Few Quick Questions...

[fterh]

I'm almost done with the diffuser, but the thing is that the cut is not perfectly straight. It goes up a little then down a bit, so it's uneven and bumpy. And also I did not follow the specifications on the maggie muggs design page, does this mean it won't work?

[larry cottrill]

Don't worry about following the exact specs -- it was all guesswork to begin with, anyway. Roughness and/or bumpiness inside the diffuser could be a problem, but I wouldn't be bothered by it until you've tried it. If the diffuser action is poor, you'll probably see some rough combustion, or maybe combustion "leaking" forward into the diffuser section on that side or something. But, if not too severe, I'll bet it will still work fairly well for you.

L Cottrill

[fterh]

Don't worry about following the exact specs -- it was all guesswork to begin with, anyway. Roughness and/or bumpiness inside the diffuser could be a problem, but I wouldn't be bothered by it until you've tried it. If the diffuser action is poor, you'll probably see some rough combustion, or maybe combustion "leaking" forward into the diffuser section on that side or something. But, if not too severe, I'll bet it will still work fairly well for you.

L Cottrill

Oh no no.. INSIDE the diffuser is super smooth, what I meant is that the cut is not in a straightline and it goes up and down. I don't have a clamp so I drew a line (which is not that straight) freehand and saw according to it. When placed cut-side down, it's still generally straight but it won't be a perfect fit like the machined-cut.

[fterh]

I just thought of something omg.

After JB welding the diffuser to the flameholder and combustion end of the ramjet there is simply no way to take it apart and make adjustments? So if I did something wrong I gotta re do from scratch?

[larry cottrill]

Yeah, that's about it. I guess you could run it enough without adequate cooling to soften the stuff up and pull it apart with appropriate hand tools, but sounds kind of messy to me ;-)

L Cottrill

[fterh]

Okay this is my flameholder and my diffuser. Any comments howsoever? Like, anyway my diffuser/flameholder can be improved, should the flameholder go [ or ] (intake from left, exhaust to the right).. etc..

[larry cottrill]

I like the diffuser except for the rolled region at the front. Remove that part completely, and hammer the edge outward to form a SMALL flared intake -- just a few mm long x a few mm outward from the basic curve of the "cone". True, a flared inlet will take away a little of the overall diffuser ratio, but it will help smooth out the flow of the intake air going in, and will also strengthen the front edge. Otherwise, looks good, I think.

L Cottrill

[fterh]

I like the diffuser except for the rolled region at the front. Remove that part completely, and hammer the edge outward to form a SMALL flared intake -- just a few mm long x a few mm outward from the basic curve of the "cone". True, a flared inlet will take away a little of the overall diffuser ratio, but it will help smooth out the flow of the intake air going in, and will also strengthen the front edge. Otherwise, looks good, I think.

L Cottrill

Fast reply ^.^

Omg.. more sawing?! You gotta be kidding

Okay .. I'll saw that part away. And will my flameholder work? I think I'll weld it in this manner:

] (intake from the left)

same as your design. If it doesn't work, then good luck to me trying to pry the whole thing apart.

And for the flared intake, maybe if it's too hard I won't do it first, after I run it for the first time maybe I'll try flare it to improve performance.

[larry cottrill]

Omg.. more sawing?! You gotta be kidding :( :( :(
Okay .. I'll saw that part away. And will my flameholder work? I think I'll weld it in this manner:
] (intake from the left)
same as your design. If it doesn't work, then good luck to me trying to pry the whole thing apart.
And for the flared intake, maybe if it's too hard I won't do it first, after I run it for the first time maybe I'll try flare it to improve performance.

One thing you might invest in is a Dremel grinder. That makes an easy job of something like this: You just grind away that innermost rolled corner, working your way gradually around the circumference. Before you know it, Voila! The unwanted piece just falls off. And if you've been careful, you end up with a perfect finished edge -- no buckling, distortion, or any such.

I think your flameholder should work. Somewhat larger holes / less steel would be better, but this should be fine for a first shot at it.

I think mine (grinder plus several tools and many replaceable disks, drums, etc.) was about $70 US, purchased a few months ago at Lowe's. Once you start using a tool like that, you'll never cease to think of new jobs for it. I know as a model builder many years ago, I found innumerable applications. For cutting music wire (that is some HARD steel wire!), the Dremel tool with one of the little paper-thin abrasive cutoff disks has no peer. I used that very tool for cutting the large breathing port in the chamber of the Big Test Lady engine. Took about 15 minutes fo careful work.

L Cottrill

[fterh]

Think I'll skip that. I made this for fun and I am not prepared *now* to spend unnecessary money on this construction

And I don't have a vacuum cleaner that blows or a leafblower.. will a really really powerful PC 120mm fan hooked up to a 12V battery work? Hmm.

[larry cottrill]

Any fan that provides enough flow at a reasonably high speed should work. Axial fans will have a lot of torque (twist) to the output flow, so on these you should provide a reasonably long "flow straightener", even if just crudely put together from a bundle of tubing scraps. A centrifugal fan probably won't have much torque, but can suffer from somewhat uneven stream velocities, which could be corrected just by providing a long, slightly converging "nozzle" (which is what makes the typical leaf blower such a nice choice).

It is practically impossible to get any fan design to produce a perfectly laminar flow stream right out of the exit port; you always need to condition it to some degree.

L Cottrill

[fterh]

Any fan that provides enough flow at a reasonably high speed should work. Axial fans will have a lot of torque (twist) to the output flow, so on these you should provide a reasonably long "flow straightener", even if just crudely put together from a bundle of tubing scraps. A centrifugal fan probably won't have much torque, but can suffer from somewhat uneven stream velocities, which could be corrected just by providing a long, slightly converging "nozzle" (which is what makes the typical leaf blower such a nice choice).

It is practically impossible to get any fan design to produce a perfectly laminar flow stream right out of the exit port; you always need to condition it to some degree.

L Cottrill

And what's the diff between axial and centrifugal fan? And is this:



axial or centrifugal?

[larry cottrill]

That's an axial fan - a fan that blows air parallel to its shaft axis. "Centrifugal fan" is an engineering term for what most people call a "blower", but also covers an "impeller" (like the little spinner inside a leaf blower). Either one can be made in a high volume and/or high speed form, but of course that requires a lot of power (big motor). The two types have VASTLY different operating characteristics, but that's not a problem for what you're doing.

The type you show will be too slow, unless it is a REALLY big one and you provide a long nozzle cone to radically speed up the flow stream. These cooling fans are actually designed for fairly slow air motion, because slowly moving air gives highly effective cooling and requires little power to run.

See if you can find something in a junk shop or some such. Either a leaf blower, shop vac, or some kind of industrial blower stripped from small equipment of some kind (a lot of them are used on equipment with gas burners). You'll never make it work unless you get something that moves a LOT of air at pretty high speed.

Another thing I thought of one time was mounting the engine on the outside of a car moving down the Interstate (driven by ANOTHER person!) while I provide fuel and ignition and observe it through the side window. I abandoned this as basically impractical, however.

L Cottrill

[fterh]

That's an axial fan - a fan that blows air parallel to its shaft axis. "Centrifugal fan" is an engineering term for what most people call a "blower", but also covers an "impeller" (like the little spinner inside a leaf blower). Either one can be made in a high volume and/or high speed form, but of course that requires a lot of power (big motor). The two types have VASTLY different operating characteristics, but that's not a problem for what you're doing.

The type you show will be too slow, unless it is a REALLY big one and you provide a long nozzle cone to radically speed up the flow stream. These cooling fans are actually designed for fairly slow air motion, because slowly moving air gives highly effective cooling and requires little power to run.

See if you can find something in a junk shop or some such. Either a leaf blower, shop vac, or some kind of industrial blower stripped from small equipment of some kind (a lot of them are used on equipment with gas burners). You'll never make it work unless you get something that moves a LOT of air at pretty high speed.

Another thing I thought of one time was mounting the engine on the outside of a car moving down the Interstate (driven by ANOTHER person!) while I provide fuel and ignition and observe it through the side window. I abandoned this as basically impractical, however.

L Cottrill

Check out the delta fans.. their dB are crazy and their cfm is like, 150+? Those are PC fans too.. but they are crazy powerful.

And why will a long nozzle cone speed up the flow stream? I mean the long cone just provide more friction for the stream to "rub against" while moving down only right?

[larry cottrill]

And why will a long nozzle cone speed up the flow stream? I mean the long cone just provide more friction for the stream to "rub against" while moving down only right?

Not so. A nozzle is the opposite of a diffuser, in terms of its action on the air flow. Air enters a diffuser at one speed and exits at a lower speed -- but its static pressure is elevated. A nozzle, on the other hand, is driven by pressure -- the air enters at a certain speed and pressure but exits at a higher speed at reduced pressure. The pressure difference across the nozzle (end to end) is the driving force; the effect is gas acceleration. This why we use a diffuser at the front end of our ramjet (to obtain stable combustion at a significantly elevated pressure) and a nozzle at the rear end (to maximize exit velocity, using the static pressure AND taking advantage of the gas expansion from heat of combustion).

The reason the length of a long nozzle would be good in this case is that it provides time and space for turbulence and other flaws in the flow to "even out". A long nozzle with carefully designed internal straightening vanes would provide almost perfect laminar flow at the exit. Of course, there would be drag losses as you have suggested, but that is a price worth paying to get that kind of flow stream.

L Cottrill