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Teaching Heat Transfer and Fluid Mechanics

by means of CFD

Keynote lecture atHEFAT 2012

by

Brian Spalding

of CHAM Ltd

i.e. Computational Fluid Dynamics

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The message:

Contents

To:

The proposal:

Teachers of HEFAT who don’t “do CFD”.

You can. It’s easy,

Here’s how,and better.

and why.A CFD-for- Teaching

club.

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The purpose is to prepare students

The purpose of teaching

who will design the most efficientand environment-friendly

equipment and processes.

to become engineers,

What is the best way?

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The necessary content of the teaching

The laws of conservation of mass, momentum and energyThe laws of transport : (diffusion, viscosity, heat conduction, convection and radiation).

The equations to which they lead, either finite-volume

How to solve the equations when applied to real-life processes and equipment.

or infinitesimal.

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Infinitesimal volumes were chosen. So the equations were differential ones.

How to solve the equations:1. Before the electronic computer

To understand these, mastery of differential calculus was necessary.

Therefore only the analytically adept were allowed even to study engineering.

This precluded many from entering the profession who might well have succeeded in it;

for practising engineers rarely use calculus.

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Heat transfer is still taught by way of differential equations because (a few) analytical solutions exist;

The consequences

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.Widely-used heat-exchanger-design software provides ‘correction factors’; but it is still based on pre-computer idealisations.

Basing predictions on finite-volume analysis (it will be argued) can therefore bring important benefits.

This has world-wide (and bad) economic and environmental significance.

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Another aspect of pre-computer teaching

A further mainstay of traditional teaching is reliance on dimensional analysis

e.g. this handbook item about heat transfer and pressure drop in banks of finned-tubes:

which should certainly not be relied upon because ......

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How many parameters are needed to describe a finned-tube bundle?

Here are 10 parameters for the geometry alone.Others are 3: velocities, 3 conductivities, etc.

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(1) The total number of dimensionless parameters needed for finned-tube bundles is at least 15 .

Finned-tube fact and fiction

(2) The army of experimentalists needed systematically to explore this 15-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  where a, b, c, e, g, i, k and m are constants, & D, F,

H, J & L dimensionless parameters acting independently.But detailed-geometry CFD can work out the interacting influences of all parameters for the particular case in question.

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Finite volumes can now be chosen. So the equations can be solved by arithmetic.

How to solve the equations:2. Using the electronic computer

Every store-keper can understand in- and out-flows.

Computers perform the laborious arithmetic.

And insight can be gained as well as numbers.

The unrealistic assumptions can be dispensed with.

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1. Traditional teaching demands rare analytical skills and provides little true design capability.

Summary of the argument so far

2. Finite-volume-based CFD plus sum-gobbling computers provide more-realistic design capability and make less intellectual demands.

3. FV-based heat-transfer teaching will therefore prepare more students to become better engineers.

The remainder of this lecture is devoted to how this may be achieved.

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1. Special-purpose packages (e.g. those marketed by HTRI and Aspen) which re-cast the pre-CFD design methods in software form. These can not serve the present purpose.

There are 3 kinds of heat-transfer-related computer software

2. General-purpose CFD packages (PHOENICS, FLUENT, etc). These can serve only as hidden-from-user ‘CFD engines’.

3. New!! User-adapted simulation-scenario packages, called ‘simulets’ in the printed paper.They can be down-loaded for use by lecturers and students who are not CFD specialists.

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Example: a file for flow & heat transfer in tubes.

What are down-loadable ‘Simulation-Scenario’ files?

Its 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.

A ‘SimScene’-viewer package reads it; and then shows this screen:

The lecturer uses the SS-Viewer throughout.

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 one of eight menus

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.

A closer look:

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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 computed results are ready for display.

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

which the class can then discuss.

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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 home-work’ sessions.

and will well know that CFD has its own limitations, viz:

Later, as professional engineers, they will be readier to use finite-volume-based simulation for design;

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 home-work 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: e.g. that fine grids are need for realism, as seen here:

Exploring the influence of uncertain inputs

So they can enlarge their students’ knowledge (and their own) by saying: ‘Run each turbulence model, at various Reynolds numbers; then compare results’. TubeFlow makes this easy. Here is its multi-run screen:

This 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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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 CFD code will serve.

(c) Step 4 requires knowledge of the engine’s language; but it is mechanical in essence (students can do it).

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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. Editors 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 HEFAT community? Perhaps in co-operation with like-minded others?

For, if ‘many’ participate, a cloud 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 is better perceived.

Author, date and institution would be useful additions.

.If a HEFAT-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 explored.

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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.

My offer: I will make TubeFlow and HeatEx downloadable by anyone who asks for them in the hope of spreading my enthusiasm and promoting the creation of new SimScenes.

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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 surely survive!

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and to all of you for your kind attention.

And now my thanks

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