Study On The Air Distribution And Optimization Of Mechanical Ventilation In Two Typical Greenhouses

Ventilation is an important means to regulate the environment of greenhouses and to create a suitable microclimate for plants.It not only affects the heat balance in the greenhouse,but also affects the relative humidity and CO₂ concentration in the greenhouse.A reasonable ventilation plan should provide a suitable growing environment for plants in the greenhouse on the basis of less energy consumption.At present,the most widely used greenhouses in China mainly include arched greenhouses（including single arch greenhouse and multi-span greenhouse）and single slope greenhouse.Among them,the multi-span greenhouse is composed of a plurality of single arch greenhouse,which is a special form of single-arch greenhouse.In this paper,the study of ventilation is only discussed for single-arch greenhouse and single-slop greenhouse.This paper selects the"Chinese jujube town"Da Li winter jujube arched greenhouse and the"Chinese vegetable town"Shou Guang single slope（winter warm type）vegetable greenhouse as an example to carry out greenhouse ventilation research,through field test and CFD numerical simulation to study the microclimate distribution and air distribution characteristics of mechanical ventilation in the greenhouse.The specific contents are as follows:In summer,the microclimate environment of winter jujube greenhouse was tested,including temperature,relative humidity,solar radiation intensity,ambient wind speed,ventilation outlet wind speed and ground temperature inside and outside the greenhouse,and the test results were analyzed.The test found that the average temperature in the greenhouse during the day exceeded 45℃,the ground temperature exceeded 50°C,there were"burning"fruits and fruit trees.Based on the test data of winter jujube greenhouse,the mechanical ventilation research on the winter jujube greenhouse was studied.The main contents of the research were as follows:According to the structure and planting characteristics of the winter jujube greenhouse,the ventilation modes of the air supply upper and lower ventilation of the fiber duct was proposed.The advantages and disadvantages of the two ventilation modes were analyzed by comparison with the test.In addition,the different outlet forms of fiber duct air supply were compared and studied,and the influence of the position of the outlet on the flow field is analyzed.The study found that both the upper and lower ventilation of the fibre duct had a cooling effect of 6⁷℃in the direction of the length of the greenhouse compared with the natural ventilation.The winter warm vegetable greenhouse belongs to a single slope greenhouse,and its structural feature is that the back wall is vertical.According to this structural feature,the Air Curtain Ventilation（ACV）is used.First,the influence of the position of the outlet on the flow field in the greenhouse is discussed.Based on the fact that plants grow continuously in the greenhouse,in order to send the outside air directly into the plant growing area,the technology of vertical wall attached to the air supply plant working area is put forward.Through the study of three different diversion plate heights,it is found that the flow field is relatively good when the diversion plate height is 1.0 m in the greenhouse;through the study of three different diversion plate widths,it is found that the flow field effect is the best when the diversion plate width is 0.2 m.By studying the influence of plant planting direction on the flow field in greenhouse,it was found that the flow field parallel to the flow field in greenhouse was better.By studying the effects of four different plant heights on the flow field in greenhouse,it is found that with the increase of plant heights,it is necessary to adjust the height of guide plate.By studying the influence of different air supply outlets on the flow field of greenhouse,it is found that there is no obvious difference between the continuous air outlet and the interval air outlet in the flow field,but considering the fabrication and application of the air duct,the interval air outlet will be better.By studying the influence of three different inclination of the back wall on the flow field of greenhouse,it is found that when the inclination angle of the back wall isα=5°,the wind speed was higher and the temperature was lower in the greenhouse plant growing area.Finally,the traditional mechanical side air exhaust and mechanical rear air exhaust are studied,and the ventilation effect of the two ventilation systems is compared with AVC.It was found that in the aspect of air distribution efficiency,the rear exhaust was the best in plant growth area,while the side exhaust was the worst.In the aspect of flow field uniformity,the velocity uniformity of vertical wall attached air supply was the best,and the velocity uniformity of lateral air exhaust was the worst.In the aspect of velocity targeted value,the velocity targeted value of vertical wall attached ventilation was the smallest,while the lateral air exhaust was the largest.In the aspect of air age,the air age of rear exhaust air was the smallest in plant growth area,while that of side exhaust air is the largest.