Measuring with pressure sensitive paint in time-varying flows

Abstract: Increasingly tougher legislation on internal combustion engine emissions drives the development towards smaller engines with higher efficiency where an important component is the gas-exchange system and especially the turbocharger. The flow in the gas-exchange system is inherently pulsating and unsteady and the present thesis aims at investigating if and how pressure sensitive paint (PSP) can be used for internal unsteady flows of relevance for engine applications. PSP is an optical non-intrusive technique for pressure measurements on surfaces and in the thesis different acquisition, evaluation and signal-to-noise-raising methods have been evaluated and developed with focus on unsteady internal flow. In particular it describes a path towards measurements of unsteady pressure distributions on the impeller blades of turbocharger compressors appearing in compressors at surge.As a first step, dynamic calibration of a polymer/ceramic pressure sensitive paint (PC-PSP) was made using a shock tube. The cut-off frequency for the tested ruthenium-based formulation was found to be a few kilohertz; sufficient for resolving unsteady compressor behaviour such as surge and rotating stall.The same PC-PSP was used for measurements of the pressure on the inside wall of a y-junction sized to resemble the exhaust manifold of a car engine. The intensity method was used where a LED array provided excitation light and luminescent intensities was acquired using a CCD camera. Phase averaging was made in-camera by summing the intensity from several LED flashes phase locked to the flow pulses.A filtering technique based on singular value decomposition (SVD) was also developed. As a test case the fluctuating pressure field due to unsteady vortex shedding on the side of a square cylinder was evaluated. The data was captured using a high speed CMOS camera and continuous LED light. The result was a reduction of pixel noise on the order of two magnitudes that made it possible to recover vortex shedding behaviour otherwise submerged in noise.Due to complex geometries and high rotational speeds, pressure measurements on the impeller blades are unfeasible using traditional pressure taps and transducers and here the pressure was measured with PSP on the impeller blades of a rotating compressor. For this study, point measurements using a scanning laser for excitation and a photomultiplier tube for the acquisition of the luminescence was used and evaluated with the so called lifetime method. The measurements were able to capture the surge frequency as well as its spatial distribution.