Acoustical characteristics of single and two-phase horizontal pipe flow through an orifice

In this work, the acoustic effects of horizontal airwater flow through an orifice are investigated experimentally. Single phase flow (air) and two-phase flow (air and water) tests are performed for two sets of orifices. One set of straight edged and one set of upstream rounded orifices. For each set, the diameters of the orifices were 2, 5, and 10mm, with a thickness of 5 mm. The two-phase flow is generated by injecting water at a rate of 0 to 40 g/s to air in a pipe with diameter of 25 mm. The air rate is fixed in the range from 5.8 to 14 g/s, where the upstream pressure varies from 1.5 to 4 bar at ambient temperature. Unsteady pressure fluctuations are recorded at two upstream and two downstream position. The valve noise standard NEN-EN-IEC for dry gas is assessed by means of experimental data in dry conditions at fixed air mass flow rate. Predictions of sound power spectra by means of standard NEN-EN-IEC (60534-8-3, 2011) are found to be more accurate compared to those obtained following Reethof & Ward (1986), also in conditions of a choked orifice. For the multiphase case, the majority of the conditions are visually recognized as stratified and stratified-wavy with slugs arising at water mass flow rate larger than 23.6 g/s. The slug frequency lies between 0.6 and 1.1 Hz. For a fixed gas rate, at increasing liquid rate, the efficiency (ratio of acoustic power to flow energy) at the downstream position initially decreases while rapidly increasing at higher liquid rates, even discounting the increased total mass flow rate. The frequency spectrum changes significantly by introducing liquid, increasing the low frequency energy content. A peak at Sr ~0.3 remains but likely radial modes are excited in some of the conditions. The influence of the edge rounding is minimal and changes effective the contraction ratio of the effective choke area.