supercharged ramjet

[serverlan]

Hi all,

I'm usually on the valveless PJ forum, but I guess I must be leaking a bit.

Subsonic ramjets don't work well because they can't get the pressure up high enough to work efficiently.
I was wondering if it would be practical to bump up the pressure intermittently using high amplitude sound waves (from a pulsejet, of course) to intermittently push the intake pressure up high enough for the ram jet to work efficiently.

A horn shaped intake could collect sound waves and concentrate them to higher pressures.

It would be important to distinguish the pressure pulses from the gas flow from the PJ, as we want the pressure, but not the burnt exhaust, to go into the ramjet.

The ramject would efectively look like an active or powered augmentor to the pulsejet, and the PJ would look like a pulsed pressure source to the ramjet.

Lots to work out, but I guess the main questions are whether the sound level (150db at 1 meter?) is sufficient to make a significant difference to the ramjet. Also, would the air intake and the pressure horn be the same thing (similar shapes) or would we need both somehow?
blah blah...

any thoughts?

don

[Harabec]

hmmmmm I never did think of that. What is causing the pressue? the PJ exaust? why not use HPA to caus an increase in pressure?

[larry cottrill]

A horn shaped intake could collect sound waves and concentrate them to higher pressures.

It would be important to distinguish the pressure pulses from the gas flow from the PJ, as we want the pressure, but not the burnt exhaust, to go into the ramjet.

But Don - there's nothing wrong with the exhaust gas going into the ramjet. For one thing, there is always unconsumed oxygen available after lean combustion (consider afterburners). For another, your horn-shaped intake, if properly spaced, will entrain way more air mass than the massflow of the exhaust. Thus, there will be plenty of free oxygen available for combustion of added fuel. The air entrainment value of an augmentor (ejector) is something only the mass flow can do - the pressure wave won't accomplish that.

The ramjet would efectively look like an active or powered augmentor to the pulsejet, and the PJ would look like a pulsed pressure source to the ramjet.

Not as "pulsed" as you might think. The air mass entrainment has a slowing and blending effect on the massflow. Only the pressure wave itself would get through as a fully "pulsed" phenomenon.

Lots to work out, but I guess the main questions are whether the sound level (150db at 1 meter?) is sufficient to make a significant difference to the ramjet. Also, would the air intake and the pressure horn be the same thing (similar shapes) or would we need both somehow?

A multi-stage horn intake might help, but I think it's been established that as you add stages, you gain less per stage. Another problem is that the pressure wave expands spherically from the pulsejet tailpipe, not just rearward. It would be interesting to see if reflectors, baffles, etc. could be designed to efficiently channel more of the wave energy to the rear. My gut feel is that such gimmickry would tend to be counterproductive, except perhaps for a stationary engine application.

any thoughts?
don

This is exactly the idea I have proposed on a couple of occasions in the past - a ramjet "super augmentor / afterburner" fed by a much smaller pulsejet. Almost nobody thought it was worth doing, as I recall. I still like the idea very much, however (but then I like motorjets, too ;-) Of course, I was concerned with utilizing the mass flow, not the wave energy per se. Like all such ideas, it undoubtedly hides many surprises for the developer. It might turn out that the pressure wave going through the ramjet is the last thing in the world you should want! Who knows, until someone tries it?

Would the ramjet then need to be designed as a resonant element? Ha ha ha ...

L Cottrill

[serverlan]

Hi Harabec,

what is HPA? High Pressure Air?

Eon

[serverlan]

You are probably right. Maybe not optimal, but heaps for a proof of concept.

Quote:
The ramjet would efectively look like an active or powered augmentor to the pulsejet, and the PJ would look like a pulsed pressure source to the ramjet.

Not as "pulsed" as you might think. The air mass entrainment has a slowing and blending effect on the massflow. Only the pressure wave itself would get through as a fully "pulsed" phenomenon.

True. But I am looking for the pressure wave to provide higher ignition pressures, and hence greater fuel economy.
A horn shape will increase the pressure magnitude of the sound wave.
A quick UFLOW model inidicates that it may even be possible to get the pressure up into the self-ignition diesel ranges. It all depends on how much constriction of the through can be tolerated.

Quote:
Lots to work out, but I guess the main questions are whether the sound level (150db at 1 meter?) is sufficient to make a significant difference to the ramjet. Also, would the air intake and the pressure horn be the same thing (similar shapes) or would we need both somehow?
A multi-stage horn intake might help, but I think it's been established that as you add stages, you gain less per stage. Another problem is that the pressure wave expands spherically from the pulsejet tailpipe, not just rearward. It would be interesting to see if reflectors, baffles, etc. could be designed to efficiently channel more of the wave energy to the rear. My gut feel is that such gimmickry would tend to be counterproductive, except perhaps for a stationary engine application.

Pressure wave can be directed by using multiple exhaust nozzles on the pulsejet, and using interference patterns to concentrate the energy. A bit like antenna arrays.
How about exhausts that direct the sound pressure in a particular direction (up?) and away from neighbours? Needs a new topic back in valveles forum



Quote:
any thoughts?
don

This is exactly the idea I have proposed on a couple of occasions in the past - a ramjet "super augmentor / afterburner" fed by a much smaller pulsejet. Almost nobody thought it was worth doing, as I recall. I still like the idea very much, however (but then I like motorjets, too ;-) Of course, I was concerned with utilizing the mass flow, not the wave energy per se. Like all such ideas, it undoubtedly hides many surprises for the developer. It might turn out that the pressure wave going through the ramjet is the last thing in the world you should want! Who knows, until someone tries it?

Too late again .... typical. Pooh.
But if we could get it to work -
All the benefits of a ramjet that runs stationary,
improved fuel economy over other jets due to high pressure combustion (a la the BCVP concept)
still no moving parts
Noise reduction by shoving it through another engine.

Would the ramjet then need to be designed as a resonant element? Ha ha ha ...

Not until I can make a fortune off the first version and hire someone else to do the math!

Don