ME462
Thermal Design

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Catalog Description: (Prerequisities - ES202 and ME302) Applications of the thermodynamic, heat transfer, and fluid flow principles to the modeling and design of thermal systems. These systems include pumps, fans, and heat and mass exchangers. A project which includes designing, constructing and testing a heat exchanger provides the focus for the course.


 
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  Stuff   Concept Questions    

Week 1

  • What is the default guess value for a variable in EES? What are the default upper and lower limits for a variable?
  • Does the "Viscosity" function in EES return dynamic viscosity or kinematic viscosity?
  • What is the syntax for finding the entropy of water at 101 kPa and 30°C in EES?
  • If you want to use the Parametric Table in EES, can you have as many equations as unknowns in the Equations Window? Why or why not?


Week 2

  • Starting with the ConAps energy equation, derive the Mechanical Energy Equation. Remember the assumptions and make use of these facts:
    • h = u + pv = u + p /ρ
    • emech,loss has got everything related to internal energy in it. After all, this is the Mechanical Energy Equation.
  • What is a streamline?
  • What is the difference between the energy form of the Mechanical Energy Equation and the head form? How is hloss related to eloss? How is hin related toWin,s? to Wdot,in?
  • Simply put, viscosity is nothing more than a constant of proportionality. Between what and what?
  • You have a length of pipe that is designated "schedule 40." What do you know about the pipe dimensions?
    1. Outside diameter
    2. Inside diameter
    3. Wall thickness
    4. All of the above
    5. None of the above
  • You have a length of pipe that is designated "2 inch nominal." What do you know about the pipe dimensions?
    1. Outside diameter
    2. Inside diameter
    3. Wall thickness
    4. All of the above
    5. None of the above
  • You have a length of pipe that is designated "2 inch nominal, schedule 40." What do you know about the pipe dimensions?
    1. Outside diameter
    2. Inside diameter
    3. Wall thickness
    4. All of the above
    5. None of the above
  • What is the interpretation of the Reynolds number?
  • What kind of forces dominate in laminar flow? turbulent flow?
  • In general, how does friction factor vary with Reynolds number? with relative roughness?
  • What is meant by the term complete (or wholly) turbulent flow?
  • Why are the terms "major" loss and "minor" loss sometimes misleading?
  • How do you calculate Reynolds number if your duct is not round?
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Week 3

  • How might you change the wetted perimeter of a pipe/duct without changing the flow area?
  • Let's say you see a minor loss coefficient expressed as C·fT. What do you think the the physical significance of C is?
  • When looking at parallel piping systems, we still the use the Mechanical Energy Equation, yet this equation is strictly true for 1 inlet/exit systems. How do we get around this?
  • In parallel piping systems, how does your pressure drop change in value for different routes? How does it change in form?
  • In tank draining problems, we use the M.E.E. to find exit pipe velocity. This should bug you. Why? How can we justify this tactic?
  • Describe the construction of a double pipe HXR and explain the how it works in layman's terms.
  • What is the difference between a parallel flow and a counter flow HXR? Which one has greater heat transfer potential? Why?
  • In the context of double pipe HXRs, what does the log mean temperature difference (LMTD) represent? (Don't tell me how to calculate it [although you should know that too] tell me what it means.)
  • For the same cold and warm fluid inlet and outlet temperatures, which flow arrangement has the greater LMTD, parallel or counter-flow?
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Week 4

  • Why is characterizing the UA of a heat exchanger (HXR) not a simple matter? (Hint: Consider what the UA depends on and then think about how a HXR is actually used in practice.)
  • Describe some other types of heat exchangers besides the good ole double-pipe kind.
  • The log mean temperature difference (LMTD) was specifically derived for a double pipe HXR. How can you use the LMTD method for a HXR which is not a double pipe design?
  • Let's say you've got a 2-12 shell and tube heat exchanger with 10 tubes. The shell side fluid is changing phase. Without consulting any charts, what is the LMTD correction factor?
  • Let's say you are designing a new HXR to increase the temperature of 3.0 gpm of water by 50°F. (Hey, wait a minute - you are!) Let's also say that the heating is accomplished using a known flowrate of Paratherm NF at an inlet temperature of 382°C. Do you use the LMTD-F method for sizing the HXR, or the ξ-NTU method? Why?
  • Let's say you have already built the HXR you designed in the last question. Let's also say that you have 2.5 gpm of water coming in at a known temperature as well as a known flowrate of hot oil coming in at 200°F. You want to predict q and the exit fluid temperatures. Do you use the LMTD-F method or the ξ-NTU method? Why?
  • You calculate the LMTD of a HXR and you get 0/0! What does this mean?
  • You have a heat exchanger for which you have calculated C = (mdotcp)min/(mdotcp)max to be zero. Regardless of configuration (shell-tube, double pipe, parallel flow, etc.) what is the expression for effectiveness, ξ?
  • What is the LMTD correction factor, F, for the HXR in the last question?
  • Is the converse of the last question true? That is, if you have a HXR for which the LMTD correction factor, F, is ___, does it imply that C = 0? Does it imply that the effectiveness, ξ, is independent of configuration?
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Week 5

  • Hey kids! Loads more concept questions on your classwork/homework entitled "Convection Concepts Review (Mostly)."
  • Describe the relative thickness of the thermal boundary layer to the velocity boundary layer for fluids with:
    1. Pr < 1
    2. Pr > 1
    Why is this the case?
  • What is the thermal resistance due to convection? Let us now add a fin array to our surface. How can you modify the thermal resistance to account for the increased heat transfer from the fins? Let's write an expression for the corrected thermal resistance. (Hint, fin efficiency and/or effectiveness will play a role here.
  • Sometimes adding insulation to the outside of a cylinder (or sphere) can increase heat transfer. Why? When can you ensure that adding insulation will decrease heat transfer (like you want it to)?
  • What's the basic reason why fins increase heat transfer rates?
  • Consider a length L of pipe with inside and outside radii of ri and ro, respectively. If the pipe has a conductivity k, what is the thermal resistance of the pipe for 1-D radial conduction? (Oh that's so easy, you should be insulted...)
  • Why would a HXR, designed for a specific task, perform poorly a year or so after it was originally built? How can you plan ahead for this?
  • What is fouling? How does it affect a HXR's UA, and thus its performance? What does it do to Δp?
  • When is the approximation U = (1/hi + 1/ho)-1 valid?
  • What is the purpose of adding baffles to shell and tube HXRs?
  • In a shell and tube HXR, what limits the number of tubes you can place in a shell?
  • U-bend and floating head shell and tube HXRS are both examples of a way to circumvent what problem?
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Week 6 & 7

  • When you have one or more degrees of freedom in a design problem (design variables) how can you use this to optimize your design?
  • Describe the Simple Payback Period (SPB) concept.
  • What is the difference between an average (or gross) SPB and an incremental SPB? Which one is more useful? Why?
  • Describe the Present Discounted Value (PDV) concept.
  • In general, how does PDV compare to annual savings times the number of years? When are these values equal?
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Weeks 8 & 9: 

  • How does the head delivered by a positive displacement pump vary with flowrate? How does it vary for a dynamic (momentum) pump?
  • How does the head loss in a piping system vary with flowrate?
  • Pump efficiency as defined in this course is the mechanical power that actually reaches the fluid divided by the mechanical power delivered to the pump. Is this the same as the isentropic pump efficiency you have seen in other courses (e.g., ES202, ME301)? Why or why not?
  • Pumps require a mechanical input which is usually supplied by an electrical motor. Electric motors also have conversion efficiencies. Give an expression for an overall pump efficiency where electrical power is now the input.
  • The efficiency of a particular dynamic (momentum) pump at fixed speed is measured to be 65% at flowrates of both 200 gpm and 400 gpm. Is this a good pump to use with a flowrate of 500 gpm? Why or why not?
  • A centrifugal pump of fixed size operating at 3600 rpm delivers 20 ft of head at a flowrate of 800 gpm.
    • If the flowrate is held constant, decreasing the speed to 2700 rpm will
      1. increase the delivered head rise
      2. decrease the delivered head rise
      3. have no effect on the delivered head rise
      4. any of the above
    • If the delivered head is held constant, increasing the speed to 4000 rpm will
      1. increase the delivered efficiency
      2. decrease the delivered efficiency
      3. have no effect on the delivered efficiency
      4. any of the above
  • Billy-Bob-Joe-Bob-Betty has a big hole in her back yard. Twelve meters down this hole is reservoir of water at standard conditions. (I.e., p = 1 atm, ρ = 1000 kg/m3.) Betty proposes using a pump located on the earth's surface to suck the water out of the hole. Is this a good idea? Why or why not?
  • What is the difference between suction head and suction lift?
  • Which pump arrangement, suction head or suction lift, would be more likely to cavitate? Why?
  • A particular piping system has a predicted head drop of 20 ft at a flowrate of 200 gpm. A centrifugal pump chosen for the system delivers a head rise of 22 ft at 200 gpm. Is the actual flow rate greater than, less than or equal to 200 gpm? Is the actual head drop greater than, less than or equal to 20 ft?
  • A particular piping system has a predicted head drop of 20 ft at a flowrate of 200 gpm. A centrifugal pump chosen for the system delivers a head rise of 20 ft at 180 gpm. Is the actual flow rate greater than, less than or equal to 200 gpm? Is the actual head drop greater than, less than or equal to 20 ft?
  • The time it takes Matt Smith to swim 100 yards is a function of the length of his arms, how long he has slept the night before, pool water temperature, and the starting block height. How many pi groups are needed to describe this functionality?
  • The pump affinity laws are most accurate when we assume that our pump operates at or near it's best efficiency point. Why?
  • When comparing the pump affinity laws as derived in class and those used by industry , we see that a D2 keeps popping up (or missing). What effect do you think this represents? (Hint: D2 has the dimensions of [L]2.)
  • What is velocity pressure? static pressure? total pressure? How are they related?
  • Performance curves for axial flow fans look a lot like those for pumps except for a little dip in the head (total pressure) curve at relatively low velocities. What is this dip all about?
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