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Published data on flow field variation caused by various blade design patterns are scarce. Most designs exhibit significant flow separation and adverse pressure gradients effects that lower mixing efficiency. In view of the design potentials of the CFD methodology, the flow field variations caused by different blade designs could be classified in order to be able to predict the spread of the low pressure regions behind blades while retrofitting existing equipment towards energy-saving performance without decreasing the impeller blending and dispersing capacity related to the geometry considered.

The aim of the present study is to reveal such variations for some conventional flat blade modifications. The performance of three flat and hollow blade design modifications comprising slotted and perforated blades are examined. The specific power drawn, pumping capacity, deformation rate and turbulence intensity are determined and compared. The impeller power effectiveness is discussed in terms of the strain deformation rate produced. Evidence for enhanced performance of slotted and perforated designs is presented.

Impeller blade geometry is a basic precondition for effective mixing, as well as an important optimization target. In view of the recent trends towards greenhouse gas limitation,1 the successful design of a stirrer has become extremely important bearing the insight of possible energy-saving. On the other hand regarding productivity, the spread of composite materials have allowed the realisation of more complex forms and have lifted up the limits on shape. Thus, development of impeller design has remained a priority objective.

While it is impossible to quote all relevant works considering impeller designs, one could refer to a recent interest on blade shape, as manifested in several representative studies.2–8 Yet, a systematic
description of the relationship between shape modification and mixing impeller flow field is lacking. More comparative information is needed in order to be able to discriminate between the properties assigned
to different geometry. In view of the design potentials of the computational fluid dynamic (CFD) methodology,9,10 the flow field variations caused by different blade designs can be classified.
In order to allow such classification, the characteristic deviations of impeller flow field parameters caused by typical modifications of the classified blades, e.g. flat, inclined, fluid-foil ones, should be examined.

The present study is focused on modified flat and hollow blade designs. Work on blade modifications in a similar context has been reported by Smith and coworkers. We have extended these studies by looking at the effect that slots on the blades could produce on the pressure distribution at the blade rear. Design hints of slots were borrowed from the fluid dynamics of airfoils. The aim of this paper is to characterise the hydrodynamic properties of impellers with slots following a systematic CFD evaluation of the parameters of the blades’ flow field.

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