Flow And Heat Transfer Characteristic In Air Layers And Its Application In Building Envelopes

Building envelope is the the key factor which greatly influences the energy comsumptions of HVAC systems. Thus, it’s vital to improve the heat transfer property of this envelope to reduce the thermal load of a building, and to realize building energy efficiency. Employing air layers in external enclosures of a building, contributes a lot in increasing the thermal resistence, as well as achieving passive cooling, hot air supplying, natural ventilation and fresh air distributing in envelope components. So it has been widely adopted in modern buildings. However, the theoretical basis of air layers is still scarce at present, consequently, when designing or construcing an air layer involved envelope, there is no theoretical foundation to fulfill an energy saving design, and there are not any guiding strategies to support an optimization study on the heat transfer problem.This paper concluded a Ra judgement standard for the flow pattern of natural convection in enclosed or circulating air layers with different aspect ratios, based on numerically simulating the flow and heat transfer characteristics of air layers in building envelopes. This standard provides a foundation and direction for optimizing the heat transfer of air layers.Employing the vortex-stream function method, numerical modelings were performed for flow and heat transfer conditions under different aspect ratios and Ra numbers. Analyzing the changes of flow characteristics, a Ra judgement standard, which can be used for differentiating the flow regimes of air layers, was concluded. Using this standard, the flow patterns in enclosed and circulating air layers were determined.A coupled model govering lamilar natural convection and radiation was established to study the heat transfer property of enclosed air layers. To analyze the variation tendencies of convective and radiative heat transfer, numerical simulations were carried out for air layers with different aspect ratios and different boundary conditions. Results show that the rate of radiative heat transfer accouts more than 60 percent and this rate rises along with the increase in air layer thickness. The layer height and thickness, the inclination angle, the temperature difference of the sidewalls and the surface emissivity will affect the heat transfer of enclosed air layer. Two measures can be taken to improve the thermal resistence of an enclosed air layer, to optimize the layer thickness or to reduce the surface emissivity. For air layers with the heights from 0.8m to 1.2m, the optimal layer thickness is 20 mm.To simulate the flow and heat transfer condition in circulating air layers, a turbulent natural convection and radiation combining model was built. The influences of layer height and thickness, the inclination angle and the surface flux on the velocity and temperature field, the air flowrate and the temperature rise were investigated. Results indicates that the heating effect only occour in the nearwall region. The air flowrate increases with the rise of layer height, layer thickness, heat flux and the inclination; but the temperature rise will decreas when the layer thickness and inclination angle increases. For air layers with the heights from 2m to 4m, when used for hot air supplying, the layer thickness should be less than 0.2m; for ventilation-inducing air layers, the optimal thickness is 0.6m.Based on the flow and heat transfer theory obtained, the investigation of the performances of the multilayer window, the Trombe wall and the wall solar chimney in cold regions were processed. Numerical simulations on these air layer involved envelopes were conducted for typcical indoor and outdoor conditions. The heat transfer property of multilayer window, the hot air heating capacity of Trombe wall and the ventilation capacity of wall solar chimney were discussed. The results and analyses provide design and construction guidings for these structures.Establishing the conjugate heat transfer model of building space, the heat transfer condions of enclosed and circulating air layer involved walls and indoor space were modeled. Based on the simulation results, the influence of air layers on indoor thermal environment and indoor air distribution was evaluated. Compared with a normal room, the employment of air layers not only improved the indoor temperature, but also increased the inner surface temperature of the outer wall, thus, the indoor thermal comfort level can be double upgraded.Based on experimental test and CFD numerical modeling, the performance of an especially designed solar wall heating and ventilation system for rual dwellings in cold regions was evaluated. Results prove that the three hot air supply modes of the solar wall system can help to improve the indoor temperature and to reduce the temperature fluctuation. Furthermore, a CCP partial heating program was proposed to substitute the heated kang for the purpose of reducing the heating energy comsumption. The performance of the patial heating program was also studied using the CFD technique.The outcomes of this study not only enrich the methods of thermal insulation technologies in building envelopes, but also provide a supplement for passive building energy efficiency strategies. Therefore, this study has important theoretical value and practical significance.