Compressor Surge and Choke

[syler]

I'm having a hard time wrapping my brain around the idea of putting 100% combustion power to the turbine without driving the compressor into either the surge or choke zones.  One would think that because an auto turbo is designed to work in a fairly narrow range based on car motor exhaust, and with the need for a waste gate, that the output of the compressor should massively over drive the turbo.  That of course would turn your O2 pump into a heat pump meaning you are pumping O2 sparse air into your chamber.  So what mechanism stops the turbo from going hyperbolic?

Also, why doesn't anyone use any type of intercooler? Isn't O2 dense air better in these motors? 

Anyway, my main question is if anyone has tried allowing just the right amount of combustion product to slip past the turbine?  This could be achieved by widening the exhaust outlet so you get more flow while keeping the turbo in its efficient zone.  Of course this would also mean more O2 for the afterburner.

[Johansson]

You can´t just use any turbo off a car and make a gas turbine from, a turbocharged IC engine built following the "pulse drive" idea with short primary exhaust manifold tubes and a small turbine section has a turbo that is useless for our purposes. It needs to have the right balance between compressor and turbine to work properly, that is why it can stay in the comp map happy zone all the time.

Intercooling should be done ahead of the comp inlet to be of any real use, that would increase mass flow which cooling air already passed through the compressor won´t do. Cooling the already compressed air would also take away heat from it that has to be added again by injecting extra fuel in the combustor.

Instead of widening the turbine outlet you should use a turbine suitable for the job, opening up the turbine housing clearance would only lower the efficiency of the turbine stage. And I can´t see why that would affect the amount of oxygen in the exhaust.

I know it is difficult to drop the IC engine thinking when it comes to this, my suggestion is that you get hold of some books on the subject and dig through them. Thomas Kamps has written a good book that is relatively easy to grasp.

Cheers!
/Anders

[racket]

Hi Syler

To prevent the compressors output from overpowering the turbine stage we put backpressure on the turbine stage ( jet nozzle or freepower turbine stage) lowering the pressure ratio across the turbine stage , this "inbalance" of pressure ratios "balance" the power requirements of the compressor to the power output from the turbine .............easy :-)

And/or we simply burn less fuel and feed the turbine cooler gases with less energy .............again, easy :-)

Cheers
John

[syler]

I started considering that backpressure against the turbine would keep the turbo in the proper zone. It's starting to look like these motors are really afterburners that use a turbo to input O2. That makes me wonder if pressure on both sides of the turbine could be raised. Obviously, more pressure means a faster reaction rate which means more power. There must be better designs though. If not going in an aircraft with a high bypass, it might be worth water cooling the system. A water jacket around the combustion chamber keeping things cool would mean higher possible pressure and more O2 per CFM which of course translates to a bigger bang in the end. I may not be an expert at home made jets but I do understand the chemistry of combustion and more O2 + more fuel always means more power.

[gidge348]

Jan 26, 2014 0:13:25 GMT -5 syler said:

A water jacket around the combustion chamber keeping things cool would mean higher possible pressure and more O2 per CFM which of course translates to a bigger bang in the end.

Cooling the combustor to increase power ?...... that would decrease power, if anything maybe "insulate" the combustor to maintain as much heat as possible.


That is why in high performance IC engines piston tops, combustion chambers and valves are ceramic coated to keep as much heat (energy) in the combustion area as possible and not let it dissipate into the water jacket.

Ian...

[racket]

Hi Syler

You're still thinking "automotive" .................forget pressure rise during combustion , the opposite happens in ALL gas/jet turbine engines , even those made by the big boys at GE, Allison, Pratt Whitney , Rolls Royce etc etc etc

Turbine engines are "internally cooled" , unlike auto engines which need cooling systems , even aircooled motorcycle engines have a "cooling system" in the form of fins on cylinders and heads ...........we need none :-)

Cheers
John

[syler]

I'm thinking chemistry. After combustion the heat help accelerate the expanding hot gasses. Before combustion it's a simple matter of having more O2 to combust which results in more energy. My understanding is that commercial jet engines are cooled by the bypass air. Also, their multi stage compressors must be far better at dissipating heat than a turbo.

Regardless, keeping the temps down will allow the parts to be pushed harder. If your AB could withstand super high temps, the heat would increase pressure. Regardless of what mechanism you put it through, the air from the turbo will produce more bang if it's cooler - that is non-negotiable.

Think about this. If your AB had fins which gave your AB 30% more strength and this enabled you to blast a 200HP nitrous shot into it without molten shrapnel flying through the air, would you have more power or less?

[racket]

Hi Syler

How many times do I have to tell you ..........WE CANNOT INCREASE PRESSURE BY BURNING FUEL .............PLEASE, STOP THINKING "AUTOMOTIVE"

When we accelerate the gases in our combustors ( pressure stays "constant" ) energy is required for to do that accelerating , in a gas turbine that requires a pressure drop (fundamental loss) to produce the accelerating energy .

Commercial jet engines use high pressure bleed air from late in the compression process for cooling the various internal parts and is reincorperated into the main air/gas flow of the engine , bypass air is the air from the fan that bypasses the combustion process , it has nothing to do with cooling the engine as its at too low a pressure .

A 200 hp nitros shot would provide an increase in thrust if injected into the afterburner because of the increased mass flow, not because of any increase in pressure or temperature ,neither of which will occur , but you may as well just construct an afterburner and burn it in that without making the gas turbine engine . .................but then it'd be a liquid fuelled rocket not an afterburning jet engine .

The gas turbine process is relatively simple , for us DIY'ers it really is only a couple of fans on a common shaft with some fuel burnt in between , but it does require us to "forget" what we learnt about turbos on a IC engine , we've all had to come to terms with that , you're not the only one , it took me years and a lot of mistakes , costly at times, to learn whats required to make a successful engine , we can shorten that process for you, thas what this Forums for ............. keep asking questions :-)

Cheers
John

[finiteparts]

The only way to get more O2 into the engine is to get more air in. Air is roughly 21% O2 by volume, so to get more O2, you would have to cool the inlet charge so that the density would drop and you could pull in more air volume. Simply cooling parts of an engine doesn't create high O2 concentrations.

Commercial engines have to use materials that had in the past been used to make turbine parts, due to the high temperatures produced when compressing air to over 40 atms or more. The higher the temperature of the air going into the combustor, the less fuel you can burn before the combustion exit exceeds the allowable turbine inlet temperature. Since the combustion is the energy input to the system, the more limited the amount of work from turbine becomes and quickly the compressor work absorbed approaches the turbine work...thus less work available to turn a large fan or produce jet exhaust. So their problems are much different than ours.

~ Chris