A “diffuser” is a venturi-like element with a well-formed contraction followed by a small-angle diffuser. When liquid flows through two diffusers connected in series various flow states are possible depending on whether or not cavitation occurs in the diffusers. It is shown that, in the absence of strong “Reynolds-number effects,” one of just two possible sequences of flow states can occur for a particular pair of diffusers. Denoting the diffusers by “upstream” and “downstream,” cavitation can occur as follows with increasing flow: 1. Neither; upstream only; both. 2. Neither; downstream only (and upstream never). Once this classification is known it is easy to predict the characteristics of the circuit which can then be used to define an “equivalent single diffuser” to represent the pair of diffusers. Experimental data verifying the theory are included.

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