Nusselt Number Correlations

October 21, 2024 Post a Comment

Nusselt number correlations

268): “Then the Nusselt number may be interpreted as the ratio of heat transfer by convection to conduction across the fluid layer of thickness L. A larger value of the Nusselt number implies enhanced heat transfer by convection.”

What is the dittus Boelter correlation?

The Dittus–Boelter equation (for turbulent flow) is an explicit function for calculating the Nusselt number. The Nusselt number is the ratio of convective to conductive heat transfer across (normal to) the boundary.

Why Nusselt number increases with Reynolds number?

It is due to the increase in temperature difference between wall temperature and bulk temperature. Similarly heat transfer rate increases (Fig. 17) with the increase of Reynolds number.

Where is Nusselt number in Reynolds number?

That you can use to calculate the nusselt. Number.

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.

Why is Nusselt number negative?

Nusselt number can be negative, since it is defined as the dimensionless temperature gradient at the wall. The slop may be negative or positive, depending on the direction of heat flux.

What is the Gnielinski correlation?

The Gnielinski correlation, however, simplifies the evaluation of the inside heat transfer coefficient of heating tubes only by using friction factor and hydraulic diameter. Nowadays, enhanced tubes, are used in an absorption refrigeration system to improve the thermal performance of the system.

What does Graetz number tell us?

The Graetz number is a dimensionless number that characterizes laminar flow in a conduit. The Graetz number equals the product of the specific heat capacity, diameter, characteristic length, characteristic speed and the mass density divided by the thermal conductivity and length.

What is correlation in heat transfer?

The heat transfer correlations are established on the basis of heat transfer mechanism, and a vast majority of correlations are intended for use only for normal HT and improved HT regimes. They can be categorized into two types: the empirical type and the semiempirical type.

What is the relationship between Nusselt number and Reynolds number?

In turbulent flow, the boundary layer gets thinner as Reynolds number increases in the same pipe, thus, the Nusselt number depends on Reynolds number.

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.

Is higher Nusselt number better?

A Nusselt number of value one (zero) represents heat transfer by pure conduction. A value between one (zero) 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 is the formula of Nusselt number?

where α is convective heat transfer coefficient, L is representative dimension (e.g., diameter for pipes), and λ is the thermal conductivity of the fluid. Nusselt number is a measure of the ratio between heat transfer by convection (α) and heat transfer by conduction alone (λ/L).

Why is Nusselt number 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!

Does Nusselt number change with temperature?

At the solid-liquid interface, the heat conduction through the ice-layer must balance with the convection heat flux from the liquid region. The Nusselt numbers increase as the cooling temperature decreases. A lower cooling temperature produces a thicker ice-layer, henceforth the liquid layer becomes thinner.

What is the difference between local and average Nusselt number?

The local Nusselt number is a maximum when γ=180° and it is at the upper end of the vertical cylinder. Although the local Nusselt number is a maximum at γ= 180°, the maximum average Nusselt number is when γ=90°, that is when the cylinder is horizontal. This is due to a single primary vortex throughout the cylinder.

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.

What is a high Nusselt number?

A large Nusselt number means very efficient convection: For example, turbulent pipe flow yields Nu of order 100 to 1000. (From Frank M. White, Heat Transfer)

Is Nusselt number a local heat flux?

Local and average Nusselt number The local Nusselt number thus describes the local heat transfer coefficient and thus the local heat flux. However, the heat transfer can also be related to the entire system as such or to a longer section.

What is the significance of Sherwood number?

The Sherwood number (Sh) (also called the mass transfer Nusselt number) is a dimensionless number used in mass-transfer operation. It represents the ratio of the convective mass transfer to the rate of diffusive mass transport, and is named in honor of Thomas Kilgore Sherwood.