Transition to a Compressor-less Engine — Ramjets
#propulsion #ramjet #compressor #aerospace
Disclaimer: Readers are assumed to have some technical knowledge and all information given here is written for non-academic audiences
Gazing out of the transparent glass window panels at the airport, a majestic sight greets you. A grand fleet of airplanes… You must be bored. Even X Æ A-12 excites you more. Well, this introduction isn’t really necessary. I really just wanted to talk about aircraft engines. Let’s cut to the chase.
If you have noticed, you should have seen that in the engines of the commercial airliners (turbofans) that we utilize in our air travels, they contain a huge fan-like component that has enormous blades. That is the inlet fan that draws air into the engine as it rotates. But behind that, is a complex system of turbo-machinery that can be depicted with this schematic diagram.
The focus of this article would be on the first major turbo-machinery component that is fundamental in turbofan/jets, which is the mechanical axial compressor. Let me first bring you through some fundamentals.
What does the mechanical compressor do? (Thorough explanation of the entire engine can be read here)
The compressor essentially squeezes the air that is coming through the fan and increases the pressure of the flow with its multi-staged compressor fans. These fans are equipped with small airfoil-shaped blades that decrease in length as it progresses towards the next stage, which meant that the air is flowing into an even smaller space as it flows across the compressor stages. The more stages there are, the higher the compression.
With this pressure rise, it allows more energy to be generated from the combustion of this air as it passes through the combustion chamber. How so? You can think of it this way. As you compress the air, the air particles would act against this force and tries to expand. You can imagine the air molecule getting squeezed and packed close to each other, vibrating at rapid speeds, which raises the potential energy. Assuming that the ideal gas law stands, an increase in pressure would result in an increase in temperature. So these air particles would also be hotter than before.
Also, with these air particles so densely packed, it reduces the volume that they initially occupy, allowing more air particles to join the mix. Remember, engine thrust is proportional to the mass flux. More air, more thrust, more efficient!
As the pressure rises, the velocity of the air reduces. One way to imagine this, is that if someone were to exert an opposing force on you while you are jogging, you would slow down, or maybe you’ll stop to teach that person a lesson for blocking your way (just kidding!).
When the velocity reduces, it aids in the combustion process. As air flows with a reduced velocity, sufficient time is allowed for full combustion of the fuel with the air as it travels through the combustion chamber after the compressor. If the air is travelling too fast, combustion might not be complete and it becomes inefficient. Also, as the air particles are more densely packed, the fuel particles that are injected into the mix in the combustion chamber would be able to react with a greater mass of air per unit volume, maximizing combustion.
To summarize, the mechanical compressor does these:
- Increase pressure (Increase compression ratio)
- Increase temperature
- Decrease velocity
Resulting in:
- Increase combustion efficiency
- Increase thermodynamic efficiency
- Thermodynamic efficiency for an Ideal Brayton Cycle (Assuming maximum efficiency for compressor and turbine):
Hence, we can easily discern that as compression ratio increases (P2/P1), the thermodynamic efficiency would increase. Having a compressor is necessary for efficiency!
But what if I told you, after explaining so much of what the mechanical compressor does, that I am about to throw it away?
Throw away the compressor!
Now, here is the interesting part. The turbofan/jet engines are designed to fly around the range of Mach ~ 0.8 to 2, with design augmentations like variable inlet geometries and modified diffusers, which will be covered in another article separately. However, above the speed of Mach ~ 2, something fascinating happens.
The sheer speed of the aircraft at Mach > 2 is able to provide the incoming air flow with sufficient kinetic energy, such that when the flow is simply slowed down through the use of a Converging-Diverging Inlet Diffuser, sufficient compression takes place (think of the diffuser as a fixed design component with no actuation, compared to a mechanical compressor that needs to expend energy in order to be actuated). This means it is not necessary to use a mechanical compressor to generate the pressure increase.
Initially, for a conventional turbofan, the speed of the air flowing into the engine is just not sufficient to provide enough kinetic energy that can be converted into ‘potential energy’ (pressure) by simply using a fixed design component, hence requiring a mechanical compressor.
What does this all mean? It means that there is no need for a turbine to power the compressors through a shaft, or rather, there is no need for a mechanical compressor at high speeds! And what do you call an engine that has no mechanical compressors, no mechanical turbines and fly at Mach > 2? Ramjets! This frees up substantial amount of weight and reduction in energy consumption that can increase efficiency and allow more sophisticated modifications to be added on the engine for supersonic flights.
In a nutshell, the essential takeaway from this article is:
At Mach > ~2, sufficient compression of air occurs without the need for turbo-machineries.
Future articles would elaborate on the operation of Ramjets and its internal components, including the above-mentioned Converging-Diverging Inlet Diffuser (involves sucking shock-waves into the engine!). If you are interested in Scram-jets and why it is challenging for practical implementation, you can read my other article here.
Continue to be fascinated with the technologies of our world! And nerd out at home during this pandemic!