Laminar flow combustion chamber

What does the combustion chamber actually do? It's a place for the
propellants to mix and burn subsonicly. The mixing typically requires
ferocious turbulence.

The problem I have with mixing and burning simultaneously is that a
lot of the mixing is happening at high temperatures, where the gases
have large volumes and require an enormous amount of turbulence to
mix. My guess is that this turbulence is a way to lose energy. An
order of magnitude less turbulence is required to mix the propellants
before burning.

Suppose the propellants are not hypergolic. Why not mix them, then
burn them, more like a bunsen burner? My understanding is generally
that any gas acceleration that happens while heat is added to the gas
(example: bunsen burner) increases entropy, but that acceleration that
happens with no heat added (example, rocket nozzle) does not. This
means you want the burning to happen at the lowest gas velocities
possible.

Deflagration in premixed gas phase propellants is pretty slow. It
gets faster as the initial pressure increases, and as the initial
temperature increases. I'm going to guess that methane-oxygen is
around 16 m/s at 10 atm. The burned gas velocity is going to be
around 160 m/s. Detonation pressures for methane are 20-40 atm, which
is why these operating pressures seem low.

To capture the flame, I'd use a tube that flares, perhaps with wires
crossing it to form a flameholder (and to provide initial ignition).
The flaring section needs have a large enough area ratio that the
flame is still trapped for any expected variation in flame speed due
to startup temperature and pressure transients.

After the flaring tube, I'd have the flow reconverge in a de Laval
nozzle. So the engine ends up looking basically like all rocket
engines do, except instead of a flat injector plate I have a flaring
tube, and the propellants injected at the end of that tube are
injected gas-phase. I might have other (not hot) wire grids in the
throat to increase the low-temperature turbulence and mixing. The
area ratios of injector throat to combustion chamber to nozzle throat
might be 1:10:1.

Now consider start-up. Begin running 100% fuel (gas phase somehow) at
full operational mass rate. Run current through the ignition wires to
get them hot, then add oxygen progressively until you get to operating
mixture ratio. The flame starts at the wide end of the flare, then moves
down near the throat as chamber pressure builds.

Advantages:
- Less noisy
- Potentially better Isp across smaller chamber/ambient pressure
drops, from lower turbulence
- More complete burning
- Short "combustion" chamber
- More laminar flow near walls means
- less heat transport to walls
- film cooling works better, with less wasted fuel
- less scouring action
- rocket engine scales down better (same Isp at smaller size)
- Reliable, nonexplosive startup
Disadvantages:
- Extra drag from larger nozzle convergence ratio
- Need to boil propellants before injection
- More constraints on cooling system (film cooling less usable)
- Smaller input/output temperature ratio will lower Isp
- Larger effect at higher pressures

How thermodynamically efficient is accelerating a boiling two-phase
mixture? My guess is that boiling the propellant in the combustion
chamber is not critical to efficiency, since the SSME injects
gas-phase hydrogen. Perhaps higher-temperature methane, propane, and
oxygen are a different story.

If you haven't figured it out already, this idea is targetted to a
restartable upper-stage engine that is simple enough for amateur work.

In article ,
Iain McClatchie wrote:
Suppose the propellants are not hypergolic. Why not mix them, then
burn them, more like a bunsen burner?

Provided you can arrange for both to be gases at the time, there is some
merit to this... although there is also some risk of explosion. (People
did experiment with premixing injectors for *liquids*, and gave up on the
idea because it tended to make brief unexpected loud noises. :-)) Bunsen
burners run at rather lower pressures and densities...

To capture the flame, I'd use a tube that flares, perhaps with wires
crossing it to form a flameholder (and to provide initial ignition).

Note that the function of a flameholder is partly to induce turbulence,
which somewhat defeats your intent. Gaseous fuel mixes burn much faster
if you stir some of the combustion products back into the incoming mix;
it's thought that they catalyze otherwise-slow steps in the combustion
process.

Advantages:
- Less noisy

If (dim) memory serves, rocket noise is thought to be more a function of
the supersonic exhaust jet hitting air, than of the process in the
chamber.

How thermodynamically efficient is accelerating a boiling two-phase
mixture? My guess is that boiling the propellant in the combustion
chamber is not critical to efficiency, since the SSME injects
gas-phase hydrogen. Perhaps higher-temperature methane, propane, and
oxygen are a different story.

People have run RL10s on methane and propane without difficulty, using
the same basic injection strategy (gaseous fuel, liquid oxidizer).

There is little experience with gas/gas injectors in rockets, though.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Henry, thanks for the thoughtful reply.

HS Provided you can arrange for both to be gases at the time, there is some
HS merit to this... although there is also some risk of explosion.

I know that premixed LOX/liquid hydrocarbon is an explosive. But premixed
gas-phase O2/CH4 is reputed to be fairly tame. Certainly if you dilute it
with 4 parts N2 and run it at 1 atm it's pretty reasonable.

The danger, I think, is that liquid injection can allow a substantial mass
of propellant to build up in the combustion chamber. With gas phase
injection the amount that can build up is two orders of magnitude smaller.

HS Note that the function of a flameholder is partly to induce turbulence,
HS which somewhat defeats your intent.

Mmmm. My intent is actually to figure out something amateurs can fool with
at less risk to their heads. In any case, you are right, and the
flameholder turbulence would be supplied by a wire grid. That turbulence
would be nothing like the turbulence in the usual combustion chamber design.

HS Gaseous fuel mixes burn much faster if you stir some of the combustion
HS products back into the incoming mix; it's thought that they catalyze
HS otherwise-slow steps in the combustion process.

I didn't know this. I know that one other thing flameholders do is provide
streams of mixture below flame speed, so that the flame always has a chance
to burn back down the flow and doesn't get blown off.

For this design, I actually don't want the flame speed to be very high,
because higher flame speed = higher flow rate = more acceleration through
the flame = less efficiency. But flame speeds below 20 m/s don't actually
involve a lot of pressure drop, and there is quite a way to go between
16 cm/s for CH4/air at STP, and 20 m/s. (That 20 m/s is a pretty weak limit,
btw. Pressure drop builds slowly with velocity.)

There is another design point that I had initially considered. If the
injected gas is higher density (pressure and cold), the flame speed should
increase. At some point (turbopumps required), the flame speed should get
to the point that the combustion products are supersonic. Note that the
flame is subsonic -- this is not a detonation. In any case, since the
post-flame flow is supersonic, you can go directly to a diverging nozzle
and eliminate the combustion chamber altogether. It sure *looks* simple!

The problem with this design point is that it's apparently not very
efficient to accelerate gas by adding heat. So you apparently want to
minimize the flow velocity when burning, and then accelerate the gas.
This inefficiency shows up in my equations as a pressure drop across the
flame.

This is why I suspect that accelerating boiling two-phase is not an
efficient process (either in the cooling jacket, or in the combustion
chamber). It's probably better to keep the fluid under enough pressure
to keep it a liquid, then drop the pressure across an injector, where the
fluid flashes to vapor.

Do you know if the H2 in the SSME / RL10 / etc boils before it gets to
the injector face? The SSME has a very low injector pressure drop -- 5%
or so -- it would be awfully impressive to heat the H2 to within a few
percent of boiling but not over. I guess you could heat into into two
phase, and that's a pretty wide heat capacity target, but then you'd have
to deal with variable volumetric flows from various ratios of the two
phases.

Ooh you can really see how injector design gets so sensitive to operating
conditions.

HS People have run RL10s on methane and propane without difficulty, using
HS the same basic injection strategy (gaseous fuel, liquid oxidizer).

Ah, but with methane the liquid volume ratio is completely different.
Did they get good Isp out of the RL10 running LOX/CH4?

(Henry Spencer) writes:

Note that the function of a flameholder is partly to induce turbulence,
which somewhat defeats your intent. Gaseous fuel mixes burn much faster
if you stir some of the combustion products back into the incoming mix;
it's thought that they catalyze otherwise-slow steps in the combustion
process.

Also, in most cases the flame-front speed is slower than the gas-flow speed
(this is one of the reasons a bunsen-burner flame has the shape of an acute
rather than an obtuse cone, and why you have to fiddle hard with it to get
the flame to jump down into the pipe). Hence, unless the combustion products
are recirculated to provide a continuous source of re-ignition for the incoming
fuel and oxidizer, the flame-front will simply blow out the back of the engine...


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

In article ,
Iain McClatchie wrote:
HS ... although there is also some risk of explosion.

I know that premixed LOX/liquid hydrocarbon is an explosive. But premixed
gas-phase O2/CH4 is reputed to be fairly tame. Certainly if you dilute it
with 4 parts N2 and run it at 1 atm it's pretty reasonable.

Depends on the mixture ratio. :-) Explosions in coal (etc) mines are
usually methane, and they can be pretty bad. But they, of course,
involve a very large volume of mixture.

HS Gaseous fuel mixes burn much faster if you stir some of the combustion
HS products back into the incoming mix; it's thought that they catalyze
HS otherwise-slow steps in the combustion process.

I didn't know this.

It's one of those occasional cases where for some reason, the full,
accurate explanation for something is found only in specialist literature,
while the accounts of the underlying theory in the engineering-design
books are oversimplified to the point of being actively misleading.

(The classic example of this is the notion that the molecular weight of a
chemical rocket's exhaust is important. All the rocket-design books say
so. Only when you read a book written by a propellant chemist -- Clark's
"Ignition!" being the canonical example -- do you find the real story.)

I know that one other thing flameholders do is provide
streams of mixture below flame speed, so that the flame always has a chance
to burn back down the flow and doesn't get blown off.

That's significant, although you would think that it could be overcome by
use of a hypergolic fuel, or continuous laser-spark ignition.

Do you know if the H2 in the SSME / RL10 / etc boils before it gets to
the injector face?

Yes and no. It doesn't boil, in the sense of a phase transition, because
pressure is well above hydrogen's critical pressure throughout -- there is
no sharp phase transition at such pressures. (Use of such pressures is
deliberate: boiling in the cooling passages is problematic because it's
unstable, with the constant danger that a brief increase in heat flux will
cause a transition to film boiling, which greatly *reduces* heat transfer
and generally leads to rapid destruction of the chamber wall.)

However, the hydrogen temperature at the injector is high enough that the
behavior is gas-like, with relatively low density and high flow velocity.
The injectors exploit this, with each element providing a slow-moving jet
of LOX at the core of a fast-moving jet of hydrogen, and the velocity
difference providing very effective atomization and mixing of the LOX.
(And the ends of the "LOX posts", the tubes that introduce the LOX into
the hydrogen stream, tend to function as flameholders.)

HS People have run RL10s on methane and propane without difficulty, using
HS the same basic injection strategy (gaseous fuel, liquid oxidizer).

Ah, but with methane the liquid volume ratio is completely different.
Did they get good Isp out of the RL10 running LOX/CH4?

They did, although they needed minor modifications to the RL10 to make it
work properly with methane or propane.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

(Iain McClatchie) wrote in message . com...
...
Do you know if the H2 in the SSME / RL10 / etc boils before it gets to
the injector face? The SSME has a very low injector pressure drop -- 5%
or so -- it would be awfully impressive to heat the H2 to within a few
percent of boiling but not over.

This question is irrelevant. Even the relatively modest chamber
pressure of the RL-10 is above the critical pressure of hydrogen. The
SSME chamber pressure is high enough to dwarf the critical pressure of
oxygen, too.

Oren

(Iain McClatchie) wrote in message . com...

What does the combustion chamber actually do? It's a place for the
propellants to mix and burn subsonicly. The mixing typically requires
ferocious turbulence.

The problem I have with mixing and burning simultaneously is that a
lot of the mixing is happening at high temperatures, where the gases
have large volumes and require an enormous amount of turbulence to
mix. My guess is that this turbulence is a way to lose energy.


How does turbulence result in an energy loss?

The integral scale of the turbulence is very small (say, the diameter
of the injection orifice), so the dissipation of vorticity to
viscosity will be fast (I would guess faster than the residence time
in the combustion chamber). The turbulence dissipates to heat (and
the purpose of the nozzle is to convert heat to directed kinetic
energy), and this heat is negligible in comparison to the heat release
of combustion anyway.

Combustion chambers in propulsion devices are well approximated by the
combustion concept of a "well-stirred reactor" and have little direct
relation to flame propagation. Well-stirred reactors are more stable
(i.e., wider "flame-out" range) with higher energy densities than
laminar flames, which is why they are used instead of laminar flames
in propulsion.
--
Andrew J. Higgins Mechanical Engineering Dept.
Assistant Professor McGill University
Shock Wave Physics Group Montreal, Quebec CANADA
http://www.mcgill.ca/mecheng/staff/academic/higgins/