2 replies to this topic

I'm just revising for a heat transfer test this afternoon. I think the nerves may be getting the better of me as I can't seem to answer the following, which should be a simple heat flow question

The wall of an oven is made from steel sheets with 50 mm mineral insulation between them. The internal temperature in the oven is 300°C and the room air temperature is 15°C. The thermal resistance of the steel sheets can be neglected.

Data
Film heat transfer coefficient of the hot air inside the oven: 15 W/ m2 °C
Film heat transfer coefficient of the air outside the oven: 15 W/ m2 °C
Thermal conductivity of mineral insulation: 0.08 W/ m °C

Calculate the heat flow through the oven wall (W/ m2).

I believe that the only way you can figure it out is to apply the basic heat flow equation;

q = (-k*A*deltaT)/x

where;

k =0.08 W/mC

A = we can assume it to be 1 m²

deltaT = (300-25) = 285 C

x = 0.05 m

Although my lecturer's answer is 376 W/m² _, mine is 456 W/m² , however the lecturer is certain his answer is correct.

Any advice PLEASE?

Kel

[demank]

You should calculate base on convective heat transfer.
In this case natural convection.

For example,
Churchill and Chu [Int. J. Heat Mass Transfer, 18, 1323 (1975)] recommend, for laminar flow on isothermal vertical wall follow equation: (Perry 8-05, page 5-8)

Edited by demank, 27 August 2009 - 07:10 PM.

[breizh]

kelly ,
Transfert from inside the oven (air) by convection then trough the wall by conduction and then to outside .
Q=U*A*Dt with U global heat transfer coefficient , I guess you know how to calculate U

Breizh