Question

Natural convection is characterised by

• A. Grashof number
• B. Peclet number
• C. Reynolds number
• D. Prandtl number

Hint:

Natural convection is governed by buoyancy force, and buoyancy effect is represented by Grashof number.

Answer 1

The Correct Option is A

Natural convection occurs due to density difference in a fluid. When a fluid is heated, its density decreases. The lighter hot fluid rises and the heavier cold fluid comes down. Thus, natural convection is caused by buoyancy force. The dimensionless number which represents the ratio of buoyancy force to viscous force is called Grashof number. It is written as: \[ Gr=\frac{g\beta\Delta T L^3}{\nu^2} \] where, \[ g=\text{acceleration due to gravity} \] \[ \beta=\text{coefficient of volume expansion} \] \[ \Delta T=\text{temperature difference} \] \[ L=\text{characteristic length} \] \[ \nu=\text{kinematic viscosity} \] So, natural convection is characterised mainly by: \[ \text{Grashof number} \] Reynolds number is mainly used for forced flow. Prandtl number relates momentum diffusivity to thermal diffusivity. Peclet number relates convective transport to diffusive transport. Therefore, the correct answer is: \[ \text{Grashof number} \]