Planar Laser Diagnostics Research Articles

Reconstructing soot fields in acoustically forced laminar sooting flames using physics-informed machine learning models

This work reports an application of physics-informed machine learning models on reconstructing key parameters of acoustically forced, time-varying laminar sooting flames, highlighting the potential of the machine learning methods as a complementary tool to conventional laser diagnostics. First, a physics-informed neural networks (PINNs) model was developed to reconstruct the fields of velocity and temperature in the region where is inaccessible with laser-based diagnosing methods due to soot scattering. The PINNs model was trained using experimental data from planar laser diagnostics and constrained with the momentum and energy conservations. The model shows effective capability of fulfilling the velocity and temperature fields. Second, an Autoencoder (AE)-based Deep Operator Network (DeepONet), also as a physics-informed model, was developed to predict the planar distribution of soot volume fraction in the flames. The AE-DeepONet framework was trained using planar images of temperature and hydroxyl radical (OH) with a hybrid way by combining physics-informed and data-driven approaches. The AE-DeepONet model outperforms the conventional data-driven-only machine learning models. The results show that, constrained by physical laws, machine learning based models can properly predict soot distribution, velocity and temperature in unsteady laminar flames, shedding light on the physics-informed machine learning methods as a complement to laser diagnostics.

A beam energy homogenizing method with total reflection beam-homogenized cavity for planar laser diagnostics

The non-uniformity of sheet-beam energy seriously limits the quantitative measurement in planar laser combustion diagnostics. A novel beam energy homogenizing method with total reflection beam-homogenized cavity is proposed to improve the uniformity of sheet-beam. The process of energy homogenizing in total reflection homogenizing cavity has been studied theoretical based on the proposed virtual light source superposition method. The influences of structure parameters and beam coupling parameters on the uniformity has been investigated. By establishing the shaping optics of the total reflection homogenizer, the sheet-beam with height of 30 mm has been achieved experimentally. The experimental results show that the uniformity of sheet-beam can be well maintained in the range of designed detection length of 60 mm. It is found that the maximum uniformity can reach up to 97.1%, which is much better than other known methods.

Experimental investigation of flame surface density and mean reaction rate during flame–wall interaction

Flame–wall interactions (FWI) of a turbulent side-wall quenching (SWQ) V-flame is characterized by planar laser diagnostics. Velocities and flame front positions are measured simultaneously by two-component particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) of the OH radical. Flame surface density and reactive flame surface density are derived from experimental data and evaluated in a physical and flame progress variable domain. The impact of the wall on flame structures is clearly observed. Additionally mean reaction rates are derived using both flame surface densities and a wall-influenced local consumption speed. Approaching walls reaction rates strongly decrease. Reaction rates are important for quantifying incomplete combustion near walls and impose a challenge for improved modeling approaches in numerical flame simulations. These experimental results are compared to previous direct numerical simulation (DNS) studies and flame surface density models from literature. Despite different Reynolds-numbers overall a good agreement between experimental data and modeling results is achieved. This is an indication that simplified chemical kinetic modeling in DNS is sufficient for representing kinematics of turbulent flames near walls.

All-diode-pumped quasi-continuous burst-mode laser for extended high-speed planar imaging

An all-diode-pumped, multistage Nd:YAG amplifier is investigated as a means of extending the duration of high-power, burst-mode laser pulse sequences to an unprecedented 30 ms or more. The laser generates 120 mJ per pulse at 1064.3 nm with a repetition rate of 10 kHz, which is sufficient for a wide range of planar laser diagnostics based on fluorescence, Raman scattering, and Rayleigh scattering, among others. The utility of the technique is evaluated for image sequences of formaldehyde fluorescence in a lifted methane-air diffusion flame. The advantages and limitations of diode pumping are discussed, along with long-pulse diode-bar performance characteristics to guide future designs.

New Perspectives on Turbulent Combustion: Multi-Parameter High-Speed Planar Laser Diagnostics

Over the past three decades laser combustion diagnostics have guided an improved understanding of turbulent combustion processes. Until recently, this was based on statistically independent sampling using sampling rates much slower than typical integral time-scales of turbulent flames. Recent developments in laser and camera technology enabled an increase in sampling rates by more than three orders of magnitudes. Using these new instruments for particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) at high sampling rates (high-speed diagnostics) allowed the resolution of integral time-scales of turbulent flames. This statistically dependent sampling is increasingly used to temporally track transients in turbulent combustion, such as flame extinction, ignition, flashback and cycle-to-cycle variations in IC engines. The simultaneous application of flow and scalar field measurements makes insights into these transients possible that were not when using statistically independent sampling with low data acquisition rates. Conditioning on distinct flame features with high-speed diagnostics enables the inclusion of time as an additional dimension. This paper reviews the emerging field of multi-parameter, high-speed, planar laser diagnostics in combustion applications. The benefit of high data acquisition rates in turbulent combustion applications is discussed in detail as well as requirements and constraints imposed by the time-scales of the investigated phenomenon are addressed. Recent developments in laser and detector hardware are highlighted, as these are the limiting factors of the sampling rate. Finally, multi-parameter high-speed measurements in combustion are summarized, with a few examples discussed in more detail.

Shock-bubble interactions: Features of divergent shock-refraction geometry observed in experiments and simulations

The interaction of a planar shock wave with a spherical bubble in divergent shock-refraction geometry is studied here using shock tube experiments and numerical simulations. The particular case of a helium bubble in ambient air or nitrogen (A≈−0.8) is considered, for 1.4<M<3.0. Experimental planar laser diagnostics and three-dimensional multifluid Eulerian simulations clearly resolve features arising as a consequence of divergent shock refraction, including the formation of a long-lived primary vortex ring, as well as counter-rotating secondary and tertiary upstream vortex rings that appear at late times for M⩾2. Remarkable correspondence between experimental and numerical results is observed, which improves with increasing M, and three-dimensional effects are found to be relatively insignificant. Shocked-bubble velocities, length scales, and circulations extracted from simulations and experiments are used successfully to evaluate the usefulness of various analytical models, and characteristic dimensionless time scales are developed that collapse temporal trends in these quantities. Those linked directly to baroclinicity tend to follow time scales based on shock wave speeds, while those linked to interface deformation and vortex- or shear-induced motion tend to follow a time scale based on the postshock flow speed, though no single time scale is found to be universally successful.

Improvement of planar laser diagnostics by the application of a beam homogenizer

For planar laser diagnostics, a most uniform beam profile is highly desirable for two reasons: first, subsequent corrections for an inhomogeneous intensity distribution are time consuming and prevent on-line engineering assessment and second, temporal fluctuations cannot be corrected anyway. However, in general for combustion and flow diagnostics pulsed laser sources are used to achieve a high temporal resolution which typically possess a rather poor beam quality compared to continuously emitting laser sources. And, pulse to pulse fluctuations of the beam profile directly increase the noise in single-shot measurements. In this contribution we show the application of a micro-lens array based beam homogenizer whereby an almost homogeneous illumination of the region of in interest is achieved. This enables the on-line evaluation of the measured data without subsequent corrections. Thus a general advance of laser techniques towards engineering practice is achieved. Additionally, statistical fluctuations of the beam profile are strongly reduced by the homogenizer what directly improves the local standard deviation of the measurement. These benefits are demonstrated by means of planar laser-induced fluorescence (LIF) experiments.

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.

Experimental Investigation of Primary and Secondary Features in High-Mach-Number Shock-Bubble Interaction

Experiments to study the compression and unstable evolution of an isolated soap-film bubble containing helium, subjected to a strong planar shock wave (M=2.95) in ambient nitrogen, have been performed in a vertical shock tube of square internal cross section using planar laser diagnostics. The early phase of the interaction process is dominated by the formation of a primary vortex ring due to the baroclinic source of vorticity deposited during the shock-bubble interaction, and the mass transfer from the body of the bubble to the vortex ring. The late time (long after shock interaction) study reveals the presence of a secondary baroclinic source of vorticity at high Mach number which is responsible for the formation of counterrotating secondary and tertiary vortex rings and the subsequent larger rate of elongation of the bubble.

Application of a beam homogenizer to planar laser diagnostics

The first application of a microlens array beam homogenizer to planar laser measurement techniques in combustion diagnostics is demonstrated. The beam homogenizing properties of two microlens arrays in combination with a Fourier lens for widespread applications are presented. An uniform line profile with very little temporal fluctuations of the spatial intensity distribution was generated resulting in a significant reduction of measurement noise and enabling an easier and faster signal processing.

Inertial-Fusion-Related Hydrodynamic Instabilities in a Spherical Gas Bubble Accelerated by a Planar Shock Wave

Experiments studying the compression and unstable growth of a dense spherical bubble in a gaseous medium subjected to a strong planar shock wave (2.8 < M < 3.4) are performed in a vertical shock tube. The test gas is initially contained in a free-falling spherical soap-film bubble, and the shocked bubble is imaged using planar laser diagnostics. Concurrently, simulations are carried out using a compressible hydrodynamics code in r-z axisymmetric geometry.Experiments and computations indicate the formation of characteristic vortical structures in the post-shock flow, due to Richtmyer-Meshkov and Kelvin-Helmholtz instabilities, and smaller-scale vortices due to secondary effects. Inconsistencies between experimental and computational results are examined, and the usefulness of the current axisymmetric approach is evaluated.