الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 174 |  |  |
| | | from | 174 | | |
Abstract
Rib geometry was investigated experimentally and numerically in the present work to give an enhancement of heat transfer coefficient and fluid flow characteristics at equal mass flow rate and pumping power constraints through the cooling passage. The combined effects of rib pitch geometry, and alignment on the static pressure distributions, heat transfer coefficient, and friction losses around rib surface for fully developed turbulent flow in a rectangular duct with ribbed-walls were determined for Reynolds number ranging from 12,000 to 65,000. The rib height to channel hydraulic diameter ratio (e/De) and angle of attack were fixed at 0.081 and 900, respectively.
A special test rig was built up for fluid flow and heat transfer measurements, special simulated models were built up also, for flow visualization. The rib pitch to height ratios were 10, 20, and 40; the rib height to each duct hydraulic diameter (E/de) and duct height ratios (e/H) are kept at 0.081 and 0.0625, respectively. Three different rib shapes (trapezoidal, square, and inverted trapezoidal) were studied to investigate its effects on the fluid flow and heat transfer characteristics (local static pressure, reattachment length, friction losses, velocity distributions, and both local and average Nusselt numbers). ANSYS FLOTRAN CFD computer package was applied to predict the flow separation, recirculation, reattachment, and wake regions to support the experimental results from the flow visualization and the ribbed-duct measurements.
A full field measuring technique was used to measure the velocity vectors magnitude and directions. The fluid velocity could be determined from the predicted seeding particle velocity. The recirculating flow around the ribs with corner vortices upstream and downstream is difficult in measuring while that can be described by flow visualization. The flow visualization processes were carried out to simulate channels with two ribbed-walls (e/H = 0.0625) with different rib pitch to height ratios (P/e = 5, 10, 15, and 20) and shapes (square, trapezoidal, and inverted trapezoidal ribs).
A comparison between the experimental and computational results and also with the previous results was investigated where good agreement was found between them. Experimental correlations for the average static pressure coefficient (Cp). average friction factor (f), and average Nusselt number ratio (Nu/Nu,) were developed in terms of Reynolds number and rib pitch to height ratio for fully developed turbulent flow in ribbed-channel with different rib shapes.
The static pressure coefficient has lower values for inverted trapezoidal rib than square and trapezoidal ribs. The average static pressure coefficient on rib surface decreases as the rib pitch decreases. The static pressure coefficient for in-line rib alignment (symmetric) has lower values than those for staggered alignment and one ribbed-wall (asymmetric) has lower values than those for staggered alignment and one ribbed-wall (asymmetric) while it was independent on Reynolds number.
The local temperature distributions had lower values on the inverted trapezoidal rib surface than that on the grooved square rib. The grooved square ribs provide heat transfer enhancement in comparison with the solid square ribs by about 23%for asymmetric rib alignment. Also, the heat transfer augmentations of grooved square rib were about 2.46-2.66 and 1.22- 1.26 times those of the smooth duct and ribbed-duct with solid square rib, respectively, while pay about 8 times the friction loss penalty of smooth duct. The inverted trapezoidal rib increases the average Nusselt number ratio about 2.79-2.6, 2.77-2.55, and 2.05-1.788 times over that of smooth duct for p/e=10,20,and 40, respectively, while pay about 7.5 times the friction loss penalty of smooth duct, At same pumping power constraint, the inverted trapezoidal rib shape had a higher heat transfer enhancement at p/e = 20 than that at p/e = 10, this is reserved for same mass flow rate.
It could be concluded that, the rib shape has a high significant effect on the local heat transfer coefficient on rib surface where the heat transfer enhancement depends on the turbulent transport which was influenced by the rib shape while the frequency of flow acceleration had a no significant effect.