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CONTRIBUTION TO CHARACTERISATION OF INSECT-PROOF SCREENS: EXPERIMENTAL MEASUREMENTS IN WIND TUNNEL AND CFD SIMULATION

2005; International Society for Horticultural Science; Issue: 691 Linguagem: Inglês

10.17660/actahortic.2005.691.53

2406-6168

Diego Luis Valera Martínez, F. Molina, A.J. Álvarez, Juan Pablo Aguilar-López, J.M. Terrés-Nicoli, Adrián Feria Madueño,

Irrigation Practices and Water Management

The aim of the present study was to evaluate geometrical characteristics and airflow resistance of eleven different insect-proof screens by three different experimental procedures: equipment based in water-flow suction, low-speed wind tunnel, and CFD simulations. The two first arrangements had the same principle, in that air was forced through the test samples in order to create a pressure drop. Last analyses were carried out by numerical simulations of airflow through insect-proof screens using a commercial fluid dynamics code based in Finite Element method (ANSYS/ FLOTRAN v8.0). Previously, an analysis images system, called EUCLIDES v1.1, was designed with MS Visual Basic 6.0 running under MS Windows, for the analysis of the screens samples images captured with a microscope. A geometrical characterization of the eleven screens materials was carried out using this software tool. The software allows to determine all the geometric parameters that characterize the screens, as thread diameter and distances between two adjacent threads in two directions, from the four coordinates that defined each pore. The results obtained in this work show that the eleven screens can be classed in three groups, corresponding with the fibre density, with similar porosity and airflow properties (permeability and inertial factor). However, sample 8 has a small thread diameter and screen thickness that decreased the pressure drop coefficient. The results suggest that equations based on the porosity of the screen and the Reynolds number can be used to calculate the pressure drop coefficient. INTRODUCTION In an integrated pest management system, exclusion of pests should be one of the first tactics considered to reduce the need for other control measures. Whiteflies (Bemisia tabaci) and thrips (Frankliniella occidentalis) are among the most important pests of greenhouse crops in Almeria (Acebedo, 2004). As in other parts of the world (Taylor et al., 2001), most of the losses produced in Spain by Bemisia tabaci are due to its role as a virus vector (Guirao et al., 1997). Tomato yellow leaf curl virus (TYLCV) was reported for the first time in Spain in the autumn of 1992 (Moriones et al., 1993). Because of pest-acquired resistance, management practices that rely on insecticides are growing increasingly less effective, and less environmentally and economically appropriate. Reductions in pest populations (Baker and Jones, 1989) and lower incidence of insect-transmitted diseases (Baker and Jones 1989, 1990) have been documented when screening is used. Exclusion screens for the greenhouse may become a necessary alternative to pesticide use. However, airflow resistance, primarily a function of hole or mesh size, reduces the ventilation rate. Many efficacious screens have a small hole size and are more resistant to airflow than are more open-meshed screens (Bethke and Paine, 1991; Bell and Baker, 2000). The aim of the present study was to evaluate geometrical characteristics and airflow resistance of eleven different insect-proof screens by three different experimental procedures: low-speed wind tunnel, equipment based in water-flow suction, and CFD simulations. Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 442 MATERIALS AND METHODS In order to obtain the airflow characteristics of porous screens we measured the pressure drop caused by the insect-proof screen for different velocities in the range 0.1 to 12 m s. The first experiments were carried out in a wind tunnel with a cross-section of 420 mm × 360 mm and 5.2 m long. A helicoidally fan of 460 mm diameter driven by multi-speed 2.2 kW 3-phase induction electric motor HCT-45 (Sodeca S.A., Sant Qurze de Besora, Spain). Airflow was controlled by a Micromaster 420 AC inverter (Siemens Energy & Automation Inc., Alpharetta, USA) that allow decreased the fan motor speed from 0 to 2865 rpm, with digital microprocessor control and a set point resolution of 0.01 Hz. The static pressure drop through the screen was measured by a pressure transducer SETRA (Setra Systems Inc., Boxboruogh, USA), connected to two Pitot tubes, one 430 mm upstream and one 430 mm downstream from the tested screens (Terres-Nicoli et al., 2004.). Air velocity was determined connecting the static pressure and total pressure tapings of the upstream Pitot tube to another pressure transducer MKS (MKS Instrument Inc., Andover, USA). For air velocity lower to 1 m s the screen samples (diameter 115 mm) were mounted in a test duct (length 220 mm) separated by PVC rings of 10 mm thickness, 70 mm internal and 115 mm external diameter containing 20 screen samples (Fig. 1). Originally, the downstream tube was connected to the upper side of a water reservoir by a flexible pipe. The measurements are based on the pressure drop caused by natural suction of air through the samples as a result of water flow induced by gravity (Miguel et al., 1997). Subsequently, we repeated the test for the sample number 1, for air velocity between 0.1 and 1 m s, connecting the downstream tube to a fan NMB-4715KL (NMB Technologies Inc, Chatsworth, USA). The airflow supplied by the fan was regulated by controlling the rotational speed of the fan, function of the voltage that was varied from 3 V to 12 V with a DC power supply HY-3010 (DavJones Technology, Singapore). No statistical differences were observed between the tests made with the fan and the water reservoir for sample number 1. For reason of simplicity, we use the fan for air supply with the rest of samples (from 2 to 11). The pressure drop was measured using an inclined tube manometer AIRFLOW type 504 (Airflow Developments Limited, Buckinghamshire, England). The manometer, with a full-scale range of 125 Pa and an accuracy of 1 Pa, was connected to two Pitot tubes (150 mm upstream and 90 mm downstream). The measurement of air velocity was taken using a multifunction digital handheld instrument TESTO® 445 (Testo S.A., Cabrils, Spain) with a hot-bulb probe. This instrument has a measurement range from 0 to 10 m s with an accuracy of ±0.03 m s and resolution of 0.01 m s. The equipment also contains a temperature probe (thermistor NTC) with a range of –20 to 70oC and an accuracy of ±0.4oC. Last analyses were carried out by numerical simulations of airflow through insectproof screens using a commercial fluid dynamics code based in Finite Element Method (ANSYS/FLOTRAN v8). In the simulation, it was assumed that a woven screen comprises a large number of small pores, and a similar flow passed through each. The pore was modelled as the intersection of four cylinder (Teitel and Shklyar, 1998). Previously, an analysis images system, named EUCLIDES v1.1, was designed with MS Visual Basic 6.0 running under MS Windows, for the analysis of the screens samples images captured with a microscope DMWB1 (Motic Spain S.L., Barcelona, Spain). We used a plan 4X achromatic lens that provided images with a resolution of 0.0105 mm/pixel. Once the image was captured a different program (Photo Finish 4.0) was used to convert the images in true colour to black and white. A geometrical characterization of the eleven screens materials was carried out using the EUCLIDES software. This software allows to determine all the geometric parameters that characterize the screens, as thread diameter, d, and distances between two adjacent threads in two directions, Dhx and Dhy, from the four coordinates that defined each pore, recognised automatically by the software. The program also measures the porosity as the ratio of geometric pore area, calculated from the vertices coordinates, to whole area. For all screens we analysed three samples of 1 cm size.