PRESENTATION ON TURBOJET

Prepared by:

Gokarna basnet

B.tech( ME 4TH sem)

Ansal university.

What is turbojet?

The turbojet is an airbreathing jet engine, usually used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor, a combustion chamber, and a turbine (that drives the compressor).

The compressed air from the compressor is heated by the fuel in the combustion chamber and then allowed to expand through the turbine.

The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust

INTRODUCTION

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DESIGN OF A TURBOJET

Air intake - An intake, or tube, is needed in front of the compressor to help direct the incoming air smoothly into the moving compressor blades. The intake is shaped to minimise any flow losses when the compressor is accelerating the air through the intake at zero and low aircraft speeds, and to slow the flow down for the compressor when the aircraft is operating above Mach 1. The air flowing into a turbojet engine must always be subsonic, regardless of the speed of the aircraft itself.

Compressor: The compressor is driven by the turbine. It rotates at high speed, adding energy to

the airflow and at the same time squeezing (compressing) it into a smaller space. Compressing the air increases its pressure and temperature. The smaller the compressor the faster it turns.

Turbine: Hot gases leaving the combustor expand through the turbine. The turbine vanes and

blades have internal cooling passages. Air from the compressor is passed through these to keep the metal temperature within limits.

In the first stage the turbine is largely an impulse turbine and rotates because of the impact of the hot gas stream. Later stages are convergent ducts that accelerate the gas. Energy is transferred into the shaft through momentum exchange in the opposite way to energy transfer in the compressor. The power developed by the turbine drives the compressor as well as accessories, like fuel, oil, and hydraulic pumps that are driven by the accessory gearbox.

OPERATION OF TURBOJET

WORKING OF TURBOJET Large amounts of surrounding air are continuously brought into the

engine inlet. At the rear of the inlet, the air enters the compressor. The compressor acts like many rows of airfoils, with each row producing a small

jump in pressure. At the exit of the compressor, the air is at a much higher pressure than free stream. In the burner a small amount of fuel is combined with the air and ignited. Leaving the burner, the hot exhaust is passed through the turbine. The turbine

works like a windmill and extracts energy from a flow of gas by making the blades spin in the flow.

The turbine takes some energy out of the hot exhaust, but there is enough energy left over to provide thrust to the jet engine by increasing the velocity through the nozzle.

Because the exit velocity is greater than the free stream velocity, thrust is created . For a jet engine, the exit mass flow is nearly equal to the free stream mass flow, since very little fuel is added to the stream

APPLICATION The jet engine is only efficient at high vehicle

speeds, which limits their usefulness apart from aircraft. Turbojet engines have been used in isolated cases to power vehicles other than aircraft, typically for attempts on land speed records. Where vehicles are 'turbine powered' this is more commonly by use of a turboshaft engine, a development of the gas turbine engine where an additional turbine is used to drive a rotating output shaft. These are common in helicopters and hovercraft.

Cycle improvements The operation of a typical turbojet is modelled approximately

by the Brayton Cycle. The efficiency of a gas turbine is increased by raising the

overall pressure ratio, requiring higher temperature compressor materials, and raising the turbine entry temperature, requiring better turbine materials and/or improved vane/blade cooling. However, when used in a turbojet application, where the output from the gas turbine is used in a propelling nozzle, raising the turbine temperature increases the jet velocity. This reduces the propulsive efficiency giving a loss in overall efficiency, as reflected by the higher fuel consumption, or SFC.