Gas Turbines - Introduction — Page 212, Lesson 278

Let’s start with the core idea that drives everything in a gas turbine: torque on the turbine blade. The torque applied to the turbine blade depends, among other things, on the rate of gas flow into it. So the faster we can make the gas flow into the turbine, the more torque we can transfer to it. That’s the fundamental relationship we’re working with. Now, logically, if we convert some of the considerable pressure energy of the gas stream into kinetic energy, it will be more efficient in imparting a turning effect upon the turbine and its shaft. So we’re trading pressure for speed, and that speed is what does the work on the turbine. Let’s look at the airflow through a pure straight turbojet engine. Figure 13.7 shows a single spool axial flow compressor turbojet engine. When a compressor and turbine are joined on one shaft, the unit is called a spool. This type was for a long time considered the most useful where an engine with a small frontal area was required, such as in fighter aircraft where high forward speed was the main criterion. There were, however, problems with controlling the smooth flow of air through the engine throughout its rotational speed range — more on that later. The flow follows conventional patterns. From the compressor, the air is fed into the combustion chambers, just as with the turboprop engine, and similarly fuel is now added to give the substantial increase in volume required. The energy required to drive the compressor is now extracted from the gases as they pass through the turbine. The remaining energy is extracted to act as thrust as the gases pass to atmosphere via the end of the jet pipe. Now let’s move to the turboprop engine. Figure 13.8 illustrates both a centrifugal compressor turboprop engine and an axial flow compressor turboprop engine. The output from a turbo-propeller engine is the sum of the shaft power developed at the turbine and the residual jet thrust. This is called Equivalent Shaft Horsepower, or ESHP. The major difference between the turboprop and the turbojet is how, in the former, almost all the energy in the gas stream is converted into mechanical power. In the turbojet, a high proportion of the gas stream energy is utilized to drive the compressor, as it is in the turboprop. But whereas in the turbojet the energy that remains is used as thrust, the energy that remains in a turboprop engine is used to drive the propeller. Only a small amount of ‘jet thrust’ is available from the exhaust system of a turboprop with an efficient turbine — it can be described as ‘residual thrust only’. Apart from this difference, the airflow through the engine is virtually the same in either case. The compressor passes the air to the combustion chamber, where the fuel is added and a substantial increase in the volume of the air is obtained at a nominal constant pressure. The gas is now expanded in the turbine, where a drop in temperature, pressure, and velocity is exchanged for the mechanical energy to drive the compressor(s) and the propeller through its reduction gear. So, to tie it together: in both engines, we compress air, add fuel, burn it to increase volume at roughly constant pressure, then expand the gas through a turbine. The turbine extracts energy — in the turbojet, leftover energy becomes thrust; in the turboprop, almost all the energy drives the propeller, with only residual thrust left. And the key lever throughout is that gas flow rate into the turbine, which controls the torque we can transfer.