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ChEg 355 Transport Phenomena

Lecture 28/24/00

Mark J. McCreadyProfessor and Department Chair

Chemical EngineeringB.Ch.E. University of Delaware, 1979

Ph.D. University of Illinois, 1984mjm@nd.edu 219-631-7146

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Review from last time

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Favorite quotes• Sufficiently advanced technology is

indistinguishable from magic• An engineer can always befuddle a

financial person with technology, but a financial person can never overwhelm an engineer with numbers

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ChEg 355 homepage• http://www.nd.edu/~mjm/cheg355.html

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Favorite quotes• The more you know, the better engineer

you will be!• I knew then as I can confirm now,

nothing would ever be as hard to understand as transport phenomena and the Navier-Stokes equations. I was never again intimidated by quantitative problems and I appreciated the need to formulate simpler problems that gave useful results.

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Favorite quotesThe history of modern science has shown repeatedly that a quantitative description of nature can often be achieved most successfully by first idealizing natural phenomena, i.e. by setting up a simplified model, either physical or mathematical, which crudely describes the essential behavior while neglecting details. (In fact, one of the outstanding characteristics of great contributors to modern science has been their ability to distinguish between what is essential from what is incidental) ........"

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Who said them• A top design engineer from DuPont• A CEO of a steel company• One of the most influential chemical

engineering professors of all time• A top research/technology engineer

from Shell• An ND Cheg Alum who is a VP for

General Mills

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Previous Conclusions• The niche in fluid mechanics for chemical

engineers is understanding the effects of small scale phenomena that cause large scale outcomes including systems with reaction and mass transfer

• This course possesses great depth and should be useful to your educational development even if you do not use it day to for a living

• Cr≡ How Smart YouAre

How Smart You Think You Are

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Dimensional analysis• Outline

– A few general cogitations– Motivation

• Dealing with complex problems that do not allow solution of governing equation

• Can learn some physics the easy way• Sometimes just the easiest thing to do

– How to do it• Pick variables, form into dimensionless

groups

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Some cogitations• Chemical Engineering at Notre Dame• From an external report: 1998:• “The undergraduate program in chemical engineering is excellent. The faculty

regard undergraduates as an essential part of the overall program… • Chemical Engineering is perhaps the most demanding major in any university

because of the number and difficulty of courses in a typical program. … • This is one of the ‘elite’ chemical engineering programs with a carefully selected

student population equivalent in quality to those of the best private schools in the country. The faculty recognize this challenge and meet it.

• The chemical engineering program is tough, at least as difficult as those in the schools represented by the Panel (Princeton, Delaware, Minnesota, Penn, Caltech)…

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Course Goals• http://www.nd.edu

/~mjm/cheg355goals.html

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Dimensionless Confucius Proverb

• He who knows not and knows he knows not is a child, teach him, Cr~1

• He who knows not and knows not he knows not is a fool, shun him, Cr<<1

• He who knows and knows not he knows is asleep, awaken him, Cr>>1

• He who knows and knows he knows is wise, follow him Cr~1

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Flow past a sphereWhat is the drag?

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Typical Problem• For a CSTR, how to make it “S”?

– How fast to stir?– What size stirrer?

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Solve pipe flow• http://www.nd.edu

/~mjm/dimensional.analysis.nb• We will get some more physical

arguments, look at some (fake) data, then see how to do dimensional analysis using Mathematica

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Pipe flow data

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Notes for chalk lecture• I hope you took notes as these are

illegible!

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Chalk notes• http://www.nd.edu

/~mjm/cheg355_lect2_notes.pdf

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Conclusions• Dimensional analysis is often a good

starting point for correlating data and making a model

• If done correctly, the dimensionless groups have important physical significance as ratios of competing effects

• Number of groups comes from Buckingham Pi, G =P-D

• Procedure is straightforward, please try a couple.