Engine Performance Significantly - Turbomachinery Aerodynamics - Lecture Slides, Slides of Turbomachinery

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In this lecture...

• Inlet distortion and its effect on compressor stability
• Control of instabilities

Inlet distortion

• Intakes of civil and military combat aircraft have very different geometries.
• Combat aircraft intakes can have very complex geometries leading to inherent problem of flow non-uniformity.
• Inflow non-uniformity or distortion is detrimental to engine operation.
• Several aircraft in the past, that were operating with engines not designed for distortion have had serious operational issues including several engine failures.
• Some of these are F100 (1954), F101 (1954), Hunter (1955), Britannia (1956), F111 (1966) etc.

Inlet distortion

Transport aircraft intakes

Military aircraft intakes

Inlet distortion

• Inflow distortion can lead to earlier initiation of instabilities: rotating stall and surge.
• Flow distortion causes local change in incidence angles.
• If these angles exceed the critical angles, one or more adjacent blades falling in this zone of distortion, stalls.
• The stall cell(s) propagate if these are able to withstand the system dynamics. Else, they dissipate and the system does not undergo any instabilities.

Inlet distortion

Parallel compressor theory

Inlet distortion

Distortion coefficient definition

Distortion coefficient

• Based on the sector angle chosen, there are different ways of defining distortion coefficient.
• The sector angle that is most commonly used is 60o^ and therefore the distortion coefficient is DC 60.
• Other angles like 45 o^ and 90o^ are also sometimes used.

Effect of inlet distortion

Effect of circumferential distortion on surge line (Hercock and William,1974)

Effect of inlet distortion

• Static/steady inlet distortion is more common form of distortion.
• Considers spatial non-uniformity of time- averaged total pressure.
• Usually circumferential non-uniformity is considered more severe as it significantly affects the incidence angle.
• Radial distortion is likely to occur due to thickening of the boundary layer. A certain amount of radial distortion will be present due to the presence of boundary layer.

Effect of inlet distortion

Engine surge caused by dynamic distortion (Hercock and William, 1974)

Swirl

• Many of the military aircraft have engines that are offset from the intake centerline.
• Such intakes referred to as S-type or Y- type intake, inherently suffer from strong secondary flows.
• In the absence of guide vanes, the flow entering the compressor is likely to have some amount of swirl.
• This swirl may get amplified under certain operating conditions, leading to severe inflow distortion.

Swirl

Structure of secondary flows in S-duct diffusers

Control of instabilities

• Compressor instabilities limit the operating range of an engine.
• Operating the compressor too much away from the surge line compromises the efficiency.
• Ability to operate the compressor close to high efficiency points (and possibly closer to the surge line) is of immense interest.
• This would require methods of preventing or controlling the occurrence of instabilities.
• The other way of preventing this altogether is to control the inflow from the inlet by flow control methodologies.