Question

Under what conditions does natural convection enhance forced convection, and under what conditions does it hurt forced convection?

Short answer

Answer: Natural convection enhances forced convection when their fluid motions act in the same direction, typically occurring when the temperature difference between the surface and the fluid creates a buoyancy force that complements the external force causing forced convection. Natural convection hurts forced convection when their fluid motions act in opposite directions, which happens when the temperature difference between the surface and the fluid generates a buoyancy force that opposes the external force causing forced convection.

Step-by-step solution

Introduction to Convection

Convection is the mode of heat transfer that occurs in fluids (such as liquids and gases) due to the combined effects of fluid motion and heat diffusion. Convection can either be natural or forced, depending on what causes the fluid motion.

Natural Convection

Natural convection occurs when fluid motion is caused by variations in density due to temperature differences within the fluid. In the presence of a gravitational field, the warmer, less dense fluid will rise, while the cooler, denser fluid will sink, resulting in a cyclic motion. The heat transfer rate in natural convection depends on the fluid properties, temperature difference, and gravity.

Forced Convection

Forced convection occurs when fluid motion is induced by external forces, such as fans or pumps. This type of convection is independent of temperature differences in the fluid. The heat transfer rate in forced convection depends on fluid properties, flow speed, and the surface's geometry over which fluid moves.

Enhancement of Forced Convection by Natural Convection

Natural convection enhances forced convection when their fluid motions act in the same direction, leading to an increased overall heat transfer rate. This typically happens when the temperature difference between the surface and the fluid creates a buoyancy force that complements the external force causing forced convection.

Detrimental Effect of Natural Convection on Forced Convection

Natural convection hurts forced convection when their fluid motions act in opposite directions. It leads to a decreased overall heat transfer rate as the two forces work against each other. This situation typically occurs when the temperature difference between the surface and the fluid generates a buoyancy force that opposes the external force causing forced convection.

Conclusion

Natural convection can either enhance or hurt forced convection, depending on the direction of the fluid motion caused by each type of convection. When both motions act in the same direction, natural convection enhances forced convection, leading to a higher overall heat transfer rate. Conversely, when the fluid motions act in opposite directions, natural convection hurts forced convection and results in a lower heat transfer rate. The conditions under which natural convection has a positive or negative impact on forced convection depend on factors like surface and fluid temperatures, fluid properties, and gravity.