Experimental Investigation Of Flow Mixing In Pebble Beds And Bundles Of Twisted Rods
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S. Rimkevicius, E. Uspuras & A. Bagdonas
The experimental results of the thermal hydraulic characteristics of pebble beds and bundles of twisted oval rods is an issue of high importance in the designing of effective heat exchangers, modular gas-cooled or low-capacity mobile nuclear reactors, in chemical and other industries. This paper concentrates on the test sections made up of axially streamlined cylinders with two kinds of a filling in a space between cylinders - pebble bed or bundles of twisted oval rods. Two types of pebble beds, as well as two types of bundles of twisted rods, were investigated in order to find the influence of different packing on the flow mixing and hydraulic drag. The flow mixing factors were computed by comparing the calculated and measured temperature distributions produced by a heat spot in the bed. The drag coefficient of the pebble bed with a regular rhombic structure was measured to be 2.6-3 times higher than that of the random pebble bed. The mixing factors in the case of the randomly packed bed on the average are 2.3 times higher than in the case of the regular rhombic packing. Thus, the randomly arranged pebble bed is preferable against the regular rhombic structure from the point of view of design simplicity, drag coefficient and flow mixing. Hydraulic drag of bundle with twisted rods at s/d = 12.2 (s – pitch of twisting, d – large diameter of twisted oval rod) was found to be about 5 % higher than in the case of s/d = 24.4. Mixing factors values are about 2 times higher in the bundle with twisted rods at s/d = 12.2. Thus, the bundle of rods with lower pitches of twisting is preferable due to considerable intensification of flow mixing at a slight increase of hydraulic drag. Keywords: pebble bed, flow mixing, hydraulic drag, thermal hydraulic, spheres, twisted rods, experimental investigation.
pebble bed, flow mixing, hydraulic drag, thermal hydraulic, spheres, twisted rods, experimental investigation.