Research On The Heat Transfer Model And The Effecting Factors Of Thermal Insulation Properties On Thick Cover In Solar Greenhouse

Thermal thick cover is one of the heat preservation methods for solar greenhouse in China. The research on heat transfer properties of various materials and combination at different condition is very important to development, application and engineering design of thermal thick cover. It’s also useful for environment analyzing, simulating and controlling. The main research contents and conclusions of the paper are as follows:(1) A model of heat transfer through thermal thick cover of greenhouse was developed in this paper. The simulation of heat transfer process in the forms of radiation, heat convection and conduction were established. The heat transfer coefficient K of thermal thick cover can be calculated using the model, if the radiometric properties of the cover surface, heat conductivity coefficient of the cover materials, the geometric parameters of thick cover and greenhouse, and the environment parameters are given. The predicted K agreed with the measured values.(2) The influencing factors of K value of thermal thick cover in the study. The results show that K value of thermal thick cover is decreased as the reflectivity value of its surface increased. When the surfaces of thermal thick cover have different radiometric properties, the reflectivity value of upper surface (towards to the sky) of thermal thick cover has more significant effect on the K value than that of the under surface (towards to the ground of greenhouse). K value of thermal thick cover is decreased with the thickness of thick cover increased and the trend became weaker.K value of thermal thick cover is increased with the heat conductivity coefficient of thick cover increased and the trend became weaker also. K value of thermal thick cover is decreased with wind speed increased (neglected the heat loss of the overlap joints) when emissivity of the upper surface is high and the heat transfer coefficient is low; K value of thermal thick cover is increased with wind speed increased when emissivity of the upper surface is low and the heat transfer coefficient is high; And the trends became weaker when wind speed is more than4.0m/s. There is no significant influence on the K value, when air temperature difference between the inside and outside of greenhouse. K value of thermal thick cover is increased as the sky radiation on background temperature increased, and variation is more obviously in higher temperature.(3) Heat preservation performance of some typical thermal thick covers in Shandong was tested and analyzed in this research. The conclusions as follows:heat preservation performance of pearl is better than that of cotton blanket. The thermal thick cover with small apparent density has good heat preservation performance as the same thickness. Thickness and mass area ratio of the thick cover have negative relation with heat transfer coefficient.(4) The influence of overlap joints to thermal properties of thick cover. Test results show that thermal preservation performance of thick cover is decreased by overlap joints. Its significant when the wind to lap seam.(5) The influence of gas barrier film to thermal properties of thick cover. Test results show that there is almost no effect of gas barrier film to heat preservation performance of thick cover as thick cover with closed pore materials (such as pearl cotton), is significant influence to thick cover with connected pore materials (such as recycle cotton). (6) An empirical formula of heat transfer coefficient was promoted, based on the analysis of the relationship between factors and heat transfer coefficient. And the parameters in the formula were obtained by regression method.