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

Natural convection is a heat transfer process that occurs due to the natural movement of fluid, resulting from density differences caused by temperature variations in the fluid. This process relies solely on buoyancy forces without any externally applied forces. On the other hand, forced convection involves the heat transfer process in which fluid is forced to flow over a surface or through a tube by an external force like a fan or a pump.

To determine under which conditions natural convection enhances or hurts forced convection, we need to analyze their heat transfer processes. The heat transfer for both natural and forced convection can be given by the following equation: Q = hA(T_s - T_inf), where Q is the heat transfer rate, h is the heat transfer coefficient, A is the surface area, T_s is the surface temperature, and T_inf is the temperature of the surrounding fluid. The heat transfer coefficient, h, depends on the specific convection type, fluid properties, flow velocity and geometry of the system. Comparing the heat transfer coefficient for natural and forced convection will give us an idea of whether natural convection enhances or hurts the forced convection process.

Natural convection can enhance forced convection when its heat transfer coefficient is high enough to supplement the forced convection process. This usually occurs when the fluid's thermal properties and flow patterns in the system are favorable for natural convection. The conditions that enhance forced convection are typically: 1. High temperature differences between the surface and the surrounding fluid. 2. Fluid properties like high thermal expansion coefficient and low viscosity. 3. Favorable geometry, such as vertical surfaces aligned with buoyancy-driven flow.

Natural convection can hurt forced convection when its heat transfer coefficient is low, and the flow patterns interfere with the forced convection process. This can occur when the fluid's thermal properties and flow patterns in the system are not favorable for natural convection. The conditions that hurt forced convection are typically: 1. Low temperature differences between the surface and the surrounding fluid. 2. Fluid properties like low thermal expansion coefficient and high viscosity. 3. Unfavorable geometry, such as horizontal surfaces not aligned with buoyancy-driven flow, which creates opposing flow patterns. In summary, natural convection can either enhance or hurt forced convection depending upon the conditions present, including temperature difference, fluid properties, and system geometry. For enhancement, the conditions should favor buoyancy-driven flow, while for deterioration, the conditions should not favor buoyancy and may result in opposing flow patterns.