Question about Convective Heat Transfer Coefficient?

In summary, the problem involves calculating the thermal resistance of a wall with an air gap of 6cm between two 14cm thick brick layers. The thermal conductivities of the brick and air are given, but the convective heat transfer coefficient is not. However, since the air is still, convective heat transfer can be neglected and the thermal resistance can be calculated using R=L/k. The resulting thermal resistance is 2.1861 °C/W. There is also a possibility of natural convection within the air pocket, which would decrease the heat transfer resistance of the wall.
  • #1
LaReina
6
0

Homework Statement


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.

Homework Equations



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

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.
 
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  • #2
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
Chestermiller said:
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

Hi Chet
Thank you for your response.

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:
  • #4
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
 
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  • #5
Thank you so much! :)
 

Related to Question about Convective Heat Transfer Coefficient?

1. What is the convective heat transfer coefficient?

The convective heat transfer coefficient is a measure of the rate at which heat is transferred between a fluid and a solid surface in a convective heat transfer process. It takes into account factors such as fluid velocity, temperature difference, and fluid properties.

2. How is the convective heat transfer coefficient calculated?

The convective heat transfer coefficient can be calculated using various empirical or theoretical equations, depending on the specific circumstances of the heat transfer process. Some common methods include the Nusselt number correlation and the Reynolds number correlation.

3. What factors affect the convective heat transfer coefficient?

The convective heat transfer coefficient is influenced by several factors, including fluid properties (such as density, viscosity, and thermal conductivity), fluid velocity, temperature difference between the fluid and the surface, and surface roughness.

4. How does the convective heat transfer coefficient impact heat transfer?

The convective heat transfer coefficient plays a crucial role in determining the rate of heat transfer between a solid surface and a fluid. A higher convective heat transfer coefficient indicates a more efficient transfer of heat, while a lower coefficient means a slower transfer.

5. Can the convective heat transfer coefficient be controlled?

Yes, the convective heat transfer coefficient can be controlled to some extent by manipulating the factors that affect it. For example, increasing the fluid velocity or using a fluid with higher thermal conductivity can result in a higher convective heat transfer coefficient, thereby increasing the rate of heat transfer.

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