**General comments**

This lecture focused on introducing the steady flow energy equation (SFEE). The steady flow energy equation is an expression of conservation of mass and energy for an open thermodynamic system. It is common for people to have difficulty both with units and with signs when applying the equation to solve a problem. Therefore, I used a PRS question to emphasize the importance of getting this right. We then moved on to one of the more confusing concepts -- the distinction between flow work and other forms of external work (shaft work being the most common example). When we express the SFEE in terms of shaft work and flow work, the flow work can be combined with the internal energy terms to arrive at enthalpy. There were several questions on the mud cards about this. I have tried to clarify some of the confusion below.

**Responses to 'Muddiest Part of the Lecture Cards'**

(41 respondents out of 66 students)

1) * What exactly do Q-dot and W-dot
mean?* (1 student) Rate of heat added to system and rate of work
done by the system (units are J/s).

2)* Could you explain again which components
of the engine go with which processes?* (1 student) Please see
the notes.

3) * What is shaft work? *(1
student) A flow of energy into or out of a system via a shaft (such as a shaft
attached to a compressor or turbine). Note I use this in the equations in the
notes because it is the most common situation, but in general ws should be replaced
with all external forms of work other than flow work.

4) **How does
follow from ?*** (and other questions of confusion regarding these
equations)* (4 students) Please see the notes.

5) * Is p1=p2 in the system for the
control volume?* (1 student) No. In general the conditions at
the inlet and exit of the control volume are different.

6) * For the jet engine, in the combustor
and leg 4-1, it is obvious that q=cpdeltaT from the definition of cp, we don't
have to go through h.* (1 student) Correct.

7) * How do blades compress the air
in a gas turbine engine?* (1 student) We will learn about this
in the spring.

8) * Why do we talk about ideal gases?
Do they really exist?* (1 student) Most gases we deal with behave
as ideal gases most of the time.

9) * How can Win=-p1V1? shouldn't it
be Win=-p1dV? *(3 students) p1V1 is the integral of p1dV for pushing
a chunk of fluid into the c.v. (think of it starting at V=0 and going to V=V1
all at a constant pressure of p1).

10) * Is it correct to say that fluid
flows in and becomes part of the control volume, while fluid from the control
volume flows out the other end? So if the fluid flowing is part of the c.v.,
how is there flow work?* (1 student) There is fluid that is part
of the c.v. , but it is also flowing in and out. In doing so, it has work done
on it and does work on the surroundings.

11) * In
the SFEE why does the difference in energy equal q-w?* (1 student)
If the energy of the c.v. is not changing (it is steady), then the only flows
of energy into and out of the system are those associated with the inlet flows,
the outlet flows, the heat and the work. So they must all balance.

12) * I did not understand your last explanation
of w=-p1v1 and w=p2v2. The pressures p1 and p2 are the pressures of what? The
volumes v1 and v2 are the volumes of what? Where does the volume of the c.v.
play into this? *(2 students) p1 and v1 are the thermodynamic
state at the inlet of the c.v., p2 and v2 are at the outlet. The volume of the
c.v. does not play a part in this. The flow work comes about because each unit
of gas (say a kilogram) that is pushed into the system requires a different
amount of work (in general) than is gained from each unit of gas that is pushed
out of the system.

13) * No mud* (17 students). Good.