Continuous detonation ramjet, is it possible?

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Continuous detonation ramjet, is it possible?

Post by Maikkel » Wed Jan 24, 2007 2:10 pm

Hello. I have been wonderin that is continuous detonation possible in theory? Now let's stay purely on a theoretical level. If it is has anyone thought about a ramjet that instead of deflagrating the mixture would detonate it? Just like a pde but would keep the detonatio going.

I would just want to know should i waste time for this thought/idea any more.

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CD ramjet

Post by heada » Wed Jan 24, 2007 2:38 pm

I could be wrong, but a sub/trans/supersonic ramjet slows the air to subsonic speeds, lets combustion occur and then expels the exhaust at sub/trans/super sonic speeds.

A hypersonic ramjet (scramjet) does not slow the air and lets combustion occur at supersonic speeds. In the raw sense of the word detonation, this would qualify, would it not?


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Detonation vs. Deflagration

Post by jthompso » Wed Jan 24, 2007 2:55 pm

Unfortunately not--just because the combusting fuel/air mixture is moving through the scramjet at supersonic speeds does not mean that the actual combustion process is proceeding faster than the speed of sound (detonation)

Think total velocity in respect to relative velocity--the relative velocity of the combustion to the airflow is still very low.

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Post by Zippiot » Wed Jan 24, 2007 5:45 pm

SCRamjets do slow the air but the air is still moving at supersonic speeds.
That is why they are moving mach 5ish, so they can slow the air to mach 1 enableing a pressure change.

I think PDE is the only detonating engine at the moment...
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Post by Maikkel » Wed Jan 24, 2007 9:24 pm

Hi. I was pretty sure before i made the topic that someone would say that scramjet is the engine i was asking about. I used to think so also but i checked it from the scramjet topic. This is the reason i asked about it, i haven't seen this kind of an engine proposed anywhere so i wondered is it even possible?

Am I alone or has anyone thought about it? Does anyone know is it against the laws of physics or is it possible at least in theory? And aren't diesel engines also detonation engines although in a completley differen't category?

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Post by Glenn Olson » Tue Feb 06, 2007 12:19 am

For a ramjet (not scramjet) continuous detonation would not be possible because in detonation, as has been stated, the flame front travels faster than the speed of sound. In a subsonic air flow this means that the flame front would travel to the fuel supply then extinguish or revert to combustion. You may get pulsations of detonation but never a continuous detonation.

For a scramjet, if you can get the air moving through fast enough and under the right conditions you might be able to match the airflow to the detonation speed and thereby maintain a standing detonation wave, but I really doubt it. Yes, you are fighting the laws of physics.

As for diesel engines, if you are getting detonation then it's tuned wrong and you will hear engine knock. This reduces the life of the engine and reduces its efficiency. In a well tuned diesel engine the fuel will begin combustion as soon injection begins and stay that way throughout the injection period. If fuel ignition is delayed then there will be an unburned fuel/air mixture in the cylinder which can detonate well into the injection period.

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Post by Maikkel » Tue Feb 06, 2007 2:30 pm

Hello. Thank you for your answer. At last I got an answer to the question I've been thinking about for the last year. So you think that it might be possible to get the detonation wave started but you can't make it sustain. I think I understand your point.

Here's a bunch of other questions. At what speed does the detonation wave actually travel? And what kind of a pressure ratio would we need to detonate for example jet a1 or diesel. And if it isn't the pressure ratio that matters in detonation then what? If the same amount of mixture was detonated and deflagrated would the total energy released be the same in both or does detonation release energy more efficently in the process? Has the idea of continuous detonation scramjet been proposed? And finaly what is the reason for unexistance of a detonation piston engine, afterall if the engines sometimes detonate the mixture accidentally, couldn't they make it do detonation on purpose? I suspect that the reason is structural.

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