# Homework Help: Calculating temperature and heat flow in a wall

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1. Mar 23, 2015

### bohire

1. The problem statement, all variables and given/known data
Consider the following multi-layer wall consisting of an interior insulation layer and an external light-weight concrete layer. Calculate the heat flow thorugh the structure and the temperature at the interface between the two materials.

Layer 1: Thermal insulation; Thickness (d): 100mm; λ: 0.033W/mK
Layer 2: Light-weight concrete; Thickness (d): 100mm; λ: 0.14W/mK

2. Relevant equations
Heat flow: Q=K*(T1-T2) [W] (But no temperature is given in the question)
Conductance: K=λ*A/d

3. The attempt at a solution
I don't know how to get there but the answer is Q=40.1W and Tm=-4.3C°.
I don't know which equations to use because there is no temperature nor wall area given in the question.

Last edited: Mar 23, 2015
2. Mar 23, 2015

### Svein

Your formula for conductance is wrong, it should be λ/d (dimensional analysis). So the thermal resistance is d/λ, giving Rθ1=0.1/0.033°K/W≈3°K/W and Rθ2=0.1/0.14°K/W=0.714°K/W. Thus the wall has a thermal resistance of 3.714°K/W⇒thermal conductivity of ≈0.27W/°K.

3. Mar 23, 2015

### bohire

Thank you Svein but do you know where I go from there? I still need to find the heat flow and temperature between the materials.

4. Mar 23, 2015

### Staff: Mentor

Actually, your equation for the conductance was correct, and Svein was not correct. He was mistaking Q for the heat flux rather than the total heat flow.

The key to this problem is recognizing that the same rate of heat flow Q passes through both wall layers. So you can do each wall separately, and then combine the results to get the heat flow Q and then the temperature Tm between the materials. Let layer 1 be to the left, and let layer 2 be to the right. The two temperatures across wall 1 are TL and Tm, and the two temperature across wall 2 are Tm and TR. Write your heat conduction equation (algebraically) for each of these layers, using the same Q for each. Now, for each layer, solve for the temperature difference across the layer. Then add the temperature differences together to eliminate Tm. This will give you an equation that you can use to solve for Q. Let's see what you come up with.

Chet

5. Mar 24, 2015

### Svein

OK. The last time I did heat flow was in 1964.

6. Mar 24, 2015

### bohire

Thank you for your answer Chet, this is how far I get
K11*A/d1=0.033A/0.1=0.33A
K22*A/d2=0.14A/0.1=1.4A

Q=0.33A(TL-TM)=1.4A(TM-TR)
Not sure if I'm supposed to do the rest or how I'm supposed to eliminate TM
1.4/0.33≈4.24
TL-TM=4.24(TM-TR)
... TM=(TL+4.24TR)/5.4
Then I'm pretty much stuck..

Last edited: Mar 24, 2015
7. Mar 24, 2015

### Staff: Mentor

Didn't they tell you what TL and TR are equal to? How do they expect you to do the problem if they don't tell you the inside and outside wall temperatures?

Chet

8. Mar 24, 2015

### bohire

No and that is exactly why I'm struggling to solve it

I also tried this
from Q=0.33A(TL-TM)=1.4A(TM-TR)
I get TM=TL-Q/0.33A And TM=Q/(1.4A)-TR
⇒TL-Q/0.33A=Q/(1.4A)-TR ... ⇒ Q=(A/3.74)(TL-TR)
But the problem with the missing temperatures still remains.. Do you think that the question might be fault or impossible to solve because no temperature is given?

9. Mar 24, 2015

### Staff: Mentor

Undoubtedly.

Chet

10. Mar 24, 2015

### bohire

Well that's pretty irritating.. I also see now that all the other exercises in the same chapter do give us interior and exterior temperatures. Anyway thank you for your time and have a good day.

11. Mar 24, 2015

### CWatters

The temperatures aren't the only thing missing.

How can the answer be 40.1W if they don't give you the area of the walls? I believe the answer should be in W/m^2 (Watts per square meter).

12. Mar 24, 2015

### bohire

I have no idea but I doubble checked and 40.1W is the answer given. All that is given in the question is in my original post. I guess Carl-Eric Hagentoft (author of the book) is to blame for coming up with a nonsense exercise.

13. Mar 24, 2015

### CWatters

Not unusual for there to be errors in books. Is there an earlier problem that might have provided the temperatures and wall area?

14. Mar 24, 2015

### bohire

Yes, all the other ones in that chapter did.