| Solar greenhouse is a special type of facility for winter vegetable production in the north of China. The greenhouse air temperature, relative humidity, soil temperature and other environmental conditions are the key factors that affect the growth of crops. The 3-D numerical simulation was utilised based on the experimental observation of the solar greenhouse with tomatoes in Jinan. The spatial and temporal variation characteristics of climate factors such as temperature and humidity in greenhouse were simulated and analyzed using the computational fluid dynamics(CFD) numerical techniques, put forward the control strategy of the greenhouse environment under different weather conditions, and provide the theoretical basis for the solar greenhouse production management.1. Contrast of meteorological elements inside and outside the greenhouseBased on the inside and outside of the greenhouse air temperature, air relative humidity, soil temperature, wind speed and wind direction observation data and analysis show that the greenhouse and dominant wind direction is northeast wind, the wind speed between the 0.5-4.1 m/s; winter greenhouse air temperature inside and outside the difference 6.5-15℃, humidity is the smallest difference of 5%-10%;The temperature and humidity in the greenhouse had some heterogeneity in both horizontal and vertical directions, and the middle and lower layers of the greenhouse was lower, and the humidity was higher; the soil temperature increases with the depth of wave attenuation, and tends to be stable.2. Construction and validation of CFD modelBased on the tested parameters of greenhouse structure, combined with experimental data and construct the boundary conditions, using standard κ-ε turbulence model, CFD simulation model is constructed through solving continuity equation, momentum equation and energy equation. The temperature and humidity were simulated by the model, and the simulation results show that the average relative error is 5.6%, relative humidity simulated value and measured value of average relative error is 7.1%.The model can correctly reflect the changes of micro-environment in sunlight greenhouse. The model can provide technical support for the further study of the influence of different ventilation conditions on the micro-climate in the greenhouse.3. Effects of crops on natural ventilationWith the outside wind direction as the northeast wind, the wind speed is 2.0m/s as an example, the simulation of crop and non crop cultivation in greenhouse was simulated. Compared with the no crop planting condition, when the crops are planted. In terms of wind speed, the vertical direction of wind velocity gradient in three vertical sections of greenhouse, the western wind speed decreases 0.1-0.2m/s, the central wind speed decreases 0-0.2m/s, the eastern wind speed decreases 0-0.2m/s; the difference between the north and the south wind speed on the horizontal plane from the ground 0.5m is increased by 0.2m/s.In terms of temperature, the vertical direction of temperature gradient in three vertical sections of greenhouse, the western temperature increases 2-4℃,the central temperature increases 3-8℃, the eastern temperature increases 2℃;The temperature on the south side of the 0.5m horizontal plane is higher than that of the ground, reaching more than 30℃,The maximum temperature difference on the whole level is reduced by 5℃,temperature difference between the central and western North and South increased by 3℃,the eastern temperature rises 0-3℃.In terms of humidity, similar to temperature distribution, the vertical direction of humidity gradient in three vertical sections of greenhouse, In the western part of the greenhouse, the humidity is reduced by 4%, the middle humidity is reduced by 6%, and the eastern humidity is reduced by 5%;From the ground 0.5m level of the south side of the relatively low humidity, for about 48%,the central and western North and South humidity difference increased by 5%, the eastern humidity decreased by 3%. Crops have a certain effect on the airflow in the greenhouse, which can not be ignored in the study.4. Effect of wind speed on natural ventilationWhen the wind is the north and the wind speed are 0.5,1.0,2.0 and 3.0m/s, the effects of natural ventilation on microclimate in greenhouse were simulated, the results show that the wind speed increases slowly with the increase of wind speed, the external wind speed increases by 1.0m/s, The air inlet of the three vertical profiles in the greenhouse and the wind speed in the middle of the greenhouse increased by 0.1 m/s, the variation of the wind speed range in the East and west of the greenhouse is not large, the wind speed of the crop canopy is increased by 0.1m/s, and the wind speed range of the other crops is not changed.In terms of the temperature, temperature gradient in the three vertical sections of the greenhouse in the direction of the north and south under the different wind speed, the temperature distribution of the east and the west is more uniform, when the outside wind speed increases by 1.0m/s,the temperature of the air inlet is reduced by 2℃, and the temperature of the East and the west of the greenhouse is reduced by about 1℃, and the temperature difference between the north and the south of the greenhouse is increased by 1℃, and the temperature of the crop canopy air inlet is reduced by 2℃, and the temperature of the other crop canopy is reduced by about 1℃.In terms of humidity, the humidity gradient in the three vertical sections of the greenhouse in the direction of the north and south under the different wind speed, when the outside wind speed increases by 1.0m/s, the humidity difference between the north and south of the weathern greenhouse is reduced by 2%,the north and south humidity difference is unchanged in the middle eastern, the humidity of the whole greenhouse had no regular change; the humidity of the crop canopy decreased slowly and there was no obvious change. The change of wind speed has an important influence on the wind speed and the temperature and humidity.5. The impact of wind directionIn the external wind speed 2.0m/s, the wind direction is set to the north, northeast, east, The effects of natural ventilation on microclimate in greenhouse were simulated, the results show that the wind speed of three vertical profiles is basically maintained between 0.1-0.4m/s, The central, western and central regions of the greenhouse is the wind inlet region when the wind direction is the north wind, the northeast wind and the east wind respectively, thus forming the vortex, there is a vertical direction of wind speed gradient in other areas; the wind speed of the crop canopy is 0.45m/s,0.65m/s,0.50m/s when the wind direction is the north wind, the northeast wind, the other crop canopy areas are maintained between 0.05-0.2m/s.In terms of the temperature, the temperature gradient exists in the central, central and western, western regions of the three vertical profiles in the greenhouse when the wind direction is the north wind, the northeast wind and the east wind, the temperature difference between the north and the south is 5℃,3℃ and 4℃ respectively; the temperature difference between the crop canopy and the wind outlet is 8,3 and 5 respectively when the wind direction is the north wind, the northeast wind and the east wind, when the wind direction is the northeast wind, the temperature difference between the whole and the crop canopy is small, which is favorable for the growth of the crops.In terms of humidity, the humidity gradient exists in the central, central and western, western regions of the three vertical profiles in the greenhouse when the wind direction is the north wind, the northeast wind and the east wind,the humidity difference between the north and the south is about 8%,6%,3% respectively; the relative humidity difference between the wind and the wind area was 8%,3% and 2%, respectively. When the wind direction is east wind, the difference between the whole and the crop canopy is small, which is favorable to the growth of the crop. Therefore, the greenhouse microclimate control strategy should be adjusted according to the specific direction.6. Solar Greenhouse Control StrategyTaking Tomato in greenhouse as an example, according to the flower and fruit of suitable growth temperature 20-25℃ index, CFD simulation method was used to study the average wind speed of 7 dominant wind directions in Ji’nan area and the temperature of the suitable growth temperature of the crops under different wind speeds, the ventilation time were 7-22min, 9-21min,7-20min,12-21min,6-16min,4-23min,8-19min which can reach the appropriate temperature of crops when the wind direction is NE、ENE、E、ESE、SE、SSW and SW, when the wind direction is NE and ENE, E, ESE, SE, SSW and SW, and the first and second grade wind ventilation time were 23-26 min,21-24min,21-24min,18-21min,15-18min,24-28min, 15-18min. The third and forth grade wind ventilation time were 12-18min,10-16min,10-16min, 8-14min,9-12min,18-21 min,10-13min can reach the appropriate temperature of crops. When the average temperature of the indoor temperature reached 25℃, there was no significant difference in the temperature distribution of different wind speeds; the average temperature in the greenhouse is reduced to 20℃, the greater the wind speed, the lower the temperature in the western part of the greenhouse, the greater the difference of temperature between the East and the West.Under different ventilation conditions, although the representative point temperature in the greenhouse to achieve the appropriate state, but there are still differences between the different regions, the temperature is low, the upper layer temperature is high, and the temperature difference between the north and the south is big which is effected by the bottom vent ventilation, When the wind direction is east or west wind, the temperature difference between the east and the west is big, When the wind direction is south wind, the overall temperature distribution is uniform in the greenhouse. Different ventilation modes, different wind speed and wind direction have important influence on the ventilation time and temperature distribution of the greenhouse.The above research results show that using the above numerical calculation method can simulate and analyze the distribution of micro environment in the greenhouse, and can effectively control the solar greenhouse in Ji’nan area. |
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