Abstract
A study was made of the pumping capacity of pitched blade impellers in a cylindrical pilot plant vessel with four standard radial baffles at the wall under a turbulent regime of flow. The pumping capacity was calculated from the radial profile of the axial flow, under the assumption of axial symmetry of the discharge flow. The mean velocity was measured using laser Doppler anemometry in a transparent vessel of diameter T = 400 mm, provided with a standard dished bottom. Three and six blade pitched blade impellers (the pitch angle varied within the interval a Îá24°; 45°ń) of impeller/vessel diameter ratio D/T = 0.36, as well as a three blade pitched blade impeller with folded blades of the same diameter, were tested. The calculated results were compared with the results of experiments mentioned in the literature, above all in cylindrical vessels with a flat bottom. Both arrangements of the agitated system were described by the impeller energetic efficiency, i.e, a criterion including in dimensionless form both the impeller energy consumption (impeller power input) and the impeller pumping effect (impeller pumping capacity). It follows from the results obtained with various geometrical configurations that the energetic efficiency of pitched blade impellers is significantly lower for configurations suitable for mixing solid-liquid suspensions (low impeller off bottom clearances) than for blending miscible liquids in mixing (higher impeller off bottom clearances).
Highlights
The pumping capacity of a pitched blade impeller (PBT) is defined [1] as the amount of liquid leaving the rotor region of the impeller, i.e., the cylindrical volume circumscribed by the rotating impeller blades, per unit time
The pumping capacity of a PBT can be measured by the indirect “flow follower” method [1] and calculated from the measured mean time of liquid primary circulation, or calculated [3, 5, 6] from the known radial profile of the axial component of the mean velocity in the impeller discharge stream leaving the impeller rotor region by means of integration over the circular cross section of the impeller rotor region
The mean velocity field in the impeller discharge flow just below the impeller rotor region was measured with a laser Doppler anemometer (LDA)
Summary
The pumping capacity of a pitched blade impeller (PBT) is defined [1] as the amount of liquid leaving the rotor region of the impeller, i.e., the cylindrical volume circumscribed by the rotating impeller blades, per unit time. This quantity is an important process characteristic of the PBT and plays an important role when calculating the blending or homogenization time of miscible liquids in mixing [2,3], in the design of continuous-flow stirred reactors [4] and in calculating the process characteristics of solid-liquid suspensions [5], i.e. the impeller frequency for just off bottom suspension. Where r is the density of the agitated liquid This quantity is independent of the impeller Reynolds number when it exceeds ten thousand. The velocity profile will be determined by laser Doppler anemometry
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