Heat transfer problem!

[inoxsteel]

Hello :),

I am trying to calculate the global coefficient of transference in cube in which, a side located on the ground and the back side on a separated wall 15 centimeters, circulate in frontal sense a 22.2 airflow of m/s.
The interior of cube is to a temperature of 8ºC with a speed of the air of 1 m/s circulating to 0ºC. To all this the sun of the noon with a room temperature of 42ºC is added to him.

However, when arriving at the convective coefficients, attempt to calculate it like a tube of noncylindrical section, but in the hydraulic diameter I lose myself, so that the air affects 3 of its faces (lateral and superior) and the frontal face affects (treatment directly to calculate it like a heat exchanger, but when finding me the section of air passage I realize that the section is the atmosphere.
It was using for this case the Dittus-Boelter correlation in a intervalue of 40 < L/Dh | 2300 < Re < 10^6

You would know to me to say that it forms and that correlations are used in this case?:confused:

To pardon my English but use a translator. :o

Thanks;)

[mohamed khamis]

Becus i found many difficulties to understand ur english so could u rewrite it in an understandable form and I would help u:

There are some questions should be answerd to give more understanding:

1- U mentioned ur cube is located in room temperture of 42, this room walls surface temperture or the room air temperture?,

2- The back side of the cube is placed on separte wall (thermal conductivity = ?), dimensions of this wall = ?, what is ambient temperture of the circulated air arround this wall

3- I don not know how the air will circultate with a velocity of 1 m/s inside the cube?

Anyhow My suggestion is to treat each side separtely like that:

1- The back side will be subjected to thermal resistances of Forced convection outside the 15 cm wall + conduction in 15 cm + conduction in the cube side + forced convection for the air at 0C inside the cube

use Dittus bolter equation for flow over flat plate in this case

2- The ground side is conduction from the soil + conduction thru the bottom side of the cube

3- The other will be treated as

Free convection from room air to the cube + conduction thru the cube walls + Forced convection thru the interior air

Note that in the free convection treat the issue of facing up or facing down when use the corrlations and the surface temperture of 8C is used to calacuet the bounency force in the natural convection

The overall heat transfer is the inverse of the sum of all those resistence times 6 * cube length ^2

Cheers

[inoxsteel]

Hello,

I respond to its questions:

1) The cube is located in the street, at temperature 42.

2) The back side is displaced 15cm of the wall

3) The "cube in" is cooled to 8º with a speed of air when coming out of the evaporator of 1 m/s

Thanks