What losses are due to the interaction of air molecules with the sides of the duct?

What losses are due to the interaction of air molecules with the sides of the duct?

• A. Dynamic Losses
• B. Friction Losses
• C. Contractions
• D. Natural Ventilation

Answer: (B)

Friction losses are the energy losses when air interacts with the duct walls. Because air sticks to the walls, a velocity gradient forms from zero at the wall to the flow speed away from it. This viscous interaction produces shear stresses that dissipate some of the flow’s pressure energy as heat, creating a pressure drop along the duct. In duct design, these major losses depend on duct length, diameter, roughness, and flow rate (often described with the Darcy-Weisbach approach). The other options describe different phenomena—sudden area changes (contractions) or air movement driven by outdoor conditions (natural ventilation)—and do not capture the wall-friction effect.

Friction losses are the energy losses when air interacts with the duct walls. Because air sticks to the walls, a velocity gradient forms from zero at the wall to the flow speed away from it. This viscous interaction produces shear stresses that dissipate some of the flow’s pressure energy as heat, creating a pressure drop along the duct. In duct design, these major losses depend on duct length, diameter, roughness, and flow rate (often described with the Darcy-Weisbach approach). The other options describe different phenomena—sudden area changes (contractions) or air movement driven by outdoor conditions (natural ventilation)—and do not capture the wall-friction effect.