Q1: Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the

ANS:D - none of these.

The answer "none of these" suggests that none of the provided options adequately describe the primary mechanism responsible for fluid motion in natural convection heat transfer between a solid surface and a fluid. In natural convection, fluid motion is driven by buoyancy forces arising from density differences within the fluid due to temperature variations. As a fluid near the solid surface is heated, it becomes less dense and tends to rise, while cooler fluid from the surroundings moves in to take its place. This process creates a continuous circulation pattern known as a convection current, which enhances heat transfer by bringing fresh fluid into contact with the heated surface. While thermal boundary layers do play a role in convection by influencing the rate at which heat is transferred across the boundary between the fluid and the solid surface, they do not directly drive fluid motion. Thermal boundary layers are regions near the surface where the velocity and temperature gradients are significant. Similarly, the buoyancy of bubbles produced at active nucleation sites is more relevant in boiling heat transfer, where bubbles form due to localized boiling rather than in natural convection. Therefore, the correct answer "none of these" indicates that the primary mechanism driving fluid motion in natural convection is not explicitly captured by any of the provided options. Instead, it is the density-driven buoyancy forces resulting from temperature differences that predominantly govern the fluid motion in natural convection.