Question

In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural convection or forced convection? Why?

Short answer

Answer: Forced convection usually has a higher convection heat transfer coefficient than natural convection. This is due to the increased fluid velocity and enhanced mixing provided by externally driven forces in forced convection, which leads to more efficient heat transfer compared to natural convection that relies solely on buoyancy-driven fluid movement.

Step-by-step solution

Understand Convection Heat Transfer Coefficient

Convection heat transfer coefficient (h) is a measure of the ability of a fluid to transfer heat through the process of convection. It depends on the fluid properties, temperature difference, and flow characteristics.

Define Natural Convection

Natural convection is a mode of heat transfer that occurs when fluid motion is driven by buoyancy forces caused by temperature differences within the fluid itself. For instance, when a heated object is immersed in a liquid or in the presence of a gas, the fluid in contact with the hot surface expands, becomes less dense, and rises. The colder, more dense fluid then flows in to replace the rising warm fluid, creating a natural circulation pattern.

Define Forced Convection

Forced convection is a mode of heat transfer where the fluid movement is driven by an external force, such as a fan, pump, or wind. It involves mixing the fluid, which enhances heat transfer by reducing the thickness of the stagnant boundary layer around the heated object.

Compare the Convection Heat Transfer Coefficient

Forced convection usually has a higher convection heat transfer coefficient than natural convection.

Reason for the Higher Coefficient in Forced Convection

The reason for this is that the fluid flow in forced convection is driven by external forces, which increases the fluid velocity and creates turbulence. This turbulent flow enhances the mixing of the fluid and reduces the thickness of the stagnant boundary layer around the heated object, leading to more efficient heat transfer. On the other hand, natural convection relies only on buoyancy forces due to temperature differences, which results in relatively lower fluid velocity and weaker convection currents, leading to a lower convection heat transfer coefficient. In conclusion, the convection heat transfer coefficient is usually higher in forced convection compared to natural convection because of the increased fluid velocity and enhanced mixing provided by externally driven forces in forced convection.