The Research On Equivalent Heat Transfer Coefficient Of Precast Concrete Sandwich Wall

With the booming development of prefabricated buildings and the improvement of building energy conservation in China,as an important component of prefabricated building envelope,the precast concrete sandwich wall has been widely used and shows vast potentials for future development.Different from traditional cast-in-place wall,the continuity of precast concrete sandwich wall is interrupted due to the existence of connectors.The accurate evaluation on the impact of multi-point thermal bridges on thermal performance of the wall has significant influence on the development and application of precast concrete sandwich wall as well as the prefabricated buildings.The connectors and their thermal bridge effect could lead to multiple non-uniform composite areas in the precast concrete sandwich wall.Therefore,the classical thermal conductivity calculation method of composite wall could not be directly applied to the calculation of heat transfer coefficient of precast concrete sandwich wall.At present,the research on the heat transfer coefficient of precast concrete sandwich wall is mostly by the means of experimental test and numerical simulation,it is difficult to analyze the influence mechanism of physical parameters of connectors on heat transfer coefficient.Due to the variety of connectors’ material and geometry,current engineering calculation methods like insulation thermal conductivity correction method could not accurately reflect the influence of changeable connectors on the heat transfer coefficient of precast concrete sandwich wall.There is still a lack of universal model and a more accurate calculation method of heat transfer coefficient for the precast concrete sandwich wall with connectors.In this context,this paper analyzed the equivalent heat transfer coefficient of precast concrete sandwich wall by theoretical analysis,experimental test and numerical simulation,the main research contents include:(1)To describe the thermal conductivity characteristics of the non-uniform composite area generated by the connectors and their thermal bridge effect,the composite area was equivalent to a composite material,based on the proposed shape factor of connector,a general calculation method for equivalent thermal conductivity of composite area with connectors was obtained.Furthermore,a general calculation model of equivalent heat transfer coefficient of precast concrete sandwich wall based on shape factor was developed.(2)The heat transfer unit model with a connector was established.The experimental test on thermal performance of precast concrete sandwich wall was carried out by the hotbox-heat flow meter method.The accuracy of the model was verified by experimental results.Based on the model,the influence of the connector on the temperature distribution of the wall surface was analyzed.Then,the equivalent heat transfer coefficient of the wall was predicted.The accuracy of the general calculation model of equivalent heat transfer coefficient was verified by the experimental results.(3)Considering the influence of the physical parameters of connectors as well as the wall materials on the thermal bridge effect of connectors and heat transfer coefficient of precast concrete sandwich wall,based on the heat transfer unit model,the effects of the thermal conductivity,spacing and cross-sectional area of the connector,as well as the thermal conductivity of insulation and concrete,the thickness of the insulation layer on the size of the thermal bridge area and the heat transfer coefficient of the wall were simulated and analyzed.The prediction model of influence area of thermal bridge was established.(4)Based on the general calculation model of equivalent heat transfer coefficient of precast concrete sandwich wall and the building energy saving design requirements in different climate zones,the climate applicability study of precast concrete sandwich wall was carried out and the recommended parameter design ranges of connectors were analyzed,which provided a basis for the selection and optimization of connectors in engineering applications.Through the research of above contents,the main conclusions are as follows:(1)Comparing the simulation results of the wall heat transfer unit model with the measured results by experiments,the error was less than 5% and the maximum error was 2.8%,which indicated that the accuracy of established model was high.The temperature distribution of the wall surface was analyzed by using the unit model,and the size of thermal bridge influence area caused by the connectors in specimen1#-4#and 6# was obtained.(2)Based on the thermal bridge area of connector and the equivalent heat transfer coefficient calculation model,the heat transfer coefficients of the specimens1#-4# and6# were predicted.Comparing the predicted results with the measured results,the error was less than 10% and the maximum error was 5.9%.Comparing the values of current insulation correction method with measured values,the errors were between 10.2%-24.5%.It showed that the accuracy of equivalent heat transfer coefficient calculation model was high.(3)Based on the heat transfer unit model,the quantitative correlation between thermal bridge area and thermal conductivity of connector,connector spacing and crosssectional area of connector,as well as thermal conductivity of concrete,thermal conductivity of insulation and thickness of insulation layer was obtained.The prediction model of influence area of thermal bridge was established,which provided support for the equivalent heat transfer coefficient general calculation model.Comparing the equivalent heat transfer coefficient based on the prediction model with experimental values,the error was less than 5% and the maximum error was 4.7%,which indicated the accuracy of the prediction model.(4)The applicable range of general calculation model of equivalent heat transfer coefficient was obtained based on quantitative analysis and calculation,it was found that when the thermal conductivity of the connector was between 4W/(m·K)-60W/(m·K),the accuracy of equivalent heat transfer coefficient general model was high.Therefore,for the precast concrete sandwich wall with low thermal conductivity connectors(0.32W/(m·K)≤ thermal conductivity of the connector<4W/(m·K)),the equivalent heat transfer coefficient value was revised to improve the versatility and accuracy of the model.(5)Based on the equivalent heat transfer coefficient calculation general model,under the same mechanical condition,according to different thermal design zones,the recommended value ranges of thermal conductivity and cross-sectional areas of connectors under different insulation layer thicknesses were proposed.Therefore,a reference for the application strategy and optimized direction of connectors in different climate areas was provided.In this paper,the research on equivalent heat transfer coefficient of precast concrete sandwich wall was carried out,according to the propose of connector shape factor,a general calculation method for equivalent thermal conductivity of composite area with connector was established,thus,a general calculation model of equivalent heat transfer coefficient for precast concrete sandwich wall with different connectors based on shape factor was proposed,which gave full consideration to the influence of physical parameters of connectors and wall materials on heat transfer coefficient of precast concrete sandwich wall,the accuracy and universality of the heat transfer coefficient of precast concrete sandwich wall was improved.The research results will provide a method and theoretical support for the evaluation of thermal performance of precast concrete sandwich wall and the application strategy of connectors.