Nusselt Number Calculation

October 21, 2024 Post a Comment

Nusselt number calculation

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.

Why do we calculate Nusselt number?

If the range is more, it is active convection with turbulence in the 100–1000 range. Nusselt number on the other hand is a non-dimensional heat transfer coefficient. It is used to determine whether the heat transfer is conduction or convection.

What is K in Nusselt number?

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 is difference between Biot No & Nusselt no?

Biot numbers much larger than 1 indicate more difficult problems due to non-uniformity of temperature fields within the object. It should not be confused with Nusselt number, which employs the thermal conductivity of the fluid and hence is a comparative measure of conduction and convection, both in the fluid.

What is Prandtl number and Nusselt number?

Nusselt number is the ratio of convection heat transfer to pure conduction heat transfer, (K- Thermal conductivity of fluid) Reynolds number is the ratio of the inertia to the viscous forces avting on a fluid. Prandtl number is the ratio of the momentum and thermal diffusivities. Cite.

How is Nusselt number calculated using Reynolds number?

For fully developed (hydrodynamically and thermally) turbulent flow in a smooth circular tube, the local Nusselt number may be obtained from the well-known Dittus-Boelter equation. To calculate the Nusselt number, we have to know: the Reynolds number, which is ReDh = 575600. the Prandtl number, which is Pr = 0.89.

Can Nusselt number be zero?

Moreover, convective heat transfer (V>=0) must be equal to or greater than the conductive heat transfer (V=0). That's why Nu must be equal to or greater than 1. Correction: Nu should be greater than 1. As convection is defined as the heat transferred between a surface and a moving fluid, velocity must not be 0.

How is Prandtl number calculated?

To calculate the Prandtl number: Multiply dynamic viscosity with the specific heat of the fluid. Divide the product by the thermal conductivity of the fluid.

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.

Can the Nusselt number be less than 1?

However, you have also been told that the Nusselt Number is the “ratio of convective to conductive heat transfer”, and are wondering if there was only a tiny amount of motion, then the additional heat transfer would be less than that by conduction, so the Nusselt Number would be less than 1. You are absolutely right!

What is Prandtl number used for?

A dimensionless number approximating the ratio of momentum diffusivity to thermal diffusivity. The Prandtl Number is often used in heat transfer and free and forced convection calculations. It depends on the fluid properties.

What is K in heat transfer?

k: The factor k is called the thermal conductivity constant. The thermal conductivity constant k is larger for materials that transfer heat well (like metal and stone), and k is small for materials that transfer heat poorly (like air and wood).

What is Biot and Fourier number?

Fourier number is also known as dimensionless time and have effect on temperature before the steady state achieved. Biot number arises when we use third kind of boundary condition (i.e convective heat transfer in presence of external fluid surface). Both have significant effect on temperature distribution.

Why is Biot number important?

The numerical value of Biot Number (Bi) is a criterion which gives a direct indication of the relative importance of conduction and convection in determining the temperature history of a body being heated or cooled by convection at its surface.

How is Biot number calculated?

We can compute the characteristic length Lc knowing the volume V and the area A of a surface through which the material is heated up (or cooled down): Lc = V / A . For example for a copper pan of water with characteristic length Lc = 15 cm , the Biot number is Bi = 2.807 .

What is the unit for Reynolds number?

Reynolds Number Units The Reynolds number is unitless and dimensionless. That it is unitless means that it has no direct standard of measurement, the Reynolds number is determined by the properties of fluid flow.

How is Reynolds number calculated?

The Reynolds number (Re) of a flowing fluid is calculated by multiplying the fluid velocity by the internal pipe diameter (to obtain the inertia force of the fluid) and then dividing the result by the kinematic viscosity (viscous force per unit length).

What Reynolds number is laminar flow?

As indicated previously, if the Reynolds number is less than 2000, the flow is considered to be laminar. This is also known as viscous flow.

What does Reynolds number mean?

Well, the Reynolds Number signifies the nature of the flow of a fluid across a cross-section. On a wide basis, there are two types of flowing styles that a liquid can flow in. One is laminar flow and the other is turbulent flow. Reynolds number is used to determine the type of flow of a fluid.