Reaction Engines and
Thermodynamic Processes

Reaction Engines:

A family of engines uses the principle of Newton's third law to create a force called thrust to move vehicles. These engines are called reaction engines because the force these engines generate is a reaction to the expelling of hot gasses in the opposite direction. This family of engines includes engines such as; 
1. Ram jet 

2. Pulse jet 

3. Gas Turbine Jet engine 

 

4. Rocket engine

Before understanding the Gas turbine Engine it is important to understand or recall the Thermodynamic processes. So here we will first briefly study the thermodynamic processes.

Thermodynamic Processes:

When a system undergoes change from one state to final state due change in properties like temperature, pressure, volume etc, the system is said to have undergone thermodynamic process. Various types of thermodynamic processes are: isothermal process, adiabatic process, isochoric process, isobaric process and reversible process. These have been described below:

1. Constant Volume Heating (Isochoric)

A gas, heated in a fixed enclosed space, will remain at constant volume and the process is said to be isochoric. The heating will be accompanied by a rise in pressure and temperature. As there is no change in volume there will be no external work done by the gas; the whole of the heat supplied will be stored in the gas in the form of internal energy.

2. Constant Pressure Heating (Isobaric)

Heating of a gas at constant pressure is termed as isobaric process. A gas may be heated whilst enclosed in a cylinder containing a sliding piston, on which a constant force is acting; this will maintain a constant pressure in the cylinder, the heating of a gas under constant pressure causes an increase in volume and temperature. External work is done owing to the increase in volume.

3. Constant Temperature Heating (Isothermal Expansion)

If heat is supplied to a gas whilst maintaining temperature constant the gas will expand, doing external work equal to the amount of heat supplied. An expansion at constant temperature is known as an isothermal expansion. When a gas is compressed or expanded in accordance with the Boyle's Law, the change is said to take place isothermally. Such a process can only be approached in practice if the change takes place slowly so that all the heat of compression or expansion was absorbed or supplied by an external source. In practice changes takes place quickly and there is always a change in temperature so that the isothermal change has no practical significance in connection with our consideration of the Gas Turbine Engine.

4. Adiabatic Expansion

When a gas expands, doing external work, in such a manner that no heat is supplied or rejected during the expansion, the expansion is called adiabatic. The adiabatic expansion may take place in an engine cylinder, in which case no heat passes through the cylinder walls and work is done on the piston as the gas expands.

The following three conditions must be satisfied in this type of expansion:

(a) No heat is supplied or rejected during the expansion.

(b) Work of some nature, must be done by the gas in expanding.

(c) The expansion is assumed to be frictionless.

As the gas is not receiving heat, the work done must be performed at the expense of its own supply of internal energy. Hence, it follows, that the temperature will fall during an adiabatic expansion, and will rise during an adiabatic compression.

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