Heat Transfer from a hollow sphere to air

[okimoin]

I am looking for heat transfer from a heated hollow sphere to air by convection. say sphere temperature is 40 deg air temperature is 30. what would be convective heat transfer coeffeicent for sphere? how much heat will removed from the air if sphere diameter is .15 meter?

 

[Ashereng]

Is this a homework/school assignment?

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[25362]

I estimate that if the sphere is constantly kept at 40^oC, and no radiation effects considered, the free convection htc to air would be in the range of 3-4 W/(m²K).

If it is becoming cooled with time (unsteady state heat transfer) the heat lost to air would be m[×]C[×]10,

where m is the mass of the sphere, C its heat capacity, and 10 the original difference in temperature.

In this case, the htc comes progressively down as the sphere cools and approaches the air temperature.

 

[okimoin]

ty folks. but I forgot to mention air is blowing at 5 m/s around the hollow sphere and I calculated htc which is around 130 W/m^2.K. so how much heat will loss? isn't it hct*10*A where A is the surface area of the sphere?

 

[25362]

The formula you mentioned expresses the heat transferred per unit time. It is applicable when the [Δ]T is fixed.

But if the sphere is cooling down to ambient conditions, ie, an unsteady state heat transfer condition, the total heat (no time reference) transferred to the air stream equals the heat lost by the sphere: m.C.[Δ]T.

There is practically no way to calculate analytically the htc due to the air flow (boundary layer) separation processes around the sphere.

An estimate for the htc -based on a correlation from experimental data by Hilpert- is lower than yours, around 25 W/(m²K).

 

[corus]

There will be no heat removed from the air as the sphere is at a higher temperature than the air. Conversely, the air will heat up. The sphere remains at a constant temperature presumably because it's heated. I'd look in heat transfer books for heat transfer coefficients.

corus

 

[okimoin]

25362 thanks for your tips. Infact I m gonna make a program to calculate heat balance of a hollow black sphere which is under sun. So I have to add direct solar radiation, diffuse solar radiation, longwave sky radiation and longwave radiation from surrounding surfaces as radiation input on the sphere. The sphere has 0.15 m dia. and radiation out would be heat removed by air flow and radiation hear loss of the sphere itself. I have understood other parameters except calculation of direct solar radiation and heat loss by air flow. If someone can explain me these two it would be a great help or refer me some books or programs. Thanks

 

[Ashereng]

I am curious about the application of a heated hollow black sphere in the sun, radiating heat out to the air.

Can you provide more info on the application?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[okimoin]

well Ashereng for your concern, black globe is a device widely use to estimate radiation temperature which can be used with air temperature to measure thermal comfort temperature both for indoor and outdoor spaces.Its a topic of architecture concern about human thermal environment. thanks

 

[Ashereng]

Didn't know that. Thanks.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?