Nusselt Number For Turbulent Flow

May 22, 2024 Post a Comment

Nusselt number for turbulent flow

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.

What is the formula of Nusselt number?

the outer diameter of the cladding is: d = 2 x rZr,1 = 9,3 mm. the Nusselt number, which is NuDh = 890. the hydraulic diameter of the fuel channel is Dh = 13,85 mm. the thermal conductivity of reactor coolant (300°C) is: kH2O = 0.545 W/m.K.

What does Nusselt number 1 mean?

If the Nusselt number is about 1, it represents that the heat transfer is conduction only, but if the value is between 1 and 10, then it shows laminar of slug flow. If the range is more, it is active convection with turbulence in the 100–1000 range.

What is the relationship between Nusselt number and Reynolds number?

Nusselt number is an important parameter that can contribute to a better rate of heat exchange. It is basically a function of Reynolds and Prandtl number. The correlation is provided in Eq. (6).

What is Reynolds number for turbulent flow?

Whenever the Reynolds number is less than about 2,000, flow in a pipe is generally laminar, whereas, at values greater than 2,000, flow is usually turbulent.

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 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.

What is Nusselt number and its significance?

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.”

Why is the Nusselt number a function of Reynolds and Prandtl?

Nusselt number is a measure of the ratio between heat transfer by convection (α) and heat transfer by conduction alone (λ/L). Convective heat transfer relationships are usually expressed in terms of Nusselt number as a function of Reynolds Number and Prandtl Number.

Can Nusselt numbers 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 average Nusselt number?

For a laminar flow over a flat plate, average Nusselt number is given by: 0.664 ( P r ) 1 3 ( R e L ) 1 / 2.

How do you calculate turbulent flow?

The Reynolds number NR can reveal whether flow is laminar or turbulent. It is NR=2ρvrη N R = 2 ρ v r η . For NR below about 2000, flow is laminar. For NR above about 3000, flow is turbulent.

What is meant by turbulent flow?

turbulent flow, type of fluid (gas or liquid) flow in which the fluid undergoes irregular fluctuations, or mixing, in contrast to laminar flow, in which the fluid moves in smooth paths or layers. In turbulent flow the speed of the fluid at a point is continuously undergoing changes in both magnitude and direction.

Is turbulent flow faster than laminar?

Turbulent flow is faster than Laminar flow. It is because of the higher Reynolds number at which Turbulence occurs.

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.

Is Prandtl number dimensionless?

The Prandtl number is an example of a dimensionless number that is an intrinsic property of a fluid. Fluids with small Prandtl numbers are free-flowing liquids with high thermal conductivity and are therefore a good choice for heat conducting liquids.

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.

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.

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 .