Recent advances in high-speed planar Rayleigh scattering in turbulent jets and flames: increased record lengths, acquisition rates, and image quality

Abstract

In this manuscript, we describe recent advances made within our research group to obtain long-record-length, high-speed (10–40 kHz), two-dimensional planar Rayleigh scattering image sequences in turbulent non-reacting and reacting flows with high image quality. Specifically, high-speed planar Rayleigh scattering is used to obtain the time-varying mixture fraction field in non-reacting turbulent jets and the time-varying temperature field in turbulent non-premixed flames. This study highlights the significant improvements obtained with the use of a new, high-energy pulse burst laser system (HEPBLS). The HEPBLS, which can output ultra-high pulse energies at 10–50 kHz (e.g., >500 mJ/pulse at 532 nm and 10 kHz in this study) and long burst durations (e.g., up to 25 ms), allows for high-resolution, long-duration imaging of two-dimensional, time-correlated scalar fields using planar Rayleigh scattering in turbulent flows and flames. The combination of the high-energy output from the HEPBLS and an optimized optical collection system allows for the use of an un-intensified CMOS camera for all measurements, which greatly improves the spatial resolution and the signal-to-noise ratio (SNR) as compared to previous high-speed, two-dimensional mixture fraction and temperature imaging (Patton et al. in Appl Phys B 106:457, 2012; 108(2):377, 2012). Resolution and SNR tests are presented quantifying the benefits of the new HEPBLS/optical collection system. The improved spatial resolution and SNR allow detection of smaller-scale turbulent features (e.g., turbulent fluctuations) as well as improved measurements of scalar gradients as demonstrated in sample mixture fraction and temperature image sequences in highly turbulent flows. Improvements in SNR of a factor of seven are demonstrated in the turbulent non-reacting jets, and results from non-premixed flames demonstrate “single-shot” SNR of 90 in air and 35 at 2,000 K, which represents a 3.5 times improvement as compared to previously published work. Beyond visualization, the potential of the time-correlated, two-dimensional mixture fraction and temperature data is discussed in terms of new, multi-point, multi-time statistics.