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Post by racket on Feb 19, 2014 19:04:53 GMT -5
Hi Rex
With 2-5% of the comp discharge bled off via a pitot tube in the comp scroll near its outlet to maximise the P2 pressure and supplied behind the turb heat shield in which suitably drilled holes could inject the air against the back of the turb wheel near the blade roots , there should be a cool blanket of air over the hub of the wheel minimising heat absorption , any blade heating from the higher gas temps would have a heat sink to conduct into , hopefully lowering blade temps on the way .
Its something worth trying :-)
Cheers John
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Post by racket on Feb 20, 2014 3:17:59 GMT -5
Hi Rex
Theres an ~25% gain in thrust by having an extra 300 deg C temp increase going into the turbine stage, plus the chance to produce "diamonds" in a supersonic exhaust , normally we don't have quite enough pressure in the jetpipe to produce supersonic velocities but there'll be enough if using a modern high performance turbo with higher temps .
Cheers John
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rexhunt
Member
Joined: August 2012
Posts: 32
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Post by rexhunt on Feb 20, 2014 18:03:42 GMT -5
Hi John,
What do you mean by "diamonds"? I imagine it's a term used to describe the way a supersonic exhaust appears.
Regards, Josh
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Post by racket on Feb 20, 2014 18:25:28 GMT -5
Hi Rex
Yep , the shock waves in the exhaust
Cheers John
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Feb 20, 2014 19:54:30 GMT -5
Dancing Diamonds
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Post by Johansson on Feb 21, 2014 11:05:36 GMT -5
Dancing Diamonds Gahh, I think I just ejaculated...
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syler
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Joined: January 2014
Posts: 39
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Post by syler on Feb 22, 2014 19:50:57 GMT -5
I understand what you guys are saying. I also understand the conventional wisdom floating around on these boards.
What I'm trying to do is iron out some fundamental misunderstandings. First, commercial manufacturers always have numerous engineering considerations so you can't necessarily presume their designs are always the best for our purpose. I have a friend who is a professor of embedded system designs and his work is in nearly every car on the road today. I hear about it all the time.
It seems that a lot of you are under the belief that change in temp = change in heat. That is incorrect. One can increase heat 3 fold while decreasing temperature. Work, is a matter of heat EXTENSIVITY measured in Joules. Temp is a measure of INTENSITY measured in C. Two entirely different animals. Also, the work being done in an IC motor comes from the repulsive force of the newly formed combustion products. When these new products form, they give off heat which has an intensive quality and an extensive quality. The heat comes from the molecular changes and physical chemistry involved. The heat is therefore a byproduct. Now, the heat does help expand our newly formed gasses, so it gives a bonus. And yes, according to Charles' law, if you reduce temp C, you reduce pressure.
The problem - our engine components such as AB pipe have a specific heat capacity. That means that according to the material and surface area, the metal is able to give off only so much heat. As it becomes heat soaked, the temp will rise until it reaches the temp inside at which point it weakens. If the metal has sufficient surface area to give off 2X the heat, it could burn 2X the fuel and oxygen, and give 2x the power while remaining within feasible temp range.
The problem appears to be that somehow heat in joules became convoluted with temp in C. Temp C has zero to do with work output. Joules are what you need. An AB that can radiate 2x the heat can pass 2x the joules at the same temp. That means zero temp rise but 2x the work. Physics doesn't lie - experience usually does.
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syler
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Joined: January 2014
Posts: 39
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Post by syler on Feb 22, 2014 19:54:48 GMT -5
If you want proof, get a chunk of pipe, run air through it and measure the thrust. Then, heat it from the outside while air is running through it. See how much more output you get from the addition of heat alone.
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Post by racket on Feb 22, 2014 22:51:21 GMT -5
Hi Syler
LOL.......You're playing semantics ................give us some worked examples so we can test your theories ..............numbers please :-)
As for heating air in a pipe , you'll need an initial pressure to force the air through the pipe so that theres a Pressure Ratio ( PR ) across the unit , then theres the problem of choking if too much heat is added and theres not enough initial pressure to overcome the pressure drop required to accelerate the heated air ............if you'd like the calculations and equations they're in the Cohen and Rogers books I recommended, and which I've read many times over the past couple of decades .
And if you'd like the equations for heat transfer through a metal plate , I've got them here as well ( boiler plate calcs)...............but because the A/B operates with high speed gases theres the Boundary Layer to contend with , also the Luminosity factor of the fuel used which is sorta independent of the boundary layer with regards heating the A/B wall.
Simply increasing the A/B surface area will only achieve one thing , lower power due to increased losses from surface friction on all that extra metal :-(
Worked examples please , so we can show you the error of your ways, and hopefully enlighten you to just how sophisticated our DIY units really are .
Cheers John
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Feb 23, 2014 3:04:04 GMT -5
Hi Syler,
At the risk of repeating myself......
John, Anders and a host of others have spent a lot of time explaining how turbines work and why your theories wont work.... But that being said it's a free world so why not do some small scale experiments and test your theories and post the results?
Build your "cooling air bypass engine" or "water cooled combustion chamber" or "thermite powered turbine" and see what happens? You may be right? Some of the greatest advances is science came when people said it could not be done...... and someone "just did it."
Good luck Ian
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Feb 23, 2014 3:05:23 GMT -5
Dancing Diamonds Gahh, I think I just ejaculated... Gahh, I think I just threw up.....
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syler
Member
Joined: January 2014
Posts: 39
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Post by syler on Feb 23, 2014 12:32:44 GMT -5
Trial and error isn't a good way of learning about such a complex topic. Experience means little because there is no guarantee you have been doing it right the whole time. In fact, the power being produced by all these motors suggests that is the case. FYI, commercial jet engines do use cooling. They do it with bypass air and with combustion chamber designs and other means. Good experimentation starts with a fundamental understanding of the science involved. You want numbers, here they are. Taken from Wikipedia.
Combustion /kəmˈbʌs.tʃən/ or burning[1] is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can produce light in the form of either glowing or a flame.
In a complete combustion reaction, a compound reacts with an oxidizing element, such as oxygen or fluorine, and the products are compounds of each element in the fuel with the oxidizing element. For example: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) The standard enthalpy of reaction for methane combustion at 298.15 K and 1 atm is −802 kJ/mol.[2] A simple example can be seen in the combustion of hydrogen and oxygen, a reaction commonly used to fuel rocket engines: 2H2(g) + O2(g) → 2H2O(g) The result is water vapor, with a standard enthalpy of reaction at 298.15 K and 1 atm of −242 kJ/mol.[2]
Notice that the units are in KJ/mol. A mole, is a unit of 6.022 X 10^23 molecules. What this means is that when we burn methane we get 802 kJ for each mole of methane we burn. We also get Co2 + water which as you know has mass.
So as you see, heat (work) is a function of how much fuel you burn. Now, as in the Carnot Cycle, a change in temp of A GIVEN PORTION OF GAS is translated into work. BUT, optimally heating one mole of gas as much as possible is a mere constellation prize when you can process twice as many moles. Plus, you can even conserve that benefit if you can keep your steel cool at the same time the heat of the gas rises. Reverse design combustion chambers do this.
The point is, if we keep our steel cool, we can ultimately pass more moles which as you see above, does more work. Might a little be lost from cooling - sure. But it is a fair price to pay for doubling output. An AB constructed with heat sink rings on the outside would provide strength and cooling and might allow use of nitrous without destruction. You don't think a shot of nitrous will add power? With cooler, stronger steel, you could use it. The idea is simple, gas goes to higher temp but improved steel doesn't.
Another easy way to do this would be simple water injection. While this isn't feasible for an aircraft, it ought to be a fantastic benefit on a go cart. Or how about this? How about a flame tube with a valve for cooling air? That way you could over clock the entire motor for short bursts and then cool the turbine when max power isn't needed.
If we repeat what everyone else on the internet is doing or trust that some guy who published a toy jet engine book (which anyone can do these days) has all the answers, we end up with no more than what we have. If that is all that can be had, one would be smarter to put around with a chain saw motor and use an old CO2 bottle plumbed with nitrous for a simple rocket thruster.
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Post by finiteparts on Feb 23, 2014 13:11:52 GMT -5
I understand what you guys are saying. I also understand the conventional wisdom floating around on these boards. What I'm trying to do is iron out some fundamental misunderstandings. First, commercial manufacturers always have numerous engineering considerations so you can't necessarily presume their designs are always the best for our purpose. I have a friend who is a professor of embedded system designs and his work is in nearly every car on the road today. I hear about it all the time. It seems that a lot of you are under the belief that change in temp = change in heat. That is incorrect. One can increase heat 3 fold while decreasing temperature. Work, is a matter of heat EXTENSIVITY measured in Joules. Temp is a measure of INTENSITY measured in C. Two entirely different animals. Also, the work being done in an IC motor comes from the repulsive force of the newly formed combustion products. When these new products form, they give off heat which has an intensive quality and an extensive quality. The heat comes from the molecular changes and physical chemistry involved. The heat is therefore a byproduct. Now, the heat does help expand our newly formed gasses, so it gives a bonus. And yes, according to Charles' law, if you reduce temp C, you reduce pressure. The problem - our engine components such as AB pipe have a specific heat capacity. That means that according to the material and surface area, the metal is able to give off only so much heat. As it becomes heat soaked, the temp will rise until it reaches the temp inside at which point it weakens. If the metal has sufficient surface area to give off 2X the heat, it could burn 2X the fuel and oxygen, and give 2x the power while remaining within feasible temp range. The problem appears to be that somehow heat in joules became convoluted with temp in C. Temp C has zero to do with work output. Joules are what you need. An AB that can radiate 2x the heat can pass 2x the joules at the same temp. That means zero temp rise but 2x the work. Physics doesn't lie - experience usually does. I debated if it was even worth my effort to comment on this, but the pointed nature of these comments, that seems to imply that the users of this forum are somehow uninformed, made me feel that I should. First, the guys on here are making some really cool projects and do not deserve to be talked down to...making it happen verses thinking about wins in my book everytime! So syler, what have you built? Second, I see that you have finally stopped using "conformal"...but you still seem to think that the big volume change is due to intermolecular forces even at the pressure of an IC engine...they may be of some significance for a single gas being compressed, but the major driver is the change in density due to the temp change and the fact that complex combustion reactions are occurring where the molecules formed after combustion are primarily much smaller than the original molecules, makes me really doubtful that the intermolecular forces even have the small effect noticed in single gas compression tests...not to mention, in the past you were saying that it was more important than even phase changes, which is total BS in my book...have you found anything that you can share to support your idea? I would really like to see it and those Wiki articles on combustion. Third, an intensive property is one that does not depend on the mass in the system, where an extensive property does depend on the mass of the system. Temperature, pressure and density are intensive and total volume in an extensive property, so I agree with you there, but I am not sure that really has to anything to do with the argument. Work is defined as a force applied over a distance, so it is easy to see how it can be converted to pressure times a change in volume. Work is a form of energy transfer to or from a system and thus is not a property of the system. You can't say that a pound of air contains a certain amount of work and thus it is neither an extensive or intensive property of the system. The basic statement of the first law of thermodynamics states that work and heat are equivalent...so similarly, heat is not a property of the system and thus it is neither an EXTENSIVE OR INTENSIVE PROPERTY! Now to the "change in temp = change in heat", I think you meant change in temp = heat. By definition the units of work are based on temperature. James Joule did the work to find the proportionality factor between work and heat, (778.17*ft-lbf = 1 Btu)...where one Btu is defined as the energy required to raise 1 lbm of water from 59.5F to 60.5F or in the metric version, the energy required to raise 1 gr of water from 14.5 C to 15.5 C. When the SI took over, the calorie was changed to the derived unit the Joule = 1 Nm, so that 1 C = 4.184 J. The whole basis of the work done by Joule and others was to describe the mechanical equivalent of heat and move from the archaic caloric theory of matter. But even moving past the units, the first law of thermodynamics may gives us some insight here. If we assume a steady state process with negligible changes in kinetic and potential energy we get that the work done on the control volume equals the heat transferred across the boundary. Since work and heat can be described as the change in internal energy (U), we get U2-U1 = Cv(T2-T1). So it seems that heat and temperature are related. Even the heat capacity is a impacted by a change in temperature, or HEAT. Since the constant volume heat capacity can be calculated as Cv=1/2(nR(3+vr*+2vv*) where vr* is the number of active rotational modes available and vv* is the number of vibrational modes available to the molecule as a function of temp. In case you are unfamiliar with this, as a molecule is subjected to radiation, it interacts with the molecule and transfers energy to the molecule in the form of increased translation energy (kinetic energy, KE = 1/2(mv^2)), vibrational energy, rotational energy or finally electrical energy. When a molecule gains kinetic energy, the increased translation velocity is what we call temperature. When a group of molecules meets another group of molecules with a different kinetic energy, the exchange of kinetic energy is referred to as heat flow. When a molecule absorbs the radiation energy or impact energy of colliding molecules as vibrational, rotational or electronic energy, the temperature rise is limited since that energy does not contribute to the translation speed and are often given off as IR, UV or other spectral emissions (thus the use of spectroscopy to measure atomic and molecular energies). So, your statement "Temp C has zero to do with work output." is sort of correct, but a change in temp, which is what everyone here is talking about, has everything to do with work output! You are correct about cooling the components allows them to mechanically be more capable, but you are not looking at the whole system. There is only a certain amount of heat that you can introduce into the afterburner before it thermally chokes. Compressible flows accelerate as heat is added to the system and once the flow reaches sonic velocity at the nozzle exit plane, no upstream changes can push any more flow through the nozzle. Additionally, burning twice the fuel for a given flow rate (and thus a given amount of oxygen) would suggest that you are trying to burn hotter, thus a temp thing....that being said, burning fuel in the vitiated air of the turbine exhaust is limited to even closer flammability limits due to the reduce oxygen, so just adding fuel might push you into a rich condition that just causes unburned fuel to exit the AB. Another consideration of the system is the compressor operability. The sudden backpressure of the AB igniting can send a pressure wave back upstream to the compressor, which then suddenly sees a higher pressure ratio for the same mass flow. This may push the compressor into surge if it doesn't have enough margin to the surge line, or if the diffuser is on the edge of flow separation. This was a common problem on the early J-57s in the F-100s and F-105s and the TF-40 engines installed in the FB-111 and F-14s. When the pilot pushed it into AB, depending on altitude and compressor operating condition, the ignition caused a pressure wave to move upstream from the AB causing the compressor to surge. The pilots would have to reduce poser and drop to lower altitudes to get denser air into the engine to clear the surge...not an ideal move when in combat! I really question your understanding of the fundamentals. I really would like to know what your background is on this stuff. You make statements as if you know something to be true, but your statements are way off base. I am not trying to discourage your creativity, by all means keep throwing out ideas. But when you make statements like "What I'm trying to do is iron out some fundamental misunderstandings." and then give out incorrect information, it is sort of bothersome.
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Post by finiteparts on Feb 23, 2014 14:17:21 GMT -5
Not the numbers I meant...I can balance an equation and solve for the energy. I was looking for numbers to support this comment from you, "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."
I can tell that you are not an engineer. Sometimes trial and error is the way to go, especially if the home user doesn't have access to the proper information and combustors/ABs are notoriously hard to design. I should know, I designed combustion hardware for seven years at major US engine manufacturer. There is a lot of things going on, chemical kinetics, turbulence, pressure pulsatons, mechanical life issue, etc. And yes, we do spend a lot of time and money designing and developing cooling schemes, because cooling is a direct hit on the thermal efficiency of the engine and that drive fuel economy. The holy grail over all the gas turbine industry for the past 15-20 years has been ceramics that could allow the engine parts to be uncooled. But, in commercial engines, some parts are operating in gas flows with stagnation temperatures hundreds of degrees above the metals melting point, whilst having to meet lifing that may keep them from being inspected for long periods of time.
There is a term in the industry, "analysis paralysis" that many projects fall into. The fundamentals are important and I think that the guys on here have a good grasp for what they need. I am spending a lot of time designing my next engine because I am an engineer and I love to build CAD models, do stress analysis and I have quite a collection of design literature that I assembled during my Masters work that I really want to go through. But my first engine was build from bits and pieces I made to fit and then some tweaking.
You do realize that there is a limited amount of oxygen in the mass flow from the engine exhaust with which to burn in the AB? You just can't put 2X the fuel in and get 2X out! There are other things involved...
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Post by racket on Feb 23, 2014 17:00:35 GMT -5
Hi Syler I really have to take offence at your comment that ...... " .............in fact, the power being produced by all these motors suggests that is the case........." My TV84 turbo based thrust engine produced 110 pounds of thrust from a mass flow of 1.8 lbs/sec , the thrust was very accurately measured and the mass flow through a 3.5" comp inducer is at 1.8 lbs/sec for best efficiency ...................this makes the Specific Thrust ( ST) 61.11 lbs of thrust per pound of mass flow . By comparison the Detroit Diesel Allison J71 had a ST of 62.5 , the Fairchilds J44 was at ~40 , the GE J85 was at ~54.5 , the Pratt and Whitney J60 was at 60 , the Teledyne CAE J69 at ~52 , its missile derivative at ~60 , Williams Research W24 at ~57 ..............and the venerable Detroit Diesel Allison J33 was at a rather pathetic 55 , some 10% less than my turbo based engine ..................so cut your crap and get your facts right. My Reference ............ www.amazon.com/Aircraft-Gas-Turbine-Engine-Technology/dp/0028018281/ref=sr_1_1?s=books&ie=UTF8&qid=1393191997&sr=1-1&keywords=aircraft+gas+turbine+engine+technology+treagerBuy yourself a copy ...............but beware , the bindings aren't real good , mine is falling apart after >20 years of constant reading . Another good book, with slightly better bindings that have survived the years of wear and tear, is......... www.amazon.com/Aircraft-Turbine-Powerplants-Charles-Otis/dp/0884873110/ref=sr_1_sc_1?s=books&ie=UTF8&qid=1393192325&sr=1-1-spell&keywords=aircraft+gas+turbinepowerplantsThis is also an interesting volume........... www.amazon.com/Aircraft-Missile-Propulsion-Volume-Turboprop/dp/B000QAA64Y/ref=sr_1_1?ie=UTF8&qid=1393192484&sr=8-1&keywords=aircraft+and+missile+propulsion+zucrow ..............lotsa good ideas in it Yep , I started out using "trial and error", but progressed a little over the years....................I'd suggest you purchase some of these volumes and do a bit of reading , then come back and tell us where we're going wrong :-) Cheers John
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