Question

Typical rayleigh numbers for onset of convection

Answer 1

In fluid mechanics, the Rayleigh number (Ra) for a fluid is a dimensionless numberassociated with buoyancy-driven flow, also known as free convection or natural convection. When the Rayleigh number is below a critical value for that fluid, heat transfer is primarily in the form of conduction; when it exceeds the critical value, heat transfer is primarily in the form of convection.

Answer 2

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Hence it may also be viewed as the ratio of buoyancy and viscosity forces multiplied by the ratio of momentum and thermal diffusivities. It is closely related to the Nusselt number. For most engineering purposes, the Rayleigh number is large, somewhere around 106 to 108.