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Research Papers: SPECIAL SECTION PAPERS

Estimating the Thermal Radiative Properties of Shading Nets Under Natural Outdoor Conditions

[+] Author and Article Information
Ahmed M. Abdel-Ghany

Department of Agricultural Engineering,
College of Food and Agriculture Sciences,
King Saud University,
P.O. Box 2460,
Riyadh 11451, Saudi Arabia
e-mail: aghany@ksu.edu.sa

Ibrahim M. Al-Helal

Department of Agricultural Engineering,
College of Food and Agriculture Sciences,
King Saud University,
P.O. Box 2460,
Riyadh 11451, Saudi Arabia
e-mail: imhelal@ksu.edu.sa

M. R. Shady

Department of Agricultural Engineering,
College of Food and Agriculture Sciences,
King Saud University,
P.O. Box 2460,
Riyadh 11451, Saudi Arabia
e-mail: mshady@ksu.edu.sa

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received September 10, 2014; final manuscript received December 6, 2014; published online June 1, 2016. Assoc. Editor: Ziad Saghir.

J. Heat Transfer 138(9), 091004 (Jun 01, 2016) (6 pages) Paper No: HT-14-1607; doi: 10.1115/1.4032953 History: Received September 10, 2014; Revised December 06, 2014

The perforated nature of nets makes it impossible to measure the thermal radiative properties (emittance, εn, transmittance, τn, and reflectance, ρn) correctly by using any measuring device under natural conditions. In this study, a theoretical model was developed and validated to predict εn, τn, and ρn precisely by solving the model equations simultaneously. The net was tacked onto a wooden frame; thermal radiation balance was applied below and above the net surfaces and above a black substrate underneath the frame. The downward and upward thermal radiation fluxes were measured below and above the net to be used as input parameters to the simulation. Nets with different porosities (ϕ) and colors were used for the study. The results showed that the estimated εn ranged from 0.41 to 0.82 and τn ranged from 0.16 to 0.55 for the nets tested, whereas the reflectances were very low (ρn0.08). The color and porosity together affect the properties of the net. Even though under equilibrium conditions of a net with the surrounding environment, the emittance of the net is equal to its absorptance. However, the absorbed thermal radiation by the net dose not equal to the emitted radiation and the difference is a convected heat.

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References

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Figures

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Fig. 1

Magnified scanned photos for the tested nets with black background except the green-50 net with a white background

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Fig. 2

Schematic diagram of the experimental setup and the locations of the measuring devices used to measure the required parameters, dimensions in m, not to scale

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Fig. 3

Simplified illustrations of thermal radiation exchanges among the sky dome, the net sample, and the substrate

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Fig. 4

Time course for the estimated total hemispherical emittance (εn) for the four nets tested

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Fig. 5

Time course for the estimated total hemispherical transmittance (τn) for the four nets tested

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Fig. 6

Ternary plots representing the thermal radiative properties, estimated under natural conditions of solar and thermal radiation, for the four nets tested

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Fig. 7

Time course for the estimated thermal radiation emitted (2En) and absorbed (An) by the nets: (a) is for white-50 net; (b) is for green-50 net; (c) is for beige-80 net; and (d) is for dark green-80 net

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