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6: IDEAL RAMJET CYCLE

7: GAS TURBINE ENGINE ANALYSIS

At subsonic Mach numbers, and in fact upto about Mach 2.5, the pressure ratio achievable from ram compression (through deceleration of air to near-stagnation conditions) is too low for efficient cycle operation. Hence mechanical compressors are needed. To run these compressors, they are connected to turbines which extract work out of the expanding hot gas flow. Thus, we note that the turbine must be able to extract enough work from the hot gases to run the compressor.

To increase propulsive efficiency, bypass engines such as turbofans, turboprops, and propfans are used, where a large mass flow rate of air bypasses the combustor, and just goes through a propeller or fan, where the pressure of the air is increased. The air is then accelerated through a nozzle (real or virtual) until the static pressure reaches the ambient. In this case, the turbine work must be at least equal to the sum of the compressor work and the fan work.

Note that the turbojet is thus a limiting case where the fan work is zero, and the bypass ratio is zero, and the ramjet is a limiting case where the compressor pressure ratio is unity.

 Region Process Ideal Actual a to 1 External acceleration or deceleration. To, Po constant. 1 to 2 Diffuser: Adiabatic compression, with no work done except volume change Isentropic: p, T increase; To, po constant. s constant. po drops due to shocks and friction. s increases. 2 to 3 Compressor: Adiabatic work addition Isentropic. Work added until stagnation pressure is increased to a specified level. Po, To rise. These are related by isentropic relation Work required is greater than that for isentropic compression to specified pressure. Thus To rise is greater than that computed from isentropic. 2 to 8 Fan same principle as compressor same principle as compressor 3 to 4 Burner: Heat addition in the core flow Constant pressure. To, T increase; po constant. po drops. 4 to 5 Turbine:adiabatic work extraction isentropic work extraction. To, po drop, related isentropically po drops more than predicted by isentropic relation for same work extraction. 5 to 6 Afterburner constant pressure heat addition po drops due to: a) Rayleigh line losses of heat addition to flowing fluid b) friction and wake losses of duct and flameholders. 6 to 7 Hot nozzle: Adiabatic expansion. No work extracted except that of volume change. To, po constant. T, p drop. po drops slightly. s increases slightly. 8 to 9 Fan exit: cold nozzle To, po constant. T, p drop. po drops slightly. s increases slightly.
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