للكاتب : 


Mech. Eng. Dept., Faculty of Eng. (Shoubra), Benha Univ.,

108 Shoubra St., Cairo, Egypt


Experiments were conducted to investigate the thermal performance of a turbulent flow of air in a

vertical rectangular duct filled with a porous medium (packed or fluidized beds). The test section

was a vertical rectangular duct of cross section 360 mm x 40 mm with an aspect ratio of 9 and

has a hydraulic diameter of 72 mm and 800 mm total length. The test section consists of two

principle heating surfaces and two Plexiglas side walls. Heat is supplied to the test section

through only the two principle heating surfaces while the other two side walls are unheated. The

two principle heating surfaces were made of highly polished brass plates of 700 mm length,

350 mm width and 3 mm thick. The porous media used in the present work were PVC spheres

with a nominal diameter of 12 mm. For the packed bed, the randomly arranged sphere particles

filled up in the test section with a porosity ε = 0.43. The fluidized bed was simulated by stringing

nylon thread through the spherical particles in the test section. The porosity can be adjusted by

changing the space between the particles. Particles were strung with different spaces for fluidized

beds with ε = 0.875 ~ 0.99. Reynolds number based on the duct hydraulic diameter was ranged

from 13,000 to 40,000, and the particle Reynolds number was varied from 2,500 to 7,000.

Generally, a porous medium provides a large thermal dispersion and a solid-fluid contact area

many times greater than the duct surface area, thus greatly enhancing the heat transfer. The

results show that the heat transfer coefficient increases with the decrease in the porosity and the

increase in the flow Reynolds number. A maximum Nusselt number enhancement ratio of about

3.086 corresponding to a 41.87 fold increase in the friction factor was obtained for the packed

bed with ε = 0.43 for the flow with Reynolds number of 13,000. For the simulated fluidized bed,

the highest Nusselt number enhancement ratio of about 2.288 corresponding to a 25.827 fold

increase in the friction factor was obtained for the simulated fluidized bed with ε = 0.875 for the

flow with Reynolds number of 16,364. The simulated fluidized bed duct has a higher thermal

performance than the packed duct for all bed configurations studied. The greatest efficiency

index was found for the simulated fluidized bed duct with ε = 0.99 and flow Reynolds number of

15,791, where, a maximum efficiency index of about 0.235 was found. The present experimental

data were compared against those available in the literature and good correspondence was

noticed. Generalized correlations for the average Nusselt number and the flow friction factor

were obtained as functions of Reynolds number and the porosity of both the packed and fluidized


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