Pulsating flow velocity profile measurement at an acoustically reflecting and non-reflecting open pipe end using laser doppler anemometry (LDA)

Depending on the geometry and operating condition, occasionally strong and destructive acoustic resonances occur in the pipe spool piece between the high-pressure discharge of screw compressors and the connected pulsation damper/silencer inlet. To avoid these resonances, the use of a nozzle with defined cross-section reduction is investigated. By designing the contraction ratio as a function of the nozzle inlet Mach number, a non-reflecting termination/transition can be provided. For a defined operating point, acoustic reflection from the nozzle can be avoided in this way, thus preventing the acoustic resonance in the spool piece. Within this paper the pulsating flow field directly downstream an acoustically reflecting open pipe end is investigated using laser doppler anemometry (LDA) at the air test rig at the Chair of Fluidics at TU Dortmund University. In the modified experimental setup an operating point-specific designed nozzle is installed at the open pipe end to create a non-reflecting termination. By means of LDA measurements, the velocity profile is determined experimentally at the nozzle outlet for the designed operating condition. The 2D measurement results (radius dependant velocity and phase distribution) extend established 1D decomposition of pressure measurement signals into upstream and downstream travelling waves and allow a deeper understanding of the acoustic behaviour of non-reflecting nozzles.