Basic fluid flow

[PerKr]

I'm trying to understand what happens when the air is flowing inside a refrigerator but without knowing the basics, that is kind of hard. All I can do is measure the temperatures and guess what is happening. That needs to change.
I need links to sites which can help me understand what happens when the air flows across different structures. I just feel like I'm spending way too much time guessing...

 

[rb1957]

i imagine that you are looking at how things cool down in a fridge ?

not much fluid flow, there isn't a gale blowing inside the fridge ... i think it's much more a problem of heat radiation.

i imagine that there's software out there (like CFD packages) that'll allow you to model the thermal problem.

 

[PerKr]

this is definitely a fluid flow problem. The product has two fans; One radial fan at the top blowing air through the evaporator and into the refrigerator compartment (sucking it back up in order to blow it through the evaporator again) and one axial fan at the bottom sucking air through the evaporator and blowing it into a low-temperature compartment. So the cooling of foodstuff is done by circulating cold air around them.

I wish it was simply a matter of heat radiation...

 

[rb1957]

ok, sorry, i was thinking of a conventional fridge.

now it looks like quite a complicated creature. I imagine that you're into some sort of CFD model. A difficulty i can see is checking the model's predictions (flow rates and temperatures throughout the fridge) ... that'll take a bunch of testing. obviously this is going to be very dependent on how the fan recirculates the air inside the fridge; maybe you'll need baffles to direct flow, maybe you'll need to reduce the solidity of some shelves to improve flow about the fridge.

anyways, good luck !

 

[Zapster]

Have you considered some books?

“Heat and Mass Transfer” and “Fluid Mechanics” both by Frank M. White

 

[crysta1c1ear]

I am just having a look in here so you can ignore this posting in favour of something by the experts, but isn't air going to flow like in a normal room, ie as follows ...

Air on a cold window is denser and drops. It moves from the high density wall to the low density wall along the floor. Sit on the floor and you can feel it - it's called a draught. Then the warmer internal wall warms the air and it rises. The flow is completed by the warmer air flowing along the ceiling to make a circuit.

Assuming the cooling radiator is at the back of the fridge and the door is warmer, then I'd assume flow in a fridge is
down the back,
along the floor towards the front,
up the front,
back along the top.

You can test the temperature at various points in the fridge with a thermometer.

If you've got an ice box at the top with initial / extra cooling there, then the air flowing along the top would cool and start to drop before it gets near the back so the top shelf would be cold for quite a distance and not just at the very back.

That's what I think is common sense, but the guys who study this should be able to correct me or say a lot more.

I'd assume that from the fridges consumption and efficiency etc one would be able to calculate how much air is cooled at the back of the fridge and thus how fast it is likely to drop, due to increased density, etc.

 

[magicme]

the airflow inside the refrigerator will change pattern every time the food is shuffled around (re-arranged).
you will never get a fixed pattern of air flow for all combinations of bottles, containers, etc.
the convection heat transfer to each item will change with each new pattern.
best you can hope for is some average flow velocity over the surfaces, at which point you can calculate "typical" convection coefficients (these will also depend on the *shape* of each item and how it is oriented to the flow).

regards,

magicme

 

[PerKr]

magicme: fortunately, we don't have to take much consideration to the way people place their food when it comes to energy consumption. This is measured in accordance with an ISO standard which states that the temperatures are measured at 3 points: 1/3 of the height from the roof, 1/3 of the height from the bottom shelf and 25mm above the bottom shelf.

Now, seeing as how I have a cabinet which apparently does not want to function properly (the energy consumption is way off when comparing to the rest of my prototypes), I'm considering to use this to take a closer look at the air flow. How would you suggest I go about this? I'm thinking I could replace the back and door of the cabinet with clear plastic (only the flat surfaces) and then use smoke and some sort of cotton thread (any other suggestions?) to visualise the flow. Of course, I would be running the fans only, disconnecting the compressors (I would have to remove critical parts of the refrigeration system anyway). Good idea? Bad idea?

 

[GregLocock]

why not use a camera rather than windows

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IRstuff]

Smoke is certainly a tried and trued approach to visualizing air flows

TTFN

 

[berkshire]

perKr

You could also try setting up shadow graphs to visualise flow over objects within your prototype.

http://www.diracdelta.co.uk/science/source/s/h/shadowgraph/source.html

I remember seeing years ago a fascinating example of the airflow around the tongue of a person eating ice cream visualised by this method.
B.E.