Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.

ANS:D - Prandtl & Grashoff

The critical value of the Grashoff number governs the transition from laminar to turbulent flow in free convection heat transfer. The Grashoff number (Gr) represents the ratio of buoyancy forces to viscous forces in a fluid flow. In free convection, where fluid motion is driven by buoyancy forces due to density variations, the Grashoff number plays a significant role in determining the flow regime. When the Grashoff number is below a critical value, the flow is typically laminar. As the Grashoff number increases beyond this critical value, the flow transitions from laminar to turbulent. Therefore, the correct option is: Grashoff number.