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6: IDEAL RAMJET CYCLE

T-s Diagram

Example Problem

A turbofan engine operates at intermediate thrust (afterburner off) with the following characteristics:

Flight condition:

Ambient temperature = 298.15K

Ambient pressure = 101300N/m2.

Bypass ratio b = 0.6

Compressor pressure ratio pc = 25

Fan pressure ratio pf = 3.375

Fuel heating value qr = 19000Btu/lbm

Turbine inlet temperature = 2000K

Diffuser pressure recovery factor rd = 0.9

Compressor efficiency hc = 0.89

Fan efficiency hf = 0.91

Combustion efficiency hb = 0.98

Burner pressure recovery factor rb= 0.95

Turbine efficiency ht = 0.98

Nozzle efficiency hn = 1.0

Calculate the thrust per unit mass flow rate, and the thrust specific fuel consumption. Later, for a specified value of thrust, determine the area at the choked turbine exit, which limits the mass flow rate.

Such a problem can be solved by going station by station through the engine and using simple thermodynamic relations. Since the component efficiencies are specified, we don't have to go into detailed multi-dimensional flow and chemical analyses for each component.

ANALYSIS

Station 1: inlet

p01 = p0a: isentropic external acceleration or deceleration

T01 = T0a: no work done, except that of expansion or contraction.>

Station 2: end of diffuser, beginning of compressor & fan:

T02 = T01: adiabatic, no work

p02= rdp01: includes the given stagnation pressure loss

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