Study On Heat And Mass Transfer In Porous Media With Non-uniform Heat Flux

Due to its special characteristics such as thermostability,antioxidation,high specific surface and high thermal conductivity,porous media is ideal material for heat transfer enhancement in solar cavity receivers.Porous media can be use d in solar thermal high temperature thermochemical conversion process reactors and volumetric heat sinks of solar thermal power generation systems.Different from the traditional given temperature and heat flux boundary condition,in this contribution,we apply the Gaussian Distribution Model(GDM)as the heat flux boundary to simulate the methane steam reforming for hydrogen production in the solar dish receiver-reactor.Furthermore,we propose two new structures of receiver-reactor,which are the fractal tree-like structure and the swirl channel structure,respectively.We analyze the effects of structure,numbers and layers of baffle plate,and radian and numbers of blades of the receiver-reactor on maximum radial temperature difference and methane conversion rate.The numerical results show that the structure of the reactor has a significant influence on the radial temperature difference and methane conversion rate.When the inlet velocity remains constant,the swirl channel structure has the highest methane conversion rate,the fractal tree-like structure has the second highest methane conversion rate and the traditional conical cavity receiver-reactor has the lowest methane conversion rate,which are 79%,85.6%,85.9%,respectively.In addition,the traditional conical cavity receiver-reactor has the largest radial temperature difference,i.e.,182.53 K,the maximum radial temperature difference of fractal tree-like structure is 168.61 K,and the swirl channel structure has lowest radial temperature difference,i.e.,167.72 K.Based on aforementioned discussion,we analyze the effects of structural parameters on chemical performance of fractal tree-like structure and swirl channel structure receiver-reactor.The results show that there are no obvious effects on radial temperature distribution and methane conversion rate through increasing the numbers and layers of baffle plate and radian and numbers of blades of fractal tree-like structure receiver-reactor.Besides,there are no obvious effects on radial temperature distribution and methane conversion rate through increasing the radian and numbers of blades of the swirl channel structure receiver-reactor.It also shows that structural type of receiver-reactor is the most important effect factors on maximum radial temperature difference and chemical reaction performance.Thereinto,the new type of receiver-reactor with swirl structure can enhance the fluid disturbance and improve the heat and mass performance and reduce the radial temperature difference to maintain the activity of the catalyst and improve the reliability of dish solar receiver-reactor.Porous media can also be used in volumetric solar energy absorbers.Most scholars at home and abroad have studied the effects of porosity,pore size and Reynolds number on flow and heat transfer in porous media.Few studies have been done on the performance of heat absorbers in porous media with non-uniform porosity distribution.Therefore,this paper innovatively designs a variety of porous media heat absorbers with non-uniform porosity distribution.The Gauss distribution model is adopted,and the radiation transport equation in porous media is considered.Combining with the theory of porous media flow and heat transfer,based on the assumption of local non-thermal equilibrium(porous media temperature and fluid temperature are different),a correlative model of heat and mass transfer in porous media is established.Three-dimensional numerical simulation of heat and mass transfer in porous media heat absorber with non-uniform porosity distribution is carried out.The effects of flow velocity,inlet temperature of fluid,non-uniform porosity distribution and thermal conductivity of porous media on the heat absorber performance of porous media are studied and analyzed.The simulation results show that the outlet temperature of the heat absorber with convex quadratic function distribution is the highest among the designed non-uniform porosity distribution types,which is 10.07% higher than that of the heat absorber with uni form porosity distribution.The analysis shows that the high porosity at the entrance leads to more solar radiation entering the porous media,which reduces the reflection of solar radiation and the heat loss at the entrance.The outlet fluid temperature o f concave quadratic function distribution heat absorber is higher than that of linear distribution heat absorber,increasing by 3.2%.Because the porosity of the concave quadratic function distribution heat absorber at the outlet is small,the heat transfe r coefficient can be improved by reducing the porosity of the porous medium according to heat transfer coefficient formula,and the heat transfer performance of the heat absorber with concave quadratic function distribution is better at this time.