Practical Combustion Environments Research Articles

Tomographic measurements of temperature and species concentration via deep learning and wavelength modulated absorption spectroscopy

To achieve the 2-D nonintrusive measurements of temperature and species concentration in the combustion field, a new framework, combining calibration-free wavelength modulation absorption spectroscopy (CF-WMS) with a designed convolutional neural network (CNN), was developed. The principle of the CF-WMS, along with the architecture of the CNN net, the training, and the performance of the network, has been investigated. The region of interest was discretized into 24 × 24 pixels2, and 48 probing beams with six targeted frequencies were used to verify the feasibility of the designed CNN with WMS 2f/1f signal for temperature and species concentration reconstruction. 20 000 samples of temperature and water vapor concentration distributions are randomly fabricated, featuring three randomly positioned Gaussian distributions. Reconstructed images of the phantoms agreed well with the original distributions with the relative error of about 5.0%–9.2% and 8.0%–12.4% using 17 000 training datasets with different beam arrangements for temperature and H2O species, respectively. Several representative beam arrangements with a limited number have been examined and compared. The beam arrangement BA2 and BA3 exhibited better performance than BA1 with average errors of about 5% and 8% for temperature and H2O species, respectively. Such a method can provide an effective way to achieve spatially and temporally resolved, real-time, in situ monitoring in practical combustion environments.

Time-resolved multi-parameter detection in a kerosene-fueled combustor using tunable diode laser absorption spectroscopy

Non-contact and real-time measurements for species concentration and temperature with high-time resolution in practical combustion environments are desired. A laser absorption system was developed for simultaneous sensing of CO, H2O concentrations and temperature with 1 kHz in a centrally staged kerosene-fueled combustor. An absorption feature near 4.98 μm in the mid-infrared region was selected for CO detection to minimize spectral interference with other combustion species in the kerosene combustion environment. Two spectral lines of H2O near 1.4 μm were used for temperature and H2O detection using a direct scan absorption strategy. The method of the developed multi-parameter measurement was validated in a laboratory-scale flat flame under various equivalence ratios. Comparisons were made between the experimental and theoretical results within the error of 5%. Time-resolved and in-situ measurements of temperature, H2O and CO concentration in the practical kerosene-fueled staged combustor were demonstrated. The transient characteristics of CO emission in different conditions were captured experimentally. It is found that the fluctuation amplitude of CO concentration increases as the equivalence ratio decreases during the pilot stage work only. There is a dramatic rise in CO emissions during the transition of combustion conditions. Strong fluctuation occurs with a low frequency at 32 Hz near lean blowoff. The location of the flame surface and the dynamic pressure have a significant coupling relationship with CO emission by analyzing measured data with high time resolution simultaneously.

Y-Net: a dual-branch deep learning network for nonlinear absorption tomography with wavelength modulation spectroscopy.

This paper demonstrates a new method for solving nonlinear tomographic problems, combining calibration-free wavelength modulation spectroscopy (CF-WMS) with a dual-branch deep learning network (Y-Net). The principle of CF-WMS, as well as the architecture, training and performance of Y-Net have been investigated. 20000 samples are randomly generated, with each temperature or H2O concentration phantom featuring three randomly positioned Gaussian distributions. Non-uniformity coefficient (NUC) method provides quantitative characterizations of the non-uniformity (i.e., the complexity) of the reconstructed fields. Four projections, each with 24 parallel beams are assumed. The average reconstruction errors of temperature and H2O concentration for the testing dataset with 2000 samples are 1.55% and 2.47%, with standard deviations of 0.46% and 0.75%, respectively. The reconstruction errors for both temperature and species concentration distributions increase almost linearly with increasing NUC from 0.02 to 0.20. The proposed Y-Net shows great advantages over the state-of-the-art simulated annealing algorithm, such as better noise immunity and higher computational efficiency. This is the first time, to the best of our knowledge, that a dual-branch deep learning network (Y-Net) has been applied to WMS-based nonlinear tomography and it opens up opportunities for real-time, in situ monitoring of practical combustion environments.

Diffuse-reflection-based single-ended laser absorption sensor for H2O temperature and concentration in kerosene-fuelled combustor

The design, optimization and demonstration of a compact, single-ended laser absorption sensor based on a diffuse-wall-reflected signal is presented for temperature and H2O concentration measurements in a kerosene-fuelled aero-combustor. The challenges of laser measurements in such harsh, practical combustion environments involve strong beam-steering, low signal-to-noise ratio (SNR), and constrained optical access. We present here a detailed characterization and optimization procedure of a single-ended optical configuration using a holed off-axis parabolic mirror as the pitch and catch optics, and the coked wall surface of the combustor as a diffuse reflector. A near-infrared distributed-feedback diode laser near 1.4 µm scanned at 1 kHz for direct absorption detection was used to access two H2O transitions for concentration and temperature measurement. An optical system design and adaptive Savitzky–Golay signal processing algorithm were used for noise suppression and SNR amelioration. The sensor was demonstrated under practical conditions in a kerosene-fuelled swirl combustor. High signal fidelity was achieved and allowed for a single-scan H2O concentration detection limit of 122 ppm MHz−1/2, despite a low reflected laser intensity level of ~50 µW. In situ, time-resolved measurements detected temperatures ranging from ~1100 K to 1300 K, and H2O concentration ranging from ~8% to 12% as global equivalence ratio varied between 0.2 and 0.45. The results revealed an expected trend while demonstrating much faster response and less delay versus a comparative thermocouple. This successful field manifestation provides direct proof for the robustness and reliability of a diffuse-reflection-based single-ended sensor system in engine combustion chamber environments.

Baseline reduction algorithm for direct absorption spectroscopy with interference features

A baseline reduction algorithm for determining gas temperature and concentration from absorption spectra with interfering features using direct absorption spectroscopy is proposed. Empirical baselines are obtained through non-absorbing baseline transform, and background subtraction is done by reducing the first three components of the orthogonal polynomial expansion of the measured spectra. The algorithm is first validated by numerical simulation using the absorption lines of CO (R(9) and R(17)) located between 2179 and 2180 cm−1, which provides a correct estimation of temperature and CO concentration. Then the method is demonstrated in premixed CH4/air flat flame. The temperature is determined with a precision of 20 K, and the detection limit for CO is 400 ppm at 1835 K with 4.70% CO and 4.70% CO2 interference. Both the simulation and experimental results show that the algorithm is well suited for interpreting absorption spectra with neighbouring interference features as are often encountered in practical combustion environments.

Modeling Nitrogen Species as Pollutants: Thermochemical Influences.

To simulate emissions of nitrogen-containing compounds in practical combustion environments, it is necessary to have accurate values for their thermochemical parameters, as well as accurate kinetic values to describe the rates of their formation and decomposition. Significant disparity is observed in the literature for the former, and we therefore present herein high-accuracy ab initio gas-phase thermochemistry for 60 nitrogenous compounds, many of which are important in the formation and consumption chemistry of NOx species. Several quantum-chemical composite methods (CBS-APNO, G3, and G4) were utilized to derive enthalpies of formation via the atomization method. Entropies and heat capacities were calculated from traditional statistical thermodynamics, with oscillators treated as anharmonic based on ro-vibrational property analyses carried out at the B3LYP/cc-pVTZ level of theory. The use of quantum chemical methods, along with the treatments of anharmonicities and hindered rotors, ensures accurate enthalpy of formation, entropy, and heat capacity values across the temperature range 298.15-3000 K. The implications of these results for atmospheric and combustion modeling are discussed.

Single‐shot thermometry using fiber‐based picosecond coherent anti‐stokes Raman scattering (CARS) spectroscopy

Coherent anti‐Stokes Raman scattering (CARS)‐spectroscopy‐based thermometry techniques developed for laboratory flames face a stiff challenge when implemented in practical combustion environments such as engine test facilities. In addition to limited optical access, the harsh environments associated with these test facilities (i.e. uncontrolled humidity, vibration, and large thermal gradients) often restrict the operation of sensitive laser systems. To circumvent this problem, we have developed a fiber‐based, picosecond (ps) CARS system employing long‐length (up to 6 m), multimode silica fibers that permits the laser system to be isolated from the high heat and vibration of the test hardware. Proof‐of‐principle, single‐laser‐shot temperature measurements using a 3‐m‐long fiber system are demonstrated in atmospheric pressure, near‐adiabatic, laboratory H2–air flames. Limitations that are imposed by long‐length multimode‐fiber beam delivery for CARS thermometry are identified as nonlinear effects during beam propagation along the fiber, depolarization of the laser beam, and degradation of the beam quality at the output of the fiber. These effects are investigated in detail, and potential improvements are suggested. The current study shows promise for implementation of fiber‐based ps‐CARS spectroscopy in harsh environments such as those encountered in combustors and afterburners in practical gas‐turbine‐engine test facilities. Copyright © 2013 John Wiley & Sons, Ltd.

Size and Morphology of Soot Particulates Sampled from a Turbulent Nonpremixed Acetylene Flame

Soot processes within a turbulent nonpremixed flame burning acetylene/air were investigated by conducting thermophoretic sampling experiments at various axial and radial locations. Analyses of transmission electron microscope images yielded the mean soot spherule diameter, number of spherules per aggregate, and fractal morphology within this highly luminous turbulent flame. Specifically, translucent particles were observed at low-to-intermediate heights above the flame with the formation and evolution of young soot precursors. The soot spherule diameter peaked at 34 nm halfway along the centerline, identifying the flame regions of surface growth and oxidation processes. In the meantime, the aggregation was continuous along the flame axis with the mean number of spherules per aggregate reaching 150 at the highest sampling location. Size ranges of spherules and aggregates were narrow and broad, respectively, while the relative widths of both size distributions remained similar throughout the flame. In contrast to the observed axial variations, the radial changes of the mean spherule and aggregate sizes appeared to be small. Aggregate morphologies were universally characterized by a fractal dimension of 1.82 and a fractal prefactor of 1.9 for all the flame positions. In comparison to a lightly sooting ethylene flame, these measurements in the acetylene turbulent flame revealed that the fuel type mainly affected the axial evolution of spherule diameters but not their range, and enhanced the aggregate sizes but not their morphology. The effective decoupling of spherule and aggregate sizes permitted the separation of soot surface growth and oxidation from aggregation. This key aspect provided a stringent test for the existing particulate diagnostics and predictive models in their ability to quantify the actual particle surface area, particularly in optically thick conditions that are encountered in many practical combustion environments.

COMPARISON OF TURBULENT DIFFUSION FLAME CARS TEMPERATURE MEASUREMENTS AND REYNOLDS-STRESS MODEL PREDICTION

Coherent Anti-Stokes Raman spectroscopy (CARS) is a well-established diagnostic technique for probing flames and practical combustion environments. This paper, presents the results of CARS temperature measurements in a turbulent natural gas diffusion flame. The experimental setup and CARS theory are briefly discussed. Experimental single-shot results are shown in the form of mean temperature and standard deviation of temperature versus normalized flame radius and are compared to theoretical values obtained from a Reynolds-stress model with a chemical equilibrium assumption.

Determination of Relative Number Densities and Quenching Rates of Sodium in an Atmospheric Flame by Asynchronous Optical Sampling

Pump/probe methods are commonly employed to measure subnanosecond excited state processes in liquid and gas phase systems. Asynchronous Optical Sampling (ASOPS) is a newly-developed pump/probe method that will potentially allow the determination of number densities and relaxation rates in turbulent, high-pressure flames. In addition, ASOPS should yield a better signal-to-noise ratio than laser-induced fluorescence in practical combustion environments. The ASOPS method utilizes a coherent, signal-carrying beam and thus requires no more optical access than LDV measurements. The current ASOPS instrument consists of two dye lasers synchronously pumped by two frequency-doubled, mode-locked Nd:YAG lasers. The two lasers operate at slightly different repetition rates, causing a relative phase walk-out between the pump and probe beams. This strategy allows the mapping of subnanosecond excitation processes in a time scale on the order of milliseconds.

Coherent Anti-Stokes Raman Scattering As A Probe In Reactive Media

Coherent anti-Stokes Raman scattering (CARS) has become a very powerful tool in the measurement of temperature and concentrations in practical combustion environments. The technique is also proving to be useful in the study of other types of reactive media including plasmas, photodissociation studies, and excited-state relaxations. This paper reviews several research areas involving reactive or transient species where the spectral and temporal properties of CARS have been exploited.