In the equation, Qm represents what?

• A. Mass flow rate of the fluid flowing through a hole
• B. Volume flow rate
• C. Heat transfer rate
• D. Mass of the fluid

Answer: (A)

Qm denotes the mass flow rate—that is, how much mass passes through a cross section per unit time. In fluid systems you can describe flow by volume per time (volume flow rate) or by mass per time (mass flow rate). The mass flow rate is related to volume flow rate by Qm = ρ Qv, where ρ is the fluid density, so Qm is typically measured in kilograms per second. In an equation about a hole or nozzle, using Qm signals that the term represents the rate at which mass moves through, not the volume of fluid or the energy transferred. Volume flow rate would be in cubic meters per second, heat transfer rate would be in watts, and mass is in kilograms.