Flow rate measurement by an orifice in a slowly reciprocating gas flow

It is shown that small density changes can give rise to misinterpretation of flow rate signals in unsteady (reciprocating) flows. Basically a flow rate measured at some point AA cannot simply be assigned to a remote point BB. Depending on the way of plotting a hysteresis appears which, in fact, does not exist. Unsteady conservation of mass is applied to a volume and orifice flow system to obtain an equation which explains and predicts the apparent hysteresis. The equation in dimensionless form contains a key parameter ββ which holds the flow determining quantities. Experiments are conducted with respect to a wide spread of ββ. It is shown that the equation predicts reality quite well.

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► Flow rate measurement in reciprocating flows is possible with orifices.
► Then Reynolds number and Strouhal number effects are negligible.
► Small density changes are important.
► An apparent hysteresis shows which has to be explained.
► Experimental results support the theoretical calculations.