Frequently Asked Questions

CM3110 Transport Processes I

Michigan Technological University

Dr. Faith A. Morrison

1. 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.
2. 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^oC is (1.0050 = Pa s 10³⁾, i.e. the viscosity of water at 20^oC is 1 X 10^-3 Pa s.
3. 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.
  

1. 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