Convection and Thermal Resistance

[sonofptolomey]

I have a question...

Suppose you have a thin and long rectangular piece of aluminum.

You heat one end (pt A). The thermal resistance (steady state) from one end (pt A) to the other (pt B) would be

θ=ΔT/W
Where W= Watts of heat energy.
ΔT = temperature of ptA - temperature of ptB

Now suppose you expose ptB to significant convection, cooling it down and letting the system reach steady state.

Would the thermal resistance be different? Or is it constant?

I'm just confused conceptually.

I can easily argue both ways. I'm on the fence.

 

[sonofptolomey]

Really? No replies?

 

[gsal]

thermal resistance is a material property...so, depending on the material and the operating conditions you are interested in, you may consider its value a constant (if it does not vary much) or may have keep track of it if it varies significantly with temperature.

for your aluminum rod, with a temperature differential between one end and the other one...you probably have different thermal resistance values along the entire length...sure, you can always estimate a total, lumped value for it by taking dT/W.

 

[sonofptolomey]

Thanks, that's what I expected. So in the case of extreme convection, the aluminum rod temperature will steady (if convection continues unchanged) such that the ΔT/W will be constant.

Thanks!

 

[sardar]

I can understand your confusion about the thermal resistance in this scenario. Let me provide some clarification.

Thermal resistance is a measure of how difficult it is for heat to flow through a material or system. In this case, the thermal resistance is determined by the temperature difference (ΔT) and the amount of heat energy (W) being transferred through the aluminum piece.

When you heat one end (pt A) of the aluminum piece, the thermal resistance from pt A to pt B is determined by the temperature difference (ΔT) between the two points. This means that the thermal resistance will be higher if the temperature difference is larger, and vice versa. This relationship is represented by the equation θ=ΔT/W.

Now, when you expose pt B to significant convection, the temperature at pt B will decrease due to the cooling effect of the convection. This means that the temperature difference (ΔT) between pt A and pt B will decrease, resulting in a lower thermal resistance.

In other words, the thermal resistance will be different when pt B is exposed to convection because the temperature difference (ΔT) has changed. This is a result of the cooling effect of convection, which makes it easier for heat to flow through the aluminum piece. Therefore, the thermal resistance is not constant in this scenario.

I hope this explanation helps to clarify the concept of thermal resistance for you. Remember, thermal resistance is dependent on the temperature difference and the amount of heat energy being transferred, so any changes in these factors will result in a different thermal resistance.