Hi all,
Can any one clarify the relationship between thrust and fuel consumption at different frequencies? I've read the paper "Jet Propulsion" but I didn't fully understand it.
Thanks in advance!
/Alex
The relationship between thrust and fuel consumption?
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Mike Everman
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Maybe you could tell us which Jet Propulsion paper you're talking about. There are so many posted in this forum.alex wrote:Maybe I was a bit unclear. What I'm asking about is the text at page 16 in the Jet propulsion paper.
Maybe M. could give you something solid. All I have are theories. Thrust does not necessarily get better with higher frequency on the same duct. That much I do know. You can make horrible gas guzzlers easily, mizerly motors with great difficulty and fine tuning.
That's about all I can come up with. Naught but generalities, I'm afraid.
Mike Often wrong, never unsure.
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I'll simplify my question:
The frequency, and thus the thrust, can be controlled by reducing and giving more fuel right?
From what I've understood from the text I spoke about earlier a lower frequency increases the air charge per cycle, because its speed into the engine is lower and therefore the static pressure and density are higher.
If an engine is throttled down the effect would be that the amount of air increases and the fuel decreases. This gives a leaner mixture ratio.
I'm wondering if there is an optimum frequency where the mixture ratio is 14,7:1. Is this point where the maximum amount of thrust is achieved?
The frequency, and thus the thrust, can be controlled by reducing and giving more fuel right?
From what I've understood from the text I spoke about earlier a lower frequency increases the air charge per cycle, because its speed into the engine is lower and therefore the static pressure and density are higher.
If an engine is throttled down the effect would be that the amount of air increases and the fuel decreases. This gives a leaner mixture ratio.
I'm wondering if there is an optimum frequency where the mixture ratio is 14,7:1. Is this point where the maximum amount of thrust is achieved?
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Viv
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A deceptively simple question that I think will again show the complexity of the system and the problem of focusing on one aspect.
That said;-) we have to remember that in flight and static are very different conditions, thrust is normally measured static and more importantly optimized for static measurement.
Its not the wight of air ingested and fed through the engine with a suitable addition of heat that is the total thrust producing component.
Suck back of cold air in to the tailpipe is critical to the engines working mass and critically to its peak cycle amplitude, its that amplitude swing that controls how much air is ingested hence how much gets mixed with the fuel in your question.
Frequency is a big factor as you correctly say but peak cycle amplitude is the larger factor and that depends on the pressure presented to the inlet and tailpipe and the differential between them, this is why flight speed needs to be included in your question.
Frequency is decided by size/geometry/temperature,heat release rate of the fuel, temperature is affected by stoitimetric fuel ratio (as you pointed out) but also the ambient air temperature and local cooling capacity of the engine walls must be included, for Lockwoods I always feel we need to include the cooling effect of the liquid propane in the inlet tract;-)
All of the above will affect the dimensions of the engine and hence its operating point, change any of them and you change the operating point!
Look at the Melenric patents for high speed pulse jet drone engines, this is a good example of why your simple question ends up being an apple and oranges problem:-)
To answer your last question on mixture I will say yes! this after all is the design operating point of an engine but you need to add geometry and dimensions as the missing factors so that I can get away with saying yes:-)
Viv
That said;-) we have to remember that in flight and static are very different conditions, thrust is normally measured static and more importantly optimized for static measurement.
Its not the wight of air ingested and fed through the engine with a suitable addition of heat that is the total thrust producing component.
Suck back of cold air in to the tailpipe is critical to the engines working mass and critically to its peak cycle amplitude, its that amplitude swing that controls how much air is ingested hence how much gets mixed with the fuel in your question.
Frequency is a big factor as you correctly say but peak cycle amplitude is the larger factor and that depends on the pressure presented to the inlet and tailpipe and the differential between them, this is why flight speed needs to be included in your question.
Frequency is decided by size/geometry/temperature,heat release rate of the fuel, temperature is affected by stoitimetric fuel ratio (as you pointed out) but also the ambient air temperature and local cooling capacity of the engine walls must be included, for Lockwoods I always feel we need to include the cooling effect of the liquid propane in the inlet tract;-)
All of the above will affect the dimensions of the engine and hence its operating point, change any of them and you change the operating point!
Look at the Melenric patents for high speed pulse jet drone engines, this is a good example of why your simple question ends up being an apple and oranges problem:-)
To answer your last question on mixture I will say yes! this after all is the design operating point of an engine but you need to add geometry and dimensions as the missing factors so that I can get away with saying yes:-)
Viv
"Sometimes the lies you tell are less frightening than the loneliness you might feel if you stopped telling them" Brock Clarke
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Monsieur le commentaire
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Monsieur le commentaire
Thanks Viv!
"Frequency is a big factor as you correctly say but peak cycle amplitude is the larger factor and that depends on the pressure presented to the inlet and tailpipe and the differential between them, this is why flight speed needs to be included in your question."
Should the differential pressure be great or small? It's great when static and then decreases in flight as ram pressure builds up?
"Frequency is a big factor as you correctly say but peak cycle amplitude is the larger factor and that depends on the pressure presented to the inlet and tailpipe and the differential between them, this is why flight speed needs to be included in your question."
Should the differential pressure be great or small? It's great when static and then decreases in flight as ram pressure builds up?
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Viv
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The differential pressure is low when static and high in flight basically due to all that wind blowing in the intake creating ram pressure and past the tailpipe causing a partial vacuum.alex wrote:Thanks Viv!
"Frequency is a big factor as you correctly say but peak cycle amplitude is the larger factor and that depends on the pressure presented to the inlet and tailpipe and the differential between them, this is why flight speed needs to be included in your question."
Should the differential pressure be great or small? It's great when static and then decreases in flight as ram pressure builds up?
The effect of a high differential pressure is to blow the active combustion zone out of the combustion chamber area and in to the tailpipe, a bit more and its blown out the tailpipe and your engine stops.
This is why Melenric and others go to such lengths to reduce the differential pressure in their patents.
Heres some thing to add to your thoughts, having a cold plug of air in the tailpipe is good for engine performance, that dense mass gives the combustion event in the combustion chamber something to work against, this ultimately raises the pressure in the CC during combustion! thats nice as what happens is as the pressure goes up the combustion rate also goes up! the flame propagation rate is a key factor in the operating frequency of the engine and the geometry/dimensions have to compliment it.
As you can see frequency is dependent on a large set of interrelated factors, the key here is that they are all interrelated! alter one you alter them all!
For an engine running at a good operating point (the balance of all these factors) you get a nice steep pressure curve and fast combustion event, all that fuel gets converted to heat and then the heat has time to get added to the working mass of the engine.
A good positive pressure event leads to a good negative pressure event, that improves the next induction cycle.
All this has been written and discussed on the forum before but its a pain to find it all and get it in context, what you should see now is that a pulse jet is a positive feedback device, its a chaotic oscillator depending on what happened before for what happens next! think of an avalanche and you wont be far wrong:-)
Viv
"Sometimes the lies you tell are less frightening than the loneliness you might feel if you stopped telling them" Brock Clarke
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The thrust increases in flight due to the bigger diffrential pressure. The thrust available in flight depends on the velocity of the air plane and the velocity of the hot gases leaving the tailpipe.
Does the thrust increasement exceed the decreasing available thrust? I hope you understand what I mean.
How does it come that the frequency increases while speed is increasing? Is THIS the product of a bigger differential pressure?
Does the thrust increasement exceed the decreasing available thrust? I hope you understand what I mean.
How does it come that the frequency increases while speed is increasing? Is THIS the product of a bigger differential pressure?
Viv, or any one,
You wrote:
Heres some thing to add to your thoughts, having a cold plug of air in the tailpipe is good for engine performance, that dense mass gives the combustion event in the combustion chamber something to work against, this ultimately raises the pressure in the CC during combustion! thats nice as what happens is as the pressure goes up the combustion rate also goes up!
Does this mean that a longer tailpipe decreases the freguency and thus the thrust, but at the same time increases the thrust because of a plug of air in the pipe which increases the combustion chamber pressure?
You wrote:
Heres some thing to add to your thoughts, having a cold plug of air in the tailpipe is good for engine performance, that dense mass gives the combustion event in the combustion chamber something to work against, this ultimately raises the pressure in the CC during combustion! thats nice as what happens is as the pressure goes up the combustion rate also goes up!
Does this mean that a longer tailpipe decreases the freguency and thus the thrust, but at the same time increases the thrust because of a plug of air in the pipe which increases the combustion chamber pressure?
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larry cottrill
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Densities In the Pipe
alex -
It isn't the length per se but the volume of the tail section that creates the desirable plugging effect. This volume of air is sometimes called the 'tailpipe piston' or the 'tail piston' of the pulsejet. A big cone is far more effective than a pipe of the same length - partly because of the larger volume, and partly because of the lower velocity needed at the end to bring in the slug of air. I think this is also why a tail-end flare is good on a straight pipe - it increases the in-breathing effectiveness of the pipe and produces a somewhat denser piston that goes deeper into the pipe at full density.
Here's an early attempt to show from UFLOW1D analysis what happens after the blast in a "linear FWE" type engine. The setup of the next blast would actually be somewhere above the bottom of the picture, about where the highest gas density (very dark grey) is reached in the front of the chamber right behind the intake (at about .00304 sec), and while the tail piston is still in place (I can't simulate the next explosion with UFLOW1D, so the pipe just keeps oscillating).
It isn't the length per se but the volume of the tail section that creates the desirable plugging effect. This volume of air is sometimes called the 'tailpipe piston' or the 'tail piston' of the pulsejet. A big cone is far more effective than a pipe of the same length - partly because of the larger volume, and partly because of the lower velocity needed at the end to bring in the slug of air. I think this is also why a tail-end flare is good on a straight pipe - it increases the in-breathing effectiveness of the pipe and produces a somewhat denser piston that goes deeper into the pipe at full density.
Here's an early attempt to show from UFLOW1D analysis what happens after the blast in a "linear FWE" type engine. The setup of the next blast would actually be somewhere above the bottom of the picture, about where the highest gas density (very dark grey) is reached in the front of the chamber right behind the intake (at about .00304 sec), and while the tail piston is still in place (I can't simulate the next explosion with UFLOW1D, so the pipe just keeps oscillating).
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- FWE_densities_2.jpg
- Densities simulated with UFLOW1D for a "linear FWE" style valveless engine. The darker the region, the higher the density. Graphic Copyright 2005 Larry Cottrill
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