Heat Transfer Features And Simulation Of Building Envelope

The research on heat transfer characteristics of building envelope has always been one of the important basic topics in the field of architecture,and the previous research has laid a solid theoretical foundation for solving the basic problem s in this field.In recent years,the building envelope structure has been greatly developed.Most of the new building envelope systems adopt a variety of embedded heat sources to improve thermal performance.But it is also the introduction of embedded hea t sources that makes the traditional heat transfer simulation method of building envelope structure meet bottlenecks in the process of handling the new system.Specifically,the influence of embedded heat source on non-uniform heat flow and radiation flow of enclosure structure,and the complex relationship between embedded heat source itself and the internal environment of enclosure structure are highly coupled.In view of the basic problems encountered in the development of building envelope structure,this study proposes a new model and application of new ideas and theories for heat conduction and radiation transfer of built-in heat sources in building envelope structure.The main contents are as follows:Aiming at the heat conduction problem of built-in heat source in building envelope structure,starting with solving the point heat source and surface heat source conduction problem,two kinds of analytical heat transfer models for typical problems are deduced,namely,two-dimensional and three-dimensional unsteady temperature field of flat plate under the action of heat source.The basic technical route is to deal with the differential equation of heat transfer control specially.The point heat source heat transfer model incorporates the radiation,heat co nduction and convection terms of two surfaces and the environment of the flat plate into the internal heat source terms of the differential equation,thereby reducing the original three-dimensional heat transfer problem to two-dimensional plane heat transfer problem;for the surface heat source heat transfer model,the control differential equation is decomposed into a set of non-homogeneous steady-state equations and a homogeneous unsteady equation.The original temperature field is restored by superpositi on of those solutions.In the treatment of boundary and dynamic heat sources,virtual mirror heat source method and Duhamel's theorem method are used respectively,which can easily and flexibly convert the solution on an infinite plate into the temperature field determined by unsteady point heat source or surface heat source.By applying the two-dimensional model and three-dimensional model to the thermoelectric radiation plate system and comparing the numerical results,it is found that the error of the tw o-dimensional model increases with the increase of the thickness of the plate,then decreases and finally tends to be stable.When the working current of the thermoelectric chip increases from 1A to 1.6A,the maximum temperature error can be increased from 0.403 to 0.749 ?,and the maximum surface average temperature error corresponding to the two-dimensional model can be increased from 0.458 to 0.849 ?.Aiming at the radiation problem of built-in heat source in building envelope structure,this research divides the geomet ric characteristics of various louvers into three categories,and uses mathematical induction method,Gauss integral function and radiation balance equation of louver space to determine the radiation source term of blade by ray tracing method.The transmission laws of radiation-direct radiation,direct radiation-scattering radiation and scattering-scattering radiation are solved by using matrix equation.The radiation transmission and absorption laws under arbitrary louver geometry and optical characteristics of arbitrary surface are solved in a unified analytical way.With this as the core,the solar position model,inclined radiation conversion model and single-layer glass radiation are combined.The radiation transfer model and the multi-layer translucent medium radiation transfer model are used to construct a complete simulation method for radiation transfer of complex envelope structures with embedded radiation heat sources.Applying the radiation transfer model established in this study to the analysis of radiation transfer characteristics of photovoltaic louvers,it is found that the environmental radiation parameters have a significant linear relationship with the radiation characteristics of the system.With the increase of louver inclination,the rad iation absorption rate in louver space will increase slightly,which is similar to the characteristics of ordinary louvers,but the radiation absorption rate in louver space will increase slightly.The difference lies in the "turning point" in the curve.Another new discovery is that the louver angle at the turning point is equal to the residual angle of the solar altitude angle of the incident light,i.e.90-h.In addition,the study also found that smaller solar altitude angle has a significant effect on the smoothness of the curve,which weakens with the increase of solar altitude angle.Furthermore,the theoretical models of heat conduction and radiation transfer with embedded heat sources are applied to the simulation of two kinds of new building envelope systems,namely photovoltaic thermoelectric wall and photovoltaic louver window system.For the photovoltaic thermoelectric wall system,the explicit function solution of the five-parameter model of photovoltaic cell,the state-space model of heat transfer of each layer material and the two-dimensional analytical heat transfer model with thermoelectrics as an embedded heat source are coupled,and the corresponding computer program is compiled to realize the photovoltaic-photothermal coupled transfer model of photovoltaic thermoelectric wall.The influence of air passage depth,insulation material parameters and external resistance on photovoltaic thermoelectric wall was analyzed.For photovoltaic louver window system,the five-parameter model of amorphous silicon cell is coupled with analytical solution,zonal heat transfer model,power law of air flow and photovoltaic louver radiation transfer model,and the corresponding computer program is compiled.The photovoltaic-photothermal coupling transfer model of photovoltaic louver window system is established.In addition,the daylighting performance,thermal performance and power generation performance of photovoltaic shutter windows are comprehensively analyzed and discussed.In order to further explore the energy-saving potential of the photovoltaic thermoelectric enclosure structure composed of photovoltaic thermoelectric wall and photovoltaic shutter window,the traditional glass window and mass wall are selected as the reference system according to the building energy-saving code.The typical urban climate in hot summer and cold winter areas is taken as the input parameters,and the annual cooling and heat load is taken as the evaluation index.The thermal performance evaluation and energy consumption simulation analysis of photovoltaic thermal envelope structure show that the energy saving rate of the relative reference system of photovoltaic thermal envelope structure reaches 11.1% in winter and 15.4% in summer,respectively.The annual cooling and heat load per unit area of photovoltaic thermal envelope structure system is larger than that of the reference system.The energy-saving rate of the photovoltaic thermal envelope structure in five typical cities is 28.1%-55.4%.In order to verify the accuracy of various models,a full-scale experimental platform was built,and multi-working conditions were tested.A complete experimental testing system was built,and the uncertainty of each direct measurement was analyzed.Considering that there are many models and sub-models involved in this paper,the overall idea of the experimental study is to verify the sub-models separately,and then test the photovoltaic wall and photovoltaic window system.By inputting the recorded hourly indoor and outdoor air temperatu re and solar radiation data into the corresponding model,and then comparing the simulation value with the experimental value,the accuracy of heat transfer simulation of various building envelope sub-models and system models are verified respectively.Finally,the research can provide basic theoretical support for thermal performance and energy consumption analysis of building envelope structure and provide a reference simulation module for building energy consumption simulation software.