(98b) The Effect of Tip Chord Angle On Hydrofoil Efficiency At Suspending Solids

Solids suspension mixing applications exist throughout the chemical industry.  The hydrofoil impeller is the impeller of choice for such applications due to its axial flow pattern and low torque requirements.  Although often lumped together as one style of impeller, regardless of specifics, the exact combination of hub chord angle, blade twist and thus tip chord angle leads to different torque requirements for a given suspension application.  Minimizing both power and torque leads to an optimized mixer design from both an initial capital cost and annual operating costs. 

The A510 is a three bladed hydrofoil impeller that can be designed over a range of tip chord angles.  The flow patterns established at each tip chord angle are similar to one another as shown by LDV data.  As tip chord angle is increased, the ratio of flow generated versus power changes (i.e. the impeller power number and flow number vary).  This allows the design engineer to optimize impeller selection for a given application, which considers not only initial and operating costs but also considers mechanical design details such as critical speed and impeller wear.  For example, impeller wear can be an issue in slurry suspension applications and thus minimizing tip speed is important.

This study compares the performance of the A510 in a solids suspension application where the tip chord angle is varied.  Solids were modeled using two sizes of glass beads (to model two different settling velocities) with the liquid being water.  A 0.45 m diameter tank with a torispherical bottom and four 38 mm wall baffles was used for the test set up.  Three 150 mm A510 impellers were tested, with the only variable being the final blade angle on each.  The just suspended speed was determined visually in each case and the data was used to compare the performance of each on a power and torque basis.  Although a qualitative study due to the small sample size, this study demonstrates how small differences in hydrofoil design can lead to a more optimized mixer selection.