Articles
THE EFFECT OF SCREEN TEXTURE ON AIR FLOW AND RADIATION TRANSMITTANCE: LABORATORY AND FIELD EXPERIMENTS
Article number
1015_4
Pages
45 – 51
Language
English
Abstract
The area of cultivation under screen covers and in screenhouses is constantly increasing in the semi-arid regions of Israel, Morocco and other countries.
Banana screenhouses are currently equipped with transparent shading screens, with a nominal shading of about 8-15%. Two types of screens are widely used, woven and knitted, which differ in their texture and hence in their radiative and aerodynamic properties.
However, the market is abundant with a large variety of screens and farmers do not have sound criteria to choose the optimal one.
The goal of the present research was to investigate two types of screens of the same nominal shading level, 10%, but different texture: woven and knitted.
Wind tunnel experiments and radiation measurements on a roof of a building were done to study pressure drop and radiative properties of the screens, respectively.
In a field study a large commercial banana plantation in Northern Israel was covered with patches of the different screen types.
In the field, air velocity and global radiation were measured simultaneously below the two different screens.
Wind tunnel results showed that over the range of upstream air velocities of 0.4-2.5 m s-1 the pressure drops on the knitted screen were 56% of the values measured with the woven one.
Transmittance of the knitted screen sample to global radiation was 97% of the woven screen.
In the field, under the knitted screen, horizontal mean air velocity and global radiation were on average larger by 18 and 7%, respectively, than those measured under the woven one.
The change in radiation transmittance ratio between the two screens measured in the field as compared to the ratio measured on the roof was probably due to enhanced dust accumulations of the woven screen in the field.
Banana screenhouses are currently equipped with transparent shading screens, with a nominal shading of about 8-15%. Two types of screens are widely used, woven and knitted, which differ in their texture and hence in their radiative and aerodynamic properties.
However, the market is abundant with a large variety of screens and farmers do not have sound criteria to choose the optimal one.
The goal of the present research was to investigate two types of screens of the same nominal shading level, 10%, but different texture: woven and knitted.
Wind tunnel experiments and radiation measurements on a roof of a building were done to study pressure drop and radiative properties of the screens, respectively.
In a field study a large commercial banana plantation in Northern Israel was covered with patches of the different screen types.
In the field, air velocity and global radiation were measured simultaneously below the two different screens.
Wind tunnel results showed that over the range of upstream air velocities of 0.4-2.5 m s-1 the pressure drops on the knitted screen were 56% of the values measured with the woven one.
Transmittance of the knitted screen sample to global radiation was 97% of the woven screen.
In the field, under the knitted screen, horizontal mean air velocity and global radiation were on average larger by 18 and 7%, respectively, than those measured under the woven one.
The change in radiation transmittance ratio between the two screens measured in the field as compared to the ratio measured on the roof was probably due to enhanced dust accumulations of the woven screen in the field.
Authors
J. Tanny, M. Pirkner, M. Teitel, S. Cohen, Y. Shahak, O. Shapira , Y. Israeli
Keywords
shading, banana, screenhouse, radiation, air velocity
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