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Frequently Asked Questions
CM3110 Transport Processes I
Michigan Technological University
Dr. Faith A. Morrison
- Administrative
- Will this be on the exam? Answer:
Everything
that we discuss, and everything in the assigned readings may be on the
exams. The exams will focus on testing if you have learned the
material
sufficiently well to be able to apply what you have learned to problems
that are different from those you have seen already.
- Is the final cumulative? Answer:
yes.
- Can I get a copy of the slides
you
use in lecture?Answer.
I have put some of the
slides up on the web. You can see them and
print them from the main
page.
- General
- What are the rules for significant figures in this class?
Answer
- How do you read the units in tables in Geankoplis when
there is a
multiplier given in the table heading? For example, how do you
read
viscosity in Geankoplis' Table A.204 on page 855? Answer.
Geankoplis
uses the method that you equate the value in the table with the heading
above the column. Thus, the viscosity of water at 20^{o}C
is (1.0050 = Pa s 10^{3)}, i.e. the viscosity of water at 20^{o}C
is 1 X 10^{-3} Pa s.
- Fluid Mechanics
- In
the Hagen-Poiseuille equation (pressure drop/flow rate for laminar flow
in a tube) is sometimes written with gravity and sometimes
without. Which is correct or which should I use? Answer
- Where does that formula for average velocity come from?
Answer
- What
is the relationship between flow rate and velocity? Answer; see also my YouTube channel, Dr.MorrisonMTU
- How do you derive the formula for that double-well
manometer on Homework
1 1999? Answer: The
derivation of that formula is given in the solution to problem
number 1 of homework 2 1999.
- It
didn't even occur to me last night to use the mechanical energy balance
to solve the second problem. Do you have
any advice on what I could review so that I don't get the problems
wrong on the remaining exams? Answer:
I'm sorry that
the exam did not
treat you well. I think it's helpful
to make a list of all your tools: MEB, macro and micro momentum
balances, Hagen-Poiseuille, Moody chart, drag coefficient chart, etc.
Then as you study, see which problems go with which methods.
Even
after you figure out which goes with which now try to figure out why
the others are NOT helpful for the problem at hand. Maybe they
are and
there are two ways to solve the problem; maybe the alternate methods
are not solvable. By looking at all problems this way, you train
yourself (like an athlete) to address new situations.
- For
the second problem on the second mini-exam, I used the mechanical
energy balance and calculated the correct pressure. However I thought I
had missed something, as the direction of flow was up the pressure
gradient. Is there a reason that the fluid would flow up a pressure
gradient, without having work done to it, or was the flow drawn
opposite the direction it would naturally occur?
Answer:
The idea that flow
only goes down a pressure gradient
comes from thinking about straight pipes. When the flow is in a
straight pipe, with the pressure driving the flow, the flow direction
is down the pressure gradient. No subtleties here. When the flow
is not in straight lines, and when the velocity is not constant (cross
sectional area changes) then the story changes. The flow enters
the expanding bend and it must slow down as the cross sectional area
grows. The volumetric flow rate is constant (Q_in=Q_out) but
because the cross sectional area is larger on the exit, the flow
decellerates (slows down; velocities were given). If we for the moment
neglect the elevation change and just look at the other parts of the
MEB with no friction, no work, we see that delta P/rho + delta v^2/2
=0. Thus, if velocity goes down, pressure must go up. Note
again, this is only true with no gravity effect, no friction, and no
work. When we add the gravity change back in there is a
modification that we can account for in a straightforward manner.
Keeping track of pressures in flows is kinda mind-bending. Please
see the "Pressure fields and fluid acceleration" video at this
website for a more complete discussion of pressures: http://web.mit.edu/hml/ncfmf.html
- I
was just looking at Fall 2008, miniexam 3, problem 1, part b and
noticed you didn't change the log mean temperature into Kelvins. In
order to cancel out the Kelvins in "U" we would need to convert that
but you left it in Celsius. I was wondering if this is an error or if
it's suppose to be left in Celsius? Answer: In
problems that include temperature with the meaning "temperature
difference", degrees Kelvin and degrees Celcius are the same. If
you have degrees Farenheit, you need to convert 1.8oF=1oC. The
equations that have "temperature difference" include Q=m cp delta T and
q/A=h (T_bulk-T_wall). Notice that the temperature units
correspond to a difference in temperatures. The offset (273.16oC)
gets subtracted away. In problems that include "temperature on a
scale" such as the ideal gas law (PV=nRT) and, as we shall see when we
study radiation, the laws for radiative heat transfer (proportional to
T(K)^4), you must use absolute temperature units. If you do not
use absolute temperature units, your answer will be wrong. For
these problems you may use K or oR (Rankine scale, which is absolute
temp for the Fareinheit degree size). If you would like a
convincing example, calculate deltaT log mean with the temps in oC and
then calculate it with the temps in K. The answer is the same.
- Heat Transfer
- Is the symbol for heat transfer coefficent h, or a U?
You
use both
and I cannot find the difference between them. Answer
- Is a 1-1 shell-and-tube heat exchanger the same as a
double-pipe
heat exchanger? Do they have the same heat-exchanger correction
factor,
F_{T}? Answer
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