[BrowserUk]

I'm looking for somewhere to ask some very basic questions about the initial design of a contra-rotating ducted fan.


I'm not yet at the CFD point, that'll come later.


eg.

The optimisation point for my ducted fan is a dead hover. Assuming an all up weight of 1kg, I need to accelerate 1kg/s of air downward.

At sealevel, that ~1.2m³. The swept area of the duct is ~0.012m², so the required efflux rate is 100m/s.

So to determine the blade inlet & exit angles, a velocity triangle is draw using the flow velocity (100m/s) and the tangential velocity of the blade.

But for the air flow to exit, it must also enter, so the axial flow rate at both leading and trailing edges (of both contra-rotating blades) would be the same; which would mean no camber angle, and that cannot be correct.

I guess the difference is in the pressure change between the leading and trailing edge -- ducted fans are axial compressors after all -- but how can you determine what pressure change will happen until you've determined the stagger and camber angles?



Can anyone enlighten me; or point me to somewhere where I can ask such basic questions on topic?


Thanks Buk

[flotus1]

I'd call the question fundamental rather than basic. Moved to the main CFD forum, since this should give the best visibility, and also is the best spot to ask these general questions.

[LuckyTran]

You're missing one more performance parameter and in turbomachinery that is the blade efficiency usually labeled eta. That gives you how much of the total enthalpy rise across the fan goes in the static enthalpy (temperature) and how much is available for mechanical work.

[BrowserUk]

Originally Posted by LuckyTran

You're missing one more performance parameter and in turbomachinery that is the blade efficiency usually labeled eta. That gives you how much of the total enthalpy rise across the fan goes in the static enthalpy (temperature) and how much is available for mechanical work.

Thanks, but I cannot know the blade efficiency, until I know the input and output blade angles, thus the camber and stagger angles ...


Can I?

[LuckyTran]

You can iterate


Even root finding algorithms need to iterate. That doesn't stop us from finding roots.

[BrowserUk]

Originally Posted by LuckyTran

You can iterate


Even root finding algorithms need to iterate. That doesn't stop us from finding roots.

So true.


But even the Newton–Raphson version of the Sharaf al-Din al-Tusi or Babylonian method requires a starting point; an initial guess.


And if that initial guess is too far away from reality, then the algorithm will never converge.


That's my problem here in a nutshell. I need authoratative guidance from someone clearly well versed in the problem.


You do not seem to be that person; but I am most grateful for your response, because yours is one of only two responses I received, and the only one that has attempted to answer the non-meta question in my post.

[LuckyTran]

I assumed you have some secret handbook with a correlation for the blade efficiency. The efficiency is not something you can predict with absolute certainty a priori from just geometry alone. Even if you had the blade camber an stagger angles, you still wouldn't know eta. For example, knowing the diameter and volumetric displacement of an IC engine doesn't (from theory alone) give me the mileage of the car. In real life, you have to go analyze and measure these things. With that being said, I can close my eyes and pretty much guess a number for eta that is very realistic just like I can guess the mileage of a modern car. All you need to do is assume that the design is not stupid. Other than where to find the eta (which is proprietary), are there other points of confusion as to how to design a fan?

If you like, I can refer you to a textbook on Turbomachinery that has tons of examples of how to do calculations using velocity triangles.

I am sorry that I am not the authority you seek. I guess it is not enough that I have written 1.5D codes to design multistage turbines using radial equilibrium theory. Maybe try not to insult the only person that is helping you by saying they are not well versed when you have no idea of what the person has already done or is capable of doing.

[BrowserUk]

Originally Posted by LuckyTran

Maybe try not to insult the only person that is helping you by saying they are not well versed when you have no idea of what the person has already done or is capable of doing.

Sorry. I thought I had made my gratitude for your response very clear.

[LuckyTran]

You've made it very clear I'm not good enough for you.

Do you have any easier questions that I might be qualified enough to answer?

[BrowserUk]

Originally Posted by LuckyTran

You've made it very clear I'm not good enough for you.

Do you have any easier questions that I might be qualified enough to answer?

That was never my intent. I am truely sorry I've offended you.

[BrowserUk]

Rather than ask my (slightly) off topic question on an open, active forum.


I chose to post in the 'lounge' where I hoped I would receive guidance on the best place to ask my questions.


Instead, one member -- maybe senior, maybe junior, maybe ... -- responded to my actual problem with non-information*.


*Ie. Nothing that helped me; nor anything that I recognise as intended to help me. He then took umbrage at:

"I need authoratative guidance from someone clearly well versed in the problem. You do not seem to be that person;"

And now my post has been moved from that 'low traffic, off-topic is okay' to a main thread by person or persons unknown.


What is the likelyhood of me getting actually useful answers to my question; given all these circumstances that are entirely beyond my control or influence?

[BrowserUk]

[QUOTE=LuckyTran;830288 I guess it is not enough that I have written 1.5D codes to design multistage turbines using radial equilibrium theory.[/QUOTE]


How could I possibly know that from your "Suck it and see" (ie. Guess), responses to my question?


(And why is your ego so fragile, that you are offended by someone a) asking you a question and b) questioning your response.)?

[flotus1]

Moved this thread back to the offtopic "Lounge" forum upon request.

And now my post has been moved from that 'low traffic, off-topic is okay' to a main thread by person or persons unknown.

I stated loud and clear in my first response here that I moved your thread to the main forum. And also why I did it.
It was an attempt at helping you get more eyes on your question, or any eyes at all. This whole forum isn't super active, and posts in the Lounge forum don't get much attention from the few people who actively engage with questions. The main forum is the best place to post fluid mechanics questions, even if they don't involve CFD yet.
If you need anything else from moderation, please let me know. Either here or via private message. Or contact any of the other mods, we are here to help after all.

Reminder to both of you: please keep it civil.

[BrowserUk]

Originally Posted by flotus1

I stated loud and clear in my first response here that I moved your thread to the main forum.
Reminder to both of you: please keep it civil.

Looking back I see that you said "Moved to..." not as I misread it "Move to...".


I did not noticed it had moved until I logged out and closed the tab; and then came back to look for the thread in the lounge and it wasn't there.

Originally Posted by flotus1

please keep it civil.

I think we've had a very polite argument.



So polite that I didn't realise I was involved in an argument for the first 2 or 3 exchanges.


I still do not understand what I did to offend LuckyTran. If "You do not seem to be that person;" is such a bad thing to say given the limited knowledge I had of him (two very short posts that contained little information upon which to base any conclusion), then I can do no more than apologise; which I have already done.


And go find somewhere else to ask my questions as I'm obviously not going to get any further help here.

[LuckyTran]

For an isentropic meanline design all you need is (for each rotor) the 8 parameters: absolute and relative flow angles at the inlets and exits and the rotor speed; in other words, you need the velocity triangles. The detailed blade shape doesn't matter. You have a counter rotating fan so you have rotors. The rothalpy across a rotor stage is constant (if you had stators the stagnation enthalpy across a stator would be constant) and you can use this to get the static enthalpy rise across the rotor stage. You do this calculation and you are done because you can get the pressure and temperature from an h-s diagram and now you know the pressure rise. This much is trivial.

For a real system, you need the blade efficiency or the loss coefficient to determine the difference between enthalpy rise vs pressure rise. Now this loss coefficient does depend on the blade shape and really the actual flow around the blade and requires 3D CFD. So we're back where we started, you need the blade efficiencies. Or did you actually have trouble with the first part?