Biosystems Engineering (2004) 88 (4), 479490doi:10.1016/j.biosystemseng.2003.10.006

Available online at www.sciencedirect.com

SE}Structures and Environment

Effect of Vent Arrangement on Windward Ventilation of a Tunnel Greenhouse

T. Bartzanas1; T. Boulard2; C. Kittas1

1Department of Agriculture Crop Production and Agriculture Environment, University of Thessaly, School of Agriculture Science,Fytokou St., 38446, N.Ionia Magnisias, Greece; e-mail of corresponding author: ckittas@uth.gr

2 INRA, Unite Plantes et Systemes de Culture Horticoles, Domaine St Paul, Site Agroparc, 84914 Avignon Cedex 09, France;e-mail: boulard@avignon.inra.fr

(Received 22 May 2003; accepted in revised form 8 October 2003; published online 1 July 2004)

The effect of ventilation conguration of a tunnel greenhouse with crop on airow and temperature patternswas numerically investigated using a commercial computational uid dynamics (CFD) code. The numericalmodel was rstly validated against experimental data collected in a tunnel greenhouse identical with the oneused in simulations. The airow patterns were measured and collected using a three-dimensional sonicanemometer and the greenhouse ventilation rate was deduced using a tracer gas technique. A good qualitativeand 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 ventilatorcongurations on the natural ventilation system. The ventilation conguration affects the ventilation rate ofthe greenhouse and the airow and air temperature distributions as well. For the different congurations,computed ventilation rates varied from 10 to 58 air changes per hour for an outside wind speed of 3m s1 andfor a wind direction perpendicular to the openings. Likewise, the simulations highlight that while the mean airtemperature at the middle of the tunnels varied from 282 to 2988C, for an outside air temperature of 288C,there are regions inside tunnels 68C warmer than outside air. Average air velocity in the crop cover variedaccording to the arrangement of the vents from 02 to 07m s1. The consequences of the marked climateheterogeneity on plant activity through the variation of crop aerodynamic resistance as well as the inuence ofthe vent congurations on the efciencies of ventilation on ow rate and air temperature differences betweeninside and outside, are also discussed.# 2003 Silsoe Research Institute. All rights reserved

Published by Elsevier Ltd

1. Introduction

Greenhouse tunnels are widely used in the wholeworld due to their low cost, simple structure and easymanagement. However spatial heterogeneity of airow,air temperature and humidity strongly vary in thesestructures. The distribution of microclimate variablesinside the greenhouse cause non-uniform productionand quality but generates also problems with pests anddiseases (Bot, 2001). Therefore, quantitative under-standing of this heterogeneity can help to optimisegreenhouse production in terms of cost efciency, cropquality and quantity.Natural ventilation directly affects the transport of

heat and mass between the environment and the interiorof the greenhouse so that it strongly inuences the inside

greenhouse climate. Thus ventilation performance is amajor factor in production, inuencing both climatecontrol and yield quality over much of the year. Itsdriving force is the combination of buoyancy and windeffects, and their relative importance depends on thewind speed and the insideoutside temperature differ-ence.Several studies on natural ventilation were based on

estimations of a global air exchange rate from tracer gasmeasurements (Bot, 1983; Kittas et al., 1995) and energybalance (Kindellan, 1980; Wang & Deltour, 1996;Demrati et al., 2001). However, these methods do notallow clear mapping of airow patterns and temperatureproles. Moreover, little information is available on theproper design of ventilation systems, because it is verydifcult to establish fairly identical and stable conditions

ARTICLE IN PRESS

1537-5110/$30.00 479 # 2003 Silsoe Research Institute. All rights reservedPublished by Elsevier Ltd

in a eld experiment. More recently, sonic anemometrywas used to measure airow distribution in roof vents ofa double-span greenhouse (Boulard et al., 1997) and tomap airow patterns in an empty tunnel greenhousewith discontinuous vent openings (Boulard et al., 2000)and in a cultivated tunnel with continuous roof vents(Wang et al., 1999; Haxaire, 1999).On the other hand recent progress in ow modelling

by means of computational uid dynamics programs(CFD) allows one to investigate and analyse airowdistribution and to predict ventilation rates in green-houses. Actual weather conditions and structuralspecications could be simulated and changed in theCFD model while maintaining stable and intenticalboundary conditions. Computational uid dynamicssimulations can be a valuable tool for analysingthe internal airow and understanding the effectsof the greenhouse structural characteristics with respectto ventilation. The signicant advantage of CFD isnot only the prediction of the ventilation rate and thusthe greenhouse performance, but the detailed investiga-tions of airow and temperature distribution in the

greenhouse interior and especially in the region near thecrop.Short (1996) introduced the use of commercial CFD

models for solving naturally ventilated greenhouseventilation problems. Mistriotis et al. (1997a, b)analysed the ventilation process in greenhouses withouta crop. Their numerical results were agreed with theexperimental results of Sase et al. (1984) and of Boulardet al. (1997). Boulard et al. (1999) investigated thenatural ventilation (thermal and wind driven) in a small-scale greenhouse by comparing their results withexperimental data from the same greenhouse. Alwaysin greenhouses without a crop, Reichrath and Davies(2002) simulated a two-dimensional full size commercialmulti-span glasshouse comprising 60 spans and theycompared their numerical results with the experimentaldata of Hoxey and Moran (1991). Woodruff (1997),Kacira et al. (1998), and Lee and Short (2000, 2001)studied various naturally and mechanically ventilatedgreenhouses types by using a CFD numerical model.They mainly investigated the effects of weather condi-tions, greenhouse structural specications, internal and

ARTICLE IN PRESS

Notation

A vent opening area, m2

C tracer gas concentration, ppmCD drag coefcientCm model constant of the ke modelCp specic heat of air, J kg

1 8C1

Cm constant tting parameterC1e model constant of the ke modelC2e model constant of the ke modelDi saturation vapour pressure decit, Pad characteristic leaf length, mF tracer gas uxG ventilation rate, m3 s1

h reference height, mILA leaf area indexK von Karman constantk turbulent kinetic energy, m2 s2

L leaf area density, m2m3

N air changes per hourQsen sensible heat exchange, Wm

2

Qlat latent heat exchange, Wm2

Rgi internal global solar radiation, Wm2

ra aerodynamic resistance, sm1

rs stomatal resistance, sm1

S surface area, m2

SF source termT air temperature, 8CTi inside air temperature, 8C

Tc crop temperature, 8CTo outside air temperature, 8Ct time, sU, V, W components of velocity vectorUinl inlet velocity, m s

1

u air velocity, m s1

uh reference velocity, m s1

u* friction velocity, m s1

Vg greenhouse volume, m3

wa air absolute humidity, kg kg1

wc crop absolute humidity, kg kg1

Y non-linear momentum loss coefcientz height, mzo friction length, ma crop permeablility, m2

G diffusion coefcientDT temperature difference, 8CDT mean temperature difference, 8Ce dissipation rate of turbulent kinetic energy,

m2 s3

l latent heat of water vaporisation, J kg1

m dynamic viscosity kgm1 s1

r air density, kgm3

s standard deviationsk turbulent Prandtl number for turbulent

kinetic energyF concentration of transported quantity

T. BARTZANAS ET AL.480

external shading screens, number of spans and presenceof plants and benches on the air exchange rate.Bartzanas et al. (2002) investigated the three-dimen-sional air ux and temperature effects in a tunnelgreenhouse equipped with an insect screen with specialattention devoted to the inuence of the wind direction.However, all these simulations basically suffer from a

lack of realism due to the lack of modelling of the heatand water exchanges between the crop cover and itsenvironment. More recently, Boulard and Wang (2002)used a commercial CFD code (CFD2000) and incorpo-rated solar radiation and transpiration models into theoriginal code together with the modelling of the dynamiceffect of the crop by the equivalent macro-porousmedium. They validated their numerical results withexperimental data and found a good agreement betweenmeasured and computed for both climatic and croptranspiration elds. With the same approach andboundary conditions measured in the experimental study,Fatnassi et al. (2001) investigate the three-dimensional airux, temperature and humidity effects in a very large-scale greenhouse (1/2 h) equipped with an insect screen.Based on the same realistic approach of CFD

simulation which considers both the dynamic, thermaland transpiration exchange between the greenhousecrop and its environment, the aim of present paper is,after validating the code and the method againstexperimental data, to determine the effects of differentvent arrangements on windward ventilation of a tunnelgreenhouse with mature tomato crop.

2. Materials and methods

2.1. Experimental greenhouse

The measurements were carried out in an experimentalNS oriented tunnel greenhouse located at the Universityof Thessaly near Volos, (Latitude 398440, Longitude228790) on the coastal area of Eastern Greece. Thegeometrical characteristics of the greenhouse were asfollows: eaves height of 24m; ridge height of 41m; totalwidth of 8m; and total length of 20m. The greenhousewas covered with a polyethylene sheet and was equippedwith two continuous side openings (roll-up type) located06m from the ground with a maximum opening of09m. The greenhouse was cultivated with a tomato crop,which reached a height 15m during the experiments.

2.2. Measurements

Two different types of measurements were conductedin order to validate the simulations: (a) measurements of

the three components of air velocity; and (b) measure-ments of the ventilation rate of the greenhouse.

2.2.1. Air velocity measurementsRapid uctuations in air velocity were measured by

means of one three-dimensional (3-D) sonic anem-ometer (omnidirectional, R3, research ultrasonic anem-ometer, Gill R&D). The three components of windvelocity (U, V, W) were measured at six positions in themiddle of the greenhouse and along its eastwest widthat 1, 2, 3, 5, 6 and 7m from the east side. The height ofthe measurements was at 11m above ground, whichcoincided with the middle height of the full opening. Themanufacturers calibration was accepted for U, V, Wmeasurements. Sampling frequency was 5Hz. The timeduration of each measurement record was about 5min.Dry and wet bulb temperatures were also recorded atthe same points using an aspirated psychrometer. Thepsychrometer was placed within 02m of the samplingvolume of the sonic anemometer in order to minimisethe ow distortion.A weather station tower was installed outside the

greenhouse to measure the local climate such as dry andwet bulb air temperatures, wind speed (AN1-UM-3,Delta-T devices Cambridge, UK), wind direction (AN1-UM-3, Delta-T devices Cambridge, UK) and solarradiation (CM-6, Kipp and Zonen, Delft, Netherlands).Outside dry and wet air temperatures were measured at11m above ground, air speed and direction as well assolar radiation at 1m above the top of the greenhouse.The above variables were also measured each secondand averaged over the length of each record.As the CFD simulations describe only steady-state

conditions, so data for the validation were collectedwhen weather conditions such as wind speed, winddirection and solar radiation were stable, mainlybetween 1 h before and after solar noon. Table 1presents the mean value (in a time interval of 5min) ofclimate conditions during the measurements with thesonic anemometer.

2.2.2. Tracer gas measurementsThe decay rate method, using N2O as tracer gas, was

used to deduce the ventilation rate of the greenhouse. Adetailed description of the procedure for these measure-ments as well as the corresponding equations can befound in Boulard and Draoui (1995). The tracer gas wasinjected into the greenhouse, its concentration washomogenised using the fans of the heating system; thenventilation openings were opened at a known height andsimultaneously wind velocity, wind direction and gasconcentration were recorded during the tests. Airsamples were continuously taken at six points in thegreenhouse, by means of six equally distributed plastic

ARTICLE IN PRESSVENTILATION OF A TUNNEL GREENHOUSE 481

pipes of the same length, located at a height ofapproximately 18m from the ground. The air fromthe six positions was then mixed and pumped to aninfrared gas analyser (model 7000, ADC gas analyser,analysis up to 200 vpm, accuracy at 5 vpm). Theduration of each experiment varied depending onenvironmental conditions and on ventilation opening,ranging between 5 and 20min. During the experi-ments wind speeds varied between 15 and 5m s1 andventilation opening from 0 to 090m.

2.3. Numerical model

The CFD method allows the explicit calculation ofthe average velocity vector eld of a ow by numericallysolving the corresponding transport equations. Thethree-dimensional conservation equations describingthe transport phenomena for steady ows in freeconvection are of the general form:

@UF@x

@VF@y

@WF

@z Gr2F SF 1

In Eqn (1), F represents the concentration of thetransport quantity in a dimensionless form, namely thethree momentum conservation equations (the NavierStokes equations) and the scalars mass and energyconservation equations; U, V and W are the componentsof velocity vector; G is the diffusion coefcient; and SF isthe source term. The governing equations are discretisedfollowing the procedure described by Patankar (1980).This consists of integrating the governing equations overa control volume.The commercially available CFD code Fluent v.5.3.18

(Fluent, 1998) was used for this study. As the prevailingwind direction was parallel to the greenhouse during theexperiments, a 3-D model was rst built in order tocompare the numerical results with the experimentaldata. All the other simulations, used for case studies,were two-dimensional since the selected wind direction

for the simulations was perpendicular to the axis of thegreenhouse.To achieve an accurate result, second-order upwind

discretisation schemes were used for momentum andturbulence equations. A semi-implicit method forpressure linked equations algorithm was used for thecoupling between pressure and velocity. The conver-gence criterion for all variables was 1 106.

2.3.1. Mesh and boundary conditionsFor the geometry, a control volume was selected

representing a large domain including the greenhouse.The grid structure was an unstructured, quadrilateralmesh with a higher density in critical portions of theow subject to strong gradients. Body-tted coordinateswere also applied to exactly conform the grid to thecontours of the boundary conditions. After several trieswith different densities, the calculations were based on a48 by 20 by 80 grid. This results from an empiricalcompromise between a dense grid, associated with along computational time, and a less dense one,associated with a marked deterioration of the simulatedresults.The boundary conditions prescribed a null pressure

gradient in the air, at the limits of the computationaldomain, and wall-type boundary conditions along theoor and the roof whereas the side walls were treated asadiabatic (Table 2). The Boussinesq model (Launder &Spalding, 1974; Fluent, 1998) was activated for thebuoyancy effect in the computational domain.As shown by the measurements of turbulent airows

and microclimate patterns in a greenhouse tunnel(Boulard et al., 2000), the airows were highly turbulent.Consequently, turbulent models must be introduced inthe Reynolds equations written to separate the meanow from its uctuating components. The standard kemodel (Launder & Spalding, 1974) assuming isotropicturbulence was adopted to describe turbulent transport.This choice is a good compromise for a realisticdescription of turbulence and computational efciency(Jones & Whittle, 1992). The complete set of the

ARTICLE IN PRESS

Table 1Mean values (in a 5min interval) of climate conditions during the measurements with the sonic anemometer

Measurement Positions Temperature, 8C Solar radiation, W m2 Wind Speed, m s1 Wind direction* deg

Inside air Outside air

1 2900 2820 789 370 302 3020 2980 843 206 403 3090 2960 867 330 454 3120 2910 874 330 505 3130 2820 843 260 356 3150 2850 835 27 30

*0 denotes wind direction parallel to the greenhouses axis.

T. BARTZANAS ET AL.482

equations of the kemodel can be found in Mohammadiand Pironneau (1994) and their commonly used set ofparameters (empirically determined) are (Cm 009,C1e 144, C2e 1,91, sk 1) (Fluent, 1998).The wind direction was normal to the ridge for the 3-

D model and perpendicular to the ridge for the 2-Dsimulations. A reference velocity was chosen to be3m s1 at a reference height (10m). At the inlet of thecomputational domain a wind prole was imposed. Inletvelocity was dened as:

Uinl u

Kln

z zozo

2

with

u Kuh

lnh zo=zo3

with Uinl the inlet velocity in m s1, u* the friction

velocity in m s1, K the von Karman constant(K 042), z the height in m, zo the friction length inm, Uh the reference velocity in m s

1 and h the referenceheight in m. The friction length zo was chosen as 001mcorresponding to a ploughed up eld. The distributionof turbulent kinetic energy, k in m2 s2 and of theturbulent dissipation rate, e in m2 s3 in the incomingwind prole are described by the relationships:

k u2Cm

p 4

e u3

Kz z05

where Cm is a constant tting parameter.

2.3.2. The equivalent porous medium approachThe crop was simulated using the equivalent porous

medium approach by the addition of a momentumsource term, due to the drag effect of the crop, to thestandard uid ow equations. The drag force perunit volume of the crop can be expressed as (Wilson,1985):

SF rLCDu2 6

where: u is the air velocity in m s1; L the leaf areadensity in m2m3; r the air density in kgm3, and CDthe drag coefcient. The source term SF is composed oftwo parts, a viscous loss term (Darcy), and an inertialloses term. In the case of a simple homogenous porousmedia the source term was described as:

SF ma

u Y1

2rjuju 7

where: a is the permeability of the porous medium (crop)in m2; Y the non-linear momentum loss coefcient;and m the dynamic viscosity in kgm1 s1. In the caseof crop, for reasons of simplicity, it was assumedthat pressure forces contributed the major portionof total canopy drag (Thom, 1971). Using wind tunnelfacilities, for a mature greenhouse tomato crop with aleaf area index ILA of 4, Haxaire (1999) has evaluatedthe total drag of the canopy (CD 032). Usingthe relationships in Eqns (6) and (7) (Boulard &Wang, 2002) the appropriate values for permeabilityand non-linear momentum loss coefcient can bededuced.The exchange of heat and water vapour between crop

and air was considered through the heat and massbalance of crop with the air. The sensible heat Qsen inWm2, from the crop was calculated using the followingequation:

Qsen 2ILArCpTc Ti

ra8

where: ILA is the leaf area index; Cp is the specic heat ofair at constant pressure in J kg1K1; Tc and Ti are thecrop and air temperatures in 8C; and ra is theaerodynamic resistance of the crop in sm1. Notethat performing leaf heat and water vapour balancesmake it necessary to introduce a new phenomenologicalvariable Tc.Following Campbell (1977), if the interior air speed

u501m s1, then:

ra 840d

jTc Tij

0259

else:

ra 220d02

u08

10

ARTICLE IN PRESS

Table 2Boundary values used in the simulations

Parameters Numerical value

Wind direction3-D model Parallel to the ridge2-D model Perpendicular to the

ridgeTemperatureOf the cover, 8C 3200Of inside ground, 8C 4500Of outside ground, 8C 3000Of outside air, 8C 2800

Inlet airVelocity, m s1 300Relative humidity, % 4000Density, kgm3 122Gravitational acceleration, m s2 981Specic heat, J kg1 8C 100400Thermal conductivity, Wm2 8C1 00263

Plant canopyPressure drop coefcient 0395Inertial loss factor 020

VENTILATION OF A TUNNEL GREENHOUSE 483

where: d is the characteristic length of the leaf in m; andu is the local air speed in the same mesh at the givenlocation in m s1.The latent heat exchange between crop and air Qlat in

Wm2 was calculated according to the followingequation:

Qlat ILArlwc wara rs

11

where: wc and wa are the absolute humidity of crop andair in kg kg1 also in the same mesh; l is the latent heatof water vaporisation in J kg1; and rs is the cropstomatal resistance in sm1, which was calculatedaccording to the relationship found by Boulard et al.(1991) for greenhouse tomato leaves:

rs 200 11

exp0005Rgi 50

1 011 exp 034Di

100 10

12

where: Rgi is the internal global solar radiation inWm2; and Di is the saturation vapour pressure decitin Pa. In this relationship, only the radiation and airhumidity dependence of the stomatal conductance areconsidered. The rst factor on the right-hand side ofEqn (12) shows that stomatal resistance decreaseswith the solar radiation Rgi. When Rgi is greater than100Wm2, rs is maintained at a constant value of200 sm1. The second factor on the right-hand sideof Eqn (12) shows that leaf stomatal resistanceincreases with the air drying over 10 Pa of saturationdecit.The numerical model was customised in C++ in

order to perform the balance described by Eqns (8)(12),based on the local, computed air speed and climaticconditions within each mesh of the porous medium(crop cover).

2.3.3. Simulation of the ventilations ratesThe ventilation rate of the greenhouse was numerical

calculated by means of the continuity equation for everyinternal cell for a virtual tracer gas (N2O):

dC

dt FF dS 13

where: C is the concentration of the tracer gas in a cell inppm; t is the time in s; and F the tracer gas ux. On theright-hand side of Eqn (13) is given the surface integralof the tracer gas ux F through the surface area of thecells in m2. The average values of the air velocity can beused for calculating F.In fact, the results of the virtual tracer gas measure-

ments were obtained in two major steps. First aconverged solution under steady-state conditions isobtained. Then, the ow is considered unsteady and

the species model is used to inject the virtual tracer gas.Initially all the cells in the greenhouse have a xed tracergas concentration equal to unity and all the externalcells equal to zero. In this way, if a time step, dt, isselected, the continuity equation can be solved as adifference equation with respect to time. The tracer gasconcentration decreases in the greenhouse at a ratedepending on the local value of the air velocity. Then,the average tracer gas concentration %CC of that volume iscalculated as a function of time. This function usuallyexhibits an exponential decay. For this reason, it is ttedby an exponential of the form

%CCt %CC0eNt 14

The identied exponent N value is the decay rate ofthis function; therefore it describes the ventilation rateof the studied volume (air changes per hour).

2.4. Vent configurations used for simulation

The following commonly found vent congurationshave been used for the simulation of the inuence of thevent design on windward ventilation of a tunnel typegreenhouse (Fig. 1). In order to characterise the openingof the vent, the chord of the opening was dened as thedistance between the free end of the vent to its restplace on the greenhouse when the vent is closed and thewindow aperture area as the product of the length ofthe vent by the chord of the opening. In all cases thedistance between the articulation of the vent and its restplace on the greenhouse structure when the vent isclosed measures 09m.

2.4.1. Configuration (a): side openings only(roll-up type)

The greenhouse is equipped with two continuous roll-up type openings located at 06m above ground with anopening height of 09m. This conguration leads to atotal opening area of 36m2.

2.4.2. Configuration (b): side only openings(pivoting door type)

The greenhouse is equipped with two continuouspivoting door type side openings. The base of thewindow is at 06m above ground and the height of thewindow is 09m. The aperture angle is 458, which leadsto an opening area of 275m2.

2.4.3. Configuration (c): roof opening onlyThe greenhouse is equipped with a pivoting type roof

opening. The free end of the opening is at the ridge ofthe greenhouse and the articulation at 09m leewardfrom the ridge. When opened, the opening faces the

ARTICLE IN PRESST. BARTZANAS ET AL.484

wind and its chord is 09m. This conguration leads toan opening area of 18m2.

2.4.4. Configuration (d): combined roof and sideopenings (roll-up type)

This conguration combines the roll-up side openingsof conguration (a) with the roof opening of congura-tion (c). This conguration leads to a total opening areaof 54m2.

3. Results

3.1. Numerical model validation

Figure 2 shows both the experimentally and thenumerically obtained average transverse horizontalcomponent of the normalised air velocity along thegreenhouse width at a height of 11m at the middle ofthe greenhouse. The normalised air velocity wasobtained by the ratio of the interior air velocity to themean external wind speed. For roll-up openings (type(a) of Fig. 1) and a wind direction parallel to thegreenhouse axis, both computed and simulated valuesshow that air speed has relative high values nearthe openings and reduced values near the centre of thegreenhouse. However, when the wind is parallel to thegreenhouse axis both openings acted simultaneously asinlets and outlets. Air then entered the greenhousethrough the leeward section of the openings and exitedthrough the windward part. A similar airow patternwas measured in a greenhouse with a continuous roof(Boulard et al., 1997) and this phenomenon is alsocomparable to the side wall effect deduced from tracer

gas measurements (Fernandez & Bailey, 1992). Thewindward gable end induced a positive static pressureeld whose relative contribution to the whole ventilationrate is inverse to the size of the greenhouse in thedirection of the wind.

Figure 3 presents the simulated and experimentalventilation rates (air changes per hour) versus theproduct of ventilation opening area and outside windspeed. Good agreement was also found in this case andthe differences between simulated and measured ventila-tion rate varied only by 1215%. In all cases the valuededuced from CFD simulations was larger than thevalue deduced from the experimental measurements.The explanations for this are twofold: (a) The experi-mental wind velocity represents an average value overthe measurement period and therefore neglects theturbulent part, while the estimated value by the CFDmodel includes the turbulent part even if the used kemodel is a rough approximation of the reality; and (b)The homogenisation of the tracer gas, in the experiment,is not perfect although fans were used for this purpose,whereas in the CFD model a perfect homogenisationis assumed. According to Boulard et al. (1996) thisnon-perfect homogenisation leads to the fact thattracer gas techniques allow the determination of theeffective airow, which can be lower than the realairow.

Figure 4 presents the computed and measured airtemperature difference Ti2To in a horizontal plane atthe middle of the greenhouse. For an external windspeed parallel to the axis of the greenhouse, themeasured and CFD-computed results indicated thatthe inside air temperature gradually increases from theside wall to the middle of the greenhouse where its valuestarts to decrease.

ARTICLE IN PRESS

00.020.040.060.08

0.10.120.140.160.18

0 3 7Greenhouse width, m

Nor

mal

ised

velo

city

21 654 8

Fig. 2. Experimentally (& & &) and numerically obtained(& & &) average transverse horizontal component of the airvelocity along the greenhouse width at a height of 11 from the

greenhouse ground normalised by the outside wind speed

Fig. 1. Geometries of the four different configurations for theventilation efficiency study:(a) roll-up type openings; (b)pivoting door type openings; (c) roof only openings; and (d)

side and roof openings.

VENTILATION OF A TUNNEL GREENHOUSE 485

3.2. Airflow patterns and temperature distributionfor different ventilator configurations

Considering that the model was globally validated asillustrated by the good t between experimental andnumerical obtained values, we have used the CFD forinvestigating the inuence of the arrangement of variousvent openings on airow and temperature distributionsin the greenhouse tunnel.

3.2.1. Configuration (a): side openings only(roll-up type)

This rst conguration is similar to the greenhouseused for the model validation when the vents are fullyopened. Only the wind direction (now perpendicular tothe ridge) was changed. Figure 5 presents the computedcontours of the air velocities obtained for this case. Itwas characterised by a strong air current near theground and a recirculation loop with slower speedsituated near the roof and owing counter current with

respect to the wind outside. This recirculation loopimproves the air mixing but most of the air leaves thegreenhouse volume without a good homogenisation.Above the height of the ventilator (i.e., 1.5m) the airvelocities were strongly reduced. The internal ow isdifferent from that which was observed during thevalidation study. These results indicate that the winddirection clearly inuences the air velocity inside thegreenhouse and thus its ventilation rates. Figure 6presents the distribution of temperature for this case.It is clear that temperature distribution follows the airprole and in regions with small air velocities (especiallynear the corners of the greenhouse) the air temperaturewas 318C compared with a outside temperature of 288C.In the region covered by the crop the air temperature issimilar to the outside (285298C) due to the strong airmovement in this region.

3.2.2. Configuration (b): side only openings(pivoting door type)

To prevent the incoming jet (through the windwardopening) from impinging directly on the crop, the typeof the openings were changed from roll-up (congura-tion a) to side-open. Airow patterns show that theincoming air through the windward side ventilator tendsto move up immediately by the inuence of inclinedventilator ap and mainly follows the inner surface ofthe roof. In the space to be occupied with a crop, thereverse ow due to secondary circulation results inthe signicant decrease of the air velocity (Fig. 7).The distribution of temperature for this case is presentedin Fig. 8. Due to low air velocities near the greenhouseoor there are high air temperatures in this region. Thetemperature elevation in the corners of the greenhousewas 48C higher than outside air temperature, and in themiddle of the greenhouse the air temperature was 18Chigher than the temperature of outside air.

ARTICLE IN PRESS

00.20.40.60.8

11.21.41.61.8

2

0 2 5 7Greenhouse width, m

Tem

pera

ture

diff

eren

ce (T

i - T o

), C

431 6 8

Fig. 4. Experimentally (& & &) and numerically obtained(}}}) air temperature difference (Ti To) along thegreenhouse width at a height of 11 m from the greenhouse

ground

Fig. 5. Computed contours of the air velocities of a tunnelgreenhouse with side (roll up type) openings only (configura-

tion a)

0

5

10

15

20

25

0 20 40 60 80 100 120Opening surface x wind velocity (Su), m3 s-1

Air

chan

ges p

er h

our

Fig. 3. Experimentally (& & &) and numerically obtained(& & &) air changes per hour; S, surface area (m2), u air

velocity (m s1)

T. BARTZANAS ET AL.486

3.2.3. Configuration (c): roof opening onlyThe efciency of only roof openings was examined

with this conguration. The incoming air from the roofopening guided by the greenhouse walls follows a semi-spiral trajectory and leaves the greenhouse by followingthe internal surface of the walls and the roof. Still airconditions prevails at the centre of the greenhouse(Fig. 9). As a result of the low values of air velocitieswith this conguration the air temperature inside thegreenhouse reached very high values. Air temperature atthe leeward wall of the greenhouse was 68C higher thanoutside. Due to a better air mixing with this congura-tion, caused by the air circulation cell developed at thegreenhouse interior, air temperature was uniformlydistributed in most of the greenhouse but it was 28Chigher than outside air temperature (Fig. 10).

3.2.4. Configuration (d): combined roof and sideopenings (roll-up type)

With this conguration the inuence the combinationof sides and roof openings was tested. Qualitatively theairow was similar to conguration (a) because littleexchange was observed through the roof opening asthe external ow passed directly through the sideopenings. Air velocities were slightly higher in the inletand outlet of the greenhouse and almost the same in the

rest of greenhouse interior compared with the airvelocities of conguration (a). Temperature distributionfollowed the airow pattern with warm sections neargreenhouse oor (38C warmer than outside air) andsections with similar to outside air temperatures in themiddle of the greenhouse (Fig. 11).

ARTICLE IN PRESS

Fig. 6. Temperature distribution of a tunnel greenhouse withside (roll-up type) openings only (configuration a)

Fig. 7. Computed contours of the air velocities of a tunnelgreenhouse with side (sliding door type) openings only (config-

uration b)

Fig. 8. Temperature distribution of a tunnel greenhouse withside (sliding door type) openings only (configuration b)

Fig. 9. Computed contours of the air velocities of a tunnelgreenhouse with roof openings only (configuration c)

Fig. 10. Temperature distribution of a tunnel greenhouse withroof openings only (configuration c)

VENTILATION OF A TUNNEL GREENHOUSE 487

4. Discussion

The ventilation of a greenhouse is the exchange of airbetween the inside and outside in order to: (1) dissipatethe surplus heat; (2) enhance the exchange of carbondioxide and oxygen; and (3) to maintain acceptablehumidity levels. For the four tested congurations,conguration (d) (combined roof and sides openings)achieves the highest ventilation rate, and conguration(c) (roof opening only) the lowest. As it was stated fromthe introduction, the largest ventilation rates are not, apriori, the best indicator for the ventilation perfor-mances of a greenhouse. The air velocity near the cropand the temperature difference that a given type canachieve must also be taken into account since there areimportant factors inuencing the uniform growth ofcrop. Spatial heterogeneity of air velocity and climateinside greenhouse interfere with plant activity andinuence largely crop behaviour through their effectson crop gas exchanges, particularly transpiration andphotosynthesis. For instance increasing air velocityinside the greenhouse increases convective heat transferand hence reduces the leafair temperature differences.Furthermore, air velocity might be expected to increasephotosynthesis because of the reduced boundary layerresistance to the transport of carbon dioxide, unless it isnot limiting. Waggoner et al. (1963) have observed withsugar cane plants that an airow rate of 05m s1 at aCO2 concentration of 200 vpm, gave growth equivalentto 300 vpm. If the increased air speed raises transpira-tion to such extent that water stress and hence stomatalclosure occurs, then photosynthesis will be reduced as aconsequence. High air velocities (>1m s1) in the cropcover should also be avoided since they lead toreduction in leaf area and dry matter accumulation(Kalma & Kuiper, 1966). Figure 12 presents the averagetransverse horizontal component of the air velocity forthe four different congurations along the greenhouse

width at the middle of the greenhouse normalised by theoutside wind speed and Fig. 13 the corresponding cropaerodynamic resistance which was calculated afterCampbell (1977). For the congurations (a) and (d)the normalised air velocity at the middle of thegreenhouse has relative high values (0609m s1)resulted in low values of crop aerodynamic resistance(7090 sm1). Conversely with conguration (b) and (c),the normalised air velocity is relatively low (00502m s1) resulting in high values of crop aerodynamicresistance (600900 sm1). For the same outside climateconditions a reduction of 90% of the crop aerodynamicresistance was observed which will lead to an increase ofthe convective heat transfer to the same extend. For themass transfer the effects are not so obvious becausestomatal resistance, which depends on other microcli-mate factors, plays an important role too.

ARTICLE IN PRESS

Fig. 11. Temperature distribution of a tunnel greenhouse withcombined roof and side (roll- up type) opening (configuration d)

00.10.20.30.40.50.60.70.80.9

1

0 2 3 6Greenhouse width, m

Nor

mal

ised

velo

city

1 54 87

Fig. 12. Average transverse horizontal component of the airvelocity for the four different tested configurations along thegreenhouse width at the middle of the greenhouse normalised bythe outside wind speed: configuration (a) (}}}), configura-tion (b) (& & &), configuration (c) (m m m) and

configuration (d)4 (- - - -)

0

200

400

600

800

1000

1200

0 2 4 5 6 7 8Greenhouse width, m

Crop

aer

odyn

amic

re

siste

nce,

sm-1

1 3

Fig. 13. Variation of crop aerodynamic resistance for the fourdifferent tested configurations along the greenhouse width at themiddle of the crop cover: configuration (a) (}}}), config-uration (b) (& & &), configuration (c) (m m m) and

configuration (d) (- - - -)

T. BARTZANAS ET AL.488

To better exploit our CFD analysis, the results wereexpressed with respect to the effects of the differentcongurations on (i) air exchange and (ii) temperaturedifference between inside and outside. Likewise for eachventilation type, the efciency of the ventilation wasconsidered by reducing the global ow rate Q or thetemperature difference DT between inside and outside bythe vent opening area A. The homogeneity of thetemperature distribution has been evaluated by reducingthe standard deviation of DT (s(DT)) by its means valueDT. A summary of the main results for the fourcongurations is presented in Table 3.It is rst clear that conguration (c), with only roof

opening gives the worst results and presents the lowestventilation efciency by elementary surface of opening.On contrast, conguration (d) with both roof and sideventilation presents the best ventilation efciency. Sideopenings only have very similar results.Referring to the efciency of ventilation on the

temperature difference between inside and outside, itcan be stated that an even larger difference betweentypes (d) and (a), the combination of side and roofventilations again giving the best results, with anefciency of about 55 times more than that for theconguration with only a roof opening and 15 timesmore than that for conguration (a). For the sideopenings congurations, the type (a), with roll-upopenings is about two times more efcient than the type(b) with pivoting vent openings. For conguration (d),the high ventilation efciency is not only due to thehighest total vent opening surface, but mainly to thehigh efciency of the combination of both openingtypes. This is a very important point because a large partof the greenhouse cost is due to the cost of the openingsand the determination of a good opening efciency isprimordial.Considering the homogeneity of the temperature eld,

conguration (c) gives the best results and conguration(d) the worst. The highest is the efciency on the cooling,the lowest is the homogeneity of the temperature eldand conversely.

5. Conclusions

The inuence of vent arrangement on windwardventilation of a tunnel greenhouse was numericallyinvestigated using commercial uid dynamics code. Thenumerical model was rst validated against experimen-tal data. Four different congurations of ventilatorswere investigated resulting in different ventilation ratesand different airow and temperatures patterns. Theseresults indicate that the highest ventilation rates are notalways the best criterion for evaluating the performanceof different ventilation systems in greenhouses. Suchcriteria are: the air velocities in the region covered by thecrop, the corresponding air aerodynamic resistances aswell as the efciencies of ventilation on ow rate and theair temperature differences between inside and outside.For the congurations studied in this work, the abovecriteria show that the best solution is the combination ofroof and side openings. Whenever there are only sidewindows available for ventilation, roll-up openings aremore efcient than pivoting window openings.The numerical model simulates reasonably well the

ventilation performance of greenhouses. For a givengreenhouse type the CFD model can be used as adesign tool to propose the ventilation openings design(typessizeposition) in order to achieve a well-venti-lated greenhouse and uniform climate conditions in thecrop cover. However, one must keep in mind that theresults presented in this paper concern only the specicexamined cases. With a different wind direction or adifferent greenhouse type, results could be different.

References

Bartzanas T; Boulard T; Kittas C (2002). Numerical simulationof airow and temperature distribution in a tunnel green-house equipped with a insect-proof screen on the openings.Computers and Electronics in Agriculture, 34, 207221

Bot G P A (1983). Greenhouse climate: from physicalprocesses to a dynamic model. PhD Dissertation, Agricul-tural University of Wageningen, Netherlands, 240pp

ARTICLE IN PRESS

Table 3Recapitulation of the main results of the four studied cases

Conguration Air exchange density (G/A), m s1 Temperature difference

Specific rise (DT/A) 8C/m2 Homogeneity (s(DT)DT)

1 015 0022 0712 013 0040 0563 007 0090 0334 016 0016 073

G, ventilation rate in m3 s1; A, opening surface area in m2; DT temperature difference between inside greenhouse air and outsideair in 8C; DT , mean value of temperature difference between inside greenhouse air and outside air in 8C, s(DT), standarddeviation of temperature difference between inside greenhouse air and outside air in 8C.

VENTILATION OF A TUNNEL GREENHOUSE 489

Bot G P A (2001). Developments in indoor sustainable plantproduction with emphasis on energy saving. Computers andElectronics in Agriculture, 30, 151165

Boulard T; Baille A; Mermier M; Villette F (1991). Mesures etmodelisation de la resistance stomatique foliaire et de latranspiration dun couvert de tomates de serre [Measure-ment and modelling of stomatal resistance and transpirationin a greenhouse tomato canopy]. Agronomie, 11(4), 259274

Boulard T; Draoui B (1995). Natural ventilation of a green-house with continuous roof vents: measurements and dataanalysis. Journal of Agricultural Engineering Research, 61,2736

Boulard T; Haxaire R; Lamrani M A; Roy J C; Jaffrin A(1999). Characterization and modeling of the air uxesinduced by natural ventilation in a greenhouse. Journal ofAgricultural Engineering Research, 74, 135144

Boulard T; Meneses J F; Mermier M; Papadakis G (1996). Themechanisms involved in the natural ventilation of green-houses. Agricultural and Forest Meteorology, 79, 6177

Boulard T; Papadakis G; Kittas C; Mermier M (1997). Air owand associated sensible heat exchanges in a naturallyventilated greenhouse. Agriculture and Forest Meteorology,88, 111119

Boulard T; Wang S; Haxaire R (2000). Mean and turbulentair ows and microclimate patterns in an empty green-house tunnel. Agricultural and Forest Meteorology, 100,169181

Boulard T; Wang S (2002). Experimental and numerical studieson the heterogeneity of crop transpiration in a plastictunnel. Computers and Electronics in Agriculture, 34,173190

Campbell G S (1977). An Introduction to EnvironmentalBiophysics, p 159. Springer, New York.

Demrati H; Boulard T; Bekkaoui A; Bouirden L (2001). Naturalventilation and climatic performance of a large-scale bananagreenhouse. Journal of Agriculture Engineering Research,80(3), 261271

Fatnassi H; Boulard T; Bouirden L (2001). Simulation of airux and temperature patterns in a large scale greenhouseequipped with insect-proof nets. Acta Horticulturae, 578,329337

Fernandez J E, Bailey BJ (1992). Measurements and predictionof greenhouse ventilation rates. Agricultural and ForestMeteorology, 58, 229245

Fluent, 1998. FLUENT, v.5. Fluent Europe Ltd, Shefeld,UK

Haxaire R (1999). Characterization et modelisation desecoulements dair dans une serre. [Characterizationand modeling of the air uxes in a greenhouse] PhD Thesis,149p

Hoxey R P; Moran P (1991). Full scale wind pressure and loadexperiments}Multispan 167 111m glasshouse (Venlo).Divisional Note 1594, AFRC Institute of EngineeringResearch, Wrest Park, Sisloe, Bedford, January

Jones P J; Whittle G E (1992). Computational uid Dynamicsfor building airow prediction. Buildings and Environment,27, 321338

Kacira M; Short T H; Stowell R (1998). A CFD evaluation ofnaturally ventilated multi-span, sawtooth greenhouses.Transactions of the ASAE, 41(3), 833836

Kalma J D; Kuiper F (1966). Transpiration and growth ofPhaseoulus vulgaris L. as affected by wind speed. Mededelin-gen Landbouwhogeschool, Wageningen, 66(8), 19

Kindellan M (1980). Dynamic modeling of greenhouseenvironment. Transactions of the ASAE, 23, 12321239

Kittas C; Draoui B; Boulard T (1995). Quantication of theventilation of a greenhouse with a roof opening. Agricultur-al and Forest Meteorology, 77, 95111

Launder B E; Spalding D B (1974). The numerical computationof turbulent ows. Computer Methods in Applied Me-chanics and Engineering, 3, 269289

Lee I B; Short T H (2000). Two-dimensional numericalsimulation of natural ventilation in a multi-span greenhouse.Transactions of the ASAE, 43(3), 745753.

Lee I B; Short T H (2001). Verication of computational uiddynamic temperature simulations in a full-scale naturallyventilated greenhouse. Transactions of the ASAE, 44(1),119127

Mistriotis A; Arcidianoco C; Picuno P; Bot G P A; ScarasciaMugnozza G (1997b). Computational analysis of the naturalventilation in greenhouses at low wind speed. Agriculturaland Forest Meteorology, 88, 121135

Mistriotis A; Bot G P A; Picuno P; Scarascia Mugnozza G(1997a). Analysis of the efciency of greenhouse ventilationwith computational uid dynamics. Agricultural and ForestMeteorology, 85, 317328

Mohammadi B; Pironneau O (1994). Analysis of the kepsilonTurbulence Model. Research in Applied Mathematics.Wiley, New York, Masson, Paris

Patankar S V (1980). Numerical Heat Transfer. Hemisphere,Washington

Reichrath S; Davies T W (2002). Computational uiddynamics simulations and validation of the pressuredistribution on the roof of a commercial mutli - spanVenlo-type glasshouse. Journal of Wind Engineering andIndustrial Aerodynamics, 90(3), 139149

Sase S; Takakura T; Nara M (1984). Wind tunnel testing onairow and temperature distribution of a naturally venti-lated greenhouse. Acta Horticulturae, 148, 329336

Short T H (1996). Selecting the greenhouse structure your cropneeds. Grower Talks, Summer Issue, July, 8-9

Thom A S (1971). Momentum absorption by vegetation.Quarterly Journal of Royal Meteorology Society, 97,414428

Waggoner P E; Moss D N; Hesketh J D (1963). Radiation inthe plant environment and photosynthesis. AgronomieJournal, 55, 3639.

Wang S; Boulard T; Haxaire R (1999). Air speed proles in anaturally}ventilated greenhouse with a tomato crop.Agricultural and Forest Meteorology, 96(4), 181188

Wang S; Deltour J (1996). An experimental ventilationfunction for large greenhouse based on a dynamicenergy balance model. Agricultural Engineering Journal, 5,103112

Wilson JD (1985). Numerical studies of ow through awindbreak. Journal of Wind Engineering and IndustrialAerodynamics, 21, 119154

Woodruff V I (1997). Analysis of vent designs for naturallyventilated gutter connected greenhouses. MS Thesis, OhioState University, Colombus, OH

ARTICLE IN PRESST. BARTZANAS ET AL.490

Effect of Vent Arrangement on Windward Ventilation of a Tunnel GreenhouseIntroductionMaterials and methodsExperimental greenhouseMeasurementsAir velocity measurementsTracer gas measurements

Numerical modelMesh and boundary conditionsThe equivalent porous medium approachSimulation of the ventilations rates

Vent configurations used for simulationConfiguration (a): side openings only (roll-up type)Configuration (b): side only openings (pivoting door type)Configuration (c): roof opening onlyConfiguration (d): combined roof and side openings (roll-up type)

ResultsNumerical model validationAirflow patterns and temperature distribution for different ventilator configurationsConfiguration (a): side openings only (roll-up type)Configuration (b): side only openings (pivoting door type)Configuration (c): roof opening onlyConfiguration (d): combined roof and side openings (roll-up type)

DiscussionConclusionsReferences