Laser-induced Thermal Grating Spectroscopy Research Articles

Simultaneous thermometry and velocimetry for a shock tunnel using homodyne and heterodyne detection

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.

Laser-induced thermal grating spectroscopy based on femtosecond laser multi-photon absorption

Laser-induced grating spectroscopy (LIGS) is for the first time explored in a configuration based on the crossing of two focused femtosecond (fs) laser pulses (800-nm wavelength) and a focused continuous-wave (cw) laser beam (532-nm wavelength). A thermal grating was formed by multi-photon absorption of the fs-laser pulses by hbox {N}_{{2}} with a pulse energy around 700 upmu J (sim 45 TW/hbox {cm}^{2}). The feasibility of this LIGS configuration was investigated for thermometry in heated nitrogen gas flows. The temperature was varied from room temperature up to 750 K, producing strong single-shot LIGS signals. A model based on the solution of the linearized hydrodynamic equations was used to extract temperature information from single-shot experimental data, and the results show excellent agreement with the thermocouple measurements. Furthermore, the fluorescence produced by the fs-laser pulses was investigated. This study indicates an 8-photon absorption pathway for hbox {N}_{{2}} in order to reach the hbox {B}^{3}Pi _{g} state from the ground state, and 8 + 5 photon excitation to reach the hbox {B}^{2}Sigma _{u}^{+} state of the hbox {N}_{2}^{+} ion. At pulse energies higher than 1 mJ, the LIGS signal was disturbed due to the generation of plasma. Additionally, measurements in argon gas and air were performed, where the LIGS signal for argon shows lower intensity compared to air and hbox {N}_{{2}}.

Laser Induced Thermal Grating Spectroscopy (LITGS) による温度定量計測の基礎と応用

Laser Induced Thermal Grating Spectroscopy (LITGS) による温度定量計測の基礎と応用

Mid-infrared laser-induced thermal grating spectroscopy of hot water lines for flame thermometry

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.

Pressure measurement in combusting and non-combusting gases using laser-induced grating spectroscopy

The measurement of pressure using laser-induced thermal grating spectroscopy, LITGS, with improved accuracy and precision is reported. Pressure values are derived from the record of the time-profile of LITGS signals by fitting of modelled signals to experimental data. The procedure is described for accurate modelling of the LIGS signals involving a sequence of calculation steps with appropriate weighting and calibration to determine the best-fit value of pressure-dependent parameters for averaged and single-shot measurements. Results are reported showing application of this model-fitting method to measurements of pressure in static cells using LITGS generated from NO in mixtures containing N2 at pressures in the range 0.5–5.0 bar with accuracy of 1–3% and single-shot precision of 4–7%. Time-resolved measurements of pressure, using LITGS signals generated in toluene-seeded fuel vapour, during the compression and expansion strokes of a motored optically accessible engine are reported with pressure-dependent accuracy ranging from better than 10 to around 20% over the cycle and single-shot precision in the range 5–15% over the same range. Measurements in the engine under firing conditions were obtained over a limited range and slightly increased uncertainties associated with varying composition resulting from exhaust gas residuals. The method was found to be of limited utility for measurements in high temperature flames at around ambient pressures.

High Frequency Measurement of Temperature and Composition Spots With LITGS

Temperature and composition spots in a turbulent flow are detected and time-resolved using laser-induced thermal grating spectroscopy (LITGS). A 355 nm wavelength particle image velocimetry laser is operated at 0.5–1 kHz to generate the thermal grating using biacetyl as an absorber in trace amounts. In an open laminar jet, a feasibility study shows that small (≃ 3%) fluctuations in the mean flow properties are well captured with LITGS. However, corrections of the mean flow properties by the presence of the trace biacetyl are necessary to properly capture the fluctuations. The actual density and temperature variation in the flow are determined using a calibration procedure validated using a laminar jet flow. Finally, traveling entropy and composition spots are directly measured at different locations along a quartz tube, obtaining good agreement with expected values. This study demonstrates that LITGS can be used as a technique to obtain instantaneous, unsteady temperature and density variations in a combustion chamber, requiring only limited optical access.

Quantitative measurement of temperature in oxygen enriched CH4/O2/N2 premixed flames using Laser Induced Thermal Grating Spectroscopy (LITGS) up to 1.0 MPa

The application of laser diagnostics to high pressure combustion phenomena is particularly challenging, especially in practical combustors such as rocket motors. In this study, temperature measurements using Laser Induced Thermal Grating Spectroscopy (LITGS) are demonstrated in oxygen enriched CH4/O2/N2 premixed laminar flames at pressures up to 1.0 MPa. We use a previously developed OH absorption LITGS technique to determine product gas temperatures from 0.3 to 1.0 MPa, for both high temperature oxygen-enriched and pure-oxygen flames, for measurements up to 3000 K. Further, we demonstrate how it is necessary to correct the measurements for the local absorption of laser light to obtain accurate temperatures, and offer a technique for producing the correction by using different laser energies. Once the correction is applied, we demonstrate that the measurements at 0.5 MPa are within 1.6% of the adiabatic non-strained flame temperatures, with a standard deviation of about 160 K, thus offering a competitive method for the challenging conditions at high pressures and temperatures. The values obtained at derived temperatures at 1.0 MPa were lower than the adiabatic unstrained flame temperatures, which could possibly be attributed to loss mechanisms.

Mid-infrared laser-induced thermal grating spectroscopy in flames

For the first time, laser-induced thermal grating spectroscopy (LITGS) in the spectral range around 3µm is demonstrated as a versatile diagnostic tool. This spectral region is of particular interest in combustion diagnostics as many relevant species such as hydrocarbons and water exhibit fundamental vibrational modes and hence can be probed with high sensitivity. Another benefit of the IR-LITGS is that it allows performing spectroscopy in the infrared combined with signal detection in the visible. Hence, the strong thermal radiation inherent in flames does not represent an interference. As the first step, we present the application of IR-LITGS to cold gas flows, where traces of ethylene and water vapor are detected. The time-resolved LITGS signals, which can be acquired in a single laser shot, are rich in information and allow deriving temperature and to some extend chemical composition. In the second step, the IR-LITGS technique is applied to ethylene/air flames stabilized on a flat flame burner. A proof-of-concept study is carried out, in which the temperature is determined in the burned region of flames with systematically varied equivalence ratio (0.72 < Φ <2.57). Moreover, in a highly sooty flame, LITGS signals were recorded as a function of height above the burner and allowed the determination of the temperature profile. The proposed IR-LITGS method has the potential for enabling single-shot measurements of several parameters at a time. Its applicability to sooty flame environments opens up new opportunities to study the complex formation of carbonaceous particles in flames.

Mid-Infrared Pumped Laser-Induced Thermal Grating Spectroscopy for Detection of Acetylene in the Visible Spectral Range.

We present mid-infrared laser-induced thermal grating spectroscopy (IR-LITGS) using excitation radiation around 3 µm generated by a simple broadband optical parametric oscillator (OPO). Acetylene as a typical small hydrocarbon molecule is used as an example target species. A mid-infrared broadband OPO pumped by the fundamental output of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was used to generate the pump beams, with pulse energies of 6-10 mJ depending on the wavelength. The line width of the OPO idler beam was ∼5 cm(-1), which is large enough to cover up to six adjacent acetylene lines. The probe beam was the radiation of a 532 nm cw solid state laser with 190 mW output power. Signals were generated in atmospheric pressure gas flows of N2, air, CO2 and Ar with small admixtures of C2H2 A detection limit of less than 300 ppm was found for a point measurement of C2H2 diluted in N2 As expected, the oscillation frequency of the IR-LITGS signal was found to have a large dependency on the buffer gas, which allows determination of the speed of sound. Moreover, the results reveal a very strong collisional energy exchange between C2H2 and CO2 compared to the other gases. This manifests as significant local heating. In summary, the MIR-LITGS technique enables spectroscopy of fundamental vibrational transitions in the infrared via detection in the visible spectral range.

Simultaneous single-shot measurement of temperature and pressure along a one-dimensional line by use of laser-induced thermal grating spectroscopy

We report a new technique, based on laser-induced thermal grating spectroscopy (LITGS), for time- and space-resolved simultaneous measurement of temperature and pressure along a one-dimensional line. LITGS signals generated in NO2/N2 mixtures along a 5 mm line produce a time-varying image that is recorded on a streak camera. The temperature is derived with a precision of 0.3% from the streak images using a rapid Fourier method with a spatial resolution of 150 microm along the line. The principle of pressure measurement is demonstrated using a sequence of images, and a simple extension of the method to single-shot pressure measurement is discussed.

Single-shot measurement of temperature and pressure using laser-induced thermal gratings with a long probe pulse

We report accurate and precise single-shot measurements of temperature and pressure using laser-induced thermal grating spectroscopy (LITGS). The use of a flashlamp-pumped dye laser (FLPDL) as a probe for the induced gratings is described and a fast method of data analysis using Fourier methods is reported. Thermal gratings were induced by 8 ns pump pulses from a frequency-doubled Nd:YAG laser. The signals were produced by scattering 2-μs-long pulses, of energy 200 mJ, from the FLPDL. LITGS signals were obtained from NO2/N2 mixtures at pressures in the range 1–40 bar and temperatures between ambient and 400 K. Temperature values were derived from the data with an accuracy of 0.42% and a single-shot precision of 0.16%. Pressure values were obtained with an accuracy of 5.7% and a precision of 1.4% limited by uncertainties in the gas kinetic parameters.

Time‐resolved laser‐induced thermal grating experiments induced by short pulse CO2‐laser radiation

AbstractUsing a high pressure carbon dioxide laser with pulse lengths of 80 ns laser‐induced thermal grating spectroscopy (LITGS) experiments have been carried out in gas mixtures consisting of an absorbing species (ethylene) and buffer gases (nitrogen/helium) at various temperatures and pressures. The grating structure is written into the medium by absorption and subsequent collisional relaxation of CO2‐laser radiation. Transport properties in these mixtures such as thermal conductivity and the speed of sound have been determined from the time‐resolved signal generated by Bragg scattering of a continuous wave HeNe‐laser beam off the grating structure. Accurate numerical values for transport coefficients are obtained from fitting analytical expressions to the measured oscillating signal traces and are compared with literature values.

Applied laser spectroscopy in technical combustion systems

AbstractSeveral laser‐based diagnostic techniques for nonintrusive measurements of species concentrations and temperatures were developed during recent years and have become valuable tools to study technical combustion processes. Data gained from such experiments are the basis for a comparison with detailed mathematical modelling of chemical processes in laminar and turbulent flows including heat and species transport. Examples for the application of laser‐induced fluorescence (LIF), laser‐induced thermal grating spectroscopy (LITGS), Rayleigh scattering (RS), differential absorption (DIAL) and emission in the infrared in ignition processes, internal combustion engines, municipal waste incinerators, and pressurized fluidized bed reactors are described.

Thermal contributions in infrared laser induced grating experiments at high pressure

AbstractDegenerate four wave mixing (DFWM) and laser induced thermal grating spectroscopy (LITGS) have been used to detect ethylene and methanol in mixtures with nitrogen, argon or helium in the pressure range between 0.005 and 18 bar. The grating forming step for both techniques was accomplished via absorption of radiation in the interference region from two beams from a single longitudinal mode CO2 laser. For DFWM the same CO2 laser and for LITGS a diode pumped Nd: YAG laser were employed to read out the transient grating. Using photo thermal deflection spectroscopy (PDS) as an independent spectroscopic method thermal effects were confirmed for total pressures above 0.1 bar. For both DFWM and LITGS signal intensities increased with buffer gas pressure due to collision induced transfer of optical excitation energy into translational degrees of freedom. The temporal behaviour of the IR‐grating generation and thermalization processes and the advantages of hydrocarbon trace species detection from thermal gratings at high pressures is discussed.