Three-dimensional pipe flows with elbows and tees are calculated to estimate the effect of upstream flow distortion on measuring accuracy of orifice flow meter. Axisymmetric flows through orifice are calculated first to evaluate the performances of various numerical schemes and turbulence models. In three-dimensional calculations of the flow in pipe system, it is evaluated how the pressure difference across the orifice is dependent on the length of upstream straight pipe in a branch. From the results, it is found that, regardless of flow rate, the effect of the length can be neglected for the lengths larger than thirty diameters although there still remain significant swirl at the orifice.

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