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# What does Rayleigh number tell us?

## What does Rayleigh number tell us?

In fluid mechanics, the Rayleigh number (Ra) for a fluid is a dimensionless number associated with buoyancy-driven flow, also known as free or natural convection. It characterises the fluid’s flow regime: a value in a certain lower range denotes laminar flow; a value in a higher range, turbulent flow.

### What happens when Rayleigh number increases?

As the Rayleigh number increases, the isotherms become more distorted, and the stream function values increase; this increase is caused by the domination of the convective heat transfer mechanism at higher Rayleigh numbers.

#### What is the Rayleigh number for laminar flow?

The values of the constants C and n depend on the geometry of the surface and the flow regime, which is characterized by the range of the Rayleigh number. The value of n is usually n = 1/4 for laminar flow and n = 1/3 for turbulent flow.

What is Rayleigh number in heat transfer?

Rayleigh number, Ra, is a dimensionless term used in the calculation of natural convection. where g is acceleration due to gravity, β’ is coefficient of thermal expansion of the fluid, ΔT is temperature difference, x is length, ν is kinematic viscosity and κ is thermal diffusivity of the fluid.

What is the Reynolds number for turbulent flow?

While the essential Reynolds number for turbulent flow in a pipe is 2000, the critical Reynolds number for turbulent flow over a flat plate when the flow velocity equals the free-stream velocity ranges between 10 5 and 10 6. In the case of Newtonian fluids, the Reynolds number also predicts the viscous behavior of the flow.

## What is the Rayleigh number?

The Rayleigh number is defined as the product of the Grashof number, which describes the relationship between buoyancy and viscosity within a fluid, and the Prandtl number, which describes the relationship between momentum diffusivity and thermal diffusivity. Hence it may also be viewed as the ratio…

### What is the Reynolds number?

The Reynolds number, abbreviated as Re, determines whether the fluid flow is laminar or turbulent. Let’s consider a pipe where a fluid is flowing through it and a connection has been done with a separate line from where dye or ink is provided.

#### What happens when the Rayleigh number is below critical?

When the Rayleigh number, Ra, is below a critical value for a fluid, there is no flow and heat transfer is purely by conduction; when it exceeds that value, heat is transferred by natural convection.