What factors affect the heat transfer coefficient?

[theBEAST]

1. Homework Statement and the attempt at the solution

Alright, here is a picture of the problem along with my solution:

My solutions gives the correct answer but the only part I don't understand is why the heat transfer coefficients are the same. If the two rods are made of different materials don't you have a different coefficient? Or is coefficient independent of the material... If so, what does it depend on? Shape?

 

[Chestermiller]

1. Homework Statement and the attempt at the solution

Alright, here is a picture of the problem along with my solution:

My solutions gives the correct answer but the only part I don't understand is why the heat transfer coefficients are the same. If the two rods are made of different materials don't you have a different coefficient? Or is coefficient independent of the material... If so, what does it depend on? Shape?

The resistance to heat transfer is going to be dominated by the resistance on the air side of the interface. So the heat transfer coefficient is going to be essentially the heat transfer coefficient on the air side. With this situation, then yes, the overall heat transfer coefficient is virtually independent of the rod material. For forced air convection over the fins, it is determined by the cross flow reynolds number and prandtl number of the air.

Chet

 

[theBEAST]

The resistance to heat transfer is going to be dominated by the resistance on the air side of the interface. So the heat transfer coefficient is going to be essentially the heat transfer coefficient on the air side. With this situation, then yes, the overall heat transfer coefficient is virtually independent of the rod material. For forced air convection over the fins, it is determined by the cross flow reynolds number and prandtl number of the air.

Chet

In what case would it be different then? When the fluids are different? So whenever there are different materials exposed to convective heat transfer with the surrounding air can we use the same heat transfer coefficients all the time?

 

[Chestermiller]

In what case would it be different then? When the fluids are different? So whenever there are different materials exposed to convective heat transfer with the surrounding air can we use the same heat transfer coefficients all the time?

You can use the same heat transfer coefficient on the air side, but there may also be some heat transfer resistance that needs to be taken into account on the "inside the cylinder" side. For example, if you have a tube with liquid or gas flowing through it, rather than a metal rod, you will need to consider the resistance on the tube side of the interface, particularly in the case of a gas. You can have a heat transfer coefficient on the tube side that can be of the same order as on the air side. So the overall heat transfer coefficient will be substantially lower than on either side.