A fluid is compressible when its density changes with pressure. The following terms are often used is connection with the flow of compressible fluids.

Mach Number N _Ma This is the ratio of the fluid velocity to the velocity of sound in the same fluid under identical temperature and pressure.

where

Critical Pressure Ratio For a constant pressure, the flow rate of a compressible fluid increases as the downstream pressure is lowered until a downstream pressure is reached at which the flow rate is the maximum and the fluid velocity equals the sonic velocity. At this condition, the ratio of downstream pressure to the upstream pressure in absolute scale is called the critical pressure ratio. For gases obeying ideal-gas law, the critical pressure ratio is given by

If the downstream pressure is less than what is calculated from the above equation, the flow rate becomes independent of the downstream pressure.

Critical Velocity The critical velocity of a gas through a pipe or an orifice is the sonic velocity. It is given by

For the estimation of the flow of compressible fluids, the following rules may be used:

1. When...