Q1: Heat transfer by natural convection is enhanced in system with

ANS:B - high co-efficient of thermal expansion.

A high coefficient of thermal expansion (CTE) would actually inhibit natural convection rather than enhance it. Here's why:

1. Expansion and Density Changes: A high coefficient of thermal expansion means that a material expands significantly when heated and contracts significantly when cooled. As the material expands or contracts, its density changes accordingly. When the density changes, it can disrupt the density gradients within the fluid, which are essential for driving natural convection currents.
2. Buoyancy Effect: Natural convection relies on buoyancy forces generated by density differences within the fluid. If the fluid's density changes too rapidly due to thermal expansion or contraction, it can disrupt the stability of the fluid and inhibit the development of buoyancy-driven flow.

Therefore, a high coefficient of thermal expansion tends to destabilize the fluid and can suppress natural convection rather than enhance it. The correct choice for enhancing heat transfer by natural convection is: d) low density change with temperature.