Time-Resolved, Two-Dimensional Imaging of Scalar Mixing in Turbulent Gas-Phase Jets

The objective of this work is to examine the dynamics of scalar mixing in turbulent, gas-phase jets using kHz-rate laser diagnostics. The research is underpinned by a new High Energy Pulse Burst Laser System (HEPBLS), which is capable of delivering more than 150 high-energy ($>500mJ$) pulses with repetition rates exceeding 10 kHz. The unique system allows for the extension of traditionally low repetition-rate planar laser techniques such as Rayleigh scattering and Planar Laser-Induced Fluorescence (PLIF) to high-speed imaging applications. In this study, two turbulent jets with Reynolds number equal to $10,000$ and $15,000$ (based on jet diameter) are used to study time-dependent scalar mixing and dissipation processes. Temporally-resolved, two-dimensional images of the mixture fraction and scalar dissipation rate fields are obtained at axial positions of $\frac{x}{D} = 10$ to $\frac{x}{D} = 40$, revealing the highly transient mixing topology within turbulent jets. Averaged results are validated against similar imaging techniques at low repetition rates and known turbulent scaling laws. In addition to ``real time'' visualization, the scalar mixing dynamics are characterized with temporal and spatial statistics.