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【范围】
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1.1 Scope
The fluid flow treated in this section is isothermal, subsonic, and incompressible. The effects of
heat addition, work on the fluid, variation in sonic velocity, and changes in elevation are
neglected. An incompressible fluid is one in which a change in pressure causes no resulting
change in fluid density. The assumption that liquids are incompressible introduces no appreciable
error in calculations, but the assumption that a gas is incompressible introduces an error of a
magnitude that is dependent on the fluid velocity and on the loss coefficient of the particular duct
section or piece of equipment. Fig. 1A-1 shows the error in pressure drop resulting from assum
ing that air is incompressible.
With reasonably small loss coefficients and the accuracy that is usually required in most calcula
tions, compressible fluids may be treated as incompressible for velocities less than Mach 0.2. At
higher velocities and for large loss coefficients (Kt and 4/L/D), compressible flow analysis
should be used.
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