PRESSURE DROP AND HEAT TRANSFER IN PACKED BEDS WITH SMALL TUBE-TO-PARTICLE DIAMETER RATIO

Measurements of overall voidage, velocity distribution, and pressure drop in cylindrical packed beds of spherical particles in which the tube-to-particle diameter ranged from 5 to 50 and the particle Reynolds number ranged from 30 to 1,700 are reported. Owing to the change in the packing structure -within the bed, the overall voidage decreases with increasing diameter ratio, becoming constant for values of the latter greater than about 16. After exit from the bed, the velocity distribution changes continuously with distance downstream of the bed, toward that typical of flow in an empty pipe. As such, measurements made above the bed are not truly representative of flow within the bed. The pressure drop across the bed can be correlated by the Ergun-type equation, but the coefficients in this equation vary with diameter ratio. These variations can be explained by considering the effect of the tube wall on the local packing structure. Results of measurements on the heat transfer from an electrically heated rod, placed coaxially within an annular bed of spherical particles, to air flowing in an outer annulus external to the can containing the particles are presented for a range of channel Reynolds number between 4 X 10^3 and 1.7 X 10^4. The use of a porous can instead of a smooth, nonporous one results in an increase in the rate of heat transfer, and this effect increases with increasing Reynolds number.