Ack! Where to Begin! TurboFan Design

[seeking_aeolus]

I am starting to get addicted to this forum.

 

So my bind is, what is the least size flow tube to use in order to achieve 100 lb thrust force? Part of me thinks that is based on the nozzle, but if the exit is too small, then I might blow the whole thing up due to pressure, but then I want to maximize thrust meaning that the hole must be small enough to produce the most thrust. First off, I now know I cannot use the Law of conservation of momentum to start with because that requires that the net force is zero. Since the engine is to be accelerating a majority of the time, I know that I will not have a net force of zero. I guess I can determine from force and estimated mass the accelleration of the craft forward? I dunno. And then that also depends on the amount of fuel being burned, which in turn, is based on the fuel mixture ratio with the air coming in. I am just....lost now. I guess trial and error is going to have to be the best way to go.

 

For a turbofan engine:

Some constraints:

Minimum Ignition speed - No more than 75 mph

Wieght limit - 5000 lbs

Fuel efficiency - 20 mpg at least

100+ lbs thrust

Cost (materials) to produce - No more than $25,000/$5,000 Mass production

Compressor stages - No more than 5

Compression Ratio - ? I think 5:1 is decent.....?

Length - No more than 6 feet

Inlet diameter - No more than 36"

Temperature - ? But this will help determine what type of metal

Pressure(s) -? Will help determine thrust, type of connections

Assembly - Reletively easy to assemble/disassemble

Parts & Pieces :eek_big: It would make sense to custom-create (monopolize) but as simple as possible for now.

 

I will re-define these to better precision once I get to working on it.

 

I think I will just start small, and narrow it down. I will begin with mass flow rate coming in. Since the speed will be less than Mach 1, I know I dont have to worry about compressablility effects due to speed.

 

Consider this: Air Entering a 12" Inlet at 50 MPH

I think by finding the mass flow rate, I can determine what my temperatures and pressures will be. The compressure will be a challenge. I am not even touching the fan right now.

 

I wanted to show a picture of my testing engine, but, I dont feel like drawing it. I will get my wifes camera and post pics later.

 

I will post my calculation later. Tell me if you think this is a good starting point, or if you know one that is better.

 

I have already built part of a test engine that I will still try and use. The key elements to this are Mass, Distance, and Time. From this, and my handy physics book, I can remember what I need to determine Force, and then work my way from there.

 

If you have any information, feel free to drop me a note! Thanks.

 

~Seeking_Aeolus

 

 

"How strong art thou? Strong enough to ride the clouds? ~Hudson Soft

[GAHD]

Erm, I just want to make shure you understand the safty issues with building your own engine; above all else, Balance of your turbines is imperitive! With the speeds they rotate at the centrifugal force can litterally rattle an engine apart if it isn't perfectly blanced.

 

That being said I'm curious what approch you're going to take on it, are you looking at a complete from-scratch build (hope you have access to a really good macheinist & shop) or are you gonna go the junkyard route and steal turbocharges out of existing engines and convert them?

 

One big thing in reducing exit aperature to increase thrust: a tighter exit also produces more backpressure in the engine, make shure it isn't so high that it chokes-off the engine's airflow.

[seeking_aeolus]

New Update:

 

I have created an excel document that makes it easier to plug in these numbers (for all you that are OCD about units, I have already made sure I converted to the proper units), so now all I hafta do is just plug in some numbers and see what I get. I am noticing a dramatic increase in Thrust Force however, due to pressure, and yet the pressure isnt even that large! I guess what I am interpreting is that if a constant pressure is maintained despite the size increase in the nozzle, then the pressure thrust is astronomical, which makes sense, however, if you have the same flow rate going through a larger size pipe, there should be a sufficient enough pressure loss to actually lower the thrust force. (I think) I wonder if I am calculating this correctly. if you want to download the document, just email me ([email protected]) and maybe you can provide some insight. If you know anything about it, he heh.

 

Anyways, it is a fairly useful tool, because I can try different numbers out. I am still trying to determine how to find the exhaust velocity. I might need to provide the amount of force I am looking for (about 450 Newtons, or 100 lbs) and then work backwords with my known variables. And still there is a derivation of the formulas posted earlier that eliminate the need for pressures, which is one less varibla I must try to figure out! hehe.

 

Good news though. I learned why bio-diesel has so many more btu's than gasoline...or octane.

 

The chemical formula for octane is simple - C8H18, and completely burning in air yields heat, Carbon Dioxide, and Water. As we know most do not completely burn, there are still CO's and NO's and C's and O's and what have you.

 

Well!, Bio-Deisel consisting of soybean fatty acid compounds consists of the following in these percentages (I have neglected 11% I know)

 

For every 100% of Soy based biodeisel, 12% is Palmite, 25% is Oliec, and 52% is Linoliec. These are some LOOOOONG Chemical formulas. However, the former two have a CO2 and a NH3 (Ammonia) attatched that I am sure do not burn....the ammonia might, but I have considered the heat from it negligable. We are talking for every 4 moles of Palmite, 71 Moles of Oxygen are needed to burn it; For every 4 moles of Oliec, 101 moles of Oxygen is needed to burn it; and for every mole of Linoliec, 15 moles of oxygen are needed!! This makes the fuel ratio as such: 200 moles Soy Based Fuel : 4157 moles of oxygen!! This brings the fuel ratio to .048, which is OUTSTANDING fuel economy. However, this requires a cold-starting system much like standard diesel engines. But imagine! traveling 300 miles on 8 gals of fuel that only costs virtually $1.75 per gallon! SWEET.

 

I found this to be a fairly neat understanding.

 

More later, let me know if you know something I dont, pertaining the the upper part of this log.

 

~seeking_aeolus

[seeking_aeolus]

Erm, I just want to make shure you understand the safty issues with building your own engine; above all else, Balance of your turbines is imperitive! With the speeds they rotate at the centrifugal force can litterally rattle an engine apart if it isn't perfectly blanced.

 

That being said I'm curious what approch you're going to take on it, are you looking at a complete from-scratch build (hope you have access to a really good macheinist & shop) or are you gonna go the junkyard route and steal turbocharges out of existing engines and convert them?

 

One big thing in reducing exit aperature to increase thrust: a tighter exit also produces more backpressure in the engine, make shure it isn't so high that it chokes-off the engine's airflow.

 

To Gahd,

 

I realize what you mean about proper balancing of the drive-shaft, turbine assembly, fan assembly, however, I have also been considering this:

 

On a larger scale, the big engines that produce 30,000 lbs of thrust are more prone to have higher mass due to mounting systems, bolts, nuts, the works. Also, for example, since all of the parts are much bigger (the fan itself being over 72" in diameter, there is a much higher angular momentum, due to the mass. If you reduce the mass, (by means of reducing blade count, blade density, blade length, width, height, etc.), but maintain the same angular velocity, you get less angular momentum, yet the same amount of comparable power output. If the same strength rated mounting system could be used, then it would not be nearly as hazardous to have a slight imbalance in the fan, drive-shaft, axial compressor, turbine assembly, etc. The tangential Force is a lot less, the torque arm from the length of the blade along the moment of inertia (center of mass, longitudinal axis, whatever tickles your pickle) has less torque output; all of these factors are affecting vibration due to imbalance.

 

I'm not sure about vibration forces, but analyzing the angular velocity and momentum would have me find the Force, which would probably be a tangential force at any point where the imbalance is a maximum. Much like the balancing of a new tire at the auto shop. I suppose I could use the wheel balancing machine at my uncles' shop, maybe add the wieghts necessary, once this thing gets finished...?

 

As far as the blowing-up (back-pressure) is concerned, I will give myself about 50 feet, and a block wall in between me and my power plant haa haa....

Really, I will actually start with a nozzle that has only a slight throat and work my way down, measuring the position vs. time, which will give me acceleration, and by multiplying mass the force. When this result is most closest to the theoretical value, I will stop. I will figure out what Exit Area where my exhaust gases reach MACH 1, and then I will know that there are compressability effects present which will not help the flow, only choke it (back-pressure). I will also put 200 lb spring-check valves before the combustion chamber, with a 100 lb test bleed valve on the chamber to automatically discharge the pressure if it gets that high. (actually I will have 4) If I use 150 lb steam rated fittings, then I shouldn't have a problem!

 

Chances are, to scale, for what I am working on, I will not get a nozzle that I can accelerate the gases to Mach 1 exiting the combustion chamber, simply because the pipe companies dont make one THAT small.

 

write back, do you think my hypothosis is correct regarding the balance principle...or does that sound totally crazy?

 

~seeking_aeolus

 

"How strong art thou? Strong enough to ride the clouds?" ~Hudson Soft

[GAHD]

To me the balance issue is one of the most important things, but thats from having a ex-race car engine mechanic constantly drill into my head that everything needs to be as balanced as possible.

 

Consider this scenario, there is a slight imbalance in the turbine you've mounted in the engine, we'll say it's because the machinist who ground out the bearing-groove ground it to within 6 one thousandths of an inch(haphazard number) of spec for one half, but the other half was a sloppy 1 onehundreth of an inch(haphazard number) too deep. You fire up the turbine, it gets going to 2200 Rpm with no real noticeable problems, so you feed it more fuel. We'll say the miss-grind produces a (haphazard number) net 1 newton destabilized force, but you don't notice this 1 newton vibration because of the rediculous noise the engine is making and the vibrations caused by the turbulence of the air. At this point everything the turbine is connected to is getting a 1 newton shock 2200+ times per minute.

 

Now consider what the difference between 1 newton of force applied constantly, vs 1 newton of force jack hammering 2200 times per second will do.

 

So after running it here for a few minutes you decide to feed it more gas. The turbine goes up in RPM smoothly, and as it does that one measly newton of force goes higher too, as well it's times per second affecting the mounting brackets go up too. The stress on the metal is cumulative, the miss-ground half lets the slip keep happening and the mounting points slowly get nicked and dinged, the bearings get little pits and warps from being alternately slammed into the grooves until they can't so easily go through the smooth section and start tearing it up, at this point you might notice a fuel enconomy loss, but probably chalk it up to something in the air mix. Eventually though one of the bearings seizes, then explodes as the force of the shaft forces it to wrench through the track it now doesn't fit, the peices of hot bearing interfere with the other bearings and do eventually lock that section of shaft to the mounting point, but all along the shaft the sudden stop is just too much for the shaft's material to withstand. The torque twists and splinters the shaft, sending your carefully crafted blades into the ducting walls of the engine bending and snapping them while all the while they chew through the casing itself like a saw-blade. The various peises rip loose and explode out either side as buckshot if the container walls are strong, otherwise they rip through the walls and fly outwards at speeds comparable to long-barrel and high caliber rifles.

 

I don't want to be a doom-sayer but I *do* want to stress that accuracy in every aspect (from grinding to balance) is a necessity with any high-rev machine. Take a tour of your local race track's pits, talk to the crews (especially small-block high-rev crews) about the importance of balance in every aspect from pston to crankshaft to drive shaft. if *any* one of those things is out by more than a few hundred thousandths of (insert measure here) it can literally rattle and fatigue the metal until one day you see the shaft explode and spike into the tarmac, or get a piston blowing clear through the engine and landing about a minute or two later.

 

Ask any seasoned machinist to take a grinding disk designed for say, 600 RPM and use one that has just a tiny piece chipped out of it. If they still have all their fingers and eyes they probably know not to, if they don't have all their fingers and eyes they probably tried to at some point and now know that damaged disks are paper weights at best. A grinding disk in this manner directly simulates what happens when you take anything that's unbalanced up to high rev: the force multiplies exponentially and either the unbalanced item will explode due to unequal forces, or rattle the rest of the machine apart.

 

While thou sleepeth, the dread-god Murphy plotteth.

 

When playing with ANYTHING that is the one law I keep foremost in my brain and I would suggest you do too. Don't dismiss anything as negligible, eliminate it completely.

 

Edit: And no, I don't think a tire-balancing rig is sensitive enough.

[seeking_aeolus]

To me the balance issue is one of the most important things, but thats from having a ex-race car engine mechanic constantly drill into my head that everything needs to be as balanced as possible.

 

Consider this scenario, there is a slight imbalance in the turbine you've mounted in the engine, we'll say it's because the machinist who ground out the bearing-groove ground it to within 6 one thousandths of an inch(haphazard number) of spec for one half, but the other half was a sloppy 1 onehundreth of an inch(haphazard number) too deep. You fire up the turbine, it gets going to 2200 Rpm with no real noticeable problems, so you feed it more fuel. We'll say the miss-grind produces a (haphazard number) net 1 newton destabilized force, but you don't notice this 1 newton vibration because of the rediculous noise the engine is making and the vibrations caused by the turbulence of the air. At this point everything the turbine is connected to is getting a 1 newton shock 2200+ times per minute.

 

Now consider what the difference between 1 newton of force applied constantly, vs 1 newton of force jack hammering 2200 times per second will do.

 

So after running it here for a few minutes you decide to feed it more gas. The turbine goes up in RPM smoothly, and as it does that one measly newton of force goes higher too, as well it's times per second affecting the mounting brackets go up too. The stress on the metal is cumulative, the miss-ground half lets the slip keep happening and the mounting points slowly get nicked and dinged, the bearings get little pits and warps from being alternately slammed into the grooves until they can't so easily go through the smooth section and start tearing it up, at this point you might notice a fuel enconomy loss, but probably chalk it up to something in the air mix. Eventually though one of the bearings seizes, then explodes as the force of the shaft forces it to wrench through the track it now doesn't fit, the peices of hot bearing interfere with the other bearings and do eventually lock that section of shaft to the mounting point, but all along the shaft the sudden stop is just too much for the shaft's material to withstand. The torque twists and splinters the shaft, sending your carefully crafted blades into the ducting walls of the engine bending and snapping them while all the while they chew through the casing itself like a saw-blade. The various peises rip loose and explode out either side as buckshot if the container walls are strong, otherwise they rip through the walls and fly outwards at speeds comparable to long-barrel and high caliber rifles.

 

I don't want to be a doom-sayer but I *do* want to stress that accuracy in every aspect (from grinding to balance) is a necessity with any high-rev machine. Take a tour of your local race track's pits, talk to the crews (especially small-block high-rev crews) about the importance of balance in every aspect from pston to crankshaft to drive shaft. if *any* one of those things is out by more than a few hundred thousandths of (insert measure here) it can literally rattle and fatigue the metal until one day you see the shaft explode and spike into the tarmac, or get a piston blowing clear through the engine and landing about a minute or two later.

 

Ask any seasoned machinist to take a grinding disk designed for say, 600 RPM and use one that has just a tiny piece chipped out of it. If they still have all their fingers and eyes they probably know not to, if they don't have all their fingers and eyes they probably tried to at some point and now know that damaged disks are paper weights at best. A grinding disk in this manner directly simulates what happens when you take anything that's unbalanced up to high rev: the force multiplies exponentially and either the unbalanced item will explode due to unequal forces, or rattle the rest of the machine apart.

 

While thou sleepeth, the dread-god Murphy plotteth.

 

When playing with ANYTHING that is the one law I keep foremost in my brain and I would suggest you do too. Don't dismiss anything as negligible, eliminate it completely.

 

Edit: And no, I don't think a tire-balancing rig is sensitive enough.

 

Well, that was a very vivid portrait of how important it is to maintain a high standard in accuracy on every aspect of this Turbofan.

 

I do appreciate it, for that helped me realize the practicality of the situation.

 

I am really not ready to tackle that beast right now anyway. Right now, my main focus is working on the design, and more specifically, determining what my componants dimensions are going to be, and the only way I know to do that is either by trial-and-error, or by using painfully educational means that someone else has already figured out!

 

No, I am not trying to rip someone off, but we learn on a building-block system anyway, if you give a 5 yr old the task of learning the word "jet" he is going to be clueless without some kind of supplimental assistance!? right?

 

Anyway, I am stuck wondering why my ambiant pressure being 13.178 psi and my exit pressure is -14.87.....is that possible??

 

hmm, I'll just keep working at it....

 

thanks, Gahd

 

~seeking_aeolus

 

"How strong art thou? Strong enough to ride the clouds?" ~Hudson Soft

[seeking_aeolus]

I am stuck trying to figure out how to calculate the flow rate of a pipe with two different diameters.....for my diffuser, I have 18" inlet, and I want to reduce down to 4" and I have no idea how to find the flow rate! help please.

 

"How strong art thou? Strong enough to ride the clouds?"