Study of leakage flow in oil-free positive displacement rotary machines using particle image velocimetry

Rotary positive displacement machines (PDM) are widely used in industry. Oil free PDM's are used in applica-tions where presence of oil is not allowed, but they still are not much utilised due to challenges with reliability and efficiency related to extensive thermal loads. Their efficiency is important to meet carbon footprint targets while increasing reliability will allow their wider use. The main contributor to losses is the internal leakage through clearance gaps between stationary and rotating parts driven by the differential pressure between chambers. Due to the transient nature of leakage flows in running conditions, the measurement of velocity flow field inside the clearance gap is challenging but can provide significant information about flow physics. Thus, understanding of the velocity flow field is essential for developing methods to increase efficiency without compromising reliability.The flow field in clearance gaps can be captured using Particle Image Velocimetry (PIV) for optically accessible flows. The basic PIV system consists of a light source, camera, tracer particles, and analysis algorithms. In this study the Roots blower is used as a representative of the oil-free PDM for which the velocity field is measured inside the radial clearance gap in the running condition using the PIV technique. An extensive testing campaign has been carried out at various speeds and pressure ratios of the machine. PIV measurements were taken inside, upstream, and downstream of the clearance gap. Post-processing of PIV images allows the generation of velocity fields. These pioneering measurements of the velocity field inside the clearance in the machine operating con-ditions reveal interesting flow physics. Moreover, the large amount of collected experimental data provides an excellent basis for validation of simulation models and further developments.