Abstract
A study was made of the pumping capacity of pitched blade impellers (two, three, four, five and six blade pitched blade impellers with pitch angles α = 35° and 45°) coaxially located in a cylindrical pilot plant vessel with cylindrical draught tube provided with a standard dished bottom. The draught tube was equipped with four equally spaced radial baffles above the impeller pumping liquid upwards towards the liquid surface. In all investigated cases the liquid aspect ratio H/T = 1.2 - 1.5, the draught tube / vessel diameter ratios DT /T = 0.2 and 0.4 and the impeller / draught tube diameter ratio D/DT = 0.875. The pumping capacity of the impeller was calculated from the radial profile of the axial component of the mean velocity in the draught tube below the impeller at such an axial distance from the impeller that the rotor does not affect the vorticity of the flow. The mean velocity was measured using a laser Doppler anemometer with forward scatter mode in a transparent draught tube and a transparent vessel of diameter T = 400 mm. Two series of experiments were performed, both of them under a turbulent regime of flow of the agitated liquid. First, the optimum height of the dished bottom was sought, and then the dependences of the dimensionless flow rate criterion and the impeller power number on the number of impeller blades were determined for both pitch angles tested under conditions of optimum ratio HT /DT. It follows from the results of the experiments that the optimum ratio HT /DT = 0.25 when the cross sectional areas of the horizontal flow around the bottom and the vertical inflow to the draught tube are the same. For all the tested pitched blade impellers the impeller power number when α = 45° exceeds the value of this quantity when pitch angle α = 35°, while the flow rate number when α = 35° exceeds this quantity when α = 45°. On the other hand, the absolute values of the impeller power number when the draught tube was introduced correspond fairly well to the dimensionless impeller power input measured in a system without a draught tube. However, the absolute values of the flow rate number found in the former system are significantly lower than the dimensionless impeller pumping capacity determined in the latter system. The hydraulic efficiency of pitched blade impellers N3Qp/Po for the investigated geometry of the agitated systems does not depend on the number of impeller blades, but it is significantly lower than the quantity determined in an agitated system with a dished bottom but without the draught tube.
Highlights
The pumping capacity of a pitched blade impeller (PBT) in a draught tube is defined as the amount of liquid leaving the outlet base of the draught tube per unit time
The pumping capacity of a PBT can be measured by the indirect ”flow follower” method [5] and calculated from the measured mean time of liquid primary circulation, or calculated from the radial profile of the axial component of the mean velocity in the impeller discharge stream [4, 6] or in the draught tube [1, 2] by means of integration over the cross section of the impeller rotor region or over the cross section of the draught tube
The flow rate of the agitated liquid in a cylindrical draught tube with up flow PBT does not depend on the height of the slot between the draught tube and the dished bottom, when the cross sectional area in the slot is the same or greater than the cross sectional area of the draught tube
Summary
The pumping capacity of a pitched blade impeller (PBT) in a draught tube is defined as the amount of liquid leaving the outlet base of the draught tube per unit time. Where r is the density of the agitated liquid This quantity is independent of the impeller Reynolds number when it exceeds ten thousand. A combination of the dimensionless quantities and Po gives the so called hydraulic efficiency of the impeller, defined either as [6] The former definition is suitable for agitated systems with a draught tube [1], and the latter for systems without this internal. This study analyses the pumping and energetic efficiency of various pitched blade impellers in tall vessel with a draught tube, a suitable geometry for industrial applications where high homogeneity of solid-liquid suspension and temperature distribution are desirable. The velocity will be determined by the laser Doppler anemometer and the impeller power input by means of a strain gauge torquemeter
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