Modelling combustion stability on a combustion chamber 2

[Lumbisno]

Hello, hope you are all doing well, I'm starting an internship and as my first task I have been told to analytically model the combustion stability on a combustion chamber in a spacecraft from scratch, but if I'm honest, I do not know where to begin. I do know that that some of the parameters that I have to use are mass flux of my fuel, chamber pressure, injection pressure among others, but I'm stuck at the start due to lack of knowledge, and I was wondering if any of you have some advice for where to begin, be it a book that has the theory of it or anything else, I'm not looking for the solution to the problem because I want to do the project by myself, but I'm lost at where to start.
Thank you all in advance.

 

[btrueblood]

"I have been told to analytically model the combustion stability on a combustion chamber in a spacecraft from scratch"

Somebody is playing a joke on you. Go the the NASA NTRS server and search on some of the terms above. You will find the topic covered since the 1960's and possibly earlier, with continuing research to the present day, along with survey reports (dating from the 60s to present day) bemoaning the lack of a priori predictive ability of most or all models of the problem.

Good luck, we're all counting on you.

 

[MintJulep]

There are probably only a small number of people in the world who know how to do this.

Hopefully at least one of them works at the company you are interning for.

Find that person.

The purpose of internships is to provide near-slave labor for companies teach students real-world skills they don't learn in school and develop a better talent pool for the employer.

Perhaps your immediate boss doesn't understand that.

As a starting point:

https://www.nas.nasa.gov/SC18/demos/demo19.html

 

[btrueblood]

I mean, it's cool that they gave you carte blanche to expend your time and their resources in such a snipe hunt. It's been an intractable problem for over half a century, and you never know if a fresh pair of eyes will have an insight that nobody else has had. But then, look at the link Mint posted - 1.1 million processor hours to simulate an event that takes a tenth of a second or so in a full scale rocket engine...and while that was a time-dependent model, it does not simulate (nor intend to simulate) unsteady acoustics-driven combustion transitioning to detonation, which is what your problem is.

But the point of the excersize is probably to teach you how to solve real world problems...and the first step for any engineer with that task is research, thus my first post.

Do feel free to post back here with questions as you learn more, or if you need some names/contacts. Tell Fred I said hello.

 

[Lumbisno]

okay, then probably I'm misunderstanding the assignment and/or failing at translating what has been asked (or maybe they are indeed just playing a trick on me but before confronting them I will have to be sure of it), I have been told that with the measurements of the units I mentioned earlier I should iterate while varying the functioning state of the motor, that way finding when the combustion becomes unstable, does that still sound like and impossible task? i ask because my limited understanding of the topic does not allow me to see if it is or not, thank you!

 

[Lumbisno]

please do keep in mind that this is supposed to be a starting task, so maybe I'm being asked something along those lines but easier (or again, maybe someone is just playing a prank on me to see when I notice)

 

[btrueblood]

Is this a mono- or bi-propellant engine? Liquid or gas phase? Do you know the injector geometry, chamber dimensions, throat size? If bipropellant, where is it operating in terms of stoichiometric mixture ratio? There are many ways for a rocket to be unstable - you can have propellant feed system oscillations, coupling between injector and combustion chamber acoustic modes, or full blown acoustic instability in the combustion chamber.

"I have been told that with the measurements of the units I mentioned earlier I should iterate while varying the functioning state of the motor, that way finding when the combustion becomes unstable, does that still sound like and impossible task"

Iterate what? Do you have test data that covers ranges where the behavior is both stable and unstable? If so, that sounds like you are tasked with correlation, not a priori prediction. See if you can map unstable and stable conditions to the propellant mixture ratio and chamber pressure.

 

[Lumbisno]

Thank you, you have shed some light in just how little info i was given about this project, i will ask for more data and information about the conditions of this model so i can start to work on it, because as you said, there are many instabilities to account for, and i have very little data

 

[3DDave]

The general topic is called "turbulence."

British mathematician and expert on fluid flow, Horace Lamb, is said to have hoped that God might enlighten him on quantum electrodynamics and turbulence, saying that “about the former I am rather optimistic.”

http://philipball.blogspot.com/2014/07/a-feeling-for-flow.html

If you do manage to solve it, it's much, much easier to predict such variations in the stock market - get rich and buy a rocket company.

 

[Lumbisno]

Haha! Amazing find, will definitely keep that in mind if God decides to give me a little advice with this, thank you for the laugh

 

[btrueblood]

Well...you don't necessarily need to solve the turbulence problem to fix some or most instability modes...but yeah, solve the problem and expect to be rich.

 

[CWB1]

The chemical-kinetics of combustion is an interesting niche to lose yourself in but IMO unless your employer has a long-established research team and millions annually budgeted to it, you're chasing your tail giving it any effort beyond an introductory review. That's not to say that many with little experience, understanding, or budget haven't proclaimed to the world that their garbage in = magic out analysis will revolutionize industry in a manner that teams of Phds working with massive budgets for decades have not.