Understanding Airflow and Pressure Changes in Jet Engine Design

[Jet-designer]

Hello,
I'm Arman, new user here. I'm a jet-engine designer and I'm really interested in building and running different types of jet engines.

I have quetion now: Some amount of air enters a cone shaped pipe with a minimum diameter of 6 cm and a maximum diameter of 10 cm, and a length of 7 cm at 30 m/s. We know that beacuase of the increased volume, the air slows down. can you tell me how much does the air slow down and what changes are made in the pressure of air?

Thanks,
-Arman

 

[Astronuc]

Here is a page on ramjet propulsion -
http://www.grc.nasa.gov/WWW/K-12/airplane/ramth.html
http://www.grc.nasa.gov/WWW/K-12/airplane/ramjet.html

I don't know if you'll find the specific calculation in which you are interested. One must solve the simultaneous continuity (mass), momentum and energy equations to obtain the correct state point (pressure, temperature, momentum/velocity) in the flow.

from the propulsion page -
http://www.grc.nasa.gov/WWW/K-12/airplane/shortp.html

Here is some basic information on rocket propulsion - http://exploration.grc.nasa.gov/education/rocket/shortr.html

 

[FredGarvin]

Not to be a stick in the mud here, but you're a jet engine designer and you're asking how to calculate the pressure rise in a diffuser?

 

[Jet-designer]

Any Problem? I'm not a Ramjet designer and that's why I'm not that professional in designing ramjets...

 

[FredGarvin]

Any Problem? I'm not a Ramjet designer and that's why I'm not that professional in designing ramjets...

Just asking. As you must know, you described a diffuser which a very common element in jet engines, not just ramjets.

 

[Danger]

That sounds a tad peculiar to me too, Fred.
Jet, what exactly is your education in this field? It's a lot easier to help if we know what level we're dealing with.

 

[Clausius2]

Not to be a stick in the mud here, but you're a jet engine designer and you're asking how to calculate the pressure rise in a diffuser?

Jet, I'm NOT a Ramjet designer, neither every of us. But yeah, we know the answer, it belongs to "general culture" for an engineer. The answer is: who knows. You are only specifying one of the three variables (v,P,T) needed for solving the thermofluid-dynamic state of a gas.

 

[3trQN]

Hey, id like to get involved in making my own designs, and simple propulsion models, as a hobby. Any advice from anyone that's done somthing similar? i.e. with hobby models and small scale experiments.

Im currently trying to further my understanding of Thermodynamics, mechanics and fluid mechanics, and my maths also. I think a project would help keep me motivated & focused.

 

[FredGarvin]

Hey, id like to get involved in making my own designs, and simple propulsion models, as a hobby. Any advice from anyone that's done somthing similar? i.e. with hobby models and small scale experiments.

Im currently trying to further my understanding of Thermodynamics, mechanics and fluid mechanics, and my maths also. I think a project would help keep me motivated & focused.

There are some pretty cool kits out there as well as a lot of homebuilt websites. Do some surfing to see what's out there. You can get into it pretty cheaply by getting into the CNG powered burners. I remember seeing a website that had a guy in New Zeland that used a homemade turbine to cool his beer in his garage.

 

[berkeman]

Just asking. As you must know, you described a diffuser which a very common element in jet engines, not just ramjets.

I think what he meant to say is that he would like to become a jet engine designer...

Be sure to check out the homepage he lists in his bio for more information:

http://air.blogfa.com/


EDIT -- There are even some turbojet and ramjet drawings mid-way down that home page...

 

[Jet-designer]

Yeah, I'm not a real jet engine designer, I'm an analyst in aviation science and my homepage is a lovely and really enthusiastic weblog in Iran, and I'm a writer for a lot of country-published aviation magazines and the question I've made was about a new project I'm working just for fun. Thank you all...

 

[Jet-designer]

Hi,
I'm quite disappointed now, because no one gave me what I exactly wanted.
Tell me what variables are needed to calculate and solve the problem I talked about?

 

[FredGarvin]

To start, look here:
https://www.physicsforums.com/showthread.php?t=68337

 

[Jet-designer]

Dear Fred, I've already read that, can you offer me a simple solution to solve the problem?

 

[Mech_Engineer]

<SIGH>

 

[Hawknc]

http://www.grc.nasa.gov/WWW/K-12/airplane/mass.html

Look at the fluid dynamics section. That, at least, will explain how to obtain velocity. From that, since it's a nice slow air flow, you could use simple incompressible flow equations to find pressure which pretty much every flight/fluid textbook/website ever invented has.

 

[Astronuc]

Or

http://www.grc.nasa.gov/WWW/K-12/airplane/wcora.html

 

[Hawknc]

Sure, if you want to be all complicated about it. :p Things get a whole lot more interesting with compressible flow.

 

[Astronuc]

Sure, if you want to be all complicated about it. :p Things get a whole lot more interesting with compressible flow.

I suppose with the parameters given in the OP, the compression will be very little. Then one only needs to employ the continuity equation, which is NBD.

Besides, 30 m/s (~100 ft/s) isn't much of a ramjet.

 

[FredGarvin]

I'm an analyst in aviation science and my homepage is a lovely and really enthusiastic weblog in Iran, and I'm a writer for a lot of country-published aviation magazines

Do you have an english version of your blog? I would be interested in reading it, especially from the viewpoint of someone who lives in Iran.

 

[berkeman]

Dear Fred, I've already read that, can you offer me a simple solution to solve the problem?

Google Bernoulli. I did, and got lots of hits that would appear to answer your question. Like this one:

http://home.earthlink.net/~mmc1919/venturi.html

 

[RainmanAero]

Dear Fred, I've already read that, can you offer me a simple solution to solve the problem?

Are you interested in learning, or do you just want someone to solve the problem for you? Most folks who are earnest about engineering prefer understanding the how/why behind physical situations and actually solving the problem themselves, even if they ask for help from others.

Are you familiar with the fundamentals of incompressible flow and Bernoulli's equation? Not really any high-tech secrets here.

Rainman

 

[Jet-designer]

Thank you all for your response...

Dear Fred, Sorry, the weblog and the essays I've written are all in persian or farsi, but if you want to get familiar with point of view of an Iranian aviation analyst, let's start a topic and get into the discussion, I'm ready (by the way do know that IIAF or Imperial Iranian Air Force was the third powerful airforce of the world after US and USSR, with about 80 high-tech, deadly F-14 Tomcats...I'll talk about them soon..)

 

[FredGarvin]

I am well aware of the IIAF having some F-14s. I am also aware that they are now all piles of junk ever since the Shah was overthrown.

 

[Jet-designer]

Fred, I have started a new topic about "Iranian Air force", let me answer you there. I will be thankful if you and other friends take part in our discussion.