Abstract: The objective of this work was to experimentally investigate the influence of vent type (side, roof, or both) and of anti-aphid insect screens on airflow, air temperature, and air vapor pressure deficit distribution in a round arch, mono-span greenhouse with vertical side walls. The greenhouse was equipped with two side roll-up vents and a flap roof vent. A tomato crop planted in double rows was cultivated inside the greenhouse. The three components of air velocity were measured by a 3-D sonic anemometer, and the air temperature and relative humidity were simultaneously recorded at several positions inside the greenhouse. Concerning the effect of insect screens, it was found that the mean value of the normalized air velocity was 58% lower in the greenhouse with insect screens on the side vent openings than in the case of a greenhouse without screens. Furthermore, the spatial heterogeneity of the microclimate variables was reduced with screens in the vent openings. When the ventilation was provided by side openings only, the air velocity inside the greenhouse was characterized by a strong air current near the greenhouse ground and low air velocity near the roof; when the ventilation was provided by roof vents, a circulating current prevailing at the center of the greenhouse was observed. The combined use of roof and side openings increased air velocity and decreased air temperature inside the greenhouse but also increased the spatial heterogeneity of the greenhouse microclimate compared to the cases with side or roof vents only. The most homogeneous climate conditions were achieved with the use of roof openings only. The results of this study provide a better understanding of the plant environment behavior under different vent configurations and a high-resolution database for validating on-going efforts with computer simulations.

Abstract: A methodology approach in order to simulate numerically using computational fluid dynamics (CFD) tools a greenhouse equipped with fan and pad evaporative cooling system is presented. Using the main aspects of evaporative cooling systems in terms of heat and mass transfer, the flow and boundary conditions of the simulation model are identified integrating both the external and internal climatic conditions. The crop (tomato) was simulated using the equivalent porous medium approach by the addition of a momentum source term. The temperature and humidity of incoming air and the operational characteristics of fans were specified to set up the CFD model. The numerical analysis was based on the Reynolds-averaged Navier-Stokes equations in conjunction with the realizable k-ε turbulence model. The finite-volume method (FVM) was used to solve the governing equations. The 3D full scale model was solved in several differencing schemes of various orders in order to examine its accuracy. The simulation approach was used to identify the critical parameters of microclimate of greenhouse and the regions were these have to be measured during the experimental processes. The simulation model was validated against the experimental data concerning the air temperature inside the greenhouse in 23 points for three ventilation rates showing a good qualitatively agreement. Results show the influence of the different airflow rates on greenhouse microclimate, indicating that the proper choice of ventilation rate is crucial factor in order to improve the efficiency of evaporative cooling systems in greenhouses

Abstract: In this paper, the efficiency of two different greenhouse ventilation opening configurations on greenhouse microclimate during dehumidification process with simultaneously heating and ventilation was analysed by means of computational fluid dynamics (CFD) using a commercial program based on the finite volume method. The numerical model was firstly validated against experimental data collected in a tunnel greenhouse identical with the one used in simulations. A good qualitative and quantitative agreement was found between the numerical results and the experimental measurements. The results of the simulations performed for an outside wind direction perpendicular to the greenhouse axis show clearly the influence of ventilation opening configurations on the velocity, temperature and humidity distributions inside the greenhouse. With the first ventilation configuration (roll-up type) maximum air velocity inside the greenhouse was reached in the greenhouse near the ground, with the lowest values observed near the greenhouse roof. As a result, temperature and humidity decreased first near the ground and afterwards in the rest of the greenhouse volume during the dehumidification process. The exactly opposite pattern was observed with the second configuration (pivoting door type). The maximum air velocities were observed near the greenhouse roof where air temperature and humidity were decreased first during the dehumidification process. Energetically the first configuration is proven to be better since the ratio of latent to sensible exchanges during the dehumidification process was higher than the first configuration.

Abstract: Full-scale experimental data and computational fluid dynamics (CFD) methods are used to determine the accuracy of four different turbulence models [standard k–ε, k–ε renormalisation group (RNG), k–ε realisable, Reynolds stress model (RSM)], which are used to describe the turbulent part of air in problems concerning the natural ventilation of buildings. Ventilation rates were measured in a livestock building using the decay tracer gas (CO2) technique. Airflow and temperature patterns were mapped out in a greenhouse with a tomato crop using a three-dimensional sonic anemometer and a fast-response temperature sensor. A commercially available CFD code was used to evaluate the different turbulence models. Average values from experiments were used for boundary conditions. The numerical results are compared with the experimental data, and they showed a good agreement, especially when the k–ε RNG turbulence model was used. The computations of the flow field using the different turbulence models showed noticeable differences for computed ventilation rate, air velocity and air temperature confirming the importance of the choice of the closure model for turbulence modelling.

Abstract: The objective of this work was to experimentally investigate the influence of vent type (side, roof or both) and of an anti-aphid insect screen used to prevent insect intrusion on the ventilation rate of a round arch with vertical side walls, polyethylene covered greenhouse. The greenhouse was equipped with two side roll-up vents and a flap roof vent located at the University of Thessaly near Velestino in the continental area of Eastern Greece. Microclimate variables as well as the airflow rate were measured during summer. Two measuring methods were used for the determination of ventilation rate: (a) the decay rate ‘tracer gas’ method, using nitrous oxide N2O as tracer gas, and (b) the greenhouse ‘energy balance’ method. In order to study the effect of vent type on ventilation rate, in a greenhouse with an anti-aphid insect screen in the vent openings, airflow was determined during periods with ventilation being performed by: (i) roof, (ii) side or (iii) both roof and side vents. Furthermore, in order to study the effect of insect proof screen on airflow, measurements were also carried out during periods that ventilation was performed by side vents without a screen in the openings. A good correlation was found between the air exchange rate values calculated using the two methods, with the values obtained by the tracer gas method being slightly lower than those obtained by the energy balance method. Furthermore, the data of ventilation rate obtained by the tracer gas method fitted better to the model used for the prediction of ventilation rate. In addition, the use of anti-aphid screen in vent openings caused a 33% reduction in greenhouse ventilation rate. From greenhouse ventilation performance point of view, it was found that the most effective vent configuration was the combination of roof and side vents, followed by side vents only (46% reduction in ventilation), while the least effective was roof vent (71% reduction ventilation).

Abstract: In this paper the effect of insect screens and vent configuration of an arch plastic covered greenhouse cultivated with a tomato crop on airflow and temperature patterns was numerically analyzed using a commercial computational fluid dynamics (CFD) code. The numerical model was firstly validated against experimental measurements which were carried out in an arch plastic covered greenhouse with continuous side openings. The three components of air velocity and the spatial distribution of air temperature were measured using a sonic anemometer and a fast response temperature sensor respectively. After the good agreement between experimental and numerically obtained results the numerical code was used for parametric studies concerning the effect of different insect screens and vent configurations on the greenhouse microclimate. The experimental data were used to define proper and realistic boundary conditions in the numerical model. A gradual increase of air temperature and a decrease of air velocity were observed as the porosity of the tested insect screen was reduced. It was found that vent configuration affects the ventilation rate and the climate distribution inside the greenhouse

Abstract: The influence of the heating system method on greenhouse microclimate was investigated overnight, using an experimental greenhouse tunnel with a tomato crop. The heating system consisted of plastic heating pipes located close to the gutter holding the growing substrate and an air heater, located 2·6 m above the ground. All the measurements were performed during two periods: (a) a period when only pipe heating was used; and (b) a period with pipe and air heating together. Heating pipes only could maintain the desired inside air temperature up to a temperature difference between inside and outside air of 10 °C, whereas this difference was increased to 15 °C with the addition of the air heater. Energy consumption with a system combining heating pipes and an air heater was 19% higher. The use of the air heater enhanced the vapour pressure difference and thus the crop transpiration. For both cases crop temperature was lower than air temperature but this difference was larger with the air heater and resulted in an increase in crop aerodynamic conductance. The use of the air heater for dehumidification purposes was also investigated. It was shown that with the air heater, although the mass transfer conductance to the cover was higher, condensation flux was smaller which resulted in less condensation at the inner surface of the cover. These results indicate that the use of a mixed system is favourable in greenhouse tunnel conditions since the use of the air heater, although increasing slightly the energy consumption, improves the control of both air temperature and humidity, particularly by keeping the inside air dew point temperature lower than the cover temperature and preventing the occurrence of condensation on the plastic films.

Abstract: The effect of ventilation configuration of a tunnel greenhouse with crop on airflow and temperature patterns was numerically investigated using a commercial computational fluid dynamics (CFD) code. The numerical model was firstly validated against experimental data collected in a tunnel greenhouse identical with the one used in simulations. The airflow patterns were measured and collected using a three-dimensional sonic anemometer and the greenhouse ventilation rate was deduced using a tracer gas technique. A good qualitative and quantitative agreement was found between the numerical results and the experimental measurements. After its validation, the CFD model was used to study the consequences of four different ventilator configurations on the natural ventilation system. The ventilation configuration affects the ventilation rate of the greenhouse and the airflow and air temperature distributions as well. For the different configurations, computed ventilation rates varied from 10 to 58 air changes per hour for an outside wind speed of 3 m s−1 and for a wind direction perpendicular to the openings. Likewise, the simulations highlight that while the mean air temperature at the middle of the tunnels varied from 28·2 to 29·8°C, for an outside air temperature of 28°C, there are regions inside tunnels 6°C warmer than outside air. Average air velocity in the crop cover varied according to the arrangement of the vents from 0·2 to 0·7 m s−1. The consequences of the marked climate heterogeneity on plant activity through the variation of crop aerodynamic resistance as well as the influence of the vent configurations on the efficiencies of ventilation on flow rate and air temperature differences between inside and outside, are also discussed.

Abstract: The main drawback of greenhouse evaporative cooling systems based on cooling pads and extracting fans is the thermal gradient developed along the direction of the airflow. High-temperature gradients of this type can markedly affect plant growth, and growers often combine cooling pads with shading. To predict the temperature gradients along a greenhouse, a simple climate model is proposed which incorporates the effect of ventilation rate, roof shading and crop transpiration. In order to calibrate the proposed model, measurements were performed in a commercial greenhouse equipped with fans and pads and shaded in the second half. Experimental data show that the cooling system was able to keep the greenhouse air temperature at rather low levels. However, due to the significant length of the greenhouse (60 m), large temperature gradients, (up to 8°C) were observed from pads to fans. The model was calibrated by fitting temperatures in the middle and at the end of the greenhouse. The model was validated on experimental data different from those used for the calibration and then it was used to study: (i) the influence of different ventilation rates combined with shading on air temperature profiles along the greenhouse length; and (ii) the influence of the outside air temperature and humidity on the performance of the cooling system. High ventilation rates and shading contribute to reduce thermal gradients. Despite its simplicity, the model is sufficiently accurate to improve the design and the management of the cooling pad systems.

Abstract: Ventilation is the primary method of controlling greenhouse climate. Natural ventilation is the most widely used system for greenhouse climatization since requires less energy, less equipment and operation and is much quieter than other ventilation systems. However little design information is, until now, available for naturally ventilated greenhouses. A proper and effective greenhouse ventilation system is crucial both to reduce excessive temperatures and to regulate humidity in acceptable levels. The objectives of this study were to evaluate the consequences of three different ventilator configurations in greenhouse microclimate by using a commercial available computational fluid dynamic (CFD) package. The boundary conditions were representative of a hot sunny day. The dynamic effect of the plants on the flow was simulated by means of the porous medium approach and a standard k-ε model was used to simulate turbulence. The study focused on the effects of ventilations type on ventilation rates, airflow, and temperature and humidity distribution in a tunnel greenhouse. The results show the influence of ventilation type on greenhouse microclimate. It shows that, for a given greenhouse type, the CFD code can be used as a design tool to optimize ventilation design (types-size-position) in order to achieve a well ventilated greenhouse and uniform climate conditions at the level of the crop

Abstract: An analysis of the ventilation process in a tunnel greenhouse equipped with an insect-proof screen in the side openings was performed with the use of a commercial computational fluid dynamics (CFD) package (Image 2000®). The aim of the study was to investigate how the screen influences airflow and temperature patterns inside the greenhouse. The screens on the greenhouse inlets and outlets, as well as the crop were simulated using the porous medium approach. The first simulations were carried out with a wind direction perpendicular to the side openings. Insect screens significantly reduced airflow and increased thermal gradients inside the greenhouse. Maximum air velocity values inside the greenhouse were observed near the openings, whereas air velocity was lowest in the middle of greenhouse. Airflow rates reduced by half in the greenhouse equipped with screen. These differences were also important in the region covered by crop, thus screen affected the sensible and latent exchanges between crop and air. The effect of different wind directions was also investigated. Wind direction considerably affected climatic conditions inside the greenhouse, as contrasted air flow and temperature patterns were observed for various wind regimes, especially when the greenhouse was equipped with insect screens.

Abstract: The influence of an insect screen on ventilation rate was experimentally investigated in a multispan glass-covered greenhouse equipped with a continuous roof vent, located at the University of Thessaly near Volos in the coastal area of eastern Greece. Microclimate variables as well as the ventilation rate were measured during summer. Two measuring techniques were used for the determination of ventilation rate: (1) the decay rate tracer gas technique, using N2O as tracer gas, and (2) the water vapor balance technique. The influence of the insect screen on ventilation rate was studied using a wind-related coefficient identified by fitting a simple linear model to the experimental values. The two measuring techniques gave similar results, but the water vapor balance technique provided a better fit to the experimental data. The wind-related coefficient significantly decreased when an insect screen covered the vent. Finally, the influence of the insect screen on the discharge coefficient was investigated. The discharge coefficient was correlated to the aerodynamic properties of the screen using porous media flow analysis

Abstract: Temperature and humidity gradients were investigated during summer in a commercial greenhouse producing cut roses, equipped with a ventilated cooling-pad system and a half-shaded plastic roof. In a steady regime, the cooling process reached 80% efficiency and succeeded in maintaining greenhouse temperatures that were cooler (up to 10 C lower) than outside. In the unshaded half of the greenhouse, the inlet air temperature increased from the pads to the middle of the greenhouse, while the humidity content remained constant. In the shaded half of the greenhouse, plant transpiration humidified the air, but the air temperature increased only moderately. In early morning and late afternoon, the moist and cool air coming from the pads seemed to induce condensation on the soil, especially in the first (unshaded) half of the greenhouse. The physical data were compared with those predicted by an analytical model describing the greenhouse as a heat exchanger. The model helped to understand the particular temperature and humidity profiles of the air flow along the greenhouse. It also suggested that greenhouse roof shading could be avoided in dry climates because the evaporative cooling process was sufficient to prevent overheating