When testing large fans in the field, it is often necessary to make measurements at locations where the distributions of gas velocity, pressure, and temperature are highly irregular. When this occurs the fan pressure rise or fan specific energy is usually expressed in terms of average values of pressure, density, and temperature. This paper proposes criteria for the definition of such averages and develops defining equations for them. The averages proposed allow the continuity, energy, and ideal gas state equations to be written in simple one-dimensional form without correction factors. Performance variables such as total pressure are defined in the customary way from the averages of the fundamental flow properties.

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