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Robert Clark
science forum Guru Wannabe
Joined: 30 Apr 2005
Posts: 129
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 Posted: Fri May 19, 2006 4:28 pm Post subject:
How is the nitrogen in air accelerated in ramjets/scramjets?
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In the general thrust equation, in order for a jet engine, either
turbojet, ramjet, or scramjet, to produce net thrust the speed of the
exiting exhaust has to be greater than the incoming speed of the air
flow. Then the net thrust is the mass exhaust rate times exhaust speed
minus the air mass rate flow in times the air flow speed in.
But it is only the oxygen and that gets combusted with the fuel. So
you would think only the oxygen portion of the air is getting
accelerated. But there is 4 times as much nitrogen as oxygen. Is the
exiting nitrogen still going to flow at the previous incoming speed
thus contributing nothing to the thrust?
For the ramjet, the supersonic air is slowed down to subsonic speeds
for combustion, and for the scramjet the hypersonic air is slowed to
low supersonic speeds for combustion.
But if the larger mass of nitrogen is also thus greatly slowed down,
how is the combustion of the oxygen alone going to also get the exiting
nitrogen back up to above the entering speed?
Said another way, the "ram drag" term includes the much greater mass of
nitrogen. How is the oxygen alone being accelerated going to overcome
the force in the opposite direction from this larger mass of nitrogen
impinging on the aircraft?
A couple of explanations. One is that the nitrogen is also going to be
accelerated to comparable speeds to the combusted oxygen+fuel from the
heat of combustion? It seems to me this can't be very efficient since
there is much greater mass of nitrogen that has to be heated from
conduction, convection, a rather slow process. When you light a
fireplace, the entire room is not heated to the temperature of the
flame.
The lower temperature will serve to lower the exhaust speed.
The degree of this mixing and heating would seem to be an important
part of the thrust capability of the engine and its efficiency would
depend greatly on how well this is accomplished. Curious you never see
it stated specifically what this heating and velocity of the nitrogen
is.
If this is the explanation it's puzzling because whenever I see the
thrust produced given for jet engines, they only give the thrust for
the oxygen+fuel reaction products! For instance when they give the ISP
for such engines.
However, an interesting question occurs to me. Will the total thrust
from the greater mass of the nitrogen+combustion products but at
lowered temperature be the same as the thrust from the combustion
products alone but at higher temperature?
It's not obvious to me that it would. In any case if this is what
happens this is something that should be mentioned in discussing the
mechanism of ramjet and scramjet propulsion.
A second possible explanation is that when you slow down the incoming
air, the nitrogen and oxygen are greatly heated and compressed. Then
though they are greatly slowed from their entering speeds initially,
the heating and compression allows them to exit at nearly the same
speed at which they entered. That is, even if you combusted no fuel,
the exiting air would still move at close to the entering speed. In
this case you would not have to add any extra velocity to the nitrogen
from combustion.
However, I've not seen anything that suggests this is what happens
either.
- Bob Clark |
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Paul F. Dietz
science forum beginner
Joined: 08 May 2005
Posts: 9
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| Posted: Fri May 19, 2006 5:22 pm Post subject:
Re: How is the nitrogen in air accelerated in ramjets/scramjets?
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Robert Clark wrote:
| Quote: | But it is only the oxygen and that gets combusted with the fuel. So
you would think only the oxygen portion of the air is getting
accelerated.
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Why would one think that? There are no processes in a typical jet
engine that would significantly 'unmix' nitrogen from the gas. Moreover,
combustion doesn't (by itself) accelerate gas, combustion *heats* gas,
and that heat is not isolated to the oxygen or the combustion products.
Paul |
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martin dowd
science forum beginner
Joined: 01 Oct 2005
Posts: 30
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| Posted: Sat May 20, 2006 3:42 am Post subject:
Re: How is the nitrogen in air accelerated in ramjets/scramjets?
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I don't remember any details, but standard 1D models add heat to the
flow at the combustion point, and use analytic thermodynamics to
compute the state along the flow, from which the thrust can be
computed. One reference with a readable self-contained treatment is
W.H.T. Loh's book on propulsion, ca 1968.
- Martin Dowd |
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bombardmentforce
science forum beginner
Joined: 20 Aug 2005
Posts: 4
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| Posted: Sat May 20, 2006 12:59 pm Post subject:
Re: How is the nitrogen in air accelerated in ramjets/scramjets?
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In the Orion's Ulam-Taylor motor the ram effect is created without
slowing down the oncoming air flow.
Expected ISP: 15,800 seconds or 4 Hours
http://spacebombardment.blogspot.com/2005/10/orion-isp-at-low-altitude.html
Robert Clark wrote:
| Quote: | In the general thrust equation, in order for a jet engine, either
turbojet, ramjet, or scramjet, to produce net thrust the speed of the
exiting exhaust has to be greater than the incoming speed of the air
flow.
- Bob Clark |
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tadchem
science forum Guru
Joined: 03 May 2005
Posts: 1348
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| Posted: Sat May 20, 2006 2:05 pm Post subject:
Re: How is the nitrogen in air accelerated in ramjets/scramjets?
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The jet is a combustion engine When the fuel and the oxidizer react,
great amounts of heat are produced. Heat is thermal motion - average
kinetiec energy per particle. This causes the individual molecules of
the mixture to accelerate greatly.
The other components of the air (argon, CO2, et cetera) are also
accelerated proportionately.
The device is engineered so that the cool (slow) molecules come on one
way, are heated to become fast molecules, and exit the other way.
Newton's third law allows us to see that there will be a net force on
teh device because of this.
Tom Davidson
Richmond, VA |
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N:dlzc D:aol T:com (dlzc)
science forum Guru
Joined: 25 Mar 2005
Posts: 2835
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| Posted: Sat May 20, 2006 4:18 pm Post subject:
Re: How is the nitrogen in air accelerated in ramjets/scramjets?
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Dear Robert Clark:
"Robert Clark" <rgregoryclark@yahoo.com> wrote in message
news:1148056130.596689.143820@u72g2000cwu.googlegroups.com...
....
| Quote: | But it is only the oxygen and that gets combusted
with the fuel. So you would think only the oxygen
portion of the air is getting accelerated.
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Where is this "oxygen portion" located? Isn't it distributed in
the entire gas stream? Doesn't the combusted fuel also combine
with the gas stream? Is there something in the air that prevents
"dispersion" or "mixing"? Aren't there high velocity gradients
in the engine to achieve just such mixing?
The heating of the inlet air by an inlet compressor is a very
small part of the heating the entire gas stream is exposed to in
a standard turbine.
The problem isn't getting the combusted gasses to "thermally mix"
with the rest of the gas. The problem in a ramjet/scramjet is to
get the combustion to complete while still in the engine at
multi-mach speeds. If it is still burning after it has left the
nozzle, it is lost thrust, wasted fuel.
David A. Smith |
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Tom Sanderson
science forum addict
Joined: 22 Dec 2005
Posts: 55
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| Posted: Tue May 23, 2006 3:59 pm Post subject:
Re: How is the nitrogen in air accelerated in ramjets/scramjets?
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"Robert Clark" <rgregoryclark@yahoo.com> wrote:
| Quote: | But it is only the oxygen and that gets combusted with the fuel.
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No, it is only the oxygen that chemically reacts with the fuel (ignoring NOx
production)...the nitrogen (and other air fractions) are still participating
in the dynamics of the flame. Pure oxygen burns *very* differently than
air.
| Quote: | you would think only the oxygen portion of the air is getting accelerated.
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No, the thermal energy released by the combustion reaction is distributed to
all of the molecules in the flame front.
| Quote: | Is the
exiting nitrogen still going to flow at the previous incoming speed
thus contributing nothing to the thrust?
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No, the nitrogen, unburned air fractions, and combustions byproducts all
move at, essentially, the same speed.
| Quote: | But if the larger mass of nitrogen is also thus greatly slowed down,
how is the combustion of the oxygen alone going to also get the exiting
nitrogen back up to above the entering speed?
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An efficient inlet trades velocity for pressure. That pressure provides
much of the reacceleration in the nozzle. The added energy from combustion
is what gets the exhaust velocity up and over the freestream velocity.
You're just distributing a given amount of energy you got from reacting fuel
with oxygen into a given mass of working fluid (which includes nitrogen).
| Quote: | How is the oxygen alone being accelerated going to overcome
the force in the opposite direction from this larger mass of nitrogen
impinging on the aircraft?
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The oxygen isn't accelerated alone. The thrust of the engine and the
combustion in the combuster are totally unrelated processes (from a
thermodynamics point of view). A cold working fluid enters the intake, is
compressed, heat is added, and the hot working fluid exits. It happens that
the heat is added by burning fuel with some of the oxygen in the working
fluid, but the thermodynamics of the engine (and hence the thrust) don't
care how the heat got added. If you could put a good electric heater in
between the compressor and the turbine the engine would work exactly the
same way.
| Quote: | One is that the nitrogen is also going to be
accelerated to comparable speeds to the combusted oxygen+fuel from the
heat of combustion?
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Yes
| Quote: | It seems to me this can't be very efficient since
there is much greater mass of nitrogen that has to be heated from
conduction, convection, a rather slow process.
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Conduction and convetion within a flame front are extremely fast processes.
That's what makes flames possible.
| Quote: | When you light a
fireplace, the entire room is not heated to the temperature of the
flame.
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No kidding. And in a jet engine the entire working fluid is not heated to
the flame temperature either. The oxygen/hydrocarbon reaction is capable of
creating heat significantly in excess of what the engine materials can
withstand. You have to run the combustion byproducts at lower than
stoichiometric flame temperature or you burn the engine.
| Quote: | The degree of this mixing and heating would seem to be an important
part of the thrust capability of the engine and its efficiency would
depend greatly on how well this is accomplished.
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Combustor design is, indeed, a vital part of jet engine design. All the
modern versions are very good at this.
| Quote: | Curious you never see
it stated specifically what this heating and velocity of the nitrogen
is.
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You *always* see it...the nitrogen is heated to the same temperature as all
the other exhaust products (aka EGT, Exhaust Gas Temperature). The nitrogen
velocity is the same as the exhaust velocity.
| Quote: | If this is the explanation it's puzzling because whenever I see the
thrust produced given for jet engines, they only give the thrust for
the oxygen+fuel reaction products!
|
You *never* see thrust for a jet engine given as only oxygen+fuel reaction
products. It's always given for the total mass flow, which includes the
nitrogen. Rocket thrust, by definition, is only combustion products because
rockets don't use air as a working fluid.
| Quote: | Will the total thrust
from the greater mass of the nitrogen+combustion products but at
lowered temperature be the same as the thrust from the combustion
products alone but at higher temperature?
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The energy is the same but, all other things being equal, it would be more
efficient to run the combustion at higher temperature. You would be able to
extract more of the energy as useful work (thrust). That would require the
only slightly challenging technical hurdle of separating nitrogen from
oxygen at extremely high speed.
| Quote: | A second possible explanation is that when you slow down the incoming
air, the nitrogen and oxygen are greatly heated and compressed.
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In supersonic flight, this is true. For subsonic, the inlet compression is
not particularly significant compared to the compression from the compressor
stages.
| Quote: | Then
though they are greatly slowed from their entering speeds initially,
the heating and compression allows them to exit at nearly the same
speed at which they entered.
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Correct.
Tom. |
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Robert Clark
science forum Guru Wannabe
Joined: 30 Apr 2005
Posts: 129
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| Posted: Tue May 23, 2006 4:40 pm Post subject:
Re: How is the nitrogen in air accelerated in ramjets/scramjets?
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N:dlzc D:aol T:com (dlzc) wrote:
| Quote: | Dear Robert Clark:
"Robert Clark" <rgregoryclark@yahoo.com> wrote in message
news:1148056130.596689.143820@u72g2000cwu.googlegroups.com...
...
But it is only the oxygen and that gets combusted
with the fuel. So you would think only the oxygen
portion of the air is getting accelerated.
Where is this "oxygen portion" located? Isn't it distributed in
the entire gas stream? Doesn't the combusted fuel also combine
with the gas stream? Is there something in the air that prevents
"dispersion" or "mixing"? Aren't there high velocity gradients
in the engine to achieve just such mixing?
The heating of the inlet air by an inlet compressor is a very
small part of the heating the entire gas stream is exposed to in
a standard turbine.
The problem isn't getting the combusted gasses to "thermally mix"
with the rest of the gas. The problem in a ramjet/scramjet is to
get the combustion to complete while still in the engine at
multi-mach speeds. If it is still burning after it has left the
nozzle, it is lost thrust, wasted fuel.
David A. Smith
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Yes, the oxygen is thorougly mixed with the nitrogen already so it
shouldn't be a problem of applying the heat of combustion to the
nitrogen as well.
I also looked at a textbook that gave a calculation for a turbine jet
engine and it calculated the thrust produced by simply applying the
heat produced to the total air.
However, we can estimate the loss of efficiency by the fact that the
nitrogen is not contributing to the heat produced, i.e., it is in
effect just being carried along.
Let's say the temperature *increase* of the oxygen+fuel mix would be
3000K above the temperature of the incoming oxygen if it were just pure
oxygen being mixed with the fuel. The space shuttle main engines for
example produce temperatures in this range. However, the space shuttle
combustion chamber operates at 200 atm's, much higher than that of
ram/scram -jets so our temperature increase probably won't be this
much.
The oxygen coming in is quite hot for (sc)ramjets because it has to be
severely slowed down from supersonic or hypersonic speeds. Let's call
it 1000K. Then the total temperature would be 4000K. I'll assume the
fuel is hydrogen for simplicity.
Now we imagine this heat is being distributed to the nitrogen as well
as the exhaust which is now water vapor. Now the heat given to a gas is
proportional to the temperature increase, the specific heat of that
gas, and the mass. The specific heat of water (vapor) is rather high,
higher than for nitrogen gas. This means it is actually easier to raise
the temperature of nitrogen for a given heat input than water.
However, taking into account water's higher molecular weight over
oxygen, the mass ratio of the nitrogen to the water is about 3 to 1. So
the total mass is about 4 times that of the water. So I'll estimate the
temperature change for the mixture as only half. (The specific heat for
the mixture can be calculated from those of the nitrogen and of the
water and therefore the exact temperature increase for the mixture can
be calculated knowing the mass is 4 times as much, but for simplicity
I'll estimate it as about half of the pure water vapor case.) Then the
temperature for the nitrogen+water vapor mix is 1000K + 1500K = 2500K.
This page gives the formula for the exhaust velocity dependent on
temperature:
ROCKET PROPULSION.
Combustion & Exhaust Velocity
http://www.braeunig.us/space/propuls.htm#combustion
The formula is often written in approximate form as Ve = sqrt(2RT/M),
where R is the universal gas constant, T is the temperature in kelvin
and M is the molecular weight of the exhaust. So if the temperature is
smaller by a factor of 2500/4000 and the molecular weight is larger by
a factor of 30/18, the exhaust velocity will be smaller by a factor of:
sqrt[(2500/4000)*(18*30) ] = sqrt(.375) = .6124
This may seem acceptable when the mass of the exhaust is 4 times as
high, making the exhaust thrust 4*.6124 = 2.45 times as high as the
oxygen+fuel only case. But the drag due to the larger incoming mass is
also 4 times as high. The thrust then is not increased by as large
amount as is the drag.
Bob Clark |
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Tom Sanderson
science forum addict
Joined: 22 Dec 2005
Posts: 55
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| Posted: Tue May 23, 2006 7:51 pm Post subject:
Re: How is the nitrogen in air accelerated in ramjets/scramjets?
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"Robert Clark" <rgregoryclark@yahoo.com> wrote
| Quote: | However, we can estimate the loss of efficiency by the fact that the
nitrogen is not contributing to the heat produced, i.e., it is in
effect just being carried along.
|
There is no difference between what you're discussing above and running a
rocket engine lean (excess oxygen). Any molecules not used in combustion
still provide mass and a place to absorb the thermal energy of the combusion
reaction...it doesn't matter if it's oxygen or nitrogen.
| Quote: | However, the space shuttle
combustion chamber operates at 200 atm's, much higher than that of
ram/scram -jets so our temperature increase probably won't be this
much.
|
The change in temperature through the combustor does not depend on the
temperature of the incoming gas. You're adding a fixed amount of energy,
and heat energy scales linearly with temperature (provided you don't change
state).
| Quote: | Then the
temperature for the nitrogen+water vapor mix is 1000K + 1500K = 2500K.
This page gives the formula for the exhaust velocity dependent on
temperature:
ROCKET PROPULSION.
|
There's a huge problem with this analysis...rockets don't carry nitrogen (or
other inert reactants). There's not much point in comparing the thrust
efficiency of rockets to turbines.
| Quote: | But the drag due to the larger incoming mass is
also 4 times as high. The thrust then is not increased by as large
amount as is the drag.
|
Rockets don't have ram drag (the drag due to slowing the incoming air
stream), so the argument is, again, irrelavant for a rocket. Turbines
always have ram drag, and it does not depend on the molecular composition of
the working fluid.
A jet engine in normal air takes in a certain mass of fluid, heats it, then
spits it out. The total amount of heat you can add via combustion is
limited, in theory, by the oxygen percentage (in practice, it's limited by
the temperature the turbine can stand). If you ran that same jet engine in
a pure oxygen environment with the same mass flow and same fuel flow, you'd
get essentially the same thrust. There would be a lot of unburned oxygen
running through behaving just like nitrogen does in a conventional engine.
The only major change you'd get in the pure oxygen environment is that, if
you weren't limited by material properties, you could put a lot more fuel
into the combuster, get a much larger temperature rise, and get more thrust
out of the same engine, but you're no longer running at the same design
point so you can't sensibly compare the two.
Tom. |
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