Combustion Design Jun 19, 2023 14:46:22 GMT -5
Post by finiteparts on Jun 19, 2023 14:46:22 GMT -5
I have come to a few tentative conclusions after reading the material presented here, as well as Lefebvre and quite a few other texts on gas turbine and combustor design. I'd be interested to hear what others think.
There is an old adage in engineering - and in other fields of endeavor too - that an ounce of practice is worth a ton of theory. Do not read that sentence and immediately think that I am decrying theoretical knowledge. I am not - I have always tried to base the design of anything I make on sound theoretical principles.
What I am saying is that when it comes to building a gas turbine from an automotive turbocharger, there are so many uncertainties and variables that we may as well use a broad brush tool like JetSpecs and not get too hung up on the minutiae. It seems to me that we are dealing with something of a "black art". For example, I have heard about the "20% rule" when it comes to selecting a turbocharger as a suitable candidate for gas turbine conversion. I understand well the thermodynamics as to why this would be the case. And yet the internet is full of units that do not obey this rule but run well. This state of affairs reminds me of another old saying that an aerodynamics lecturer I once had was fond of. That bees are aerodynamically incapable of flight, it is just that no one has told them.
Similarly, Chris's analysis of John's combustor, which has clearly been built in accordance with good principles in terms of airflow regime shows that it is less than ideal in many respects, with low Cd values caused by too high an annulus flow velocity for some of the flame tube holes. And yet, this combustor clearly works and works extremely well, and is rightly held up as "best practice" on this site.
I had always felt that the "typical" amateur gas turbine combustor design that is seen on this site and online in various guises was poor, in terms of the compressor entry being tangential and thereby giving rise to a swirling flow inside the can which will reduce static pressure at the very point where we want to increase it. That said, looking at some of the CFD plots showing different arrangements given in this thread, tangential entry, albeit improved by the addition of a diffuser, would seem to be a more than viable option.
JetSpecs has been called an unscientific tool or maybe a sledgehammer to crack a nut. Given the broad tolerances we are dealing with when it comes to turbocharger gas turbine conversions, this may be the best approach. Get into the ballpark with JetSpecs and then refine based on performance. An empirical approach that is informed and advised by the theory.
I would be very interested to see the mathematical basis behind JetSpecs and the assumptions that it is based on. Would anyone have that information, and if so could it be published here?
I'd be very interested to hear what others think.
I read through this a few times because I was having a hard time really reducing it to it's primary thesis, it seemed to waver a bit. Correct me if I got this wrong, but I think you are trying to make a case for simple tools like Jetspecs as opposed to doing more detailed calculations for the general user?
If that is indeed the message, I would generally agree with that for the majority of readers. Over the years it has become clear that the majority of readers are not trying to design there own engines and are just wanting to build a cool little noise maker to wow their friends....and little gas turbines are great for that. The immense power that they output is evident when you are standing right next to these little fire-breathers! That is why you usually watch for them to back up once it fires up!
But, there are a few on this site that are expanding the state of the art for homebuilt gas turbine engines and that is who I was targeting this information to. Jetspecs does an excellent job giving users a good workable combustor, but it produces sub-optimal combustors for those trying to make compact combustors for more advanced homebuilt engines.
The statement "There is an old adage in engineering - and in other fields of endeavor too - that an ounce of practice is worth a ton of theory". I am definitely a proponent of this in engineering as I have seen too many times, projects confuse increased model fidelity with reduced design risk. My career has been almost exclusively in new technology integration and the rub there is that it is NEW technology. So getting to test as early as possible is key because we don't know what we don't know.
But, not doing the fundamental calculations based on first principles or previous test data, is a way to invite disaster. You need to do your due-diligence to make your best design and pre-test predictions, because when the testing starts to deviate from your predictions, you at least have a grasp of the fundamentals, and a general sense of things to try or data to collect to help you reduce the test data in a meaningful way.
Using this statement is a bit misleading. Almost all the data and calculations that I have presented from Lefebvre and others in the combustion field is reduced from test data...i.e. from practice and NOT theory. I feel that this statement also sends the wrong message, as it appears to justifies not doing any real calculations at all, just go to test. I have had the good opportunity to design and build some small combustors in both the academic and industry environments and Lefebvre's has served me well, even when some of the design data was a SWAG (scientific wild ass guess), so I can assure you that it is not an exercise in futility.
As for the discrepancy between my evaluation and John's design, you will see that I assumed a few things in that analysis that were not true and John provided his corrected data after that. I need to update that on this page, but with the new numbers, his annulus entrance velocity dropped to 129 ft/s and the Cd's came up to typical values. So the operability of his combustor aligns well with the design space of the data based parameters from Lefebvre and Mellor.
A final thought, I have come to realize over time that I enjoy doing the calculations almost as much as the engine design and building. Doing cycle calculations, CFD, thermal design, secondary flows, test predictions, etc., at work has made me enjoy doing more of this at home too, so part of my craft is the design and theory stuff.
I enjoy sharing this when I see an opportunity to help others in this hobby.