ANS:B - natural convection

Natural convection is a mode of heat transfer that occurs due to density differences within a fluid, resulting in buoyancy forces that drive fluid motion. It is a spontaneous process that does not require any external intervention or mechanical devices to induce flow. Here's an explanation of natural convection:

1. Density Differences: When a fluid is heated, it expands and becomes less dense, causing it to rise. Conversely, when a fluid is cooled, it contracts and becomes denser, causing it to sink. This density difference creates buoyancy forces that drive fluid motion.
2. Buoyancy-Induced Flow: In natural convection, heated fluid rises while cooler fluid sinks, creating circulation patterns known as convection cells. As heated fluid rises, it transfers heat away from the hot surface through convection. Meanwhile, cooler fluid replaces the rising fluid, completing the circulation loop.
3. No External Force: Unlike forced convection, where fluid motion is induced by an external force such as a fan or pump, natural convection relies solely on buoyancy forces generated by temperature differences. As a result, natural convection occurs spontaneously in any fluid medium where temperature gradients exist.
4. Applications: Natural convection is commonly observed in various natural and engineering systems, including atmospheric circulation, ocean currents, building ventilation, and heat transfer in electronics and appliances. It is particularly important in applications where forced convection may not be feasible or desirable.
5. Effect of Geometry: The geometry of the heated surface and the surrounding fluid plays a significant role in natural convection. Surfaces with larger areas or inclined orientations tend to promote stronger convection currents, while confined spaces or obstacles can disrupt fluid flow and inhibit heat transfer.
6. Enhancement Techniques: While natural convection is inherently limited by the buoyancy-driven flow, it can be enhanced through passive means such as finned surfaces, which increase the surface area available for heat transfer and promote fluid mixing.

Overall, natural convection is a fundamental mechanism of heat transfer driven by buoyancy forces within a fluid medium. It plays a vital role in many natural and engineering processes and is an important consideration in the design and operation of various thermal systems.