[AR] Re: Combustion instability and injector patterns
- From: Richard Garcia <GalaxyNGC1672@xxxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Mon, 4 Jan 2016 07:40:15 +0000
like-on-like injectors are probably the most common selection an amateur would
make
Actually, out of the several amateur injector designs I've seen, I've never
seen one that was like-on-like. I've seen doublets, triplets, and split
triplets but they where all unlike impinging injectors. However as you've
mentioned most big professional rockets use like-on-like. I don't know why
there is this difference in preference. Most of the injector designs I've seen
in detail are from the the southern California area. This may explain much of
their similarity as these designs have influenced each other. My own designs
thus far have been unlike elements because I'm sticking close to what I know
worked for others and I have the details on how to make them.
The second most common amateur injector I've seen is the pintle type. These of
course sidestep the whole combustion stability issue at the cost of some c*
efficiency and typically longer chambers. Getting the most performance out of
them usually takes an empirical test program with some of its own rules of
thumb and art, but it isn't too bad. Although I don't think that would be
necessary for an amateur engine.
The third most common I've seen is the use of spray nozzles, which I would not
recommend. Those spray nozzles probably come from just not knowing the simpler
ways to make impinging or pintle injectors that amateurs typically use.
-Richard
________________________________
From: arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> on behalf of
Andrew Burns <burns.andrew@xxxxxxxxx>
Sent: Friday, January 1, 2016 8:25 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Combustion instability and injector patterns
Thanks for all the help so far guys, lots of useful info. To expand on the
initial post I'm imagining LOX/Hydrocarbon and like-on-like injectors are
probably the most common selection an amateur would make, primarily due to the
vast amount of information out there but also because of relative ease of
manufacture (compared to coaxial) and propellant supply chain reasons.
I did just dredge up one piece of information I think is interesting from a
patent regarding Russian LOX/HC coaxial injector designs:
http://www.google.com/patents/EP1022455B1?cl=en
Patent EP1022455B1 - Liquid-propellant rocket engine chamber and its
casing<
http://www.google.com/patents/EP1022455B1?cl=en>
www.google.com
In particular when you look at the picture of the injector distribution, other
than some longer posts which form baffles (also used in SSME) you'll notice
that the remaining injector elements are all members of three different groups.
This paragraph explains the groups:
*
[0041]
The injectors, except those located near the internal fire wall of the chamber
casing, should differ in mutual location and definite step in the fuel flow
rate, and therefore they are divided into groups according to fuel flow rate
growth (three groups are given consideration as an example). In this case, the
injectors of different groups are made in such a way that the mass flow rates
of fuel for adjacent groups at the nominal operation mode of the engine differ
by no less than 3% and no more than 10%. The introduction of injectors with
different flow rates is necessary in order to reduce the effects of
high-frequency oscillations at operating modes of the engine. During the
mounting of the injectors 13 on the mixing head 1, the injectors are arranged
so that injectors of different groups are adjacent each other, the groups being
positioned in the chamber according to a definite law (for example, by cyclic
sequential helical repetition of the injector arrangement from the first to the
last group). A schematic representation of the arrangement of injectors 13 on
the mixing head is shown in Fig. 2 as a section A-A. Wherein, each injector of
a corresponding group is designated by a circle with a wholly colored sector of
corresponding size:
* An injector of the first group is designated by a colored sector which
is 1/4 the size of the circle;
* An injector of the second group is designated by a colored sector
which is 1/2 the size of the circle;
* An injector of the third group is designated by two diametrically
opposite colored sectors which are 1/4 the size of the circle.
So they deliberately vary the O/F ratio of each adjacent element by no LESS
than 3% specifically to mitigate the effects of high frequency combustion
instability. They also make mention of a 'definite law' with regards to how the
elements are varied (mentioning a spiral pattern as an example, possibly not in
reality). This would seem to confirm my thoughts of using element pattern
variations to introduce 'soft spots' into the combustion region to absorb
acoustic wave energy, although I was thinking of much more organized/large
scale discontinuities while this patent would appear to show more of a 'white
noise' type O/F ratio variation across the injector.
Andrew
On Sat, Jan 2, 2016 at 4:42 PM, Henry Spencer
<hspencer@xxxxxxxxxxxxx<
mailto:hspencer@xxxxxxxxxxxxx>> wrote:
On Sat, 2 Jan 2016, Andrew Burns wrote:
...I'm not sure that there's really enough information available
in public literature to properly estimate the tendency of a given injector
design to instability. From what I've seen the traditional process has been
testing and semi-trial and error redesign based on hot-fire results. That
said there are plenty of examples of calculated 'stability maps' so somebody
must have either a theoretical or empirical method for estimating stability.
Lots of them. The relation of any of those methods to reality, however, is a
different question. Combustion stability is still a black art, with no
convincing theory and at best rough empirical correlations, so it really is
ultimately based on testing and rule-of-thumb redesign.
(To be more precise: that's the situation for high-frequency instability,
which is typically driven by interaction with combustion; low-frequency
instability, typically driven by interaction with propellant flow and
injection, is better understood and typically less problematic.)
The bible on the subject comes in two parts. The Old Testament :-) is Harrje &
Reardon, "Liquid propellant rocket combustion instability", NASA SP-194, 1972,
a massive phonebook-sized slab that's long out of print but can sometimes be
had secondhand, and is probably available online as a PDF now (but since I have
the slab I haven't looked). The New Testament :-) is Yang & Anderson, "Liquid
rocket engine combustion instability", AIAA Progress in Astronautics and
Aeronautics vol. 169, 1995, which I think can still be had from AIAA at an
excessive price, might be findable used, and almost certainly *isn't* available
free (AIAA doesn't do free). Y&A was meant as an update to SP-194, so for
completeness one wants both.
It's getting to be about time for there to be a Koran :-) to follow the New
Testament, but if anybody's done one, I haven't heard about it yet.
Both books have considerable useful advice, but the most interesting bit I
found in either one is in the Muss paper in Y&A: plots (page 84), for two
different LOX/hydrocarbon injector types, of strong correlations between D/V
(orifice diameter over injection velocity) and a critical frequency, with
stability problems to be expected if the chamber is big enough to let that
frequency (or a lower one) resonate.
- The chance of instability is increased as injector performance increases...
Most tactics to increase performance also decrease stability and vice-versa.
This is pretty definitely the case for the like-on-like injectors used in most
big non-LH2 engines. Whether it is universally true is less clear. In
particular, other injector types (triplet and coaxial, in particular) probably
don't entirely escape from this, but might shift the tradeoff enough that it's
no longer a big problem for a particular design.
For this topic in particular, beware of assuming that the design space is well
explored. Much of the conventional wisdom is empirical correlations only,
often derived from fairly small regions of the design space and quite possibly
not applicable elsewhere. Discovery of major improvements remains possible;
indeed, there are hints that some of the newspace companies may have made real
advances here.
Clearly there are also traditional approaches to controlling instability.
For example using baffles to compartmentalize the injector face...
Acoustic cavities can also be added around the injector face...
Another possibility: as you may have noticed in another discussion, although
transpiration cooling is generally in disrepute, there is extensive use of it
for cooling LOX/LH2 injector faceplates in particular, and some of those
engines appear to be unusually stable. There has been speculation that the
porous surface may function as an acoustic absorber to some extent.
- Is there anything out there to read or that people know with regards to
controlling instability purely by injector pattern or element design?
The two books noted above have quite a bit of this, although it's not all
universally applicable or entirely consistent -- more of a collection of hints
than a coherent design methodology.
example it's my understanding that deliberately delaying mixing and
combustion such that the heat release zone is away from the injector face
increases stability but also decreases performance.
Delay certainly tends to decrease performance. How much it really affects
stability, except in the well-studied case of like-on-like injection (where it
does improve it markedly), is less clear. See note above about limited
exploration of the design space.
- I've seen references to linear and non-linear instability however I'm
still not entirely sure about what the difference is, anyone able to
explain?
As in many other things, it's basically the distinction between things we can
mathematically analyze (linear systems make that *so* much easier) and things
we have only vague clues about. There is unfortunately a lot of nonlinearity
in combustion instability, which is why the mathematical analysis is of very
limited help in practice.
- Most thrust chambers are simple cylinders with flat injector faces however is
it possible to modify the geometry of the combustion chamber to increase
damping of acoustic modes or to re-direct their energy?
In principle, yes. It's poorly explored. (Note that many injectors are at
least somewhat dished for other reasons.)
Henry
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