PLIF Imaging Research Articles

100-kHz carbon monoxide TP-PLIF imaging using ultra-narrow-linewidth burst-mode OPO.

A 100-kHz rate two-photon planar laser-induced fluorescence (TP-PLIF) imaging of carbon monoxide (CO) is successfully demonstrated utilizing a narrow-linewidth optical parametric oscillator (OPO) generating light at ∼230.1 nm. A specially designed injection-seeded burst-mode OPO was constructed and characterized for this purpose. This OPO efficiently converts the 355-nm output of a high-energy nanosecond burst-mode laser to ∼230.1 nm following parametric splitting and mixing processes. Generation of an ultra-narrow-linewidth 230.1 nm laser pulse is crucial for effectively exciting CO via a two-photon process from the ground X1Σ+ to the B1Σ+ electronic state-enabling PLIF imaging over a large area. The experimental setup is capable of tracking high-speed flow structures of a CO/N2 mixture, showcasing detection speeds 100 times greater than those achieved with previous femtosecond laser sources. This substantial increase in repetition rate will allow time-resolved CO-TP-PLIF measurements in highly dynamic hypersonic boundary layers and detonation-driven combustion processes for revealing chemical kinetics and turbulent aerodynamics.

Hybrid rich- and lean-premixed ammonia-hydrogen combustion for mitigation of NOx emissions and thermoacoustic instabilities

The large-scale direct utilization of two carbon-free fuels, ammonia and hydrogen, is currently attracting significant interest in the context of the development of new gas turbine combustion technologies, with paying close attention to the reduction of nitrogen oxides and unburned ammonia emissions. Motivated by recent observations that rich-premixed conditions tend to mitigate excessive NOx emissions from ammonia combustion, and that uniformly blended ammonia-hydrogen fuel/air mixtures tend to increase NOx production exponentially, here we propose a hybrid configuration of hydrogen-doped rich-premixed ammonia-air flames (inner stage) and lean-premixed pure hydrogen-air flames (outer stage) in a radially stratified primary reaction zone. This fuel staging scheme offers a possible mechanism for chemical kinetics-controlled NOx abatement and asymmetry-induced thermoacoustic instability suppression. The relevance and feasibility of the unconventional premixing approach are rigorously evaluated based on detailed measurements of exhaust gas concentrations and self-excited pressure fluctuations, in conjunction with OH*/NH2*/NH* chemiluminescence and OH PLIF imaging measurements. We observe that drastically different non-homogeneous reaction regions can be stably established in a comparatively compact combustion volume without producing negative flame stabilization effects – such as blowoff of less reactive ammonia flames and flashback of more reactive hydrogen flames. As compared with the uniform-blend baseline condition, the hybrid staging method is shown to significantly mitigate total nitrogen oxides emissions, from 7764 to 310 ppmvd (96 % reduction), and achieving an approximately two-fold reduction in dynamic pressure amplitude. Interestingly, hydrogen-enriched rich-premixed ammonia flames are revealed to exhibit anomalous oscillatory states originating from the preferential diffusion of hydrogen molecules and reaction rate-dependent separation of reactive layers, enabling interacting non-homogeneous reaction zones with markedly different characteristic time scales to resist the growth of intense pressure perturbations.

Megahertz-Rate Imaging of Hypersonic Boundary-Layer Instabilities in a Mach 10 Shock Tunnel

The transition of the boundary-layer state from laminar to turbulent on a 7° sharp cone in Mach 10 flow is measured using several high-speed optical diagnostics capable of time-resolved measurements of the second-mode instability with high spatial resolution. Specifically, megahertz-rate nitric oxide planar laser-induced fluorescence (NO PLIF) imaging is done at 1 MHz to provide a non-path-integrated visualization of second-mode associated structures in the transitioning boundary layer. A recent variation of focused laser differential interferometry (FLDI), namely, linear array FLDI (LA-FLDI), is also applied for multipoint measurements of frequency content in the transitioning boundary layer. From each data set, the frequency of the most unstable second-mode instability is obtained along with the phase velocity of the second-mode waves. Measurements show good agreement relative to each other and also with surface pressure sensors and high-speed schlieren imaging. Furthermore, the 1 MHz NO PLIF imaging demonstrated here shows great potential for visualization of transitional and turbulent boundary layers when path-integrated diagnostics cannot be used.

Simulation of Plasma Assisted Supersonic Combustion over a Flat Wall

This work presents a simplified methodology to couple the physics of a nanosecond pulsed discharge to the process of supersonic combustion in a flat wall combustor configuration. Plasma and supersonic combustion are separately simulated and then coupled by seeding plasma-generated radicals on the combustion domain. The plasma model is built assuming spatial uniformity and considering only the kinetic effects of the nanosecond pulsed discharge. Therefore, a zero-dimensional kinetic scheme accounting for the generation of plasma species is utilized. For the combustion model, the complete set of Favre-averaged compressible Navier Stokes equations along with finite rate chemistry is solved through a control-volume based technique via the commercial software Ansys Fluent. The computational results are compared against experimental studies showing that the proposed methodology can capture the main kinetic effects of the nanosecond pulsed discharge on supersonic combustion. OH concentration contours reveal the presence of an enhanced flame when the plasma is applied following the trends from experimental OH PLIF images. In addition, time evolving temperature and OH concentration contours show that the ignition delay time is reduced with the application of the discharge.

Hydroxyl radical dynamics in a gliding arc discharge using high-speed PLIF imaging

Plasma discharges can be transient and randomly distributed where a few investigations have been carried out using laser-induced fluorescence to capture snapshots of plasma-produced radicals in the near vicinity of the discharge. Radical distribution dynamics, however, are challenging to study in situ with high spatial and temporal resolution to fully capture the interactions between the discharge and the gas. We here demonstrate a planar laser-induced fluorescence method that can capture molecular distributions of ground state hydroxyl radicals in a discharge plasma and follow how the distribution develops in time with a repetition rate of 27 kHz. The technique is demonstrated by monitoring, in real-time, how the tube-like distribution of ground state OH radicals, surrounding a gliding arc plasma, is affected by flow dynamics and how it develops as the high voltage is turned off at atmospheric pressure. The method presented here is an essential tool for capturing radical-distribution dynamics in situ of chemically active environments which is the active region of the plasma induced chemistry.

PLIF and chemiluminescence in a small laminar coflow methane-air diffusion flame at elevated pressures

Fluorescence and chemiluminescence are significant diagnostic tools in combustion research. This experimental work studies OH PLIF and the OH electronically excited state OH* that provide beneficial insights regarding the physical extent and progress of elevated pressure combustion, particularly as regards these heat release zone and reaction zone markers. CH* chemiluminescence is also imaged since this optical diagnostic often serves as the first visual evaluation of a flame. A high-pressure combustion facility is built to study the fluorescence and chemiluminescence in a laminar coflow methane/air flame within its steady limits up to 11.4 atm. This research demonstrates how diagnostics in high-pressure combustion environments pose particular challenges because of the difficulty in accounting for quenching and line broadening in the measurements. The overall imaging results show that as the pressure rises there is a consistent thinning of the reaction zone while maintaining the same flame height, and there is an increasing concentration of soot near the flame tip. This work presents detailed high-pressure experimental design and procedures, including a discussion of the intrinsic physical challenges for soot interference associated with the optical interrogation of hydrocarbon fuel flames at these pressures. In addition, the main contributors to the fluorescence signal are identified and quantified versus pressure. Finally, we compare the experimental PLIF images with simulated PLIF obtained from CFD.

Study on the effects of narrow-throat pre-chamber geometry on the pre-chamber jet velocity using dual formaldehyde PLIF imaging

The effects of narrow-throat pre-chamber geometry on the main chamber combustion were investigated on a heavy-duty optical engine fueled with methane. Simultaneous dual formaldehyde PLIF imaging and OH* chemiluminescence imaging were applied to characterize the early-stage formaldehyde jet and flame jet discharge process. The formaldehyde jet velocity on the jet boundary was quantified by dual formaldehyde PLIF for the first time. Three narrow-throat pre-chambers with two rows of orifices on the nozzle and with different pre-chamber volume and inner throat diameter were studied under the same pre-chamber to main chamber fuel ratio and global excess air ratio of 2.0. The results indicate that the narrow-throat pre-chamber performance is not very sensitive to the pre-chamber volume. The pre-chamber with a larger volume produces only slightly higher pressure buildup in the pre-chamber (ΔP) and similar main chamber combustion phasing and engine efficiency. The inner throat diameter is the key dimension in the narrow-throat pre-chamber design. A larger inner throat diameter produces a smaller peak ΔP and a slower jet penetration, resulting in longer combustion duration and lower engine efficiency. The formaldehyde PLIF shows that the main chamber combustion can be generally classified into two stages: the initial flame ignition and the following flame propagation. The maximum local formaldehyde jet boundary velocity of the narrow-throat pre-chamber with an inner throat diameter of 3.3 mm is up to about 280 m/s and it decreases dramatically when the inner throat diameter is increased to 5.3 mm. The flame jet characteristics of lower and upper orifices can be significantly different due to the local pressure difference between them in the pre-chamber throat. A smaller inner throat diameter results in a larger pressure difference between the lower and upper row orifices and could lead to weak upper-row flame jets. The narrow-throat pre-chamber design with an optimized inner throat diameter can produce highΔP, fast jet penetration, and short combustion duration, which favor the lean limit operation and high engine efficiency.

Planar laser-induced fluorescence measurement of the angular pattern of the cone-shaped spray

In the present work, a PLIF-based approach for a large cone-shaped spray patternation ispresented and experimental results of its application are discussed. The patternation approach is based on simple time-averaged PLIF imaging. Special attention is paid to the image and data processing in order to simplify a patternation results interpretation. Experimental testing was performed on a water spray, formed by centrifugal nozzle at atmospheric ambient pressure. Circumferential relative concentration profiles in spray crosswise plane are presented in the paper and their validity is analysed.

Combustion dynamics of multi-element lean-premixed hydrogen-air flame ensemble

The crux of the technical issues facing the development of hydrogen gas turbine engines is the problem of how to enable low NOx, low dynamics combustor operations without detrimental flashback events in extremely fast lean-premixed hydrogen flames. While flashback mechanisms and nitrogen oxides emissions have been investigated extensively for high hydrogen content flames, the self-excited dynamics of lean-premixed pure hydrogen flame ensembles remain unclear. Here we use phase-resolved OH* chemiluminescence and OH PLIF imaging in a series of experiments in a multi-element injector configuration consisting of 293 small-scale injectors with an inner diameter of 3.0 mm. We collect a large experimental dataset to explore a variety of collective phenomena of the lean-premixed hydrogen-air flame ensemble, and perform systematic investigations of self-induced combustion instabilities. Our observations demonstrate that ultra-compact pure hydrogen flames generate high-amplitude pressure perturbations over a broad range of characteristic frequencies between 400 and 1800 Hz, corresponding to the third to tenth order eigenmodes. Low-frequency flame dynamics developed under relatively low equivalence ratio conditions involve a complex balance among several coexisting phenomena, including strong vortex interactions and periodic extinction-reignition processes, giving rise to large-scale asymmetric oscillations of the entire reaction zone. By contrast, the flame surface dynamics at an intermediate frequency of ~600 Hz exhibit prominent symmetric oscillations accompanied by merging and pinch-off of the constituent flames. Unexpectedly, high-frequency instabilities at approximately 1720 Hz (screech tones) are not influenced by such structurally complex flame dynamics, but by exceptionally simple, seemingly linear, flame surface motion without sudden flame area annihilation events like those observed for lower frequency cases. Despite the extremely low level of heat release rate fluctuations, on the order of less than 1%, the clustered premixed hydrogen flames are capable of producing disproportionately large pressure perturbations in excess of 12 kPa, originating from the synchronous phase dynamics of acoustic pressure and clustered flames’ heat release rate oscillations. These findings provide new insight into the driving mechanisms underlying high-frequency combustion dynamics of densely arranged pure hydrogen-air flames.

10 kHz two-color OH PLIF thermometry using a single burst-mode OPO.

10-kHz hydroxyl radical (OH) two-color planar laser-induced fluorescence (TC-PLIF) thermometry was demonstrated with a single burst-mode optical parametric oscillator (OPO) and a single camera. A fast, dual-wavelength switched seed laser enabled a high-energy, high-repetition-rate burst-mode laser to generate two 10-kHz pulse trains at wavelengths of ${\sim}{354.8}\;{\rm nm}$. The two pulse trains are colinear with 3 µs time interval between the pulse pairs. The injection-seeded OPO efficiently converts the burst-mode laser output to 285.62 and 285.67 nm to excite the ${Q}_2({12})$ and ${P}_1({8})$ OH transitions. PLIF images were collected from each of the two excitation transitions, and intensity ratios from the images were used to determine local temperatures. The development of fast, dual-wavelength switching, burst-mode OPO technology significantly reduces the experimental complexity of the high-speed TC-PLIF thermometry and simplifies its implementation in harsh combustion and flow test facilities.

A comparison of the blow-out behavior of turbulent premixed ammonia/hydrogen/nitrogen-air and methane–air flames

Ammonia has been identified as a promising energy carrier that produces zero carbon dioxide emissions when used as a fuel in gas turbines. Although the combustion properties of pure ammonia are poorly suited for firing of gas turbine combustors, blends of ammonia, hydrogen, and nitrogen can be optimized to exhibit premixed, unstretched laminar flame properties very similar to those of methane. There is limited data available on the turbulent combustion characteristics of such blends and important uncertainties exist related to their blow-out behavior. The present work reports experimental measurements of the blow-out limits in an axisymmetric unconfined bluff-body stabilized burner geometry of NH3/H2/N2-air flame, comprised of 40% NH3, 45% H2, and 15% N2 by volume in the “fuel” blend. Blow-out limits for the NH3/H2/N2-air flames are compared to those of methane–air flames. OH PLIF and OH chemiluminescence images of the flames just prior to blow-out are presented. Furthermore, two large-scale Direct Numerical Simulations (DNS) of temporally evolving turbulent premixed jet flames are performed to investigate differences in the turbulence-chemistry interaction and extinction behavior between the NH3/H2/N2-air and methane–air mixtures. The experiments reveal that the blow-out velocity of NH3/H2/N2-air flames is an order of magnitude higher than that of methane–air flames characterized by nearly identical unstretched laminar flame speed, thermal thickness and adiabatic flame temperature. Results from the DNS support the experimental observation and clearly illustrate that a methane–air mixture exhibits a stronger tendency towards extinction compared to the NH3/H2/N2-air blend for identical strain rates. Furthermore, the DNS results reveal that, even in the presence of intense sheared turbulence, fast hydrogen diffusion into the spatially distributed preheat layers of the fragmented and highly turbulent flame front plays a crucial role in the enhancement of the local heat release rate and, ultimately, in preventing the occurrence of extinction.

High-resolution velocity measurements in turbulent premixed flames using wavelet-based optical flow velocimetry (wOFV)

Wavelet-based optical flow velocimetry (wOFV) is used to estimate two-dimensional velocity fields from tracer particle images acquired simultaneously with CH PLIF imaging within two highly turbulent premixed methane-air flames. Unlike conventional correlation-based PIV approaches, the current wOFV approach yields a dense (i.e. one vector per pixel) estimation of the velocity field which offers the possibility of greatly enhanced spatial resolution as compared to correlation-based PIV. Results from the wOFV method and correlation-based PIV are directly compared in terms of instantaneous fields, flow statistics, energy spectra, and derived gradient quantities such as vorticity and 2D strain rate. An assessment of the results indicates an increase in spatial resolution and dynamic range of almost one order-of-magnitude when using wOFV as compared to high-resolution PIV. In fact, the use of wOFV permits resolution of the smallest dissipative scales within the premixed flames with turbulent Reynolds numbers of O(104). A particular focus concerns velocity measurements near and within the CH layers, which are excellent markers of the local flame front but are spatially narrow and intermittent. The high-resolution velocity results from the wOFV method provide an opportunity for improved velocity estimations, derivative quantities, and their statistical characterization at and near the flame front.

On the bi-stable nature of turbulent premixed bluff-body stabilized flames at elevated pressure and near lean blow-off

This study considers turbulent premixed bluff-body stabilized flames at elevated pressures. Specifically, the lean blow-off (LBO) limit of such flames is determined for a range of bulk velocities (5 ≤ U ≤ 50 m/s) and operating pressures up to 3 bar. Two key observations emerge from this stability assessment. The first is that considering elevated pressure leads to two stability regimes: one at atmospheric conditions and those with elevated pressure and U ≳ 20 m/s (regime-a), and another at elevated pressures with U ≲ 20 m/s (regime-b). The second observation is that within these regimes, LBO limits are insensitive to pressure. Flames in regime-a (S-flames) are found to be more stable than those in regime-b (U-flames). Advanced image-based diagnostics were employed to understand reasons for this difference in stability. Flow field measurements indicate that U-flames are associated with an outer recirculation zone (ORZ) that formed as pressure increased but receded from the burner as U surpassed ∼ 20 m/s. PLIF images of CH2O and OH demonstrated that the ORZ interacts with U-flames such that their downstream regions are prevented from collapsing to the inner recirculation zone (IRZ). Furthermore, analysis of the OH-PLIF images indicate that U-flames possess larger turbulent consumption rates, helping them form large IRZs and rendering them more susceptible to influence from the ORZ. Results of high-speed OH* imaging demonstrate that LBO events differ between U- and S-flames. Namely, while S-flames collapse to their IRZs during LBO, U-flames lift off from the burner, depleting their anchoring regions of reactions and hot products. Losing back-support in this region is what ultimately reduces the stability of U-flames. Finally, the reason U-flames lift off from the burner during LBO is elucidated by joint flow-flame measurements. Specifically, the anchoring regions of U-flames reside in regions of large axial velocity, which likely stems from their enhanced burning rates.

An Experimental Investigation of Scalar Mixing and Flame Structure of a Partially Premixed Flame

ABSTRACT Experimental investigation of a partially premixed flame stabilized in a combination of bluff body and swirling flowfields is reported. Simultaneous measurement of 2D fuel distribution (Acetone-PLIF) and flame structure (OH-PLIF) are performed with varying thermal power. Two flame stabilization modes – swirl influenced conical shaped, and, bluff-body influenced toroidally shaped – are observed in OH chemiluminescence measurements. Upstream propagation of the flame with decreasing global equivalence ratios is noticed, with the flame being transferred from the swirl-influenced to the bluff body influenced flowfields. The average flame stabilization location in both modes is found to be approximately close to the stoichiometric surface with the flame base oriented toward the inner shear layer. The degree of fuel/air mixing calculated from Acetone PLIF images by determining the unmixedness shows that the stabilization locations overlap with regions of perfect mixing. The swirl-influenced flames are stabilized at the farthest distance from the burner exit, with the flame base closer to the stoichiometric surface, whereas the bluff body influenced flames oriented along the stoichiometric surface in the inner shear layer.

Experimental laminar burning velocity of syngas from fixed-bed downdraft biomass gasifiers

Laminar burning velocity is considered as being one of the fundamental properties of a premixed flame and reliable data are constantly required for practical applications. Although it is possible to find an extensive amount of experimental laminar burning velocity data for fuels containing one or two components, data for fuels with three or more components are scarce. The goal of this study is to help fill this gap by providing experimental laminar burning velocity data for a fuel mixture with five components (H2:CO:CO2:CH4:N2). The fuel composition utilized is proposed as surrogates of the fuel provided by a downdraft gasifier, the most common and the most efficient type of gasifier. Experimental measurements were carried out for different fuel-to-air equivalence ratios (0.88 < ϕ < 1.74) at atmospheric conditions, 954 mbar and 298 K. The method utilized was the conical-flame surface using OH PLIF images (Planar-Laser-Induced Fluorescence imaging of OH). The area method provides a good approximation of the unstretched laminar burning velocity. Experimental data were compared with the simulated results obtained from a CHEMKIN chemical kinetics software. The highest experimental laminar flame speed of a downdraft syngas air mixture was 0.3491 m s−1 and occurred when ϕ ∼1.3.

500 Hz ultraviolet dye laser with pulse energy 1.7 mJ and potential for PLIF imaging

ABSTRACTThis paper describes a high-pulse-energy frequency-doubled ultraviolet dye laser operating at a repetition rate of 500 Hz. The pump source is a laser-diode side-pumped Q-switched Nd:YAG laser with a pulse energy of 29 mJ at 532 nm. A master oscillator power amplifier is employed to amplify the output pulse of the dye laser to 8.1 mJ at 566 nm, and by frequency doubling with BBO crystal a pulse energy of 1.7 mJ at 283 nm is achieved with a pulse width of 8 ns. This is more than four times the largest reported pulse energies generated by other fixed-frequency dye lasers when operating at repetition rates of more than 1 kHz. The conversion efficiency and stability of dye laser are discussed, which show the potential for high-speed laser diagnostics in the fields of combustion and turbulent flow detection.

Assessing Blending of Non‐Newtonian Fluids in Static Mixers by Planar Laser‐Induced Fluorescence and Electrical Resistance Tomography

AbstractPlanar laser‐induced fluorescence (PLIF) and electrical resistance tomography (ERT) were applied simultaneously to monitor the mixing performance of a KM static mixer for the blending of non‐Newtonian fluids of dissimilar rheologies in the laminar regime. The areal distribution method was used to obtain quantitative information from the ERT tomograms and the PLIF images. Comparison of the ERT and PLIF results demonstrates the ability of ERT to detect the mixing performance in cases of poor mixing within the resolution of the measurement, though the accuracy decreases as the condition of perfect mixing is approached. Thus, ERT has the potential to detect poor mixing within the confines of its resolution limit and the required conductivity contrast, providing potential rapid at‐line measurement for industrial practitioners.

10 kHz, 10 ns, 13.4 mJ Nd:YAG based frequency-tripled burst-mode MOPA source at 355 nm

We demonstrated a Nd:YAG based Q-switched burst-mode MOPA laser with frequency-tripled ultraviolet output by nonlinear harmonic generation. The individual pulse energy of 13.4 mJ with a repetition rate of 10 kHz at 355 nm was obtained. The short pulse width of ∼10 ns provides a high peak power up to 1.3 MW at 355 nm. This high-energy and high-repetition-rate ultraviolet source has the potential in applications such as pumping the dye laser systems for PLIF imaging.

HCCI and SICI combustion modes analysis with simultaneous PLIF imaging of formaldehyde and high-speed chemiluminescence in a rapid compression machine

Homogeneous Charge Compression Ignition (HCCI) and Spark Induced Compression Ignition (SICI) of a lean iso-octane air mixture are investigated through simultaneous measurements of planar laser-induced fluorescence at 355 nm and high-speed chemiluminescence in the parallelepipedic combustion vessel of a rapid compression machine (RCM). A radiofrequency igniter with a high energy deposit (305 mJ) is used to investigate the SICI combustion phenomena in lean conditions (Φ = 0.5), relatively close to the frontiers of the SICI regime. Fluorescence images enable to monitor both the development of the cool flame process and the topology and dynamics of reaction fronts during the second stage of ignition. The results are first analyzed from a phenomenological point of view, bringing insights into the understanding of the both HCCI and SICI combustion processes as they take place in the RCM. Additional data are gathered from double-pulse 355 nm PLIF imaging, with focus on the temporal evolution of the cool flame and on the reaction front propagation during hot ignition. From a more quantitative point of view, an analysis of apparent velocities of the reaction zones is then presented, and large variations of these values are observed depending on the experimental conditions. These local quantities are closely related to the global heat release rate which is a key parameter for practical applications of HCCI and SICI combustion modes. The proposed simultaneous diagnostics finally lead to a better understanding in the local reaction modes, namely deflagration, spontaneous ignition fronts and bulk auto-ignition – e.g. volumetric auto-ignition -, which are implied in the combustion processes. The results highlight the complex aerothermal interactions taking place in the RCM vessel, in particular through the pre-ignition thermal stratification. The results suggest the latter strongly affects the HCCI combustion process, but also drives the heat release rate during the second stage of the SICI combustion mode. Furthermore, deflagration fronts are found to be significantly affected by cool flame chemistry, as well as by the large and small scale structures of the fluid flow.

高速PLIF成像诊断应用的脉冲串激光器研究进展

高速PLIF成像诊断应用的脉冲串激光器研究进展