Research On The Influence Of Wind Driven Rain On The Hygrothermal Performance Of Building Wall

Most of the building walls are made of porous media.The heat and mass transfer processes exist simultaneously in the wall.The heat and moisture transfer are coupled with each other.The mechanism of heat and moisture transfer is complex and the model is highly nonlinear.In the rainy,hot and humid south China area,wind driven rain(WDR)will affect the distribution of temperature and moisture content in the wall,resulting in problems such as moisture accumulation,condensation and mould growth in the wall.The research on the influence of wind driven rain on the hygrothermal performance of the wall is of great significance to accurately calculate the cooling and heating load transmitted through the wall,optimize the wall insulation thickness,and improve the indoor air quality problems related to the wall moisture transfer.Based on the theory of heat and mass transfer in porous media,the mathematical model of coupled heat and moisture transfer under overhygroscopic condition is established.The hygrothermal performance of building wall under the influence of wind driven rain is systematically studied.The main research contents include:(1)This paper explores the mechanism of coupled heat and moisture transfer in porous media.According to the moisture storage characteristics and moisture transfer mechanism of porous media under overhygroscopic,the driving potential of moisture transfer is expressed as the function of temperature and capillary pressure.Taking temperature and capillary pressure as driving potential,the coupled heat and moisture transfer model suitable for overhygroscopic condition is established.The established mathematical model is solved by using the partial differential equation solving tool.(2)By comparing the simulation results of the model established in this paper with the results of the EU HAMSTAD benchmark case,the experimental results of foreign climate chambers and the actual building measurement results,the accuracy of the coupled heat and moisture transfer model with capillary pressure as driving potential under overhygroscopic condition was verified.(3)The difference of heating and cooling load transmitted through the wall was calculated under three simulation conditions:moisture transfer with wind driven rain(CHM-WDR),moisture transfer without wind driven rain(CHM-NOR)and pure transient heat conduction(TH).The influence of wind driven rain on the heating and cooling energy consumption caused by heat transfer of unit area exterior wall was compared and analyzed.(4)The difference of the optimal insulation thickness of the wall was calculated under three simulation conditions:moisture transfer with wind driven rain,moisture transfer without wind driven rain and pure transient heat conduction.The influence of moisture transfer with or without wind driven rain on the optimal insulation thickness of the wall was compared and analyzed.(5)According to the coupled heat and moisture transfer model established in this paper,the risk of capillary condensation in the wall was calculated and compared to the traditional steady-state condensation method.(6)According to the coupled heat and moisture transfer model established in this paper,the risk of mold growth in the wall was calculated with and without wind driven rain.Considering the influence of wind driven rain,two models with capillary pressure and relative humidity as moisture driving potential were used to calculate the difference of mold growth index in the wall.(7)According to the coupled heat and moisture transfer model established in this paper,the risk of wood decay at the interface between the wall and pine board were calculated with and without wind driven rain.Considering the influence of wind driven rain,two models with capillary pressure and relative humidity as moisture driving potential were used to calculate the the difference of wood decay risk at the interface between wall and pine board.Through the above research,the following conclusions were concluded:(1)When the relative humidity in the wall is close to 100%,the simulation result of coupled heat and moisture transfer model with capillary pressure as moisture driving potential is more accurate than the model with relative humidity as the moisture driving potential.The coupled heat and moisture transfer model with capillary pressure as moisture driving potential established in this paper is suitable for the calculation of hygtothermal performance of porous media wall under overhygroscopic condition.(2)Compared to the pure transient heat conduction,when considering the influence of wind driven rain,the total cooling load transmitted through the east,south,west and north orientation brick walls calculated with CHM-WDR model separately increased by-2.48%,1.77%,-7.87%,and-46.08%.Compared to the pure transient heat conduction,when considering the influence of wind driven rain,the total heating load transmitted through the east,south,west and north orientation brick walls calculated with CHM-WDR model separately increased by 3.15%、3.24%、63.28%and 71.53%.(3)When considering the pure transient heat conduction,the annual heating and cooling energy consumption generated by heat transfer of the east,south,west and north orientation brick wall is 7662.3,6694.2,8443.8 and 7401.7 k W·h/m~2 respectively.When considering the influence of wind driven rain,the annual heating and cooling energy consumption generated by heat transfer of the east,south,west and north orientation brick wall separately increased by 1.53%,2.77%,33.38%and 42.48%compared to the pure transient heat conduction.(4)When considering the pure transient heat conduction,the optimal insulation thickness of the east,south,west and north brick walls calculated with TH model is 0.070,0.065,0.075and 0.070m respectively.When considering the influence of moisture transfer with wind driven rain,the optimal insulation thickness of the four orientation brick walls calculated with CHM-WDR model is 0.075,0.070,0.090 and 0.080m respectively.(5)When the influence of wind driven rain is ignored,the condensation capacity at the interface between insulation layer and mortar of the east,south,west and north orientation brick walls in summer calculated with the steady state condensation method are 0.3、0.2、0.6、0.1 kg/m~3 respectively.When considering the influence of wind driven rain,the condensation capacity at the interface between insulation layer and mortar of the four orientation brick walls in summer calculated with the capillary condensation method are 0、53.0、55.9、80.9kg/m~3 respectively.(6)When the influence of wind driven rain is ignored,the the highest mold growth index on the outer surface of brick wall,the interface between brick wall and pine board and the inner surface of brick wall are only 0.88,0.18 and 0.30.When considering the influence of wind driven rain,the highest mold growth index on the outer surface of brick wall,the interface between brick wall and pine board and the inner surface of brick wall can reach 2.38,4.10 and 0.39 respectively.(7)When considering the influence of wind driven rain,the mass loss rate of wood decay at the interface between brick wall and pine wood board reached 7.9%in two years.When the influence of wind driven rain is ignored,there is no mass loss of wood decay at the interface between brick wall and pine board.