Relation between Pressure and Convection Heat Transfer 1

[adgirard]

I would like to know the relation between Pressure and Convection Heat Transfer. Especially the rough pressure when the convection becomes less significant. I did not find satisfying results on the forum.

I have to study the global heat transfer through the wall (5 mm) of a cold chamber at 5 degree C, low pressure, and the environment at 22 degree C, atmospheric pressure.

Thank you very much for any help or suggestion.

 

[25362]

One of the following threads may be helpful:

thread391-255227
thread391-332444
thread378-232020

 

[adgirard]

Hello,

Sorry for misunderstanding and thus confusing a little bit. I will make it more clear, I hope.

As far as I understood, for insulating purpose, at normal to low pressure (lets say 1000 Pa, I cannot reach less probably) the main heat transfer between two plate is convection. Then, if I want to reduce the heat transfer, I first have to reduce the size of the convective cells between the 2 panels, right ?

And if I increase the distance between the 2 walls, the heat transfer will in fact be increased due to the characteristic length in the convection transfer?
Thus, for air (low pressure) insulation the main factor in more to reduce the characteristic length of convection rather than reduce the pressure ?
Is there really any relevant convection reduction when I reduce the pressure ?

Thank you a lot

 

[adgirard]

Ok, I apologize I misunderstood the way to ask question here because of course like every beginner I did not imagine that it was so complicated. Let me please ask my question again, in a more convenient way. I hope it will be Ok. I also read more stuff on this subject, thanks to 25362.

I try to figured out how heat transfer will occurs in my specific case. I am doing research to insulate a box and thus there is a lot of parameters that can change, because I will adapt the box according to the solution.

What I cannot change:
The inside of the box has to be kept at 5 degree C and the outside temperature is at 22 degree C. Then I have a box with dimension equal to 0.5 x 0.5 x 0.5 [m] in aluminium and the thickness of the panels is equal to 10 mm. There is no forced convection, neither inside or outside the box.

I though about two solutions to make a better insulation. The two solutions are apparently not good according to what I read in some posts, but it does not matter I would like to know how to break these idea.

1) Vacuum panel : low pressure available cannot be less than 500 Pa. So a simple sandwich layers : alu 1 mm, vacuum 8 mm, alu 1 mm.

2) Vacuum inside the box : low pressure available cannot be less than 500 Pa.

As far as I investigated a 500 Pa pressure will not affect at all conduction, and the convection will then depends on the characteristic length, for instance the solution "vacuum box" is then probably worth than the first one. Right ?
And basically, if I remove convection effect (e.g. with honey comb) I can keep atm pressure and thus simplify the solution.

I saw as well that radiation could be the main heat transfer sometimes, does it could be the case here ? Is there a way to neglect it without checking it properly ?

Thank you very much for your suggestions,
And once again sorry for my first post, I really thought the answer could be very simple.

 

[IRstuff]

The convection coefficient does drop as a function of pressure, but it's pretty slowly varying, and at 1000 Pa, it's still probably close to natural convection.

Several points:
> VIPs don't necessarily require that you provide vacuum. Glacier Bay used to do that, but they seem to have vaporized:

http://www.kollmann-marine.com/insulation.aspx

> You have not specified how you keep the 5ºC temperature, i.e., what removes the heat? and what's its cooling capacity. THIS is what dictates how much insulation you really need.

> You have not specified how big the 5ºC object is. Presumably, the gap between it and the walls can be filled with polystyrene foam, or even aerogel.

> Multilayer insulation (MLI) is used for ultracold objects, usually in vacuum, but even in just low pressure, MLI would still work; that's the principle behind layering up in cold weather. The objective is to keep the air from moving, which then limits the thermal loss/gain to the conduction of the air and the conduction of the insulation.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[adgirard]

Thank you very much for your answer.

The convection coefficient does drop as a function of pressure, but it's pretty slowly varying, and at 1000 Pa, it's still probably close to natural convection.

Could you please provide me some reliable documentation to double check?

> One of the solution I imagined is using an active vacuum panel, which means (for me) 2 sheets of aluminium airtight sealed and a low pressure inside. Nothing else. I do know that there is other way (probably best) to make vacuum panel. It was just an other idea, and I want to understand why it is not a good one.

> I did not specified how the temperature is kept because it is an other problem, and I want to understand technologies for insulation and technologies for cooling separately. Actually, I will preferably compensate the insulation by a cooling principle, thus first I need to insulate it well.

> No the box cannot be filled with foam or other stuff. I need to place something inside but the size may vary and is thus not an important material. It could be separated by tray like in a fridge.

Thank you very much

 

[IRstuff]

"I did not specified how the temperature is kept because it is an other problem, and I want to understand technologies for insulation and technologies for cooling separately. Actually, I will preferably compensate the insulation by a cooling principle, thus first I need to insulate it well."

I suggest you do more research; this is not an academics site.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[adgirard]

Seriously I cannot get your point, I am not an academic student!

True this is my first job and this is why I lack experience in this field. But still I would like to know since my first job all the knowledge I will need for my career.

I am doing research for an industrial company and if I keep it a lit vague around this this is just because I cannot tell more information due to the policy of the company.

 

[prex]

A general expression of the heat transfer with natural convection is:
N[sub]Nu[/sub]=a(N[sub]Gr[/sub]N[sub]Pr[/sub])[sup]m[/sup] where a and m are experimental numbers.
Now the Prandtl number is quite constant, the Grashof number contains the fluid density that of course changes with pressure and the Nusselt, that contains the heat transfer coefficient, contains also the thermal conductivity of the fluid, whose change with low pressure is quite limited.
By using that relationship and a handbook for gas properties you can derive the change of h.t. coeff. with pressure, but don't expect large variations, unless you go towards vacuum.
The heat loss by radiation will always be present: mirrored surfaces are often used to reduce it.
Your option with walls made by sandwiched alu panels, with a honeycomb inside, or simply a good insulating material, and polished surfaces seems a viable solution.

prex
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[adgirard]

Thank you very much
This is definitely the kind of answer I was looking for even if I am quite forced to go through a hand book to solve it.