An Effect of a Baffle Length on the Power Consumption in an Agitated Vessel

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Abstract

An effect of the baffle length L on power consumption has been studied for an agitated vessel of inner diameter D=0.6 m equipped with short baffles. The measurements were carried out for the following agitators: Rushton and Smith turbines, pitched blade turbine (β=45°, Z=3) and propeller. Power characteristics Ne=f(Re) for different geometrical parameter L/H of the baffles were obtained within the turbulent regime of the fluid flow in the agitated vessel. The results of the experiments were approximated by means of , , , , , , .

Introduction

An electric power Pe required to drive an electromotor coupled with the shaft of the agitated vessel has been calculated on the basis of the knowledge of the power characteristics which are determined for a given type of impeller. The experimental results of the power consumption are usually presented in the form of a dimensionless equationNe=CReAFrBwhere dimensionless numbers are defined as follows:Ne=Pn3d5ρ;Re=nd2ρη;Fr=n2dgand power consumption P is determined from the equationP=2πnMwhere P<Pe, M [Nm]=torque and n [s−1]=impeller speed. For turbulent fluid flow in the agitated vessel equipped with baffles, exponents A and B are approximately equal to zero, then Eq. (1)simplifies toNe=CThe coefficient C depends on the geometrical parameters of the agitator, baffles and vessel. The power curves for different agitators working in the vessel equipped with standard baffles (i.e. baffle length L is equal to liquid height H in the vessel) were published in the literature 1, 2, 3, 4, 16, 17, 18, 19, 20, 21, 23, 24.

During recent years, the studies of the power consumption for the impeller-vessel systems of different geometry have been continued by many research workers. Kamieński [5], Saito et al. [6]and Ibrahim and Nienow [7]determined power characteristics for new types of the high-speed impellers. The results of the power consumption for the low-speed impellers—anchor, ribbon, helical-screw and gate agitators—were presented in the papers 8, 9, 10, 11, 12, 13, 14. Problems of the dependency on scale of power input in the agitated vessels were analysed in the papers 15, 25, 26. An influence of the shape of the vessel 22, 27, 28, 29and bottom [30]of the agitated vessel on the power consumption was also tested.

The shorter baffles (L<H), located at a certain distance p (p≥0) from the bottom of the vessel, can appear to be an interesting solution for such applications as, for example, the producing of the suspension. Power consumption for the agitated vessel equipped with the short baffles was tested by Strȩk and Karcz [31]only. The investigations [31]were performed for the several impellers; therefore, the studies should be continued for the other types and geometries of the agitators.

In the paper, the experimental results of the measurements of an effect of a baffle length on the power consumption in the agitated vessel with different high-speed agitators have been presented.

Section snippets

Experimental

The experiments were carried out in the agitated vessel of inner diameter D=0.6 m, filled with the tap water up to height H=D. Four planar baffles of width B=0.1D and different length L were mounted in the vessel with the flat bottom. As Fig. 1 shows, the geometrical parameter L/H was varied within the range L/H〈0; 1〉, i.e. six series of the measurements were conducted for the L/H=0; 0.33; 0.5; 0.67; 0.83 and 1, i.e. p/H=1; 0.67; 0.5; 0.33; 0.17 and 0. The propeller, Rushton and Smith turbines

Results and Discussion

In total, 600 experimental data of the power number Ne as a function of Reynolds number Re were obtained. The power characteristics Ne=f(Re) for different agitators and a given value of the geometrical parameter p/H are presented in Fig. 4Fig. 5Fig. 6Fig. 7Fig. 8Fig. 9Fig. 10 and, in detail, the values of the power numbers are collected in Table 2. The results of the experiments have revealed that power characteristics Ne=f(Re) depend on the length L of the baffle, where L=H−p, and H=const. As

Conclusions

The results of the experimental studies of the power consumption in the agitated vessel with the short baffles can be summarised as follows:
Within the turbulent regime of the fluid flow, the power number Ne depends strongly on the length L of the baffle.
The greatest effect of the geometrical parameter p/H on the power number Ne is observed for the system with Rushton turbine.
The dependencies of the power number on the geometrical parameter L/H (or p/H) were approximated by means of , , , , , ,

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