Temperature and multi-species measurements by supercontinuum absorption spectroscopy for IC engine applications

Time-stretch spectroscopy for fast infrared absorption spectra of acetylene and hydroxyl radicals during combustion

We have developed a diagnostic that uses time-domain spectroscopy to measure transient infrared absorption spectra in gases. Using a time-stretch Fourier transform approach, we can determine pressure, temperature, and gas concentrations with sub-microsecond time resolution for over two milliseconds. We demonstrate high-resolution (0.015 nm), time-resolved spectral measurements in an acetylene-oxygen gas mixture undergoing combustion. Within a 5 µs period during the reaction, the acetylene line intensities decrease substantially, and new spectra appear that are consistent with the hydroxyl (OH) radical, a common by-product in the combustion, deflagration, and detonation of fuels and explosives. Post-reaction pressures and temperatures were estimated from the OH spectra. The technique measures spectra from 1520 to 1620 nm using fiber optics, photodetectors, and digitizers. No cameras or spectrometers are required.

Time-stretched photonic Doppler velocimetry

Inertial confinement fusion facilities generate implosions at speeds greater than 100 km/s, and measuring the material velocities is important and challenging. We have developed a new velocimetry technique that uses time-stretched spectral interferometry to increase the measurable velocity range normally limited by the detector bandwidth. In this approach, the signal is encoded on a chirped laser pulse that is stretched in time to reduce the beat frequency before detection. We demonstrate the technique on an imploding liner experiment at the Sandia National Laboratories' Z machine, where beat frequencies in excess of 50 GHz were measured with 20 GHz bandwidth detection.

High-speed combustion diagnostics in a rapid compression machine by broadband supercontinuum absorption spectroscopy

The first results under fired internal combustion engine conditions based on a supercontinuum absorption spectrometer are presented and discussed. Temperature, pressure, and water mole fraction are inferred simultaneously from broadband H₂O absorbance spectra ranging from 1340 nm to 1440 nm. The auto-ignition combustion process is monitored for two premixed n-heptane/air mixtures with 10 kHz in a rapid compression machine. Pressure and temperature levels during combustion exceed 65 bar and 1900 K, respectively. To allow for combustion measurements, the robustness of the spectrometer against beam steering has been improved compared to its previous version. Additionally, the detectable wavelength range has been extended further into the infrared region to allow for the acquisition of distinct high-temperature water transitions located in the P-branch above 1410 nm. Based on a theoretical study, line-of-sight (LOS) effects introduced by temperature stratification on the broadband fitting algorithm in the complete range from 1340 nm to 1440 nm are discussed. In this context, the recorded spectra during combustion were evaluated only within a narrower spectral region exhibiting almost no interference from low-temperature molecules (here, P-branch from 1410 nm to 1440 nm). It is shown that this strategy mitigates almost all of the LOS effects introduced by cold molecules and the evaluation of the spectrum in the entirely recorded wavelength range at engine combustion conditions.

Supercontinuum based absorption spectrometer for cycle-resolved multiparameter measurements in a rapid compression machine

A broadband supercontinuum (SC) based absorption spectrometer capable of cycle-resolved multiparameter measurements at internal combustion (IC) engine conditions is presented. Three parameters, temperature, pressure and water mole fraction, were extracted from broadband near-infrared H₂O absorption spectra, spanning the wavelength-range from 1340 to 1405.5 nm, which exhibits a large number of specific H₂O transitions. The spectrometer is based on spatial domain detection and features a near-infrared line scan camera as a detector. Measurements were performed during a compression cycle of a rapid compression machine comprising a pressure and temperature range from 2.5 to 65 bar and 300 to 900 K, respectively. With the new spectrometer, we are for the first time, based on the authors' knowledge, able to perform measurements based on SC radiation over a complete compression and expansion stroke at measurement rates up to 50 kHz. A detailed overview is provided about the best match algorithm between theory and experiments, including parameters from two different spectral databases, namely the Barber-Tennyson database (BT2) and HITRAN2012. The results indicate that spectral broadening effects are not properly described by theory, especially at pressure levels exceeding 20 bar, which culminates in a clear underestimation of the derived pressure data by SC absorption spectroscopy. Nevertheless, temperature can be determined accurately by performing a three-parameter fit based on water mole fraction, temperature, and pressure. In contrast, making use of pressure transducer data as look-up values and varying only temperature and H₂O mole fraction to find the best match leads to a clear overestimation of temperature at elevated pressures.

Minute Concentration Measurements of Simple Hydrocarbon Species Using Supercontinuum Laser Absorption Spectroscopy

Minute concentration measurements of simple hydrocarbon gases are demonstrated using near-infrared supercontinuum laser absorption spectroscopy. Absorption-based gas sensors, particularly when combined with optical fiber components, can significantly enhance diagnostic capabilities to unprecedented levels. However, these diagnostic techniques are subject to limitations under certain gas sensing applications where interference and harsh conditions dominate. Supercontinuum laser absorption spectroscopy is a novel laser-based diagnostic technique that can exceed the above-mentioned limitations and provide accurate and quantitative concentration measurement of simple hydrocarbon species while maintaining compatibility with telecommunications-grade optical fiber components. Supercontinuum radiation generated using a highly nonlinear photonic crystal fiber is used to probe rovibrational absorption bands of four hydrocarbon species using full-spectral absorption diagnostics. Absorption spectra of methane (CH4), acetylene (C2H2), and ethylene (C2H4) were measured in the near-infrared spectrum at various pressures and concentrations to determine the accuracy and feasibility of the diagnostic strategy. Absorption spectra of propane (C3H8) were subsequently probed between 1650 nm and 1700 nm, to demonstrate the applicability of the strategy. Measurements agreed very well with simulated spectra generated using the HITRAN database as well as with previous experimental results. Absorption spectra of CH4, C2H2, and C2H4 were then analyzed to determine their respective measurement accuracy and detection limit. Concentration measurements integrated from experimental results were in very good agreement with independent concentration measurements. Calculated detection limits of CH4, C2H2, and C2H4 at room temperature and atmospheric pressure are 0.1%, 0.09%, and 0.17%, respectively.

Supercontinuum high-speed cavity-enhanced absorption spectroscopy for sensitive multispecies detection

Cavity-enhanced absorption spectroscopy is promising for many applications requiring a very high concentration sensitivity but often accompanied by low temporal resolution. In this Letter, we demonstrate a broadband cavity-enhanced absorption spectrometer capable of detection rates of up to 50 kHz, based on a spatially coherent supercontinuum (SC) light source and an in-house-built, high-speed near-infrared spectrograph. The SC spectrometer allows for the simultaneous quantitative detection of CO₂, C₂H₂, and H₂O within a spectral range from 1420 to 1570 nm. Using cavity mirrors with a specified reflectivity of R=98.0±0.3% a minimal spectrally averaged absorption coefficient of α_min=1·10^-5  cm^-1 can be detected at a repetition rate of 50 kHz.

High-resolution bispectral imager at 1000 frames per second

We describe a bispectral, 1000-frames per second imaging instrument working simultaneously in two spectral bands. These bands may be selected for a specific application; however, we implement a pair centered at 4.3 μm and 4.66 μm. Synchronization is accomplished by employing a single focal plane array. To demonstrate the performance of the bispectral imager, we apply it to the methane flame of a Bunsen burner in a near conjugate configuration with flame image length subtending at about 200 pixels. The instrument detects bispectral puffing at 2 Hz, pulsations, and bispectral radiation oscillations, first reported here in two spectral intervals. The period of oscillatory spectral components in two bands is the same, about 3 Hz for this flame, with delay of a quarter period between them, first reported here. With 1-ms integration time, we detect significant formation of turbulence and vortices, especially pronounced in the region where the flame transitions into a plume. We display bispectral ratioed images of flames in near-real time with either the laboratory or the field device.

Broadband supercontinuum laser absorption spectrometer for multiparameter gas phase combustion diagnostics

We report on the development and application of a broadband absorption spectrometer utilizing a pulsed supercontinuum laser light source and dispersion compensating fiber with a single-pass absorption path to obtain absolute methane mole fractions in a laminar nonpremixed CH(4)/air flame supported on a Wolfhard-Parker burner. The basic principle of supercontinuum broadband absorption spectroscopy (SCLAS) provides advantageous means of combustion diagnostics since the broad spectral coverage allows for use in high-pressure high-temperature environments. Furthermore, a previously validated tunable diode laser absorption spectroscopy fitting algorithm was applied to the recorded spectra and found to be applicable to SCLAS measurements as well, by comparison of fitted methane gas concentrations to reference measurements on the Wolfhard-Parker burner. The spectrometer reached spectral resolutions of up to 0.152  cm(-1), while providing a spectral coverage of over 110  cm(-1), with an absorption path length of only 41 mm. First measurements of absolute CH(4) mole fractions showed the suitability of SCL-based spectroscopy for combustion diagnostics with short absorption path lengths in the nIR spectral region. Here, we achieved in-flame methane mole fraction resolutions of 3%(Vol.) (1210 ppm·m) and optical resolutions of up to 1.1×10(-2). Based on this first validation, this method can now be extended to other species and combustion parameters such as temperature and pressure.