Ramjets and Turbojet combustors

[Viv]

Maybe I was just being a bit slow but I was just looking at the Ramjet forum and thinking what a quite lonely place it is as very few of use post here:-(

Then it occured to me that a ramjet engine could be looked upon as a turbo jet combustor.

Thay have the same problems and requirments but the ramjet has the advantage of size to get it all working were the turbo jet combustor has to do it in the minimum of space.

Entry velocites are near enough the same so we can look upon the two as kindred spirits.

What about a traped vortex ramjet engine then?

Viv

[Bruno Ogorelec]

What about a traped vortex ramjet engine then?

Sounds good to me. It would probably work at lower speed than conventional ramjets, too. An interesting idea. Don't know enough about the trapped vortex concept to judge, but it sure sounds good.

[Glenn Olson]

There are similarities between ramjet combustion chambers and those of turbojets. However, modern turbojet combustion chambers tend toward the complex with swirl vanes, high pressure spray nozzles, surface cooling boundary layers, and etc. Production ramjets are more akin to afterburners with simple flame holders, for a number of tradeoff reasons.

I've looked at a number of more complex ideas for flame holders and tried a few. As an amateur ramjetter I prefer the more simple and inexpensive approaches.

I've looked at the trapped vortex type of flame holders before and, because of their simplicity, may have an application in amateur ramjets. They're certainly worth considering. Just thinking off the top of my head, it may be advantageous to build a multi-step version, each step being smaller than the previous in a quasi-conical form. I don't think the drag would be significant and a test unit shouldn't be too hard to fabricate and test. Hmmmm!

[Viv]

There are similarities between ramjet combustion chambers and those of turbojets. However, modern turbojet combustion chambers tend toward the complex with swirl vanes, high pressure spray nozzles, surface cooling boundary layers, and etc. Production ramjets are more akin to afterburners with simple flame holders, for a number of tradeoff reasons.

I've looked at a number of more complex ideas for flame holders and tried a few. As an amateur ramjetter I prefer the more simple and inexpensive approaches.

I've looked at the trapped vortex type of flame holders before and, because of their simplicity, may have an application in amateur ramjets. They're certainly worth considering. Just thinking off the top of my head, it may be advantageous to build a multi-step version, each step being smaller than the previous in a quasi-conical form. I don't think the drag would be significant and a test unit shouldn't be too hard to fabricate and test. Hmmmm!

Hi glen

No I had more in the single traped vortex combustor that NASA are funding to replace the standard turbojet combustor can with all its complexities.

I cant remember the link but a google search for traped votex combustor will find it, it has a single step with a large vortex area.

So for ramjets even better, no drag from the flame holders so lowering the losses

Viv

[Viv]

The diameter of the engine would be larger, which would be a significant problem at ramjet velocities.

The ramjet by geometry and operation is significantly larger in diameter than a turbo jet combustor to start with due to the large diffuser.

The traped vortex combuster works at higher flow rates than a ramjet flame holder is capable of, the the trade offs are still worth while.

As the traped vortex chamber is annular it would fit in a ramjet body with out that much problem as the space it occupys is less than the traditional flame holders and less disruptive to the flow field and with better combustion.

Viv

[Glenn Olson]

Trapped Vortex flame holders and the combustor designs that use them are a relatively broad area of research. Such research projects abound throughout the world, by government agencies, universities, and companies. They come in many different sizes and shapes. I've seen them with both small and large steps. Even within NASA I'm sure there have been many studies.

The latest NASA report I've seen is NASA/TM-2004-212507, "Experimental and Computational Study of Trapped Vortex Combustor Sector Rig With Tri-Pass Diffuser," by R.C Hendricks, et. al. This one does appear to use a large volume for the trapped vortex relative to the main flow, making it unsuitable for most ramjet applications.

The trapped vortex designs I've seen for supersonic combustion, on the other hand, have much smaller and shallower steps. These may be quite adaptable to ramjets.

Of course, if you have a specific design or report you would like to discuss in this thread it would be helpful to know just which one you are referring to.

[Hank]

Hello- Flame cavity research is being done at Stanford by several folks under Dr. Ron Hansen.
The link for the NACA document posted above is:
gltrs.grc.nasa.gov/reports/2004/TM-2004-212507.pdf
Single burners such as Ramjets and Pulsejets are more sensitive to side loading than their multi-combustor bretheren. Picture the inside of the burner as one goes into yaw at some high mach number. The flame front gets pulled away from the wall or flameholder. The use of a turbulator or heat retaining cavity enhances the engines ability to sustain under g forces not normal to the direction of flight. These gizmos have valid application. Hank

[Stuart]

There are similarities between ramjet combustion chambers and those of turbojets. However, modern turbojet combustion chambers tend toward the complex with swirl vanes, high pressure spray nozzles, surface cooling boundary layers, and etc. Production ramjets are more akin to afterburners with simple flame holders, for a number of tradeoff reasons.

I've looked at a number of more complex ideas for flame holders and tried a few. As an amateur ramjetter I prefer the more simple and inexpensive approaches.

I've looked at the trapped vortex type of flame holders before and, because of their simplicity, may have an application in amateur ramjets. They're certainly worth considering. Just thinking off the top of my head, it may be advantageous to build a multi-step version, each step being smaller than the previous in a quasi-conical form. I don't think the drag would be significant and a test unit shouldn't be too hard to fabricate and test. Hmmmm!

Glad to see you are still kicking around Glen. I haven't been on the boards for quite a while, but I now have a working model of something I think is unique. It could probably benefit from vortex flow. Hell, I don't even have a flameholder yet and it runs.