Mechanism of Circumferential Static Pressure Oscillation in a Centrifugal Compressor With Volute

Under the action of an asymmetric volute structure, a non-uniform flow field is formed in the circumferential direction of the centrifugal compressor. During the throttling process of the compressor at different rotational speeds, the static pressure presents a double-peak distribution of two high static pressure strips, one of which is induced by the volute tongue. However, the formation mechanism of the other high static pressure strip remains unclear. In this regard, computations of the steady and unsteady flows in a centrifugal compressor with and without a volute are performed. The purpose of removing the volute is to simplify the boundary conditions at the diffuser exit, eliminate the circumferential pressure gradient distribution in the volute, and retain the circumferential local high static pressure region induced by the VT; thereafter, the circumferential static pressure distributions in the diffuser and impeller are observed. The results indicate that after eliminating the pressure gradient at the diffuser exit along the rotation direction, only local high static pressure boundary conditions can result in the formation of two high static pressure strips in the diffuser and impeller. The local high static pressure at the exit redistributes the mass flow rate at the impeller outlet, forming two regions with high airflow velocity in the diffuser; this leads to the appearance of two high static pressure strips in the circumferential direction. With the increase in the pressure amplitude of the high static pressure at the diffuser exit, the oscillation amplitude of the circumferential pressure is intensified, and the pressure peaks of the two high static pressure strips increase. However, the circumferential positions of the two static pressure peaks practically remain constant. At large mass flow rates, the pressure reduction along the circumferential direction at the diffuser exit preclude the formation of two circumferential high static pressure strips in the diffuser and impeller.

Thrust Force Measurements in an Axial Steam Turbine Test Rig

Large mechanical drive steam turbines used in the oil & gas industry are operating at increasingly higher inlet pressure, generating higher shaft power. Those higher power requirements result in larger disk diameters and surface areas. High thrust forces can be a result, due to both the high inlet pressure and large disk surface area. Industry standards require oversizing of thrust bearings to handle uncertainty in thrust predictions. These factors make improvement in thrust prediction accuracy and mitigation strategies important. A full-size, axial flow steam turbine test rig capable of measuring turbine thrust, and static pressure in the upstream rotor-stator cavity was built and commissioned. The test rig was operated in single stage configuration for the tests reported here. The rotor disk had balance holes and stationary axial face seals near the disk rim. The face seals divide the upstream rotor-stator cavity into inner and outer circumferential cavities. The rotor-stator cavity upstream of the rotor disk was instrumented, on the stationary wall, to measure the radial and circumferential pressure distribution. Bearing thrust was measured with load cells. Tests varied nominal pressure ratios (1.2, 1.5, 2.0 and 3.0), velocity ratios (0.35–0.6), admission fractions (0.25–1.0) and shaft leakage flow rates. Circumferential pressure asymmetry, due to partial admission operation, was confined to the outer cavity. The inner cavity pressure coefficient was circumferentially uniform at all operating points. The average pressure coefficient in the upstream rotor-stator cavity generally decreased as the shaft leakage flow rate coefficient increased. Increased leakage flow rate coefficient also increased the magnitude of the upstream directed or negative thrust.

Pressure Profile Measurements Within the Gas Film of Journal Foil Bearings Using an Instrumented Rotor With Telemetry

Foil bearings are strong candidates to support oil-free turbomachinery. Although foil bearings are a widely used technology, models describing their behavior are not validated using the film pressure, which is the fundamental variable of any fluid film bearing. This paper presents pressure profiles measured within the gas film of a journal foil bearing. The pressure is measured using an instrumented rotor with embedded pressure probes and wireless telemetry. The measurements yield the simultaneous circumferential pressure profiles at two axial positions inside the bearing. Proximity probes on the bearing allowed the measurement of the corresponding rotor orbits. The bearing under investigation is a bump-type compliant journal bearing, with a nominal diameter of 40 mm, an L/D = 1, and was tested up to 37.5 krpm. Load-displacement and break-away tests were performed on the test bearing in order to identify bearing parameters necessary for reproducibility. The pressure profiles are compared to a frequency domain foil bearing model. This paper is a step toward further fundamental understanding of the foil bearing behavior and the validation of the rich modeling literature.

Pressure Profile Measurements Within the Gas Film of Journal Foil Bearings Using an Instrumented Rotor With Telemetry

Foil bearings are strong candidates to support oil-free turbomachinery. Although foil bearings are a widely used technology, models describing their behavior are not validated using the film pressure, which is the fundamental variable of any fluid film bearing. This paper presents pressure profiles measured within the gas film of a journal foil bearing. The pressure is measured using an instrumented rotor with embedded pressure probes and wireless telemetry. The measurements yield the simultaneous circumferential pressure profiles at two axial positions inside the bearing. Proximity probes on the bearing allowed the measurement of the corresponding rotor orbits. The bearing under investigation is a bump type compliant journal bearing, with a nominal diameter of 40mm, an L/D = 1, and was tested up to 37.5 krpm. Load-displacement and break-away tests were performed on the test bearing in order to identify bearing parameters necessary for reproducibility. The pressure profiles are compared to a frequency domain foil bearing model. This paper is a step towards further fundamental understanding of the foil bearing behavior, and the validation of the rich modelling literature.

Effects of endwall profiling near the blade leading edge on the sealing effectiveness of turbine rim seal

Endwall profiling designed to reduce secondary flow loss may change the local pressure distribution which has an impact on the sealing effectiveness of a rim seal. This paper presents a numerical comparison of the sealing effectiveness of the rim seal and the aerodynamic performance of the blade with five different endwall profiling near the blade leading edge. Three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) equations coupled with a fully developed shear stress transport (SST) turbulent model are utilized to investigate the sealing effectiveness and the flow characteristics of turbine rim seal. The numerical method for the pressure field and sealing performance of turbine rim seal is validated on the basis of published experimental data. The total-to-static efficiency of the blade and the minimum sealing rates of the rim seal with five endwall profiling near blade leading edge are compared. The baseline, convex and concave cases are selected to investigate the transient variation of the sealing effectiveness and the flow field in the disc cavity. In comparison with baseline case, the convex endwall makes the high pressure area move forward, increases the mainstream circumferential pressure fluctuation, and reduces the sealing effectiveness. The concave endwall reduces the local pressure and the mainstream circumferential pressure fluctuation, and increases the sealing effectiveness. However, the concave endwall profiling enhances the vortex in the blade passage and increases the secondary flow loss. The flow field near the rim seal with different endwall profiling is illustrated and analyzed.

Fluid-mechanic interactions between a pipe flow having circumferential pressure variations and a piezometer ring

PurposeThis paper aims to investigate and understand the fluid mechanics of piezometer rings, a device frequently encountered in engineering practice.Design/methodology/approachThe investigation, implemented by numerical simulation, is based on turbulent flow in a pipe with a 90-degree bend. The pipe Reynolds numbers ranged from approximately 50,000 to 200,000. Two rings, with different dimensions, were investigated. Each ring consisted of four radially deployed straight segments of tubing which connect the pipe to a surrounding circular ring. The interconnections between the pipe and the ring were situated at 90-degree intervals around the circumference of the pipe.FindingsThe focus was directed to optimal circumferential locations of the radial connections, the optimal circumferential locations for accurate pressure measurements and the pressure drop penalty incurred by the use of a piezometer ring. For both of the investigated piezometer ring configurations, it was found that measurement locations situated just beyond the points of intermediate circumferential pressure variations were suitable for determining accurate values. The pressure drop was seen to increase because of the presence of the ring. For the smaller ring configuration, the increase in relative pressure drop was on the order 15 per cent, whereas the larger ring configuration lead to a 10 per cent increase.Originality/valueThis is the first attempt known to the authors to investigate and understand the fluid mechanics of piezometer rings.