الفهرس 
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Abstract
Experimental investigation of the natural convection in rectangular enclosure differentially heated for solar simulation was carried out. The effect of Rayleigh number, angle of inclination and enclosure aspect ratio on the Nusselt number were evaluated. The use of rectangular and tubular compound honeycomb to reduce heat losses from the enclosure that heated from below and its effect on the enhancement of natural convection heat transfer was also studied. Closed enclosure with low vacuum conditions as a low molecular density space for enhancement of the heat transfer process was also studied. Natural convection along upward facing heated flat plate was also analyzed and reported to check the test rig performance as a preliminary study. Test rig was developed for laboratory tests to measure the temperature distributions and heat transfer from the bottom surface to the air naturally flowing inside the enclosure. The arrangement is composed essentially of an electrically heated aluminum plate. Another aluminum plate containing a copper tubing water-cooling coil is made to provide a temperature gradient. A pump is used to circulate the cooling water from the coil to a reservoir to prevent cooling water temperature fluctuation. A constant heat flux is directed inward to the air in the enclosure between the two aluminum plated. A guard heater to prevent back heat losses is mounted below the main heater. A special pivoted mechanism is designed to control and adjusts the tilt angle of the test section assembly. All experiments are performed at steady state conditions. Measurements are made using the specially made apparatus with 0.2x0.4-m closed enclosure employing five aspect ratios of 8, 10, 13.3, 20, and 40. The Rayleigh number was varied up to 3.6x105, and the tilt angle with respect to horizontal were 0, 15, 30, 45, 60, 75 and 90deg. The flow inside the enclosure of 13.3 aspect ratio was simulated by means of ” ANSYS-FLORTAN” CFD (Computational Fluid Dynamics) computer package which utilized the finite element approach for computation. Stream function and temperature contours in the model region were obtained. The effect of tilt angle on the flow geometry and heat transfer characteristics was investigated.
The results showed that there is an inverse relation between the average Nusselt number and the enclosure tilt angle until a certain angle lies in an interval that starts at 600. Increasing the tilt angle any more slightly affecting the average Nusselt number is observed.
Decreasing the enclosure aspect ratio from 20 to 8, increases the averages Nusselt number by about 2-3 times. A correlating equation is derived and found to fit very closely the present data for the inclined enclosure. This correlation is valid for a range of Rayleigh numbers of 3.6x 103 to 3.6x105 , tilt angle of 150≤0≥ 750, and aspect ratio of 8 to 40.
Using of the compound honeycomb layers in helpful for convection suppression in solar collector applications. It reduces the average Nusselt number by a factor of about 25% compared with the closed rectangular enclosure. A negligible effect of the compound honeycomb shape on the average Nusselt number for closed enclosures can be concluded especially at the working range of solar collections tilt angle (about 300).
Using of enclosure with low vacuum conditions reduces the surface temperature difference. An empirical correlating equation was derived. It was valid for tilt angle of 150≤0≤750, and aspect ratio of ≤AR≤40.
Computational between the results obtained in the experimental and computational phases showed good agreement, indicating that the computer package is reliable as a powerful tool for computational in such cases.