Researchs On The Effects Of Moisture Content On Thermal Performance Of Walls

In the current situation of energy crisis and building energy-saving, the development and research of self thermal insulation system of external walls in "hot summer and cold winter" areas is of great significance. Most of the self thermal insulation wall materials belong to the scope of the non-saturated porous media. On the one hand, the relationship between its effective thermal conductivity and moisture content is not recognized enough in the relevant standards. On the one hand ,at present, only the absolutely dry and wet saturated are frequently mentioned in existing research, while there is still a lack of accepted experimental data and predictive models of the effective thermal conductivity of non-saturated porous materials,.Studies on effective thermal conductivity of porous building materials.In this paper, to take ceramic reinfored aerated building block, thermal insulation mortar and normal mortar for example, theoretical and experimental studies on the relationship between effective thermal conductivity and moisture content of self thermal insulation wall materials and further researchs on the effects of moisture content on thermal performance of external walls are carried out. Simultaneity, preliminary experimental study of heat consumption of mortar joints thickness are conducted.This paper extends parallel model, series model and WM model to three-phase state, derives theoretical calculation of effective thermal conductivity of non-saturated porous media and relates effective thermal conductivity in absolutely dry state with effective thermal conductivity in wet saturated state from the macro level to make it closer to engineering practice. At the same time, the measured values of effective thermal conductivity at different moisture states of three kinds of materials are compared with theoretical calculation curves of three kinds of models to analyze their respective trends and appropriate forecasting model . Exact calculation formula of effective thermal conductivity of reinfored aerated building block, thermal insulation mortar and normal mortar are fitted for the first time. Influence of moisture on stability of test data, equilibration time of the rest and the rate of water content of thermal transfer Plate are also discussed .In the theoretical calculation of heat transfer coefficient, the author try to replace general values of thermal conductivity given in the existing norms with thermal conductivity in actual moisture conditions of the materials in order to improve the defect that the material moisture content is ignored in the heat transfer coefficient calculations in the existing norms.At the same time,there is still a vision that the projection of heat transfer coefficient is viable by real-time control of moisture content changes of the materials,which is of relatively realistic significance to the detection and assessment of wall thermal performance.It is found that the measured values of heat transfer coefficient of the reinfored aerated building block walls are larger than the theoretical values through further theoretical projections and experimental studies and determined that the correction factor of the theoretical calculations averaged about 1.493. The higher the moisture content of wall materials is, the higher the heat transfer coefficient test value is.Within the scope of moisture content of the test, which is at a value of 20% or less, when the average moisture content of the wall materials increased by 1%, the values of heat transfer coefficient of wall 1 #, wall 2 #, wall 3 # and wall 4 # , respectively built by thermal insulation mortar with 10mm mortar joint thickness, thermal insulation mortar with 5mm mortar joint thickness, nomal mortar with 10mm mortar joint thickness, nomal mortar with 5mm mortar joint thickness , increase by 4.21%, 3.35%, 2.67% and 2.56% based on the the original corresponding moisture content. The greater mortar joint thickness is, the greater the growth rate of heat transfer coefficient is.Mortar joints influence coefficients of wall 1 #, wall 2 #, wall 3 # and wall 4 # are respectively 1.960,1.896,1.884 and 1.592.This study shows that moisture content of wall materials is the one of the determinants of wall heat transfer coefficient.With the moisture content increases, the wall heat transfer coefficient increasing, thus the design calculation of heat transfer coefficient should take impact of moisture content into account. At the same ,detection and assessment of wall heat transfer coefficient is also quite possibly achieved through the"bridge" of material moisture content.In self-insulation system engineering practice, the heat consumption of walls caused by water will be reduced when developing new self thermal insulation wall materials with lower moisture absorption and taking damp-proof measures to curb the moisture content growth of wall materials in engineering practice. During the promotional course of self-insulation wall materials, the use of matched thermal insulation mortar, reasonably controlling and choosing mortar joint thickness will helps to prevent heat consumption caused by thermal bridges.