A Master of Science Thesis in Mechatronics Engineering Submitted by Dhafar S. Mohammed, "Suppression of Rotating Stall by Throttle Rings Control in Vaneless Diffusers of Centrifugal Compressors," April 2006. Available are Both Soft and Hard Copies of the Thesis.
Centrifugal compressor systems are widely used in many industrial applications. However, the operation of such systems is limited at low-mass flow rates by fluid flow instabilities. These instabilities lead the compressor to rotating stall or surge. Compression systems of gas turbines can exhibit several types of instabilities: impeller flow instabilities, surge and finally flow instabilities in the diffuser known as rotating stall which this investigation is restricted to. These instabilities limit the flow range in which the compressor can operate. Surge and rotating stall also restrict the performance and efficiency of the compressor. iv Experimental techniques are used for investigating a model of radial vaneless diffuser at stall as well as stall-free operating conditions. The speed of the impeller was kept constant while the mass flow rate was reduced gradually to scan the steady and unsteady operating conditions of the compressor. Generally speaking, for stall-free conditions only, the flow rate through the compressor was kept constant at its optimum design value while studying the flowfield for each of the three-impeller speeds tested ranging from 1000 to 2000 RPM. For each speed, the flow rate through the compressor was gradually decreased until flow instability is initiated at the diffuser. The flow rate was further reduced to study the characteristics of rotating stall. These measurements were also repeated for diffuser diameter ratios, Do / Di, of 1.5, 1.75, and 2.0 with diffuser width ratio, b / Di, of 0.055 to determine the effects of diffuser diameter ratios on rotating stall characteristics. At stall-free operating conditions were also reported. Furthermore, the frequency of the stall cells, as well as its strength, was found to increase as the impeller speed increases, irrespective of the number of stall cells. For a fixed diffuser width, the frequency of the stall cells increases as the diffuser diameter ratio was decreased. At a constant impeller speed and lower flowrates than the critical flowrate, the rotating stall pattern with one stall cell was dominant over the pattern with two stall cells. In addition, the instability in the diffuser was successfully delayed to a lower flow coefficient when throttle rings were attached to either / or both of the diffuser walls in order to reduce the diffuser exit flow area. Finally, the results showed that an increase of the blockage ratio improves the stability of the system. This work involves new technologies and offers opportunities to develop techniques that can handle the analysis of unsteady flow signals and pattern recognition techniques. This work might play a major role in air-conditioning industries as well as small gas turbines using centrifugal compressors.