Airflow Diversion Design Of An Air Duct With Spray And Cooling Effects For Horticultural Crops

In recent years,continuous high temperature climate frequently appears in summer in China.Plants with poor heat resistance,especially horticultural plants,suffer from high temperature heat damage seriously.The cooling effect and application scale of traditional measures such as shading and irrigation are all limited.At present,agricultural cooling equipment mainly based on ventilation cooling and spray cooling has some problems such as short distance of air supply,small coverage and low cooling rate.In this study,aiming at the problem of heat damage caused by continuous high temperature in summer in China,taking the tea garden as the object,the cooling effect of the existing spray dryer in the tea garden was tested first,and the problem was analyzed.Then,the diversion structure is designed,the three-dimensional model of the whole duct is established,the internal airflow field is simulated and the key diversion parameters are optimized.Optimization of spray working parameters based on multiphase flow numerical simulation.Finally,a spray cooling experimental platform was built in the tea plantation,and the spray cooling effect was tested.The chief research contents and results are as follows:（1）Meteorological characteristics of high temperature in tea garden analysis of spray cooling in air ductThe Spatiotemporal distribution of temperature and relative humidity（RH）in tea canopy during elevated temperature were studied It was found that the daily maximum temperature generally appeared in the sunny side of the top or middle of the canopy.By analyzing the whole day temperature changes of typical high temperature days,it is found that 10:00 to 15:00 is the time when the air temperature changes the most.During this time,the temperature in the tea plantation increases and the RH decreases,and the daily thermal peak generally appears between 13:00 and 14:30.Based on the existing spray tube developed by Jiangsu University,the spray cooling experiment of tea garden was carried out.The results showed that the cooling rate of the spray fan was above 2℃at a distance of about 18.0 m at 1.5 m height,and the cooling rate was smaller after 18.0 m.Because the air flow has a certain winding speed after leaving the fan,resulting in the loss of winding kinetic energy,rapid attenuation of wind speed,low air and fog supply capacity,and then reduce its cooling ability.In order to improve the performance of spray duct,increase the air supply distance and the effect of droplet evaporation,it is necessary to design the air duct diversion.（2）Experimental study on cooling effect of diversion sprayBased on the existing air spray principle,the preliminary design of the guide vane,core tube and conical cylinder is carried out according to the aerodynamics principle.The flow direction is improved by using the diversion effect,and the air supply capacity is improved,thus improving the droplet distribution and evaporation effect.The multi reference frame（MRF）model in fluent is used to simulate the internal flow field of fan diversion.Through the design of response surface test,the length of cone,the diameter of air outlet and the number of guide vanes are optimized.The results show that when the length of cone is 450mm,the diameter of air outlet is 928 mm and the number of guide vanes is 8,the diversion effect is better,the outlet thrust of fan can be increased by 13.66%,the flow rate is only decreased by 0.96%,and the efficiency is increased by 23.90%.When there is no diversion,the air flow has a certain winding speed after passing through the impeller,forming a"three blade fan"shaped low speed zone in the center of the fan outlet surface,and the circumferential velocity component is divergent.When there is diversion,the cone and the core make the air flow to the center converge and transport forward.When the air flow passes through the guide vane,the winding speed is converted into the axial speed,which improves the thrust of the air duct,and the velocity distribution on the outlet surface of the fan is more uniform.（3）Optimization of spray working parameters for cooling air ductThe three-dimensional model of the local airflow field of the diversion spray duct and tea garden is established.After the treatment of the model domain,Boolean operation and mesh generation,the CFD model of spray cooling for tea garden is constructed by combining the relevant models in the Fluent software.The temperature error is between 0.2～1.8℃,the maximum relative error is 6.1%,and the average relative error is 2.2%.A L₉（3⁴）orthogonal experiment was designed to study the effects of droplet size,spray flow,initial droplet temperature and nozzle arrangement on the cooling effect.The results show that when the droplet size is 15～45μm,the spray flow is 4.5 kg/min,the initial droplet temperature is 25℃,and the nozzle is double circle,it can cool down 2.0～11.2℃at the distance of 35.0 m along the central line of the fan in the tea garden.The cooling effect of the spray cooling is numerically simulated.Compared with that without diversion,the air supply distance is increased by 30.15%,the maximum cooling rate is increased by 2.7℃,and the effective cooling distance is increased by 10.11 m.（4）Experimental study on cooling effect of diversion sprayThe experimental platform of diversion spray cooling was built in the tea garden.The optimized diversion and spray working parameters were selected to verify the wind speed distribution and test the effect of the spray cooling.The results of wind speed distribution verification show that the wind speed of the fan on the central axis is greatly improved when there is diversion,and the error between the simulation value and the experimental value is less than 3.1%;The results of the spray cooling effect test show that the cooling effect is quite different at different heights in the tea garden.At 1.5 m height,the temperature range of 36.0 m in front of the central axis of the wind duct is reduced by 1.1～9.8℃.At the top height of canopy,the temperature drop was 2.7～8.2℃within 24.0 m in front of the air duct.At the height of the canopy bottom,the temperature drops 4.2～8.6℃within 18.0 m in front of the air duct.In addition,the cooling range of the air duct will decrease with the increase of the rotary range of the air duct.