Inter-scale energy transfer in turbulent premixed combustion

Abstract

Turbulent premixed combustion is widely used for energy conversion in power generation and propulsion devices. However, our understanding of the underlying fluid dynamics, combustion, and their interaction is still incomplete. The complexity of turbulent combustion arises from the non-linear, multi-scale, and multi-physics nature of the problem, which involves interactions between fluid dynamic and chemical processes across a myriad of length and time scales. The existing literature demonstrates that the dynamics of reacting turbulence does not necessarily follow the same phenomenology as in non-reacting incompressible turbulence. One of the key differences in reacting and compressible flows is the reversal of the classical turbulent energy cascade in a process termed as ‘backscatter’. Moreover, backscatter was shown to potentially depend on the magnitude of the pressure gradients across the flame; this is reflected in the sub-filter-scale pressure-work. Previous studies have predominantly focused on flames in homogeneous isotropic turbulence (HIT), in which the pressure gradients are associated with the flame and turbulence themselves. In contrast, practical combustors have mean pressure fields generated by the flow, which can induce significantly different turbulence dynamics as compared to non-reacting turbulence. The presented research explores the conditions at which energy backscatter occurs in an aerospace relevant configuration and attempts to identify the underlying physical mechanisms that have a leading order impact on these processes. This is done through systematic variation of the global equivalence ratio, the jet flow velocity, and the swirl number. The impact of these controlling parameters on turbulence production and energy backscatter is assessed through the analysis of filtered kinetic energy transport equations. Tomographic particle image velocimetry (TPIV) and planar laser induced fluorescence (PLIF) are used to measure the 3D velocity fields and planar distribution of formaldehyde, respectively; the relevant thermodynamic properties (e.g., density and progress variable) are estimated from PLIF data. Ultimately, this work provides both an assessment of the validity of current turbulence modeling paradigms employed in aerospace relevant combustion, as well as the data necessary to develop and validate new models if required.