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Computational Heat Transfer in Engineering Education

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 ?.

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Computational Heat Transfer in Engineering Education

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  1. Keynote lecture at International Symposium CHT- 2012 Computational Heat Transferin Engineering Education by Brian Spalding of CHAM Ltd

  2. 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? Because finite-volume thinking has not yet trickled down to pre-university level. We should help it to do so.

  3. The tank-and-tube concept embodies finite-volume thinking Upwind-differencing eluded mathematicians for many years. To ‘tank-and-tubists’ it was obvious: fluid on the upstream side of the ‘tubes’ flowed in to the ‘tank’; fluid fromwithin the tank flowed out. Not for the first time, physical intuition turned out to be at least as productive as mathematical training.

  4. A later tank-and-tube idea: the X-cell Intuition suggested that grids made of cells like this might reduce numerical diffusion. It was right In the same flow X-cells do not smear at all. Doubling the number of square cells has much less effect Square-cell grids smear badly in diagonal flow

  5. A 1954 example of finite-volume success: 1-D flame propagation Solving simultaneous differential equations was hard. Dusinberre’s 1949 book used finite volumes for heat-conduction; so why not use them for combustion too? even in graphical Schmidt-method form! Temperature profiles on the right show: (a) response to the initial discontinuity; Finite -volume thinking had solved the problem. . (b) Temperature-dependent sources then promote flame propagation.

  6. A summary of the argument to be presented Heat transfer is taught by way of differential equations because (a few) analytical solutions exist; 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 they recognised that industry-standard design software is still based on the unrealistic presumptions. This has world-wide (and bad) economic significance. Therefore CHT specialists have a duty: to promote the finite-volume formulation throughout education

  7. Education through simulation in the lecture-room Today’s teachers are ‘computer-savvy’, using lecture-room computers, Power-Point, GOOGLE searches, etc 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.

  8. What the CHT community could provide: down-loadable ‘Simulation-Scenario’ files d Example: a file for flow & heat transfer in tubes; which a ‘SimScene’-viewer package reads; and then shows this  interactions with which constitute the lecture. 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. But the input data must first be inspected.

  9. About the TubeFlow package: data input via menu Clicking on the appropriate bar and then on the seventh left-hand box causes this menu to appear. 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.

  10. Handling temperature-dependent properties Heat-transfer text-book formulae connect Nusselt, Reynolds and Prandtl numbers, each containing thermo-physical property values, treated as constants. 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.

  11. How the in-classroom simulations are performed Just click on the running man icon; then in a few seconds the results are available to be displayed. 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

  12. Some merits of the down-loadable SimScene system The lecturer needs only minimal computer skills; and he/she can deliver ‘as is’ or with own embellishments. Graphical displays make more impact on students’ minds than algebraic derivations. Moreover students can make explorations for themselves in ‘SimScene homework’ sessions. Later, as professional engineers, they will be readier to use finite-volume-based simulation for design; and will have learnt that CHT/CFD has limitations too:- computer-size needs; viz. grid-fineness effects; turbulence-model uncertainties; and human error.

  13. Example of a homework assignment 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 The student might obtain this  Not bad. But how explain the drooping of the Nusselt No curve (bottom-right)  Unravelling puzzles promotes understanding.

  14. Exploring the influence of uncertain inputs Lecturers should know enough about CHT to explain its sources of uncertainty: too-coarse grids, turbulence models, multi-phase effects. 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.

  15. 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: They represent someone’s long-ago hopeful guess; and they are copied from book to book without criticism. Likewise, figures like this,  with impossibly low Nusselt Nos.

  16. . 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 ...

  17. (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 Letcetera are dimensionless parameters. Only a SimScene package devoted to finned-tube-bundle geometries can work out the interacting influences of all parameters.

  18. How to create a SimScene package; what’s involved? Step 1. Decide whatparameters define the scenario, e.g. shapes, sizes, materials, thermal conditions. 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.

  19. How to create a SimScene package; some details 1. Knowing only the PHOENICS Input Language (PIL), I used it; but repeat: SimScenes can use any CFD engine. 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?

  20. Regarding possibility, Sir Francis Bacon wrote: “I take it those things are to be held possible which may be done by some person though not by every one; “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.

  21. Another heat-transfer-related SimScene: HeatEx Its top page looks like this: 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.

  22. The purpose of HeatEx HeatEx is designed to teach students about shell-and-tube heat exchangers, like this one: Influences of tube and baffle number and positioning are among those to be simulated.

  23. Available flow configurations As well as the text-book-standard options,: parallel-, counter- and cross-flow, it has oblique, two-baffle, leaky baffle and four-baffle options.

  24. Data-input facilities It has a menu structure similar to that of TubeFlow. Here the flow configuration is being selected.

  25. Its multiple-grid feature For economical programming and display, it uses twogrid segments to cover the same space: one for the shell- and one for the tube-side fluid.

  26. What students can learn from the HeatEx SimScene Simulations run in the classroom by the lecturer or as home work by the students reveal: 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-counterfloweffectiveness but raise the pressure drop. 4.That phase-change effects can be taken account of.

  27. Last words about simulation-scenario packages A few such packages exist; and I hope to create more. 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.

  28. The last slides The only mathematics which is essential for understanding CHT is that of the storekeeper; and he has computers to keep his books for him. So it no longer makes sense to bar from studying engineering university entrants for whom calculus is too high a hurdle. ‘Infinitesimal’ is like ‘truly fine enough’. Beyond our reach!

  29. A final speculation We laugh because medieval scholars debated how many angels could stand on the head of a pin. One day we may scornmathematicians for their obsession with the infinitesimal. But finite-volume-based simulation will survive!

  30. Thank you To AlexeyGinevskyfor conceiving and creating the SimScene-viewer package and the PIL-editor; and to all of you for your attention. to Elena Pankovafor her work on TubeFlow and HeatEx; The End

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