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

In heat transfer, there are two modes of convection - natural convection and forced convection. Natural convection happens when the flow of the fluid is driven by buoyancy forces caused by temperature differences in the fluid. It occurs in the absence of any external force like fan or pump. Examples include the flow of air due to temperature differences around a hot object or the natural circulation that occurs in a liquid when heated. Forced convection, on the other hand, occurs when fluid flow is driven by an external force, such as a fan, pump, or a mechanical stirring device. Examples include heat transfer in HVAC systems where air is circulated by fans or heat transfer in a car radiator where coolant is circulated by a pump.

The convection heat transfer coefficient (h) is an indicator of how efficient a particular mode of convection is in transferring heat between a solid surface and a fluid. A higher convection heat transfer coefficient means more effective heat transfer. In general, forced convection exhibits higher convection heat transfer coefficients compared to natural convection. This is because forced convection relies on an external force (fan, pump, etc.) to induce fluid flow, resulting in higher fluid velocities and more turbulent flow. The increased turbulence and fluid velocities enhance the mixing of fluid layers, which in turn improves the heat transfer process.

As mentioned earlier, forced convection is characterized by higher fluid velocities and more turbulent flow, which leads to better mixing of fluid layers and, in turn, more efficient heat transfer. In contrast, natural convection relies solely on buoyancy forces resulting from temperature differences, which tend to result in slower fluid velocities and less efficient heat transfer. To summarize, the convection heat transfer coefficient is usually higher in forced convection compared to natural convection. The main reason for this is the increased fluid velocities and turbulence associated with forced convection, which enhance the mixing of fluid layers and improve heat transfer efficiency.