 The idealized model of the gas turbine engine is called the Brayton cycle and in considering the Brayton cycle let's first look at a very simple gas turbine engine. We have the same general principles that we encountered when we were looking at reciprocating engines. We are combusting fuel to heat up air and we are taking advantage of the fact that the hot air tries to expand to produce power. Therefore we have a combustion chamber where fuel is intermixed with the air and combusted a regular internal combustion process and then we run the air through a turbine wherein we produce power and we can improve upon this process by adding a compression process prior to the combustion process and that will also be an axial device. We have a compressor which is compressing the air it enters a heat addition process and then it expands to produce power and like with the auto and diesel cycles we are going to be simplifying our analysis with the air standard and part of that involves closing the cycle. Furthermore we treat the input of heat as an external heat addition process despite the fact that we are actually combusting fuel that is intermixed with air. Therefore our four processes of the Brayton cycle are isentropic compression, unless we are given enough information to deduce otherwise, isobaric heat addition, isentropic expansion and then isobaric heat rejection. And remember this process doesn't actually exist it's just there to complete the cycle so that represents exhausting the exhaust gases to the atmosphere allowing them to cool back down before they are brought back into the engine again. Unlike the auto and diesel cycle the Brayton cycle is made up of steady flow devices so we have a rate of mass flowing through each device and the rate of mass flow rates at all state points is going to be the same unless we have some sort of complex situation where we are splitting the streams apart. Note that in this type of analysis compression ratio is no longer a very useful parameter instead it's more useful to talk about a pressure ratio. The pressure ratio is going to be the pressure ratio across a compression process or across an individual compressor. Pressure ratio is abbreviated RP and is not compression ratio. They sound very similar they are not the same. Do not make that mistake. So like I said it could be describing a single compressor or it could be describing a series of compressors that are working together to accomplish some bigger pressure ratio. You could have like three compressors back to back each of which had an individual pressure ratio of three and that would give you an overall pressure ratio of 27 because each one would multiply it by three so it's three times three times three. Also note you'll notice that we're talking about the Brayton cycle for basically the rest of the chapter. We start off with what we call the simple Brayton cycle that is so named not because it's easy but because it is the least complicated of the variations of Brayton cycle that we're going to be considering and then we start adding devices to try to improve upon the performance or to try to accomplish some specific task. Also note that the Brayton cycle could be used in stationary gas turbine engines say something burning natural gas to produce power for a municipality or a region but it could also be used to produce thrust that's what we will be talking about down here it's all just variations on the Brayton cycle. Let's try an example.