Natural convection typically yields what heat transfer rate compared with forced convection?

• A. Higher
• B. Lower
• C. The same
• D. Variable

Answer: (B)

The main idea is how strongly the fluid is moved. In natural convection, fluid motion comes from buoyancy due to temperature differences, which usually isn’t as vigorous as an externally driven flow. This weaker motion means the thermal boundary layer near surfaces stays thicker, the convective heat transfer coefficient is smaller, and the overall heat transfer rate is lower.

In forced convection, an external pump or fan pushes the fluid, often achieving higher speeds and sometimes turbulence. This drives a thinner boundary layer and a larger heat transfer coefficient, leading to a higher transfer rate for the same geometry and temperature difference.

So, natural convection generally yields a lower heat transfer rate than forced convection (though special cases with very large temperature differences or particular geometries can change the comparison).