True or False: In flowing gases the density is generally constant.

True or False: In flowing gases the density is generally constant.

• A. True
• B. False
• C. Not determinable
• D. Sometimes

Answer: (B)

Gases are compressible, so their density changes with pressure and temperature along a flow. In steady flow, the mass flow rate must be constant, expressed as ρ A v = ṁ. If the velocity changes as the gas moves through a duct, nozzle, or diffuser, the density must adjust accordingly to conserve mass (or the cross-sectional area must change). The ideal gas relation ρ = p/(R T) shows directly that even small changes in pressure or temperature alter density. So density is not generally constant in flowing gases. Only in special cases where the flow is effectively incompressible or pressure/temperature remain nearly constant would you approximate density as constant.

Gases are compressible, so their density changes with pressure and temperature along a flow. In steady flow, the mass flow rate must be constant, expressed as ρ A v = ṁ. If the velocity changes as the gas moves through a duct, nozzle, or diffuser, the density must adjust accordingly to conserve mass (or the cross-sectional area must change). The ideal gas relation ρ = p/(R T) shows directly that even small changes in pressure or temperature alter density. So density is not generally constant in flowing gases. Only in special cases where the flow is effectively incompressible or pressure/temperature remain nearly constant would you approximate density as constant.