Axial Jet

[johndoe1776]

Ok this is for a mostly personal, partially school project. I’m trying to build an Gas Turbofan with an Axial compressor. Is there a specific angle your compressor blades should be decreasing in length on. I tried to make that sound right. Angle for the compressor. I know the core of the engine, beginning of the compressor starts at 2in diameter. Any help would be great.

[turboron]

johndoe1776, We need a lot more information to help you answer any questions you have about gas turbines. For starters, what is the design mass flow or horsepower?

Thanks, Ron

[racket]

My advice ...............do not attempt this , the complexity of both design and manufacture will be horrendous and theres virtually no way a 2" inlet axial comp will work

There have been very experienced builders on the GTBA that have constructed axial comps www.gtba.co.uk/ but its not a build for a first timer .

[johndoe1776]

Thank you for your advice I really appreciate it. I know it will be hard but I think with a little help I can do it or at least give this engine a run for it’s money. I’ve already got about $60 into parts. My shaft is 1/4 in threaded pipe and I made a 1/2in in diameter, cylindrical bearing housing for the compressor shaft. I’m a highschool student who’s only made it through Chem, about 1/2 way through physics and 3/4 through AP Calc before this whole virus hit. That’s about as much math as I know, I’ll be perfectly honest I don’t know Flow Rate, or Nozzel Angles. I was into model rocketry for a short time. I don’t see why a jet engine needs all those fans. Why don’t they just use a standard 45 degree converging nozzel with a fan and a combustion chamber to create a vacuum and then add your 15 degree diverging nozzel out the back. Like an Air Breathing rocket jet? Furthermore since we are compressing the air with the fans, the longer the compressor the better right? Thus a lower angle. Something akin to 20 degrees right or wrong?

[johndoe1776]

I made the area of the entrance to the combustion chamber a 1/4 of the area air could enter through my inbetween my 2in intake and the 1/2 bearing. I came up with about 1.163825in or approximately (1 1/8 in). I don’t know if that’s right but the area it can enter the combustion chamber is 1/4 of that my intake. This is looking at this from a 2d perspective, I don’t know if you can do that. You tell me your the experts.

[racket]

DO NOT CONTINUE WITH THIS PROJECT, IT IS DOOMED TO FAILURE ............. heres a "simple" explanation of whats required 164.100.133.129:81/econtent/Uploads/04-PT12-AxComp-1%2064%20%5BCompatibility%20Mode%5D.pdf

[turboron]

johndoe1776, I agree with John that you are doomed to failure on two counts. First, your size is too small to result in a working device due to tolerances(they don't scale) and the difficulty of machining. Consider the Rolls-Royce/Allison Model 250 helicopter gas turbine's 6 stage axial compressor. It the smallest axial compressor in production and its inlet is about 6" in diameter. Second, you need to achieve at least a pressure ration of 4/1 to have a successful gas turbine as Whittle proved 90 years ago. A well designed single stage axial compressor would be a 1.45 pressure ratio.

Thanks, Ron

[finiteparts]

Why don't you try to build a turbocharger based engine first, then work towards an axial design after you get a better understanding of what is happening?

From your comments, I feel that you might not understand this process at even the fundamental level, so let's talk about this and see if we can help you out.

You stated:

"I don’t see why a jet engine needs all those fans. Why don’t they just use a standard 45 degree converging nozzel with a fan and a combustion chamber to create a vacuum and then add your 15 degree diverging nozzel out the back. Like an Air Breathing rocket jet? Furthermore since we are compressing the air with the fans, the longer the compressor the better right? Thus a lower angle. Something akin to 20 degrees right or wrong?"

First, if we create a vacuum at the combustion chamber, how would we derive power from the turbine or even flow out to create the propulsive jet?  Gases flow from high pressure to low pressure, so we couldn't flow from the combustor through the turbine or out an exhaust. Not sure what you were going after there, but at a completely fundamental level this doesn't work.

Second, a 15 degree diverging nozzle is not really a "nozzle".  A diverging passage in subsonic airflow acts to reduce the flow speed and increase the static pressure, we call that a diffuser. 

Third, a single fan blowing into a funnel does not compress the air to the level needed to run an engine.  As Ron said, there is a limit to the pressure ratio that a fan can produce before the flow through it completely breaks down and stalls.  In a axial compressor, the rotor stages add energy to the fluid by accelerating it.  Adding energy to the flow does increase the total pressure of the gas flow, but the accelerated flow has a lower static pressure, thus the addition of stator vanes.  These stators act to diffuse the flow and correct the flow angle so it can enter the next rotating stage efficiently.  The ability of these two components to generate a stage pressure sets the number of stages that you need in the compressor assembly.  The are change through the compressor is done to keep the axial velocity  similar through the entire assembly as the local gas density is changing.  A homemade compressor stage will likely be very inefficient and thus you will need quite a few stages.

There is a lot of work that goes into designing a decent axial compressor and there may be a infinitesimal chance that you could stumble onto a workable design, but I can guarantee you that you will be less efficient than any old turbocharger compressor impeller.  This is why you don't see a bunch of small axial turbojets flying around.  Manufacturers and experts in the industry have know this for years, do a quick search on specific speed of compressors. Small axial compressors can be done, but it is orders of magnitude harder to do.

Now I don't want to discourage you in any way, but if you are asking for help, we are here to help.  If you choose to ignore our advice, then the question has to be asked as to why you are asking for advice anyway?

Good luck,
Chris

[turboron]

Chris, well said.

Thanks, Ron

[johndoe1776]

Alrighty then, I guess I’ll start with the centrifugal compressor. Sorry if I’m a little hesitant to switch after already putting some money into it. Money doesn’t grow on trees. Also I’d like to point out that the idea of a rocket-jet engine combination is not as stupid as you make it sound. Apparently I’m not the only one who has come up with this idea. futurism.com/revolution-flight-jet-rocket-engines-combined-new-prototype. Anyways any suggestions that pertain to building a regular jet engine would be helpful, as I have stated before, I’m not an expert. And maybe a little help with those calculations, because looking at those is incredibly discouraging, I’m stumped.

[turboron]

johndoe1776, here is a link we recommend to get an understanding of the basics. Have a look on the Yahoo DIY Turbines Site groups.yahoo.com/neo/groups/DIYGasTurbines/info

There are also some inexpensive books on building a centrifugal based gas turbine using a small turbocharger impeller. I recommend the English version of Thomas Kamps book "Model Jet Engines (Modeller's World)". It is available used from Amazon for approximately $20.


Thanks, Ron

[finiteparts]

I was not saying that your "idea" was stupid, I was just trying to help you understand the actual physics.  Hybrid propulsion systems are not new and there are many good ideas out their that allow mode switching of propulsion system types.

From your comments, I was a bit concerned that you might have a misunderstanding of the basic physics that drive any of these propulsion systems, so I was only trying to help you get on a better footing to build your own systems.

Here is a very good set of videos by Roddy Mc Namee that walks through the basic calculations of  an ideal turbojet.  If you click his name and go to his channel, you will see he has quite a few good  lectures on turbojets and turbofans.  This should help you get up to speed quickly and easily.

www.youtube.com/watch?v=Fm5orZfz3rQ

Also, if you search for videos on thermodynamics, they will be very helpful in understanding such concept as enthalpy, entropy and available work.  Now, keep in mind that thermodynamics is usually a sophomore or junior year class taken by undergraduate engineering students so it is ok if it seems a bit unclear or tough.  Your high school physics and Calculus courses should have you in a good spot to be ready to start learning thermodynamics and heat engine design.  You have a much better opportunity than I had when I was in high school, being that you have a wealth of knowledge and teaching at your fingertips.  I had to learn on my own from books and sometimes, things didn't "click" in my mind for years after reading them.  

As an example, here is a Thermo class from MIT that you can watch and learn now, well before having to take the classes in college.  

www.youtube.com/playlist?list=PLA62087102CC93765

Like I said above, if you can do ok in physics or calculus, you can learn Thermodynamics, which is the basic building block of heat engine design.  Now don't feel like I am saying that you have to understand thermo, it is just a way to give you more thorough knowledge of how the design equations come to be, how the basic systems behave, etc.

I hope that helps and good luck!

Chris

[finiteparts]

Also, these videos are great for seeing and understanding the "basics".

www.youtube.com/playlist?list=PLHbe2KtPdMP1Oowt2lukiR87katXyA7ix

Enjoy!

Chris