Two-Dimensional Tomographic Simultaneous Multi-Species Visualization—Part I: Experimental Methodology and Application to Laminar and Turbulent Flames

In recent years, the tomographic visualization of laminar and turbulent flames has received much attention due to the possibility of observing combustion processes on-line and with high temporal resolution. In most cases, either the spectrally non-resolved flame luminescence or the chemiluminescence of a single species is detected and used for the tomographic reconstruction. In this work, we present a novel 2D emission tomographic setup that allows for the simultaneous detection of multiple species (e.g., OH*, CH* and soot but not limited to these) using a single image intensified CCD camera. We demonstrate the simultaneous detection of OH* (310 nm), CH* (430 nm) and soot (750 nm) in laminar methane/air, as well as turbulent methane/air and ethylene/air diffusion flames. As expected, the reconstructed distributions of OH* and CH* in laminar and turbulent flames are highly correlated, which supports the feasibility of tomographic measurements on these kinds of flames and at timescales down to about 1 ms. In addition, the possibilities and limitations of the tomographic approach to distinguish between locally premixed, partially premixed and non-premixed conditions, based on evaluating the local intensity ratio of OH* and CH* is investigated. While the tomographic measurements allow a qualitative classification of the combustion conditions, a quantitative interpretation of instantaneous reconstructed intensities (single shot results) has a much greater uncertainty

Spatially and Temporally Resolved Measurements of NO Adsorption/Desorption over NOx‐Storage Catalyst

The two‐dimensional (2D) temporal evolution of the NO‐concentration over a NOx‐storage catalyst is investigated in situ with planar laser‐induced fluorescence (PLIF) in an optically accessible parallel wall channel reactor. Signal accumulated phase‐correlated 2D‐recordings of repetitive adsorption/desorption cycles are obtained by synchronizing the switching of the NO gas flow (on/off) with the laser and detection system, thereby significantly increasing the signal‐to‐noise ratio. The gas compositions at the reactor outlet are additionally monitored by ex‐situ analytics. The impacts of varying feed concentration, temperature and flow velocities are investigated in an unsteady state. Transient kinetics and the mass transfer limitations can be interpreted in terms of the NO concentration gradient changes. The technique presented here is a very useful tool to investigate the interaction between surface kinetics and the surrounding gas flow, especially for transient catalytic processes

DNS of Near Wall Dynamics of Premixed CH4_4/Air Flames

This work presents a numerical study on the effect of flame-wall interaction (FWI) from the viewpoint of flame dynamics. For that purpose, direct numerical simulations (DNS) employing detailed calculations of reaction rates and transport coefficients have been applied to a 2D premixed methane/air flame under atmospheric condition. Free flame (FF) and side-wall quenching (SWQ) configurations are realized by defining one lateral boundary as either a symmetry plane for the FF or a cold wall with fixed temperature at 20 oC for the SWQ case. Different components of flame stretch and Markstein number regarding tangential, normal (due to curvature) and total stretch, Ka$_s$ , Ka$_c$ and Ka$_tot$ = Ka$_s$ + Ka$_c$, as well as their correlations with respect to the local flame consumption speed SL have been evaluated. It has been shown that the FWI zone is dominated by negative flame stretch. In addition, S$_L$ decreases with decreasing normal stretch due to curvature Ka$_c$ while approaching the cold wall. However, S$_L$ increases with decreasing Ka$_c$ while approaching the symmetry boundary for the free flame case, leading to an inversion of the Markstein number Ma$_tot$ based on Ka$_tot$ from positive in the free flame case to negative in the SWQ case. The quenching distance evaluated based on wall-normal profiles of S$_L$ has been found to be approximately equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature. The flame speed has been confirmed to scale quasi-linearly with the stretch in the FWI zone. The results reveal a distinct correlation during transition between FWI and FF regarding flame dynamics, which brings a new perspective for modeling FWI phenomena by means of flame stretch and Markstein number. To do this, the quenching effect of the wall may be reproduced by a reversed sign of the Markstein number from positive to negative in the FWI zone and by applying the general linear Markstein correlation (S$_L$/S$_{L,0}$ = 1− Ma · Ka), leading to a decrease of the flame speed or the reaction rate in the near-wall region

Carbon nanostructure and reactivity of soot particles from non-intrusive methods based on UV-VIS spectroscopy and time-resolved laser-induced incandescence

The objective of this study is to derive morphological and nanostructural properties of soot as well as the reactivity against low-temperature oxidation by O₂ from easily measurable optical properties. First, ex-situ experiments utilizing thermogravimetric analysis (TGA) and high-resolution transmission electron microscopy (HRTEM) serve to evaluate the kinetics of soot oxidation with O₂ and relate reactivity to particle morphology and nanostructure. Second, ultraviolet–visible (UV-VIS) absorption spectra provide wavelength-dependent absorption cross sections and refractive-index functions E(m~,λ). From these, optical band gap energies, EOG, and coefficients ξ∗ for single parameter functions describing the wavelength-dependency of E(m~,λ) are obtained. Third, from time-resolved laser-induced incandescence (TR-LII) ratios of the refractive-index functions E(m~,λi)/E(m~,λj) at three excitation wavelengths and primary particle size distributions are acquired.The ex-situ experiments show that the size of the graphene layers predominantly determines soot reactivity against oxidation. Graphene layer size and, therefore, soot reactivity are reflected in the UV-VIS absorption spectra and E(m~,λ), EOG, and ξ∗, respectively. Similarly, scattering-corrected ratios E(m~,λi)/E(m~,λj) from TR-LII also reflect graphene layer size and, hence, soot reactivity. The established strong correlations between the optical properties, nanostructural characteristics and reactivity against oxidation make UV-VIS spectroscopy as well as TR-LII useful fast in-situ diagnostic methods for soot reactivity

Turbulent impinging jets on rough surfaces

This work presents direct numerical simulations (DNS) of a circular turbulent jet impinging on rough plates. The roughness is once resolved through an immersed boundary method (IBM) and once modeled through a parametric forcing approach (PFA) which accounts for surface roughness effects by applying a forcing term into the Navier–Stokes equations within a thin layer in the near-wall region. The DNS with the IBM setup is validated using optical flow field measurements over a smooth and a rough plate with similar statistical surface properties. In the study, IBM-resolved cases are used to show that the PFA is capable of reproducing mean flow features well at large wall-normal distances, while less accurate predictions are observed in the near-wall region. The demarcation between these two regions is approximately identified with the mean wall height of the surface roughness distribution. Based on the observed differences in the results between IBM- and PFA-resolved cases, plausible future improvements of the PFA are suggested

Experimental and numerical investigation of NO oxidation on Pt/Al₂ O₃- and NOₓ storage on Pt/BaO/Al₂ O₃-catalysts

Planar laser-induced fluorescence with laser synchronized flow control is employed as a non-invasive in situ technique to investigate a NOₓ storage catalyst, especially to grant a deeper insight into the reaction dynamics and its interaction with mass transfer. In addition to visualizing the spatial and temporal NO evolution over a Pt/BaO/Al₂O₃ catalyst, the spatially resolved NO distribution over a Pt/Al₂O₃ catalyst in the steady-state is measured to understand the oxidation of NO to NO₂ as a precursor step of NOₓ storage. The experimental results are compared with corresponding numerical simulations using transient one- and two-dimensional reactor simulations with detailed surface reaction mechanisms. The thermodynamic equilibrium for NO oxidation over Pt/Al₂O₃ approaches between 623 K and 723 K, as a reduced conversion is observed at a higher temperature. The NOₓ storage on the Pt/BaO/Al₂O₃ catalyst decreases with time, which is partly due to the reduced storage capacity, but also strongly limited by the NO oxidation rate

Numerical Study of Quenching Distances for Side-Wall Quenching Using Detailed Diffusion and Chemistry

The numerical investigation of quenching distances in laminar flows is mainly concerned with two setups: head-on quenching (HOQ) and side-wall quenching (SWQ). While most of the numerical work has been conducted for HOQ with good agreement between simulation and experiment, far less analysis has been done for SWQ. Most of the SWQ simulations used simplified diffusion models or reduced chemistry and achieved reasonable agreement with experiments. However, it has been found that quenching distances for the SWQ setup differ from experimental results if detailed diffusion models and chemical reaction mechanisms are employed. Side-wall quenching is investigated numerically in this work with steady-state 2D and 3D simulations of an experimental flame setup. The simulations fully resolve the flame and employ detailed reaction mechanisms as well as molecular diffusion models. The goal is to provide data for the sensitivity of numerical quenching distances to different parameters. Quenching distances are determined based on different markers: chemiluminescent species, temperature and OH iso-surface. The quenching distances and heat fluxes at the cold wall from simulations and measurements agree well qualitatively. However, quenching distances from the simulations are lower than those from the experiments by a constant factor, which is the same for both methane and propane flames and also for a wide range of equivalence ratios and different markers. A systematic study of different influencing factors is performed: Changing the reaction mechanism in the simulation has little impact on the quenching distance, which has been tested with over 20 different reaction mechanisms. Detailed diffusion models like the mixture-averaged diffusion model and multi-component diffusion model with and without Soret effect yield the same quenching distances. By assuming a unity Lewis number, however, quenching distances increase significantly and have better agreement with measurements. This was validated by two different numerical codes (OpenFOAM and FASTEST) and also by 1D head-on quenching simulations (HOQ). Superimposing a fluctuation on the inlet velocity in the simulation also increases the quenching distance on average compared to the reference steady-state case. The inlet velocity profile, temperature boundary condition of the rod and radiation have a negligible effect. Finally, three dimensional simulations are necessary in order to obtain the correct velocity field in the SWQ computations. This however has only a negligible effect on quenching distances