Mohamed Alqaradawi

Unsteady CFD Simulation for High Speed Centrifugal Compressor Operating Near Surge

Centrifugal compressor surge and stall are an instabilities flow and system phenomenon, which can lead to a loss of engine power, large pressure transients and, in some cases, failure of the compressor and bearing system. Three-dimensional, compressible, unsteady Navier-Stokes equations are solved, using the sliding mesh at the interface between the moving and stationary parts. The investigated computational domain is composed of inlet pipe, inlet bell, impeller followed by a diffuser. The impeller side cavities are included in the computational domain, to make the simulation more close to the actual case. The present article focuses on the unsteady pressure and velocity results within the impeller and diffuser, at an operating condition close to the surge of the compressor. The computational data compares favorably with the measured data, from literature, for the same compressor and operational point. The pressure fluctuation results are analyzed for the surge and design operating conditions. Through the acquisition of both velocity and transient pressure data, the time evaluation of the unsteady flow during surge is revealed.Copyright

Numerical Simulation and Control of Rotating Stall in a Transonic Centrifugal Compressor

This study presents a numerical simulation of precursors and characteristics of rotating stall in a centrifugal compressor with vaned diffuser. The aim is to further understand the causes of stall inception, characteristics of stall process and to investigate reliable simulation of stall control methods and aid in their optimization. Initial numerical results show that the tip leakage flow and early separation in the impeller blades are causing weakness of radial velocity and deviation of fluid angle at impeller exit.Also, results show that the vaneless region between impeller and diffuser near shroud surface is the key factor of developing radial velocity deficiency into stall cells leading to pressure fluctuations on impeller blades. The present study confirms that injection of air at the shroud surface of the vaneless region has an effect on modifying fluid angle entering diffuser and on delaying of stall inception to occur at lower mass flow rates.Copyright

Numerical Investigation of Steady Air Injection Flow to Control Rotating Stall in Centrifugal Compressors

This paper concerns the role of air injection method in stabilization and stall control in centrifugal compressors. The main aim is to find the best arrangement of air injection parameters such as injection angle and injection mass flow rate in order to optimize the injection performance for stabilizing the compressor and increasing the surge margin. Numerical model was built to simulate high speed transonic centrifugal compressor working at an operating point close to surge. Air was injected at 12 locations at the vaneless region between the impeller and the diffuser at shroud surface with 5 different injection angles and 3 different injection mass flow rates. Results showed that the best injection method is when using an injection angle of 30° with injection mass flow rate of 1.5% of the design mass flow rate and the worst injection method is the injection at angle of 180° (reverse tangent injection). Results also indicated that by using air injection, the number of stalled diffuser passages is decreased compared to the case of no injection. The most significant result of this paper is that using an angle of injection around twice the value of the diffuser vane angle gives the best results and makes the ideal correction of the fluid kinetic energy and fluid angle at the diffuser inlet. It was found that injecting air at an angle less than the diffuser vane angle weakens the effect of injection and doesn’t increase kinetic energy of the fluid at diffuser inlet. It was also found that injecting air at an angle larger than the diffuser vane angle corrects the fluid direction but, at the same time, decreases the fluid kinetic energy at diffuser inlet.Copyright

Numerical Simulation of Stall Development Into Surge and Stall Control Using Air Injection in Centrifugal Compressors

This study presents a numerical simulation of the formation of rotating stall and the initiation of surge in order to study the connection between stall and surge in centrifugal compressors. Also, the current paper introduces an optimization of the air injection method as a way to increase the surge margin. Results showed that during stall, the compressor is exposed to velocity and pressure fluctuations varying with time, and these fluctuations are increased suddenly and causing surge initiation. The major part which is responsible for the sudden increase in fluctuations is the vaneless region because it was found that the problem starts at the impeller exit near the shroud surface and then transfers to the impeller inlet. Results also showed that during surge, forces on the impeller blades increase to nearly double of its initial value and then decrease again. By using air injection at the vaneless region with different injection angles, it was found that injection with angle of 30° has a good effect on preventing surge and minimizing the pressure fluctuations comparing to other injection angles results. Results showed finally that the surge margin can be increased by using the injection with angle of 30° and with injection mass flow rate of 1% of the design inlet mass flow rate and this causes the surge limit to shift from 4 kg/s to 3.9 kg/s.Copyright

Numerical Investigation of Rotating Stall Characteristics and Active Stall Control in Centrifugal Compressors

This paper focuses on providing better view for the understanding of rotating stall phenomenon in centrifugal compressors by using numerical simulations and presents a study of the role of air injection method in delaying stall inception by using different injection parameters aiming at increasing the efficiency of this method. Results showed that the formation of stall begins at the impeller inlet due to early flow separation at low mass flow rates and due to the increase of the turbulence level and the absence of fluid orientation guidance at the vaneless region. The flow weakness causes back flow that results in the formation of the tip leakage flow which causes stall development with time. Results also showed that using air injection at specified locations at the vaneless shroud surface at injection angle of 20° and with injection mass flow rate of 1.5% of the inlet design mass flow rate, can delay the stall onset to happen at lower mass flow rate about 3.8 kg/s comparing with using injection with angle of 10° with different injection mass flow rates and also comparing with the case of no injection.Copyright

Numerical Simulation of Using Combined Active and Passive Stall Control Techniques in Centrifugal Compressors

This study presents a numerical simulation of stall control by using the casing treatment method which represents the passive control technique and also by using the air injection method which represents the active control technique. The casing treatment types used are the holed casing treatment and the casing skewed slots. There are four proposed numerical models; the first model is the regular shape model without casing treatments, the second model combines the holed casing treatment with the skewed slots, the third model is similar to the second one but enhanced by air injection at specified locations at the slot wall while the fourth model contains casing slots only. Results showed that the holed casing treatment helps in reducing the tip leakage flow by transferring it from the blades tip area to the impeller inlet. Also, it was found that the casing slots can cause a flow circulation and increase the kinetic energy around the position of slots and this leads to decreasing the number of stall cells. Results indicated also that the air injection at the casing slots increases the circulated flow rate inside the slots and minimizes the flow separation. It was found that the third model which contains casing slots only is the best comparing with other models in terms of decreasing the stall areas and increasing the compressor stability. Final results confirmed also that the casing treatment method increases the stability but reduces the isentropic efficiency.© 2014 ASME

Identifying Crack Parameters in Slow Rotating Machinery Using Vibration Measurements and Hybrid Neuro-Particle Swarm Technique

Low-cycle fatigue-initiated cracks may result in failure in slow-rotating equipments. Online monitoring to identify such fault/crack parameters, namely crack size and crack location, would be critical in providing an early warning signal to the operator and would be used in calculating estimate about the remaining safe life of the equipment in operation. In an earlier study, a scaled-down slow-rotating washer drum was constructed to experimentally investigate the vibrations of a cracked rotor and/or drums. Cracks were simulated using the bolt removal method (BRM), and the vibration signals identifying signatures of certain cracks were measured. Thereafter, a 3D finite element model was used to solve the forward analysis of the inverse problem of crack identification. In this paper, the scaled-down experimental setup is introduced to cracks at different locations of the drum/rotor. Vibration signals identifying signatures of such cracks are measured. Since noisy signals, similar patterns of faults, and similar vibration fault signals create particular challenges for feature extraction systems, two techniques for feature extraction are considered and compared in this work. The fast Fourier transform (FFT) of the vibration signals showing variation in amplitude of the harmonics as time progresses are presented for comparison with the full time signal feature extraction. A hybrid particle-swarm artificial Neural Networks (neuro-particle swarm) is used to identify both the crack size and crack location. The hybrid neuro-particle swarm technique is compared with the previously investigated fuzzy genetic algorithms.Copyright