| Now plastic arch shed is a facility type widely used in Chinese facility agriculture.Because of its low construction cost,fast construction speed and high land utilization rate,it is widely distributed in northern and southern regions of China.However,in recent years,with the increasing demand for mechanization in facility agriculture,the traditional arch sheds can no longer meet the requirements of mechanized operations.This puts urgent demands on increasing the space of the arch.Therefore,the concept of long-span thermal shed was formed.And the practice and exploration of this idea have begun in some areas of our country.However,there is no theoretical guidance for the use of asymmetric long-span arch sheds and operations are still being explored.And there is no detailed study of the temperature field and velocity field inside the asymmetric large shed.Based on the above questions,in this research,a new type of asymmetric large-span arch shed in Jinan is taken as the research object.Based on CFD,the temperature field and velocity field distribution in the asymmetric long-span arch shed were studied.Analysis the internal environment of the shed under different weather,shed structure,vent openings and external wind speeds.It provides corresponding theoretical basis for the use and design of asymmetric large-span arch sheds.1.Two asymmetric large-span test arch sheds in Jinan were tested under low temperature and typical sunny weather.It was found that the average daytime and nighttime temperature of the 13m+7m arch shed increased by 1.8℃and 1.6℃,respectively compared to the 15 m +5m arch shed.It shows that the structure of arch shed can affect the temperature inside the arch shed.In typical high-temperature weather,the difference between two arch shed was not much.2.Take 13m+7m arch shed as an example.The model of greenhouse is built by meshing with Gambit.Built CFD model of arch shed.Turbulence model adopts standard k-ε model,discrete format adopts second-order upwind,use solar ray tracing simulate sunlight,DO sun model is adopted to simulate sun radiation.The ground temperature is considered a constant temperature.The heat transfer on both sides of the shed membrane is solved by coupling.Crops in greenhouses are considered porous media.The steady state solution is adopted by SIMPLE(semi-implicit method for pressure-linked equations).Comparison between simulated and measured values,it found that the errors at the three measurement heights of1.5m,3.0m and 4.5m are 6.10%、6.30%、3.69%.It shows that the established CFD model was effective.3.Based on CFD model,the temperature fields and velocity fields in two asymmetric large arch sheds were considered.It is concluded that 13m+7m has a better temperature gradient distribution in the vertical direction and along the east-west span direction of the shed.The temperature was higher and has better ventilation.And simulation of asymmetric long-span arch sheds under different conditions.The temperature of the crop area in the shed increased by about 2℃ while consider the influence of crop.The structure of 11m+9m arch shed has the best thermal insulation performance in Jinan.The external wind speed and the size of the opening of the roof vent have a significant effect on the temperature in the shed in a low temperature environment.The math model between planting area temperature inside the arch shed and arch shed structure,outside velocities,top vent size were built by response surface method,which can provide theoretical reference and guidance for the design of different region and different request.4.In order to further improve the thermal insulation performance of asymmetric large arch sheds in low temperature and severe weather.The effect of internal insulation on the temperature in the shed was studied by numerical simulation.After solving the calculation,it was found that the temperature in the shed increased by about 2℃.The result shows that can better meet the conditions required for crop growth. |
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