Big Turbo based engine and Afterburner questions......

[CH3NO2]

Oct 5, 2017 13:09:10 GMT -5 mjb777 said:

Oct 5, 2017 9:42:53 GMT -5 CH3NO2 said:

Thanks a bunch Tony!

I am very grateful for the vast amount of knowledge and support here, and also the fact that the sheer numbers of guys who also enjoy this pursuit means I'm not that abnormal after all! haha

Those numbers for the evaporator are great and very much appreciated! I hope you realise that you have just been placed on my "people to pester" list!

Matt.

Hey Matt,

If you want to learn more about turbines and combustion systems, I also recommend reading Finiteparts Chris posts.  Particularly his threads on small turbines, vortex generators/combustion design, turboprops, etc.   

Tony  

[CH3NO2]

Oct 8, 2017 15:31:08 GMT -5 racket said:

Hi Tony

The FT parts are only tack welded mainly because I didn't want to contend with buckling from a full weld , also tack welds can be easily ground out if I needed o change parts of the FT , because both the top cap and tapered tertiary pieces have their "laps" bent over a steel die , the straight wall section is seated firmly and being on the "inside" will expand outwards into the lap for a decent seal, its irrelevant if theres a few leaks here and there , the whole FT is full of holes, also having a lap means the weld is easier to do than a butt weld where the chances of blowing a big hole are always prevalent...........unless you're a very competent welder like Anders ;-)

....

Cheers
John

Hi John,

I wont be able to use dies to fabricate the same way you did...But I can laser cut sheet metal parts designed for butt welds. 
Would there be any other inherent drawbacks to using a fully butt welded flame can fabricated in SS 304, 16 gauge (0.062”)?  I can weld this thickness without blowout or excessive warping.  

If the hole layout proves to function well, I can always refabricate the flame can in a better alloy later. If needed.

Tony

[racket]

Hi Tony

You should be OK with stainless of that gauge , I used 0.5mm -0.020" thick 304 for my bike as well as the 2 shaft kart flametubes , though at that thickness theres really not enough metal to cope with the pressure differences across a red hot flametube , for a large dia FT like yours will need to be , 1.2mm - 0.050" SS would be a minimum .

With reasonable care taken to provide some wall cooling, stainless steel should be adequate for our FT expected life spans.

Cheers
John

[finiteparts]

Hi Tony,

In reference to your questions,
"On the orifice inlet, sharp edge Vs radiused inlet. Assuming equal mass flow, and I assume velocity, they should have the same momentum right?"

Technically, you would have the same "total" momentum. But the jet would not be "controlled"...meaning that the jet will not flow in a normal direction to the hole, it will lean over. When you have a larger l/d (hole length/hole diameter), the flow will follow the orifice direction better, or more accurately, you get the momentum going in the right direction. You see this in some of the smaller r/c combustors where they really want to direct the jet, so they will use a small tube as an orifice. Rolls Royce uses deeply plunged holes often for this same reason...for example in their RB199 dome section.

"Do the different shape inlets create different velocity profiles or Reynolds numbers at their outlet?"

Absolutely. The plunged holes have less vena contracta, so the velocity profile will be less center peaked. Reynolds number is probably not impacted that much...but it also depends on what you use as your characteristic length.

As for the liner thickness, I have run the numbers on the buckling requirements for other projects and the thin 304 stainless does alright up to about 1300F, even though it strength properties are falling fast at that temperature. If you check the numbers, the liners are not pressure loaded much due to the pressure drop across them. The real problem for buckling is how you mount the liner, it has to be fixed only on one end, the other end has to slip. If it binds up at all, the thermal stresses get large quickly and you end up with a mess.

You can do a quick check on the critical buckling of a cylindrical pressure vessel pretty easy using Roark's "Formulas for Stress and Strain". As an example, lets look at a 6 inch diameter liner, 12 inches long, 0.020 inch wall thickness and with 5% pressure drop across it assuming a Pt3 = 55psi.

If we use Eqn 19b from Table 35 to find the critical buckling pressure for a short cylinder with external pressure loading, we end up with a pressure required to buckle the tube of 15.46 psi. This is 5.6 times the pressure load that the liner will see for a 5% pressure drop (2.75 psi) across it at the stated Pt3. This assumes that the modulus of elasticity has been reduced to 20 x 10^6 psi due to the metal temperature reaching 1300F.

For that data, see page 17 of:

www.nickelinstitute.org/~/Media/Files/TechnicalLiterature/High_TemperatureCharacteristicsofStainlessSteel_9004_.pdf

Then, if we use Eqn 17 from Chapter 11, we see that the estimated buckling stress is 31,933 psi (taking the quoted 40% value).

So, you can see that using 0.062 inch is WAY more than required. I will put up those equations later, when I have more time.

I hope that helps,

Chris

[CH3NO2]

Oct 10, 2017 15:49:22 GMT -5 racket said:

Hi Tony

.....

With reasonable care taken to provide some wall cooling, stainless steel should be adequate for our FT expected life spans.

Cheers
John

Hi John,

That's good to hear.  Since it will be laser cut, I can easily load it up with lots of very small holes for transpiration cooling.  A thin boundary layer with a large thermal gradient.

Tony

[CH3NO2]

Thanks for the link to the PDF Chris. It's a good one.

[racket]

Hi Chris

LOL..........guess the two shaft karts flametube was a "tad" hotter than 1300 F when one side collapsed in :-( ........... , maybe all the holes in it changed the strength, don't really know , but it certainly wasn't from axial compression due to not being able to expand , the outlet snout was long and parallel walled with plenty of room to expand axially into the thick mounting base plate, and it was cooled "radially??" by the incoming air from the discharge tube from the comp preventing excessive growth in that direction .

The turbine bikes flametube of similar wall thickness and diameter was made of welded "segments" , the overlap louvres probably provided extra wall strength to prevent problems .

Strange things can happen with a prototype jetandturbineowners.proboards.com/attachment/download/25 and was the reason behind recommending to Tony thickish material for the flametube , a couple of extra pounds weight is cheap insurance .

Cheers
John

[CH3NO2]

John,
This 6041BL turbo is already so heavy I dont mind an extra couple pounds of weight.
The best part is I like working with 0.062" 304. I can weld it without stressing on a blow out.

Chris,
I'll design the flame can assembly to allow for jam free axial expansion. I've seen how SS304 can grow so much when it gets hot and in this application, its going to get incredibly hot. In fact, I'm anticipating intergrannular corrosion. When the flame can reaches its end of life I'll rip it apart just because its neat to see how embrittled SS can become when its exposed to extreme heat.

I've seen sheet SS321 off an old turbine crumble in my hands because it was so roasted. It was incredible to see because it looked like it was made from pressed metallic sand.

[finiteparts]

John,

I was trying to show how your 0.020 inch wall is plenty adequate, but then...well...hummmm...

That doesn't look like buckling to me...it looks like you experienced a bad case of high temperature oxidation. Am I missing something?

You can also buckle the liner due to local hot spots. The "cold" portion of the liner will resist the expansion of the hot spot and then the hot spot will buckle because it can't expand out. You can also get a bending of the liner due to the local hot spot. When the hot side expands and the cold side resists the expansion, the liner can bend causing it to bind up in the slip joint and ratcheting up the stresses. Mounting and controlling stresses on hot parts is a really challenging problem.

Remember, if it is a thermally induced stress, such as something due to a hot spot or a severe thermal gradient, you can't make the part thick enough to control the stress loads...but you can design in compliance so that they can handle the large thermal deflections without building up huge stresses. Thick parts can actually make things worse by trying to resist the thermal expansion.

All that being said, the 0.062 inch is good. That is what I used on my Senior Design combustor, (304 SS tubing) without any issues. I had a fuel nozzle mount controlling the axial location of the dome, spring centering to control the radial location of the dome (but also allowing the center-line to tilt) and a slip joint at the aft end allowing for the axial growth. On propane it did great...barely any oxidation. But when we switched to kerosene, the radiational heating must have been pretty strong, because it got that blackish looking oxide layer near the primary zone. I put in some dome cooling holes, but I am not sure how well they helped because the liner was already oxidized.

Good luck!

Chris

[racket]

Hi Chris

That damaged flametube was from my 10/98 engine , it was the result of a "blow out" where the pressure between shaft tunnel and inner wall was less than the pressure within the flametube , the flames exited the 0.032" thick Series 800 sheeting flametube , melted the steel shroud surrounding the ceramic blanket protecting the shaft tunnel , melting the ceramic blanket on their way to the back of the diffuser.

I added the pic to show that "things happen" with a new engine and its advisable to add a bit of extra meat, not that it would have helped much with the 10/98 flametube , that "oxy torch" would have cut through even 0.060" stainless .

The 2 shaft kart flametube was the one that "folded in" , I normally work on allowing sufficient axial expansion for a 1,000 degree C flametube wall, so for an 18 " long flametube at least 9mm of expansion room at the outlet snout.

I think your correct about there having been some localised high temp which seriously degraded the strength of the wall , but again , thats often part of the development troubles we experience , I learn't my lesson with that flametube and went thicker in later engines , the extra "meat" also helps with that radiation heating and burning which I experienced with bits of the TV84 bike flametube , some of the overlap at the louvres in the Primary Zone got "burnt" over time .

LOL, making a flametube is a relatively easy construction , getting it to work properly and survive is the hard part , especially when other components of the build often create aberrant conditions .

I agree that 0.020" WT should be adequate and is the reason I made a couple that thick , but unfortunately the real world decided to play games with me ...........we live and learn :-(

Cheers
John

[CH3NO2]

Hi Chris,

The difference in heating you noticed between propane and kerosene.
Does kerosene typically emit more infared or was it due to a fuel rich, localized, FAR near the nozzle?
Could liquid phase kerosene or soot have been deposited onto the metal surface causing the increased heating?

Tony

[finiteparts]

Hi Tony,

The difference in liner heat load is due to the flame radiation.

In a partially premixed flame setup, gaseous fuels such as propane burn with a blue color and liquids often burn with a orange color. What's the difference? The liquid flame has to phase change to a gas before it can intermingle with the air enough to mix and burn. The fuel is heated by the surrounding gas and/or the flames radiation as it moves into the combustor, but it takes a finite amount of time to heat it up to the point that it boils off into a gas phase.

While it is heating, the liquid has regions where there is no real oxygen to speak of (inside the liquid droplets, etc) and the fuel forms soot through a pyrolysis process. The soot is essentially agglomerations of carbon particles that form when the hydrocarbon molecules are shaken hard enough (heated enough) to break the hydrogen to carbon bonds. These tiny carbon clusters glow when they are heated and because they are nearly perfect black bodies, they radiate their thermal energy in nearly all spectral bands. They emit more radiation than the product molecules in the blue flame (which are emitting mainly in the blue frequencies only)...it is not limited to the IR bands, the flames are emitting higher energies in the visible and UV spectrum also. Technically, it is termed emissivity and the blackbody soot particles have a nearly perfect emission of the energy absorbed.

- Chris

[CH3NO2]

Thanks for the explanation Chris.

It's good to know the mechanisms for radiant heating. Pyrolysis and soot formation being a big factor determining the radiation spectrum and intensity. Would adding a small amount of black dye to kerosene increase it's post injection evaporation rates?


or

If for example you used methanol in your senior design combustor, where soot formation is not possible, the flame radiation would not be as intense in the lower frequencies?

Tony

[finiteparts]

I have no idea if you darkened the color of the fuel, would it absorb more radiant energy from the flame and get to the evaporation temperature faster. It sounds like it meets the fundamental physics, but I am sure there are tiny details that change things. Primarily I am wondering if the dye is composed of carbon, then you change your fundamental hydrogen/carbon ratio which has drastic effects on fuel properties. Also, how much dye has to be used? Since the radiant energy is occurring at the molecular level, it feels like you would need the amount of dye to be near the same order as the mass of fuel. I really have no idea...Might be worth testing out.

Sure, anything you do to reduce the capacity of the reactants to form soot reduces the thermal loading on your combustor. You don't need to switch fuels, you can just run the dome at a well mixed, lean condition and get a clean blue flame or just run a vaporizer. One of the key advantages of the vaporizers used back in the day on Rolls and other engines was the lower thermal loading due to the clean blue flame. The vaporizer had already heated some of the fuel past boiling, premixed it with air and then the larger droplets that pass out the vaporizer exit usually get splattered on the combustor wall, so you are not having droplets in the combustor primary flow heating and creating soot. You can see that the combustion is a nice blue flame in the JETPOL guy's cool videos with the quartz casing videos.

www.youtube.com/watch?v=yZ2RcXOXdLw

Technically, I can't say that the flame radiation would reduce in intensity at lower frequencies. Since the soot behaves as a blackbody, it absorbs and emits across very broad spectral frequencies. Technically, there are peaks where the product species emit energy and they might be similar, but it is likely, due to other factors, that the intensity for most frequencies are lower.

One reason is that Wein's law tells us that as objects get hotter, the dominant emission frequency gets shorter...hotter atoms vibrate faster, thus they have shorter wavelengths (shorter wavelengths = higher energy). This is why the turbine wheel starts a dull red and then as it get hotter it goes orange then yellow then finally white. The white light might not be intuitive, but the reason is simple. Wein's law predicts the peak frequency that the body will emit, but Planck's Law describes the distribution of the total electromagnetic radiation emitted. By the time the temperature gets to the point that we see white light, the total energy distribution curve is covering the entire visible light frequency spectrum, thus we see white light, (which is the combination of all frequencies in the visible spectrum).

Another interesting tid-bit on radiational heating is given to us by the Stefan-Boltzmann Law.

E = dela* T^4 where delta = Stefan Boltzmann Constant = 5.67 x 10-8 W/(m^2 K^4)

It tells us that the energy emitted goes up as the temperature to the fourth power! So if the turbine metal temperature goes up from 1000 F to 1600 F (140% in terms of degree K), the turbine wheel radiant heat flux has gone up by 4 times! This is why shielded thermocouples are used in services where there is a likelihood of larger thermal radiation loads. You can calibrate an exposed TC junction for a given loading, but when the loading is changing by a fourth power driven factor, it is very unlikely that you can have any accuracy measuring gas temperatures in the presence of a glowing chunk of metal.

One last cool fact that I learned in Heat Transfer class years ago...have you even notice that on a frosty night your car hood, roof and trunk are covered with ice, but not the sides of your car? Seems odd, since they are all sitting in the same ambient temperatures. Well the reason is that those parts are radiating their heat into space. Anything above absolute zero radiates heat, so even your cold car sitting in 25F air has heat to radiate into the very cold depths of space. The upper surfaces of your car have a line of sight into space, where the side of your car has a line of sight to the garage or a tree or something that is not very different in temperature to it. So the energy that the side of the car radiates to the tree is almost the same as the energy that the tree radiates to the side of the car, thus no real change in temperature. But space doesn't radiate energy back to the top of the car at any rate similar to what the car is giving up, thus the top of the car drops in temperature and any water vapor in the air, condenses and freezes on the roof.

- Chris

[CH3NO2]

Great post Chris,

My thought is that the fuel would only need a small amount of dye to opacify it, for the fuel to then absorb the majority of radiation it recieves. But maybe its not necessary because the convective heat transfer rate is likely much higher. In the end it all gets burned anyways.

Cool video link. I havn't seen that one before. I like the idea of a transparent plenum for research and development purposes. Being able to see what is happening with the combustion process could come in handy.
I found these are available in 8" diameter. They aren't quartz but with some modification to the flanges, some thermal isolation and careful use of cooling they may be able to work.
www.aliexpress.com/item/Free-Shipping-4-102mm-OD119-High-Quality-Flow-Sight-Glass-Dioptr-Sight-Tower-Sight-Glass-Stainless/32824802370.html

Its interesting to hear about ice formation on the top of the car. I always assumed it was due to gravity pulling precipitation downward but it does make sense that the part facing to space will be slightly colder than everything else.
Makes me wonder about the atmospheres own blackbody radiation potential.

Tony