Numerical Simulation Of Coupled Heat And Moisture Transfer In Porous Building Walls

In the high temperature and humidity environment,the porous media walls are easy to accumulate moisture,which will cause many problems,such as reducing the thermal performance of the wall,increasing energy consumption,reducing indoor air quality and so on.However,on the basis of existing researches,when calculating building energy consumption,designers often neglect the influence of moisture migration and accumulation on wall heat gains,which makes the calculated load deviate from the actual amount,resulting in greater energy waste.Therefore,in order to better solve the problem of heat-moisture coupling of walls,based on the latest theory of heat-moisture coupling transfer of walls,this paper establishes a dynamic mathematical model of unsteady heat-moisture coupling to simulate the heat-moisture transfer process of porous media walls,in order to meet the needs of existing projects in calculating load.In this paper,Crank-Nicholson(Weighted Implicit)scheme is chosen to discretize the governing equations.The computer program is developed using C++ as a tool.And the detailed contents of the program are given in the appendix.Firstly,based on the principle of energy conservation and mass conservation,the heat and humidity control equations are established with temperature and relative humidity as driving potentials,respectively.In this way,we avoid the discontinuity of driving potential at the interface of multi-storey building walls.The model is solved based on the finite difference method.Because of the coupling between temperature and relative humidity,the equations are highly non-linear and difficult to solve,and many coefficients of the equations contain unknown variables to be solved after discretization.In this paper,Newton iteration method is used to solve the non-linear equations.The non-linear equations with many unknowns are transformed into linear equations,which reduces the difficulty of solving.Then the column principal elimination method is used to solve the linear equations.Then,in order to verify the rationality of the model and the correctness of the program,the results of the model calculation and the output calculated by COMSOL software are compared with the HAMSTAD verification examples and Rafidiarison's experimental data sets.Compared with the HAMSTAD verification examples and four kinds of wall experimental data,the results show that the simulation results of the new heat-moisture coupling model are in good agreement with the verification examples and experimental data,which verifies the correctness of the model.In addition,for the experimental test,the calculation results of this model program are close to those of COMSOL software,which verifies the accuracy of the program.Finally,using the meteorological data of typical years as boundary conditions,the heat flux of the South vertical brick wall in Changsha,Shanghai,Wuhan and Chongqin during the cooling and heating seasons is calculated by using this model and the traditional unsteady state model without considering wet migration:The results show that ignoring of the moisture transfer,the cumulative total heat gains of Shanghai,Wuhan and Chongqing in cooling season are underestimated by 10.04%,8.69% and 9.11%,respectively.And the cumulative total heat loss in heating season are underestimated by 6.34%,6.62% and 6.13%,respectively.It is pointed out that the coupled heat and moisture transfer is not negligible for accurately calculating the annual heat flux of building walls through the enclosure structure,and the influence of moisture transfer should be fully considered in the future energy consumption performance evaluation of buildings.