Flame-Wall Interactions at Elevated Pressure Studied with Advanced Laser Diagnostics

Despite the ongoing progress in electrification driven by the indispensable substitution of fossil fuels with renewable energy sources, the thermochemical conversion of chemical energy carriers is expected to continue playing an important role in the energy transition. This underlines the necessity for continual development of low-emission combustion technologies, grounded in a comprehensive understanding of the underlying physical processes through ongoing fundamental research efforts. Within this cumulative dissertation, the complex phenomenon of flame-wall interaction (FWI), which is an essential aspect of practical combustion systems, is investigated experimentally using advanced laser diagnostics. FWI involves the mutual interaction between chemical reaction, solid surface and fluid flow and is associated with undesired effects, such as reduced efficiency and increased pollutant emissions. The main objective of this thesis, which includes three peer-reviewed publications, is the investigation of fundamental aspects of FWI at elevated pressures and increased Reynolds numbers, mimicking operating conditions of practical combustion systems. Experiments are carried out within a novel, enclosed test rig - the pressurized side-wall quenching (SWQ) burner - which provides a reproducible, generic configuration of a premixed flame interacting with a cold, solid wall. This test rig was designed, built and commissioned within the scope of this thesis and is presented in detail. The process under investigation is examined at operating pressures ranging between atmospheric and 5 bar absolute and Reynolds numbers up to 20,000 using various laser diagnostics. Limitations of such measurement techniques resulting from the complex test rig design and process-inherent challenges arising with increasing pressure are reported. First, a characterization of the turbulent flow field and the combustion dynamics is conducted using velocity data and spatial fields of the instantaneous flame front positions, provided by high-speed and low-speed particle image velocimetry measurements and planar, laser-induced fluorescence of the hydroxyl radical (OH-PLIF). This involves the inspection of the inflow and the near-wall flow field under non-reacting and reacting conditions, as well as the examination of the transient flame front motion. Building on this, turbulent flame propagation close to the wall is further analyzed in terms of the flame surface density (FSD), a central quantity in numerical combustion modeling, which is derived from measured flame front positions (OH-PLIF) following two common approaches. Furthermore, the near-wall thermochemistry of the turbulent flame quenching process is explored through simultaneous measurements of the gas-phase temperature and mole fractions of CO2 and CO by means of dual-pump coherent anti-Stokes Raman spectroscopy and laser-induced fluorescence of CO. These measurements represent the first reported attempt to investigate the thermochemistry of FWI at pressures above atmospheric by means of multi-parameter laser diagnostics. This cumulative dissertation presents novel insights into the impact of elevated pressure and increased Reynolds numbers on turbulent FWI and provides experimental data for model validation. It furthermore, contributes to the FWI research community by exploring the limits of state-of-the-art laser diagnostics for measurements in pressurized, near-wall reactive flows

On the evolution of turbulent boundary layers during flame-wall interaction investigated by highly resolved laser diagnostics

The turbulent boundary layer behavior in the presence of flame-wall interactions (FWI) has an important role on the mass and energy transfer at the gas/solid interface. Detailed experiments resolving the turbulent boundary layer evolution in the presence of FWI are lacking, which impedes knowledge. This work presents a combination of particle image velocimetry (flow field), dual-pump coherent anti-Stokes Raman spectroscopy (gas temperature), and OH laser induced fluorescence (flame topology) measurements to study the evolution of the boundary layer structure in the presence of FWI. Experiments are conducted in a side-wall quenching (SWQ) burner. Findings reveal that the reacting boundary layer flow adheres to the linear scaling law u+ = y+ in the viscous sublayer until y+ = 5. Beyond y+ = 5, the flame modifies the velocity and temperature field such that the uz+ streamwise velocity deviates from the viscous sublayer and the law-of-the-wall scaling in the log-layer with uz+ being smaller than that of the non-reacting flow (the subscript z refers to the streamwise coordinate and is used throughout this manuscript). As the fluid approaches the flame impingement location at the wall, the gas temperature increases significantly, causing a threefold increase in kinematic viscosity, ν. Although the near-wall streamwise velocity gradient d&lt;Uz&gt;/dy|y=0 decreases, the larger increase in ν reduces uz+ and leads to the deviation from the law-of-the-wall. Downstream the flame impingement location, ν is relatively constant and uz+ values begin to approach those of the law-of-the-wall. Trends are presented for SWQ and head-on quenching flame topologies, and are intended to help development of more accurate wall models.  </div

Assessment of the impact of multiple mild-steam decontaminations on the protection performance of disposable KN95 filtering facepiece respirators

The COVID-19 pandemic caused tremendous supply bottlenecks of single-use filtering facepiece respirators (FFRs) leading to a growing need for a potential reuse. This study assesses the impact of multiple mild-steam decontaminations with 121 °C/2000 mbar/20 min on the protection performance of disposable FFRs. It focuses on FFRs of type KN95 that is recently dominating the markets, but its decontamination is not covered in the literature. It was found that up to ten cycles, only minor degradation in the filter efficiency, breathing resistance and none in the material structure is apparent, suggesting a potential for multiple decontamination cycles at almost unchanged protective properties of KN95 FFRs

Flame-Wall Interactions at Elevated Pressure Studied with Advanced Laser Diagnostics

Despite the ongoing progress in electrification driven by the indispensable substitution of fossil fuels with renewable energy sources, the thermochemical conversion of chemical energy carriers is expected to continue playing an important role in the energy transition. This underlines the necessity for continual development of low-emission combustion technologies, grounded in a comprehensive understanding of the underlying physical processes through ongoing fundamental research efforts. Within this cumulative dissertation, the complex phenomenon of flame-wall interaction (FWI), which is an essential aspect of practical combustion systems, is investigated experimentally using advanced laser diagnostics. FWI involves the mutual interaction between chemical reaction, solid surface and fluid flow and is associated with undesired effects, such as reduced efficiency and increased pollutant emissions. The main objective of this thesis, which includes three peer-reviewed publications, is the investigation of fundamental aspects of FWI at elevated pressures and increased Reynolds numbers, mimicking operating conditions of practical combustion systems. Experiments are carried out within a novel, enclosed test rig - the pressurized side-wall quenching (SWQ) burner - which provides a reproducible, generic configuration of a premixed flame interacting with a cold, solid wall. This test rig was designed, built and commissioned within the scope of this thesis and is presented in detail. The process under investigation is examined at operating pressures ranging between atmospheric and 5 bar absolute and Reynolds numbers up to 20,000 using various laser diagnostics. Limitations of such measurement techniques resulting from the complex test rig design and process-inherent challenges arising with increasing pressure are reported. First, a characterization of the turbulent flow field and the combustion dynamics is conducted using velocity data and spatial fields of the instantaneous flame front positions, provided by high-speed and low-speed particle image velocimetry measurements and planar, laser-induced fluorescence of the hydroxyl radical (OH-PLIF). This involves the inspection of the inflow and the near-wall flow field under non-reacting and reacting conditions, as well as the examination of the transient flame front motion. Building on this, turbulent flame propagation close to the wall is further analyzed in terms of the flame surface density (FSD), a central quantity in numerical combustion modeling, which is derived from measured flame front positions (OH-PLIF) following two common approaches. Furthermore, the near-wall thermochemistry of the turbulent flame quenching process is explored through simultaneous measurements of the gas-phase temperature and mole fractions of CO2 and CO by means of dual-pump coherent anti-Stokes Raman spectroscopy and laser-induced fluorescence of CO. These measurements represent the first reported attempt to investigate the thermochemistry of FWI at pressures above atmospheric by means of multi-parameter laser diagnostics. This cumulative dissertation presents novel insights into the impact of elevated pressure and increased Reynolds numbers on turbulent FWI and provides experimental data for model validation. It furthermore, contributes to the FWI research community by exploring the limits of state-of-the-art laser diagnostics for measurements in pressurized, near-wall reactive flows

Laser-based investigation of flame surface density and mean reaction rate during flame-wall interaction at elevated pressure

Underlying data of the figures in publication "Laser-based investigation of flame surface density and mean reaction rate during flame-wall interaction at elevated pressure" in Proceedings of the Combustion Institut

Laser-based investigation of flame surface density and mean reaction rate during flame-wall interaction at elevated pressure

Velocities and flame front locations are measured simultaneously in a turbulent, side-wall quenching (SWQ) V-shaped flame during flame-wall interaction (FWI) at 1 and 3 bar by means of particle image velocimetry (PIV) and planar laser-induced fluorescence of the OH radical (OH-PLIF). The turbulent flame brush is characterized based on the spatial distribution of the mean reaction progress variable and a common direct method is used to derive the flame surface density (FSD) from the two-dimensional data by image processing. As the near-wall reaction zone is limited to a smaller region closer to the wall at higher pressure, higher peak values are observed in the FSD at 3 bar. A second definition of the FSD adapted for flames exposed to quenching is utilized similar to previous studies emphasizing the impact of FWI. The influence of the wall on the flame front topology is investigated based on a flame front-conditioned FSD and its variability within the data set. In a last step, an estimate of the mean reaction rate is deduced using an FSD model and evaluated in terms of integral and space-averaged values. A decreasing trend of integral mean reaction rate in regions with increasing flame quenching is observed for both operating conditions, but more pronounced at 3 bar. Space-averaged mean reaction rates, however, increase in the quenching region, as the size of the reaction zone decreases

Characterization of flow field and combustion dynamics in a novel pressurized side-wall quenching burner using highspeed PIV/OH-PLIF measurements

Digital version of the figures in publication (insert link here

Classification of flame prehistory and quenching topology in a side-wall quenching burner at low-intensity turbulence by correlating transport effects with CO2, CO and temperature

Thermochemical states of a turbulent, lean premixed dimethyl ether/air flame were assessed for the first time using simultaneous measurements of gas temperature T, CO2 and CO mole fractions with locations as close as 120 um above the quenching wall. This is realized by combined dual-pump coherent anti-Stokes Raman spectroscopy (DP-CARS) and two-photon laser-induced fluorescence (LIF) of CO. In addition to thermochemical states, the flow and flame dynamics were measured separately using a combined two component particle image velocimetry (PIV) and planar LIF of the OH radical at high (4 kHz) and low (50 Hz) repetition rates. The data from the independent measurements was linked by the instantaneous flame front topologies, determined by the qualitative OH-LIF in both experiments. The grid-generated turbulence intensity was found to be relatively low in the bulk flow (~ 4.5%, streamwise velocity component) with the turbulent flame classified within the regime of wrinkled flamelets (w'/sL between 0.3 and 0.75, for streamwise velocity component). The flame-wall interaction could be assigned to either a side-wall quenching (SWQ)-like (~ 50%) or a head-on quenching (HOQ)-like scenario (~ 50%), with a transition between these scenarios taking place within a few milliseconds. The thermochemical states depend significantly on whether a SWQ-like or a HOQ-like scenario is present. Here, the thermochemistry of the SWQ-scenario is studied in detail, and three zones, A, B, C, could be distinguished in the state space on the basis of the (CO2,T) correlations. Zone A is characterized by strong wall heat losses and mixing influences, while zone B features less pronounced wall heat losses and mixing processes. In zone C the impacts of turbulence almost completely disappear and conditions comparable to a laminar near-wall flow are observed. The distinguishability of the three zones in the (CO,T) or (CO,CO2) correlations is less clear, which underlines the importance of the additional CO2 measurement in the DP-CARS methodology