Solution(By Examveda Team) In free convection heat transfer, Nusselt number is function of Grashoff number and Prandtl number the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid.

Which number is used in free convection?

But, in natural convection the Grashof number is the dimensionless parameter that governs the fluid flow. Using the energy equation and the buoyant force combined with dimensional analysis provides two different ways to derive the Grashof number.

What is the mechanism of free convection?

Natural convection, known also as free convection is a mechanism, or type of mass and heat transport, in which the fluid motion is generated only by density differences in the fluid occurring due to temperature gradients, not by any external source (like a pump, fan, suction device, etc.).

What are the causes of free convection?

Free convection refers to fluid motion induced by buoyancy forces. Buoyancy forces may arise in a fluid for which there are density gradients and a body force that is proportional to density. In heat transfer, density gradients are due to temperature gradients and the body force is gravitational.

How do you calculate Nusselt number in natural convection?

If you are sure about your mesh, boundary conditions and initial configurations, you can calculate Nusselt Number by deriving h from the method I mentioned. In your case Nu is Nu = hL/k. ) note that L is plate length) .

Introduction to Free Convection

Why Nusselt number is used?

The Nusselt number is a dimensionless number closely related to Péclet number and both numbers are used to describe the ratio of the thermal energy convected to the fluid to the thermal energy conducted within the fluid.

What is local 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 meant by free convection?

Natural convection, known also as free convection is a mechanism, or type of mass and heat transport, in which the fluid motion is generated only by density differences in the fluid occurring due to temperature gradients, not by any external source (like a pump, fan, suction device, etc.).

What is free and forced convection?

For free convection tests, the heated air rises from the surface and up the duct. For forced convection tests, a variable-speed fan draws air up through the duct and across the surface. Thermocouples measure the air temperature upstream and downstream of the surface and the temperature at the heat transfer surface.

What is free convective mass transfer?

Mass transfer by convection involves the transport of material between a boundary surface (such as solid or liquid surface) and a moving fluid or between two relatively immiscible, moving fluids. ... Mass transfer takes place in the two contacting phases as in extraction and absorption.

What is beta in free convection?

β = coefficient of volume expansion, 1/K. δ = characteristic length of the geometry, m. ν = kinematics viscosity of the fluid, m2/s. The role played by Reynolds number in forced convection is played by the Grashof.

Does free convection require a medium?

HEAT TRANSFER THROUGH CONVECTION.

Like conduction and unlike radiation, convection requires a medium.

What is the example of free convection?

Heating a pot of water on a stove is a good example of the transfer of heat by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated.

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 is free convection boiling?

Natural convection boiling occurs when ΔTsat < 5°C. In this boiling mode, vapour will be observed over the water surface, but usually no bubbles will be observed. ... The pioneering work on boiling was done in 1934 by S.

What are the units 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).

What is the function of heat exchanger?

A heat exchanger is a system used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact.

What is the difference between free convection and natural convection?

Summary – Natural vs Forced Convection

The difference between natural and forced convection is that, in natural convection, the natural means influence the motion of the fluid whereas, in forced convection, external means influence the motion of the fluid.

What is convection engineering?

Heat transfer between a solid and a moving fluid is called convection. ... Heat energy transferred between a surface and a moving fluid with different temperatures - is known as convection. In reality this is a combination of diffusion and bulk motion of molecules.

How do you calculate free convection coefficient?

1. Plane area A= m²
2. Plane perimeter P= m.
3. Plane height L= m.
4. Angle from vertical °
5. Diameter D= m.
6. Density ρ = kg/m³ Viscosity μ = N*s/m² Specific heat C = J/kg*K. Thermal conductivity k = W/m*K. Thermal expansion coefficient β = 1/K.

What are three types of convection?

• Natural convection.
• Forced convection.

What are two types of convection?

There are two types of convection: natural convection and forced convection. Natural convection is produced by density differences in a fluid due to temperature differences (e.g., as in “hot air rises”). Global atmospheric circulation and local weather phenomena (including wind) are due to convective heat transfer.

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

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. ... 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.

What is the 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.

What is the difference between Nusselt number and Biot number?

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.