Study On The Heat And Moisture Transfer Characteristics Of A Roof-wall Corner Building Node Structure

Due to the influence of multi-dimensional effect,the thermal resistance of building thermal bridge changes significantly,which is the area with the most heat loss in the whole building structure,and its heat loss accounts for about 10% ～ 50% of the wall heat transfer.In addition to the thermal effect,in the corner area,the frequent contact of hot and cold air will affect the mass transfer.The roof-wall corner is a complex threedimensional structure and often involves complicated heat and moisture transfer process that is different from the one-dimensional wall.The isothermal / isohumid line is nonuniformly distributed on the roof-wall corner.However,there is almost no research to consider the impact of heat and moisture coupling transfer on the thermal bridge area,which will lead to deviations in thermal calculations at building nodes.Therefore,determining the multi-dimensional heat and moisture coupling migration characteristics at building nodes is the key to accurately calculating building heat transfer losses.In this study,a multi-dimensional analysis is performed for the heat and moisture transfer on the roof-wall corner.The influence of physical parameter settings on heat and moisture transfer of double beam column building nodes is analyzed.The difference between the heat and moisture transfer of the three-dimensional thermal bridge structure(roof-wall corner structure),the two-dimensional thermal bridge structure(exterior corner structure)and the one-dimensional wall(flat wall)is compared.The heat and moisture transfer characteristics of the roof-wall corner building node under the steady boundary and the periodic boundary are studied;and the influence of the distribution of steel bars inside the node on the heat and moisture state of the inner and inner surface of the node is analyzed.For the double-beam-column building node,the effects of different structural column cross-sections and beam heights on the thermal bridge’s influence area,line heat transfer coefficient and point heat transfer coefficient under the condition of coupled heat and moisture transfer are analyzed.Finally,the main cities in the country are divided into wet zones,and the heat transfer coefficients of different types of building nodes and the overall heat flow added value correction rate are calculated for the main cities in each zone.The results show that:(1)After considering the moisture transfer,setting the thermal conductivity to a constant value will cause the total heat flow through the inner surface to be underestimated by about 10.4%.And after considering that the thermal conductivity varies with the moisture content,the influence of moisture migration on the thermal bridge is increased.Setting the thermal conductivity as a function of the moisture content can more accurately calculate the heat flow through the thermal bridge.(2)For different types of roof-wall corner building nodes,the heat flow loss rates of double beam column,ring beam,and shear wall roof-wall corner building nodes are31.0%,27.0%,and 7.6%,respectively.After considering the moisture transfer,the heat flow loss rate of the double-beam-column type,ring-beam type,and shear wall type roofexterior corner building node increases,reaching 37.9%,34.5%,and 15.2%,respectively.(3)For periodic boundary conditions,after considering the moisture transfer,the attenuation factor is greater than pure heat transfer,and the lag time is less than heat transfer alone,and the existence of thermal bridges causes the deterioration of the attenuation factor and time lag to increase,which is not conducive to creating a comfortable indoor environment.(4)For building nodes with external thermal insulation,sandwich thermal insulation and internal thermal insulation,the heat loss rates when only considering heat transfer are23.4%,26.2%,15.4%,and the heat loss rates after considering moisture transfer are30.9%,34.2%,and 23.2%,respectively.Compared with pure heat transfer,the loss heat flux of each structure increases by 46.2%,46.7% and 65.7% respectively after considering the moisture transfer.(5)For the double beam column roof-wall corner hygrothermal bridge structure,considering the moisture transfer,the influence area of the thermal bridge increases about52.4% compared with only considering the heat transfer.As the cross-section size and beam height of the structural column increase,the heat loss of the node increases55%～77%、59%～63%,respectively,compared with the pure heat transfer.The increase in the size of the structural column has a greater impact on the heat loss.(6)For major cities across the country,after considering the moisture transfer,the additional heat flow correction rates for the roof-wall corner building node of the frame structure in summer and winter are about 13%～59% and 18%～32%,respectively.The additional heat flow correction rate of the roof-wall corner building node of the shear wall structure in summer and winter is about 14%～186% and about 21%～44%,respectively.Through theoretical analysis and numerical simulation research,the multidimensional effects of building nodes are analyzed,and the effect of moisture transfer on the heat transfer of roof-wall corner building nodes is obtained,and the additional heat flow correction rates of cities across the country are obtained,which provides a reference for practical engineering applications.