heat flow through a wall?

Question

determine the heat flow through a wall
area = 100m sq
250 mm thick
change in temperature (between inside and outside temperature) = 10 degrees C
inside surface convective heat transfer coefficient = 8.3
outside surface convective heat transfer coefficient = 36
thermal conductivity of wall = 0.115

ive been given a few equations and canm figure each of those out individually but i cant seem to put it all together to beable to solve this. if any one can lend a hand that would be great

Answer 1

You have to solve the problem using "resistances", just as in electricity.
The thermal resistance of a material is given by :
thickness/conductivity*surface
The thermal resistance of the convective layer at the surface of the material is :
1/ coefficient*surface
The total thermal resistance of the wall is thus :
1/8.3*100 + 0.250 / 0.115*100 + 1/36*100
=0.0232 K per Watt.
So if the difference of temperature is 10°C = 10 K, the heat flow must be 10 / 0.0232 = 430.6 Watts.
Hope it helps.
If you need some more explanations, feel free to ask!

Answer 2

Overall Heat Transmission Coefficient
The overall of heat transmission coefficient - U - can be calculated as

U = 1 / (1 / fi + x1 / k1 + 1 / fo)

where

fi = surface conductance for inside wall (W/m2K)

x = thickness of material (m)

k = thermal conductivity material (W/mK)

fo= surface conductance for outside wall (W/m2K)

Then Q = UA(T1 - T2) = Heat Flow thru the wall

where U = from above

A = surface, or wall area

(T1 - T2) = Temperature difference

Answer 3

The thermal conductivity of the cork is a linear function of temperature with values of 0.04 W/mK at 0°C and 0.3 W/mK at 100°C. so at 5°C thermal conductivity = ( 0.3 - 0.04 ) x 5 /100 = 0.013 W/mK and at 80°C thermal conductivity = ( 0.3 - 0.04 ) x 80 /100 = 0.208 W/mK average thermal conductivity = (0.013 + 0.208)/2 = 0.1105 W/mK Heat flow = 0.1105 x ( 150 /1000) x ( 80 - 5) = 1.243 W / s ans. or 1.243 joules / s ans