Many important chemically reacting systems are inherently multi-dimensional with spatial and temporal variations in the thermochemical state, which can be strongly coupled to interactions with transport processes. Fundamental insights into these systems require multi-dimensional measurements of the thermochemical state as well as fluid dynamics quantities. Laser-based imaging diagnostics provide spatially and temporally resolved measurements that help address this need. The state of the art in imaging diagnostics is continually progressing with the goal of attaining simultaneous multi-parameter measurements that capture transient processes, particularly those that lead to stochastic events, such as localized extinction in turbulent combustion. Development efforts in imaging diagnostics benefit from advances in laser and detector technology. This article provides a perspective on the progression of increasing dimensionality of laser-based imaging diagnostics and highlights the evolution from single-point measurements to 1D and 2D multi-parameter imaging and 3D high-speed imaging. This evolution is demonstrated using highlights of laser-based imaging techniques in combustion science research as an exemplar of a complex multi-dimensional chemically reacting system with chemistry-transport coupling. Imaging diagnostics impact basic research in other chemically reacting systems as well, such as measurements of near-surface gases in heterogeneous catalysis. The expanding dimensionality of imaging diagnostics leads to larger and more complex datasets that require increasingly demanding approaches to data analysis and provide opportunities for increased collaboration between experimental and computational researchers in tackling these challenges.
Zhou, Bo; Li, Tao; Frank, Jonathan H.; Dreizler, Andreas; Bohm, Benjamin
High-speed, three-dimensional (3D) scalar-velocity field measurements were demonstrated in a lifted partially-premixed dimethyl-ether/air jet flame using simultaneous laser-induced fluorescence (LIF) of formaldehyde and tomographic particle image velocimetry (TPIV). The 3D LIF measurements were conducted by raster scanning the laser beam from a 100 kHz pulse-burst laser across the probe volume using an acousto-optic deflector. The volumetric reconstruction of the LIF signal from ten parallel planes provides quasi-instantaneous 3D LIF measurements that are synchronized with 10 kHz TPIV measurements. The temporally resolved formaldehyde-LIF and velocity field data were employed to analyze Lagrangian particle trajectories and displacement speeds at the base of the lifted flame. The particle trajectories revealed flow structures that are difficult to observe in an Eulerian reference frame. Positive and negative displacement speeds were observed at the formaldehyde-LIF surfaces at the inner and outer regions of the jet flame with a maximum displacement speed of approximately eight times the laminar flame speed of a stoichiometric dimethyl-ether/air mixture.
Fundamental chemistry in heterogeneous catalysis is increasingly explored using operando techniques in order to address the pressure gap between ultrahigh vacuum studies and practical operating pressures. Because most operando experiments focus on the surface and surface-bound species, there is a knowledge gap of the near-surface gas phase and the fundamental information the properties of this region convey about catalytic mechanisms. We demonstrate in situ visualization and measurement of gas-phase species and temperature distributions in operando catalysis experiments using complementary near-surface optical and mass spectrometry techniques. The partial oxidation of methanol over a silver catalyst demonstrates the value of these diagnostic techniques at 600 Torr (800 mbar) pressure and temperatures from 150 to 410 °C. Planar laser-induced fluorescence provides two-dimensional images of the formaldehyde product distribution that show the development of the boundary layer above the catalyst under different flow conditions. Raman scattering imaging provides measurements of a wide range of major species, such as methanol, oxygen, nitrogen, formaldehyde, and water vapor. Near-surface molecular beam mass spectrometry enables simultaneous detection of all species using a gas sampling probe. Detection of gas-phase free radicals, such as CH3 and CH3O, and of minor products, such as acetaldehyde, dimethyl ether, and methyl formate, provides insights into catalytic mechanisms of the partial oxidation of methanol. The combination of these techniques provides a detailed picture of the coupling between the gas phase and surface in heterogeneous catalysis and enables parametric studies under different operating conditions, which will enhance our ability to constrain microkinetic models of heterogeneous catalysis.
The effects of heat release on interactions between vorticity (ω) and strain rate (s) in turbulent premixed CH4/O2/N2 counterflow flames are investigated using simultaneous OH laser-induced fluorescence (LIF) and tomographic particle image velocimetry (TPIV) measurements. A comparison between the flames and a corresponding turbulent non-reacting variable density N2-vs-products counterflow reveals the impact of heat release on vorticity-strain rate alignment statistics. Vorticity and strain rate statistics in the flames and non-reacting flow are conditioned on distance from the local flame front and gas mixing layer interface (GMLI) contours, respectively. The magnitude, alignment, and spatial distribution of the vorticity and principal strain rates (s1, s2, s3) are rather different when heat release is present. Density variations without heat release enhance the ω-s2 alignment while significantly reducing the ω-s3 alignment and modestly reducing the ω-s1 alignment. In contrast, heat release at the flame front further reduces the ω-s1 alignment but increases the ω-s3 alignment and suppresses the preferential ω-s2 alignment. Furthermore, increasing turbulence diminishes the effect of heat release on this preferential alignment. In regions with the largest vorticities, both the reacting and non-reacting counterflows show an increase in the probability of ω-s2 alignment. All counterflow cases have a net positive vortex-stretching contribution to the enstrophy production with a peak production rate at the flame front or GMLI, but the peak values depend on the density variation, heat release, and turbulence level. Elucidation of the complex interplay between these factors contributes to the understanding of the dynamics of turbulence-flame interactions.
Here, the effects of heat release on interactions between vorticity (ω) and strain rate (s) in turbulent premixed CH4/O2/N2 counterflow flames are investigated using simultaneous OH laser-induced fluorescence (LIF) and tomographic particle image velocimetry (TPIV) measurements. A comparison between the flames and a corresponding turbulent non-reacting variable density N2-vs-products counterflow reveals the impact of heat release on vorticity-strain rate alignment statistics. Vorticity and strain rate statistics in the flames and non-reacting flow are conditioned on distance from the local flame front and gas mixing layer interface (GMLI) contours, respectively. The magnitude, alignment, and spatial distribution of the vorticity and principal strain rates (s1, s2, s3) are rather different when heat release is present. Density variations without heat release enhance the ω-s2 alignment while significantly reducing the ω-s3 alignment and modestly reducing the ω-s1 alignment. In contrast, heat release at the flame front further reduces the ω-s1 alignment but increases the ω-s3 alignment and suppresses the preferential ω-s2 alignment. Furthermore, increasing turbulence diminishes the effect of heat release on this preferential alignment. In regions with the largest vorticities, both the reacting and non-reacting counterflows show an increase in the probability of ω-s2 alignment. All counterflow cases have a net positive vortex-stretching contribution to the enstrophy production with a peak production rate at the flame front or GMLI, but the peak values depend on the density variation, heat release, and turbulence level. Elucidation of the complex interplay between these factors contributes to the understanding of the dynamics of turbulence-flame interactions.
Li, Tao; Zhou, Bo; Frank, Jonathan H.; Dreizler, Andreas; Bohm, Benjamin
Abstract: The development of high-speed volumetric laser-induced fluorescence measurements of formaldehyde (CH 2O -LIF) using a pulse-burst laser operated at a repetition rate of 100kHz is presented. A novel laser scanning system employing an acousto-optic deflector (AOD) enables quasi-4D CH 2O -LIF imaging at a scan frequency of 10kHz. The diagnostic capability of time-resolved volumetric imaging is demonstrated in a partially premixed DME/air lifted turbulent jet flame near the flame base. Simultaneous imaging of laser beam profiles is performed to account for the laser pulse energy fluctuation and laser sheet inhomogeneity. With the accurate registration of laser sheet positions, the volumetric reconstruction of CH 2O -LIF signals is performed within a detection volume of 17.3×11.9×2.3mm3 with an average out-of-plane spatial resolution of 250μm. A surface detection algorithm with adaptive thresholding is used to determine the global maximum intensity gradient by calculating gradient percentiles. The flame topology characteristics are investigated by evaluating the 3D curvatures of CH 2O surfaces. Curvatures calculated using 2D data systematically underestimate the full 3D curvature due to the lack of out-of-plane information. The inner surfaces near the turbulent fuel jet exhibit higher probabilities of large mean curvature than the outer surfaces. The saddle and cylindrical structures are dominant on both the inner and outer surfaces and the elliptic structures occur with lower probability. The results suggest that the damping of turbulent fluctuations by the temperature increase through the CH 2O region reduces the curvature, but the local structure topology remains self-similar. Graphic abstract: [Figure not available: see fulltext.].
Data assimilation techniques are investigated for integrating high-speed high-resolution experimental data into large-eddy simulations. To this end, an ensemble Kalman filter is employed to assimilate velocity measurements of a turbulent jet at a Reynolds number of 13,500 into simulations. The goal of the current work is to examine the behavior of the assimilation algorithm for state estimation of turbulent flows that are of relevance to engineering applications. This is accomplished by investigating the impact that localization, measurement uncertainties, assimilation frequency, data sparsity and ensemble size have on the estimated state vector. For the flow configuration and computational setup considered in this study an optimal value of the localization radius is identified, which minimizes the error between experimental data and state vector. The impact of experimental uncertainties on the state estimation is demonstrated to provide solution bounds on the assimilation algorithm. It is found that increasing the number of ensembles has a positive impact on the state estimation. In comparison, decreasing the assimilation frequency or reducing the experimental data available for assimilation is found to have a negative impact on the state estimation. These findings demonstrate the viability of assimilating measurements into numerical simulations to improve state estimates, to support parameter evaluations and to guide model assessments.
Abstract: Beam steering by index-of-refraction gradients poses a significant challenge for laser-based imaging measurements in turbulent reacting and non-reacting flows, particularly at elevated pressures. High fidelity imaging and quantitative data interpretation in turbulent flows can be considerably impeded by artefacts generated from beam steering. A wavelet-based filtering scheme has been developed to recover the underlying turbulent flow structures from imaging measurements containing severe beam-steering artefacts. This analysis technique is equally applicable to imaging measurements in reacting and non-reacting flows. It is demonstrated using mixture fraction measurements in a transient turbulent jet flow at 8 bar using Rayleigh scattering imaging at a repetition rate of 100 kHz. The corrected images reveal the temporal evolution of flow structures with negligible residual beam-steering artefacts. Tests of the sensitivity of the wavelet-based filtering scheme to noise and spatial resolution indicate that it is a robust analytic tool for correcting severe beam-steering artefacts commonly encountered in laser-based imaging measurements at elevated pressures. Graphic abstract: [Figure not available: see fulltext.].
Data assimilation techniques are investigated to determine how high-speed experimental measurements can be infused into a combustion simulation with the goal of capturing transient combustion events and isolating model deficiencies. To this end, an ensemble Kalman filter (EnKF) is employed to assimilate simultaneous measurements from tomographic PIV and OH-PLIF into a combustion LES of a turbulent DME jet flame, taking into consideration experimental uncertainties and modeling errors. It is shown that by assimilating experimental data, EnKF improves the prediction of the extinction and reignition dynamics observed in this flame. Subsequently, the capability of the assimilation method in evaluating the model performance is examined by considering an assimilation sequence. It is shown that the combustion model investigated (namely a flamelet/progress variable model) exhibits a tendency to relax towards a more reactive state, indicating a deficiency in quantitatively predicting the extent of extinction and reignition with this particular model.
Frank, Jonathan H.; Chandler, David W.; Fournier, Martin P.M.; Jaska, Mark J.
This project explored a new capability for studying collisions of electrons and molecules with unprecedented accuracy by combining high electron-energy resolution with velocity mapped imaging of electrons. Low-energy electrons were produced within a supersonic beam by photoionization of metastable krypton using a dye laser to generate electrons with tunable kinetic energy and a narrow energy spread. A new configuration for electron imaging optics was developed to enable scattering of electrons in a zero-field environment with subsequent rapidly pulsed velocity mapped imaging of the electrons. Development of this new capability will significantly enhance DOE/NNSA's ability to perform basic research on processes relevant to plasmas in atmospheric re-entry and neutron generation for weapons systems and provide fundamental understanding of electron-driven chemistry important to solar energy conversion.
The collapse or merging of individual plumes of direct-injection gasoline injectors is of fundamental importance to engine performance because of its impact on fuel-air mixing. However, the mechanisms of spray collapse are not fully understood and are difficult to predict. The purpose of this work is to study the aerodynamics in the inter-spray region, which can potentially lead to plume collapse. High-speed (100 kHz) particle image velocimetry is applied along a plane between plumes to observe the full temporal evolution of plume interaction and potential collapse, resolved for individual injection events. Supporting information along a line of sight is obtained using simultaneous diffused back illumination and Mie-scatter techniques. Experiments are performed under simulated engine conditions using a symmetric eight-hole injector in a high-temperature, high-pressure vessel at the “Spray G” operating conditions of the engine combustion network. Indicators of plume interaction and collapse include changes in counter-flow recirculation of ambient gas toward the injector along the axis of the injector or in the inter-plume region between plumes. The effect of ambient temperature and gas density on the inter-plume aerodynamics and the subsequent plume collapse are assessed. Increasing ambient temperature or density, with enhanced vaporization and momentum exchange, accelerates the plume interaction. Plume direction progressively shifts toward the injector axis with time, demonstrating that the plume interaction and collapse are inherently transient.
The collapse or merging of individual plumes of direct-injection gasoline injectors is of fundamental importance to engine performance because of its impact on fuel-air mixing. However, the mechanisms of spray collapse are not fully understood. The purpose of this work is to study the effects of injection duration and multiple injections on the interaction and/or collapse of multi-plume GDI sprays. High-speed (100 kHz) Particle Image Velocimetry (PIV) is applied along a plane between plumes to observe the full temporal evolution of plume-interaction and potential collapse, resolved for individual injection events. Supporting information along a line of sight is obtained using Diffused Back Illumination (DBI). Experiments are performed under simulated engine conditions using a symmetric 8-hole injector in a high-temperature, high-pressure vessel at the "Spray G" operating conditions of the Engine Combustion Network (ECN). Longer injection duration is found to promote plume collapse, while staging fuel delivery with multiple, shorter injections is resistant to plume collapse.
We present a series of benchmark flames consisting of six partially-premixed piloted dimethyl ether (DME)/air jet flames. These flames provide an opportunity to understand turbulence-flame interactions for oxygenated fuels and to develop predictive models for these interactions using a canonical burner geometry. The development of accurate models for DME/air flames would establish a foundation for studies of more complex oxygenated fuels. The flames are stabilized on a piloted jet burner similar to that of the partially-premixed methane/air jet flames that have been studied extensively within the context of the TNF Workshop. This series of six jet flames spans jet exit Reynolds numbers, ReD, from 29,300 to 73,300 and stoichiometric mixture fractions, ξst, from 0.35 to 0.60. Flame conditions range from very low probability of localized extinction to a high probability of localized extinction and subsequent re-ignition. Measurements in the flames are compared at downstream locations from 5 to 25 diameters above the nozzle exit. Mean and fluctuating velocity components are measured using stereo particle image velocimetry (SPIV). Simultaneous laser-induced fluorescence (LIF) imaging of OH and CH2O provides insights into the distribution of these intermediate species in partially-premixed DME/air flames. OH LIF imaging is also combined with SPIV to investigate the strain rate field across the reaction zone.
Fuel and oxidizer mixing is a key parameter influencing combustion and emission performance in diesel engines. At the same time, quantitative mixing measurements in automotive sprays are very challenging such that only a few experimental results are available as targets for the development and tuning of numerical models. The caveat is that the experimental data mainly concern the quasi-steady part of the jet, while it can be argued that the injection process in current alternative thermal engines is mostly transient. This work applies planar laser Rayleigh scattering at high-frequency to resolve the development and mixing of vaporized diesel sprays injected in a highly-pressurized environment. The state-of-the-art equipment employed for these experiments include a purposely-built high-power, high-repetition rate pulsed burst laser, optimized optics and a state-of-the-art high-speed CMOS camera. Advanced image processing methods were developed and implemented to mitigate the negative effects of the extreme environments found in diesel engines at the time of injection. The experiments provided two-dimensional mean and variance of the mixture and temperature quantities. The optical system's high spatial and temporal resolution enables tracking of the mixing field with time and space, from which temporally and spatially correlated mixing quantities can be extracted. Further analysis of the detailed mixture and temperature fields offered information about the turbulent mixing process of high-pressure diesel sprays such as scalar dissipation rates or turbulent length scales. Substantial effort was made to assess the uncertainties and limitations of such experimental results due to the optically challenging environment.
