OH Planar Laser-induced Fluorescence Measurements Research Articles

Analyzing 3D fields of refractive index, emission and temperature in spray-flame nanoparticle synthesis via tomographic imaging using multi-simultaneous measurements (TIMes)

Turbulent spray flames issuing from the SpraySyn 2.0 burner were investigated in the present work by means of tomographic imaging using multi-simultaneous measurements (TIMes). This small-scale standard burner is designed to study flame-spray synthesis of tailored nanoparticle systems. The applied measurement technique combined background-oriented schlieren tomography, computed tomography of light emission and two-color tomographic salt emission thermometry to reveal single-shot and time-averaged 3D fields of refractive index (linked to temperature, pressure and species concentration), CH* chemiluminescence (flame front indicator), SrOH emission from synthesized trace amount of Sr(NO3)2 dissolved in the liquid precursor solution (marker of the hot spray-flame products), and temperature of the hot spray-flame products, respectively. The salt emission thermometry technique was extended to tomographic measurements for the first time. Its implementation was based on the voxel-wise intensity ratio of reconstructed thermally-excited SrOH signals at two different regions in the visible spectrum that was converted to temperature using a single calibration curve based on multi-line OH planar laser-induced fluorescence measurements. The cost-effective setup for TIMes was composed of 43 machine-vision cameras equipped with bandpass spectrum filters, an innovative continuous semi-circular background pattern and LED light sources. The combined reconstructed fields provided information on the 3D flame structure in detail, showing the interaction between the flame front and spray stream, and the hot temperature regions. Spray-flame features were quantitatively analyzed, namely temperature, height, width, 3D tilt angle, and spreading half-angle of the flame plume. The temperature measurements proved to be robust against variations in the fuel/oxidizer equivalence ratio for the four investigated operating conditions. The spray flame issuing from the SpraySyn 2.0 burner at the standard operating condition was more stable than that in the previous burner generation, although more asymmetric near the nozzle exit and tilted, which might still cause heterogeneity of the synthesized nanoparticle system.

OH-Planar Laser-Induced Fluorescence Measurements in Laminar Diffusion Flames of n-Heptane with Coflow Air Diluted by N2 and CO2

The distribution of hydroxyl (OH) radicals in the laminar diffusion flame of n-heptane was studied by planar laser-induced fluorescence (PLIF). The influences of nitrogen (N2) and carbon dioxide (CO2) dilution on the formation and distribution of OH radicals were analyzed. The corresponding dilution ratios (volume fractions) of both N2 and CO2 vary from 0 to 5%. The results show that for the n-heptane flame, the OH radical is mainly concentrated in the two wings of the flame, presenting a radially approximate symmetrical distribution. Both N2 and CO2 dilutions decrease the intensity of the maximum OH radical fluorescence and the total OH radical fluorescence. Moreover, the flame temperature decreases more significantly with the CO2 dilution ratio due to the combination of the dilution effect and thermal effect.

Near blowout dynamics of a premixed, swirl stabilized flame

This paper analyzes the flame dynamics of near LBO (lean blowoff) swirl stabilized flames, using simultaneous OH and CH2O (formaldehyde) PLIF (planar laser induced fluorescence) measurements. Prior studies have shown that recirculation stabilized flames approach blowoff through two distinct stages – “stage 1” characterized by local extinction, where the overall flame and flow field remain largely unchanged, and “stage 2”, characterized by a fundamental change in the flow field, accompanied by violent flame flapping and wake disruption. This paper quantifies extinction spots along the flame edge, and entrained reactants within the combustion product region to analyze these stages in greater detail. Extinction spots were quantified by the overlapping regions of OH and CH2O – numerous such spots were found near blowoff. The entraining of unburnt reactants into the recirculation zone was quantified by detecting low intensity OH pockets that were not surrounded by CH2O. As expected, the flame near blowoff displayed significantly more entrained reactant pockets relative to a stable flame. Unexpectedly however, the volume of these pockets is tiny compared to the products, even on the edge of blowoff. Once they enter the wake, they are short-lived, suggesting that they are diluted and/or quickly react. This was surprising given the non-trivial baseflow and flame position disruption at these conditions, suggesting a striking similarity between the average composition of the wake, to that of a stable flame.

Experimental Flame Front Characterisation in a Lean Premix Burner Operating with Syngas Simplified Model Fuel

The recent growing attention to energy saving and environmental protection issues has brought attention to the possibility of exploiting syngas from gasification of biomass and coal for the firing of industrial plants included in the, so called, Integrated Gasification Combined Cycle power plants. In order to improve knowledge on the employ of syngas in lean premixed turbulent flames, a large scale swirl stabilized gas-turbine burner has been operated with a simplified model of H2 enriched syngas from coal gasification. The experimental campaign has been performed at atmospheric pressure, with operating conditions derived from scaling the real gas turbines. The results are reported here and consist of OH-PLIF (OH Planar Laser Induced Fluorescence) measurements, carried out at decreasing equivalence of air/fuel ratio conditions and analysed together with the mean aerodynamic characterisation of the burner flow field in isothermal conditions obtained through LDV (Laser Doppler Velocimetry) and PIV (Particle Image Velocimetry) measurements. The OH concentration distributions have been analysed statistically in order to obtain information about the location of the most reactive zones, and an algorithm has been applied to the data in order to identify the flame fronts. In addition, the flame front locations have been successively interpreted statistically to obtain information about their main features and their dependence on the air to fuel ratio behaviour.

OH planar laser-induced fluorescence measurements with high spatio-temporal resolution for the study of auto-ignition.

For the detailed understanding of transient combustion processes, in particular, of auto-ignition, quantitative measurements with high spatio-temporal resolution are desirable. These can, for instance, serve as validation data for time-resolved numerical simulations and in particular for the combustion models used in those simulations. In the current study, a jet-in-hot-coflow (JHC) burner, developed at the German Aerospace Center (DLR), the DLR JHC, was used to inject a turbulent methane jet into the hot exhaust gas of a lean hydrogen/air flame, and a steady state jet flame was established. In addition, fuel could be injected in a transient manner. Here, an auto-igniting jet was observed. The flame stabilization of the steady state jet flame and the auto-ignition during transient fuel injection were studied using high-speed laser-based and optical measurements. A strategy for quantifying high-speed OH planar laser-induced fluorescence is presented, and the measurement uncertainties are evaluated. The flame stabilization mechanism in steady state jet flames was assessed using probability density functions of the OH concentration at different axial and radial locations. The formation of auto-ignition kernels during transient fuel injection is evaluated based on time series of the OH concentration. It is shown how the OH concentration levels and PDF shapes can be used to characterize the chemical state of the reacting flow and to distinguish between auto-ignition and flame propagation.

Characterization of the reaction zone structures in a laboratory combustor using optical diagnostics: from flame to flameless combustion

Flame emission spectra and OH* imaging are carried out together with planar laser-induced fluorescence (PLIF) of OH and CH2O to characterize the reaction zone structures of three combustion modes, i.e. a flame mode, a transition mode and a flameless mode, established in a laboratory combustor. Spectroscopic measurements indicate that the invisibility of the flameless mode can be attributed to a moderate suppression of the CH* and C2* emissions as well as a significant increase of the continuous background from the CO2* emission. PLIF measurements of OH and CH2O show that all studied cases experience a premixing of unburnt reactants with the recirculated oxygen-containing hot burnt gases in the near-inlet region. It is found that the interplay between turbulence intensity and the availability of oxygen from both the inlet air jet and recirculation is important in establishing flameless combustion in the present combustor. Consistent with a previous chemical kinetic study, the present experimental results suggested that the flameless condition is in favor of converting a highly diluted CH4/air mixture into CO and H2, which will be further oxidized. For the flameless case, CH2O is detected slightly before the OH* region, where a relatively high level of minimal OH is observed. The reaction zone region marked by the OH* emission for the flameless case is characterized by a uniform instantaneous OH distribution with small standard deviation, suggesting a distributed oxidization of intermediate species such as CO and H2 for this combustion mode.

Simultaneous 5 kHz OH-PLIF/PIV for the study of turbulent combustion at engine conditions

Simultaneous measurements of velocity and scalar fields were performed in turbulent nonpremixed flames at gas turbine engine-operating conditions using 5 kHz particle image velocimetry (PIV) and OH planar laser-induced fluorescence (OH-PLIF). The experimental systems and the challenges associated with acquiring useful data at high pressures and high thermal powers are discussed. In this work, a wide range of operating conditions were studied, with a maximum pressure and thermal power of 1.8 MPa and 950 kW, respectively. In the PIV measurements, the high thermal power conditions were shown to cause significant defocusing of the particle images. This was the result of variations in the optical refractive index of the gas which were caused by strong temperature gradients within the inner structure of the flame. High flame luminosity also led to decreased SNR with increasing flame power. The OH-PLIF measurements did not show indication of strong laser sheet absorption at any condition tested. However, a decrease in the peak SNR was observed with increasing chamber pressure. An analysis of the true measurement resolution with respect to the scales of the flow is also given. Based on the resolved scales, the present dataset was used to study the time-averaged flow structure and its effect on flame behavior. Heat release conditioned flow statistics were studied to elucidate the flow–flame interactions in high-power flames.

Continuous hydroxyl radical planar laser imaging at 50 kHz repetition rate.

This study demonstrates high-repetition-rate planar laser-induced fluorescence (PLIF) imaging of hydroxyl radicals (OH) in flames at a continuous framing rate of 50kHz. A frequency-doubled dye laser is pumped by the second harmonic of an Nd:YAG laser to generate laser radiation near 283nm with a pulse width of 8ns and rate of 50kHz. Fluorescence is recorded by a two-stage image intensifier and complementary metal-oxide-semiconductor camera. The average power of the 283nm beam reaches 7W, yielding a pulse energy of 140μJ. Both a Hencken burner and a DC transient-arc plasmatron are used to produce premixed CH4/air flames to evaluate the OH PLIF system. The average signal-to-noise ratio for the Hencken burner flame is greater than 20 near the flame front and greater than 10 further downstream in a region of the flame near equilibrium. Image sequences of the DC plasmatron discharge clearly illustrate development and evolution of flow features with signal levels comparable to those in the Hencken burner. The results are a demonstration of the ability to make high-fidelity OH PLIF measurements at 50kHz using a Nd:YAG-pumped, frequency-doubled dye laser.

High-speed tomographic PIV and OH PLIF measurements in turbulent reactive flows

High-speed tomographic particle image velocimetry (TPIV) is demonstrated in turbulent reactive flows at acquisition rates ranging from 10 to 16 kHz. The 10-kHz TPIV measurements are combined with planar laser-induced fluorescence (PLIF) imaging of OH to mark the high-temperature reaction zone of the flame. Simultaneous TPIV/OH PLIF measurements are applied to the stabilization region of a weakly turbulent lifted dimethyl ether (DME)/air jet flame (Re D = 7,600) and the mixing layer of a turbulent partially premixed DME/air jet flame (Re D = 29,300). In the lifted jet flame, vortical structures exhibit time-dependent morphological changes and eventually dissipate as they approach the flame. In the near field of the turbulent jet flame, dynamics of localized extinction are captured as coherent structures with high compressive strain rates interact with the reaction zone and subsequently break apart. The principal axis of compressive strain has a strong preferential orientation at 45° with respect to the jet axis. The three-dimensional velocity field measurements are used to evaluate biases in two-dimensional (2D) measurements of compressive strain rates in a turbulent jet flame. The biases in the 2D measurements primarily stem from out-of-plane orientation of the principal axis of compressive strain. Comparisons with a constant density turbulent non-reactive jet (Re D = 22,600) show that the jet flame has larger coherent structures that are confined near the reaction zone. Data from the non-reactive jet are also used to evaluate effects of noise, bias, and spatial averaging on measurements of the velocity and velocity gradients.

Simultaneous dual-plane CH PLIF, single-plane OH PLIF and dual-plane stereoscopic PIV measurements in methane-air turbulent premixed flames

Simultaneous dual-plane CH planar laser induced fluorescence (PLIF), single-plane OH PLIF and dual-plane stereoscopic particle image velocimetry (PIV) measurement has been developed to investigate three-dimensional structures of methane-air turbulent premixed flame. Dual-plane CH PLIF system consists of two independent conventional CH PLIF measurement systems and laser beams from each laser system are led to parallel optical pass using the difference of polarization, and CH PLIF is conducted in two parallel two-dimensional cross-sections. Dual-plane stereoscopic PIV was established by using the difference of wavelength. The dual-plane CH PLIF and stereoscopic PIV are combined with single-plane OH PLIF. The laser sheet for single-plane OH PLIF is located at the center of two planes for CH PLIF and stereoscopic PIV. The separation between each plane is set to about 170μm. The spatial resolution of PIV is set to 175μm (about three times Kolmogorov length scale)×175μm×210μm and is same as that of general direct numerical simulations (DNSs) of turbulence. This simultaneous measurement can provide three-dimensional flame front, three velocity components and nine velocity derivatives. The measurement is conducted in methane-air swirl-stabilized turbulent premixed flames of Reλ=63.1, 95.0 and 115.0. Various complicated three-dimensional flame structures such as the handgrip and spire structures, which have been shown by previous three-dimensional DNSs, are observed in high Reynolds number turbulent premixed flames. Magnitude of strain rate, which is evaluated from nine velocity derivatives, shows high value in unburnt region except for the region engulfed by burnt gas. These results show that the simultaneous dual-plane CH PLIF, single-plane OH PLIF and dual-plane stereoscopic PIV measurement is useful for investigating the characteristics of turbulent premixed flames.

Non-linear Response of Turbulent Premixed Flames to Imposed Inlet Velocity Oscillations of Two Frequencies

This paper describes an experimental study investigating the non-linear response of lean premixed air/ethylene flames to strong inlet velocity perturbations of two frequencies. The combustor has a centrally-placed bluff body and a short quartz section. The annulus between the bluff body and the flow tube, which also housed the acoustic pressure transducers, allowed the reactants into the combustor. The inlet flow was perturbed using loudspeakers. High speed laser tomography, OH* chemiluminescence and OH Planar Laser Induced Fluorescence (PLIF) have been used for flow visualization, heat release and flame surface density (FSD) measurements respectively. The heat release fluctuations increased initially linearly with inlet velocity amplitude for a single frequency forcing, with saturation occurring after forcing amplitudes of around 15% of the bulk velocity, which was found to occur due to vortex roll up and subsequent flame annihilation. The introduction of energy at the second frequency (i.e, the harmonic) was found to change the vortex formation and shedding frequency, depending on the level of forcing. This resulted in a non-linear flame response transfer function (defined as the amplitude of unsteady heat release divided by the amplitude of velocity perturbation at the fundamental) whose amplitude depended greatly on the amount of harmonic content present in the perturbations. The introduction of higher harmonics reduced the flame annihilation events, which are responsible for saturation, thus reducing non-linearity in the amplitude dependence of the flame response. These results were further verified using sequential time-resolved OH PLIF measurements. The findings from this study suggest that the acoustic response of the flame was mostly due to flame area variation effected by modulation of the annular jet and evolution of the shear layers.

Simultaneous CH–OH PLIF and stereoscopic PIV measurements of turbulent premixed flames

Simultaneous CH and OH planar laser-induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) measurement have been developed to investigate the local flame structure of turbulent premixed flames. In this system, high-speed CMOS cameras were adopted to capture clear stereoscopic particle images without contamination by the flame radiation. The effects of scattering of CH PLIF laser by tracer particles are carefully investigated to improve signal-to-noise ratio in CH fluorescence images. The developed simultaneous two radical concentrations and three-component velocity measurement on a two-dimensional plane was applied to relatively high Reynolds number turbulent premixed flames in a swirl-stabilized combustor. All measurements were conducted for methane–air premixed flames in the corrugated flamelet regime. Simultaneous CH and OH images suggest the presence of the isolated burned gas in the unburned mixture and the isolated unburned mixture in the burned side, which have been predicted by direct numerical simulations. Detailed analysis of simultaneous CH and OH images reveals that the minimum curvature radius of the flame front coincides with the Kolmogorov scale. Strong three-dimensional velocity fluctuations, which are measured by the stereoscopic PIV, imply that misunderstanding of the flame/turbulence interactions would be caused by the analysis of two-component velocity distributions in a cross-section. Furthermore, comparisons of CH–OH PLIF and three-component velocity fields show that the burned gases do not always have high-speed velocity in a relatively high Reynolds number turbulent premixed flame. The Reynolds number dependence of the flame front was clearly captured by the simultaneous CH–OH PLIF and stereoscopic PIV measurements.

Laser imaging system for determination of three-dimensional scalar gradients in turbulent flames.

An imaging system for the measurement of three-dimensional (3D) scalar gradients in turbulent hydrocarbon flames is described. Combined line imaging of Raman scattering, Rayleigh scattering, and CO laser-induced fluorescence (LIF) allows for simultaneous single-shot line measurements of major species, temperature, mixture fraction, and a one-dimensional surrogate of scalar dissipation rate in hydrocarbon flames, while simultaneous use of two crossed, planar LIF measurements of OH allows for determination of instantaneous flame orientation. In this manner the full 3D scalar dissipation can be estimated in some regions of a turbulent flame on a single-shot basis.

Injection of single and multiple vortices in an opposed-jet burner

A thorough understanding of turbulent reacting flows is essential to the continued development of practical combustion systems. Combustor codes can be validated using data such as those generated in this study of a vortex interacting with a nonpremixed, opposed-jet hydrogen-air flame. When experimental results are compared with model predictions, the underlying flowfield must be matched carefully. Since the vortex-injection process used in the present experiments can result in many types of vortices, including multiple vortices, restrictions on the experimental operation of the burner are required as well as careful vortex characterization. Vortex-characterization data are acquired using digital, two-color particle-image velocimetry (PIV), and the hydroxyl (OH) layer produced by the flame is imaged using planar laser-induced fluorescence (PLIF). The PIV and OH PLIF measurements are performed simultaneously. Good agreement with previous numerical-modeling predictions is obtained when experiments and computations are performed using similar vortex conditions.

Regimes of non-premixed flame-vortex interactions

Detailed studies of flame-vortex interactions are extremely valuable to improve our understanding of turbulent combustion regimes. Combined experimental and numerical studies have already been performed in the premixed case during previous investigations. Therefore, we decided to carry out a detailed experimental investigation on the regimes observed during in teraction of a vortex ring and a non-premixed, diluted, hydrogen/air, laminar counterflow flame. To obtain the needed information, several optical diagnostic techniques have been used, in particular, planar laser-induced fluorescence (PLIF) of acetone to quantify vortex structure and speed, simultaneous OH PLIF and Rayleigh measurements, and simultaneous OH PLIF and particle-imaging velocimetry (PIV) measurements. A post-processing of the results combined with direct simulations using detailed chemistry and transport models to check the quality of the postprocessing procedures has led to the construction of a spectral interaction diagram. Eight interaction types were found, emphasizing the relative importance of competing physical phenomena such as straining, curvature, wrinkling, roll-up, and extinction. In particular, we observe two different types of extinction, one due to the combined action of curvature and straining, and the other purely due to straining effects. It was also observed that many vortices are too small or dissipate too rapidly to influence the flame. In other cases, the vortex ring can lead to the formation of pockets of oxidizer burning in the fuel part of the domain. These regimes and the limits between them have important implications for the modeling of turbulent non-premixed combustion.

96/05267 Laboratory study of N2O formation from 15N-isotope-marked NO while burning char particles at FBC conditions

Detailed studies of flame-vortex interactions are extremely valuable to improve our understanding of turbulent combustion regimes. Combined experimental and numerical studies have already been performed in the premixed case during previous investigations. Therefore, we decided to carry out a detailed experimental investigation on the regimes observed during in teraction of a vortex ring and a non-premixed, diluted, hydrogen/air, laminar counterflow flame. To obtain the needed information, several optical diagnostic techniques have been used, in particular, planar laser-induced fluorescence (PLIF) of acetone to quantify vortex structure and speed, simultaneous OH PLIF and Rayleigh measurements, and simultaneous OH PLIF and particle-imaging velocimetry (PIV) measurements. A post-processing of the results combined with direct simulations using detailed chemistry and transport models to check the quality of the postprocessing procedures has led to the construction of a spectral interaction diagram. Eight interaction types were found, emphasizing the relative importance of competing physical phenomena such as straining, curvature, wrinkling, roll-up, and extinction. In particular, we observe two different types of extinction, one due to the combined action of curvature and straining, and the other purely due to straining effects. It was also observed that many vortices are too small or dissipate too rapidly to influence the flame. In other cases, the vortex ring can lead to the formation of pockets of oxidizer burning in the fuel part of the domain. These regimes and the limits between them have important implications for the modeling of turbulent non-premixed combustion.

Laser Selection Criteria for OH Fluorescence Measurements in Supersonic Combustion Test Facilities

The limitations of the application of dye laser and narrow-band tunable KrF excimer laser systems to OH planar laser-induced fluorescence measurements in supersonic combustion test facilities are examined. Included in the analysis are effects of signal strength, collisional quenching, beam absorption, and fluorescence trapping on achievable measurement accuracy using several excitation and detection options for either of the two laser systems. Dye based laser systems are found to be the method of choice for planar imaging when the line integral of OH concentration along the incidence or detection paths is less than 10 16 cm/cm 3 , whereas the ArF based systems provide significant reduction in measurement ambiguity when either concentration line integral is in excess of 10 16 cm/cm 3

An experimental study of supersonic H 2-AIR combustion in a shock tunnel flow facility

The dynamics of reacting and non-reacting high temperature supersonic flowfields were investigated using advanced imaging and line-of-sight optical diagnostics. The measurements were made in a reacting flow shock tunnel facility generating Mach 3 air at static conditions of 1500 K and 0.3 atm. Hydrogen and helium were injected into the flow downstream of a rearward-facing step. Atomic oxygen in the free-stream air, produced in the reflected shock plenum, ensured prompt ignition at the H 2 -air interface. Planar Laser-Induced Fluorescence (PLIF) measurements of NO, also present in the free-stream air, determined the basic flow structure and temperature field for non-reactive flow with and without helium injection. PLIF measurements of OH revealed the instantaneous reaction zone interface for H 2 injection. The results provide insight into the turbulent mixing and burning phenomena in supersonic combusting flows.

Planar laser-induced fluorescence imaging of shock-induced ignition

Planar laser-induced fluorescence (PLIF) imaging has been developed for use in studies of shock-induced ignition. PLIF measurements of OH have been made in stoichiometric mixtures of H2 and O2 diluted in 90% Ar using two shock tube endwall configurations. These OH images, which serve as indicators of ignition, were first obtained in the weak and strong ignition limits in the gas behind a shock reflected from a planar endwall. In both limits the images exhibited planar combustion waves. The absence of observable ignition kernels in the weak ignition limit presumably results from lack of optical access to the corners of the shock tube where kernel formation is expected to occur. A grooved endwall was used to introduce thermal nonuniformities and thereby promote isolated regions of ignition within the imaged region. Sequences of PLIF images defining the temporal evolution of the ignition process were acquired in the weak and strong ignition limits for this endwall configuration. These images exhibit highly nonuniform structure and serve to illustrate the utility of 2-D imaging techniques for ignition studies.