Simultaneous High-Resolution kHz-Rate 2-D Conserved Scalar and 3-Component Velocity Field Measurements in Gas-Phase Turbulent Jets

Abstract

Turbulent scalar mixing processes are of significant interest to a broad range of fields within the fluid and thermal sciences. An improved understanding of the underlying physics of how two independent fluid streams mix under turbulent flow conditions is important for a multitude of engineering systems and natural processes ranging from the mixing of a fuel and oxidizer in combustion chambers to the dispersal of pollutants within the atmosphere. The dispersion and mixing of scalars is dependent on the local three-dimensional velocity field, which for turbulent flows, fluctuates in both space and time. In this manner, it is important to have both scalar and velocity measurements with high spatial and temporal resolution across a large dynamic range of scales. Laser-based imaging diagnostics have made it possible to investigate the underlying structure of turbulent flows with high spatial resolution; however, the majority of commonlyused scalar measurement techniques have been limited to low acquisition rates in gas-phase flows. Typically, gasphase measurements utilize techniques which rely upon high laser pulse energies (i.e., 100’s mJ/pulse) due to the relatively “weak” signals collected from the scalar of interest. Such levels of output pulse energy are not currently possible with commercially-available high-speed laser systems. In the present work we demonstrate simultaneous acetone planar laser-induced fluorescence (PLIF) and stereoscopic particle imaging velocimetry (sPIV) at 10 kHz. The acetone acts as a tracer to mark the conserved scalar field and the sPIV measurements yield all three components of the turbulent velocity field. The PLIF measurements are facilitated by a High-Energy Pulse-Burst Laser System (HEPBLS), which can generate 266-nm pulse trains for burst durations exceeding 20 ms at 10 kHz with >140 mJ/pulse. The fourth-harmonic output of the HEPBLS is approximately an order of magnitude higher than any previously-reported kHz-rate 266-nm output. The high pulse energies available from the HEPBLS allows our preliminary measurements to be performed with low tracer seed levels (4% acetone in the main jet and 0.75% acetone in the co-flow) which significantly reduces absorption effects and facilitates quantitative measurements. A sample image set of the scalar field, where the velocity field is left off for visual clarity, is shown in Fig. 1 demonstrating the detail in both time and space that is possible with the current measurements. The sPIV measurements are made with an EdgeWave IS80-2-LD double-pulsed PIV laser capable of 40 Watts of average power per laser head. While PIV measurements are possible using the HEPBLS, an independent laser system is employed for the PIV measurement to allow for an optimization of the temporal spacing between the PIV double pulses and an accurate temporal placement of the PIV laser pulses in reference to the scalar measurement. Both of these considerations are important when attempting to accurately correlate an inferred velocity field to an instantaneous scalar measurement. The two laser systems are coupled with two Vision Research VR710 (sPIV) and one VR711 (PLIF) cameras for the data collection. The PLIF imaging system includes a 240-mm focal length acromat lens coupled with an 85 focal length f#1.4 Nikkor camera lens to maximize the signal while maintaining the desired resolution of 55 um x 55 um per pixel. The sPIV cameras are each mounted with a scheimpflug adapter and a 200-mm focal length Nikkor Macro lens leading to a resolution of 28 um x 28 um per pixel. For the given repetition rate of 10 kHz the fields of view of the PLIF and PIV cameras are 27.5 mm x 60.5 mm and 11 mm x 25 mm, respectively. The current work is centered on developing a new measurement capability for examining the time-dependent coupling between the turbulent velocity field and conserved scalar mixing in non-reacting gas-phase axisymmetric jets over a broad range of Reynolds number. Our preliminary results show significant promise for the methods described above. Simultaneous 3-component velocity and 2-D scalar imaging will provide the necessary data (both visualization and spatio-temporal statistics) to study the complex interaction between the velocity and scalar fields. Beyond an increased understanding of the underlying physics, new spatiallyand temporally-resolved data will provide new, detailed information for assessing and validating turbulence models.