# Homework Help: Question about Convective Heat Transfer Coefficient?

1. Jun 21, 2014

### LaReina

1. The problem statement, all variables and given/known data
The cross section of a wall is shown in the figure below. To reduce the heat flow, an air gap (still air) of 6cm is created between the brick wall layers of 14cm thick, each. The thermal conductivities of the brick wall and the still air are: Kair = 0.032 W/m.K and Kbrick = 0.9 W/m.K, respectively.

Calculate thermal resistance.

This is only part of the question as I have no problem with the rest. The convective heat transfer coefficient has not been given thus making it very difficult for me to even begin.

2. Relevant equations

R=1/hA
R=L/kA
Q=T1-T2/Rtotal

3. The attempt at a solution
As no convective heat transfer coefficient has been given, I can't even begin to calculate the thermal resistance. It's probably something very simple but I just can't get my head around it.

2. Jun 21, 2014

### Staff: Mentor

Hi LaRiena. Welcome to Physics Forums.

They are only asking for the thermal resistance of the wall, so the convective heat transfer resistance on either side of the wall is not relevant to the question. Since the air within the wall is still, convective heat transfer within the air compartment is negligible. This is a straightforward conduction problem.

Chet

3. Jun 22, 2014

### LaReina

Hi Chet

Correct me if I'm wrong but since the convective heat transfer coefficient is negligent would I use R=L/k instead to find the thermal resistance?

So Rtotal=((0.14/0.9) x 2) + 0.06/0.032
Rtotal=2.1861 °C/W

Last edited: Jun 22, 2014
4. Jun 22, 2014

### Staff: Mentor

Yes. This looks correct to me.

When you are talking about the possibility of convective heat transfer within the air pocket, natural convection there would increase the rate of heat transfer and reduce the heat transfer resistance of the wall.

Chet

5. Jun 23, 2014

### LaReina

Thank you so much! :)