Mode of heat transfer by buoyancy-driven motion Heat transfer caused by the actual movement of fluid particles due to density differences produced by temperature variations is called:

Introduction / Context:
Heat in fluids moves by both diffusion (molecular conduction) and advection (bulk motion). When bulk motion arises naturally from buoyancy, the mechanism has a specific name with distinct correlations.

Given Data / Assumptions:

• Temperature gradients create density gradients in a gravitational field.
• No external devices (fans or pumps) drive the flow.

Concept / Approach:
Free (natural) convection occurs when buoyancy forces due to density differences overcome viscous resistance, causing motion and convective heat transfer. The characteristic dimensionless groups are Grashof and Rayleigh numbers, contrasting with Reynolds for forced convection.

Step-by-Step Solution:

Verification / Alternative check:
Examples include heating of still air by a radiator, cooling of a hot vertical plate, and plume rise from a candle—each occurs without fans or pumps.

Why Other Options Are Wrong:
Conduction involves energy transfer by molecular interaction without bulk motion. Forced convection requires an external driver (fan, pump). Radiation transfers energy by electromagnetic waves and does not require a medium.

Common Pitfalls:
At low Grashof numbers, natural convection may be weak and easily swamped by small external drafts—then the regime becomes mixed convection.

Final Answer:

free convection