| Glasshouse greenhouse cultivation has become an important part of agriculture production in our country. However, the cooling problem is always a trouble for it. The mechanical ventilation for this is problem with high energy loss and cooling effect is not uniform and the nature ventilation is with less energy consumption and its cooling effect is uniform. But the nature ventilation cooling was difficult to achieve great cooling effect because the traditional glass greenhouse’s nature ventilation effect was poor. Foreign researchers have developed the type greenhouse of fully open-roof,which is with excellent natural ventilation performance. The greenhouse completes the cooling in summer only with the use of natural ventilation. In recent years, our country introduced this type greenhouse.But because of the lack of the study of the greenhouse’s flow field in China’s climate and environment, the use of the fully open-roof type greenhouse in our country lacks theoretical guidance, so that the cooling performance cannot be a good play.A fully open-roof greenhouse in the Jinshan district Shanghai was treated as the test object and the CFD model was built and used to analysis the internal temperature distribution and air flow condition in the current greenhouse. The cooling improvement experiment of the greenhouse was to be done with no change the greenhouse ventilation structure and with redesign the greenhouse ventilation structure using the CFD model. Through researching CFD simulation experiment, theory support was provided for greenhouse to adapt the high solar radiation and weak wind weather in summer.By the research, the following conclusion was got in this paper :(1) when fully open-roof greenhouse and ordinary Venlo type greenhouse was almost with the same atmospheric,outdoor radiation and shading net conditions, the temperature different of fully open-roof greenhouse between indoor and outdoor was 3.6 ℃ and the temperature different of ordinary Venlo type greenhouse between indoor and outdoor was 0.9 ℃.Obviously, the cooling performance of fully open-roof greenhouse is better than ordinary type Venlo greenhouse. The difference of the cooling ability was 2.7℃ contrasted the two type greenhouse in the experiment of this research.(2) After the physical model of fully open-roof type greenhouse was simplified, the model of CFD was built and the Standard k-? turbulence model was selected to simulate the air turbulent motion in the numerical simulation method. The DO radiation model was adopted to the simulate the effect of radiation from the sun. The method converting the measured radiation was adopted to simulate the influence of shading net and dry wet curtain was set as porous media in the simulation. After comparing the measured values and measured values,it was found that the two group values was almost with the same variation trends at each height. The maximum error between the two group values was 2.4 ℃,the average error was 0.1℃and the average relative error was 2.4%.It shows that CFD model was effective.(3) The opening angle of skylight has a huge impact on the greenhouse temperature for fully open-roof greenhouse. The flow field of the greenhouse with different angle of skylight was analyzed. When the angles of skylight were 44°, 48°,52°, the temperature of the greenhouse were 42.8 ℃, 38.7 ℃, 38.4 ℃. Through analyzing the airflow field and temperature field of greenhouse with the three angle of skylight, it was found that 52 °was best for greenhouse’s flow field among the three angles. But the temperature not reached the requirements of plant growth when the greenhouse was with the this ventilation condition. Under the working condition that opening angle of skylight was 52°,the CFD simulation was used to simulate the condition that the greenhouse with adding one layer shading net more. The average temperature of the inside greenhouse was 37.6 ℃when setting the greenhouse receiving total radiation of 176.3 W/m2.(4)Because shading net cooling limits the light and should not use for a long time, cooling improvement experiment of the greenhouse was to be done with changing the greenhouse’s ventilation structure using CFD model. The height of wet curtain and fan was improved in the simulation design greenhouse. Both sides of window in the north and south wall were set. Simulated and compared the flow fields of the greenhouse with 45 °, 60 °, 75 ° three different angles of skylight, it was found that the flow field in the greenhouse was the best when the angles of skylight was 60 °. When the current condition of greenhouse’s flow field was contrasted with the improved condition of greenhouse’s flow field, it was found that the average air velocity became 0.893 m/s after the greenhouse used improved condition, it was 2 times than the greenhouse’s average air velocity when it used current condition. Greenhouse’s average temperature was 36.9 ℃ when it was used improved condition and the temperature was decreased 1.4 ℃ contrasted greenhouse under the current condition. The temperature under the improved condition reached the demands of heat resistant plants grow basically which require the greenhouse to be under 38 ℃. |
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