Mike Everman wrote:To my mind, the neat thing about a motor jet at flight speed would be that you could run the fan at whatever speed you want to get the diffusion you want through it. ie, running it slowly when you are going fast(?) Skip the whole diffuser grid and let it be the fan blades.
To me, the trouble here is that you're boosting velocity, not pressure (I mean, if you eliminate the diffuser by just substituting the fan for it). If your speeds are slow and you have a really well-designed flameholder for it, you might get stable combustion, but what would that do for you without measurable pressure gain? (I grant you that in a low-speed engine, any gain will be small, but it seems to me you have to have SOME pressure gain to make the expansion in the chamber mean anything.) I guess another way to say it is that you don't want to end up with a ducted fan followed by a thrustless burner! Recall that even in turbojets, you have to have a heck of a diffuser behind that big "compressor" section -- it really isn't a compressor without it! (I suppose in some cases, the diffuser is considered the front end of each combustion chamber, but it's still there in some form.)
Secondarily, I would also worry about the roughness coming off the fan, even if it's just idling in the wind. It seems to me that any roughness is going to create paths for the flame front to break up and push forward, which can't be very good.
Then again, maybe I'm misunderstanding what you're saying. A slow ramjet has to be thought through in a lot different way from a fast one. In a fast ramjet, the problem is keeping the flame front forward in the chamber where you want it, so you can take full advantage of the gas expansion. In a slow ramjet, the problem is going to be keeping the flame front back
where you want it, i.e. prevent it from surging forward into the diffuser section (however you happen to configure it).