Modeling Installation Effects on Motorized Impeller Performance

Abstract

Motorized impeller performance is typically measured with the impeller exhausting into a plenum. In practice, impellers are usually packaged in obstructive modules or fan trays. The exhaust is usually obstructed at the sides of the impeller, and above and below it. Restricting exhaust to either the front or the rear is particularly detrimental. In this paper, a relatively simple CFD impeller modeling approach, designated the distributed blade model, is described and used to predict the measured performance of three packaged impellers, ranging in size from 133 mm in diameter to 225 mm. This new approach does not model the individual impeller blades but rather approximates their effect by using a rotating volumetric resistance to force the flow within the impeller bladed region to follow the approximate blade camber lines. Deviation (slip) is permitted to match the impeller torque to the known torque. This computationally inexpensive model produces results in good agreement with measured values of torque and pressure rise for the impellers operating with a free (unobstructed) discharge, as well as with the performance reduction due to packaging in modules. Minimal data are required to predict packaging (installation) losses - impeller torque and RPM at a single mid-to-high flow, and an assumed or measured average blade angle, are sufficient. Pre-calculations with a free discharge to obtain such things as the impeller inlet axial-to-radial flow turning loss, needed with some other modeling approaches, are unnecessary.