Research On The Effect Of Moisture Transfer On Heat Transfer Coefficient Of Building Enclosure

The heat transfer calculation of building enclosure is usually based on the heat transfer theory and mostly ignores the influence of moisture transfer on heat transfer progress. There is a deviation between the calculated heat transfer capacity by heat transfer theory and actual heat transfer capacity, which will lead indoor heat and humidity unable to meet the expected design requirement or cause energy waste. In order to facilitate the calculation of cooling-heating load and energy consumption and improve its accuracy, the influence of moisture transfer on heat transfer capacity is equal to correction factor for heat transfer coefficient in this thesis.Based on heat and mass transfer theory in the porous media, this thesis, taking the humidity transfer and the changes of humidity parameter brought by temperature and humidity into consideration, establishes coupled heat and moisture transfer equations.Through numerical analysis, the influence of different factors(such as the humidity and heat parameters of material, the humidity and temperature environment of indoor and outdoor and the structure of building enclosure)on heat transfer process under the action of moisture transfer across building enclosure is analyzed. And clay brick wall is taken as the research object to do detailed analysis. According to the research, following conclusions are achieved:(1) The correction for heat transfer coefficient rises with the rise of water vapor permeability coefficient, and decreases with the rise of thermal conductivity. When the water vapor permeability coefficient and thermal conductivity of clay brick is 2.5 times to the original parameters, the correction for heat transfer coefficient increases by 5.8%and decreases by 0.4% respectively.(2)The larger the differences between indoor and outdoor ambient temperature and humidity are, the greater the influence of moisture transfer on heat transfer is. When the temperature and humidity change cyclically, the effect of moisture transfer on heat transfer and heat transfer coefficient is huger with a higher fluctuation of temperature and humidity change. Taking the typical summer day in Wuhan as the research object,compared with the building which is part time using HVAC system, moisture transfer has a huge impact on the building which is all day using HVAC system and the correction for heat transfer coefficient of the wall is about 7.3%.(3)Internal thermal insulation is set of the wall in high-temperature and high-humidity areas, the correction for heat transfer coefficient is about 4.0% lower than that of external thermal insulation; the correction for heat transfer coefficient of external thermal insulation is set of the wall in dry and cold areas is lower than set in internal insulation. The EPS thermal insulation layer increases from 20 mm to 60 mm, the correction for heat transfer coefficient increases by 2.6%.In order to acquire the correction for heat transfer coefficient of different walls in main cities in China, the building enclosures in main cities are constructed according to the envelope thermal performance in different climate zones. Main cities in China are divided into humid, middle-humidity and dry regions according to the wet bulb temperature. Taking aerated concrete wall in typical areas as an example, the research analyzes the influence of moisture transfer on heat transfer. The correction for heat transfer coefficient for humid, middle-humidity and dry regions are 11%~18%,-44%~43% and-28%~14% respectively. In winter, the correction for heat transfer coefficient for humid, middle-humidity and dry regions are 8%~31%,-24%~5% and1%~8% respectively. The correction for heat transfer coefficient of the wall in major cities in China is given in this thesis. The research results could provide accurate calculation of practical engineering of building HVAC load and energy consumption.