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Manipulating heat-transfer coefficient

Posted Feb 3, 2012, 5:56 a.m. EST Fluid & Heat, Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD), Modeling Tools & Definitions, Parameters, Variables, & Functions Version 3.5a, Version 4.0, Version 4.0a, Version 4.1, Version 4.2, Version 4.2a 7 Replies

Akmal
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Hi All,

I have 3 domains, from bottom to top iron, air and glas. Iron and Glas are defined as heat transfer in solid, Air as heat transfer in fluids. Iron being heated, the heat flow to air and glas.

i want to increase the heat flux from iron to air by manipulating the heat-transfer coefficient.

the convective cooling doesnt work here, it works only at outer boundary.

thanks. happy weekend

akmal
7 Replies Last Post Dec 11, 2015, 5:06 a.m. EST
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Feb 3, 2012, 8:20 a.m. EST
You can manipulate the thermal conductivity of the air domain instead.

Nagi Elabbasi
Veryst Engineering
You can manipulate the thermal conductivity of the air domain instead. Nagi Elabbasi Veryst Engineering
Akmal
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Posted: 1 decade ago Feb 4, 2012, 4:18 p.m. EST
Thanks for your quick answer.

The easiest thing to solve my problem is by "adjust" properties of air, but by changing it, my modell will be far than reality.

the model its about a wire (long wire, outher radius 8mm) for max 300 Ampere. no air domain are defined, but a convective cooling for horisontal cylinder is defined. To have my modell same as lab meassurement i add correction factor "ka" to the heat transfer coeficient "h". "ka" is a must because not all what happen in reality can be 100% describe in model. the correction factor are depends on wire radius. bigger radius has a bigger "ka", from 1.7 until 2.4. It means more heat are transfered to air than theoriticaly should happend.

any critics or idea to "ka"?

client want to have protection sleeve around the wire and interested on the temperatur increase that caused by protection sleeve. So i adapt my model and defined an air betwen wire and protection sleeve. here comes the problem. since i defined air, i cant apply convective cooling on the outer wire surface. i actualy looking for something on the equation that allow me to implement my "ka". so that, there are the same relation of heat amount that goes to the air. i cant find it yet.

the result, there are higher temperatur on the copper than the lab measurement (4-11 temperatur different).

any idea are welcome.

best regards
akmal
Thanks for your quick answer. The easiest thing to solve my problem is by "adjust" properties of air, but by changing it, my modell will be far than reality. the model its about a wire (long wire, outher radius 8mm) for max 300 Ampere. no air domain are defined, but a convective cooling for horisontal cylinder is defined. To have my modell same as lab meassurement i add correction factor "ka" to the heat transfer coeficient "h". "ka" is a must because not all what happen in reality can be 100% describe in model. the correction factor are depends on wire radius. bigger radius has a bigger "ka", from 1.7 until 2.4. It means more heat are transfered to air than theoriticaly should happend. any critics or idea to "ka"? client want to have protection sleeve around the wire and interested on the temperatur increase that caused by protection sleeve. So i adapt my model and defined an air betwen wire and protection sleeve. here comes the problem. since i defined air, i cant apply convective cooling on the outer wire surface. i actualy looking for something on the equation that allow me to implement my "ka". so that, there are the same relation of heat amount that goes to the air. i cant find it yet. the result, there are higher temperatur on the copper than the lab measurement (4-11 temperatur different). any idea are welcome. best regards akmal
Akmal
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Posted: 1 decade ago Feb 4, 2012, 4:21 p.m. EST
maybe i sholud add laminar flow module on my model. what do you think?
maybe i sholud add laminar flow module on my model. what do you think?
Akmal
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Posted: 1 decade ago Feb 6, 2012, 8:04 a.m. EST
Hi All,

it solved in very elegant way.

Using Form assembly to get solid and fluid virtualy separated. the solid boundary to fliud can now be used by convective cooling include "ka".

thanks to Mr Dennis Cronbach.

regards
akmal
Hi All, it solved in very elegant way. Using Form assembly to get solid and fluid virtualy separated. the solid boundary to fliud can now be used by convective cooling include "ka". thanks to Mr Dennis Cronbach. regards akmal
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Feb 6, 2012, 1:10 p.m. EST
Be careful though, when you put convection on an inner surface the heat lost (or gained) at that surface will go to the ambient not the glass at the other side of your air gap.

Nagi Elabbasi
Veryst Engineering
Be careful though, when you put convection on an inner surface the heat lost (or gained) at that surface will go to the ambient not the glass at the other side of your air gap. Nagi Elabbasi Veryst Engineering
Akmal
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Posted: 1 decade ago Feb 9, 2012, 5:00 a.m. EST
Of course, to avoid that by the convective cooling, the T.ext should be taken from temperature probe at the air domain below the acyrl, not ambient temperatur.

thanks for your input.

best regards
akmal hidayat
Of course, to avoid that by the convective cooling, the T.ext should be taken from temperature probe at the air domain below the acyrl, not ambient temperatur. thanks for your input. best regards akmal hidayat
BB
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Posted: 9 years ago Dec 11, 2015, 5:06 a.m. EST

Could you please share with us in more details how you could give air temperature as T.ext by defining a probe?
When I want to define a “boundary Probe” giving the temperature of fluid, I only have the following options as “Probe Type” in COMSOL 5.2: Average, Maximum, Minimum, and Integral. Therefore, the convection heat flux, h(T_solid – T_fluid), cannot be calculated correctly without taking the variation in the fluid’s temperature along its path with any of the available “Probe Types”.
Could you please share with us in more details how you could give air temperature as T.ext by defining a probe? When I want to define a “boundary Probe” giving the temperature of fluid, I only have the following options as “Probe Type” in COMSOL 5.2: Average, Maximum, Minimum, and Integral. Therefore, the convection heat flux, h(T_solid – T_fluid), cannot be calculated correctly without taking the variation in the fluid’s temperature along its path with any of the available “Probe Types”.

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