], [10], [11] and [12]. Full-size image (32 K) Fig. 1. Tower CSP system diagram. Figure options Full-size image (31 K) Fig. 2. Pressurized volumetric air receiver. Figure options During the working process of the air receiver, the heliostat field focuses the solar light to shoot on the interior of the air receiver and the solar energy irradiates the porous material. Then the porous material absorbs the solar energy and is heated. In the receiver, when the air flows from outside through the porous material it is heated, then the heated air flows out the receiver to produce water vapor. In the theoretical and numerical researches, the porous material structure is often simplified to ideal configuration, like a series of periodical cylinder, club, and cube [13], [14], [15] and [16]. The more complicated research models are the cube model (Dul’nev model), face center model, volume center model, Weaire–Phelan unit model, Kelvin tetrakaidecahedron model and so on [17], [18] and [19]. Wu et al. [20] used the tetrakaidecahedron model and FLUENT software to predict the convection heat transfer coefficient between the air and porous foam ceramic. From the calculation results, the relationships between the porosity, air velocity, unit size, temperature and the convection heat transfer coefficient were obtained. Petrasch et al. [21] and [22] used computed tomography (CT) method to get the true net characteristics of porous foam and numerically simulated the penetrability and interface heat transfer performance. The tetrakaidecahedron model can present the major structure characteristics of the usual porous material quite well, and can be adequately numerically simulated. So the tetrakaidecahedron model is used in this paper for the porous material study. As indicated above the porous material has an advantage of large ratio of surface area over volume, which is an important way for enhancing heat transfer. In the study of enhancement mechanism of convective heat transfer, researchers have made big progress. Guo et al. [23], [24] and [25] revealed the physical mechanism of single phase convection heat transfer and presented the field synergy principle (FSP) between velocity and temperature gradient field. According to the FSP, the intensity of fluid convective heat transfer is not only affected by the velocity and temperature gradient, but also is influenced by the synergy degree between the velocity vector and fluid temperature gradient [26]. The FSP is tested and verified via a lot of numerical calculations and experiments [27], [28], [29], [30], [31] and [32]. It can unify all existing mechanisms for enhancing single phase convective heat transfer [28]. The FSP can provide a guidance for the study of enhancing convective heat transfer. There are two irreversible processes in the convection heat transfer: momentum transfer and heat transfer. The irreversibility of the momentum transfer leads to the viscosity dissipation, and then the irreversibility of the heat transfer would bring some kind of dissipation. From the irreversibility of the thermodynamics, Bejan [33] and [34] suggested that the entropy generation is used to evaluate the irreversible performance of convection heat transfer. He pointed out that the minimum total entropy generation can be used to optimize convection heat transfer process. This is called the thermodynamic optimization. It is well-known that the entropy and entropy generation are the physical quantities that indicate the ability of transforming thermal energy to work. The minimum entropy generation is the object function of optimization which can be applied for energy conversion—from thermal energy to work. To explore the object function of optimizing heat transfer process, Guo et al. [35] presented a new physical quantity—entransy. Its physical meaning is the ability of a body to transfer its internal energy (heat) to the environment. In the heat transfer process, the energy is conserved, while the ability of transferring heat is reduced because of the thermal resistance. That is to say, there is entransy dissipation in the heat transfer process. The entransy dissipation reflects the loss of heat transfer ability caused by the irreversibility in the heat transfer process. Guo et al. proposed an entransy dissipation extremum principle (EDEP) [35]. Since then the concept entransy and the EDEP have been widely adopted to analyze heat transfer problems. Cheng et al. [36] applied the EDEP in the distribution optimization of high thermal conductivity materials in the conduction process, and they obtained the optimization result superior to that obtained from minimum entropy generation principle. Meng et al. [37] and [38] used the EDEP and the variation method to get the optimum velocity field in the laminar flow heat transfer. Wu et al. [39] presented the EDEP in the radiation heat transfer optimization, and applied it into the radiation heat transfer between two infinite flat plates in which the emissivity optimum distribution of high emissivity material was obtained for certain conditions. As indicated above this paper adopts the tetrakaidecahedron model to simulate the interior character of porous material. We use FLUENT software and the user-defined function (UDF) program to implement the simulation. Four different composite porous materials constituted with two different porosities are studied. The purpose of the study is to reveal the flow and heat transfer characteristics when air flows through the composite porous material. The SST k–ω turbulence model is adopted. Through numerical simulation, the air temperature, the wall temperature of the porous material and the local heat transfer coefficient between air flow and the porous material surface are obtained. For analyzing the flow and heat transfer characteristics in the four composite porous materials in depth, the FSP and EDEP are applied. Through the numerical simulation, the variations of the field synergy angle, Nu number, temperature difference of heat transfer, heat flux, entransy flux dissipation, and equivalent thermal resistance of heat transfer with the inlet Re number are obtained. As a result, we can obtain the best composite form which is superior to other three composites in the flow and heat transfer performance. It is also demonstrated that the FSP and the EDEP are consistent. In addition, the effect of different thermal boundary condition is studied in the flow and heat transfer performance research. The research results are of importance in the design of porous material used in the air receiver.

In this paper, the flow and heat transfer performance is researched for the four composite porous materials for the constant heat flux and constant wall temperature boundary conditions. The software of FLUENT and the UDF program are used to implement the simulation. The FSP and EDEP are used to analyze the flow and heat transfer performance. From the numerical results, following conclusions can be obtained: (1) In the four composite porous materials, the order of heat transfer intensity is: S–S > D–S > D–D > S–D. (2) In the analysis of the flow and heat transfer performance for the composite porous materials, the results analyzed from FSP and EDEP are consistent for the boundary conditions of constant heat flux and constant wall temperature. (3) For the geometric and physical model studied, the turbulent heat transfer at given wall temperature condition is better than that of the corresponding giving heat flux condition, i.e. NuT > Nuq.