436
12 Thrust Gas Turbines
Examples:
• T 0 = 225 K (10 km height), M 0 = 0.85 → v 0 ≈ 250 m/s (900 km/h), for v j = 1125 m/s
→ h p = 0.364.
• T 0 = 225 K, M 0 = 2.5 → v 0 ≈ 750 m/s (2700 km/h), for v j = 1125 m/s → h p = 0.800.
Achieving good propulsive efficiency with a single-jet engine requires a flight
Mach number of about 2.5 or above. The purpose of the second jet is thus obvious.
The gas power supplied to the hot nozzle is decreased by converting part of it into
mechanical power and by generating a second jet through a compressor, a fan or a
propeller so that dynamic power becomes available at a lower jet velocity and with
a larger mass flow rate. The higher the bypass ratio, the better the engine is adapted
to low flight speed. Of course, the higher the bypass ratio, the lower the final thermal efficiency is. But clearly, the product of thermal efficiency and propulsive efficiency (12.8) must be optimised.
Precise performance analysis requires splitting of a gas turbine based propulsion system into two parts. We discern a gas generator and a jet generator. First,
we discern a hot flow and a cold flow. The hot flow is the flow that passes through
the combustion chamber. The gas generator is the part of the engine that processes
the hot flow until a section in the turbine such that the part upstream of it drives
the hot-flow compressor parts. The compressor and the turbine parts of the gas
generator are not always easily discernible. The high-pressure compressor and the
high-pressure turbine are part of it in a double-shaft engine. In a triple-shaft engine,
it encompasses the high-pressure and intermediate-pressure compressor and turbine
parts. In a bypass engine and a turbofan, the low-pressure turbine drives the lowpressure compressor or the fan. These contribute both to the cold and the hot flows.
In practice, the term core engine denotes the engine part exclusively processing the
hot flow (normally the HP and IP parts). So, the term core engine is more restrictive
than the term gas generator. For the gas generator, we have to think of the fan in a
turbofan and the LP compressor in a bypass engine as split into a hot flow (interior
part) and a cold flow (exterior part) part. The hot flow part belongs to the gas generator and the cold flow part belongs to the jet generator. Further, we have to think
of the LP turbine as split into a front part belonging to the gas generator and a rear
part belonging to the jet generator.
The gas generator is characterised by the thermodynamic efficiency. This is the
ratio of gas power to thermal power. The jet generator is characterised by the dynamic efficiency. This is the ratio of the dynamic power (sum of the two jets) to
the gas power. The thermal efficiency of a propulsive system is the product of the
thermodynamic efficiency and the dynamic efficiency ( t
td d
h h h
=
). The overall efficiency is the product of three efficiencies:
(12.9)
Optimisation of the propulsive system may be defined as follows: the gas generator
is designed for maximum thermodynamic efficiency and the jet generator is designed for maximum product of dynamic efficiency and propulsive efficiency. The
.
td d p
h h h h
=
12 Thrust Gas Turbines
Examples:
• T 0 = 225 K (10 km height), M 0 = 0.85 → v 0 ≈ 250 m/s (900 km/h), for v j = 1125 m/s
→ h p = 0.364.
• T 0 = 225 K, M 0 = 2.5 → v 0 ≈ 750 m/s (2700 km/h), for v j = 1125 m/s → h p = 0.800.
Achieving good propulsive efficiency with a single-jet engine requires a flight
Mach number of about 2.5 or above. The purpose of the second jet is thus obvious.
The gas power supplied to the hot nozzle is decreased by converting part of it into
mechanical power and by generating a second jet through a compressor, a fan or a
propeller so that dynamic power becomes available at a lower jet velocity and with
a larger mass flow rate. The higher the bypass ratio, the better the engine is adapted
to low flight speed. Of course, the higher the bypass ratio, the lower the final thermal efficiency is. But clearly, the product of thermal efficiency and propulsive efficiency (12.8) must be optimised.
Precise performance analysis requires splitting of a gas turbine based propulsion system into two parts. We discern a gas generator and a jet generator. First,
we discern a hot flow and a cold flow. The hot flow is the flow that passes through
the combustion chamber. The gas generator is the part of the engine that processes
the hot flow until a section in the turbine such that the part upstream of it drives
the hot-flow compressor parts. The compressor and the turbine parts of the gas
generator are not always easily discernible. The high-pressure compressor and the
high-pressure turbine are part of it in a double-shaft engine. In a triple-shaft engine,
it encompasses the high-pressure and intermediate-pressure compressor and turbine
parts. In a bypass engine and a turbofan, the low-pressure turbine drives the lowpressure compressor or the fan. These contribute both to the cold and the hot flows.
In practice, the term core engine denotes the engine part exclusively processing the
hot flow (normally the HP and IP parts). So, the term core engine is more restrictive
than the term gas generator. For the gas generator, we have to think of the fan in a
turbofan and the LP compressor in a bypass engine as split into a hot flow (interior
part) and a cold flow (exterior part) part. The hot flow part belongs to the gas generator and the cold flow part belongs to the jet generator. Further, we have to think
of the LP turbine as split into a front part belonging to the gas generator and a rear
part belonging to the jet generator.
The gas generator is characterised by the thermodynamic efficiency. This is the
ratio of gas power to thermal power. The jet generator is characterised by the dynamic efficiency. This is the ratio of the dynamic power (sum of the two jets) to
the gas power. The thermal efficiency of a propulsive system is the product of the
thermodynamic efficiency and the dynamic efficiency ( t
td d
h h h
=
). The overall efficiency is the product of three efficiencies:
(12.9)
Optimisation of the propulsive system may be defined as follows: the gas generator
is designed for maximum thermodynamic efficiency and the jet generator is designed for maximum product of dynamic efficiency and propulsive efficiency. The
.
td d p
h h h h
=