Swirl Fuel Injector - for gaseous fuel?

[britishrocket]

Hello everyone,

I have a question for the assembled experts. And I know from past experience that there are a few of those here.

First of all, a bit of background. About 10 years ago I did a fairly in depth study of swirl fuel injection for rocket engines. One of my principle references was the work of Yuri Khavkin.

I was particularly interested in so called "open ended" swirl injectors in which the coefficient of discharge is unity.

Here is a link to details of my experiments and findings which you may find useful as I go on to pose my question.It is worth looking at as it at least gives background to my query:-

britishreactionresearch.blogspot.com/2013/06/further-swirl-investigations.html?m=1

During the above investigations I ran air through my swirl injector at about 100psi. I found that instead of a column of air in the centre of the fluid cone which is produced in a liquid swirl injector, this air swirl injector had a strong vacuum at it's core - which makes total sense. I never measured the vacuum at the time as the idea of a gas swirl injector wasn't part of my plan.

So my question is this. If, instead of injecting propane radially through holes into an amateur style sore thumb combustor, we inject it using a swirl device as I've outlined, would there be any benefits?

The gas would travel in a vortex along the length of the flame tube interior, but substantially perpendicular to the dilution holes.This should induce good mixing.

Further questions. Would the strong vacuum at the core of the swirling gas assist with flame holding through recirculation? Would this vacuum create a "draw" effect and help to pull in the charge air from the combustor annulus?

I don't know anything about CFD analysis so I can't model any of this. Actually, there is a large body of evidence to suggest that I don't know anything, full stop.

Hopefully someone here can give me an answer to my questions. Thanks in advance.

[racket]

What you're proposing is a hollow cone spray nozzle normally used in gas turbines when spraying liquid fuel .

Yes it creates recirculation into the "hollow" center of the spray pattern for good flame stability.

I guess it'd be a case of getting a large enough flow capacity for gasious injection rather than liquid , but can't see why it wouldn't work :-)

I once tried something similar to inject water into the comp scroll of my TV84 engine imgbb.com/hBTkxRL , I made up a swirl chamber that was screwed to the side of the scroll near , the brass plug blocking off the supply hole can be seen in the pic.

[britishrocket]

Hello and thanks for your reply. Yes that's exactly what I'm proposing, a swirl injector. The type of swirl injector I made in my old research can have a really high discharge coefficient.

I think I can make one with enough flow to inject enough gaseous fuel.

Thanks again for your reply and help. If anyone else has any further please don't hesitate. Have a look at my link to my original research in the above post first to get some background to my thoughts.

Cheers!

[CMDR_Boom]

While I haven't as much rocket experience other than what transfers between working knowledge of jet engines, most jet engines from the late 1970s and up feature 'swirl' nozzles via the combustion liners, which when paired with hollow conical-shaped fuel nozzles (usually liquid but gaseous would work about the same once it meets the air temperature present) is part of working operation in multi-group older combustor engines. Annular type combustor liners have traditionally been a bit too expensive for me to obtain, but I can ask a few colleagues if you'd like for direct reference if they are present in modern engines (I would tend to think so just from an efficiency stand-point).

If you're going after implementation for home/hobby use, swirl ring nozzles are a bit easier to machine on the air side vs the fuel size, though I've seen both done.I seem to recall seeing something similar on repurposed pressure washer nozzles and hollow conical spray heads when I was perusing nozzles before landing a few NOS jet fuel nozzles some years ago. I may even have the bookmarks active if needed.

Add: Wa ha, found it! Something like this for gaseous? 

Just for reference, I did a lot of testing that same year on more traditional hollow cone nozzles with high pressure pumps. At 2400psi for liquid fuels, that hollow cone pattern at 1 atmosphere had around a 240cm maximum cross section. Granted when I ran the later test at 300psi it was restrained to a 150mm combustor liner, but mixed beautifully in the liner with the feed air. I wasn't going for swirl induction, just the hollow conical pattern on the liquid nozzle. 

[britishrocket]

Thank you for that informative and highly interesting reply.

I've never seen an example of a swirl gaseous fuel injector, either in a rocket or gas turbine.

Swirling a percentage of the charge air as it enters the chamber is common in gt combustors as we know. It occured to me, why not swirl a gaseous fuel. I know from my previous experiments that swirling gas induces a strong vacuum in the core and I surmised this might lead to recirculation. I also wondered if it would give a bit of induction in terms of drawing the annulus air into the flame tube.

If you look at my prior research you will see that I managed to characterise open ended swirl injectors with a reasonable degree of success. I made several test pieces armed with that information. So I'm confident I can make my own swirl injector to my exact specifications. Obviating the need to press an off the shelf unit into service.

Thank you very much again for the excellent response. You've really spurred me on!

[racket]

I don't think the "vacuum" at the core will make much difference to the flow of air through the flametube wall holes , you're also feeding in a large volume of gaseous fuel , a 1,000 times more volume than liquid fuel .

If you can produce a nice spread gaseous fuel across the flametube , it'll work just fine :-)

[britishrocket]

Yes I take your point.However it will induce recirculation which should hopefully stabilise the flame. Thank you for all your input.

[racket]

Yep , its the recirculation thats important