A Master of Science Thesis in Mechatronics Submitted by Hayder J. Salem Entitled, "Suppression of Unstable Flow by Using Rough Surfaces at Small Mass Flow Rates in a Radial Diffuser," August 2006. Thesis Advisor is Dr. Saad Ahmed. Available are both Soft and Hard Copies of the Thesis.
The operation of centrifugal compressor systems is limited at low-mass flow rates by fluid flow instabilities leading to rotating stall or surge. These instabilities limit the flow range in which the compressor can operate. They also decrease the performance and efficiency of the compressor. Experiments were conducted to investigate a model of radial vaneless diffuser at stall as well as stall-free operating conditions. The speed of the impeller was kept constant at 2000 rpm, while the mass flow rate was reduced gradually to scan the steady and unsteady operating conditions of the compressor. 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 reported 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 ratio on rotating stall characteristics. The frequency of the stall cells decreases when the diffuser diameter ratio increased. At lower flow rates than the critical, the rotating stall pattern with one stall cell was dominant. The instability in the diffuser was successfully delayed to a lower flow coefficient when rough surfaces were attached to one or both sides of the diffuser. The lowest values were achieved by attaching the rough surface to the shroud. This effect was more pronounced with the increase of diameter ratio. The roughness effect was also studied by using two sandpapers with two different grit sizes. It was found that the higher roughness could achieve more reduction in the flow recorded at stall inception. Results show that the roughness has no significant effect on stall cell characteristics.