| Solar Greenhouse is widely used to grow winter vegetable in north of China. But, we still have to overcome some problems in present greenhouse, such as unreasonable greenhouse structure, low solar radiation utilization rate, poor insulation effect, and bad controllability of the indoor environment and so on. Poor insulation effect is widely criticized by the people. It is significant to study the reasonable greenhouse structure which is good for reducing heat loss, and it is also important to develop novel heat preservation covering materials of good performance and low cost.In this paper, effect on heat transfer coefficient with respect to the greenhouse structure (back-roof elevation angles, back-wall elevation angles, front-roof structure) is investigated by numerical simulation, with the condition of south wind and north wind discussed, the results of which show that:when back-roof elevation angles are15-30°, the average heat transfer coefficient of roof is lowest, the vortex two meters away from the vertex of greenhouse front-roof is very small whenever north wind or south wind; A violent vortex, which could intensify the mixing of fluid in main flow region and near bed region, thin the velocity boundary layer, appears at the back-roof on the condition of north wind. This vortex not only enhances the heat transfer but also raises the cover up even destroys it, it’s better to avoid it. With the increase of the back-wall elevation angles, the heat transfer coefficient of the front-roof turns down, but the heat transfer coefficient of the back-roof increases more and has a violent vortex; the back-wall elevation angles have little to do with the heat transfer coefficient of the front-roof and back-roof. So when we design the greenhouse, we’d better to choose a low back-wall elevation angle taking into the area it corers consideration. With north wind flowing, the curve shape of the front-roof impact little on the heat transfer coefficient of the front-roof, and the more full the curve is, the smaller the vortex is on the front-roof. The heat transfer coefficient decreases as the front shape turns full. When south wind flows, as the curve shape turns full, the heat transfer coefficient of the front-roof increases, but it influence more on the decrease of the back-roof. As a result, a full curve should be chosen when building the greenhouse.Taking nocturnal radiation into condition, a mathematical model was built to emphasize nocturnal heat losses which are influenced by the emissivity of out-cover and the thermal conductivity of the cover materials. The results are as follows:When the thermal conductivity is0.092W/(m-K), lowering emissivity is an effective way to reduce the total heat flux. The total heat transfer flux when the emissivity rate is0.1reduces by9.2%,17.5%,26.2%and33.5%respectively compared with the condition whose emissivity rate is0.3,0.5,0.7and0.9. The results shows that the reduction of emissivity could improve the temperature of the inside air obviously. The higher the emissivity rate is, the more percentage the radiation transfer flux in total heat transfer flux is. The radiation transfer flux whose emissivity rate is0.9accounts for53%. The influence of emissivity with different thermal conductivities is also studied, the results show that:the higher the thermal conductivity is, the better effect it is when reducing the emissivity rate. With the emissivity rate is fixed, the total heat flux turn down substantially as reducing the thermal conductivity of cover material. The total heat transfer flux when the thermal conductivity is0.05W/(m-K) reduces by26%,63%and77%compared with thermal conductivity is0.092,0.2and0.3. With the increase of the thermal conductivity, radiation heat flux increases too, but the percentage in total heat transfer flux reduces. It shows that the change of the convection heat transfer with surrounding atmosphere predominates in the thermal conductivity changed. |
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