Combustion Design

[theq]

Could one use the coanda effect for creating the turbulence for mixing fuel and air in a combustor? Like a flare used in oil wells?

[Edit]

I was just reminded of the potential for poor fuel efficiency by the direct injection of fuel into the chamber. From what I gather it is recommended to line up the primary ports with the injection spray cone. However this could lead to fuel droplets sticking to the walls of he flametube.
Is this is a big problem? Is it hard to evaporate the fuel that collects here or is it easily vaporized? Say one was using a heavier fuel like diesel.

[racket]

Hi

There are some basic requirements with regards the fuel and air presentations , follow them and the engine works , go off on a tangent with a radical idea and you could have years of frustration .

The liquid fuel spray should be atomised to a degree where droplet size is small enough that they don't "hit the wall" , the wall generally runs hot enough to instantly vapourise any stray droplet .

Cheers
John

[finiteparts]

Nov 20, 2018 2:45:16 GMT -5 ripp said:

Hello Chris,

Can you figure out why Combustion Disign is different?
Allison 250 / GTD350
Striking is that both combustion chamber have plenty of room after the Tertiary holes
That seems very important.


youtu.be/V10uHxJA2V0

www.users.zetnet.co.uk/gas/combust.htm



Thanks, Ralph

translate.google.com

Hi Ralph,

I am not sure exactly what you are asking on why they are different...my first thought is that they are different because they are designed by different companies. 

As for the location of the tertiary holes, my thoughts on why they are "forward" of the exit has to do with the engine design.  These engines use a single combustor feeding into the axial turbines, which is rather unique.  The dilution jets must penetrate into the hot gas flow, mix and then ideally, the mixed, cooled gas flow enters the turbine stage at a uniform temperature across the inlet plane.  The term used to describe the temperature distribution across the turbine inlet plane is "pattern factor".

So, with the large dilution jet size and relatively low number of these tertiary holes, there is a need to have some axial distance to let the dilution jets mix with the hot gas flow coming from upstream.  If the dilution holes were too close to the turbine inlet plane, there would be insufficient time to evenly mix the two gas streams and the turbine would see alternating regions of temperature as it rotated relative to the NGVs.  The NGVs would also suffer from the thermal stresses that are caused by the varying temperatures entering.  With an even inlet gas temperature the NGV structure grows out radially in an even sense, but with a varying  gas temperature, the structure doesn't grow evenly and there is larger stresses on the NGV structure due to the resulting uneven thermal growths.

I hope that answered what you were asking,

Chris

[finiteparts]

Nov 21, 2018 0:49:00 GMT -5 theq said:

Could one use the coanda effect for creating the turbulence for mixing fuel and air in a combustor? Like a flare used in oil wells?

[Edit]

I was just reminded of the potential for poor fuel efficiency by the direct injection of fuel into the chamber. From what I gather it is recommended to line up the primary ports with the injection spray cone. However this could lead to fuel droplets sticking to the walls of he flametube.
Is this is a big problem? Is it hard to evaporate the fuel that collects here or is it easily vaporized? Say one was using a heavier fuel like diesel.

I am sure there is some way that you could use the coanda effect in the combustor design...but for liquid fuels, I am not sure that is a good idea.  I actually learned about the coanda effect when a coworked had developed a premixed gas injector that was based on the coanda effect.  The fuel was very reactive and trying to premix it with air offered huge challenges, so the low blockage offered by using the Coanda design was thought to be beneficial.  I think for a homemade combustor system, there would not be sufficient turblent kinetic energy produced to generate good mixing, especially with a relatively heavy liquid fuel. 

An oil flare is not what I would use as inspiration for designing a gas turbine combustor....they are not efficient and produce relatively ragged flame structures that would be likely unstable in a combustor can.

As John said, it is desirable to have the fuel atomization to such a level that the fuel droplet size is small enough that it is carried by the airflow in the chamber.  There is a break-point where the drag of the droplet is larger than the inertial effects, so that the droplet follows the airflow rather than the path defined by its own momentum.  Using oil burner nozzles, you can get below 80 micron droplets and these are well within this range and thus make a very good choice for your fuel injection.  The level of quality and technology offered in such a low cost package is the primary advantage of using oil burner atomizer nozzles.

I am not sure that the comparison between gasoline direct injection and a gas turbine combustor is a good one.  The IC engine cylinder wall is much colder than the GT combustor, so the rapid cooling of the droplet is directly related to the unburned hydrocarbon emissions.  As John mentioned, the combustor wall temperature is usually pretty high, at least 1.5 to 2 times the first vaporization point of the fuel, at least for a while.  It is usually a bad idea to have fuel wetting the liner wall as this leads to gum and coke formation.  When the droplet impacts the wall, it spreads and vaporizes off the lighter hydrocarbon fractions.  The heavier components of the fuel, tend to take longer to vaporize and once an initiation site if found for the coke to stick, they tend to form layers of gum or coke.  Once this occurs, the heat transfer across the liner is inhibited and the rate of coke formation can increase.  So the primary byproduct of droplets impacting the wall in a gas turbine is liner build-up of coke, not unburned hydrocarbon release like the IC engines.

I hope that helps,

Chris

[CH3NO2]

Hi Chris,

By chance have you worked on ramjets? Super or hypersonic?
Just curious.

Tony

[finiteparts]

Tony,

I have never worked on ramjets.

- Chris

[CH3NO2]

Right on Chris.

The comment about "the fuel was very reactive and trying to premix it with air offered huge challenges" is a bit unusual in the context of classic turbine engines.
Leading the imagination to thoughts of Triethylboron, RJ-5, UDMH or trying to premix H2 in a hot supersonic air stream.  

HAPPY THINKS GIVING TO YOU AND THE FAMILY!
Tony

[finiteparts]

Tony,

That was actually on a land based, power generation application. It is amazing what some process plants produce as a byproduct and then want to burn in a gas turbine. I have not actually worked on any aviation projects that did not use standard jet fuel. I have to be very careful what I discuss due to proprietary information and such, so I tend to not talk about such things.

Thanks...we had a great Thanksgiving and I hope you and yours do too!

Chris

[ripp]

Hello Chris,

thanks for your answer,
what i meant specifically is the Hole arrangement and inside of the allison 250 combustion chamber.
it seems to need more air as you can see from the larger primary holes, this seems to make the gas core shorter in compared to the GDT-350 combustion chamber it has a classic hole arrangement.
Is now the Allison combustion chamber a progress, more effective, powerful?




The Allison 250 turbine is designed for thousands of hours of running time, could now reduce the Tertiary area for a more compact combustion chamber designed, with the disadvantage of a reduced runtime ?
(Is an allusion to the combustion chamber design of the HX 82 102/255 " Money Pit ")



Thanks, Ralph

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[CH3NO2]

Hi Chris,

I had a question come up when thinking about choked flow conditions. If combustion gasses are flowing through a turn just before going to a convergent nozzle how are tumbling flow and turbulence affected as they flow through a choke point?

It seems turbulence would, at least momentarily, wash out on its Z axis but not on its X or Y axis... but its not entirely clear.
What do you think?

Thanks,
Tony

[monty]

Dec 9, 2018 11:14:05 GMT -5 CH3NO2 said:

Hi Chris,

I had a question come up when thinking about choked flow conditions. If combustion gasses are flowing through a turn just before going to a convergent nozzle how are tumbling flow and turbulence affected as they flow through a choke point?

It seems turbulence would, at least momentarily, wash out on its Z axis but not on its X or Y axis... but its not entirely clear.
What do you think?

Thanks,
Tony

Tony,

Hard to say exactly, but a nozzle is typically a very nice place for flow. The pressure gradients are kind. The nozzle isn't choked until the end, and flow is accelerating. You can get away with a lot of sins with a favorable pressure gradient. The boundary layer will be well behaved and compressed.

Monty

[finiteparts]

Tony and Monty,

I agree that due to the positive pressure gradient through the nozzle section, generally the boundary layers do not grow. But your comment on the turning section does usually indicate that there will be profile to the flow entering the nozzle section. The secondary flows in a turning section tend to reduce the velocity at the convex side (suction side) and increase the velocity near the concave side (pressure side) of the turn. Depending on the severity of the turn, you can get flow separation and thus feed the nozzles with a very distorted flow.

I am not sure that you can say that the turbulence would wash out, but it is probably safe to say that it would probably not grow. But near the choke point, there may be a standing shock and the interactions of a shock wave with the boundary layer are known to cause large boundary layer growth and even local flow separation.

I hope that helps,

Chris

[CH3NO2]

Maybe these images will help clarify a comparative scenario I was considering.

With strongly tumbling flow from a miter turn entering the convergent section, at a high chamber pressure and the knowledge that flow cannot be accelerated beyond Mach 1 at the throat:

Can this cause thrust vectoring or unstable vectoring?

Would it affect barrel shock upon exiting the divergent section?

[finiteparts]

Tony,

I am really not sure on that one. The usual assumption is that the nozzle is being fed from a plenum, thus there are no real cross-stream momentum components. I would venture to guess that if the flow velocity was large, you would end up with a distorted inlet flow to the nozzle and as such, you would have a ugly discharge flow that I would assume would modify the barrel shock. But, that is just speculation as I have no real knowledge of anything remotely close to that set-up. Sorry I don't have any better insight to offer.

Good luck!

Chris

[finiteparts]

Nov 23, 2018 1:57:54 GMT -5 ripp said:

Hello Chris,

thanks for your answer,
what i meant specifically is the Hole arrangement and inside of the allison 250 combustion chamber.
it seems to need more air as you can see from the larger primary holes, this seems to make the gas core shorter in compared to the GDT-350 combustion chamber it has a classic hole arrangement.
Is now the Allison combustion chamber a progress, more effective, powerful?




The Allison 250 turbine is designed for thousands of hours of running time, could now reduce the Tertiary area for a more compact combustion chamber designed, with the disadvantage of a reduced runtime ?
(Is an allusion to the combustion chamber design of the HX 82 102/255 " Money Pit ")



Thanks, Ralph

translate.google.com

Ralph,

I am not sure that I can really make a good estimation of the differences without a better understanding of their design (actual flow splits, design inlet temperatures, fuel flows, etc.).

I would guess though that you can not reduce the tertiary holes without severely impacting the design intent of the combustor. I also doubt that it would result in a more compact flame structure.

Sorry I didn't have a better answer than that...

Chris