Experimental and Numerical Study of Flow Dynamics in the Leakage Gaps of Oil-Free Positive Displacement Machines

Rotary positive displacement machines are widely used in industry. Their efficiency is influenced by leakage through the clearance gaps between their stationary and rotating parts. The study of leakage flows through the clearances in oil-free rotary positive displacement machines (PDMs) is necessary to get a real insight into the attributes of such leakage flows. Computational fluid dynamics (CFD) can help in understanding the nature of leakage flow phenomena. But sufficient experimental results are needed to validate the CFD models. For example, it is extremely challenging to obtain the correct velocity field of the leakage flow in the running conditions of the machine. Therefore, this study focuses on the design and development of an experimental setup that can measure the velocity field with the controlled operating parameters of the machine. This study is a part of Project SECRET (Smart Efficient Compression, Reliability, and Energy Targets), which is supported by an award from The Royal Academy of Engineering, UK, and Howden Compressors to City, University of London’s Centre for Compressor Technology.

In this study, a Howden URAI-22 oil-free Roots blower is used. To have optical access to visualize the flow field inside the machine, a metallic part of the casing is replaced by sapphire glass. National Instrument’s hardware-based data acquisition system is designed to measure and control machine operating parameters such as pressure, temperature, flow, power, and speed by implementing appropriate sensors. The particle image velocimetry (PIV) technique is identified for velocity field measurement, and the setup is designed using optical components. Designed experimental setup specifically consists of Litron Bernoulli laser (200-15), Long laser guide arm (532/266nm), Beam waist adjuster, HiSense Zyla sCMOS Camera 5.5MP, UV/VIS Parallel Light Sheet Optics (H 50mm at 532 nm), Performance synchronizer, Seeder, K2 Distamax lens with CF2 and CF4 objective, an imaging system with Dynamic studio software. The clearance flow field is obtained with experiments over the range of operating conditions (Speed and discharge pressure) of the machine.

PIV results were processed and data such as gas velocity, local flow patterns, turbulent kinetic energy, and transient flow structures were then evaluated in a fully transient, three-dimensional CFD model of the test Roots Blower. An important feature of this numerical model is the deforming computational grid required for the flow solver and this was obtained using the in-house tool SCORG. The boundary conditions at suction, discharge, and the rotor operating speed were set as per the test data and the CFD model was used to analyze the resultant fluid flow and thermal field. Leakage gap flow and downstream pattern of the local field were quantitively compared with the PIV measurements in order to verify the computational model which will be used further for design improvement and detailed evaluation of leakage reduction mechanisms.