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.