turbojet thrust

 

Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. The first and simplest type of gas turbine is the turbojet. How does a turbojet work?

On this slide we show a schematic drawing of a turbojet engine. The parts of the engine are described on other slides. Here, we are concerned with what happens to the air that passes through the engine. Large amounts of surrounding air are continuously brought into the engine inlet. In England, they call this part the intake, which is probably a more accurate description, since the compressor pulls air into the engine. We have shown here a tube-shaped inlet, like one you would see on an airliner. But inlets come in many shapes and sizes depending on the aircraft's mission. 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. A compressor is like an electric fan and we have to supply energy to turn the compressor. 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. In a typical jet engine, 100 pounds of air/sec is combined with only 2 pounds of fuel/sec. Most of the hot exhaust has come from the surrounding air. Leaving the burner, the hot exhaust is passed through the turbine. The turbine works like a windmill. Instead of needing energy to turn the blades to make the air flow, the turbine extracts energy from a flow of gas by making the blades spin in the flow. In a jet engine we use the energy extracted by the turbine to turn the compressor by linking the compressor and the turbine by the central shaft. The turbine takes some energy out of the hot exhaust, but the flow exiting the turbine is at a higher pressure and temperature than the free stream flow. The flow then passes through the nozzle which is shaped to accelerate the flow. Because the exit velocity is greater than the free stream velocity, thrust is created as described by the thrust equation. 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. The amount of mass flow is usually set by flow choking in the nozzle throat.


The nozzle of the turbojet is usually designed to take the exhaust pressure back to free stream pressure. The thrust equation for a turbojet is then given by the general thrust equation with the pressure-area term set to zero. If the free stream conditions are denoted by a "0" subscript and the exit conditions by an "e" subscript, the thrust F is equal to the mass flow rate m dot times the velocity V at the exit minus the free stream mass flow rate times the velocity.
F = [m dot * V]e - [m dot * V]0


This equation contains two terms. Aerodynamicists often refer to the first term (m dot * V)e as the gross thrust since this term is largely associated with conditions in the nozzle. The second term (m dot * V)0 is called the ram drag and is usually associated with conditions in the inlet. For clarity, the engine thrust is then called the net thrust. Our thrust equation indicates that net thrust equals gross thrust minus ram drag. If we divide both sides of the equation by the mass flow rate, we obtain an efficiency parameter called the specific thrust that greatly simplifies the performance analysis for turbine engines.


You can explore the design and operation of a turbojet engine by using the interactive EngineSim Java applet. Set the Engine Type to "Turbojet" and you can vary any of the parameters which affect thrust and fuel flow. You can also explore how thrust is generated within the nozzle by using the nozzle simulator program which runs on your browser