computational heat transfer in engineering education
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Keynote lecture at International Symposium CHT- 2012. Computational Heat Transfer in Engineering Education. by Brian Spalding of CHAM Ltd. Some preliminary ideas. Which is easier to understand ?. A finite volume ?. Or an infinitesimal one ?. - PowerPoint PPT PresentationTRANSCRIPT
BathJuly 1-6
2012
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Computational Heat Transfer
in Engineering Education
Keynote lecture atInternational Symposium
CHT- 2012
by
Brian Spalding
of CHAM Ltd
BathJuly 1-6
2012
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Some preliminary ideas
Which is easier to understand?
A finite volume?
Or an infinitesimal one?
Then why do we teach students about differential equations before finite-volume ones?
We should help it to do so.
Because finite-volume thinking has not yet trickled down to pre-university level.
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Upwind-differencing eluded mathematicians for many years.
The tank-and-tube concept embodies finite-volume thinking
fluid on the upstream side of the ‘tubes’ flowed in to the ‘tank’; fluid from within the tank flowed out.
Not for the first time, physical intuition turned out to be at least as productive as mathematical training.
To ‘tank-and-tubists’ it was obvious:
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Intuition suggested that grids made of cells like this might reduce numerical diffusion.
A later tank-and-tube idea: the X-cell
It was right
Square-cell grids smear badly in diagonal flow
Doubling the number of square cells has much less effect
In the same flow X-cells do not smear at all.
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Solving simultaneous differential equations was hard.
A 1954 example of finite-volume success:
1-D flame propagation
even in graphical Schmidt-method form!
Temperature profiles on the right show: (a) response to the initial discontinuity; (b) Temperature-dependent sources then promote flame propagation.
Dusinberre’s 1949 book used finite volumes for heat-conduction; so why not use them for combustion too?
Finite -volume thinking had solved the problem. .
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Heat transfer is taught by way of differential equations because (a few) analytical solutions exist;
A summary of the argument to be presented
but only for seldom-realistic conditions (uniform heat-transfer coefficient, temperature-independent properties).If the solutions are used for design, large ‘safety factors’ must be applied.Therefore serious designers would use finite-volume-based computer simulation instead, but only if
Therefore CHT specialists have a duty: to promote the finite-volume formulation throughout education
This has world-wide (and bad) economic significance.
they recognised that industry-standard design software is still based on the unrealistic presumptions.
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Today’s teachers are ‘computer-savvy’, using lecture-room computers, Power-Point, GOOGLE searches, etc
Education through simulation in the lecture-room
But few perform live simulations: they lack tools, knowledge and confidence.
My proposal is that, for heat-transfer lecturers, CHT can and should provide all three. I shall explain how.
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Example: a file for flow & heat transfer in tubes;
What the CHT community could provide: down-loadable
‘Simulation-Scenario’ files
d
The main window contains the start of an html file, which the lecturer is free to edit .
Above are buttons enabling him/her to do live simulations of flow in tubes.
which a ‘SimScene’-viewer package reads; and then shows this interactions with which constitute the lecture.
But the input data must first be inspected.
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Clicking on the appropriate bar and then on the seventh left-hand box causes this menu to appear.
About the TubeFlow package: data input via menu
The flow formulations which can be chosen are shown here.
The lecturer may choose to explain their meanings,
perhaps after first studying the html document.
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Heat-transfer text-book formulae connect Nusselt, Reynolds and Prandtl numbers, each containing thermo-physical property values, treated as constants.
Handling temperature-dependent properties
In reality, properties vary with temperature.
A SimScene-using lecturer can explore these effects in the class-room;
and more easily than in the laboratory; both real and fictitious fluids can be investigated.
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Just click on the running man icon; then in a few seconds the results are available to be displayed.
How the in-classroom simulations are performed
Nor need the lecturer know how to operate the graphical display package; for, when the run ends, he will see this:
Clicking on the icon will activate a macro which creates images automatically, such as:
contours of temperature
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Some merits of the
down-loadable SimScene system
Graphical displays make more impact on students’ minds than algebraic derivations.
Moreover students can make explorations for themselves in ‘SimScene homework’ sessions.
and will have learnt that CHT/CFD has limitations too:-
Later, as professional engineers, they will be readier to use finite-volume-based simulation for design;
viz. grid-fineness effects; computer-size needs;
and human error.turbulence-model uncertainties;
The lecturer needs only minimal computer skills; and he/she can deliver ‘as is’ or with own embellishments.
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The task: Use TubeFlow’s multi-run capability to compute fluid flow and heat transfer for water, at 80 degC, in fully-developed flow , for various Reynolds numbers; and explain the results
Example of a homework assignment
The student might obtain this
Not bad. But how explain the drooping of the Nusselt No curve
(bottom-right)
Unravelling puzzles promotes understanding.
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Lecturers should know enough about CHT to explain its sources of uncertainty: too-coarse grids, turbulence models, multi-phase effects.
Exploring the influence of uncertain inputs
Then they can enlarge their students’ knowledge (and their own) by saying: ‘Run each turbulence model; then compare results’.
TubeFlow makes this easy. Here is its multi-run screen:
It will launch 30 runs: 5 models for each of 6 velocities.
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Further advantages: some things
need no longer be taught
Text- and hand-books are cluttered with formula which purport (implausibly and impractically) to be useful in design. For example:
Likewise, figures like this, with impossibly low Nusselt Nos.
They represent someone’s long-ago hopeful guess; and they are copied from book to book without criticism.
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.
More fiction to be binned
Finned-tube bundle Nusselt and Euler Number formulae according to Rohsenow and Hartnett:
They are neither reliable nor credible because ...
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(1) The number of dimensionless parameters needed for finned-tube bundles should be at least 12 .
The reasons for binning
(2) The army of experimentalists needed systematically to explore this 12-dimensional space has surely never been mobilised. Nor will it ever be.
(3) Even if it had been, it is highly improbable that its findings would have fitted the always-preferred form:
Nu=a*Reb*Prc*De*Fg*Hi Jk*Lm etcetera wherein a, b, c, e, g, i, k and m are constants, and D,
F, H, J and L etcetera are dimensionless parameters.
Only a SimScene package devoted to finned-tube-bundle geometries can work out the interacting influences of all parameters.
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Step 1. Decide what parameters define the scenario, e.g. shapes, sizes, materials, thermal conditions.
How to create a SimScene package; what’s involved?
Step 2. Decide what default values (or lists) shall appear in the SimScene-viewer’s menu boxes.Step 3. Decide what CFD engine will perform the flow-simulating calculations.
Step 4. Express the above decisions in the CFD engine’s Data-Input language.
Comments: (a) Steps 1 and 2 are the creative steps
(b) Re Step 3, any general-purpose code will serve.
(c) Step 4 requires knowledge of the engine’s language; but it is mechanical in essence.
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1. Knowing only the PHOENICS Input Language (PIL), I used it; but repeat: SimScenes can use any CFD engine.
How to create a SimScene package; some details
2. There does exist a PIL editor, with macros, widgets and other aids. They may exist for other engines too.3. It is Steps 1 and 2 that require agreement on format. Commercial competition should not hinder its making.
4. The current SimScene format can of course be improved; but refinement is cheaper than replacement.
5. For those who make the same choice of engine, I can provide the Editor, and how-to-use instructions.
6. My aim is to bring into existence a ‘critical mass’ of SimScenes in a short time. Can that be made possible?
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“I take it those things are to be held possible which may be done by some person though not by every one;
Regarding possibility, Sir Francis Bacon wrote:
“and which may be done by many, though not by any one;.
“and which may be done in the succession of ages, though not within the hour-glass of any one man’s life;“and which may be done by public designation, though not by private endeavour”.
What public? Could that be the CHT community? Perhaps in co-operation with like-minded others?
For, if ‘many’ participate, a plethora of SimScenes might exist sooner than ‘succession of ages’ suggests.
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Its top page looks like this:
Another heat-transfer-related SimScene: HeatEx
If all SimScenes: have similar forms, novelty of content stands out better.
Author, date and institution would be useful additions.
.If a CHT-SimScene-Creators’ Club came into existence, a first task would be to recommend an all-fitting format.
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HeatEx is designed to teach students about shell-and-tube heat exchangers, like this one:
The purpose of HeatEx
Influences of tube and baffle number and positioning are among those to be simulated.
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As well as the text-book-standard options,: parallel-, counter- and cross-flow, it has oblique, two-baffle, leaky baffle and four-baffle options.
Available flow configurations
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It has a menu structure similar to that of TubeFlow.
Data-input facilities
Here the flow configuration is being selected.
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For economical programming and display, it uses two grid segments to cover the same space: one for the shell- and one for the tube-side fluid.
Its multiple-grid feature
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Simulations run in the classroom by the lecturer or as home work by the students reveal:
What students can learn from the HeatEx SimScene
1. That finite-volume-based simulations fit the text-book formulae closely enough, if the grid is sufficiently fine.
2. That contours of temperature in cross-flow exchangers may look like this 3.That baffles bring close-to-counterflow effectiveness but raise the pressure drop.
4.That phase-change effects can be taken account of.
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A few such packages exist; and I hope to create more.
Last words about simulation-scenario packages
But hundreds are needed, for the teaching of all relevant fluid-flow, heat/mass transfer topics; and my personal ‘hourglass’ will certainly not suffice.
I hope therefore to have conveyed the vision clearly enough for some of you to share it; and to desire to turn it into reality.
Finally, I disclose that there is a sociological aspect to my ambition: properly considered, the ‘trickle down’ of finite-volume thinking into secondary schools can widen the entrance doorway of the engineering profession.
BathJuly 1-6
2012
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The only mathematics which is essential for understanding CHT is that of the storekeeper;
The last slides
‘Infinitesimal’ is like ‘truly fine enough’.
So it no longer makes sense to bar from studying engineering university entrants for whom calculus is too high a hurdle.
Beyond our reach!
and he has computers to keep his books for him.
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A final speculation
We laugh because medieval scholars debated how many angels could stand on the head of a pin.
One day we may scorn mathematicians for their obsession with the infinitesimal.
But finite-volume-based simulation will survive!
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and to all of you for your attention.
Thank you
To Alexey Ginevsky for conceiving and creating the SimScene-viewer package and the PIL-editor;
to Elena Pankova for her work on TubeFlow and HeatEx;
The End