Diode-Laser Wavelength-Modulation Absorption Spectroscopy for Quantitative in situ Measurements of Temperature and OH Radical Concentration in Combustion Gases

The in situ quantitative profiles of temperature and OH radical concentration in a postflame region of methane-air premixed counterflow flames were measured by wavelength modulation spectroscopy with a 1.5-mum external cavity diode laser. The second harmonic (2f) signal was generated from absorption by overtone vibrational-rotational transitions of OH: the ?(3/2) (v?, v?) = (2, 0) P11.5e (nu(0) = 6421.35 cm(-1)) or the ?(3/2) (v?, v?) = (3, 1) P5.5f (nu(0) = 6434.61 cm(-1)) transitions. The absorption occurred in the postflame region between methane-air premixed twin flames stabilized in a two-dimensional laminar counterflow burner (Tsuji burner) with a 60-mm line-of-sight path length. The temperature and OH concentration profiles at an equivalence ratio of phi = 0.85 were determined by least-squares fitting of theoretical 2f line shapes to the experimental counterparts and by calculation of the ratio of the line intensities of the two different OH transitions (two-line thermometry). The measured temperature and OH concentration profiles were cross checked by Rayleigh scattering thermometry, thermocouple measurements, and two-dimensional numerical prediction of premixed combustion by use of a detailed chemical kinetic mechanism. The measurements and the prediction showed reasonable agreement.     

In situ combustion measurements of CO with diode-laser absorption near 2.3 microm

In situ measurements of CO concentration were recorded with tunable diode-laser absorption spectroscopy techniques in both the exhaust and the immediate post-flame regions of an atmospheric-pressure flat-flame burner operating on ethylene air. Two room-temperature cw single-mode InGaAsSb/AlGaAsSb diode lasers operating near 2.3 microm were tuned over individual transitions in the CO first overtone band (v' = 2 <-- v" = 0) to record high-resolution absorption line shapes in the exhaust duct [79 cm above the burner, approximately 470 K; R(15) transition at 4311.96 cm(-1)] and the immediate postflame zone [1.5 cm above the burner, 1820-1975 K; R(30) transition at 4343.81 cm(-1)]. The CO concentration was determined from the measured absorption and the gas temperature, which was monitored with type-S thermocouples. For measurements in the exhaust duct, the noise-equivalent absorbance was approximately 3 x 10(-5) (50-kHz detection bandwidth, 50-sweep average, 0.1-s total measurement time), which corresponds to a CO detection limit of 1.5 ppm m at 470 K. Wavelength modulation spectroscopy techniques were used to improve the detection limit in the exhaust to approximately 0.1 ppm m (approximately 500-Hz detection bandwidth, 20-sweep average, 0.4-s total measurement time). For measurements in the immediate postflame zone, the measured CO concentrations in the fuel-rich flames were in good agreement with chemical equilibrium predictions. These experiments demonstrate the utility of diode-laser absorption sensors operating near 2.3 microm for in situ combustion emission monitoring and combustion diagnostics.     

Use of laser-induced ionization to detect soot inception in premixed flames

Experimental measurements of laser-induced ionization were performed for ethene-air premixed flames operated near the soot inception point. Soot was ionized with a pulsed laser operated at 532 nm. The ionization signal was collected with a tungsten electrode located in the postflame region. Ionization signals were collected by use of both single-electrode and dual-electrode configurations. Earlier laser-induced-ionization studies focused on the use of a single biased electrode to generate the electric field, with the burner head serving as the path to ground. In many practical combustion systems, a path to ground is not readily available. To apply the laser-induced-ionization diagnostic to these geometries, a dual-electrode geometry must be employed. The influence of electrode configuration, flame equivalence ratio, and flame height on ionization signal detection was determined. The efficacy of the laser-induced-ionization diagnostic in detecting soot inception in the postflame region of a premixed flame by use of a dual-electrode configuration was investigated. Of the dual-electrode configurations tested, the dual-electrode geometry oriented parallel to the laser beam was observed to be most sensitive for detecting the soot inception point in a premixed flame.     

OH absorption spectroscopy in a flame using spatial heterodyne spectroscopy

We demonstrate measurements of OH absorption spectra in the post-flame zone of a McKenna burner using spatial heterodyne spectroscopy (SHS). SHS permits high-resolution, high-throughput measurements. In this case the spectra span approximately 308-310 nm with a resolution of 0.03 nm, even though an extended source (extent of approximately 2x10(-7) m(2) rad(2)) was used. The high spectral resolution is important for interpreting spectra when multiple absorbers are present for inferring accurate gas temperatures from measured spectra and for monitoring weak absorbers. The present measurement paves the way for absorption spectroscopy by SHS in practical combustion devices, such as reciprocating and gas-turbine engines.     

Measurement of absolute minority species concentration and temperature in a flame by the photothermal deflection spectroscopy technique

It is experimentally demonstrated that absolute concentrations of minority species in flames can be measured by the photothermal deflection spectroscopy (PTDS) technique. In addition, the PTDS signal simultaneously yields the flame temperature the measurement point. Absolute concentration profiles of OH have been measured in a flat-flame burner with methane as fuel. The PTDS measurements agree well with those obtained independently by the absorption technique. The flame temperature measurements by PTDS are also in good agreement with those obtained by the Boltzmann distribution among the rotational levels of OH.     

In Situ Combustion Measurements of CO(2) by Use of a Distributed-Feedback Diode-Laser Sensor Near 2.0 mum

High-resolution absorption measurements of CO(2) were made in a heated static cell and in the combustion region above a flat-flame burner for the development of an in situ CO(2) combustion diagnostic based on a distributed-feedback diode laser operating near 2.0 mum. Calculated absorption spectra of high-temperature H(2)O and CO(2) were used to find candidate transitions for CO(2) detection, and the R(50) transition at 1.997 mum (the nu(1) + 2nu(2) + nu(3) band) was selected on the basis of its line strength and its isolation from interfering high-temperature water absorption. Measurements of spectroscopic parameters such as the line strength, the self-broadening coefficient, and the line position were made for the R(50) transition, and an improved value for the line strength is reported. The combustion-product populations of CO(2) in the combustion region above a flat-flame burner were determined in situ to verify the measured spectroscopic parameters and to demonstrate the feasibility of the diode-laser sensor.     

Application of infrared emission spectroscopy to the thermal transition of indium hydroxide to indium oxide nanocubes

Cubic indium hydroxide nanomaterials were obtained by a low-temperature soft-chemical method without any surfactants. The transition of nano-cubic indium hydroxide to cubic indium oxide during dehydroxylation has been studied by infrared emission spectroscopy. The spectra are related to the structure of the materials and the changes in the structure upon thermal treatment. The infrared absorption spectrum of In(OH)(3) is characterized by an intense OH deformation band at 1150 cm(-1) and two O-H stretching bands at 3107 and 3221 cm(-1). In the infrared emission spectra, the hydroxyl-stretching and hydroxyl-bending bands diminish dramatically upon heating, and no intensity remains after 200 °C. However, new low intensity bands are found in the OH deformation region at 915 cm(-1) and in the OH stretching region at 3437 cm(-1). These bands are attributed to the vibrations of newly formed InOH bonds because of the release and transfer of protons during calcination of the nanomaterial. The use of infrared emission spectroscopy enables the low-temperature phase transition brought about through dehydration of In(OH)(3) nanocubes to be studied.     

Time-resolved wavelength modulation spectroscopy measurements of HO 2 kinetics

High-frequency wavelength modulation spectroscopy (WMS) has been applied to the detection of the hydroperoxyl radical (HO2 ) in a laser photolysis and long-path absorption pump-probe kinetics reactor with a near-infrared distributed feedback diode laser. The HO2 is formed by the 355-nm photolysis of Cl2 in the presence of CH3 OH and O2 and monitored by a phase-sensitive detection of the second-harmonic (2f ) signal in the 2?1 band with a 1.5- ?m diode laser directly modulated at 5 MHz. The measured 2f WMS signal is calibrated by direct absorption and converted to an absolute number density with the known absorption line strength of the HO2 line at 6625.80cm-1 . The utility of time-resolved WMS as a second-order kinetics probe is demonstrated through the measurement of the HO2 self-reaction rate constant at 295 K.     

Visible and near-ultraviolet absorption spectrum of liquid water

The optical absorption of pure liquid water in the 300-700-nm region has been measured by use of a long (1.5-m) path-length cell. The absorption spectrum coincides well with the edge of previous data in the 200-320-nm region and provides reliable data in the 320-420-nm region that has until now been a region of considerable unreliability. The data obtained for the 420-700-nm region agree quite well with the means of the existing literature in that well-studied region. The absorptions in the 550-700-nm region show evidence for the overtones and combination tones of the well-known OH stretching frequency at 3500 cm(-1).     

Measurement of atomic concentrations in reacting flows through the use of stimulated gain or loss

Apump laser at 226 nm was used to generate a population inversion between the 3p (3)P(2) and 3s (3)S(1) states of the oxygen atom in the postflame gases of a 7-torr H(2)/O(2) flame by means of resonant two-photon absorption from the ground state. The inversion produced an amplified spontaneous emission (ASE) signal at 845 nm. A probe laser tuned to this transition was crossed at a small angle with the pump beam. The probe beam experienced gain at the expense of theASE signal. By analyzing the gain on the probe beam and the loss on the ASE signal, we have determined the density of oxygen atoms in the flame environment.     

Near-infrared spectroscopic monitoring of the diffusion process of deuterium-labeled molecules in wood. Part I: softwood

The diffusion process of several molecules (D2O, n-butanol (OD) and t-butanol (OD)) in softwood (Sitka spruce) was investigated by means of a deuterium exchange method and Fourier transform near-infrared (FT-NIR) polarization spectroscopy. The location of OH groups in different states of order of cellulose in wood was clarified by analyzing the FT-NIR transmission spectra ranging from 7200 to 6000 cm(-1). Four absorption bands were assigned to 2 x v(OH) absorptions of the amorphous regions, OH groups in semi-crystalline regions, and two types of intramolecular hydrogen-bonded OH groups in the crystalline regions, respectively. The saturation level of accessibility was very different for these absorption bands (i.e., 70-80, 60, and 40-50% for the amorphous, semi-crystalline, and crystalline regions, respectively). However, the saturation accessibilities for each absorption band varied little with molecular structure and geometry of the diffusants. The diffusion rate of D2O was much faster than that of n-butanol (OD) and t-butanol (OD) for all states of orders. The size effect of the butanols led to slight differences in the diffusive transport in the crystalline regions.     

Spectroscopic ellipsometry studies on various zinc oxide films deposited by ion beam sputtering at room temperature

Various zinc oxide films were deposited by ion-beam sputter deposition (IBSD) under different oxygen partial pressures (P(O2)) at room temperature. The as-deposited ZnO films fabricated at P(O2)>1.0×10(-4) Torr had poly-crystalline structures to absorb water on the surface at ambient condition. Simultaneously, the film surfaces were covered and smoothed by the surface layers formed with the water, hydroxyl (OH(-)) groups, and ZnO materials investigated by X-ray photoelectron spectroscopy (XPS). When the compositions of the surface layers were used in a multilayer fitting model of spectroscopic ellipsometry, the actual optical refractive index of the ZnO film deposited at P(O2)=1.2×10(-4) Torr was found to be about 1.9618 at λ=550 nm.     

Photocatalytic activity of novel AgBr/WO3 composite photocatalyst under visible light irradiation for methyl orange degradation

A novel AgBr/WO(3) composite photocatalyst was synthesized by loading AgBr on WO(3) substrate via deposition-precipitation method and characterized by XRD, SEM and DRS. The as-prepared AgBr/WO(3) was composed of monoclinic WO(3) substrate and face-centered cubic AgBr nanoparticles with crystalline sizes less than 56.8 nm. AgBr/WO(3) had absorption edge at about 470 nm in the visible light region. The optical AgBr content in AgBr/WO(3) was 0.30:1 (Ag/W) at the corresponding apparent rate, k(app), of 0.0160 min(-1) for MO degradation. The highest k(app) was 0.0216 min(-1) for 4 g/L catalyst. The OH acted as active species. Addition of H(2)O(2) within 0.020 mmol/L can efficiently trap electrons to generate more OH and further improved photocatalytic activity of AgBr/WO(3).     

Emission spectroscopy for coal-fired cyclone furnace diagnostics

Using a spectrograph and charge-coupled device (CCD) camera, ultraviolet and visible light emission spectra were obtained from a coal-burning electric utility's cyclone furnaces operating at either fuel-rich or fuel-lean conditions. The aim of this effort is to identify light emission signals that can be related to a cyclone furnace's operating condition in order to adjust its air/fuel ratio to minimize pollutant production. Emission spectra at the burner and outlet ends of cyclone furnaces were obtained. Spectra from all cyclone burners show emission lines for the trace elements Li, Na, K, and Rb, as well as the molecular species OH and CaOH. The Ca emission line is detected at the burner end of both the fuel-rich and fuel-lean cyclone furnaces but is not detected at the outlet ends of either furnace type. Along with the disappearance of Ca is a concomitant increase in the CaOH signal at the outlet end of both types of furnaces. The OH signal strength is in general stronger when viewing at the burner end rather than the exhaust end of both the fuel-rich and fuel-lean cyclone furnaces, probably due to high, non-equilibrium amounts of OH present inside the furnace. Only one molecular species was detected that could be used as a measure of air/fuel ratio: MgOH. It was detected at the burner end of fuel-rich cyclone furnaces but not detected in fuel-lean cyclone furnaces. More direct markers of air/fuel ratio, such as CO and O2 emission, were not detected, probably due to the generally weak nature of molecular emission relative to ambient blackbody emission present in the cyclone furnaces, even at ultraviolet wavelengths.     

Fullerenol-containing Materials Derived by Sol-gel Processing

Water soluble fullerenols were synthesized and incorporated in SiO2, SiO2-TiO2, GPTMS-SiO2, GPTMS-ATPS inorganic and organic-inorganic materials by sol-gel processes. The maximum concentrations of C60(OH)n and C70(OH)n in the obtained SiO2 gels were estimated to be about 0.6% and 1.5% in weight, respectively. The characteristics of the UV/visible spectra of fullerenols in H2O and various gels were measured and compared. The thermal stability of fullerenols in gels was investigated with differential thermal analysis (DTA). The results indicate that the absorption features of fullerenols in solid gels are similar to those in H2O and the fullerenols in SiO2 are stable at 400℃. The optical limiting effect of the fullerenols was investigated preliminarily.     

Absorption cross-sections of the C-h overtone of volatile organic compounds: 2 methyl-1,3-butadiene (isoprene), 1,3-butadiene, and 2,3-dimethyl-1,3-butadiene

Many molecules or transient radicals have well-documented absorption cross-sections in the ultraviolet (UV) region, but their absorption cross-sections in the near-infrared (NIR) region are much less often known and are difficult to measure. We propose a method to determine the unknown NIR absorption cross-sections using the known absorption cross-sections in the UV region, in which single-path UV absorption spectroscopy and NIR continuous wave cavity ringdown spectroscopy (cw-CRDS) are employed in a cross-arm reaction chamber for simultaneous measurements. Without knowing the actual sample partial pressures (or concentrations), the NIR absorption cross-sections can be accurately determined through the two sets of measurements. The method is demonstrated by measuring the NIR absorption cross-section of the first overtone of the asymmetric C-H stretch of 2-methyl-1,3-butadiene (isoprene) (3.24 (+/-0.16) x 10(-22) cm(2) molecule(-1)) at 1651.52 nm using the known value of the absorption cross-section at 220 nm. The diode laser wavelength was calibrated by atmospheric cavity ringdown spectra of CH(4), CO(2), and H(2)O. By comparison with sample pressure measurements, this method can also be used as a pressure calibration means for the reaction chamber, and this has been demonstrated with two additional measurements of the absorption cross-sections of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene (2.50 (+/- 0.08) x 10(-22) and 2.82 (+/-0.16) x 10(-22) cm(2) molecule(-1), respectively) at 1651.52 nm. The applicability of the method to determining absorption cross-sections using the simultaneous measurements of cw-CRDS and single-path absorption spectroscopy is discussed.     

Near-infrared diode laser absorption diagnostic for temperature and water vapor in a scramjet combustor

Tunable diode laser absorption measurements of gas temperature and water concentration were made at the exit of a model scramjet combustor fueled on JP-7. Multiplexed, fiber-coupled, near-infrared distributed feedback lasers were used to probe three water vapor absorption features in the 1.34-1.47 microm spectral region (2v1 and vl + v3 overtone bands). Ratio thermometry was performed using direct-absorption wavelength scans of isolated features at a 4-kHz repetition rate, as well as 2f wavelength modulation scans at a 2-kHz scan rate. Large signal-to-noise ratios demonstrate the ability of the optimally engineered optical hardware to reject beam steering and vibration noise. Successful measurements were made at full combustion conditions for a variety of fuel/air equivalence ratios and at eight vertical positions in the duct to investigate spatial uniformity. The use of three water vapor absorption features allowed for preliminary estimates of temperature distributions along the line of sight. The improved signal quality afforded by 2f measurements, in the case of weak absorption, demonstrates the utility of a scanned wavelength modulation strategy in such situations.     

Prognostic significance of 25‐hydroxivitamin D entirely explained by a higher comorbidity burden: Experience from a South‐Eastern European Dialysis Cohort

Vitamin D deficiency is still a common problem particularly in the elderly and in individuals with various degrees of renal impairment. The present study aimed to evaluate the association between plasma concentrations of 25(OH)D and death in a large cohort of prevalent patients on hemodialysis (HD) from south‐east Romania, a typical Balkan region. This is an observational prospective study that included a total of 570 patients on maintenance HD. Study patients were classified into three groups by baseline 25(OH)D levels: (1) sufficient 25(OH)D—i.e., >30 ng/mL; (2) insufficient 25(OH)D—i.e., between 10 and 29 ng/mL; and (3) deficient 25(OH)D—i.e., <10 ng/mL. During the follow‐up period of 14 months, 68 patients (11.9%) died, the Kaplan–Meier analysis showing significant differences in all‐cause mortality for chronic kidney disease patients in different 25(OH)D groups (P = 0.002). Unadjusted Cox regression analysis also showed significant differences in survival. The multivariate Cox regression model showed no significant differences in survival according to vitamin D levels. Hazard ratio for death in the “<10 ng/mL” group was 1.619 (P = 0.190) and in the “10–30 ng/mL” group was 0.837 (P = 0.609). In our dialysis population with a high comorbidity burden, low 25(OH)D concentration was not associated with mortality in the adjusted Cox model, suggesting that vitamin D deficiency could represent only a non‐specific marker for a poor health status, with less impact on mortality.     

Mid-infrared spectroscopic investigation of methylamines by a continuous-wave difference-frequency-generation-based system

A laser spectroscopic system based on a cw difference-frequency generation source with a ratiometric multipass absorption detection scheme was employed for high-resolution spectroscopic investigation of gas-phase monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA). Possible application of the system as a noninvasive human breath analyzer for renal and liver diseases is targeted. The system operates in the fundamental C-H stretch absorption region around 2740-2860 cm(-1). A detection sensitivity of 2 x 10(-6) cm(-1) Hz(-(1/2)) (for signal-to-noise ratio SNR=1) is achieved, corresponding to detection limits of 900 ppb (parts in 10(9)) for MMA, 450 ppb for DMA, and 120 ppb for TMA in mixtures containing H2O and CO2 with concentrations of up to those present in human breath (2% and 5%, respectively). Future developments are discussed to further improve these detection limits that are currently still about 2 orders of magnitude higher than required for direct methylamine monitoring in human breath.     

Thermal Property Measurements and Enthalpy Calculation of the Lithium Bromide+Lithium Iodide+1,3-Propanediol+Water System

The lithium bromide+lithium iodide+1,3-propanediol+water [LiBr/LiI mole ratio=4 and (LiBr+LiI)/HO(CH₂)₃ OH mass ratio=4] solution is being considered as a potential working fluid for an absorption chiller. Heat capacities at four temperatures, 283.15, 298.15, 313.15, and 333.15 K, were measured in the range from 50 to 70 mass%. In addition, the differential heats of dilution at 298.15 K were measured in the range from 45.3 to 71.8 mass%. Each individual data set was correlated with a proper regression equation with a high accuracy. A new enthalpy calculation method for the working fluids containing organics was proposed. The calculation method correlated the heat capacity (at various temperatures and concentrations) and the differential heat of dilution (at ambient temperature and various concentrations). The present method was applied for the construction of enthalpy–concentration (H–T–X) diagrams with high confidence.