Transonic Compressors - Turbomachinery Aerodynamics - Lecture Slides, Slides of Turbomachinery

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Transonic Compressors

Normally a transonic compressor has either V 1 or C 2 supersonic

V 1 or C (^2) may be supersonic at tip or at many sections of the blade

1. Supersonic flow enters the rotor.
2. Shock is contained within the rotor blade and leaves the rotor subsonically
3. Flow in the stator is subsonic

1. High stage performance
2. Flow enter both rotor and stator supersonically
3. And exit subsonically
4. Both rotor and stator blades are highly loaded

• To utilize supersonic entry flow in a controlled manner, new airfoils needed to be developed.
• Airfoils with sharp leading edges were ruled out due to requirements at off-design operations
• Controlled supersonic diffusion followed by subsonic diffusion, enable transonic compressors to achieve higher compression ratios

CDA Blades :

• Controlled Diffusion Airfoil (CDA) was conceptually derived from supercritical airfoils, first used in aircraft wings in the 60’s. The CDA were created in the 80’s using the established CFD techniques.
• Velocity or C (^) p distribution on the blade was predetermined to arrive at a 2-D cascade for smooth transition from subsonic-to- supersonic-to- subsonic flow for the minimum loss and maximum diffusion and optimized camber
• CDA blades are also referred to as wide chord blades. Longer chord allows the diffusion control.

Flow through a transonic blading would diffuse through the shocks before further diffusing and exiting as subsonic flow

DCA blades

Shocks in MCA blades

Flow through S-type blades has minimal camber but high supersonic diffusion

DCA Blades :

• At low supersonic Mach number (<1.4) the flow supersonically accelerates through a series of expansion fans after the front oblique shock and transits to subsonic through the passage shock.
• According to the model used, the shock diffusion and the supersonic expansion are approximately equal to each other and the flow regains its original entry Mach number in front of the normal shock.
• Flow parameters to be estimated across the passage shock using the normal shock theories

MCA Blades :

• MCA blades are used for compressors /fans with low solidity and higher Mach number (>1.4).
• This shape was created for greater control of the blade profile by using multiple arcs.
• These blade shapes create a bow shock.
• These MCA blades, used near the tips, are set at high stagger, due to which the inflow experiences a mildly converging (virtual) passage. The suction surface of the blade is convexly curved resulting in a series of mild shock fans.
• The entry flow through the shock fans is, thus, supersonically diffused till the passage shock , through which it finally becomes subsonic.

• The rotor loses are measured in relative frame of reference and thus relative total pressure ratio gives a measure of the losses in the rotor.
• Rotor or stage maps (characteristics) of transonic compressors are much sharper and are more sensitive to inflow characteristics

When a designer tries to maximize the absolute total pressure ratio by increasing the total temperature ratio, the relative total pressure ratio falls, depending on the design point efficiency chosen based on state of art of design capability available at hand.