Abstract
Headloss and longitudinal dispersion coefficients were investigated for steady uniform flow through rock beds with different gravel sizes in a range of flow rates or Reynolds numbers relating to operation of the rock bed contact-purification channel. The empirical equation for volume to surface of rock media was obtained to compare the equivalent spherical surface with the actual surface of a large number of rocks. The shape factor (α/β) of rock media was considerably larger than that of filtration sand, and α/β was 7.2-8.5 for the 350-1,200cm3 rocks. The headloss equation by Fair-Hatch explains approximately the experimental results by the use of the above shape factor. The dimensionless coefficients aL and bL for the longitudinal dispersion equation given by DL/ν=aL·RebL were obtained from the one-dimensional tracer tests in a Reynolds number range from 101-103 through rock beds of 9mm-113mm media. Such a kind of experimental study is necessary and important in the region of large Reynolds numbers (beyond the range of Darcy's law) because of the pore-system geometry and the effects of inertia and turbulence which can no longer be neglected as the mechanical dispersion becomes dominant. The coefficient is 12.38 for aL and 0.72 for bL within the limit of this experiment. The longitudinal dispersion coefficient is calculated to be 3.6-17.6cm2·sec-1 for the fluid interstitial velocities (=(flow rate)/(cross sectional area of voids)) of 0.1-0.9cm·sec-1 in a bed of 113mm media using the above coefficients of aL and bL.
