nusselt number

[namihabib]

why in nusselt number the lenght not thickness is used... Like we take thickness perpendicular to the direction in which heat is transferred but why not in nusselt no

 

[namihabib]

Sorry parallel to heat transfer

 

[IRstuff]

Is this for school? Wikipedia has a pretty good explanation.

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[racookpe1978]

From that Wikipedia source - which for things like this is usually adequate.
'The convection and conduction heat flows are parallel to each other and to the surface normal of the boundary surface, and are all perpendicular to the mean fluid flow in the simple case.

Nu = hL/k

where L is the characteristic length, k is the thermal conductivity of the fluid, h is the convective heat transfer coefficient of the fluid.

Selection of the characteristic length should be in the direction of growth (or thickness) of the boundary layer; some examples of characteristic length are: the outer diameter of a cylinder in (external) cross flow (perpendicular to the cylinder axis), the length of a vertical plate undergoing natural convection, or the diameter of a sphere. For complex shapes, the length may be defined as the volume of the fluid body divided by the surface area.
The thermal conductivity of the fluid is typically (but not always) evaluated at the film temperature, which for engineering purposes may be calculated as the mean-average of the bulk fluid temperature and wall surface temperature.

In contrast to the definition given above, known as average Nusselt number, local Nusselt number is defined by taking the length to be the distance from the surface boundary[1] to the local point of interest.

H * H_x/ k

I'm not sure where your question comes from, but the Nu number is used for flat surfaces being heated by fluids (gas and liquid) above or below AND for circular problems like pipes and tubes. The Nu focuses on the film between the surface and the rest of the fluid. That has to be a "thickness", doesn't it? The Reynolds Number focuses on the length a fluid travels past a surface - because that affects turbulence and the how the heat gets transferred.

 

[namihabib]

Nu=h/k/l means convection upon conduction...in conduction say we have a rectangle and heat is flowing in x direction and in x direction we have thickness t so now the direction of heat transfer and t is same that is parallel.. But in bl. Heat transfer is occuring in say y direction and we are taking thickness in X.. Why is that?

 

[IRstuff]

Because it's based on how much distance is required to develop the complete boundary layer. If you consider a box, looking at the right side wall as the source of the convected heat, the boundary layer will start with zero thickness at the bottom of the wall and only develop into a full boundary layer as you move up the wall. It's the boundary layer that's convecting the heat, so a taller wall is more efficient at removing heat because it's tall enough that the boundary layer is fully developed.

I'm guessing you didn't bother looking it up in Wikipedia, so here:

http://en.wikipedia.org/wiki/Nusselt_number#Introduction

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