Measurement of inter-scale molecular fluxes in supercritical reacting turbulence

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Understanding how fluctuations at a specific length scale in a turbulent flow affect the phenomena at other length scales is critical to accurately predict flow properties by numerical simulation. In turbulent combustion, which is the predominant mode of converting chemical energy to thermal energy for transportation and power generation applications, additional challenges arise in comparison to non-reacting turbulent flows because the fluctuating chemical reaction rates occur at very small length scales that typically are not resolved by simulations. This problem is exacerbated by the desire to operate combustion systems at increasingly high pressures, often spanning in to the supercritical pressure conditions. While increasing the pressure has important benefits for efficiency and power density, it also concentrates fluctuations at increasingly small scales and introduces strong variations in thermodynamic properties around the critical point. To better understand this phenomenon, this project will use a novel combination of laser diagnostic and data analysis techniques to measure how fluctuations in molecular nitrogen density move between length scales in premixed flames burning reactants at initially supercritical conditions. Time-resolved measurements of the spatially filtered nitrogen number density and velocity fields will be made in crossed planes using burst-mode spontaneous Raman scattering. The filtering effect of the observation process will be determined and tuned through image deconvolution. Hence, the resultant data will allow explicit determination of the flux of sub-filter fluctuations through the filter scale based on the dynamics of the filtered fields. This information will identify whether traditional modeling paradigms hold for supercritical reacting turbulence and, where necessary, enable the development of more physically accurate models. It also will provide a unique data set against which to benchmark simulations of supercritical turbulent combustion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.