Study On Regulation Mechanism Of Greenhouse Thermo-hygro Environment In Southern China During Summer

High temperature and humidity as well as the high energy consumption of environmental regulation are the common problems of the greenhouse located at Southern China in summer.Although the energy saving has turned to be a severe problem and improved by some measures,a better way to solve such problem is needed.Thus,it is necessary to investigate the regulation mechanism of thermo-hygro environment,which can not only provide the references for the energy-saving of greenhouse in summer,but also increase energy utilization rate and provide a better environment for crop growing in greenhouse.This paper focus on the environmental regulation of greenhouse located at Southern China in summer.Firstly,the variation characteristic of environment in greenhouse under summer climate was obtained by test.Secondly,the effects of single or combined environmental regulation on the correlative mechanism between air temperature and humidity was analyzed,which was based on the environmental requirement of crop growing.Thirdly,the optimization on energy saving of fan was studied by analyzing the effects of controlled air flow rate from fan-pad on the thermal and humidity environment in greenhouse.After that,the functional mechanism of roof sprinkling on the thermal and humidity environment in greenhouse was obtained,in which the change law of crop temperature under the environmental variation was studied.Finally,the effects of gas fertilization on the environmental variation were obtained by numerical simulation and test,which was based on the requirement of crop growing.The results from the researches above,by which the regulation mechanism of thermo-hygro environment in greenhouse during summer was obtained,can provide significant theoretical support and practical value for energy saving in the greenhouse located at Southern China.The main work of this paper are shown as follow.1)Test and analysis on the regulation mechanism of thermal and humidity environment in greenhouseIn order to lower the high air temperature of greenhouse in Southern China during summer,an experimental platform on environmental regulation of greenhouse was built.The effects of 5 single and 2 combined regulation measures on air temperature and humidity in the greenhouse were studied.And the characteristics of environmental regulation on thermal and humidity for greenhouse were discussed.The results indicated that the air temperature can be decreased to lower than that outside the greenhouse by fan-pad or combination of fan-pad and shading curtain.Air temperature decreased the fastest when spraying indoor was taken.And other two methods,such as shading curtain and roof sprinkling had no significant effect on air cooling in greenhouse,but they can lower the temperature of film.The measures,include combination of shading curtain and roof sprinkling,combination of shading curtain and fan-pad,combination of shading curtain and window opening,and the combination of indoor spraying and roof sprinkling,can improve the air relative humidity in greenhouse.A concept called 'Time for Air Temperature Increasing in Special Temperature Range' was generated to represent effects of the outside complex thermal environment on the air temperature in greenhouse.The air temperature in greenhouse is affected by solar angle,solar intensity,air velocity and other factors which are difficult to be measured and they change continuously.'Time for Air Temperature Increasing in Special Temperature Range' can be used to describe the intensity of effects on thermal environment by the complex weather condition at a certain moment.The results from test indicated that the time for air temperature increasing was 71 s,60 s and 41 s according to the highest air temperature in greenhouse was 35±1?,40±1? and 45±1?,respectively.2)Test on the effects of controlled fan frequency on thermal and humidity environment and energy saving in greenhouseIn order to obtain the detail of air cooling effect and energy consumption of fan-pad cooling system of greenhouse in Southern China,an experimental platform on the environmental regulation by fan-pad was built.According to the measurement,higher frequency of fan lead to more ventilation,faster air cooling speed and better air cooling effect,and that means higher energy consumption.When the time for temperature increasing?air temperature range was 34-35??was 71 s and 60 s,respectively,indoor air temperature could be cooled down to the preference temperature of crops by switching fan frequency to 20 Hz.At the same time,the energy consumption when fan frequency was 20 Hz were lower than that at 50 Hz.When the time for temperature increasing?air temperature range was 34-35??was 41 s,the indoor air temperature only can be cooled to the preference temperature of crops with fan frequency at 50 Hz.Controlled fan frequency could improve the air relative humidity in the greenhouse,which is satisfy for the requirement of crop growing.3)Numerical simulation and test on the effects of roof sprinkling flow on thermal and humidity environment in greenhouseRoof sprinkling is one of cooling measures for greenhouse in summer.In order to investigate the cooling effect of roof sprinkling on air temperature in greenhouse located at Southern China,a test was conducted on the environmental regulation experimental platform.In this paper,the effects of different sprinkling flow on indoor air temperature,relative humidity and crops temperature in greenhouse was studied.The results indicated that larger flow of roof sprinkling caused lower air temperature in greenhouse.When the roof sprinkling flow was 17.0 L/min,time for air temperature increasing?air temperature range was 34-35??was 71 s,60 s and 41 s that air temperature in greenhouse decreased 0.4?,1.1? and 2.3?,respectively.When the roof sprinkling flow was 3.5 L/min,9.5 L/min and 17.0 L/min,respectively,the air relative humidity in greenhouse was 7.3%,9.5% and 10.8% higher than the air outside the greenhouse,which indicated that the effects of different roof sprinkling on the relative humidity in greenhouse were not significant,while the temperature on the surface of crop decreased-1.8?,-1.3? and 0.8?,and the temperature on the surface of stalk decreased-1.5?,-1.4? and 0.9?,respectively.From the result stated above,better effects on temperature decrease can be achieved by enlarging the flow of roof sprinkling.A three-dimensional symmetry model was built,and k-? turbulent model,Do irrational model,SIMPLE algorithm and Multiphase model were used to simulate the effects of the sprinkle rate,0.282 kg/s,0.564 kg/s and 0.846 kg/s,respectively,on the airflow and temperature distribution in the greenhouse.The results were analyzed and some conclusions can be drawn.The temperature on the roof of greenhouse can be reduced by improving the sprinkle rate and the temperature on it will be more uniform.The decrease speed of temperature in the container can be improved by improving the sprinkle rate but it has little effects on the average temperature at the end of roof sprinkle.A decrease of 1.65 ? can be seen after the roof sprinkle is finished.And the effects of roof sprinkle on the temperature of crops are not significant.A test was made to verify the accuracy of the model and the numerical results are fit the test results well,which tells that the results of numerical model are correct.4)Numerical simulation and test on the regulation mechanism of gas environmental regulation in greenhouseThe influence of organic fertilizer fertilization on gas concentration distribution in greenhouse was studied.This paper focused on increasing greenhouse oganic fertilizer that an entitative two-dimension turbulent model for greenhouse was built and solved by software ‘Fluent' combined with SIMPLE arithmetic,in which wall-function method,porous model and component mode were used to analyze the change of CO₂ concentration and air flow field distribution in greenhouse during oganic fertilizer fertilization.The simulated results shown that oganic fertilizer fertilization can increase CO₂ concentration in greenhouse from 50 uL/L to 500 uL/L within 10 h.The air flow field distribution in crop area was uniform during fertilization.Thermal buoyancy in top area,where is near the wet-pad outlet in greenhouse,was bigger than other areas.The CO₂ concentration in crop area was bigger than other areas.The CO₂ was mainly assembled at the bottom of greenhouse.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 fertilization,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-? 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 solve the model,and 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 fertilization property were computed while the change of CO₂ concentration and distribution regularities were studied.After the simulations,some results were obtained.CO₂ fertilization process had little effect on temperature of crop area,and both temperature distribution and air velocity distribution of crop area were uniform during fertilization.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-pad because of density difference of air flow.With the influence of buoyancy lift and newly added flow,air velocity in bottom area was higher.And the biggest velocity of air flow appeared in the area between jetting nozzle and bottom area in the greenhouse.Molar mass of CO₂ is greater than O₂ and N2.And as a result of deposition effect of CO₂,the bottom area of the greenhouse had a higher CO₂ concentration while CO₂ concentration in upper area was lower,which was beneficial for the photosynthesis of crops.Moreover,CO₂ flowed to the exit of the greenhouse via bottom area.In crop area,CO₂ concentration was higher when closing to entrance.Although there was nearly no difference in first 30 s,CO₂ jetting height had a certain effect on CO₂ fertilization 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 fertilization flow,CO₂ concentration increased and CO₂ fertilization time decreased.CO₂ fertilization flow had a certain effect on CO₂ fertilization time in crop area.Thus,transforming fertilization flow or time could control CO₂ concentration of the greenhouse which would be beneficial for 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 are correct.The research results of this paper can provide references for the environmental conditioning and energy saving on the greenhouse located at Southern China in summer.