What is Nusselt number formula?

What is Nusselt number formula?

Nusselt number basically calculated by using the formula (Nu=((hLc)/k), Where h- convective heat transfer coefficient, Lc is characteristic length. In your case, the distance between parallel plates is Lc. and Finally k is thermal conductivity of the material. If the flow is laminar, conductive heat transfer dominates.

What does a Nusselt number of 1 mean?

A Nusselt number of value one represents heat transfer by pure conduction. A value between one and 10 is characteristic of slug flow or laminar flow. A larger Nusselt number corresponds to more active convection, with turbulent flow typically in the 100–1000 range.

What does Nusselt number depend on?

The Nusselt number in the fully developed laminar flow is constant and depends on the channel geometry and the wall heat transfer boundary condition.

Can Nusselt number be less than 1?

Nu can not be less than one since it gives relation between conduction and convection of the liquid.

What is local Nusselt number?

The local Nusselt number is defined as Nux=(x−x0) q/(k(Ts−Tref)), where q is the measured convective heat flux, Ts and Tref surface and reference (the location of the reference point is located just upstream of the heater) temperatures and k the thermal conductivity of air.

How is the Nusselt number of a fully developed flow changing along the position?

In developing flow inside pipes the Nu number changes along the axial position. After the flow is fully developed the Nu number remains constant along the axial position (Nu=4.36 for laminar flow, constant heat flux at wall, circular pipe). It is a coefficient indicating the rate at which a geometry transfer heat.

What does a high Nusselt number?

A Nusselt number of order unity would indicate a sluggish motion little more effective than pure fluid conduction: for example, laminar flow in a long pipe. A large Nusselt number means very efficient convection: For example, turbulent pipe flow yields Nu of order 100 to 1000.

What is average Nusselt number?

It is a coefficient indicating the rate at which a geometry transfer heat. Average Nusselt number is the mean of Local Nusselt numbers over the entire heat transfer surface. Therefore, its value depends on the geometry or surface of heat transfer.

Why is Nusselt number constant?

First, there is a laminar boundary layer, next a transition and then the turbulent boundary layer. This is an external flow and the temperature of the flow outside the boundary layer is always constant. Both the local and averaged Nusselt numbers are larger for q=const.

Why is Nusselt number constant for laminar flow?

In laminar flow in a tube with constant surface temperature, both the friction factor and the heat transfer coefficient remain constant in the fully developed region. Therefore, for fully developed laminar flow in a circular tube subjected to constant surface heat flux, the Nusselt number is a constant.

What is difference between Biot no and Nusselt no?

Biot number uses thermal conductivity of the body (not fluid), whereas Nusselt number uses thermal conductivity of the fluid. Nusselt number can be used for heat exchangers, because fluid convection dominates, calculation of the heat transfer coefficient of various flows such as in pipes.

How is Nusselt number calculated in Comsol?

Nu number only have significance on the wall surface, though, first you have to calculate h, then you put h in Nu formula to obtain the value of Nu number. In order to calculate the h, you should explain it as: – k*(Tx+Ty+Tz) /(Ts-Tm).