تجزیه و تحلیل عملکرد حرارتی از گیرنده های متخلخل با تابش خورشیدی متمرکز

The distribution of concentrated solar irradiation has a significantly impact on the temperature distribution of porous media receiver. The thermal performance of porous media receiver is investigated by combining the Monte Carlo Ray Tracing (MCRT) method and FLUENT software with user defined functions (UDFs). The MCRT method is used to obtain the heat flux distribution on the fluid inlet surface of porous media receiver. The calculated heat flux distribution is treated as the wall heat flux boundary condition of thermal performance analysis. The local non-equilibrium thermal equation (LNTE) model with Rosseland approximation is used to investigate the temperature distributions. Typical influences of the heat flux boundary condition, radiation heat loss, porosity, emissivity, flow mass and average particle diameter on the temperature distributions are investigated.

Concentrated solar thermal utilization which supplies high temperature air is a promising approach for power generation [1]. Compared to photovoltaic, concentrating solar energy onto a receiver allows cost reduction of electricity generation as the reduction requirement of materials for reflectors [2]. Since the heat transfer surface of porous media per unit volume is greatly increased compared with tube receiver [3] and the porous media can effectively damp vortex during flow [4], the utilization of porous media as solar receiver has attached much attention for research and development [5], [6] and [7]. Technology achievements of porous media receiver in solar power generation have enabled the concentrated solar flux to 1 MW/m2, with reduced weight and size of receivers, shortened startup and increased efficiency [8]. Paal et al. have developed a heat transfer performance analytical approach for volumetric porous media receiver, which has taken into the consideration of three-dimensional irradiation distribution and its influence on fluid flow [9]. The numerical results show that temperature distributions of volumetric porous media receiver are strongly influenced by solar irradiation distribution. A 1 kW thermochemical solar receiver fitted with porous foam is studied numerically by Villafán-Vidales et al. to predict the thermal transfer performance, and a Gaussian flux density distribution with 1.4 MW/m2 peak power is adopted as the concentrated solar energy [10]. The radiation heat transfer plays a dominant role in the heat transfer when the porous is placed in a high temperature environment [11]. Wu et al. have simulated the temperature distribution of the fluid and solid phase in porous media receiver using LNTE model with a Gaussian solar flux distribution boundary condition, and the radiation transport in the porous media was modeled with P1 approximation [12] and [13]. Natural convection boundary layer flow analysis of porous media receiver due to collimated beam solar irradiation is investigated by Chamkha et al. [14]. Numerical simulations of composite-wall solar collector system with porous media receiver are conducted by Chen and Liu [15] to analyze the heat transfer performance of porous media receiver. During their simulations, the heated surfaces are subjected to a uniform solar irradiation at the same time point. The numerical heat and mass transfer analysis of porous media receiver with preferable volume convection heat transfer coefficient is conducted by Xu et al. [16]. During the analysis, concentrated solar irradiation heat flux distribution on the receiver surface is appointed to be a function of temperature gradient. Badruddin et al. have conducted the heat transfer analysis of a saturated porous media enclosed in a square cavity using the LNTE model, the governing partial differential equations are non-dimensional and solved by finite element method [17]. The literature survey shows that the distribution of concentrated solar irradiation has a significantly impact on the temperature distribution of porous media receiver. However, many researchers treat the distribution of concentrated solar irradiation as simplified heat flux distribution boundary condition during the thermal performance analysis of porous media receiver, and these treatments of boundary condition can not accurately reflect the temperature distribution. In this study, the thermal performance analysis of porous media receiver which is installed on the focus plane of solar dish collector is conducted by combining the MCRT method and FLUENT software with UDFs. The MCRT method is adopted to obtain the heat flux distribution and used as the wall heat flux boundary condition of thermal performance analysis. The commercial software FLUENT was used to solve the mass, momentum and energy conservation equations in the fluid phase. For the solid phase, the energy equation and radiative heat transfer were computed using the UDFs. Typical influences of the heat flux boundary condition, radiation heat loss, porosity, emissivity, flow mass and average particle diameter on the temperature distributions were investigated.

The thermal performance of porous media receiver was investigated by combining the MCRT method and FLUENT software with UDFs. The MCRT method was used to obtain the heat flux distribution on the fluid inlet surface of porous media receiver. The LNTE model with Rosseland approximation was used to investigate the temperature distributions. The following conclusions have been drawn: 1. Heat flux distribution has a strong impact on the temperature distribution. The maximum temperature for non-uniform heat flux distribution boundary condition is 1372 K, and the maximum temperature for uniform heat flux distribution boundary condition is 1287 K. 2. The radiation heat loss on the fluid inlet surface cannot be neglected during the thermal performance analysis of porous media receiver. 3. The maximum temperature of solid phase and thickness of thermal non-equilibrium region increases with porosity increasing. 4. The maximum temperature of solid phase and equilibrium temperature decreases with the increasing of flow mass. The temperature differences between the solid phase and fluid phase increases on the thermal non-equilibrium region with the increasing of flow mass. 5. The temperature of solid phase decreases with the emissivity increasing. The maximum temperature of the solid phase increases with the particle diameter increasing.