I recently made a ramjet. I got it to run and I got a question, is it all right for a ramjet to have all its flame contained in its body or should some flame be exiting from the exhaust?
When I was reducing the winnd velocity there was flame coming out if the exhuast but when I increased thewind velocity the flame receded into the combustion chamber till it was near lean extinction.
PS:Does anyone have any information regarding the Wolf Sapphire leaf blower on how much cubic metres of air it blows out per minute becuase I do not have that information.
Question on flame containment
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larry cottrill
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re: Question on flame containment
Najm -
Theoretically, it shouldn't matter much whether the flame extends from the nozzle or not. However, that is kind of an oversimplification. Consider:
If the flame is very short at high air velocity, you might be trying to run too lean; i.e., too close to lean extinction (as you yourself have observed). This could be cured by doing something to increase the available fuel flow. In other words, right now, your fuel flow is insufficient for that air velocity.
If your flame is really long at low air velocity, it's probably just a matter of throttling back on fuel so that you aren't running too rich. It is possible to run most any type of jet engine so rich that you get yellow flame, soot, smoke, etc. !!! But of course, the power development of such a mixture is very ineffective - the combustion is very slow, and a lot of the combustion energy ends up being wasted behind the engine.
So, it's really all a matter of mixture control. I would think that, for most small ramjet designs, the ideal burn would have the visible flame projecting only a short way behind the exhaust port. Note especially that a lean running engine will kill easily if the fuel has to be throttled back suddenly at high speed. I have always believed that it would be much better to have combined air and fuel throttling, but of course that adds a degree of complexity that nobody really wants in a ramjet.
Many turbocombustor engineers will cite an "ideal" ratio of 30 kg air / kg fuel as perfect for continuous combustion (at atmospheric pressure), which is quite lean. In pulsejet work, we often hear that 15:1 is a good ratio (obviously, much farther from extinction). Either way, that is a LOT of air per unit mass of fuel burned. It just shows that in a jet engine, we want to be on "the lean side of the power curve". This gives reasonable economy for the power achieved, and makes it possible to throttle the engine by simply regulating fuel pressure and flow.
It would be fun to see a couple of pictures of your engine in action, especially if taken in low light conditions so the flame can be readily seen.
L Cottrill
Theoretically, it shouldn't matter much whether the flame extends from the nozzle or not. However, that is kind of an oversimplification. Consider:
If the flame is very short at high air velocity, you might be trying to run too lean; i.e., too close to lean extinction (as you yourself have observed). This could be cured by doing something to increase the available fuel flow. In other words, right now, your fuel flow is insufficient for that air velocity.
If your flame is really long at low air velocity, it's probably just a matter of throttling back on fuel so that you aren't running too rich. It is possible to run most any type of jet engine so rich that you get yellow flame, soot, smoke, etc. !!! But of course, the power development of such a mixture is very ineffective - the combustion is very slow, and a lot of the combustion energy ends up being wasted behind the engine.
So, it's really all a matter of mixture control. I would think that, for most small ramjet designs, the ideal burn would have the visible flame projecting only a short way behind the exhaust port. Note especially that a lean running engine will kill easily if the fuel has to be throttled back suddenly at high speed. I have always believed that it would be much better to have combined air and fuel throttling, but of course that adds a degree of complexity that nobody really wants in a ramjet.
Many turbocombustor engineers will cite an "ideal" ratio of 30 kg air / kg fuel as perfect for continuous combustion (at atmospheric pressure), which is quite lean. In pulsejet work, we often hear that 15:1 is a good ratio (obviously, much farther from extinction). Either way, that is a LOT of air per unit mass of fuel burned. It just shows that in a jet engine, we want to be on "the lean side of the power curve". This gives reasonable economy for the power achieved, and makes it possible to throttle the engine by simply regulating fuel pressure and flow.
It would be fun to see a couple of pictures of your engine in action, especially if taken in low light conditions so the flame can be readily seen.
L Cottrill
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Najm
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re: Question on flame containment
I currently don't have a digital camera so I cannot post any pics but I will try my best to post the pics in a week's time
.
By the way I was using the natural gas supply at a very low pressure(0.5 psi)
so I know there is room for improvement inthis area,do you know any way to feed it with kerosene at low pressure preferebly gravity fed.
I wasn't getting a yellow flame when I was supplying air to it, only at very low speeds the flame showed orange on its edges otherwise during throttling up the flame first changes into a solid blue cylinder type shape which extends
six to seven inches past the exhaust then it thins in diametre to about one inch and gets shorter and then at full air velocity the flame is mostly in the combustion chamber with it showing only a bit past the exhaust.After this if the position of the leaf blower's nozzle is changed the engine blows out.
There is also very little sign of the heat getting transferred to the combustion chamber at all velocities.
It got heated only at the end of the exhaust because I was aiming the nozzle slightly off center.
.
By the way I was using the natural gas supply at a very low pressure(0.5 psi)
so I know there is room for improvement inthis area,do you know any way to feed it with kerosene at low pressure preferebly gravity fed.
I wasn't getting a yellow flame when I was supplying air to it, only at very low speeds the flame showed orange on its edges otherwise during throttling up the flame first changes into a solid blue cylinder type shape which extends
six to seven inches past the exhaust then it thins in diametre to about one inch and gets shorter and then at full air velocity the flame is mostly in the combustion chamber with it showing only a bit past the exhaust.After this if the position of the leaf blower's nozzle is changed the engine blows out.
There is also very little sign of the heat getting transferred to the combustion chamber at all velocities.
It got heated only at the end of the exhaust because I was aiming the nozzle slightly off center.