Heat transfer analysis of porous media receiver with different transport and thermophysical models using mixture as feeding gas
Introduction
The greenhouse gas emission derived from the combustion of fossil fuel can be greatly reduced by the utilization of solar energy [1]. The usage of concentrated solar radiation as energy source has attracted an increasing interests worldwide [2], for example: steam generation [3], thermal power plant [4], thermochemical reaction [5] and electrical cooling [6]. Since the heat transfer surface of porous media receiver per unit of concentrated solar radiation is greatly increased [7] and porous medium can effectively damp vortex during flow [8], the application of porous media as solar receiver is an attractive option for power generation and fossil fuel reforming.
Many numerical researches have been conducted to investigate the heat transfer performance of porous media. For the correlations of fluid to solid heat transfer coefficient, Hwang, Achenbach, Dixon and Cresswell had put forward different models [9]. With the aim to study the variations of heat transfer model on temperature distribution, an investigation of variants with different transport models was performed by Alazmi and Vafai [9], in which the radiative heat transfer between the strut surfaces of solid phase is not incorporated. The numerical results indicate that velocity field and temperature distribution of porous media varies with heat transfer model change. Wu et al. had numerically investigated the convective heat transfer characteristics of air flow through ceramic foam, and proposed a local volumetric heat transfer coefficient model [10]. In 2011, Wu et al. had conducted numerical analyses to research the variation of thermal conductivity of the solid phase on temperature distribution of porous media receiver [11]. The Monte Carlo Ray Tracing (MCRT) and Finite Volume Method (FVM) coupling method were adopted by Wang et al. to solve the radiation, conduction and convection coupled heat transfer problems of porous media receiver with solar dish collector [12], [13]. A 2D computational fluid dynamic model was developed by Villafán-Vidales et al. to predict the thermal performance of thermochemical solar receiver fitted with porous medium at different operational parameters [14]. Steady state heat and mass transfer models with ignoring the radiation effects were developed by Xu et al. [15] to investigate the heat transfer characteristics of porous media receiver for solar tower power generation system. During Xu’s numerical simulation, comparisons of calculated volume convection heat transfer models with experimental data were conducted for choosing preferable heat transfer model. Chen and Liu [16] had analyzed the heat transfer performance of composite-wall porous media receiver with uniform solar irradiation at the same time point. The local thermal equilibrium model was adopted by Cheng et al. to study the coupled heat transfer performance of volumetric porous media receiver [17]. Haberman and Young [18] had conducted three-dimensional numerical simulation of the multi-species reactive gas flow in the porous media for thermochemical reaction. Parametric numerical simulations of methane reforming in porous medium were conducted By Ni [19], [20] to investigate the effects of parameters on H2 production, and the numerical results indicated that temperature distribution of porous media receiver had significantly effects on chemical reaction rates. With the aim to reveal the importance of design and operating parameters on H2 production, a fully three dimensional calculation method was developed by Yuan et al. to simulate the methane reforming reaction in porous medium [21].
For power generation, the most commonly used heat transfer medium for porous media receiver is air. However, two or more gas species are injected into porous media receiver during fossil fuel reforming. Take the methane reforming as an example, CH4 and H2O gas mixture are injected into porous media receiver as feeding gas to produce H2 [22]. Many theoretical models have been developed to predict the thermophysical characteristics of multi-species gas mixture, taking the thermal viscosity of gas mixture for example: Mass weighted mixing law [23], Wilke’s model [24].
Silicon ceramic (SiC) is chosen as the material of porous media receiver due to the advantages of light weight, high strength, large specific surface areas, high porosity and excellent thermal shock resistance performance [25]. During the process of power generation and fossil fuel reforming, the operational temperature of porous media receiver is higher than 1000 K, especially in the fluid inlet surface. The radiative heat transfer is proportionally to the fourth power of absolute temperature and plays a dominant role in the heat transfer phenomenon for porous media receiver [26]. Since the SiC porous media strongly absorbing solar radiation, porous media is large optically thickness with a short radiation transport mean free path and the optical thickness is generally larger than five [26]. The Rosseland approximation methods are commonly adopted for large optical thickness problems with less time consuming [27].
From the literature review, it can be seen that some researches have been conducted for porous media to evaluate the effects of different transport model and operational parameters, and numerous models have been put forward to predict the thermophysical characteristics of multi-species gas mixture. However, few literatures have been published on the evaluation of various transport and thermophysical models on heat transfer performance of porous media receiver under concentrated solar energy using gas mixture as feeding fluid.
In this paper, the local thermal non-equilibrium model is adopted to solve the steady state heat and mass transfer models of porous media solar receiver with CH4/H2O mixture as feeding gas. The radiative heat transfer between porous media structures is solved by Rosseland approximation. The impacts of variation in heat transfer model and thermophysical characteristics model of gas mixture on thermal performance of porous media receiver are investigated.
Section snippets
Porous media receiver description
As shown in Fig. 1, the SiC porous media receiver is placed horizontally. The front surface of porous media receiver is subjected to concentrated solar energy. In this paper, the concentrated solar energy distribution and dimensions of porous media receiver are the same as those in Ref. [14]. The concentrated solar energy is supplied by a solar parabolic furnace with 2 m diameter and 0.85 m focal length. The concentrated solar energy distribution can be presented by Gaussian distribution to
Mathematical model
The mathematical model assumes that (1) flow of gas mixture in the porous media receiver is laminar, (2) the lateral walls are well insulated (adiabatic), (3) the porous media is considered as a gray, optically thick, absorbing, emitting and isotropic scattering media with homogeneous thermo physical properties, and (4) the gas mixture flow is steady. The governing equations for the energy conservation are volume-averaged equations. As there are significant temperature differences between the
Results and discussion
The physical parameters of porous media receiver used in this study are the same as Ref. [14]: the porosity is set to be 0.90, the mean cell size is 2.0 mm, the emissivity of porous media receiver is 0.92 and the wall emissity is set to be 0.30, the density of porous media is 3200 kg/m3, the conductivity and thermal capacity of porous media are 80 W/(m2 K) and 750 J/(kg K) respectively. The Rosseland approximation is used to solve the radiative heat transfer equation. The mixing law provided by
Conclusion
The impacts of variation in transport models and thermophysical characteristics models of gas mixture on thermal performance of porous media receiver are investigated. The following conclusions have been drawn:
- (1)
The models of momentum source term for porous media receiver have significant impact on pressure drop and static pressure distribution, while the models of momentum source term have little impact on temperature distribution.
- (2)
Temperature distribution of porous media receiver varies with
Acknowledgements
This work was supported by Program for New Century Excellent Talents in University (NCET-12-0152) and the Natural Science Foundation of China (Grant No. 51106087), Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT.NSRIF.2011110, HIT.NSRIF. 2015117). A very special acknowledgement is made to Professor Meng Ni of Hong Kong Polytechnic University whose constructive suggestion has improved this paper.
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