| At present,there is a lack of control means for greenhouse thermal environment,which results in unreasonable construction of solar greenhouse and poor heat storage of wall.In order to change the lack of the theory of heat preservation and storage in the construction process of agricultural greenhouse,this paper proposes a U-shaped block bearing wall structure,which is based on the lightweight concrete hollow block to change its structural form,fully optimize the size and shape of the internal cavity to improve its thermal performance.In this paper,the mechanical and thermal properties of materials,blocks and walls are studied.(1)Material level: The results show that the longer the pre wetting time is,the lower the 7d strength of ceramsite concrete is,and the higher the 28 d strength is;the 7d failure interface will occur at the interface between ceramsite aggregate and cement mortar,and the 28 d failure interface will generally fracture along the ceramsite aggregate itself;the thermal conductivity and thermal storage coefficient of ceramsite concrete will increase with the increase of the degree of water saturation of ceramsite aggregate;the later stage of ceramsite pre wetting has water release Phenomenon,release water to supplement the interface between ceramsite aggregate and paste,and supply cementitious material for continuous hardening.Because of the different size of ceramsite,its internal porosity is also different,which results in the change of thermal and mechanical properties of concrete.The test shows that the correlation coefficient between the average particle size and the apparent density of concrete is 0.923,the correlation coefficient with 28 d strength is 0.981,the correlation coefficient with thermal conductivity is 0.833,and the correlation coefficient with heat transfer coefficient is 0.818,the correlation coefficient with thermal resistance is 0.973,and the correlation coefficient with thermal inertia is 0.893.The model is λ=y0+A1×e(φ-x0)/t1,and y0=0.6175,A1=-0.1557,x0=0.84868,t1=0.16685.It is convenient to calculate the concrete mix proportion directly through the thermal conductivity.(2)Block level: The results show that the larger the air layer size increases the equivalent stress of block along the height direction,the more likely the stress concentration will occur;the smaller the thickness of rib wall on both sides,The ductility of rib wall can be increased,but the compressive strength of block will be reduced;the size of air layer is related to the compressive strength value of U-shaped block,and the gradient is set according to the size between 25% and 45%.The larger the cavity rate is,the higher the reduction rate of block strength is,and the reduction coefficient is about 3/4~5/6.The simulation value of finite element steady-state heat transfer by ANSYS is close to the test value.The larger the cavity ratio of U-shaped ceramic concrete block is,the smaller the heat transfer coefficient is,the greater the heat transfer resistance is.Group k-1 has the best mechanical performance,poor thermal insulation performance,good heat storage and poor thermal stability.Group k-7 has the worst mechanical performance,good thermal insulation performance,good heat storage and good thermal stability.Considering the mechanical and thermal performance factors,the heat transfer coefficient of K-3 group is 1.021 w/(m·℃),the heat resistance is 0.979(m2·℃)/W,and the heat storage coefficient is 8.82 w/(m2·℃).In this paper,K-3 group blocks are selected for the wall layer.(3)Wall level: A set of simplified calculation method of heat transfer resistance of wall components-"calculation method of heat transfer area weight" is summarized,and taking the ceramsite concrete U-shaped block wall and the ordinary concrete hollow block wall as the research object to carry on the test verification.It is shown that the calculation results of "heat transfer area weight calculation method" are close to the simulation results and hot box test results,with an average error about 5%. |
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