Investigation of laser-induced grating spectroscopy of O2 for accurate temperature measurements towards applications in harsh environments

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 (Formula presented.) transition from the (Formula presented.) 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 (Formula presented.). 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

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

Investigation of ro-vibrational spectra of small hydrocarbons at elevated temperatures using infrared degenerate four-wave mixing

The ro-vibrational spectra around 3 µm of four small hydrocarbons (C2H2, CH4, C2H6 and C2H4) at 296, 550 and 820 K have been investigated using infrared degenerate four-wave mixing (IR-DFWM). The spectra were recorded in gas flows of nitrogen with small admixtures of the hydrocarbons. A fused silica glass tube surrounded by an electric heating wire was used to heat the gas flows. The recorded IR-DFWM spectra are compared with simulations using the spectral information available in the HITRAN database, in order to identify spectral lines. The measurements demonstrate good signal to noise ratio and good sensitivity even at elevated temperatures. Several weak hot lines were detected that are not included in the current database. This paper demonstrates the potential of IR-DFWM for purposes of investigating spectral lines at elevated temperatures, which is often a challenging task with conventional absorption spectroscopy techniques. The possibility of applying IR-DFWM for combustion diagnostics of small hydrocarbons is discussed from the detection limits of the measurements and the potential water line interference. Because of the non-linear nature of the DFWM technique, it provides much higher contrast for strong lines of small molecules over backgrounds of high-density weak lines, which commonly exist in hot gas flows of thermochemical reactions

Mid-infrared laser-induced thermal grating spectroscopy in flames

For the first time, laser-induced thermal grating spectroscopy (LITGS) in the spectral range around 3. μm 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

Temperature imaging in low pressure flames using diode laser two-line atomic fluorescence employing a novel indium seeding technique

The use of diode lasers for spatially resolved temperature imaging is demonstrated in low pressure premixed methane-air flames using two-line atomic fluorescence of seeded indium atoms. This work features the advantages of using compact diode lasers as the excitation sources with the benefits of two-dimensional planar imaging, which is normally only performed with high-power pulsed lasers. A versatile and reliable seeding technique with minimal impact on flame properties is used to introduce indium atoms into the combustion environment for a wide range of flame equivalence ratios. A spatial resolution of around 210 µm for this calibration free thermometry technique is achieved for three equivalence ratios at a pressure of 50 mbar in a laminar flat flame

Low-noise mid-IR upconversion detector for improved IR-degenerate four-wave mixing gas sensing

We compare a nonlinear upconversion detector with a conventional cryogenic InSb detector for the detection of coherent infrared light showing near-shot-noise-limited performance in the upconversion system. The InSb detector is limited by dark noise, which results in a 500 times lower signal-to-noise ratio. The two detectors are compared for the detection of a coherent degenerate four-wave mixing (DFWM) signal in the mid-infrared, and applied to measure trace-level acetylene in a gas flow at atmospheric pressure, probing its fundamental rovibrational transitions. In addition to lower noise, the upconversion system provides image information of the signal, thus adding new functionality compared to standard point detection methods. We further show that the upconversion detector system can be implemented as a simple replacement of the cryogenic detector. (C) 2014 Optical Society of Americ

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 N 2 with a pulse energy around 700 μ J (∼ 45 TW/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 N 2 in order to reach the B 3Π g state from the ground state, and 8 + 5 photon excitation to reach the B2Σu+ state of the N2+ 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 N 2

Utvärdering av förståelse på skriftlig tentamen

I tidigare studier har det visats att examinationen i hög grad styr hur studenterna kommer att studera under en kurs. Därför är det viktigt att examinationen inte bara testar ytliga kunskaper och beräkningsförmåga, utan även förståelse. I denna artikel diskuteras olika sätt att utvärdera studenternas förståelse genom att analysera resultaten på skriftliga tentamina. En fallstudie på Institutionen för reglerteknik vid Lunds tekniska högskola presenteras och möjliga åtgärder för att öka förståelsen hos studenterna diskuteras

Non-intrusive in situ detection of methyl chloride in hot gas flows using infrared degenerate four-wave mixing

We demonstrate the potential of infrared degenerate four-wave mixing (IR-DFWM) as a tool for non-intrusive in situ spatially resolved detection of CH3Cl in reactive hot gas flows especially feasible for applications to biomass combustion and gasification. IR-DFWM spectra of CH3Cl, by probing ro-vibrational transitions belonging to the fundamental stretching modes v(1) and v(4), have been successfully recorded in gas flows diluted with nitrogen at atmospheric pressure and elevated temperatures up to 820K. In order to identify the spectral lines of CH3Cl, the recorded IR-DFWM spectra are compared with simulations using molecular parameters extracted from the HITRAN database. The potential interference from water vapor is discussed from measurements of H2O spectrum at 820K combined with simulations of H2O IR-DFWM spectrum based on the HITEMP database, and it was found that the (Q)Q(6) line of the v(1) band is relatively free from water interference at elevated temperatures. At atmospheric pressure, the detection limits for temperatures at 296, 550 and 820K were estimated to be 2.1, 3.1 and 6.2 (x10(15) molecules/cm(3)), respectively, by scanning the (Q)Q(6) line of the v(1) band. These results show the potential of interference free detection of CH3Cl with IR-DFWM in harsh environments like combustion. Copyright (c) 2015 John Wiley & Sons, Ltd