Stagger-angle Setting Research Articles

On the Flow Physics During the Transition of a Variable Pitch Fan From Nominal Operation to Reverse Thrust Mode

Abstract The flow field during the transition of variable pitch fans (VPFs) from nominal operation to reverse thrust mode at typical “Approach Idle” engine power setting and aircraft touchdown speed of 140 knots is described in this work. An integrated airframe-engine-VPF research model that features a future 40,000 lbf geared high bypass ratio engine installed on a twin-engine airframe in landing configuration is used to explore the flow field in a fully transient unsteady Reynolds-averaged Navier–Stokes (URANS) simulation with imposed wall motion. A novel methodology that implements an adaptation of a mesh displacement equation to mimic the fan blade airfoil rotation is developed. The implementation of this method with gradual, small-step deformation along with an automated mesh update routine enables a high quality, near “real-time” simulation of the complete transition. The flow field during transition is characterized by the evolution from full typical forward flow through the engine to the development of massive recirculation regions at the feather pitch setting and finally to development of a reverse flow from the bypass nozzle to the fan passages. In the paper, the transient development of the various flow features through different stations of the engine flow path apropos the fan blade airfoil rotation to reverse thrust mode are discussed in detail. Also, the temporal development of the mass flow ingested through the engine, airframe decelerating force, and distorted flow at the core engine inlet are described. A hitherto unresolved fan power peaking during the middle of the transition and higher power requirement at reverse thrust mode is captured. The effect of fan rotational speed in terms of engine power setting and the aircraft touchdown velocity on the transition flow physics is explored. A comparison of this fully transient approach with discrete steady-state runs for different stagger angle settings is presented. The flow physics during transition to reverse thrust mode as described in this study is critical in understanding the feasibility of using VPF for reverse thrust in future aircraft. The new capability to study the transition in a fully transient simulation can be used as a design development aid to define design and control characteristics of the reverse thrust VPF.

Inception of Casing Wall Flow Instabilities and Downstream Flow Evolution in Subsonic Axial Compressor Rotors At High Rotor Stagger Angles

Two critical aspects of the casing wall flow structure in a subsonic axial flow compressor are studied in this paper: flow mechanism leading to stalling at different stagger angle configurations; and the effect of tip leakage flows on these flow phenomena. In some of the previous works, casing wall flow instabilities similar to the ones discussed in the present paper were reported, typically for high stagger angle rotor designs. However under what conditions flow tends to take such shapes were not discussed in detail. This work attempts to address this through a detailed numerical study on unswept and forward swept rotors, for different stagger angle settings, and for varied levels of rotor tip gaps. Three candidate rotors (unswept, 20° true swept, 20° axially swept) are studied with three tip clearances (0.0%, 0.7%, and 2.7% of the blade chord) for three stagger angle changes (0°, +3° and +5°). Stagger angle is found to have significant effect on suction surface flow separations. Tip clearance is observed to be critical on the inception of casing wall flow instabilities, especially at higher stagger angles. Hub endwall flow structure and the downstream flow evolution are also discussed in the paper.

Flow Distortion Into the Core Engine for an Installed Variable Pitch Fan in Reverse Thrust Mode

Abstract The flow distortion at core engine entry for a variable pitch fan (VPF) in reverse thrust mode is described from a realistic flow field obtained using an integrated airframe-engine model. The model includes the VPF, core entry splitter, complete bypass nozzle flow path wrapped in a nacelle, and installed to an airframe in landing configuration through a pylon. A moving ground plane to mimic the rolling runway is included. 3D Reynolds-averaged Navier–Stokes (RANS) solutions are generated at two combinations of VPF stagger angle and rotational speed settings for the entire aircraft landing run from 140 to 20 knots. The internal reverse thrust flow field is characterized by bypass nozzle lip separation, pylon wake, and recirculation of flow turned back from the VPF. A portion of the reverse stream flow turns 180 deg with separation at the splitter leading edge to feed the core engine. The core engine feed flow exhibits circumferential and radial nonuniformities that depend on the reverse flow development at different landing speeds. The temporal dependence of the distorted flow features is also explored by an unsteady Reynolds-averaged Navier–Stokes (URANS) analysis. Total pressure and swirl angle distortion descriptors, as defined by the Society of Automotive Engineers (SAE) S-16 committee, and, total pressure loss into the core engine are described for the core feed flow at different operating conditions and landing speeds. It is observed that the radial intensity of total pressure distortion is critical to core engine operation, while the circumferential intensity is within acceptable limits. Therefore, the baseline sharp splitter edge is replaced by two larger rounded splitter edges of radii, ∼0.1x and ∼0.2x times the core duct height. This was found to reduce the radial intensity of total pressure distortion to acceptable levels. The description of the installed core feed flow distortion, as described in this study, is necessary to ascertain stable core engine operation, which powers the VPF in reverse thrust mode.

Fan Flow Field in an Installed Variable Pitch Fan Operating in Reverse Thrust for a Range of Aircraft Landing Speeds

Abstract The installed flow field for a variable pitch fan (VPF) operating in reverse thrust for the complete aircraft landing run is described in this paper. To do this, a VPF design to generate reverse thrust by reversing airflow direction is developed for a representative 40,000 lbf modern high bypass ratio engine. Thereafter, to represent the actual flow conditions that the VPF would face, an engine model that includes the nacelle, core inlet splitter, outlet guide vanes, bypass nozzle, core exhaust duct, aft-body plug, and core nozzle is designed. The engine model with the VPF is attached to a representative airframe in landing configuration to include the effects of installation. A rolling ground plane that mimics the runway during the landing run is also included to complete the model definition. Three-dimensional (3D) Reynolds-averaged Navier–Stokes (RANS) solutions are carried out for two different VPF stagger angle settings and rotational speeds to obtain the fan flow field. The dynamic installed VPF flow field is characterized by the interaction of the freestream and the reverse stream flows. The two streams meet in a shear layer in the fan passages and get deflected radially outward before turning back onto themselves. The flow field changes with stagger setting, fan rotational speed, and the aircraft landing speed because of the consequent changes in the momentum of the two streams. The description of the installed VPF flow field as generated in this study is necessary to (a) qualify VPF designs that are typically designed by considering only the uninstalled static flow field and (b) choose the VPF operating setting for different stages of the aircraft landing run.

Inception mechanism and suppression of rotating stall in an axial-flow fan

Inception patterns of rotating stall at two stagger-angle settings for the highly loaded rotor blades were experimentally investigated in a low-speed axial-flow fan. Rotor-tip flow fields were also numerically investigated to clarify the mechanism behind the rotating stall inception. The stall inception patterns depended on the rotor stagger-angle settings. The stall inception from a rotating instability was confirmed at the design stagger-angle settings. The stall inception from a short length-scale stall cell (spike) was also confirmed at the small stagger-angle setting. The spillage of tip-leakage flow and the tip-leakage vortex breakdown influence the rotating stall inception. An air-separator has been developed based on the clarified inception mechanism of rotating stall. The rotating stall was suppressed by the developed air-separator, and the operating range of fan was extended towards low flow rate. The effect of developed air-separator was also confirmed by application to a primary air fan used in a coal fired power plant. It is concluded from these results that the developed air-separator can provide a wide operating range for an axial-flow fan.

Stall Inception, Evolution and Control in a Low Speed Axial Fan With Variable Pitch in Motion

Obtaining the right pitch in turbomachinery blading is crucial to efficient and successful operations. Engineers adjust the rotor’s pitch angle to control the production or absorption of power. Even for low speed fans this is a promising tool. This paper focuses on the inception and the evolution of the flow instabilities in the tip region which drive the stall onset in low speed axial fans. The authors conducted an experimental study to investigate the inception patterns of rotating stall evolution at different rotor blade stagger-angle settings with the aim of speculating on stable operating margin. The authors drove the fan to stall at the design stagger-angle setting and then operated the variable pitch mechanism in order to recover the unstable operation. They measured pressure fluctuations in the tip region of the low-speed axial-flow fan fitted with a variable pitch in motion mechanism, with flush mounted probes. The authors studied the flow mechanisms for spike and modal stall inceptions in this low-speed axial-flow fan which showed relatively small tip clearance. The authors cross-correlated the pressure fluctuations and analyzed the cross-spectra in order to clarify blade pitch, end wall flow, and tip-leakage flow influences on stall inception during the transient at the rotor blades’ different stagger-angle settings. The authors observed a rotating instability near the maximum pressure-rise point at both design and low stagger-angle settings. The stall inception patterns were a spike type at the design stagger-angle setting as a result of the interaction between the incoming flow, tip-leakage flow and end wall backflow.

Stall inception process and prospects for active hub-flap control in three-stage axial flow compressor

The possibility to apply the active hub-flap control method, which is a proven rotating stall control method for a single-stage compressor, to a 3-stage axial compressor is experimentally discussed, where complex rotating stall inception processes are observed. The research compressor is a 3-stage one and could change the stagger angle settings for rotor blades and stator vanes. Sixteen rotor blade/stator vane configuration patterns were tested by changing stagger angle for the stator vanes. By measurement of surface-pressure fluctuation, stall inception processes are investigated and the measured pressure fluctuation data is used as a predictive signal for rotating stall. The experimental results show that the stall detection system applied to active hub-flap control in a single-stage compressor could be usefully applied to that in a 3-stage compressor with a more complex stall inception process.

Effect of Rotor Tip Leakage Flow on Spike-Type Stall Inception

We measured pressure fluctuations and distributions on the casing wall in a low speed axial-flow fan with relatively large tip clearance to clarify the criteria for a spike-type stall inception. The blade loading at the rotor tip and the tip leakage flow were compared between the two stagger angle settings for the rotor blade. Spikes appeared at the design stagger angle setting, and did not appear at the high stagger-angle setting, which was ten degrees larger than the design value. A rotating instability, which was induced by an interaction among the incoming flow, tip leakage flow, and end-wall backflow, was confirmed near the maximum pressure-rise point at both stagger angle settings. At the design stagger angle, the interface of this interaction became parallel to the leading edge plane. This interaction moreover generated a large end-wall blockage near the leading edge. This blockage developed a leading edge separation on the stalled rotor at the tip, and the leading edge separation then grew into a spike. In contrast, at the high stagger angle, the interface of this interaction did not become parallel to the leading edge plane, and the end-wall blockage was smaller than that at the design stagger angle. The leading edge separation therefore neither developed nor grew into a spike although the rotor blade at the tip stalled. We concluded from these results that the large end-wall blockage induced by the interaction among the incoming flow, tip leakage flow, and end-wall backflow was the main criterion for the spike type stall inception.

Spike and Modal Stall-inception Patterns in a Variable-pitch Axial-flow Fan

Inception patterns of rotating stall in a low-speed axial-flow fan have been investigated by measuring pressure and velocity fluctuations. These experimental measurements were carried out at two different stagger-angle settings for the rotor blades. They show that at the design stagger angle, a short length-scale stall cell known as a “spike” appears first. This stall cell then grows into a long length-scale stall cell as flow rate decreases. At the small stagger angle, a long length-scale disturbance known as a “modal” is first observed, and the modal and the spikes then induce the long length-scale stall cell. The short length-scale stall cell and the modal are first observed at the rotor tip, because the end-wall blockage at the tip becomes large. At the small stagger angle, this blockage is smaller than that at the design angle. The modal is therefore generated before the short length-scale cells appear. It is concluded from these results that the rotating stall-inception patterns depend on the rotor stagger angle.

1314 動翼可変軸流ファンにおけるスパイク型とモーダル型旋回失速発生過程

Inception patterns of rotating stall in a low-speed axial-flow fan have been investigated by measurements of pressure and velocity fluctuations. Experiments have been carried out at two different stagger angle settings for rotor blades. At the design stagger angle, a short length-scale stall cell known as a "spike" and multiple short length-scale stall cells appear first. These stall cells then grow into a long length-scale stall cell as flow rate decrease. The spike and the multiple short length-scale stall cells induce a part-span stall, and the long length-scale stall cell induces a full-span stall. At the small stagger angle, a long length-scale disturbance known as a "modal" is observed first, and the modal and spikes then induces the long length-scale stall cell. It is concluded from these results that the rotating stall inception patterns depend on the rotor stagger angle.