Convective heat transfer correlations

[arpatil_31]

What is the difference between constant wall heat flux and constant wall temperature for convective heat transfer correlations?

 

[Chestermiller]

What is the difference between constant wall heat flux and constant wall temperature for convective heat transfer correlations?

Hi arpatil_31. Welcome to Physics Forums!
In the case of constant wall heat flux, the temperature at the wall is increasing with distance along the wall, while, in the case of constant wall temperature, the wall temperature is constant. In the constant wall heat flux case, the wall temperature has to increase along the wall in order to maintain the heat flux constant.

 

[boneh3ad]

It's a matter of boundary conditions, really. As was previously stated, for a constant temperature wall, the wall is held at a constant temperature. For a constant heat flux wall, the derivative of temperature normal to the wall is held constant at the wall.

 

[arpatil_31]

Thanks Chestermiller and boneh3ad...
OK.
That means for constant wall temperature, wall is held at constant temperature and its heat flux (Heat transfer rate) may change.
But I am slightly confused for constant wall heat flux (For e.g. Hot fluid flows through pipe):Let us consider eqn. q= h x (dT/dy).
For pipe flow at leading edge h is higher and it goes on decreasing then after some time it becomes constant. when h becomes constant the temperature difference should becomes constant by increasing the temperature of the wall to maintain constant wall heat flux.

Am I right? If not then please elaborate it.

 

[Chestermiller]

Thanks Chestermiller and boneh3ad...
OK.
That means for constant wall temperature, wall is held at constant temperature and its heat flux (Heat transfer rate) may change.
But I am slightly confused for constant wall heat flux (For e.g. Hot fluid flows through pipe):Let us consider eqn. q= h x (dT/dy).
For pipe flow at leading edge h is higher and it goes on decreasing then after some time it becomes constant. when h becomes constant the temperature difference should becomes constant by increasing the temperature of the wall to maintain constant wall heat flux.

Am I right? If not then please elaborate it.

Yes. The temperature difference between the wall temperature and the average fluid temperature becomes constant. The shape of the temperature profile remains constant radially, but the temperature at all radial locations increases uniformly and linearly with distance along the tube as the fluid continues to receive heat. So the temperature becomes a function of r plus a linear function of z. You should be able to use this info to easily derive a simple equation for the asymptotic heat transfer coefficient.

 

[arpatil_31]

Thanks Chestermiller. Now my doubt is clear.