Analysis Of Heat Transfer And Flow In Porous Solar Wall

The characteristics of discontinuity and disperse of solar radiation give a great difficulty in solar energy applications. The absorption and storage of solar radiation has important effect on the utilization efficiency of solar energy. In heating buildings and greenhouses, the solar wall is a key part to incorporate the utilization of solar energy with the building, and also subject studied by researchers at home and abroad. So, it is very important to study the solar wall. In this paper, actualities of investigations on solar wall at home and abroad are analyzed. A new porous solar wall is designed by using the excellences of the porous composite Trombe wall and the porous absorption solar wall. In addition, based on numerical simulation, the application and heat transfer performances of the porous solar wall are studied.For simplification, one-dimensional mathematic model is used to describe the heat transfer and flow in the porous solar wall. The flow and heat transfer in the porous wall with the air as heat transfer medium are investigated primarily. To reach such conclusions, the porous wall can collect and store solar energy, and heat air; The air temperature will increase with a decrease in the inlet velocity; On the premise of demand for solar radiation collection, the air temperature will increase with an increase in the porosity and permeability of the porous wall; There are different results when metal and nonmetal are used as the material of the porous matrix, respectively. The increase speed of the air temperature is quicker when aluminum is used as the material of the porous wall. In contrast, the increase speed of the air temperature becomes slower when rock is used as the material of the porous wall. Consequently, the permeability and material of the porous wall should be carefully selected to reduce the cost of the porous solar wall.Based on the thermal analysis, the structure of the porous solar wall is improved and a new porous solar wall is designed to reduce convection and radiation heat exchanges between the porous wall and the ambient. The glass plate is located between the porous wall and the ambient to form a glass duct, which can reduce heat losses and collect hot air by using "greenhouse effective". Additionally, the fan is located in the duct near the inside surface of the porous wall. Under the action of the fan, the ambient air flows into the porous wall and exchanges heat with the porous wall. Then the air is heated to certain value to demand for heating in winter. Based on the two-dimensional steady Navier-Stokes equations, Brinkman-Forchheimer Extended Darcy model and energy two-equation model for saturated porous medium, the flow and heat transfer in the new porous solar wall system with glass duct and fan are simulated. The results show that the design of the fan has significant influence on the flow field and temperature field, the flow resistance will decrease and the thermal efficiency of porous wall will increase with a decrease in the inlet velocity, the porous wall with the glass duct can decrease length-wave radiation and collect hot air. So the new porous solar wall system can obtain a higher thermal efficiency as compared with a conventional one.A new greenhouse with heat-storage porous wall is designed, in which the north wall is a heat-storage porous wall. Equal diameter, uniform and semitransparent porous balls are used as the material of the porous solar wall, which can collect and store solar energy to heat the air in the greenhouse, and condition the thermal environment in the greenhouse. Their effect both can be exerted fully by combining the greenhouse with the heat-storage porous wall, which causes the higher solar energy utilization efficiency. Based on Brinkman-Forchheimer Extended Darcy model and energy two-equation model with internal heat source for saturated porous medium, k-εturbulent model, the flow and heat transfer characteristics of solar greenhouse system are simulated. Additionally, the influence of the porous wall with layered porosity on the characteristics of the greenhouse is also analyzed by numerical simulation further. The results show that the inlet parameters of the greenhouse and the structure of the porous wall have great effect on the temperature field, the flow field and the pressure field. So, for a certain structure of the greenhouse, the porous wall should be designed and the fan operation should be also conditioned reasonably according to the admired thermal environment in the greenhouse.Two kinds of new solar heating systems with a new porous heat storage wall are designed, which have different ventilation patterns. Based on k-εturbulent model, Brinkman-Forchheimer Extended Darcy model and energy two-equation model for saturated porous medium, the coupled heat transfer and flow characteristics in the new solar heating system are simulated, analyzed and compared. The results show that ventilation pattern has great effect on temperature field and flow field, which also has important influence on the insulation of the heating system and the thermal efficiency of the porous heat storage wall. Thus in real applications, the solar heating system with porous heat storage wall should be designed properly in order to increase the thermal efficiency of porous solar wall and decrease the heating price for porous wall.The flow field and temperature field are available by simulating the flow and heat transfer in two kinds of porous solar wall heating system with partial and slope under floor distribution system, respectively. The effect of the structure and air supply vent dimension of the under floor distribution system on flow field and temperature field are analyzed. The effect of the south wall on the temperature in the heating room also is studied. The results show that uniform temperature field and flow field in the heating room can be ensured by using the under floor distribution system, while the slope under floor distribution is good to obtain uniform mass flow rate for every supply air vent. In real applications, the south main wall of building should be insulated to prevent the phenomenon of "heat-erode".Based on the mathematical model used to describe the structure filled with uniform and equal diameter porous balls, the structure characteristics of porous solar wall are investigated further. The results show that the flow resistance will decrease with an increase in particle diameter and porosity. The results also provide parameters for an optimization of the porous wall.The porous solar wall experiment system is also designed, which can be used to investigate such performance parameters as the flow resistance of the porous solar wallsystem, the absorptivity and volume convection heat transfer coefficient of the porous wall,etc. But simulation of solar radiation is so difficult that the porous solar wall experimentsystem should be developed further to test the thermal performance of the porous wallaccurately.