Lecture P7: Ideal Cycle Analysis


General comments

Very light attendance today (due presumably to the 16.070 exam later today):

I have lots of questions, but that is because I could not pay much attention. 16.070 is making my stomach hurt. (1 student)

No mud. Sorry, I have been thinking too much about .070 to concentrate. (1 student)

Those that attended class, saw me cover Chapter VII of the notes. The important part of this material is understanding the behavior of simple turbojet engine cycles. A necessary complication along the way is expressing the fairly straightforward expressions for thrust and efficiency in terms of principal design parameters, design contraints, and flight conditions--this makes them appear more complicated. We did two PRS questions (PRS #1, PRS#2). These questions are about at the level that I would like you to understand the behavior of turbojets. You should also know what the principal design parameters and constraints are for these engines (like compressure pressure ratio, and turbine inlet temperature).

It would nice to have a syllabus up on the web with lecture number, the main topics, and exactly what readings correspond to that lecture (1 student). I agree. Some of this is available (the syllabus and main topics--on the propulsion web page). The mapping to the lectures is not tabulated anywhere. I will fix this next time through. The small synopses I give in these mud responses should clarify it for past lectures. For the last two lectures, the material is covered in Chapter IX of the notes.

There was one comment on the cell phone technology. Keep these coming (that is why we are trying these, to get student and instructor feedback):

I don't like the "You lost me" button because typically if I'm lost, if I wait a few mintues you explain whatever it was I was lost about and then there's no need for alarm. Maybe if there was an "I'm unlost" button as well. (1 student) Note that the way Eric currently has it set up, the system only holds on to the "I'm lost" signals from the last ten minutes, signals that are older than that are flushed.


Responses to 'Muddiest Part of the Lecture Cards'

(30 respondents, 42 students in class)

1) What kind of metals or alloys do they use for these compressor blades? Turbine blades as well? (1 student) I will look up some specifics for you when I get back into the office.

2) What are the advantages of a two-spool bypass engine? (1 student) Greater flexibility to meet design goals. This is particularly true for bypass engines. It is desirable to spin the larger blades at lower rotational speeds (to keep the tip Mach number from being too high and to limit stresses at the blade root among other reasons). If all the blades were on the same spool they would all have to spin at the same rotational speed. This means that the performance of more of the blades would have to be compromised. Even with two spools you end up having to make a compromise on the best speed to spin at. Note Rolls-Royce builds three-spool engines!

3) Maybe I was wrong, but what was t in the T/mdot-ao equation? (1 student) Tau-sub-c was the stagnation temperature ratio across the compressor.

4) Turbojet performance graph. What does the intersection of the curves mean? The fonts on the viewgraphs are too small to see. (1 student). Good comment about the font, I need to fix these. I am not sure which intersection on which chart you are referring to. Ask me again during recitation or after lecture and I will see if I can clear it up.

5) How does the F119 engine fit into those curves? (1 student) I will look up some specifics for you when I get back into the office. Does a ramjet just use the ram pressure and completely bypass the compressor? (same student) Yes it does. Ramjets do not have compressors. Of course, then they only work at high speeds, so you need a way to get them moving first (like a rocket or some other type of propulsion system).

6) 1) f=mdot-f/mdot seems a little redundant. Why do we use such a simple relationship? (1 student). You are right, we get rid of one variable by introducing another. However, this is the conventional way it is written. Could you explain again why the thermal efficiency does not have a steep a grade as propulsive and overall efficiency? (Same student) First, overall efficiency is the product of thermal and propulsive efficiency, so its behavior is set by the behavior of the thermal and propulsive efficiency. The relative slopes of these two parameters as a function of design parameters (e.g. compressor pressure ratio) and flight conditions (e.g. Mach number) is a function of the specific engine. That is, it will be different for turbojets versus high bypass ratio engines. And different for different design paramters. So for the turbojet, it just happens that the change in thermal efficiency with Mach number (due to increased total temperature rise because of the ram effect) is less significant that the reduction in ue/uo with changes in flight Mach number. You can see this by looking at the dependencies on flight Mach number in the various equations.

7) Why is eta-thermal dependent on Mach number? (1 student). Thermal efficiency depends on the total temperature rise from ambient to the back of the compressor. This is caused by two effects: the stagnation temperature rise due to flight at M>0 (from the isentropic relationship), and the stagnation temperature rise due to the work added to the flow in the compressor.

8) No mud. What do the different colors on the engine graph mean? Same produces, Family/series?. (1 student) Each color is for a different family of engines. Is the T/mdot-ao equation the same for M>1 and M<1?. (same student). Yes it is. Note it is also valid for M=0 (and gives a finite value even though it looks like it doesn't--thetao goes to 1).

9) No mud. Is there a lot of turbulence in the combustor and does it affect combustion? (1 student).The combustor is one of the most turbulent parts of the engine--by design. The turbulence helps mix the fuel with the air (very quickly, like a big blender). Also note that in parts of the combustor the flow is designed to recirculate (i.e. form an eddy where the flow goes backwards). This is necessary because the flame speeds are on the order of 1 to 10 m/s. So low speed regions of the flow must exist to act as flameholding sites--keeping the flame from blowing out (this happens if the flow speed is greater than the flame speed--like blowing out a match).

10) No mud (23 students).