In the energy equation for steady, incompressible flow, what does head loss h_L represent?

• A. Major and minor head losses between sections
• B. Kinetic energy term
• C. Lossless energy transfer
• D. Pressure head

Answer: (D)

Head loss h_L represents the reduction in the total head of the fluid as it moves from one section to another due to friction and other resistances. In the steady, incompressible energy equation, total head (pressure head plus velocity head plus elevation head) decreases along the flow path by this amount. The losses come from both major friction in pipes and minor losses from fittings, bends, valves, etc. It’s the energy that's dissipated, not the pressure head itself, and it’s subtracted from the upstream head to give the downstream head.