Laser-induced Fluorescence Of OH Radicals Research Articles

The effects of various flame retardants on the combustion of polypropylene: Combining optical diagnostics and pyrolysis fragment analysis

The assessment of the efficacy and mode of action of flame retardants in polymers requires a thorough knowledge of the coupling of the presence of chemical inhibitors with the underlying heat and mass transport processes. In this experimental work, we combine a thermal decomposition analysis with optical combustion diagnostics in different test environments to gain a comprehensive understanding of the relevant sub-processes and demonstrate how this approach provides future opportunities for the research regarding the mode of action of flame retardants. Four different composites of polypropylene (PP) and 10 wt% of selected flame retardants as well as neat PP are prepared. Flame retardants include pentaerythritol spirobis(methylphosphonate) (PSMP), zinc diethylphosphinate (DEPZn), aluminum hydroxide (i.e. aluminum trihydrate, ATH) and ammonium polyphosphate (APP). The released substances in the gas phase are investigated by using thermogravimetric analysis coupled to Fourier-transform infrared spectroscopy (TGA-FTIR), and the burning behavior regarding carbon monoxide concentration and heat release rate (HRR) is studied in cone calorimeter experiments. The ignition and combustion of flame retarded micrometer-sized particles of PP are experimentally investigated in a laminar flat flame burner by means of simultaneous scanning planar laser-induced fluorescence of OH radicals (OH-PLIF) and diffuse back-illumination (DBI) at a repetition rate of 10 kHz. Particles with gas-phase activity show a decreased normalized OH signal intensity during combustion. A dependency of the dimensionless flame radius on the particle size is found which can be linked to the faster heating and pyrolysis of smaller particles compared to large particles and agglomerations. Eventually, the interaction with an external premixed flame and the subsequent extinction of self-sustaining flames of polymer sticks is investigated using OH-PLIF in an adapted horizontal burning test suited for the application of optical diagnostics. This approach allows to link the findings of investigations encompassing multiple scales, resulting in a thorough understanding of both polymer chemistry and combustion dynamics in flame retarded polymers.

Accurate determination of homogeneous ignition of single solid fuel particles enabled by machine learning

This work presents an experimental investigation of solid fuel combustion in a laminar flow reactor with high-speed laser diagnostics. The main focus lays on the development of image evaluation approaches for accurate determination of single-particle ignition based on optical measurement data. Homogeneous ignition of individual bituminous coal particles is visualized by simultaneous planar laser-induced fluorescence of OH radicals (OH-LIF) and diffuse backlight-illumination (DBI) techniques at 10kHz. Two coal particle sizes of 90–125μm and 160–200μm are investigated in conventional air and oxy-fuel conditions with increasing oxygen concentrations. A comprehensive experimental data set is established, containing in total 1518 single-particle events and high-quality ground truth labels of ignition delay times. The previously proposed structure and signal (SAS) analysis is evaluated. Errors and uncertainties are evaluated in a sensitivity analysis with variation in the threshold selection, implying inherent restrictions of conventional image processing methods. An improved blob detection approach is applied, enabling feature detection at various scales and higher accuracy in predicting ignition delay times. Moreover, two deep learning architectures, namely residual network (ResNet) and feature pyramid network (FPN) are implemented. Both networks are capable of detecting ignition with significantly higher accuracy and precision than other algorithms in this study. Influences of training data and depth of networks on the prediction performance of a trained model are examined. The current study shows that the hierarchical feature extraction of the convolutions networks clearly facilitates data evaluation for high-speed optical measurements and could be transferred to other solid fuel experiments with similar boundary conditions.

Experimental investigations of single particle and particle group combustion in a laminar flow reactor using simultaneous volumetric OH-LIF imaging and diffuse backlight-illumination

The volatile combustion of Colombian high-volatile bituminous coal was experimentally studied in a laboratory laminar flow reactor. The volatile flames corresponding to the single particle and particle group combustion were visualized using non-intrusive multi-parameter optical diagnostics. In the present study, high-speed laser-induced fluorescence of OH radicals (OH-LIF) was applied to study igniting particles by temporally tracking OH-LIF signals in the gas-phase flame. A novel acousto-optic deflector combined with a 10 kHz dye laser was employed for laser scanning through a probe volume with a thickness of a few millimeters. The three-dimensional OH-LIF signals were used to reconstruct the volatile flame structures of burning particles. Simultaneously, diffuse backlight-illumination (DBI) is implemented to measure the size and the spatial distribution of particles to distinguish between single and group particle combustion. For single particles, starting from the onset of ignition, the OH-LIF intensity reaches its maximum within several milliseconds, which is temporally resolved by employing a laser scanning system. The gas-phase ignition starts downstream of the particles. As the particle size increases, the flame stand-off distance increases, whereas the ratio of the flame stand-off distance and the particle diameter decreases, which ranges from 2 to 4 for the coal particles investigated. For particle groups, the flame topology is evaluated for individual reconstructions with different particle number densities (PND). As the PND increases, the volatile flames are pushed outwards to the boundary of particle clouds and a non-flammable region emerges in the center of volatile flames. Soot formation is observed and becomes increasingly intensive as the PND increases.

Characterization of Supersonic Turbulent Combustion in a Mach-10 Scramjet Combustor

Turbulent combustion remains an unsolved problem, and the simulation models that have been developed, although accurate for subsonic turbulent combustion, have limited accuracy when applied to supersonic combustion. An experiment has been developed to characterize supersonic turbulent combustion in a geometrically simple flowpath that is capable of accurately reproducing the complex and highly nonuniform flow structures present in flight-candidate scramjet engines. An inlet-fueled scramjet engine was designed with a symmetrical intake to reproduce the correct flow structures. This was tested with an inflow condition of Mach 7, using a Mach-10 flow enthalpy of to reproduce the combustion conditions in a flight vehicle operating at Mach 10. The turbulent flame was directly observed in the flow using planar laser-induced fluorescence of OH radicals. These results have been complemented by large-eddy simulations of the experimental conditions, showing that the combustion process in scramjets is multimode, where neither premixed nor non-premixed combustion dominates, and both substantially contribute to heat release. These modes are present simultaneously inside the engine. For both modes, combustion is confirmed to happen over multiple combustion regimes. This work provides new insight into the supersonic turbulent combustion process and enhances our understanding of combustion in scramjet engines.

Investigation of flame retarded polypropylene by high-speed planar laser-induced fluorescence of OH radicals combined with a thermal decomposition analysis

The combustion of micrometer-sized polypropylene (PP) particles is analyzed in situ using a combination of high-speed planar laser-induced fluorescence of the OH radical (OH-PLIF) and a thermal decomposition analysis. The gas phase is investigated by multiple analytical techniques to gain comprehensive knowledge on the decomposition products of flame retardants and their effect on the combustion process. Neat PP is compared with a formulation consisting of 10 wt% of a phosphorus-containing flame retardant (pentaerythritol spirobis(methylphosphonate), PSMP) which is known to provide gas phase activity. The decomposition of the neat flame retardant, PP and the flame retardant formulation is investigated using a simultaneous analysis (STA) consisting of a thermal gravimetric analysis and a differential thermal analysis device which is coupled to Fourier-transform infrared spectroscopy and mass spectrometry devices. By this, the release of decomposition products of the flame retardant additive can be determined. The excitation of OH radicals is used to temporally track the diffusion flame surrounding the particles during combustion in a laminar flow reactor. The radial distance to the peak reactivity zone of flame retardant containing particles increased by about 70 µm compared with neat PP particles. Tracking the peak OH signal in the diffusion flame, during ignition and the early phase of combustion, a decrease in the peak intensity is observed for flame retardant polymer particles. Additionally, cone calorimeter tests are used to evaluate the combustion behavior as a standard test in flame retardancy.

Influence of self-sustained jet oscillation on a confined turbulent flame near lean blow-out

Premixed methane–air turbulent flame is generated in a single-nozzle jet-stabilized combustor designed based on the FLOX® concept11FLOX® is a registered trademark of WS Wärmeprozesstechnik GmbH, Renningen, Germany. Confinement-induced, self-sustained jet oscillation is observed. Its influence on combustion stability near lean blow-out (LBO) is investigated using simultaneous particle imaging velocimetry (PIV), planar laser-induced fluorescence of OH radicals (OH PLIF), and OH chemiluminescence imaging at 5-kHz repetition rate. Via proper orthogonal decomposition (POD) of the velocity field and extended POD of the scalar fields, pronounced variations in the flame shape are observed during a cycle of jet oscillation. In extreme cases, flame is partially blown out in the combustor due to jet impingement on the wall during the first half of its oscillation cycle. In the subsequent half cycle following jet detachment, flame is restabilized after robust flashback and re-light. Statistical analysis shows that such pattern is by far the most prevalent mechanism for blow-out and restabilization to take place at the operating condition. Additionally, these events are found with much higher probability during slow-paced jet oscillations.

Confinement-Induced Instabilities in a Jet-Stabilized Gas Turbine Model Combustor

Self-sustained jet flapping is observed in a confined, premixed and preheated methane-air turbulent flame, generated in a single-nozzle jet-stabilized gas turbine model combustor designed based on the FLOX Ⓡ concept. The flapping frequency and its complex motion within the confinement of the combustor are characterized in detail using proper orthogonal decomposition (POD) of the flow fields measured by particle imaging velocimetry (PIV). The influence of jet flapping on combustion stability is examined using simultaneous PIV/OH chemiluminescence imaging and PIV/planar laser-induced fluorescence of OH radicals (OH PLIF) at 5 kHz repetition rate. By influencing the size and location of the recirculation zones, jet flapping modifies the flame shape and flame lift-off height. It also controls the amount of hot gas entrainment into the recirculation zones. In extreme cases, jet flapping is found to cause temporary local extinction of the flame, due to jet impingement on the combustor wall and partial blockage of burned gas entrainment. The flame is only able to recover after the jet detaches from the wall and reopens the back flow channel. The results suggest that jet flapping could play a key role in the stabilization mechanisms in similar jet-stabilized combustors.

Laser-induced breakdown spectroscopy in a partially premixed turbulent jet flame

A new approach for studying mixing and combustion phenomena in turbulent partially premixed flames using laser-induced breakdown spectroscopy (LIBS) is demonstrated. Setting the laser pulse energy just above the breakdown threshold in the cold gas allows statistical data to be obtained concerning the probability of finding cold or hot gas inside the measurement volume. Moreover, a compositional analysis utilizing the LIBS spectra allows the mixing of the different cold gas flows to be studied. A proof-of-concept experiment is carried out in a turbulent jet flame stabilized on a coaxial burner. The LIBS data agree well with results obtained by planar laser-induced fluorescence of OH radicals.

Gas temperature of capacitance spark discharge in air

Capacitance spark discharge has been widely used for studying the ignition of flammable gas caused by electrostatic discharge. In the present study, the gas temperature of capacitance spark discharge is measured. The gas temperature is an important factor in understanding the electrostatic ignition process because it influences the reaction rate of ignition. Spark discharge is generated in air with a pulse duration shorter than 100ns. The discharge energy is set to 0.03–1mJ. The rotational and vibrational temperatures of the N2 molecule are measured using the emission spectrum of the N2 second positive system. The rotational and vibrational temperatures are estimated to be 500 and 5000K, respectively, which are independent of the discharge energy. This result indicates that most of the electron energy is consumed in the excitation of vibrational levels of molecules rather than the heating of the gas. The gas temperature after discharge is also measured by laser-induced fluorescence of OH radicals. It is shown that the gas temperature increases after discharge and reaches approximately 1000K at 3μs after discharge. Then the temperature decreases at a rate in the range of 8–35K∕μs depending on the discharge energy.

Laser-induced fluorescence of OH radicals in a dielectric barrier discharge

We discuss the results of laser-induced fluorescence measurements of OH radicals in a dielectric-barrier discharge. The discharge is in parallel plate geometry in atmospheric pressure air and argon. Although the air discharge consists of discrete microdischarges, two-dimensional images show the spatial uniformity of the OH radical. Results show that with increasing power, the OH production decreases due to gas heating and increased ozone production. The addition of O2 increases the OH production at low concentrations; however, at higher O2 concentration the OH concentration decreases due to increased electron attachment.

Experimental study of extinction of nonadiabatic counterflow premixed flames

The effects of the velocity gradient and nonadiabaticity on the extinction of premixed counterflow low-pressure flames were studied using saturated laser-induced fluorescence of OH radicals. The temperature and OH concentration were determined at a stagnation point for different opposed flow velocities in nonadiabatic and near-adiabatic methane-oxygen-nitrogen flames in the range of equivalence ratio 0.6–1.2. The temperature and OH concentration profiles were measured in twin flames far from and close to extinction. Temperatures at the stagnation point vary with heat losses and velocity gradient, while OH concentrations depend mainly on velocity gradient. Spatial gradients of temperature or OH concentration profiles in twin flames are insensitive to velocity gradient and heat losses. The implication of these results for the correctness of probe sampling and mass-spectrometric analysis in low-pressure flat flames is discussed.

Measurement of the instantaneous detailed flame structure in turbulent premixed combustion

Simultaneous two-dimensional UV-Rayleigh thermometry and 2D laser-induced fluorescence of OH radicals were used to analyze highly turbulent, premixed natural gas/air flames with up to 130-kW heat release, where the turbulence conditions are approximately homogeneous in the measurement area. The turbulent Reynolds number has been varied between 60 and 2500, and the turbulent Karlovitz numbers ( Ka ), between 0.03 and 14. The simultaneously recorded single-shot images resolve the instantaneous structure of turbulent premixed flames to a spatial resolution of about 0.1 mm. For flames with Ka <1, the local flame-front structure (temperature and OH concentration profiles normal to the flame front) is similar to calculations Δ of laminar premixed flames. For flames with 1< Ka <14, the mean thermal flame-front thickness is significantly decreased compared with laminar unstrained calculations of flames with the same stoichiometry. This is contrary to the expectation of thickened flames (Klimov-Williams criterion). The mean flame thickness based on the OH concentration ascend is found to be independent of Ka . Obvious strongly localized deviations from a laminar-like flamelet structure were found for flames with Ka >5. In a few regions, temperatures above 1400 K, but no OH radicals, were observed, indicating regions with local extinction. A comparison of the size of small-scale eddies with the laminar eldovich flame-front thickness and the thermal thickness indicates that these eddies do have much smaller influence than has been expected by the Klimov-Williams criterion.

Two-dimensional visualization of the flame front in an internal combustion engine by laser-induced fluorescence of OH radicals

Two-dimensional laser-induced fluorescence (2D-LIF) imaging of OH radicals, excited at 308 nm, has been employed to visualize the flame front in an internal combustion engine burning air/propane mixtures. Light sheet thicknesses down to 70 μm have been attained for excitation. Hydroxyl radicals were detected up to pressures of 7.5 bar at engine speeds of 500 rpm. An upper limit of 300 μm for the flame front thickness was obtained from line intensity profiles.