Yanping Song

A Numerical Investigation of Boundary Layer Suction in Compound Lean Compressor Cascades

This paper is focused on the numerical investigation of boundary layer suction (BLS) via a slot on the suction surfaces of two compound lean compressor cascades with large camber angles as well as a conventional straight compressor cascade for comparison. The objective of the investigation is to study the influence of boundary layer suction on the performance of compound lean compressor cascades, thus to discuss the possibility of the application of boundary layer suction to improve their performance. An extensive numerical study has been carried out under different spanwise lengths, different axial positions of the slots, and different suction flow rates. The results show that the total loss of all three cascades is reduced significantly by boundary layer suction, and the largest reduction occurs at the highest suction flow rate. The axial locations of the slot have little effect on the total loss of the three cascades, which means the slots are opened within the optimal axial range in this case. The slot opened along the full span is the best one to obtain the largest reduction in total loss for all three cascades due to the alleviation of flow separation in the corner between the endwall and the suction surface. Moreover, the flow turning is increased, and pressure rise at the rear of the passage is recovered along the whole blade height via boundary layer suction along the full span, enhancing the working range of the highly loaded compressor cascades.

Effects of Boundary Layer Suction on the Performance of Compressor Cascades

The effects of boundary layer suction on the aerodynamic performance of compressor cascade are mainly determined by: (1) the location of the suction slot; (2) the suction flow rate; (3) the suction slot geometry; and (4) the aerodynamic parameters of the cascade (e.g. solidity and incidence). In this paper, an extensive numerical study has been carried out to investigate the effects of these influencing factors in a highly-loaded compressor cascade by comparing the aerodynamic performance of the cascade in order to give guidance for the application of boundary layer suction to improve the performance of modern highly-loaded compressors. The results show that boundary layer suction alleviates the accumulation of low-energy fluid at suction surface corners and enhances the ability of flow turning, and this improvement in flow behavior depends on the location of the suction slot and the suction flow rate. When the location of the suction slot and the suction flow rate are fixed, as the cascade solidity decreases from 1.819 to 1.364 and 1.091, the cascade total pressure loss is reduced at most by 25.1%, 27.7% and 32.9% respectively, and the cascade exit flow deviation is decreased by 3.1°, 4.2° and 5.0° accordingly. Moreover, boundary layer suction also has the largest effect in the cascade with smaller solidity at large positive incidences, which means that boundary layer suction is an effective way to widen the stable operating range of the highly-loaded compressor cascade. The suction slot geometry is described by the suction slot width and the suction slot angle with respect to the direction normal to the blade suction surface. The results show that the flow behavior is improved and the endwall loss is reduced further as the increase of the suction slot width. The suction slot angle has an obvious influence on the pressure inside the slot, therefore, should be considered in the design of the suction slot since the maximum pressure inside the slot is usually required.Copyright

Enhancing Aerodynamic Performances of Highly Loaded Compressor Cascades via Air Injection

This article experimentally studies the effects of air injection near the blade trailing edge on flow separation and losses in a highly loaded linear compressor cascade. Aerodynamic parameters of eight cascades with different air injection slot configurations are measured by using a five-hole probe at the cascade outlets. Ink-trace flow visualization is performed to obtain the flow details around the air injection slots. The static pressure distribution is clarified with pressure taps on the endwalls. The results indicate that air injection has little effect on the static pressure distribution on the endwalls, but improves the flow behavior at the corners between the suction surfaces and the endwalls with the decrease in losses at midspan. Slot positions have great effect on the compressor cascade performances. The optimal slot location is 25% of the blade span. The energy loss coefficient is reduced by 5.5% at most.

Effects of Air Injection on Performance of Highly-Loaded Compressor Cascades

The effects of air injection on the performance of highly-loaded straight blade compressor cascade and compound lean blade cascade were investigated numerically. Air injection was implemented via the hole/slot penetrating through the pressure and suction surfaces under the effect of the pressure difference between the two sides. Various injection configurations including one-hole, multi-hole and slot configurations were studied in the straight blade cascade first to find the optimum injection configuration in this case. Then the mechanism of the injection slot on the performance improvement of the straight blade cascade was discussed in detail. The results show that air injection provides the low-momentum fluid near the suction surface with kinetic energy, to enhance its ability to withstand the adverse pressure gradient within the compressor cascade. Among the hole/slot configurations, the slot configuration (the radial width of 4.0mm) has the most favorable results in through-flow capacity enhancement and total loss reduction, and is used in the compound lean cascade study. The effects of air injection on the performance improvement in the compound lean blade cascade are less significant than those in the straight blade cascade though the compound lean blade cascade with the injection slot has the most improvement in the aerodynamic performance when the slot is placed at an appropriate location in this study.Copyright

Large eddy simulation of saw tooth plasma actuator for improving film cooling efficiency

This paper presents results on a saw tooth plasma actuator for the inducement of flow topology and the improvement of flat plate film cooling efficiency. A phenomenological plasma model is constructed to generate the three-dimensional plasma force vectors of the saw tooth plasma actuator. The dynamics of airflow induced by the saw tooth plasma actuator on a flat plate in quiescent air are numerically investigated. The results show that the saw tooth plasma actuator pushes the fluids in all three directions and induces a three-dimensional jet flow with counter rotating streamwise oriented vortices that propagate downstream. The flow field characteristics of both cylindrical hole with and without the saw tooth plasma actuator are studied by large eddy simulation, and a comparison is made. The saw tooth plasma actuator improves the cold jet adherent performance and promotes the spanwise spreading rate of the coolant. Meanwhile, the streamwise vortices induced by the saw tooth plasma actuator suppress the development of counter-rotating vortex pair, thus delaying the diffusion of coolant in the crossflow. Accordingly, the centerline cooling efficiency and the spanwise-averaged cooling efficiency are improved by 36% and 144% at x/d = 15, compared with the baseline case without the saw tooth plasma actuator.

Numerical Study on Typical Wet Steam Flow Based on a New Two-Fluid Model

A new two-fluid model with the influence of inter-phase velocity-slip taken into account was proposed and a modified realizable k-e turbulence model was put forward as well to make the equation set of two-fluid model closed. Based on this two-fluid model, numerical simulations were conducted on typical wet steam flow in different cases. The good consistency of numerical result and the experimental result infers that this two-fluid model is provided with high accuracy and wide applicability. The flow field analysis also shows that there exist several particular positions along the flow direction. These particular positions illustrate the development mechanism of nucleation and droplet growing. In addition, further discussion on the flow in cascade then indicates that the occurrence of condensation has strong impact on the flow while the impact of inter-phase velocity-slip is relatively weaker. The composition of total pressure loss is present here, the majority of total pressure loss brought by condensation is about 8.78% of inlet total pressure while the inter-phase velocity-slip just results in a small part of about 0.42% of inlet total pressure, the rest of the total pressure loss is caused by pneumatic factors and this part is about 3.95% of inlet total pressure. In addition, discussion about the turbulence then shows that the turbulence intensity in two-phase flow is higher than that in single-phase flow.Copyright

Flow separation control on a highly loaded compressor cascade using endwall synthetic jets

This paper presents flow separation control conducted on a highly loaded compressor stator cascade using endwall synthetic jets. Numerical methods are employed and mechanisms of endwall synthetic jets in improving the cascade performance are discussed in detail. The influence of several actuation parameters is also investigated. Results show that endwall synthetic jets are able to improve the flows in the blade passage significantly, a maximum loss reduction of 21.63% and a pressure rise increment of 5.60% are obtained at design condition. Apart from energizing the low momentum fluid inside endwall boundary layer by streamwise momentum addition, endwall synthetic jets could induce a streamwise jet vortex and impede the transverse movement of endwall boundary layer through upwash and downwash. Hence, at the expense of slightly degraded near-wall flows, the formation and further evolution of passage vortex would be delayed and flows in the midspan region would be improved notably. The effectiveness of endwall synthetic jets relies on the proper selection of actuation position and jet angle. Flow control turns out to be the most efficient when the actuator is positioned at just upstream of corner separation region with a relatively small jet angle, and a large enough injected momentum is also necessary. Additionally, the adaptability of the actuation at off-design conditions is validated in the present study.

Effect of clearance height on tip leakage flow reduced by a honeycomb tip in a turbine cascade

In this paper, the control mechanism of the honeycomb tip structure on the tip leakage flow of a turbine cascade is studied experimentally and numerically, and the sensitivity of tip leakage flow characteristics to different clearance heights from 0.5% to 2% based on the blade span are mainly discussed. A flat tip is considered as a comparative case. The results show that a part of the leakage flow enters the tip honeycomb cavity, forming small-scale vortices and mixes with the upper leakage fluid, which increases the flow resistance within the clearance. In the range of clearance height variation investigated, honeycomb tip structure can effectively reduce the leakage flow, and reduce the size and strength of the leakage vortex, so that the loss of the cascade is reduced. At a large tip clearance height, the unstable split of the vortex cores causes the vortex in the honeycomb cavities near pressure side to grow in size, so that the vortex extends further into the upper gap, where the turbulent blocking effect of the vortices on the leakage flow is increased. However, due to the vortex movement and the mixing between honeycomb vortices and the upper clearance flow, there is no obvious advantage in reducing the total loss of the cascade compared to the small tip clearance height.

Active flow control by means of endwall synthetic jet on a high-speed compressor stator cascade

The underlying physics of the endwall synthetic jet in improving the aerodynamic performance of a high-speed compressor stator cascade is investigated in this paper. The effects of both injected momentum and actuation frequency are discussed in detail. In the investigations, the injected momentum is controlled by either changing the maximum jet velocity or modifying the tube diameter. Numerical results demonstrate that the streamwise momentum addition and flow mixing enhancement are the key factors of the endwall synthetic jet in improving the cascade performance. The high momentum fluid injected into the flow field can reenergize the passage flow, and the generated streamwise jet vortex contributes to the strengthening of flow mixing. Consequently, the momentum exchange between the low momentum fluid region and the main flow is enhanced and boundary layer separation on the blade suction surface is delayed. The loss characteristic in the corner region is improved as well. The intensified flow mixing will also increase the total pressure loss in the near-endwall region, which as a result will worsen the cascade performance, and hence the total effect of the endwall synthetic jet depends on the sum of its impacts. Moreover, the injected momentum and the actuation frequency have strong influences on the flow control effect. With the momentum coefficient and the reduced frequency being Cµ = 0.131% and F+ = 1.0, the reduction in total pressure loss coefficient and the increment in pressure rise coefficient are 7.3% and 3.3%, respectively.

Numerical investigation on impact of blowing control in flush-mounted S-shaped inlet to rear fan-stage performance

In this paper, numerical simulation of the whole structure of a half flush-mounted S-shaped air-intake and the rear fan stage was conducted for investigating the impact of air-intake exit distortion on the fan-stage performance. Considering substantial boundary layer ingesting, a scheme of blowing control imposed near the first bend of the inlet where the flow separation occurs was also carried out. The results show that the air intake internal separation is eliminated, and the distorted air region at its exit decreases significantly after blowing, simultaneously, the aerodynamic performance of the rear fan stage improves dramatically. The choked mass flow increases about 1.45%, and the maximum isentropic efficiency as well as the corresponding total pressure ratio at this point increases about 1.83% and 1%, respectively. In addition, the distorted fluid always covers several blade passages no matter with blowing control or not when it goes through the fan stage. Both relative total pressure and velocity of air in these distorted blade passages are lower than that in other uniform passages and have slightly increased with blowing. Under effect of the inlet flow angle increase caused by the non-uniform air admission and the adverse pressure gradient in the rear part of the flow passage, serious flow separation occurs at the stator blade suction side but mainly exists at 50% span and below and weakens after blowing control.