Development of fiber-coupled high-speed particle image velocimetry (PIV) for flow fields measurements in gas turbine engine hardware

Abstract

We demonstrate the design and implementation of a fiber-optic beam-delivery system using a large-aperture, tapered step-index fiber for high-speed PIV in turbulent combustions flows. The tapered fiber in conjunction with a diffractive-optical-element (DOE) fiber-optic coupler significantly increases the damage threshold of the fiber, enabling fiber-optic beam delivery of sufficient nanosecond, 532-nm, laser pulse energy for high-speed PIV measurements. The fiber successfully transmits 1-kHz and 10-kHz laser pulses with energies of ~5.3 mJ and ~2 mJ, respectively, for more than 25 min without any indication of damage. It is experimentally demonstrated that the tapered fiber possesses the high coupling efficiency (~80%) and moderate beam quality for PIV. Additionally, the nearly uniform output-beam profile exiting the fiber is ideal for PIV applications. Comparative PIV measurements are made using a conventionally ( bulk-optic) delivered light sheet, and a similar order of measurement accuracy is obtained with and without fiber coupling. Effective use of fiber-coupled, 10-kHz PIV is demonstrated for instantaneous 2D velocity-field measurements in turbulent reacting flows. We have also developed and implemented a fiber-coupled, high-speed PIV and PLIF system for measuring hydroxyl radical (OH) concentration and velocity in a realistic 4-MW combustion rig.Simultaneous OH-PLIF and PIV imaging at a data-acquisition rate of 10 kHz is demonstrated in turbulent premixed flames behind a bluff body. Our results show significant promise for the performance of fiber-coupled high-speed PIV and OH-PLIF in harsh laser-diagnostic environments such as those encountered in gas-turbine test beds and the cylinder of a combustion engine. Nomenclature DC= direct current DOE = diffractive optical element LIDT = laser-induced damage threshold NA = numerical aperture OH = hydroxyl radicals PIV = particle image velocimetry PLIF = planar LIF PMT = photomultiplier tube SNR = signal-to-noise ratio UV = ultraviolet