When heat is transferred by forced convection, the heat transfer coefficients _____.

When heat is transferred by forced convection, the heat transfer coefficients _____.

• A. Increase as fluid velocity increases
• B. Decrease as fluid velocity increases
• C. Remain constant regardless of velocity
• D. Depend only on temperature difference and not velocity

Answer: (A)

In forced convection, moving the fluid over a surface enhances heat transfer because the convective heat transfer coefficient rises with flow velocity. As velocity increases, the thermal boundary layer becomes thinner and the fluid carries more heat away, which is reflected in the common correlations where the Nusselt number (and thus h) increases with Reynolds number. In short, a faster stream improves the rate at which heat is transferred for a given temperature difference. The other ideas don’t fit because h is not constant and does depend on velocity in forced convection, and this mechanism is driven by flow, not just by the temperature difference as in natural convection.

In forced convection, moving the fluid over a surface enhances heat transfer because the convective heat transfer coefficient rises with flow velocity. As velocity increases, the thermal boundary layer becomes thinner and the fluid carries more heat away, which is reflected in the common correlations where the Nusselt number (and thus h) increases with Reynolds number. In short, a faster stream improves the rate at which heat is transferred for a given temperature difference. The other ideas don’t fit because h is not constant and does depend on velocity in forced convection, and this mechanism is driven by flow, not just by the temperature difference as in natural convection.