ExamIII-key.pdf - CBE 320 Exam III, Fall 2014 NAME 100...

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CBE 320 Exam III, Fall 2014NAME100 pointsShort questions1. (8 points) The drawings labeled (a) and (b) on Figure 1 indicate the concentration profile in a thinfilm when a heterogeneous reaction is occurring at the left boundary of the film. Which of the twocorresponds to high Damk¨ohler number? Justify your answer.Figure 1: Diffusion and heterogeneous reaction in a thin film. Left: sketch of system. Right: concentrationprofiles in different regimes of Damk¨ohler number.2. (8) Consider the cocurrent heat exchanger depicted in Figure 2. Sketch the temperature profilesTh(z)andTc(z). Label the curves, indicatingTh1,Th2,Tc1andTc2.Figure 2: Cocurrent heat exchanger.3. (8) For heat transfer from a cylinder in cross-flow, this expression has been determined:Nu = (0.4Re1/2+ 0.06Re2/3)Pr0.4.In the absence of additional information, what would you expect the analogous expression for masstransfer to be? Define any dimensionless groups you use.4. (8) Briefly describe one mechanism for species transport other than convection or molecular (Fick’slaw) diffusion.5. (8) In the problem of transient heat conduction into a slab, the thermal boundary layer thicknessδT(t) = 4(αt)a. What is the numerical value of the exponenta? Hint: recall the dimensions ofα.

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