للكاتب :
A. R. El-SHAMY
Mech. Eng. Dept., Faculty of Eng. (Shoubra), Benha Univ.,
108 Shoubra St., Cairo, Egypt
ABSTRACT
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
beds.