Quote:
Originally Posted by tkelly27
What school do you go to? I'm not going to say you're wrong, you obviously work with the thing. However, I've got my thermodynamics book in front of me, turbines are modeled as isentropic (geez, it's been a while, isn't that the right word?).
"Heat transfer from turbines is usually considered negligible..." and if you think about the amount of insulating air around the thing, it makes sense.
Also, it says that the change in pressure at the same temperature causes a change in enthalpy, and the kinetic energy (opposite of the compressor which accelerates the particles, the turbine decelerates them).
Furthermore, EGT gauge manufacturers advise that thet the sensor can be placed up to 2" from the turbine with a drop of only 200 degrees F
http://www.autometer.com/productPDF/2650-1088.pdf
"If the exhaust manifold can not be removed, install the probe 1-2 inches after the turbo exhaust outlet (Exhaust gas temps could drop over 200˚ when installing after the turbo)."
If I'm reading that right, that's the difference from 1-2" from the port vs. 1-2" from the turbine outlet... So I'm not really sure where this huge temp drop comes from.
It's been a while, go easy on me  |
Hi,
I go to Virginia Tech, they got a engine lab/dyno room there which is where I and the project team I'm on do our work.
Isentropic means that that the flow is reversible and adiabatic which means there is no heat transfer coming from the turbine (adiabtatic) and no losses in the system due to friction (reversible). This obviously is not real and is an idealization. When working on turbines in power plants and stuff like that you can usually assume that there is a little amount of heat transfer becasue of the immense amount of insulation they have.
Now for turbochargers, I believe that you can not assume them adiabatic because heat transfer is driven by temperature difference, heat always flows from high temperature to low temperature. So like the exhaust is 1300-1500 degrees F and the outside air is like maybe 150 degrees in the engine bay, and the only thing between the two temperatures is the turbine housing, which is a obviously metal. Also when drving down the road the air is moving very fast and is most likely very turbulent in the engine bay so this would make the heat transfer out of the turbine even higher.
For our project we are going to bring the turbocharger upto operating speed (140,000 rpm) using compressed air heated to a certain temperature. We are going to try to get the air upto about 400 degrees F and that should be fine for running it considering that we want the outlet temp to be no less then 32 degrees Fahrenheit. We came to this temperature by assuming that heat transfer out of the turbine was zero, we did this becasue of the smaller temperature difference from air inside the turbine (maximum of 400 degrees F) to air outside the turbine, and we also might ceramic coat the turbine housing to ensure minimal heat transfer.
You are an engineer right, I figured since why the hell else would you have a thermo book, haha. If you look at the enthalpy values for say 400 degrees F (I pick this becasue this is what we are trying to use) and then compare that to the enthalpy value for air moving at 450 ft/sec. You will see that the enthalpy value for the air veolocity is very small when compared to the enthalpy value for the temperatures.
In your thermobook there are many ways to deal with turbines, you are looking at just one. If you look at other ways to do it, you will see that it will say that velocities can usuallly be ignored, which I just described to you. I would try to tell you where to look in your thermo book but I dont have mine since i'm back home for the holidays.
If this explanation isnt good enough I can try to post this part of our final report, but it is like 10 pages long and I didnt think you would want to read all that.
Oh and for the EGT, it says that it could drop over 200 degrees Fahrenheit. So 200 degrees F or greater, which represents the temperature drop through the turbo. Now like you said the temperature drop through the turbo isnt massive but it can be quite large.
Hope this helps. Merry Christmas. Sorry for spleling errors, haha.
Bill