Effects of segmented layer suction and micro-vortex generator on a high-load compressor cascade performance

The axial location of full-span boundary layer suction is studied to explore the influences of suction slot on the cascade performance. At the design condition, the slot with 50% axial location shows a superior capability to reduce the total pressure loss. At the near stall condition, the more upstream of the suction slot is moved, the more total pressure loss is reduced, and the suction slot with a location of 0.7 axial chord length cannot effectively reduces the total pressure loss in all conditions. Moreover, a rearranged segmented suction slot according to the distribution characteristics of the flow reversal region is developed and compared with full-span boundary layer suction. The segmented suction slot shows significant advantages in delaying the stall occurrence, and the stall point is delayed from 7.9° to 10.0° compared with the baseline. According to a quantitative analysis method selected to measure the performances of flow control technologies, the wake loss is significantly reduced by the segmented suction slot. Finally, a set of micro-vortex generator is introduced in the cascade with a segmented suction slot, and the conclusion indicates that the portion near the end-wall is very effective to reduce the flow loss.

Study on the model of semi-open-end twist in compact spinning with lattice apron

The twist mechanism of the fiber strand in the condensing zone in compact spinning is complex. This paper proposes a dynamic model to evaluate the additional twist of the fiber strands. Based on the flow simulation in the condensing zone, the fiber trajectory in the suction slot was simulated and obtained. Several spinning parameters such as suction slot angle, suction slot width, negative pressure, and shape of suction slot, were varied to show their effects on the additional twist. The simulation results indicated that by increasing the suction slot angle from 5° to 10° the additional twist increased significantly. Higher negative pressure also leads to an increase in the additional twist. The suction slot width has a greater effect on the fiber trajectory than on the additional twist. An arc-shape suction slot increased the additional twist compared with a linear-shape one. An experimental test conducted revealed a precise agreement with the simulation results.

Design strategies and numerical simulation of highly loaded aspirated compressor blades

Boundary layer suction can effectively eliminate flow separations and increase aerodynamic loading of axial compressors. The design methodology of highly loaded aspirated compressor blades was developed and illustrated in this study. In this work, Computational Fluid Dynamics (CFD) methods were first validated with existing data and then used to develop the design strategy of aspirated compressor blades. Design strategies for higher blade performances, including higher loading, larger stall margin and larger blade thickness near the suction slot of aspirated blades, were investigated through analyzing a series of highly loaded aspirated cascades with diffusion factors (DF) around 0.71. Results showed that the design methodology proposed in this paper was appropriate for designing highly loaded aspirated compressor blades. Under the condition of no boundary layer suction (BLS), severe flow separations of highly loaded blades were tailored at the aft part of suction surface by adopting the “ski-slope” velocity distribution, which almost remained unchanged within a large incidence range. The “ski-slope” velocity distribution was appropriate for removing flow separations and beneficial for obtaining thicker blade. High loading of aspirated blade was achieved by the postpositional suction peak and minimum velocity distribution on pressure surface. The stall margin of highly loaded aspirated cascades could be enlarged by designing the velocity distribution upstream of the suction slot and by selecting suction peak position and solidity. A three-dimensional (3D) highly loaded aspirated cascade was designed based on a two-dimensional (2D) cascade. Both the trailing edge separation and corner separation of the 3D highly loaded aspirated cascade were eliminated successfully with coupled suction surface and endwall suction.

Effects of location, shape and width of a suction slot on the water removal performance of a hollow stator blade

Experimental tests were implemented on a wet steam test rig to investigate the effects of location, shape and width of a suction slot on the water removal performance of a hollow stator blade. A straight cascade with varying outlet Mach numbers and suction pressure differences was used for the tests. The inlet flow conditions were consistent with the real running condition before the last stage stator of a 1000-MW nuclear steam turbine. Results show that the flow Mach number and suction pressure difference affect the amount of water removed. A moderate increase in the suction pressure difference triggers water film vaporisation, which decreases water removal performance. The amount of water removed continuously increases, as the slot location moves from 0.24 to 0.42 times the axial chord in the suction surface. Compared with the straight slot, the step-shaped slot cannot improve the water removal performance. On the contrary, the result is poor when the Mach number is above 0.7 because additional sharp corner leads to more serious water vaporisation. A suction slot with an arc-shaped inlet significantly improves the water removal performance by eliminating water film vaporisation under the test conditions. A 0.35-mm-width suction slot is apt to allow water film across, and a 2-mm-width suction slot cannot form an effective suction pressure difference along the slot height, both leading to poor water removal performance. Meanwhile, 0.7- and 1-mm-width suction slots promote good water removal performance, but the latter is less affected by water vaporisation.

Mechanical modeling of an arc-shaped suction slot for compact spinning and analysis of additional twists

In this study, an arc-shaped suction slot was designed for a pneumatic compact spinning system with a lattice apron. A model was built via mechanical analysis of a fibrous strand in an arc-shaped suction slot to calculate additional twists inserted during condensing. The equations can be solved by using the Runge–Kutta method. The simulation results showed that negative pressure and frictional coefficient of the lattice apron and the fibrous strand have significant effects on additional twists of the strand in an arc-shaped slot. The curvature radius of the arc slot has some influence on the additional twists, while that of the condensing surface has no obvious effect on the additional twists. The radius of the strand may significantly influence the additional twists, but the radius of the strand itself is affected by various other factors. Spinning tests were carried out based on the model design. These results verified the additional twist model of the arc-shaped suction slot.

Comparison of Steady and Unsteady Flows in a Transonic Radial Vaned Diffuser

Boundary layer suction can be effective in delaying compressor surge, if the surge is triggered by flow separation on the shroud- or hub-casing. This work aims at positioning a suction slot in a radial vaned diffuser, which is thought to be the limiting component in a centrifugal compressor, such as the one considered here. The location of the slot is determined based on the results of both steady and unsteady flow simulations of a transonic centrifugal compressor of a turboshaft. Although the overall performance of the compressor is well-described by steady RANS, large discrepancies are observed between the steady and unsteady simulations of the diffuser flow, discrepancies imply different flow-separation scenarios. Steady results show more low-momentum fluid near the hub, whereas it is concentrated near the shroud in the unsteady simulations, hence no valid physical conclusions can be expected from the steady simulations. Analysis of the instantaneous skin-friction distribution from the unsteady simulations reveals that the separation is fixed and leads to a slot location on the shroud casing, near the diffuser main-vane suction side, so that it covers the range of separation saddle positions as the operating point is changed.

Studies on Characteristic Frequency and Length Scale of Shock Induced Motion in Transonic Diffuser Using a High Order LES Approach

Unsteady transonic flows in diffuser have become increasingly important, because of its application in new propulsion systems. In the development of supersonic inlet, air breathing propulsion systems of aircraft and missiles, detail investigations of these types of flow behavior are very much essential. In these propulsion systems, naturally present self-sustaining oscillations, believed to be equivalent to dynamically distorted flow fields in operational inlets, were found under all operating conditions. The investigations are also relevant to pressure oscillations known to occur in ramjet inlets in response to combustor instabilities. The unsteady aspects of these flows are important because the appearance of undesirable fluctuations generally impose limitation on the inlet performance. Test results of ramjet propulsion systems have shown undesirable high amplitude pressure fluctuations caused by the combustion instability. The pressure fluctuations originated from the combustor extend forward into the inlet and interact with the diffuser flow-field. Depending on different parameters such as the diffuser geometry, the inlet/exit pressure ratio, the flow Mach number, different complicated phenomena may occur. The most important characteristics are the occurrence of shock induced separation, the length of separation region downstream of the shock location, and the oscillation of shock location as well as the oscillation of the whole downstream flow. Sajben experimentally investigated in detail the time mean and unsteady flow characteristics of supercritical transonic diffuser as a function of flow Mach number upstream the shock location and diffuser length. The flows exhibited features similar to those in supersonic inlets of air-breathing propulsion systems of aircraft. A High-order LES turbulence model developed by the author is assessed with experimental data of Sajben on the self-excited shock oscillation phenomena. The whole diffuser model configuration including the suction slot located at certain axial location around the bottom and side walls to remove boundary layer, are included in the present computation model. The time-mean and unsteady flow characteristics in this transonic diffuser as a function of flow Mach number and diffuser length are investigated in detail. The results of study showed that in the case of shock-induced separation flow, the length and thickness of the reverse flow region of the separation-bubble change, as the shock passed through its cycle. The instabilities in the separated layer, the shock /boundary layer interaction, the dynamics of entrainment in the separation bubble, and the interaction of the travelling pressure wave with the pressure fluctuation region caused by the step-like structure of the suction slot play very important role in the shock-oscillation frequency.