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Research Papers

J. Turbomach. 136, 101001 (2014) (13 pages);   Paper No: TURBO-13-1061;   doi:10.1115/1.4027747

Industrial gas turbines are susceptible to compressor fouling, which is the deposition and accretion of airborne particles or contaminants on the compressor blades. This paper demonstrates the blade aerodynamic effects of fouling through experimental compressor cascade tests and the accompanied engine performance degradation using turbomatch, an in-house gas turbine performance software. Similarly, on-line compressor washing is implemented taking into account typical operating conditions comparable with industry high pressure washing. The fouling study shows the changes in the individual stage maps of the compressor in this condition, the impact of degradation during part-load, influence of control variables, and the identification of key parameters to ascertain fouling levels. Applying demineralized water for 10 min, with a liquid-to-air ratio of 0.2%, the aerodynamic performance of the blade is shown to improve, however most of the cleaning effect occurred in the first 5 min. The most effectively washed part of the blade was the pressure side, in which most of the particles deposited during the accelerated fouling. The simulation of fouled and washed engine conditions indicates 30% recovery of the lost power due to washing.

J. Turbomach. 136, 101002 (2014) (7 pages);   Paper No: TURBO-14-1070;   doi:10.1115/1.4027971

This study forms part of a program to develop a micro-electro-mechanical systems (MEMS) scale turbomachinery based vacuum pump and investigates the roughing portion of such a system. Such a machine would have many radial stages with the exhaust stages operating near atmospheric conditions while the inlet stages operate at near vacuum conditions. In low vacuum such as those to the inlet of a roughing pump, the flow can still be treated as a continuum; however, the no-slip boundary condition is not accurate. The Knudsen number becomes a dominant nondimensional parameter in these machines due to their small size and low pressures. As the Knudsen number increases, slip-flow becomes present at the walls. The study begins with a basic overview on implementing the slip wall boundary condition in a commercial code by specifying the wall shear stress based on the mean-free-path of the gas molecules. This is validated against an available micro-Poiseuille classical solution at Knudsen numbers between 0.001 and 0.1 with reasonable agreement found. The method of specifying the wall shear stress is then applied to a generic MEMS scale roughing pump stage that consists of two stators and a rotor operating at a nominal absolute pressure of 500 Pa. The zero flow case was simulated in all cases as the pump down time for these machines is small due to the small volume being evacuated. Initial transient two-dimensional (2D) simulations are used to evaluate three boundary conditions, classical no-slip, specified-shear, and slip-flow. It is found that the stage pressure rise increased as the flow began to slip at the walls. In addition, it was found that at lower pressures the pure slip boundary condition resulted in very similar predictions to the specified-shear simulations. As the specified-shear simulations are computationally expensive it is reasonable to use slip-flow boundary conditions. This approach was used to perform three-dimensional (3D) simulations of the stage. Again the stage pressure increased when slip-flow was present compared with the classical no-slip boundaries. A characteristic of MEMS scale turbomachinery are the large relative tip gaps requiring 3D simulations. A tip gap sensitivity study was performed and it was found that when no-slip boundaries were present the pressure ratio increased significantly with decreasing tip gap. When slip-flow boundaries were present, this relationship was far weaker.

J. Turbomach. 136, 101003 (2014) (14 pages);   Paper No: TURBO-13-1169;   doi:10.1115/1.4027967

Aerodynamic instabilities such as stall and surge may lead to mechanical failures. They can be avoided by better understanding and accurate prediction of the associated flow phenomena. Numerical simulations of rotating stall do not often match well the experiments as the number of cells and/or their rotational speed are not correctly predicted. The volumes surrounding the compressor have known effects on rotating stall flow patterns; therefore, an increased need for more realistic simulations has emerged. In that context, this paper addresses a comparison of numerical stall simulation in a compressor alone with a numerical stall simulation including the additional compressor rig. This study investigates the influence of the upstream and downstream volumes of the compressor rig on the rotating stall flow patterns and the consequences on surge inception in a high-pressure, high-speed research compressor. The numerical simulations were conducted using an implicit, time-accurate, 3D compressible Reynolds-averaged Navier–Stokes (URANS) solver. First, rotating stall is simulated in both configurations, and then the outlet nozzles are further closed to bring the compressors to surge. The numerical results show that when the compressor rig is accounted for, fewer cells develop in the third stage and their rotational speed is slightly higher. The major difference linked to the presence of the rig lays in the existence of a 1D low frequency oscillation of the static pressure, which affects the entire flow and modifies surge inception. The analysis of the results leads to a calculation of the thermo-acoustic modes in the whole configuration, which shows that this low frequency corresponds to the third thermo-acoustic mode of the complete test-rig.

J. Turbomach. 136, 101004 (2014) (15 pages);   Paper No: TURBO-14-1005;   doi:10.1115/1.4027968

Aerodynamic instabilities such as stall and surge may occur in compressors, possibly leading to mechanical failures so their avoidance is crucial. A better understanding of those phenomena and an accurate prediction are necessary to improve both the performance and the safety. A surge event in a compressor threatens the mechanical integrity of the aircraft engine, and this remains true for a research compressor on a test rig. As a result, few experimental data on surge are available. Moreover, there are technological, restrictive constraints that exist on test rigs and limit severely the type of data obtainable experimentally. This partially explains why numerical simulation has become a usual, complementary and convenient tool to collect data in a compressor, as it does not disturb the flow nor does it encounter technological limits. Despite the inherent difficulties, an entire surge cycle has been simulated in a high-speed, high-pressure, multistage research compressor, using an implicit, time-accurate, 3D compressible unsteady Reynolds-averaged Navier–Stokes solver. First, the paper presents the main features of the surge cycle obtained, along with those from the experimental cycle, for a validation purpose. Four phases compose the surge cycle: surge inception, the reversed-flow phase, the recovery phase, and the repressurization of the compressor flow. All of them are described, and focus is put on surge inception and the reversed-flow phase, as they induce greater risk for the mechanical integrity of the machine.

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