Project SECRET - A Novel Experimental Setup

A Novel Experimental Setup to Measure Velocity and Temperature Field in Leakage Flows of Oil Free Rotary Positive Displacement Machines


A study of leakage flows in oil-free rotary positive displacement machines (PDMs) is necessary to get a real insight into the attributes of leakage flows. It is challenging to obtain velocity field and temperature field of the leakage flow in the running conditions of the machine. Therefore, this study focuses on developing an experimental setup that can measure the velocity field and temperature field with the controlled operating parameters of the machine. This study is a part of SECRET (Smart Efficient Compression, Reliability and Energy Targets) project which is supported by an award from The Royal Academy of Engineering to City, University of London's Centre for Compressor Technology and Howden Compressors.

National Instrument-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) is identified for velocity field measurement, and setup is designed using optical components. For temperature field measurement, the Planar laser-induced fluorescence (PLIF) is selected based on a feasibility study carried out of PLIF for the current application, and the setup is designed for in-house experiments. High-speed infrared thermography is chosen to measure the surface temperature of a rotary element in operating condition. A combination of PIV, PLIF, and Infrared thermography can produce velocity field, temperature field, and surface temperature history of a lobe. Experimental results will help to generate data of aerothermal behaviour in clearance flows of PDMs, and it will provide a benchmark case for CFD validation.



2.2. Particle image velocimetry (PIV) setup

Particle image velocimetry (PIV) is an optical method of flow visualization. It is used to obtain instantaneous velocity measurements and related properties in fluids. For that optical access of clearance gap is required. For optical access, fused silica glass is the best suitable material for our application. Figure 4 (a) shows a complex shape of the optical Glass manufactured from the Fused silica. As shown in Figure 4 (b) & (c), Optical access from radial direction and side of the Roots blower (upper left portion in Figure 1(b)) is provided to visualize a flow in clearance gap between rotor tip and housing. A thin gasket is used between metal and Glass surface to avoid any direct contact to reduce risk of cracking in the Glass at higher working temperature. Glass is kept tight using an external metal plate to eliminate leakage through glass and metal mating surfaces.
figure 4

In PIV experiments the fluid is seeded with tracer particles, for sufficiently small particles, are assumed to faithfully follow the flow dynamics. The fluid with entrained particles is illuminated by laser and motion of those particles are captured by cameras. The motion of the seeding particles is used to calculate speed and direction (the velocity field) of the flow being studied. Schematic of the designed PIV setup is shown in Figure 5. Dual pulse Litron Bernoulli laser with 532nm wavelength is selected. Its maximum output energy is about 200mJ per pulse at frequency of 15Hz. This laser has a pulse-to-pulse energy stability less than 2% RMS. For imaging, HiSense zyla 5.5 MP camera, 8 bit and 40 fps having resolution of 2560 by 2160 pixels has been chosen. K2 DistaMax standard objective is used to get better magnification of small clearance gap. High performance synchronizer is used along with encoder input for cyclic synchronizer to trigger laser and image at same time precisely. Long light guide arm (2100mm) suitable for 532 and 266nm wavelength is selected to transfer laser beam from laser head to area under study. In addition, laser sheet optics is required to convert laser beam to laser sheet, for that UV/VIS Parallel Light Sheet Optics and beam waist adjuster are implemented. In this study DEHS (Di-Ethyl-Hexyl-Sebacic) particles as a tracer, it will be seeded using Liquid Seeder (type FT700CE).
figure 5


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