Near-infrared diode laser absorption sensor for rapid measurements of temperature and water vapor in a shock tube

A fast-response (100 kHz) tunable diode laser absorption sensor is developed for measurements of temperature and H₂O concentration in shock tubes, e.g. for studies of combustion chemistry. Gas temperature is determined from the ratio of fixed-wavelength laser absorption of two H₂O transitions near 7185.60 cm^-1 and 7154.35 cm^-1, which are selected using design rules for the target temperature range of 1000–2000 K and pressure range of 1–2 atm. Wavelength modulation spectroscopy is employed with second-harmonic detection (WMS-2f) to improve the sensor sensitivity and accuracy. Normalization of the second-harmonic signal by the first-harmonic signal is used to remove the need for calibration and minimize interference from emission, scattering, beam steering, and window fouling. The laser modulation depth for each H₂O transition is optimized to maximize the WMS-2f signal for the target test conditions. The WMS-2f sensor is first validated in mixtures of H₂O and Ar in a heated cell for the temperature range of 500–1200 K (P=1 atm), yielding an accuracy of 1.9% for temperature and 1.4% for H₂O concentration measurements. Shock wave tests with non-reactive H₂O–Ar mixtures are then conducted to demonstrate the sensor accuracy (1.5% for temperature and 1.4% for H₂O concentration) and response time at higher temperatures (1200–1700 K, P=1.3–1.6 atm). PACS 42.62.Fi; 42.55.Px; 42.60.Fc; 07.35.+k     

基于TDLAS的痕量CO浓度检测系统及温压补偿

利用可调谐二极管激光吸收光谱技术(TDLAS)实现气体高灵敏度,高精度的非接触式检测。为避免二次谐波信号随环境中温度和压强的改变导致实测出现较大误差,需对测得的气体浓度进行温压补偿。实验以2332nm波长作为CO的中心吸收波长,以质量分数125×10~(-6),1001×10~(-6),1701×10~(-6)的CO作为实验气体。提出了利用BP神经网络补偿模型,并采用遗传算法(GA)与粒子群算法(PSO)优化BP,修正受温压影响的标气浓度,并进行了仿真测试对比。实验结果表明,采用PSO优化BP补偿效果最好,修正后的CO浓度平均相对误差约为1.55%,有效提高了CO气体检测系统的精度。     

In situ optical measurements of bacterial endospore breakdown in a shock tube

The interaction of endospore-laden bioaerosols and shock waves is monitored with a combination of laser absorption and scattering. Tests are performed in the Stanford aerosol shock tube for post-shock temperatures ranging from 400–1100 K. In situ laser measurements at 266 and 665 nm provide a real-time monitor of endospore morphology. Scatter of visible light measures the integrity of endospore structure, while absorption of UV light provides a monitor of biochemicals released by endospore rupture. For post-shock temperatures greater than 750 K endospore morphological breakdown is observed. A simple theoretical model is employed to quantify the optical measurements, and mechanisms leading to the observed data are discussed.     

Problems of shock temperature measurements for metals by using optical radiometry method

Abstract

Several problems encountered in shock temperature measurements for metals using optical radiometry were discussed in detail. The influences of the driver/film gap upon the observed interface temperature history were formulated analytically, and the temperature relaxation behaviors at the film/window interface were also characterized. We found that the energy deposition at the driver/film gap would influence to varied degrees the film/window interface temperature profile unless the film deposited on the window can be regarded to have an infinite thickness. Modeling calculations showed that at shock pressures of a few megabars, the observed interface temperature of iron would be at least 300–600 K higher, relative to the prediction from the Grover model, provided that the gap and film were both 1 ～ 2 pm thick. Additionally, a metal-plate sample was used to measure a reliable shock temperature based on one-dimensional heat conduction model for the plate-sample(driver)/ pm-gap/window setup we proposed. Preliminary measured results for a meteoritic iron sample (kamacite, with Fe 93.65 wt.% and Ni 6.35 wt.%) using a disc sample showed that this method was practical and effective. Finally, the “heat-resistance model” proposed by Tang et al., and the experimental measurement of the heat conductivity for a transparent window (sapphire or LiF crystal) at megabar shock pressures were discussed and commented on as well.     

A multiple-pass interferometer for electron and atom density measurements in shock tube plasmas

An eleven-pass, two-wavelength interferometer for density measurements within the ionisation relaxation region of rare-gas shock waves has been developed. The minimum detectable electron density is 1·10¹⁴ cm^?3. A method to calculate the electron temperature profile across the relaxation region from the measured density profiles is described.     

Oxygen concentration and temperature measurements in N2–O2 mixtures using rotational coherent anti-Stokes Raman spectroscopy

The accuracy and precision of oxygen concentration and temperature measured by dual-broadband rotational Coherent Anti-Stokes Raman Spectroscopy (CARS) were investigated in nitrogen-oxygen mixtures at atmospheric pressure and temperatures between 290 and 1410 K. The relative standard deviation of temperatures evaluated from pure oxygen rotational CARS spectra was found to be around 5%, and the mean temperature was the same as for nitrogen CARS spectra, except for temperatures above 1000 K, where the temperature was 120 K below the correct value. The in situ calibrated oxygen concentrations were within 10% of the correct value, with a standard deviation of around 1.2% for the mixtures of 12 and 20% oxygen in nitrogen. For the lowest oxygen concentrations considered in this study (2 and 4%), the systematic errors in the evaluated concentrations were very large, and the standard deviation of repeated single-shot measurements was above 2%. However, employing weighting in the spectral fitting routine reduced the errors in the concentration and the single-shot standard deviation was lowered to 0.5%. Finally, it was shown that spectral interference (from oxygen) in a rotational CARS spectrum of nitrogen generally had little impact on the temperature evaluated from fitting the spectra to theoretical nitrogen spectra.     

Application of time-resolved spectroscopy to concentration measurements in gas mixtures

Concentration measurements using femtosecond Raman Induced Polarization Spectroscopy (RIPS) are performed in binary gas mixtures CO₂–N₂ and CO₂–N₂O at room temperature. The principle of these measurements is based on the nonlinear rotational time response of each molecular component of the mixture. The general form of this molecular response is a series of periodic transients with a period related to the rotational constant B_e. The relative strength of the individual responses allows an accurate determination of the concentration. Two techniques are presented using either two pulses (one pump and one probe) or three pulses (two pumps and one probe).     

Ethanol ignition in a high-pressure shock tube: Ignition delay time and high-repetition-rate imaging measurements

Ethanol is known to be prone to pre-ignition in internal combustion engines under high-load conditions and its ignition shows large deviations from ideal, spatially, and temporally-homogeneous ignition in shock tubes at moderate temperatures (800–950 K). In this context, the ignition of stoichiometric ethanol/O₂ mixtures with various levels of inert gas dilution was investigated in a high-pressure shock tube at ?20 bar between 800 and 1250 K. Ignition delay times were determined from spatially integral detection of chemiluminescence emission. Additionally, high-repetition-rate color imaging enabled the differentiation of the luminescence in time, space, and spectral range between various ignition modes. In the low-temperature range (800–860 K), different inhomogeneous ignition modes were identified. The addition of small amounts of helium into the undiluted fuel/air mixture was found to be efficient to mitigate pre-ignition, attributed to a variation in heat transfer and thus suppression of the build-up of local temperature inhomogeneities. The experiments in case of spatially homogeneous ignition show very good agreement with the predictions based on three detailed kinetics mechanisms (Zhang et al., CNF 190 (2018) 74, Frassoldati et al., CNF 159 (2012) 2295, and Zhou et al. CNF 197 (2018) 423), inhomogeneities, however, resulted in a shortening of the ignition delay times up to a factor of 2.6.     

Experimental study and modeling of shock tube ignition delay times for hydrogen-oxygen-argon mixtures at low temperatures

Recent literature has indicated that experimental shock tube ignition delay times for hydrogen combustion at low-temperature conditions may deviate significantly from those predicted by current detailed kinetic models. The source of this difference is uncertain. In the current study, the effects of shock tube facility-dependent gasdynamics and localized pre-ignition energy release are explored by measuring and simulating hydrogen-oxygen ignition delay times. Shock tube hydrogen-oxygen ignition delay time data were taken behind reflected shock waves at temperatures between 908 to 1118 K and pressures between 3.0 and 3.7 atm for two test mixtures: 4% H₂, 2% O₂, balance Ar, and 15% H₂, 18% O₂, balance Ar. The experimental ignition delay times at temperatures below 980 K are found to be shorter than those predicted by current mechanisms when the normal idealized constant volume (V) and internal energy (E) assumptions are employed. However, if non-ideal effects associated with facility performance and energy release are included in the modeling (using CHEMSHOCK, a new model which couples the experimental pressure trace with the constant V, E assumptions), the predicted ignition times more closely follow the experimental data. Applying the new CHEMSHOCK model to current experimental data allows refinement of the reaction rate for H + O₂ + Ar ↔ HO₂ + Ar, a key reaction in determining the hydrogen-oxygen ignition delay time in the low-temperature region.     

Turbulence measurements in a resonance tube

Experimental investigations of acoustically induced turbulence in a resonance tube have been performed. Frequency (f) and sound pressure level (I_p) effects have been studied. Measurements were made at various spatial locations on loops and nodes. Sampled data were processed to estimate the characteristics of turbulence. It is found that the acoustically induced turbulence appears when I_p exceeds 160 dB under the experimental conditions of f = 680–2740 Hz and I_p = 160–166 dB. The turbulent spectrum (F) and the wave number (κ) are found to satisfy a power law F ∝ K^s with $\text{s ? ?1·6}\text{to}\text{? 2·1}$. The r.m.s. turbulent velocity ($\text{u}\text{?}$) is experimentally found to have an $\text{I}{}_{\mathrm{p}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence, yet is relatively insensitive to the variation of f. Throughout the whole measuring range of f and I_p, the rate of energy dissipation per unit mass (ε) is estimated to be in the order of 10⁶–10⁷cm²/s³.     

Phase equilibrium measurements of acoustically levitated squalane–CO2 mixtures by Raman spectroscopy

This work describes the phase equilibrium measurements of acoustically levitated binary mixtures with concentration measurements using Raman spectroscopy without sample extraction of the autoclave. The levitator design is implemented in a Single‐Droplet Optical Cell for levitation processes under varying atmospheres. The advantages of acoustic levitation of small droplets under increased temperatures and pressure combined with spectroscopic applications like Raman spectroscopy enable novel experiments possibly relevant to the fields of chemical engineering. To the author's knowledge, this is the first use of Raman spectroscopy for phase equilibria investigations on acoustically levitated droplets under high pressure and temperature. The results show very good agreements with literature data. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental and modelling study of gasoline surrogate mixtures oxidation in jet stirred reactor and shock tube

The oxidation of several mixtures of surrogate for gasoline was studied using a jet stirred reactor and a shock tube. One representative of each classes constituting gasoline was selected: iso-octane, toluene, 1-hexene and ethyl tert-butyl ether (ETBE). The experiments were carried out in the 800-1880 K temperature range, for two different initial pressures (0.2 and 1 MPa), with an initial fuel molar fraction of 0.001. The equivalence ratio varied from 0.5 to 1.5. Each hydrocarbon sub-mechanism was validated using shock tube data. The full mechanism describing the surrogate fuel oxidation is constituted of the sub-mechanisms for each fuel components and by adding interaction reactions between different hydrocarbon fragments. Good agreement between the experimental results and the computations was observed under JSR and shock tube conditions.     

OES analysis in a nonthermal plasma used for toxic gas removal: Rotational and excitation temperature estimation

Nonthermal plasma technologies are often used for cleaning toxic gases. In this work, we will present an optical emission spectroscopy (OES) study in a dielectric barrier discharge (DBD) used to remove NO _x specifically. Rotational temperatures are calculated from the UV-OH band (A ²Σ, ν = 0 → X ²Π ν′ = 0) situated between 306–310 nm; for the rotational temperature, a fitting method was employed (comparison between experimental data with a synthetic molecular spectrum). Excitation temperatures were calculated using OII atomic lines situated in a wavelength range of 300–700 nm using a Boltzman’s plot method. From calculated temperatures, a thermal inequity characteristic of nonthermal plasma discharges has been high-lighted. The influence of the percentage of water added to the DBD reactor is also studied in the removal efficiency and in the OH band intensities and temperatures.     

Multi-band infrared CO2 absorption sensor for sensitive temperature and species measurements in high-temperature gases

A continuous-wave laser absorption diagnostic, based on the infrared CO₂ bands near 4.2 and 2.7 μm, was developed for sensitive temperature and concentration measurements in high-temperature gas systems using fixed-wavelength methods. Transitions in the respective R-branches of both the fundamental υ ₃ band (~2,350 cm^?1) and combination υ ₁ + υ ₃ band (~3,610 cm^?1) were chosen based on absorption line-strength, spectral isolation, and temperature sensitivity. The R(76) line near 2,390.52 cm^?1 was selected for sensitive CO₂ concentration measurements, and a detection limit of <5 ppm was achieved in shock tube kinetics experiments (~1,300 K). A cross-band, two-line thermometry technique was also established utilizing the R(96) line near 2,395.14 cm^?1, paired with the R(28) line near 3,633.08 cm^?1. This combination yields high temperature sensitivity (ΔE” = 3,305 cm^-1) and expanded range compared with previous intra-band CO₂ sensors. Thermometry performance was validated in a shock tube over a range of temperatures (600–1,800 K) important for combustion. Measured temperature accuracy was demonstrated to be better than 1 % over the entire range of conditions, with a standard error of ~0.5 % and µs temporal resolution.     

Double detonation drivers for a shock tube/tunnel

~~Double detonation drivers for a shock tube/tunnel~~     

Double detonation drivers for a shock tube/ltunnel

Recent progress on detonation drivers is reviewed. Performances of the forward detonation driver and backward detonation driver have been obsenred. To eliminate occurrenoe of a Taylor wave following the detonation wave in the primary driver and to improve the performance of the detonation driver, an additional backward detonation driver was proposed to attach to the end of the forward detonation driver. When the ratio of the initial pressures between the additional and the primary drivers becomes larger than or equal to a critical value, the Taylor wave will disappear, and thus a homogeneous driving gas with high pressure and high temperature can be generated. Furthermore, an over-driving detonation wave will be also obtained, which can increase the driving capability.     

TP-LIF技术测量甲烷-空气火焰中CO的相对浓度分布

 阐述了双光子激光诱导荧光(TP-LIF)技术的原理及线性模型，利用双光子过程激励CO分子B¹∑+←←X¹∑＋(0,0)带Q支的跃迁(约230 nm)，分析了B¹∑+→A¹Π荧光带的荧光光谱特性，探讨了激光功率密度、激光波长及火焰温度等因素对测量的影响，并给出甲烷-空气火焰在一定燃烧条件下CO分子浓度随火焰位置及高度的变化关系。实验结果表明，利用TP-LIF技术测量CO的浓度分布，其时空分辨率及探测灵敏度都很高。当激光功率密度较强时，TP-LIF信号和激光能量成线性关系，而且由于光电离速率的增强，大大降低了碰撞猝灭速率等环境因素对信号测量造成的影响，该特性对实验标定及定量测量都非常有帮助。