Advanced Laser Diagnostics Research Articles

Spatiotemporal characterization of evaporated atoms during electron beam melting additive manufacturing by advanced laser diagnostics

Electron powder bed fusion (E-PBF) is an additive manufacturing technology used to produce parts layer-wise for advanced aerospace, biomedical, and other applications. Precise control over the energy transferred to the powder by the electron beam is key to further process improvements. Here, we used tunable diode laser absorption spectroscopy to characterize the evaporated titanium atoms above the molten area of a TA6V powder alloy, and, thus, the effects of the energy transferred to it by the electron beam. This unconventional diagnostic tool achieves analyses at very high temporal (<1 μs) and spatial (<100 μm) resolutions, thus, offering valuable information on the microsecond-scale dynamics of the micro-melting zone and the effectiveness of the electron beam spot at diameters as small as ∼200 μm. Our measurements highlighted sharp fluctuations during the evaporation process that were independent of the power and scan speed of the electron beam; instead, the molten pool surface itself seems to drive these fluctuations. Our analysis also documented the shape and density of the vapor plume, which was oriented perpendicular to the surface under common E-PBF conditions.

Investigation of mixing processes of effusion cooling air and main flow in a single sector model gas turbine combustor at elevated pressure

Mixing processes between main flow and effusion cooling air are investigated in an effusion cooled, swirl-stabilized pressurized single sector gas turbine combustor using advanced laser diagnostics. Quantitative planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) and planar laser-induced fluorescence of nitric oxide, seeded to the effusion cooling air, (NO-PLIF) are employed in the primary zone and close to the effusion cooled liner. This data is used to identify mixing events at three stages of premixed combustion, i.e. mixing before reaction, mixing during reaction and mixing after reaction. A parametric study of swirl and cooling air mass flow is conducted to investigate the mutual interaction between flame and cooling air. Within the primary zone, a significant radial asymmetry of OH concentration is observed. This asymmetry is partly explained by the presence of effusion cooling air within the unburned fresh gas, leading to lowered OH concentration within local reaction zones and their post-flame equilibrium concentration. Near the effusion cooled liner, adiabatic mixing after reaction is the dominant process across all investigated operating conditions. Notable mixing before reaction is only observed for the first effusion hole on the center line at low swirl conditions.

Particle dynamics in a gas assisted coal combustion chamber using advanced laser diagnostics

Coal combustion is strongly influenced by the interaction of gas phase turbulence, particle dynamics and chemistry. To advance the understanding of these mutually coupled processes, experiments under well-controlled inflow and boundary conditions are needed that provide access to non-intrusive multi-parameter measurement techniques. Following this idea, in this work gas-assisted coal flames with power up to 40 kWth are investigated in an optically accessible combustion chamber including a quartz glass quarl of the swirl burner assembly. A Two-phase particle image/tracking velocimetry (PIV-PTV) technique is applied for the first time in coal combustion to measure simultaneously velocities of small and large particles. Due to the wide particle size distribution of the grinded coal small particles can be used as tracers for the gas flow while the velocity of large particles can be measured with particle tracking velocimetry (PTV) simultaneously. For this purpose Mie-scattering is imaged by a single camera. Large and small particles are separated in the post-processing based on apparent size and signal intensity of each particle individually. By measuring quasi-simultaneously laser-induced fluorescence of intermediate hydrocarbons released from coal particles during their devolatilization process, regions of intense pyrolysis are identified. Flames operated with different coal types and thermal powers in air and oxy-fuel atmospheres are compared to each other. Additionally, advantages and limits of the measurement techniques are discussed in the context of coal combustion.

Experimental and numerical study of biomass fast pyrolysis oil spray combustion: Advanced laser diagnostics and emission spectrometry

The objective of this work was to move towards developing a comprehensible Computational Fluid Dynamics (CFD) model to facilitate the predictive modeling of Fast Pyrolysis Oil (FPO) spray combustion. A CFD model was implemented from the literature and results were compared to 2D data from non-intrusive optical diagnostics involving Planar Laser Induced Fluorescence of the OH radical, Mie scattering imaging and two-color pyrometry using a laboratory-scale, CH4/air flat-flame with an air-assist atomizer. Furthermore, flame radiation and contributions from graybody sources, chemiluminescence and soot were studied experimentally using emission spectroscopy and Laser Induced Incandescence (LII). Reasonable qualitative agreement was found between experimental and model results in terms of flame structure and temperature. Emission spectroscopy and LII results revealed and confirmed earlier observations regarding the low soot concentration of FPO spray flames; furthermore, it was shown that a significant portion of flame radiation originated from graybody char radiation and chemiluminescence from the Na-content of the FPO. These suggest that the treatment of soot formation might not be important in future computational models; however, the description of char formation and Na chemiluminescence will be important for accurately predicting temperature and radiation profiles, important from the point of e.g., large-scale power applications. Confirmed low soot concentrations are promising from an environmental point of view.

Force measurement of a gas-assisted atomization using an impulse probe

The main objective of this study was to develop further insight into the mass flux characterization of an effervescent atomization. The spray is injected through an industrial effervescent nozzle, and is measured using a newly developed coupled PDPA + IS method. Mass flux measurement of microscale droplets in multiphase atomization is a challenging task. The advanced laser diagnostics such as the phase Doppler particle anemometer (PDPA) are not capable of accurately measuring the droplet mass flux in a dense spray due to the high rejection of non-spherical droplets by the PDPA. A combined measurement of momentum rate data from the impulse sensor (IS) and velocity data from the PDPA provides a fairly reasonable estimate of mass flux data in effervescent atomization. Moreover, the mass flux data obtained by the coupled PDPA + IS will facilitate the estimation of the void fraction in multiphase dense spray, which will be very beneficial to the design of industrial multiphase nozzles.

Development and testing of the ACT-1 experimental facility for hypersonic combustion research

A new pulsed-arc-heated hypersonic wind tunnel facility, designated as ACT-1 (Arc-heated Combustion Test-rig 1), has been developed and built at the University of Notre Dame in collaboration with the University of Illinois at Urbana-Champaign and Alta S.p.A. The aim of the design is to provide a suitable test platform for experimental studies on supersonic and hypersonic turbulent combustion phenomena. ACT-1 is composed of a high temperature gas-generator system and a model scramjet combustor that is installed in an open-type vacuum test section of the wind tunnel facility. The gas-generator is designed to produce high-enthalpy (stagnation temperature = 2000 K–3500 K) hypersonic flows for a run time up to 1 s. The supersonic combustor section is composed of a compression ramp (scramjet inlet), an internal flow channel of constant cross-section, a fuel jet nozzle, and a flame holder (wall cavity). The facility allows three-way optical accesses (top and sides) into the supersonic combustor to enable various advanced optical and laser diagnostics. In particular, planar laser Rayleigh scattering (PLRS), high-speed schlieren imaging and OH-planar laser induced fluorescence (OH-PLIF) have successfully been implemented to visualize the turbulent flows and flame structures at high speed flight conditions.

Nonintrusive and Multidimensional Optical Diagnostics and Their Applications in the Study of Thermal-Fluid Systems

Nonintrusive measurements are highly desirable in the study of modern thermal-fluid systems. One of the key bottlenecks of studying such systems stems from the experimental difficulty of resolving the governing processes with the necessary temporal and spatial resolution. The research and development of modern power and energy systems (e.g., aero-propulsion engine) require understanding such processes under extreme conditions, such as sub-millisecond time resolution, sub-millimeter spatial resolution, high temperature and pressure, and multiphase flows. Traditional experimental techniques are no longer capable of meeting all these requirements. Therefore, this paper discusses nonintrusive laser diagnostics and their prospective of meeting these challenges. Using laser beams as probes and being nonintrusive, laser diagnostics can be applied to any harsh environments, at least theoretically. This paper specifically focuses on the recent work from the author's group to apply tomographic laser techniques to obtain multidimensional measurements in thermal-fluid systems. Examples include two-dimensional imaging of temperature fields at 50 kHz, and three-dimensional measurements of combustion parameters using emission spectroscopy at multi-kilohertz. Lastly, as an example, this paper discusses our ongoing efforts of applying advanced laser diagnostics for the study of thermal management of batteries in a hybrid vehicle.

Advanced laser diagnostics for an improved understanding of premixed flame-wall interactions

This review discusses the role of laser diagnostics in combustion science and technology. In its first part, it may guide understanding of advanced diagnostic methods, and is particularly helpful for non-specialized experimentalists. Various challenges for future developments and applications of optical combustion diagnostics are highlighted. In the second part of this review, flame-wall interactions are selected for a more in-depth discussion. Flame-wall interactions are scientifically interesting and are of great importance to any enclosed practical combustion process. Following a description of current understanding, the focus is on using optical diagnostics to probe thermal, fluidic, and chemical properties of head-on and sidewall quenching. The review ends with a discussion of issues and implications for future experimental research and specific diagnostic needs.

Laser-based measurements of gas-phase chemistry in non-equilibrium pulsed nanosecond discharges

Detailed experimental investigation of a non-equilibrium nanosecond pulsed discharge in premixed CH 4/air mixtures at atmospheric pressure has been carried out. The experiments demonstrated significant reductions in ignition delay and increased lean burn capability relative to conventional spark ignition. Advanced laser diagnostics have been used to identify the physical processes which lead to these improvements. The electron temperature and density properties were measured using laser Thomson scattering (LTS). Temperature measurements were performed using N 2 CARS thermometry to quantify the energy transfer in the gas mixture. Effect of the discharge on the local temperature shows the existence of the ignition of the gas mixture for equivalence ratio between 0.7 and 1.3. Fast development of a flame kernel is then observed. The experiment also shows that the flame can be sustained above the discharge due the repetitive ignition of the flame at the plasma repetition rate. Finally, OH and CH PLIF experiments were performed to confirm the large OH and CH streamer-induced production over the discharge volume. To cite this article: F. Grisch et al., C. R. Mecanique 337 (2009).

Laser Diagnostics for the Model Development in Turbulent Premixed Flames

Abstract The interaction of turbulent flow and chemical processes is complex. Due to the strong turbulent fluctuations on very small scales, a direct insight into the processes has not been possible for long time. New laser diagnostics with sufficient spatial and temporal resolution allow now the answer to long discussed questions, for instance concerning the contradictory ideas of thin or thickened flame fronts in turbulent premixed combustion. More and more also numerical models allow the combined calculation of the local processes describing turbulent flow and combustion reaction. However, often unknown terms coming from the averaging procedure of the turbulence models need to have closures. Advanced application of the planar PIV measurement technique allowed the velocity field evaluation conditioned to the either unburnt or burnt gas state in turbulent premixed flames. Herewith, the direct evaluation of unclosed model terms has been possible for both RANS models as well as for models for the large-eddy simulation method. The mutual interaction of advanced laser diagnostics and numerical models is seen as one of the milestones for the development of advanced technical combustion.

Experiments for Combustion-LES Validation

Designs of future combustors increasingly rely on numerical combustion simulations. Large-eddy simulation (LES) emerged as a method to better describe turbulence than Reynolds-averaged Navier-Stokes (RANS) approaches. Processes at the subgrid scale, however, need to be modelled. Validation using comprehensive and reliable experimental data sets is therefore a crucial part in the development of combustion-LES. Using well-defined benchmark flames and advanced laser diagnostics, different physical and chemical quantities can be precisely measured with high temporal and reasonable spatial resolution. In this contribution, quantities important to combustion-LES validation and the most frequently used laser diagnostic methods are briefly reviewed. New challenges in the field of diagnostics at high repetition rates and at walls are highlighted. In a closing chapter, some critical aspects on the comparison of experimental measurands and combustion-LES-predicted counterparts are discussed.

Experimental analysis of flashback in lean premixed swirling flames: conditions close to flashback

Swirling lean premixed flames are of practical relevance due to their potential for low nitric oxide (NOx) emissions. Unfortunately, these flames have various drawbacks. One critical attribute is the possibility for flashback of the reacting flow into the nozzle. Advanced numerical simulations should be able in the future to predict the transition from stable flames to flashback. For a better understanding of the process itself and for validation of numerical simulation a well-documented generic benchmark experiment is needed. This study presents a burner configuration that has already been studied extensively in the past. By minor geometrical adaptations, and via the possibility to vary the swirl intensity in a controlled way, the transition from stable flames to flashback is now accessible to detailed characterisation using advanced laser diagnostics. In a first part of this study the different states of the flame were classified. In the second part, both a stable and a precessing flame very close to flash back were compared in terms of flow and scalar field. The variation of the swirl intensity on the flame is discussed. Because the flame is strongly influenced by its inflow conditions additional velocity measurements inside the nozzle were carried out. This is of special importance for subsequent numerical simulations to match the experimental conditions. The quantitative investigation of the flame during flashback is subjected to consecutive experiments where planar laser diagnostics at high repetition rates will be exploited.

Homogeneous charge compression ignition: the future of IC engines?

The Homogeneous Charge Compression Ignition Engine, HCCI, has the potential to combine the best of the Spark Ignition and Compression Ignition Engines. With high octane number fuel, the engine operates with high compression ratio and lean mixtures giving CI engine equivalent fuel consumption or better. Owing to pre-mixed charge without rich or stoichiometric zones, the production of soot and NOx can be avoided. This paper presents some results from advanced laser diagnostics showing the fundamental behaviour of the process from a close to homogeneous combustion onset towards a very stratified process at around 20–50% heat released. The need for active combustion control is shown and possible means of control are discussed. Results with multi-cylinder engines using negative valve overlap, variable compression ratio, fast inlet temperature control as well as dual fuel are given.

A detailed experimental investigation of well-defined, turbulent evaporating spray jets of acetone

This paper aims at investigating the detailed structure of turbulent non-reacting dilute spray flows using advanced laser diagnostics. A simple spray jet nozzle is designed to produce a two-phase slender shear flow in a co-flowing air stream with well-defined boundary conditions. The carrier flow is made intentionally simple and easy to model so that the focus can be placed on the important aspects of droplet dispersion and evaporation, as well as turbulence–droplet interactions. Phase Doppler interferometry is employed to record droplet quantities, while planar laser-induced fluorescence imaging is applied separately to obtain acetone vapour data. Measurements are conducted for four acetone spray jets in air at several axial stations starting from the nozzle exit. The combined liquid and vapour mass fluxes of acetone integrated across the jet at downstream locations agree satisfactorily with the total mass flow rate of acetone injected. The development of droplet rms velocity for the spray jets investigated is quite different in its radial and axial components. Variation in the radial rms velocity of droplets conditioned on different size-classes, v′, is subject mainly to the response of droplets to turbulent fluctuations of the carrier flow and is characterized by the droplet relaxation time, t d. Deviation of v′ from that of the carrier phase occurs when the Stokes number becomes greater than approximately unity. For the size-classified axial rms velocity, u′, the importance of the time scale for the rate of the incremental change of the axial mean velocity in the radial direction, t m, has been identified. The droplet rms velocity ratio, u′/ v′, increases substantially as the time scale ratio t d/ t m becomes greater than approximately 0.3, indicating strong anisotropy in droplet dispersion. A higher axial mean slip velocity is found for larger droplets that also intensifies their evaporation, and hence the bulk evaporation rate. Turbulence effects, however, are most effective in enhancing the evaporation of relatively small droplets that can follow closely the turbulent fluctuations of the gas flow.

Application of advanced laser diagnostics for the investigation of the ionization sensor signal in a combustion bomb

The ionization sensor is an electrical probe for diagnostics in internal combustion engines. Laser-induced fluorescence (LIF) imaging of fuel, hydroxyl (OH), and nitric oxide (NO) distributions has been employed to extend our knowledge about the governing processes leading to its signal. By monitoring the flame propagation in quiescent and turbulent mixtures, the cycle-to-cycle variations in the early sensor signal was attributed to the stochastic contact between flame front and electrodes. An analysis of the relationship between gas temperature and sensor current in the post-flame gas suggests a dominant role of alkali traces in the ionization process at the conditions under study. Significant cooling of the burned gas in the vicinity of the electrodes was observed in quiescent mixtures. Imaging of the post-flame gas in turbulent combustion revealed moving structures with varying NO and OH concentrations, which were identified as sources of variation in the sensor current.

The detailed flame structure of highly stretched turbulent premixed methane-air flames

The premixed stoichiometric turbulent methane flames are investigated on a piloted Bunsen burner with a nozzle diameter of 12 mm and mean nozzle exit velocities of 65, 50, and 30 m/s. Advanced laser diagnostics of the flow field using two-component and two-point laser Doppler anenometer (LDA), as well as of the scalar fields with 2-D Rayleigh thermometry and line Raman/Rayleigh laser-induced predissociation fluorescence (LIPF)-OH techniques, are applied to obtain both the instantaneous and mean flame structure in terms of velocity, temperature, and major species concentrations, as well as turbulent kinetic energy and length scales. In terms of their location on the combustion diagram, the three flames cover the entire range of the distributed-reaction-zones regime from the borderline to the well-stirred reactor regime to the flamelet regime. Measurements were from X/D = 2.5 above the nozzle exit plane to X/D = 12.5 downstream. Thus, a complete database is established for comparison with the numerical predictions.Within the mixing layer between the unburnt gas and the pilot flame, the instantaneous temperatures are much lower than the adiabatic flame temperature due to the short residence time and heat loss to the burner. With increasing residence time the mean flame temperature increases in the axial direction. The radial mixing of the turbulence generated with the shear layers between the nozzle jet stream and surrounding pilot stream is surpressed, such that the turbulence kinetic energy remains nearly constant on the centerline.From the two-dimensional (2D) temperature fields instantaneous iso-temperature contours are plotted showing broad regions where burnt and unburnt gas are partially mixed. These regions are interpreted in terms of the quench scale ℓq = (ετc3)1/2. The measured values of the flame brush thickness are proportional to the quench scale for the two high-velocity flames, whereas the low-velocity flame exhibits essential flamelet behavior.

Compact incinerator afterburner concept based on vortex combustion

A design concept for a compact incinerator afterburner based on actively controlled vortex combustion was developed and tested at ≈ 5 kW and ≈ 50 kW. Acoustic control of fluid dynamics was used to enhance mixing and increase the DRE (destruction and removal efficiency) for a waste surrogate. A detail study of the concept of utilizing vortex combustion for incineration was undertaken in a small-scale flame using advanced laser diagnostics to elucidate and optimize the fluid dynamic mechanisms. The system was then scaled up by ≈ 10, optimized, and evaluated for performance. The open loop active control methodology is based on the concept of combustion in periodic axisymmetric vortices. Acoustic excitation was, used both to stabilize coherent vortices in the central jet air flow and to control circumferential gaseous fuel and waste injection into the shear layer at the right time during vortex formation. The gaseous-fueled actively controlled 4.5 kW incinerator was able to surpass 99.997% DRE even when the gaseous benzene waste surrogate constituted 66% of the total combustible content and the combustible to total air ratio (ø) was 0.974; lowering ø below this increased the DRE to beyond our detection limit. The DRE for gaseous benzene exceeded 99.999% in the 50-kW system when combustible to air ratios were kept below 0.78. Parameters found critical to maintenance of high DRE at both energy scales were the fraction of circumferentially entrained air, the entrainment geometry, the forcing levels, and the phase angle of fuel injection with respect to the vortex roll-up. The 50-kW combustor was also evaluated for stack emissions and combustion efficiency. The controller improved combustion efficiency and lowered emissions: CO dropped from 2900 ppm to as low as 2 ppm. Unburned hydrocarbons were also reduced. Under some conditions the controller also reduced NO x levels down to as low as 12 ppm.

Future Directions in Turbulent Combustion Research

ABSTRACT A fantastic vision of the work of combustion and safety engineers some years from now leads into a Sophoclean debate about the shortcomings of current and future research in turbulent combustion. The objectives of future research in turbulence theory and modeling, turbulent combustion modeling, direct numerical simulation and application of advanced laser diagnostics are discussed. A plea is made for making more efficient and coordinated use of the limited resources that are available.