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6: IDEAL RAMJET CYCLE

Station 9:Fan exit nozzle:

Thrust per unit hot mass flow rate:

Thrust-specific fuel consumption:

If the required thrust is given, the limiting mass flow rate can be found, and the engine sized accordingly.


Computing the Limiting Mass Flow Rate

Usually the mass flow rate is limited by the "choked turbine" condition: i.e., the Mach number reaches 1.0 in the stator passage at the end of the final stage of the turbine. This is because the air at this station has a relatively low stagnation pressure, low static temperature, and high flow velocity, and the area available to the flow is the area between the blades in the turbine stage annulus. Thus, the stator passage is choked. The choked area is easily found from the formula for the mass flow rate through a choked throat:

 

If the thrust at some flight condition is known (for example, static or takeoff thrust at sea-level), the thrust per unit mass flow rate is first computed, from which the mass flow is determined. From the choked turbine condition above, the choked area A is found. This area remains constant at all other flight conditions, and thus the limiting mass flow rate at any other flight condition can be determined, once the stagnation pressure and temperature at the turbine exit (Station 5) is found.

The limiting mass flow rate is directly proportional to the stagnation pressure, and inversely proportional to the square root of the stagnation temperature. Thus, if more work is taken out of the flow by the turbine per unit mass, the stagnation pressure drops sharply, and the mass flow and thus the thrust drop sharply as well.

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