Project Details
Projekt
Novel experimental imaging framework for laser-optical two-phase flow analysis across thin annular rotor-stator gaps
Applicant
Dr.-Ing. Jochen Kriegseis
Subject Area
Technical Thermodynamics
Hydraulic and Turbo Engines and Piston Engines
Fluid Mechanics
Hydraulic and Turbo Engines and Piston Engines
Fluid Mechanics
Term
since 2023
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 510921053
The operation of oil-injected rotary-type positive displacement compressors (RPDC) like screw compressors is accompanied with the formation of a complex two-phase surge and gap flow across the lateral rotor-stator gap, which as yet has to be rigorously investigated. Subproject A2 provides multiple experimentally obtained information on this two-phase gap flow with moving boundaries. For this purpose, a complex imaging framework for two-phase flow diagnostics in narrow annular rotor-stator gaps is developed to deliver three-dimensional (3D) three-component (3C) velocity information of the liquid phase, while simultaneously tracking 3D-locations, size and growth as well as speed of bubbles and/or larger gaseous flow patterns. The framework builds upon three simultaneously operated measurement techniques, which are substantially modified to meet the requirements of the narrow curved annular rotor-stator gap with only single-sided optical access. The liquid phase is investigated by means of defocusing particle tracking velocimetry (DPTV), which provides 3D3C velocity fields of the liquid flow. The gas phase is studied with two additional whole-field techniques, which are purposely chosen to span from small bubbles to large gaseous patches. Interferometric particle imaging (IPI) is expanded to the operation in back-scatter mode and across the astigmatic impact of the curved surfaces to qualify the method for the investigation of bubbles in the considered thin annular gap. In addition, the size, location and also growth and velocity of the bubbles are quantified, and phenomena like coalescence or bubble-bubble interactions are evaluated with IPI. Larger gaseous patches are investigated with high-speed imaging (HSI), which reveals macroscopic flow patterns that are consequently able to classify as flow-pattern maps. All three methods are combined into a single experimental apparatus to ensure identical (simultaneous) optical access of all measurement techniques. In addition, the image- and data-processing strategies of either technique are substantially expanded to account for the complex optical transfer functions as present in the given scenario. The framework will be utilized at the compressor test rig as built in subproject A1 so as to reveal the desired flow-field information during operation of different rotor-geometries and speeds, as well as under various operating conditions. All image-processing approaches are complemented with the rapidly maturing capabilities of deep learning methods for image evaluation, such that the developed comprehensive flow diagnostics approach can take combined advantage of multiple target-specific processing strategies. Results are converted into valuable global values and local distributions of gas-liquid slip, void-fraction, near-wall velocity gradients, which is essential information for the Research Unit FOR 5595.
DFG Programme
Research Units
