Module 3 Jet Propulsion cycles and their analysis

Module -3 Jet Propulsion cycles and their analysis

Introduction • Gas turbine cycles for jet propulsion differ from shaft power cycles because if the fact that the useful power output for jet propulsion is produced, wholly or partially; as a result of expansion of gas in a propelling nozzle; wholly in turbojet engines and partially in turbo prop engines. • The second distinguishing feature is the need to consider the effect of forward speed and altitude on the performance of propulsion engines.

• The principle of jet propulsion is obtained from the application of Newton’s law's of motion. • Experience shows that only two types of fluids are particularly suitable for jet propulsion. • i) A heated and compressed atmospheric air. • ii) Another class of jet propulsion engines use a jet of gas produced by the chemical reactions of fuel and oxidizer. The fuel-oxidant mixture is called the propellant.

• From the above discussion it is clear that jetpropulsion engines may be classified broadly into two groups. • i) air-breathing engines • ii) rocket engines

Air breathing engines can be further classified as follows: • Reciprocating or propeller engines • Gas turbine engines

Reciprocating or propeller engines

Direction of Movement • Inward (Down) ------- • Outward (Up)----- • Inward (Down)------- • Outward (Up)---- Event (what happens) Intake Compression and Ignition Power Exhaust

Gas turbine engines can be classified into, i) Ramjet engines ii) Pulse jet engines iii) Turbojet engines iv) Turboprop engines v) Turbofan engines.

The Ramjet Engine

Advantages of Ramjet Engine: • Ramjet engine is very simple and does not have any moving parts. It is very cheap to produce and requires almost no maintenance. • Due to the fact that a turbine is not used to drive the mechanical compressor, the maximum temperature which can be allowed in ramjet is very high, about 2000°C as compared to about 900° C in turbojets. This allows a greater thrust to be obtained by burning fuel at air –fuel ratio of about 13: 1, which gives higher temperatures. • The specific fuel consumption is better than other gas turbine power plants at high speed and high altitudes.

Disadvantages of Ramjet engine: • Since the compression of air is obtained by virtue of its speed relative to the engine, the take-off thrust is zero and it is not possible to start a ramjet engine without an external launching device. • Very difficult to design a diffuser which will give good pressure recovery over a wide range of speeds. • Due to high air speeds, the combustion chamber requires flame holder to stabilize the combustion.

Pulse Jet Engine First part of the cycle: air flows through the intake (1), and is mixed with fuel (2). Second part: the valve (3) is closed and the ignited fuel-air mix (4) propels the craft.

Advantages of Pulse jet Engine: • Simple device next to ramjet engine and it is light in weight. • Very small and occasional maintenance. • It does not need a device for initial propulsion like ramjet engine. This engine will create more static thrust than the cruise thrust. • It can run on almost any types of liquid fuels. • This engine is relatively cheap. • It can also operate on gaseous fuel with little modifications.

Disadvantages of Pulse jet Engine: • SFC is as high as that of ramjet engine. • The biggest disadvantage is very short life of flapper valves and high rate of fuel consumption. • Operating range is limited in altitude range. • Lower propulsive efficiency than turbo jet engines. • Speed of the pulse jet is limited to a very narrow range of about 650 -800 km/h, because of the limitations in the aerodynamic design of a diffuser.

Turbojet § § Chemical energy is converted into mechanical energy 100% Thrust produced by Nozzle Operating Mach No: 1 to 2 Supersonic Aircraft (1 to 5)

CHARACTERISTICS OF A TURBOJET ENGINE • Low thrust at low forward speeds • Relatively high TSFC at low altitudes and speeds (fuel efficiency of an engine design with respect to thrust ) • Lightest specific weight (weight per unit volume) • Ability to take advantage of highest ram pressure • Lowest frontal area and almost no ground clearance problem

Advantages of turbojet engine Ø Ø Ø Ideal for long distance flight at higher speeds and altitudes Lower frontal area and shorter landing gear Lower weight per unit thrust at design speeds and altitude Pressure rise through inlet diffuser is significant Reheat can be employed for increased thrust Disadvantages Of Turbojet Engine Ø Take-off roll is longer requiring longer runway Ø TSFC is comparatively higher at low speeds and altitudes Ø Uneconomical on short distance flights Ø Lower thrust and propulsive efficiency at lower speeds.

Turboprop § § 20 to 25% of Thrust produced by Nozzle 75 to 80% of Thrust produced by Propeller Operating Mach No: 0. 4 to 0. 65 Subsonic Aircraft (0. 1 to 0. 8)

CHARACTERISTICS OF A TURBOPROP ENGINE • Very high propulsive efficiency at low airspeeds but with altitude it falls off rapidly • Most complicated design and heavier than turbojet • Lowest TSFC • Large frontal area, Longer landing gear for low wing airplanes • Highly efficient thrust reversals Thrust reverser systems are featured on many jet aircraft to help slow down just after touch-down, reducing wear on the brakes and enabling shorter landing distances

Advantages of turboprop engine Ø Higher thrust at low speeds Ø Take off roll is short Ø Propulsive efficiency within operational range is high Ø Specific fuel consumption is low Ø Thrust reversal is easily achieved Disadvantages of Turboprop Engine Ø Ø Application is limited to lower speeds and altitude Landing gears have to be longer Engine is heavier and complicated Usually centrifugal compressors are used which increases the frontal area Ø Higher weight per unit thrust

Efficiencies • Thermal Efficiency for Turbo jet engine = Propulsive power/ (Fuel flow rate* Calorific value of fuel) • Thermal Efficiency for Propeller engine = Brake power/ (Fuel flow rate* Calorific value of fuel) • Propeller Efficiency = Thrust power/ Shaft Power • Propulsive efficiency = Thrust power/Propulsive Power

• Transmission efficiency = Output of Transmission/Input of Transmission • Over all Efficiency of a Propulsive system =Useful Propulsive work/Chemical Energy supplied • i. e = Thermal efficiency*Transmission efficiency*Propulsive efficiency