增施CO_2气肥对温室流场影响的数值模拟及验证

Numerical simulation and experimental verification of effect of CO_2 enrichment on flow field of greenhouse

CO2气体是温室作物光合作用的重要原料之一。为掌握温室CO2气肥增施性能,以温室CO2气肥增施为研究对象,采用计算流体力学(computational fluid dynamics,CFD)技术建立二维紊流数值计算模型。基于FLUENT软件,结合SIMPLE算法,采用有孔介质模型、k-ε模型、离心坐标(discrete ordinates,DO)模型,添加组分传输模型,对气肥喷射高度、气肥增施流量等因素对CO2增施性能进行数值模拟,得到温室内CO2浓度变化和分布规律。研究结果表明:气肥增施过程对作物区的温度场影响较小,温室的作物区域最大温度与最小温度差值不超过0.5℃,作物区域的气流流场以及温度场分布较为均匀;由于CO2的沉积效应,温室下部区域的CO2浓度相对较高;气肥喷射高度越高,CO2扩散的范围越大,沉积在作物区的CO2相对越少,CO2的浓度也相对较低;气肥增施流量越大,作物区域的CO2浓度上升越快。试验结果表明,CO2浓度模拟值与试验值差异不大于5%,模拟结果与试验结果较吻合,证明了模型的正确性。该研究对掌握温室CO2气肥增施性能的流场变化规律,开展温室气肥增施装备的优化设计具有一定的参考价值。

Carbon dioxide（CO2） is one of the important raw materials for photosynthesis of crops in greenhouse,which can increase the harvest of the crops.In order to analyze the performance of CO2 fertilizer enrichment,this paper took indoor environment of greenhouse as the research object,and by means of unstructured mesh generation method,a two-dimensional turbulence computational model of greenhouse was built by ICEM CFD（the integrated computer engineering and manufacturing code for computational fluid dynamics） technique.After building 7532 triangle/quadrangle meshes,the skewness of mesh model was less than 0.75.To solve two-dimensional computational model of greenhouse,the CFD software FLUENT and the SIMPLE algorithm were used.Meanwhile,the porous model,the DO（discrete ordinates） model and the k-ε model were adopted.Governing equations of finite volume method were employed,including mass,momentum and energy conservation equations.In computational model,CO2 enriching inlet was set with the condition of mass flow inlet while outlet was set with the condition of pressure outlet through pre-calculation.And crop area was defined as porous material with the porosity of 0.8.Adopting steady-state solver to operate,environmental parameters such as indoor temperature,nitrogen（N2） concentration,oxygen（O2） concentration and CO2 concentration should be initialized when flow field of greenhouse was stable.Then transient solver was needed to numerical simulation when acceleration of gravity was 9.8 m/s2 and step size was 0.01 s.By such methods,the effects of CO2 jetting height,enriching flow speed and other factors on CO2 enrichment property were computed while the change of CO2 concentration and distribution regularities were studied.After the simulations,some results were obtained.CO2 enrichment process had little effect on temperature of crop area,and both temperature distribution and air velocity distribution of crop area were uniform.As CO2 enrichment process lasted only for 5 min and solar radiation affected indoor environment of the greenhouse rarely,the temperature difference in the crop area was less than 0.5℃.On the other hand,cooler air was more likely to deposit to the bottom of the greenhouse and hotter air exhausted from wet curtain because of density difference of air flow.With the influence of buoyancy lift and newly added flow,air velocity of bottom side was higher.And the biggest velocity of air flow appeared in the area between jetting nozzle and bottom side of the greenhouse.Molar mass of CO2 is greater than O2 and N2.And as a result of deposition effect of CO2,the bottom of the greenhouse had a higher CO2 concentration while CO2 concentration in upper part was less,which helped the photosynthesis of crops.Moreover,CO2 flowed to the exit of the greenhouse via bottom side.In crop area,CO2 concentration was higher when closing to entrance.Although there was nearly no different in first 30 s,CO2 jetting height had a certain effect on CO2 enrichment time.CO2 concentration could be decreased when CO2 jetting height was located at a high level.When jetting height was higher,spreading range of CO2 was larger,CO2 deposited in crop area was less and CO2 concentration was lower.However,CO2 concentration of crop area would be increased too fast when jetting height was too low.With the increase of CO2 enrichment flow,CO2 concentration increased and CO2 enrichment time decreased.CO2 enrichment flow had a certain effect on CO2 enrichment time in crop area.Thus,transforming enrichment flow or time could control CO2 concentration of the greenhouse which benefited photosynthesis of crops.A test was made to verify the accuracy of the numerical model.The simulation values and test values of CO2 concentration at the same moment were in good agreement,and the biggest difference between them was less than 5%,which demonstrated that such models were correct.Result of this paper reveals the CO2 enrichment rules and the characteristics of flow field in greenhouse,which has a certain reference for the design and optimization of CO2 enriching equipments in greenhouse.