title: Investigation of Reaction and Transport Phenomena during Flame-Wall Interaction Using Laser Diagnostics
Creators: name: Zentgraf, Florian
Publisher: name: UNSPECIFIED
Year: 2022

description: In this dissertation, flame-wall interaction (FWI) is investigated experimentally with respect to near-wall reaction and transport effects. In technical combustion, FWI processes are crucial, as they involve negative aspects like lowered efficiency or promoted pollutant formation. FWI is a highly complex interaction between surface, reaction and flow, where the most relevant processes occur a few hundred microns above the surface. So far, it is not yet fully understood. As near-wall reaction chemistry is fuel-dependent, FWI of novel alternative fuels thus requires investigation, like for the partially oxygenated dimethyl ether used here. A generic side-wall quenching (SWQ) burner for premixed atmospheric operation is used, with well-defined boundary conditions and good accessibility for experimental and numerical studies. Comprehensive measurements during FWI are performed by means of various laser diagnostics implemented simultaneously. Near-wall flame and flow dynamics are assessed simultaneously by spatially and temporally highly resolved fields of velocity (by particle image velocimetry) and flame front distribution (by laser-induced fluorescence (LIF) of OH). In turbulent operation, SWQ-like and head-on quenching (HOQ)-like events alternate randomly. Turbulent boundary layers are resolved down to y^+=1.5. During flame quenching, the inner structure (y^+≤5) remains unaffected while scaling laws are no longer valid farther from the wall. The results suggest that near-wall vortex structures promote exhaust gas recirculation (EGR) in SWQ-like events, while they push the reaction zone towards the wall in HOQ-like cases. Near-wall thermochemistry is studied by quantitatively and simultaneously measuring gas temperature and mole fractions of CO2 (by dual-pump coherent anti-Stokes Raman spectroscopy) as well as CO (by CO-LIF) along with qualitative OH-LIF. This is the first reported application of this three-parameter thermochemistry diagnostics in FWI environments. A validation of the approach yields good accuracy and precision for application during FWI. CO2 proves to be less affected by the non-adiabatic conditions during FWI than CO. Under laminar conditions, the importance of differential diffusion effects in near-wall thermochemistry is demonstrated by comparison to numerical simulation. Investigation of turbulent FWI suggests the presence of further transport mechanisms compared to the laminar case, different for SWQ- and HOQ-like events. Through coupling with the findings on vortex structures, the hypothesis on near-wall EGR is supported, as CO2 is evident upstream the quenching point. Overall, additionally measuring CO2 proves highly suitable to describe FWI and enables the identification of novel insights and phenomena. A feasibility study on partially premixed FWI reveals that both near-wall flame structure and thermochemistry change significantly compared to the fully premixed case. The presented results provide novel insights into FWI that extend beyond the state of research and yield outstanding validation data for numerical simulation.