How does increasing water temperature generally affect pipe friction for a given flow?

• A. Increases viscosity, increases friction factor.
• B. Decreases viscosity, reduces friction factor.
• C. Increases Reynolds number and increases head loss.
• D. Has no effect on friction factor.

Answer: (B)

Increasing water temperature lowers dynamic viscosity. With a fixed flow rate, the velocity in the pipe stays the same, but the Reynolds number rises because μ is smaller. Friction factor generally falls as Reynolds number increases (in laminar flow f = 64/Re, so f drops with Re; in turbulent flow, f also tends to decrease with higher Re for a given roughness). Since head loss due to friction scales with the friction factor, the reduction in f leads to lower friction losses. So warmer water reduces pipe friction for the same flow.