Power plant : Jet Propulsion

CLASSIFICATION

 The two main categories of jet propulsion systems are the atmospheric jet engine and rocket.

Fig:1: Jet propulsion engine

TURBOJET ENGINE

The turbojet engine (fig.2) is similar to the simple open cycle constant pressure gas turbine plant except that the exhaust gases are first partially expanded in the turbine to produce just sufficient power to drive the compressor. The exhaust gases leaving the turbine are then expanded to atmospheric pressure in a propelling (discharge) nozzle. The remaining energy of gases after leaving the turbine is used as a high-speed jet from which the thrust is obtained for forwarding movement of the aircraft.

Working Cycle

Air from the surrounding atmosphere is drawn in through the diffuser, in which air is compressed partially by the ram effect. Then air enters the rotary compressor and a major part of the pressure rise is accomplished here. The air is compressed to a pressure of about 4 atmospheres. From the compressor, the air passes into the annular combustion chamber. The fuel is forced by the oil pump through the fuel nozzle into the combustion chamber. Here the fuel is burnt at constant pressure. This raises the temperature and volume of the mixture of air and products of combustion. The mass of air supplied is about 60 times the mass of the fuel burnt. This excess air produces sufficient mass for the propulsion jet, and at the same time prevents the gas temperature from reaching values that are too high for the metal of the rotor blades.

 The hot gases from the combustion chamber then pass through the turbine nozzle ring. The hot gases which partially expand in the turbine are then exhausted through the discharge (propelling nozzle) by which the remaining enthalpy is converted into kinetic energy. Thus, a high-velocity propulsion jet is produced.

A typical turbojet engine cycle on T - ϕ diagram.

THRUST POWER AND PROPULSIVE EFFICIENCY

The jet aircraft draws in air and expels it to the rear at a markedly increased velocity. The action of accelerating the mass of fluid in a given direction creates a reaction in the opposite direction in the form of a propulsive force. The magnitude of this propulsive force is defined as thrust. It is dependent upon the rate of change of momentum of the working medium i.e. air, as it passes through the engine.

 The basis for the comparison of jet engines is the thrust. The thrust, T of a turbojet engine can be expressed as,

 T = m (V_j – V₀)             

where, m = mass flow rate of gases, kg/sec.

           V_j = exit jet velocity, m/sec.

           V₀ = vehicle velocity, m/sec.

The above equation is based upon the assumption that the mass of fuel is neglected. Since the atmospheric air is assumed to be at rest, the velocity of the air entering relative to the engine, is the velocity of the vehicle, V_o. The thrust can be increased by increasing the mass flow rate of gas or increasing the velocity of the exhaust jet for given V₀.

Thrust power is the time rate of development of the useful work achieved by the engine and it is obtained by the product of the thrust and the flight velocity of the vehicle. Thus, thrust power TP is given by

TP = T V₀ = m (V_j – V₀) V₀ Nm/sec

Propulsive power PP is given by

propulsive efficiency ⴄ_p is given by

RAMJET

It is steady combustion or continuous flow engine. It has the simplest construction of any propulsion engine (fig.4) consisting essentially of an inlet diffuser, a combustion chamber, and an exit nozzle or tailpipe.

 Since the ramjet has no compressor it is dependent entirely upon ram compression. Ram compression is the transformation of the kinetic energy of the entering air into pressure energy.

Fig:4: Supersonic Ramjet engine.

The cycle for an ideal ramjet, which has an isentropic entrance diffuser and exit nozzle, is the Joule cycle as shown by the dotted lines in fig. 5-6. The difference between the actual and ideal jet is due principally to losses actually encountered in the flow system. The sources of these losses are

• Wall friction and flow separation in the subsonic diffuser and shock in the supersonic diffuser.
• Obstruction of the air stream by the burners introduces eddy currents and turbulence in the air stream.
• Turbulence and eddy currents introduced in the flow during burning.

By far, the most critical component of the gram jet is the diffuser. Due to the peculiarities of streamline flow, a diffuser that is extremely efficient at a given speed may be quite inadequate at another velocity.

Fig5: T - ϕ Diagram of Ramjet engine.

PULSE JET ENGINE

• The pulse jet engine is somewhat similar to a ramjet engine. The difference is that a mechanical valve arrangement is used to prevent the hot gases of combustion from flowing out through the diffuser in the pulse jet engine.

 The turbojet and ramjet engines are continuous in operation and are based on the constant pressure heat addition (Bryton) cycle. The pulse jet is an intermittent combustion engine, and it operates on a cycle similar to a reciprocating engine and maybe better compared with an ideal Otto cycle rather than the Joule or Bryton cycle.

 The compression of the incoming air is accomplished in a diffuser. The air passes through the spring valves and is mixed with fuel from a fuel spray located behind the valves. A spark plug is used to initiate combustion but once the engine is operating normally, the spark is turned off and residual flame in the combustion chamber is used for ignition. The engine walls also may get hot enough to initiate combustion.

The mechanical valves which were forced open by the entering air, are forced shut when the combustion process raises the pressure within the engine above the pressure in the diffuser. As the combustion products cannot expand forward, they move to the rear at high velocity. The combustion products cannot expand forward, they move to the rear at high velocity. When the combustion product, leave, the pressure in the combustion chamber drops and the high-pressure air in the diffuser Forces the valves open, and fresh air enters the engine.

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