what are your engine stats?

[syler]

I'm wondering what kind of pressures, thrust and temps you guys are getting.

Specifically, what is the pressure inside your combustion chambers and afterburners? What pressure and flow rate are your compressors producing?

Do these motors use the turbo in it's efficient zones?  Also, since heat is obviously an issue - why no heat sink fins?

[Johansson]

Heat is our best friend, without it the engines would be useless.

[racket]

Hi Syler

Theres "numbers " on my GT6041 build Thread jetandturbineowners.proboards.com/thread/78/garrett-gt6041-powered-kart

Using a quality turbo, pressures can be up to 45psi in the combustor , turbine inlet temperatures of 900deg C - 1650 F , mass flow rates of up to 2.75 lbs/sec , and yes, the turbo is running in its most efficient region for the pressure ratio being used.

Cheers
John

[syler]

One thing mechanics tend to overlook is the chemistry part of combustion. Rising temps is only part of the combustion equation. The majority of pressure comes from turning dense liquid molecules into gasses that take up far more space. I'm not talking about state change, but conformational change in which the repulsive forces of the atoms themselves are harnessed. With combustion this is mainly CO2 and N. Basically, fuel is a bunch of tiny wound up springs that pop open when the reaction happens. The heat comes into play after that by making these all bounce around faster which results in pressure.

Heat however is highly over rated in terms of an explosion. Thermite gives off a lot more heat than TNT, I believe it is the hottest reaction short of nuclear. It doesn't case an explosion because it doesn't change any of the molecules into gaseous ones. This is the same reason a cutting torch doesn't break our wrist when we light it. There is no organic molecular conformation change providing substance to heat.

[gidge348]

Jan 26, 2014 0:39:13 GMT -5 syler said:

Heat however is highly over rated in terms of an explosion. Thermite gives off a lot more heat than TNT, I believe it is the hottest reaction short of nuclear. It doesn't case an explosion because it doesn't change any of the molecules into gaseous ones. This is the same reason a cutting torch doesn't break our wrist when we light it. There is no organic molecular conformation change providing substance to heat.

Whoa....., "It doesn't case an explosion because it doesn't change any of the molecules into gaseous ones" sorry thermite DOES create gaseous bi-products, it is a self sustain burn that you could call an "explosion" but a very low detonation velocity.

[racket]

Hi Syler

You're getting too complicated here , .............our engines are more similar to home heating furnaces than they are to turbocharged auto engines .

The household heater has a fan to provide air , our compressor wheel does the same job , the heater has a combustion area , so do we , if you don't need so much "energy/heat" you reduce the fuel flow , so do we , your heater has the same air/gas pressure through out it so do we , only we often change some static pressure for dynamic energy, and remove some to power the compressor .

So think simple , don't get complicated , its not :-)

Cheers
John

[syler]

What happens in an explosion or most useful combustion is largely an organic chemical reaction. The heat is just a lucky supplement. The main force comes from conversion of the dense reactant molecules into ones that take up a great deal more space. When we burn Octane for example, the main product is CO2 gas. When combustion occurs, it creates massive amounts of it very quickly and that is the explosion. The heat of course expands the gasses even more adding to the rapid release of the CO2. Plus it burns stuff.

That's why heat alone is meaningless. Thermite is useful because it forms little gaseous products. That's why it hot enough to melt rock but doesn't cause an explosion. The change isn't a state change - it's an organic reaction. Most people just don't study O-chem but that's the deal. Think if you had a semi load full of 100,000 empty airbags and suddenly they all went off.

[Johansson]

Listen to the man syler, you are of course free to try to run a gas turbine on thermite but saying that it will work like you are stating the obvious won´t get you anywhere.

[finiteparts]

Jan 26, 2014 0:39:13 GMT -5 syler said:

One thing mechanics tend to overlook is the chemistry part of combustion. Rising temps is only part of the combustion equation. The majority of pressure comes from turning dense liquid molecules into gasses that take up far more space. I'm not talking about state change, but conformational change in which the repulsive forces of the atoms themselves are harnessed. With combustion this is mainly CO2 and N. Basically, fuel is a bunch of tiny wound up springs that pop open when the reaction happens. The heat comes into play after that by making these all bounce around faster which results in pressure.

Heat however is highly over rated in terms of an explosion. Thermite gives off a lot more heat than TNT, I believe it is the hottest reaction short of nuclear. It doesn't case an explosion because it doesn't change any of the molecules into gaseous ones. This is the same reason a cutting torch doesn't break our wrist when we light it. There is no organic molecular conformation change providing substance to heat.

I totally disagree with you. I have been designing gas turbine combustion systems for many years and I did my Masters thesis on laser absorption spectroscopy of several combustion reaction intermediates...so I am not just someone throwing out an unfounded opinion. Chemical "conformation" refers to spatial variations of a molecules structure, such as rotation around a bond or the such not the transformation due to reaction processes. I think the point you are trying to say is that the products of the combustion reaction are structurally changed from the original liquid hydrocarbon chain and thus occupy a different molecular volume, which is totally correct. I have never looked at the actual numbers for this, but I would venture to guess that it is very small for our purposes here and no where close to the pressure change caused by the increase in kinetic energy of the gas molecules or the change of state...if you have data to refute this I would be very interested in seeing it.

For the design of a combustor, we look at very thin reaction zones and thus the large majority of the combustor volume can be accurately characterized as being in a chemical equilibrium state (assuming proper design), so there should be minimal repulsive forces between the atoms/molecules involve as related to the mean free pathlength since they will be in stable chemical arrangements.

If you have done any work in combustion kinetics you know that the liquid fuel must first vaporize to a gas before participating in the reaction, so the first thing is the volume change due to the change of state...but, this is usually not going to be a kerosene vapor since the thermal processing can lead to breakdown of the large hydrocarbon chain into smaller molecules. Common smaller HC chains are propane (C3H8), ethane (C2H6) and methane (CH4). So lets take the easiest one, methane. So our fuel droplet has been absorbing heat and cracked the chain into smaller molecules, maybe butane then propane then ethane then methane. As these heat, they translate farther apart due to the increase in kinetic energy (temperature) until they are far enough apart that the intermolecular forces no long hold them in close proximity and they become a gas. For this change of state in methane there is a 621.4 time increase in volume for a CONSTANT pressure. The densities of the methane (0.668kg/m^3 at STP) and CO2 (1.8714 kg/m^3), shows that there is a change in occupied volume, but as you can see, for a STP conditions the CO2 will occupy less volume per mass (remember, conservation of mass must be followed and the carbon in CO2 must come from the CH4).

The idea of constant pressure in the combustor is idealized, since the exit plane is in communication with the lower pressure turbine inlet and must be so since there must exist a pressure difference to drive the flow. (An aside comment: The pressure rise combustion that some big manufacturers are looking at, rely on detonation waves and thus the pressure rise across a shock wave, which is allowed since there is no downstream conditions felt upstream of the wave.) So with the idealized case for our combustors being constant pressure (which is not realistic due to pressure losses of the diffuser, swirlers, cooling holes, skin friction...not to mention the fundamental pressure loss caused by the addition of heat in a duct), and using the ideal gas equation, PV=nRT...we see that the pressure is directly proportional to temperature.

As for the explosion topic, heat does play into things, but it is the rate at which the reaction propagates that determines if it is a detonation (supersonic expansion), a deflagration (subsonic expansion) or just a standard combustion reaction. A sudden release of heat can cause an explosion with out the need for organic material...electric arcs, lasers, lightning all can produce explosions.  Air heated to sufficient temperatures dissociates into unstable species, thus lightning created ozone, N2 breaking down to N radicals in the reaction zone of combustion processes, etc...

~ Chris

[syler]

Chris, the gas laws, which BTW only hold at STP, would give a situation where by volume at STP would double as T(C) doubles. So you wouldn't get that many doubling events. Anyway, the heat causes the now larger molecules to collide more frequently which causes pressure. Just consider thermite which produces enormous delta T but little gaseous product. Where is the kaboom?

No, the bulk of combustion, or a bullet leaving a gun comes from the rapid expansion of gas due to the conformational change. The heat just offers an added kick. Any Wikipedia article tells us that. And on a side note. Joules are what give us work, not temp C.

[finiteparts]

syler,

Maybe you need to take thermodynamics again, but ideal gas laws do not have anything to do with standard temperature and pressure (STP). STP is just a standard point for measurement and design work. The ideal gas laws do breakdown and deviate from reality at high pressures and temps., but you should remember that Boyle and Gay-Lussac derived the ideal gas equation from a lot of experiments on real gases and it fits well enough for their theory and the kinetic theory of gases. For the purposes of designing small home built gas turbines, this is a very valid assumption. The reason I highlighted the constant pressure was to make sure that those numbers were clear and the very large effect due to the change of state was clear. Since the combustion in a gas turbine is ideally done at constant pressure, I wanted to make sure that you understood how the volume changes.

If you go back to your physical chemistry, the definition of a gas is a swarm of molecules in constant, chaotic motion. For the molecular size to have an effect, the intermolecular forces would have to be significant..."In a gas the average distance between molecules and the distance they travel between collisions is normally large relative to their diameter. This implies that intermolecular forces play only a minor role in comparison with the kinetic energy of translational motion." - P.W. Atkins Physical Chemistry, 2nd Edition. If you don't believe me, maybe you will believe a highly regarded professor at Oxford. At higher pressure the intermolecular forces become significant (~10 atms) and then very important at high pressures (>30 atms). So for detonations, the intermolecular forces would be significant, but it takes additional energy to make larger molecules and that is why chemical reactions usually breakdown into smaller species, so I still disagree with your statements. If you can produce any kind of support I would be happy to read it...

Chemical reactions are exothermic, they release heat. Sudden releases of heat cause gas densities to drop and pressure waves. You are still using "conformal" incorrectly. Conformal changes refer to spatial changes in a molecule, not completely changed molecular structures...check it out for yourself here.... en.wikipedia.org/wiki/Conformational_change

Do you have a link to that Wikipedia article on this combustion stuff that any article tells us? By the way, what's your background on this information?

[racket]

Hi Syler

Temperature change in degrees C per pound of air/gas flow per second can be converted into a horsepower "number" .

Purchase this book............ www.amazon.com/gp/offer-listing/B007T3INJG/ref=tmm_other_meta_binding_used_olp_sr?ie=UTF8&condition=used&sr=1-6&qid=1392503454

Written by the godfathers of gas turbine theory , this early 1951 editions of their books , (now in their 7th edition) makes for easier understanding of how our engines work , all the theory is in them, including the theory behind the equations used ...............a must have book ...........the "Bible" for us gas turbine engine builders

The seller won't post to Australia, otherwise I'd be buying it, I've a couple of their other editions , 1958 and 1996, but not this one ..............its your lucky day, get in quick before another Member beats you to it :-)

Cheers
John

[Richard OConnell]

I'll give you something that make things sound a lot simpler. Jets like cold air. Cold air in, hot air out. Why is that? Why would getting air to extreme temps matter and how can heat have anything to do with producing usable energy? When air is cold it becomes denser. Now take a single cubic meter of that air and heat it up. The molecules become excited and the gas tries to expand. Now that its in our engine and has no where to go and the pressures increase rapidly. The expanding gas passes over the turbine stages, causing them to rotate, turning the compressor, and then exiting the engine. If you are cooling the air again, you are playing tug-of-war with yourself.

I dont claim to be an expert on the subject by any means. To be honest, I find Racket's posts to be difficult to follow at times. But I listen. There is so much knowledge here, it is absolutely mind boggling. If I can absorb even a fraction of what these people have to offer then it will be more than I walk in with and my time will not have been wasted. You have an interactive library at your disposal, there is no need to argue with the books. Just read them. Its completely up to you how you apply the information

[racket]

Hi Richard

LOL........I'll try to simplify my answers in future ;-)

Cheers
John

[enginewhisperer]

yep,

air in ----> add energy /expand air ----> spin turbine to drive compressor ----> use remaining energy for thrust and / or freepower tubine.



The colder the air coming in the more energy you can add to it by combustion. Cooling the air after the compressor is robbing it of energy - which you would need to replace by burning more fuel to achieve the same turbine inlet temp.