Sweep Effects on Fan-Intake Aerodynamics at High Angle of Attack

2021 ◽  
Author(s):  
Ben Mohankumar ◽  
Cesare A. Hall ◽  
Mark J. Wilson
Keyword(s):  
Mach Number ◽  
Pressure Ratio ◽  
Angle Of Attack ◽  
Pressure Rise ◽  
Low Pressure ◽  
Rotating Stall ◽  
High Angle ◽  
High Angle Of Attack ◽  
Mach Numbers ◽  
High Mach

Abstract Sweep in a transonic fan is conventionally used to reduce design point losses by inclining the passage shock relative to the incoming flow. However, future low pressure ratio fans operate to lower Mach numbers meaning the role of sweep at cruise is diminished. Instead, sweep might be repurposed to improve the performance of critical high Mach number off-design conditions such as high angle of attack (AOA). In this paper, we use unsteady computational fluid dynamics to compare two transonic low pressure ratio fans, one radially stacked and one highly swept, coupled to a short intake design, at the high AOA flight condition. The AOA considered is 35°, which is sufficient to separate the intake bottom lip. The midspan of the swept fan was shifted upstream to add positive sweep to the outer span. Based on previous design experience, it was hypothesised the swept fan would reduce transonic losses when operating at high AOA. However, it was found the swept fan increased the rotor loss by 24% relative to the radial fan. Loss was increased through two key mechanisms. i) Rotor choking: flow is redistributed around the intake separation and enters the rotor midspan with high Mach numbers. Sweeping the fan upstream reduced the effective intake length, which increased the inlet relative Mach number and amplified choking losses. ii): Rotor-separation interaction (RSI): the rotor tip experiences low mass flow inside the separation, which increases the pressure rise across the casing to a point where the boundary layer separates. The swept fan diffused the casing streamtube, causing the casing separation to increase in size and persist in the passage for longer. High RSI loss indicated the swept fan was operating closer to the rotating stall point.

Fan-Intake Coupling With Conventional and Short Intakes

2021 ◽  
Author(s):  
E. J. Gunn ◽  
T. Brandvik ◽  
M. J. Wilson
Keyword(s):  
Flow Field ◽  
Potential Flow ◽  
Pressure Ratio ◽  
Current Trend ◽  
Angle Of Attack ◽  
High Angle ◽  
High Angle Of Attack ◽  
Field Distortion ◽  
Intake Flow ◽  
The Impact

Abstract The current trend in civil engine fans towards lower pressure ratio and larger diameter is accompanied by a need to shorten the engine intake length to reduce weight and drag. This paper uses full-annulus, unsteady CFD simulations of two coupled fan-intake configurations to explain the impact of flow field coupling and intake length on fan and intake performance. On-design and off-design operating points are considered at cruise and high angle of attack, respectively. The fan efficiency at cruise is shown to be determined by a trade-off between two effects. Cruise efficiency is reduced by 0.11% with a short intake due to increased potential flow field distortion, which alters the incidence and diffusion of the rotor. This is partially offset by a reduction in casing boundary layer thickness due to lower intake wetted area. At high angle of attack conditions, a short intake leads to increased potential flow field distortion and an earlier onset of intake flow separation due to a higher adverse pressure gradient approaching the fan. Both effects combine to reduce the fan thrust at such conditions, although the fan is shown to remain stable at attack angles up to 35°. The reduction in performance is shown to be dominated by flow separations in the rotor, which increase in size and severity for a given attack angle as the intake length is decreased. The fan is also shown to have a stronger influence on the form of the intake flow field in a short intake, suggesting that it is necessary to model the fan in the intake design process for a successful design.

scholarly journals Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

2021 ◽  
Vol 11 (6) ◽  
pp. 2593
Author(s):  
Yasir Al-Okbi ◽  
Tze Pei Chong ◽  
Oksana Stalnov
Keyword(s):  
Boundary Layer ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Shear Instability ◽  
Vortical Flow ◽  
High Angle ◽  
High Angle Of Attack ◽  
Layer Separation ◽  
Aerodynamic Performances

Leading edge serration is now a well-established and effective passive control device for the reduction of turbulence–leading edge interaction noise, and for the suppression of boundary layer separation at high angle of attack. It is envisaged that leading edge blowing could produce the same mechanisms as those produced by a serrated leading edge to enhance the aeroacoustics and aerodynamic performances of aerofoil. Aeroacoustically, injection of mass airflow from the leading edge (against the incoming turbulent flow) can be an effective mechanism to decrease the turbulence intensity, and/or alter the stagnation point. According to classical theory on the aerofoil leading edge noise, there is a potential for the leading edge blowing to reduce the level of turbulence–leading edge interaction noise radiation. Aerodynamically, after the mixing between the injected air and the incoming flow, a shear instability is likely to be triggered owing to the different flow directions. The resulting vortical flow will then propagate along the main flow direction across the aerofoil surface. These vortical flows generated indirectly owing to the leading edge blowing could also be effective to mitigate boundary layer separation at high angle of attack. The objectives of this paper are to validate these hypotheses, and combine the serration and blowing together on the leading edge to harvest further improvement on the aeroacoustics and aerodynamic performances. Results presented in this paper strongly indicate that leading edge blowing, which is an active flow control method, can indeed mimic and even enhance the bio-inspired leading edge serration effectively.

Roles of vanes in diffuser on stability of centrifugal compressor

2019 ◽  
Vol 233 (14) ◽  
pp. 5380-5392
Author(s):  
Wangzhi Zou ◽  
Xiao He ◽  
Wenchao Zhang ◽  
Zitian Niu ◽  
Xinqian Zheng
Keyword(s):  
High Pressure ◽  
Centrifugal Compressor ◽  
Dynamic Pressure ◽  
Pressure Ratio ◽  
Pressure Rise ◽  
Rotating Stall ◽  
Centrifugal Compressors ◽  
Compression System ◽  
The Stability ◽  
Rotating Instability

The stability considerations of centrifugal compressors become increasingly severe with the high pressure ratios, especially in aero-engines. Diffuser is the major subcomponent of centrifugal compressor, and its performance greatly influences the stability of compressor. This paper experimentally investigates the roles of vanes in diffuser on component instability and compression system instability. High pressure ratio centrifugal compressors with and without vanes in diffuser are tested and analyzed. Rig tests are carried out to obtain the compressor performance map. Dynamic pressure measurements and relevant Fourier analysis are performed to identify complex instability phenomena in the time domain and frequency domain, including rotating instability, stall, and surge. For component instability, vanes in diffuser are capable of suppressing the emergence of rotating stall in the diffuser at full speeds, but barely affect the characteristics of rotating instability in the impeller at low and middle speeds. For compression system instability, it is shown that the use of vanes in diffuser can effectively postpone the occurrence of compression system surge at full speeds. According to the experimental results and the one-dimensional flow theory, vanes in diffuser turn the diffuser pressure rise slope more negative and thus improve the stability of compressor stage, which means lower surge mass flow rate.

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