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Temperature and velocity determination of shock-heated flows with non-resonant heterodyne laser-induced thermal acoustics

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Abstract and Figures

Non-resonant laser-induced thermal acoustics (LITA), a four-wave mixing technique, was applied to post-shock flows within a shock tube. Simultaneous single-shot determination of temperature, speed of sound and flow velocity behind incident and reflected shock waves at different pressure and temperature levels are presented. Measurements were performed non-intrusively and without any seeding. The paper describes the technique and outlines its advantages compared to more established laser-based methods with respect to the challenges of shock tube experiments. The experiments include argon and nitrogen as test gas at temperatures of up to 1000 K and pressures of up to 43 bar. The experimental data are compared to calculated values based on inviscid one-dimensional shock wave theory. The single-shot uncertainty of the technique is investigated for worst-case test conditions resulting in relative standard deviations of 1, 1.7 and 3.4 % for Mach number, speed of sound and temperature, respectively. For all further experimental conditions, calculated values stay well within the 95 % confidence intervals of the LITA measurement.
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DOI 10.1007/s00340-015-6217-7
Appl. Phys. B (2015) 121:235–248
Temperature and velocity determination of shock‑heated flows
with non‑resonant heterodyne laser‑induced thermal acoustics
F. J. Förster1 · S. Baab1 · G. Lamanna1 · B. Weigand1
Received: 13 July 2015 / Accepted: 1 September 2015 / Published online: 21 September 2015
© Springer-Verlag Berlin Heidelberg 2015
Challenges arising from measurements in impulse facili-
ties are briefly addressed followed by a discussion of LITA
with respect to more established techniques. An overview
of existing studies using LITA in shock-induced flows is
given to clarify the resulting motivation of the presented
Shock tubes are widely used in a variety of scientific
applications, which feature high pressure and temperature
environments (e.g. shock-induced flows [1], combustion
chemistry [2], fluid disintegration [3] and reentry phys-
ics [4]). However, quantitative measurements under such
conditions are inherently challenging. In addition, the
very short test duration requires fast-response techniques.
Acquiring multiple gas properties from a single experiment
is desirable as turn-around times and operational costs may
be considerable.
Conventional probing techniques as employed in wind
tunnels are intrusive, which make measurements inside
the flow field difficult due to interference with the flow.
Sensors embedded into the model or shock tube wall are
commonly used to measure pressure. For temperature, this
is more challenging as—in contrast to pressure—tempera-
tures measured at the surface and inside the flow may differ
significantly due to the boundary layer [5].
Laser-based techniques can potentially overcome all
these problems. In addition to well-established tech-
niques such as tunable diode laser absorption spectroscopy
(TDLAS) [6, 7], laser-induced fluorescence (LIF) [810]
or coherent anti-Stokes Raman spectroscopy (CARS)
[11, 12], the authors evaluate LITA as a diagnostic tool
for pulsed facilities. For temperature measurements after
reflected shock waves similar to those used in this study,
Farooq et al. [13] reported a relative standard deviation of
0.5 % for TDLAS. Furthermore, mean discrepancies to the-
oretical values of 3.6 % were found for LIF [9]. Precision
Abstract Non-resonant laser-induced thermal acous-
tics (LITA), a four-wave mixing technique, was applied to
post-shock flows within a shock tube. Simultaneous single-
shot determination of temperature, speed of sound and
flow velocity behind incident and reflected shock waves
at different pressure and temperature levels are presented.
Measurements were performed non-intrusively and with-
out any seeding. The paper describes the technique and
outlines its advantages compared to more established laser-
based methods with respect to the challenges of shock tube
experiments. The experiments include argon and nitrogen
as test gas at temperatures of up to 1000 K and pressures
of up to 43 bar. The experimental data are compared to cal-
culated values based on inviscid one-dimensional shock
wave theory. The single-shot uncertainty of the technique
is investigated for worst-case test conditions resulting in
relative standard deviations of 1, 1.7 and 3.4 % for Mach
number, speed of sound and temperature, respectively. For
all further experimental conditions, calculated values stay
well within the 95 % confidence intervals of the LITA
1 Introduction
The paper presents laser-induced thermal acoustics (LITA)
as diagnostic tool for shock tube facilities. LITA combines
a number of desirable features including non-intrusive,
seedless point measurements of several flow properties.
* F. J. Förster
1 Institute of Aerospace Thermodynamics (ITLR), University
of Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Further challenges for the measurement technique are the extremely high temperature and pressure as well as the very short measurement time in the nozzle reservoir of shock tunnels, which can be overcome by optical measurement techniques. Homodyne laserinduced thermal grating spectroscopy (LITGS, also called resonant LIGS or resonant LITA) needs no seeding particles and offers the possibility of temperature measurements with a high temporal resolution [3][4][5][6][7] under single-shot conditions. In addition, only a small optical access is required for this method, which is particularly advantageous for measurements under high temperatures and pressures. ...
... In the literature, different values can be found for x and y, which also differ between the procedures electrostrictive (LIEGS, also called non-resonant LIGS or non-resonant LITA) and LITGS. In the work of Förster et al. [7], the value 2 is given for x and -3.4 for y for LIEGS. Schlamp et al. [8] report, deviating from this, y = −3 (fluid at rest) or y = −4.25 (flowing fluid). ...
... A more recent work on measurements with laser-induced gratings can be found in Förster [7]. The experimental setup used is basically based on those of Schlamp et al. [16] and Hemmerling et al. [17]. ...
Full-text available
At our institute a piston-driven shock tunnel is operated to investigate structures of space transportation systems under reentry and propelled flight conditions. For temperature measurements in the nozzle reservoir under single-shot conditions, laser-induced thermal grating spectroscopy is used to date to measure the speed of sound of the test gas. The temperature then can be calculated from this data. The existing experimental setup has already been successfully used to measure flows up to an enthalpy of 2.1 MJ/kg. Since conducting the experiments is extremely time-consuming, it is desirable to extract as much data as possible from the test runs. To additionally measure the velocity of the test gas, the test setup was extended. Besides, extensive improvements have been implemented to increase the signal-to-noise ratio. As the experiments can be conducted much faster at the double-diaphragm shock tube of the institute without any restrictions on the informative value, the development of the heterodyne detection technique is carried out at this test facility. A series of 36 single-shot temperature and velocity measurements is presented for enthalpies of up to 1.0 MJ/kg. The averaged deviation between the measured values and the values calculated from the shock equations of all measurements related to the average of the calculated values is 2.0% for the Mach number, 0.9% for the velocity after the incident shock and 4.8% for the temperature after the incident shock.
... The shock tube was spatially fixed in a foundation such that no recoil occurred to induce deflection and laser beam misalignment throughout the experiment. In a similar small-sized conventional, spatially fixed shock tube, Förster et al. [28] presented simultaneous single-shot temperature and velocity measurements by non-resonant heterodyne LIGS. Experiments in nitrogen and argon at shock Mach numbers 1.67-1.96 ...
... Alternatively, a so-called heterodyne approach uses a second interrogation beam as local oscillator (i.e., reference beam) not passing the measurement volume but being mixed with the signal beam from the measurement volume prior to detection. Interference of both signal and reference beams enables additional quantification of the local bulk fluid velocity (single component) from the oscillation beat frequency and Doppler shift-thus enabling direct, simultaneous measurement of local flow Mach number via sound speed and fluid velocity; compare Förster et al. [28]. The present study solely applies the homodyne approach as quantification of fluid residual velocity components in the high-vorticity, post-reflected shock state 5 is not considered of interest. ...
Full-text available
Experimental determination of test gas caloric quantities in high-enthalpy ground testing is impeded by excessive pressure and temperature levels as well as minimum test timescales of short-duration facilities. Yet, accurate knowledge of test gas conditions and stagnation enthalpy prior to nozzle expansion is crucial for a valid comparison of experimental data with numerical results. To contribute to a more accurate quantification of nozzle inlet conditions, an experimental study on non-intrusive in situ measurements of the post-reflected shock wave stagnation temperature in a large-scale free-piston reflected shock tunnel is carried out. A series of 20 single-shot temperature measurements by resonant homodyne laser-induced grating spectroscopy (LIGS) is presented for three low-/medium-enthalpy conditions (1.2–2.1 MJ/kg) at stagnation temperatures 1100–1900 K behind the reflected shock wave. Prior limiting factors resulting from impulse facility recoil and restricted optical access to the high-pressure nozzle reservoir are solved, and advancement of the optical set-up is detailed. Measurements in air agree with theoretical calculations to within 1–15%, by trend reflecting greater temperatures than full thermo-chemical equilibrium and lesser temperatures than predicted by ideal gas shock jump relations. For stagnation pressures in the range 9–22 MPa, limited influence due to finite-rate vibrational excitation is conceivable. LIGS is demonstrated to facilitate in situ measurements of stagnation temperature within full-range ground test facilities by superior robustness under high-pressure conditions and to be a useful complement of established optical diagnostics for hypersonic flows.
... A radial piston pump or a pressurized fluid reservoir provides the required injection pressures depending on the desired NPRs in the experiments. A more detailed description of the test facilities is given in [43,36,44]. ...
... For a detailed description of the optical arrangement the reader is referred to Förster et al. [44] and Baab et al. [36]. Here, only the essential information are given. ...
The present work provides an overview on the possible phase transitions associated with supercritical fluid injection and a detailed evaluation of the mixing process between injectant fluid and quiescent ambience. The experiments cover superheated liquid disintegration, pseudo-boiling transition and single-phase jets under different nozzle pressure ratios. Pseudo-boiling effects emerge when rapid, subsequent changes in pressure/temperature interact with the non-linear behavior of thermodynamic response functions across the Widom line. The associated density fluctuations cause a significant increase in the scattering cross section and may lead to thermo-convective instabilities. Our analysis of the mixing process demonstrates the limited applicability of the adiabatic mixing model, which is often restricted to short residence times even in highly turbulent jets (Re=O(105)). Specifically, our findings show the importance of considering all coupled transport processes in the analysis of mixing problems at high pressures, in particular in presence of large mass concentration and temperature gradients.
... This link is provided by a simple scaling constant providing a significantly simpler calibration procedure. In recent studies, we proved the effectiveness of LITA for quantitative speed of sound measurements in high-pressure atmospheres [37] and the turbulent far-field zone of extremely underexpanded jets [38]. ...
... We determined Λ from frequency measurements in pure nitrogen at ambient conditions for which the speed of sound is well-known. The uncertainties from this calibration procedure can be considered to be within 1.3 % as it was found in Förster et al. [37]. ...
This study is the first to provide a comprehensive speed of sound database for multi-component jet mixing at high pressure. It serves as a unique reference for numerical simulations of mixture preparation processes in future liquid rocket engines and internal combustion engines. We performed quantitative speed of sound measurements in jet mixing zones for five configurations with well-defined experimental conditions. The database covers three different injectant fluids (two alkanes and a fluoroketone) that were brought beyond their critical temperature and pressure prior to injection and discharged into cold nitrogen at supercritical pressure (with respect to the pure injectant properties). Here, we chose the conditions such that subsonic jets were obtained and re-condensation due to cooling of the injected fluid was prevented. Hence, we provide speed of sound data for single-phase jet mixing for three different binary systems. Quantitative data are presented along the jet centerline with sufficiently high spatial resolution to properly resolve the axial decay. In addition, two radial profiles at a position close to the nozzle allow for an assessment of the transversal mixing characteristics. The experimental speed of sound data show consistent trends, which corroborate that mixture effects are correctly resolved in the measurement.
... Three different atmospheres, namely nitrogen with a purity of 99.999 % , argon with a purity of 99.998 % , and carbon dioxide with a purity of 99.995 % , are studied. The optical setup is adapted from the one described in Baab et al. (2016) and Förster et al. (2015). ...
Full-text available
Mixing and evaporation processes play an important role in fluid injection and disintegration. Laser-induced thermal acoustics (LITA) also known as laser-induced grating spectroscopy (LIGS) is a promising four-wave mixing technique capable to acquire speed of sound and transport properties of fluids. Since the signal intensity scales with pressure, LITA is effective in high-pressure environments. By analysing the frequency of LITA signals using a direct Fourier analysis, speed of sound data can be directly determined using only geometrical parameters of the optical arrangement no equation of state or additional modelling is needed at this point. Furthermore, transport properties, like acoustic damping rate and thermal diffusivity, are acquired using an analytical expression for LITA signals with finite beam sizes. By combining both evaluations in one LITA signal, we can estimate mixing parameters, such as the mixture temperature and composition, using suitable models for speed of sound and the acquired transport properties. Finally, direct measurements of the acoustic damping rate can provide important insights on the physics of supercritical fluid behaviour. Graphic Abstract
... The technique has also been successfully applied to calibration of Two-Colour Planar Laser Induced Fluorescence imaging of temperature distributions in a firing engine [10] . Recent developments have also included applications in shock tubes and high-speed fuel injection jets and mixing [11,12] . ...
Full-text available
Crank angle-resolved temperatures have been measured using laser induced grating spectroscopy (LIGS) in a motored reciprocating compression machine to simulate diesel engine operating conditions. A portable LIGS system based on a pulsed Nd:YAG laser, fundamental emission at 1064 nm and the fourth harmonic at 266 nm, was used with a c.w. diode-pumped solid state laser as probe at 660 nm. Laser induced thermal grating scattering (LITGS) using resonant absorption by 1-methylnaphthalene, as a substitute fuel, of the 266 nm pump-radiation was used for temperature measurements during non-combusting cycles. Laser induced electrostrictive grating scattering (LIEGS) using 1064 nm pump-radiation was used to measure temperatures in both combusting and non-combusting cycles with good agreement with the results of LITGS measurements which had a single-shot precision of ± 15 K and standard error of ± 1.5 K. The accuracy was estimated to be ± 3 K based on the uncertainty involved in the modified equation of state used in the derivation from the LIGS measurements of sound speed in the gas. Differences in the in-cylinder bulk gas temperature between combusting and non-combusting cycles were unambiguously resolved and temperatures of 2300 ± 100 K, typical of flames, were recorded in individual cycles. The results confirm the potential for LIGS-based thermometry for high-precision thermometry of combustion under compression-ignition conditions.
... ment and data acquisition system is sketched in Fig. 3 . A detailed characterization of the shock tube facility can be found e.g. in Förster et al. (2015) . ...
In this study, n-hexane was compressed beyond six times its critical pressure and discharged into argon at subcritical pressure (with respect to the injectant). The injection temperature systematically varied from sub- to supercritical values to investigate near-critical disintegration phenomena of retrograde jets. Here, the ratio of the pressure in the nozzle reservoir and chamber was always above 14 leading to highly-expanded injections. We analyzed the breakup process in terms of combined shadowgraphy and light scattering measurements. Close to the critical point, different physical mechanisms have been observed. Their occurrence is predominantly determined by the expansion process resulting from the thermodynamic conditions in nozzle reservoir and chamber in combination with thermodynamic properties of the injectant. The experimental images for near- but subcritical injection temperatures imply that a fluid in liquid state expands in the nozzle and atomizes downstream of it. For sufficiently low backpressures, high liquid superheats trigger rapid vapor formation across a thin transition layer inside the nozzle that leads to a choked two-phase flow. A thermodynamic model that assumes a discontinuous phase transition layer and uses metastable fluid properties provides a physical explanation of the resulting underexpanded two-phase disintegration downstream of the nozzle exit. An increase to supercritical injection temperatures increases the compressibility of the fluid within the nozzle. An isentropic flow analysis showed that this triggers choking at thermodynamic states in the supercritical pressure regime resulting in the discharge of a sonic single-phase fluid. The expansion within the Mach barrel can lead to a supersaturated fluid state at near-critical temperature. In this case, a sharp phase transition front established approximately half a nozzle diameter downstream of the exit. We defined a dimensionless parameter to characterize the two-phase extent in underexpanded jets with near-critical phase transition based on initial injection conditions and retrograde fluid properties. We deduced the axial extent of the two-phase region from light scattering signals for a wide parameter range and demonstrate that it features a clear dependency upon the proposed parameter, which demonstrates its feasibility.
Full-text available
We present an in‐depth investigation of laser‐induced grating spectroscopy (LIGS) for temperature measurements in practical applications using a narrow‐band dye laser with 760 nm wavelength and a pulse duration of 8 ns as the source for the pump beams creating the laser‐induced grating. The pump laser wavelength was set to be either resonant with the transition from the band of O2 for generation of thermal LIGS or nonresonant for generation of purely electrostrictive LIGS. Signals were generated in ambient air as well as in high‐pressure or high‐temperature dry air mixtures. Pump laser irradiances up to 11 GW/cm2 were used, which resulted in strong electrostrictive contribution to the overall LIGS signals at atmospheric pressure, with a low thermal contribution due to the weak absorption by the singlet O2 . The advantage and disadvantage of thermal or electrostrictive LIGS for temperature measurements are discussed, as well as potential applications in high‐pressure environments. Furthermore, the precision of the temperature measurement is discussed by comparing different analysis methods. We present an in‐depth investigation of laser‐induced grating spectroscopy (LIGS) for temperature measurements in practical applications using a narrow‐band dye laser with 760 nm wavelength and a pulse duration of 8 ns as the source for the pump beams creating the laser‐induced grating. The advantage and disadvantage of thermal or electrostrictive LIGS for temperature measurements are discussed, as well as potential applications in high‐pressure environments. Furthermore, the precision of the temperature measurement is discussed by comparing different analysis methods.
Full-text available
In this work we report the impact of using mid-infrared laser-induced thermal grating spectroscopy (IR-LITGS) for temperature measurements in flames by probing hot water lines. The measurements have been performed in the product zone of laminar atmospheric CH4/H2/air flat flames with equivalence ratio ranging between 0.6 and 1.05. LITGS is a technique based on thermalization through collisions of the excited molecules for generation of a laser-induced grating, which then decays through thermal diffusion. As such, it tends to have limited application in atmospheric flames compared to flame measurements at elevated pressures due to the faster decay at low gas densities. However, by using mid-IR pump laser beams, it enables the generation of laser-induced gratings with large grating spacing, resulting in strong signal intensities and long signal durations. Single-shot IR-LITGS signals were recorded in the different CH4/H2/air flames that covered a temperature range between 1500 and 1800 K. To test the accuracy, the IR-LITGS flame temperature measurements were compared with laser Rayleigh scattering measurements and the result were in good agreement with each other. The IR-LITGS flame temperature measurements show a repetitive single-shot temperature precision better than 1% and an accuracy of 2.5% of the flame temperature. An IR-LITGS excitation scan of water in the flame shows that some ro-vibrational transitions exhibit no IR-LITGS signal, probably due to less efficient collisional energy transfer mechanism. This is important when deciding the wavelength to use for IR-LITGS flame temperature measurements using water absorption.
The compressible accelerated mixing layer of a central injector was thoroughly investigated experimentally to provide a data set that can be used for validating numerical simulations. A drop-shaped central injector was mounted upstream of a rectangular convergent-divergent nozzle, through which air was accelerated to a Mach number of 1.7. The free-stream Reynolds number at the point of injection was 6.245 × 10⁴. Four different measurement techniques - short-time illuminated schlieren imaging, laser schlieren, laser-induced thermal acoustics, and laser-induced fluorescence (LIF) - were applied to visualize the flow structures and to measure the predominant frequency of periodic flow features, the Mach number and temperature, and the injectant distribution. Instantaneous images show that the mixing layer was dominated by a series of alternating vortices. The mixing layer’s self-similarity could be proven by means of injectant mass fraction profiles, which were derived from LIF measurements. The growth rate of the mixing layer was shown to approximately follow the 1 2-power law. It was concluded from comparison to literature data that the growth rate is primarily determined by the free-stream Reynolds number, whereas the free-stream Mach number (compressibility effects) and the injectant amount play a minor role. These experimental data were used to validate three-dimensional (3D) unsteady Reynolds-averaged Navier-Stokes simulations using the shear-stress transport turbulence model. It was shown that the vortex shedding frequency and the mixing layer growth rate as well as the wake velocity deficit were underestimated by the simulations. This indicates that the flow physics of vortex formation were not entirely reproduced.
Conference Paper
To investigate structures of space transportation systems under reentry conditions, high-speed and high-enthalpy flows are simulated experimentally at the Institute for Thermodynamics at the University of the Federal Armed Forces Munich. To characterize flow conditions of the piston-driven shock tunnel, High-Enthalpy Laboratory Munich, laser-induced grating spectroscopy was chosen as the temperature measurement technique. In former works, it was shown that this technique is suited for the investigation of gas properties under single-shot conditions. In this work, single-shot measurements in a conventional medium-enthalpy shock tube are presented. For this purpose, the exact synchronization of the test bench with the optical setup was realized. Successful measurements at Mach numbers between approximately 1.4 and 2.2 leading to temperatures up to 840 K and pressures up to 2 MPa are presented. An additional test series was conducted to analyze the reproducibility of the measurements. A standard deviation of 3%was calculated for this investigated condition. The experiments were conducted in air, and a number of them were seeded with nitric oxide to improve the signal quality of the thermal grating.
Toluene is frequently used as fluorescence tracer in high-temperature combustion applications. A quantitative analysis of laser-induced fluorescence (LIF) signals requires the knowledge of photophysical properties and decomposition kinetics. Using spectrally and temporally resolved ultraviolet absorption and LIF measurements, we studied the spectral properties of toluene and its pyrolysis products behind shock waves between 810 and 1,755 K. Transient absorption spectra were acquired between 220 and 300 nm. The temporal behavior of the absorption at 266 nm was compared to simulations based on literature kinetics models of toluene pyrolysis and available high-temperature absorption cross-sections of toluene, benzyl radicals, and C7H6 as a product from benzyl decomposition. Experiment and simulation agree well at the beginning of the pyrolysis process, whereas for longer reaction times deviations occur presumably due to the build-up of high molecular weight species, which contribute to the observed absorption but have unknown spectral properties. Additionally, LIF emission spectra were recorded following 266-nm excitation at selected reaction times. From measurements up to 1,220 K, the relative fluo-rescence quantum yield of toluene was derived, extending existing data to higher temperatures. Products from toluene pyrolysis were found to be the major contributors to the LIF signal at higher temperatures.
Abstract Data on surface heat flux is critical in all hypersonic missions due to the extent of risk and the penalty associated, when it is not properly accounted for. In the present study an E-type coaxial thermocouple has been designed, fabricated, validated and benchmarked against a more established platinum thin film sensor. The study proves that coaxial thermocouples used in impulse facilities do not require cold junction compensation. The comprehensive study conducted in IIT Bombay Shock Tunnel (IITB-ST) has confirmed the performance, ruggedness and reliability of the coaxial thermocouple, ascertaining it to be an effective, impulse, heat flux sensor.
The paper provides an overview of the use of coherent anti-Stokes Raman scattering (CARS) and spontaneous Raman scattering for diagnostics of low-temperature nonequilibrium plasmas and nonequilibrium high-enthalpy flows. A brief review of the theoretical background of CARS, four-wave mixing and Raman scattering, as well as a discussion of experimental techniques and data reduction, are included. The experimental results reviewed include measurements of vibrational level populations, rotational/translational temperature, electric fields in a quasi-steady-state and transient molecular plasmas and afterglow, in nonequilibrium expansion flows, and behind strong shock waves. Insight into the kinetics of vibrational energy transfer, energy thermalization mechanisms and dynamics of the pulse discharge development, provided by these experiments, is discussed. Availability of short pulse duration, high peak power lasers, as well as broadband dye lasers, makes possible the use of these diagnostics at relatively low pressures, potentially with a sub-nanosecond time resolution, as well as obtaining single laser shot, high signal-to-noise spectra at higher pressures. Possibilities for the development of single-shot 2D CARS imaging and spectroscopy, using picosecond and femtosecond lasers, as well as novel phase matching and detection techniques, are discussed.
Femtosecond (fs)-duration laser pulses are well suited for two-photon laser-induced-fluorescence (TPLIF) imaging of key atomic species such as H, N, and O in gas-phase reacting flows. Ultrashort pulses enable efficient nonlinear excitation, while reducing interfering photochemical processes. Furthermore, amplified fs lasers enable high-repetition-rate imaging (typically 1–10 kHz) for capturing the dynamics of turbulent flow fields. However, two-dimensional (2D), single-laser-shot fs-TPLIF imaging of the above species is challenging in most practical flow fields because of the limited ultraviolet pulse energy available in commercial optical parametric amplifier (OPA)-based tunable laser sources. In this work, we report the development of an efficient, fs frequency-quadrupling unit [i.e., fourth-harmonic generator (FHG)] with overall conversion efficiency more than six times greater than that of commercial OPA-based systems. The development, characterization, and application of the fs-FHG system for 2D imaging of H atoms in flames are described in detail. The potential application of the same laser system for 2D imaging of N and O atoms is also discussed.
Recent advances in laser absorption and shock tube methodologies for studies of combustion chemistry are reviewed. First the principles of shock tube operation are discussed, and then an overview of shock tube diagnostic methods and experiments is covered. Recent shock tube developments include the use of driver inserts to counteract the small pressure gradient seen in conventional reflected shock wave experiments and the use of a constrained-reaction-volume strategy to enable the implementation of near-constant-pressure gasdynamic test conditions during energetic processes. Recent laser absorption developments include the use of a CO2 laser absorption sensor to accurately monitor temperature during shock wave experiments, the use of multi-wavelength laser absorption strategies to simultaneously monitor multiple species time-histories, and the used of isotopic labeling strategies to identify individual reaction sites during the measurement of elementary reaction rate constants. The improved ability to accurately constrain the test conditions in shock tube experiments, combined with non-intrusive, species-sensitive and quantitative laser absorption diagnostics, is enabling experimenters to provide a new generation of high-quality experimental kinetics targets for combustion chemistry model validation and refinement. The paper concludes with a brief discussion of newly emerging laser-diagnostic techniques and a summary of future research directions.
Conference Paper
This paper describes an experimental study on the propagation of premixed flames through a flow with well-defined turbulence characteristics. In this study, multi-plane OH planar laser-induced fluorescence (OH-PLIF) and stereoscopic particle image velocimetry (SPIV) were applied simultaneously at 10 kHz to measure the local three-dimensional displacement velocity of unstabilized, freely propagating flames passing through a premixed flow of methane and air. The dual-plane OH-PLIF data was used to track both reaction zone location and flame-normal orientation, while SPIV was used to track the three-component velocity field. The vectorial difference of the two produces a direct, time-resolved measurement of the local 3D displacement velocity of the flame. The use of unstabilized, freely propagating flames eliminates spatiotemporal dependence of local 3D displacement velocity associated with burner-or aerodynamically-stabilized test flames. Statistics of local 3D displacement velocities are presented. These show values ranging from slightly negative up to approximately eight times the laminar flame speed. Instantaneous flame-normal orientation with respect to the direction of local fluid velocity is identified as one parameter affecting instantaneous displacement velocities.
The first application of Laser Induced Thermal Gratings Spectroscopy (LITGS) for precision thermometry in a firing GDI optical engine is reported. Crank-angle resolved temperature values were derived from LITGS signals generated in fuel vapour with a pressure dependent precision in the range 0.1–1.0% allowing differences in evaporative or charge cooling effects arising from a variety of ethanol and methanol blends with a model gasoline fuel to be quantified. In addition, fluctuations in temperature arising from cyclic variations in compression were directly detected and measured.
To enhance the efficiency of direct injection engines, the understanding of the combustible mixture formation after a liquid fuel spray is injected is crucial; hence, the need is readily apparent for experimental methods that allow determining characteristic parameters such as fuel/air ratio and gas temperature. The paper demonstrates the potential of the laser‐induced grating technique to determine simultaneously the local vapor phase fuel concentration and the temperature during the temporal evolution of a direct‐injected spray. The precision and the accuracy of the experimental approach and of the applied data treatment procedure were evaluated by using calibration measurements in a constant volume cell under known stationary conditions. The test measurements of fuel vapor concentration and gas temperature were carried out in an injection chamber using a high‐pressure swirl injector. Liquid propane as the main component of liquefied petroleum gas was directly injected into the chamber under flash‐boiling conditions. Injection and ambient pressure were varied, whereas fuel and ambient temperature remained constant. The measurements were performed inside the fuel jet at different distances from the injector outlet along the axis of propane injection. The results are indicative of small evaporative cooling of the gas in the jet. Copyright © 2013 John Wiley & Sons, Ltd.
Degenerate four-wave mixing (DFWM) is a nonlinear optical-measurement technique that has recently received attention for its potential as a quantitative probe of gaseous media, including plasma and combustion environments. DFWM can be used to measure temperature, species concentration, velocity, and other properties non-intrusively. DFWM measurements in the gas phase have traditionally been attributed to population gratings (spatial perturbations in the level populations of absorbing species). Recently, contributions from thermal gratings (spatial perturbations in the overall density) have been observed in the gas phase. We report investigations of DFWM line intensities in the A ^2Sigma^{+}arrow X^2Pi electronic transitions of nitric oxide (NO). Using several experimental and analytical techniques, we distinguished and compared contributions from population and thermal gratings. For small quantities of NO in a strongly quenching buffer gas (CO_2 ), thermal-grating contributions dominated at room temperature for gas pressures of ~0.5 atm and higher. In a nearly non-quenching buffer (N _2), population gratings dominated at pressures of ~1.0 atm and lower. Population gratings of NO also dominated at higher temperatures in an atmospheric-pressure methane/air flame. To interpret these results, we propose a simple model for the ratio of thermal- to population-grating scattering intensities that varies as P^4T^{-4cdot4 }. Preliminary investigations of the temperature dependence and detailed studies of the pressure dependence are in agreement with this model. Measurements of the temporal evolution and peak intensity of isolated thermal -grating signals are in detailed agreement with calculations based on a linearized hydrodynamic model from the literature. In a further investigation, we used high-resolution lasers to perform detailed measurements of the shapes of DFWM spectral lines for conditions dominated by both population and thermal gratings. The observed thermal-grating spectra had larger linewidths and weaker line strength dependences than population-grating spectra. The spectra are in excellent agreement with calculations based on perturbation theory for contributions from population gratings and a Voigt -squared profile for contributions from thermal gratings. Based on this work, we discuss the relative merits of various DFWM schemes for measuring temperature and concentration in practical high-temperature environments.