Extinction strain rates of n-butanol, 2-butanol and iso-butanol in counterflow non-premixed flamelets

The extinction strain rates of three butanol isomers, (n-butanol, sec-butanol and iso-butanol) were studied experimentally in a counterflow burner, and computationally using a one-dimensional numerical model. The experimental results provided an insight in how the difference between molecular structures affects combustion of the isomers. Molecular branching made the isomers more prone to extinction. They shared a similar maximum temperature, as well as virtually identical high temperature kinetics, which implied that the underlying chemistry that produced difference in extinction strain rates lay, in the low-temperature oxidation steps. A numerical study was employed to calculate the extinction strain rate of n-butanol, and compare it with the experimental results. Good correlation was observed between the simulation and experimental data. During gradual increase of strain that led to extinction in the computations, the maximum temperature as well as the reaction zone thickness decreased. The mole fractions of H and OH radicals decreased, while HCO appeared to remain constant throughout the process. The formation of an annular edge flame at high strain rates was also observed experimentally for all the isomers. Very rich mixtures (ϕ=4 or higher), produced more resilient edge flames, than lean (ϕ=0.5 or lower) ones

Reactive flow visualization through high-speed optical laser diagnostics

Turbulent combustion is a complex phenomenon that involves the interaction between fluid mechanics and chemical kinetics. Turbulence plays an important role in combustion because it ensures a thorough mixing between the oxidizer and fuel at a molecular level. Improper mixing can lead to incomplete combustion, resulting in increased emissions and decreased flame stability. Hence, understanding and controlling the coupling between chemical kinetics and turbulence can lead to significant improvements in the design of combustion systems, improving their stability, efficiency and emissions characteristics. In this study, a method was demonstrated using temporally-resolved laser diagnostics to visualize: 1) The turbulent flow field with particle image velocimetry (PIV); and 2) the distribution of combustion radicals, by planar laser-induced fluorescence (PLIF). This allows for the visualization of the flame surface behavior, while at the same time linking it to inherent flow and combustion instabilities. Characterizing turbulence requires high temporal resolution and combining it with high-speed combustion radical imaging adds an additional diagnostics challenge. Trying to extend the capabilities of current laser diagnostics techniques to higher repetition rates to achieve the desired temporal resolution in order to resolve turbulence is a major issue due to limitations in terms of laser power scalability with repetition rate. In this work, a new approach is investigated that reduces the overall laser power required to visualize the flame structure (CH and OH radicals) while allowing for simultaneous flow field imaging at high repetition rates (10 kHz). A comprehensive study of CH-PLIF imaging using the of the CH molecule to visualize the flame reaction zone is presented here. The CH-PLIF imaging effectiveness and its suitability for use in conjunction with PIV were quantified using a laminar Bunsen flame. Due to the high Einstein coefficients, the Q-branch rotational excitation strategy has the lowest laser power requirements and hence is best suited for high-speed imaging. However, isolating the fluorescence signal from the excitation wavelength is a major concern especially in high-scattering environments, due to the inherent resonant transition that is observed in the Q-branch excitation strategy. To address this, the excitation scheme was switched to the weaker (by a factor of 5) R-branch transition, whose fluorescence signal proved to be sufficient for CH-imaging, with separation of the fluorescence and excitation wavelengths through the use of a custom-made sharp cut-off edge filter. The low laser pulse energy requirements of this transition combined with the use of the custom edge filter allowed for simultaneous 10 kHz CH-PLIF and PIV imaging in a highly turbulent Hi-Pilot burner (ReT ~ 7900). Reaction layer thicknesses were estimated from the CH-PLIF images, and their interaction with the flow field was observed. Folding of the flame sheet caused an interaction between the out-of-plane and in-plane flame sheets, manifesting in the presence of products well upstream of the flame. Additionally, preheat zone broadening effects were observed, suggesting the existence of eddies smaller than the laminar flame thickness, able to penetrate into the preheat zone and therefore enhance scalar transport, through a purely hydrodynamic straining mechanism. Finally, the ability to image three major combustion radicals (OH, CH and CH2O), using a reduced experimental setup was demonstrated in mesoscale burner array, by making use of the increased efficiency of the C-X CH transition, as well as the presence of OH lines (from the A-X (0,0) band) in the vicinity

Reactive flow visualization through high-speed optical laser diagnostics

Turbulent combustion is a complex phenomenon that involves the interaction between fluid mechanics and chemical kinetics. Turbulence plays an important role in combustion because it ensures a thorough mixing between the oxidizer and fuel at a molecular level. Improper mixing can lead to incomplete combustion, resulting in increased emissions and decreased flame stability. Hence, understanding and controlling the coupling between chemical kinetics and turbulence can lead to significant improvements in the design of combustion systems, improving their stability, efficiency and emissions characteristics. In this study, a method was demonstrated using temporally-resolved laser diagnostics to visualize: 1) The turbulent flow field with particle image velocimetry (PIV); and 2) the distribution of combustion radicals, by planar laser-induced fluorescence (PLIF). This allows for the visualization of the flame surface behavior, while at the same time linking it to inherent flow and combustion instabilities. Characterizing turbulence requires high temporal resolution and combining it with high-speed combustion radical imaging adds an additional diagnostics challenge. Trying to extend the capabilities of current laser diagnostics techniques to higher repetition rates to achieve the desired temporal resolution in order to resolve turbulence is a major issue due to limitations in terms of laser power scalability with repetition rate. In this work, a new approach is investigated that reduces the overall laser power required to visualize the flame structure (CH and OH radicals) while allowing for simultaneous flow field imaging at high repetition rates (10 kHz). A comprehensive study of CH-PLIF imaging using the of the CH molecule to visualize the flame reaction zone is presented here. The CH-PLIF imaging effectiveness and its suitability for use in conjunction with PIV were quantified using a laminar Bunsen flame. Due to the high Einstein coefficients, the Q-branch rotational excitation strategy has the lowest laser power requirements and hence is best suited for high-speed imaging. However, isolating the fluorescence signal from the excitation wavelength is a major concern especially in high-scattering environments, due to the inherent resonant transition that is observed in the Q-branch excitation strategy. To address this, the excitation scheme was switched to the weaker (by a factor of 5) R-branch transition, whose fluorescence signal proved to be sufficient for CH-imaging, with separation of the fluorescence and excitation wavelengths through the use of a custom-made sharp cut-off edge filter. The low laser pulse energy requirements of this transition combined with the use of the custom edge filter allowed for simultaneous 10 kHz CH-PLIF and PIV imaging in a highly turbulent Hi-Pilot burner (ReT ~ 7900). Reaction layer thicknesses were estimated from the CH-PLIF images, and their interaction with the flow field was observed. Folding of the flame sheet caused an interaction between the out-of-plane and in-plane flame sheets, manifesting in the presence of products well upstream of the flame. Additionally, preheat zone broadening effects were observed, suggesting the existence of eddies smaller than the laminar flame thickness, able to penetrate into the preheat zone and therefore enhance scalar transport, through a purely hydrodynamic straining mechanism. Finally, the ability to image three major combustion radicals (OH, CH and CH2O), using a reduced experimental setup was demonstrated in mesoscale burner array, by making use of the increased efficiency of the C-X CH transition, as well as the presence of OH lines (from the A-X (0,0) band) in the vicinity.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste

Preventing Undetected Train Torsional Oscillations

Tutorialpg. 135-146In the past few years the authors have encountered numerous problems and coupling failures related to unexpected torque fluctuations on applications with reciprocating compressors and on VFD, synchronous motor, and gas engine driven trains. In some cases, dangerous coupling failures have resulted causing significant lost uptime, and requiring a tremendous amount of engineering resources to discover the root problem and the corrective action. Since almost all modern turbomachinery is outfitted with proximity probes to detect lateral vibrations, but not with probes or systems to detect torsional oscillations, these failures occurred suddenly and, in some instances, without warning. In this paper, some of these cases and the work done to deduce the causes of these failure and discussed. The various methods used to measure the oscillations and the advantages and disadvantages of each are presented

Simultaneous high speed PIV and CH PLIF using R-branch excitation in the C²Σ⁺-X²Π (0,0) band

Simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) utilizing R-branch transitions in the C-X (0,0) band were performed at a 10-kHz repetition-rate in a turbulent premixed flame. The CH lines at 310.690nm (from the R-branch of the C-X band) used here have greater efficiency than A-X and B-X transitions, which allows for high-framerate imaging with low laser pulse energy. Most importantly, the simultaneous imaging of both CH PLIF and PIV is enabled by the use of a custom edge filter, which blocks scattering at the laser wavelength (below ∼311nm) while efficiently transmitting fluorescence at longer wavelengths. The Hi-Pilot Bunsen burner operated with a turbulent Reynolds number of 7900 was used to demonstrate simultaneous PIV and CH PLIF utilizing this filtered detection scheme. Instances where pockets of products were observed well upstream of the mean flame brush are found to be the result of out-of-plane motion of the flame sheet. Such instances can lead to ambiguous results when interpreting the thickness of reaction layers. However, the temporally resolved nature of the present diagnostics facilitate the identification and proper treatment of such situations. The strategy demonstrated here can yield important information in the study of turbulent flames by providing temporally resolved flame dynamics in terms of flame sheet visualization and velocity fields

Temporally resolving premixed turbulent flame structures using self-supervised adversarial reconstruction of CH-PLIF

Understanding the turbulence-flame interaction is crucial to model the low-emission combustors developed for energy and propulsion applications. To this end, a novel frame interpolation (FI) method is proposed to better resolve the spatiotemporal evolution of premixed turbulent flame structures. The framework is completely self-supervised, agnostic to optical flow, and driven by leveraging transferrable feature knowledge at lower speeds and adversarial learning to statistically map the flame dynamics across frames. The method is successfully applied on a 10 kHz CH planar laser-induced fluorescence (PLIF) dataset of highly wrinkled premixed flames with turbulent Reynolds numbers (ReT) of 1100, 1400, and 7900, by down-sampling the image sequence to 5 kHz and restoring the sequence back to 10 kHz via FI. All reconstructions recovered important flame events and displayed excellent resemblance of the corrugated CH-layer geometries to that of the ground truths, with average intersection over union (IoU) and structural similarity index (SSIM) scores of 0.49 and 0.82, which are above the high-similarity baselines of 0.36 and 0.75, respectively. The wrinkling parameters (WP) of the flames also matched the ground truths, wherein R2 was roughly 0.95 for ReT = 1100 and 1400 and 0.85 for ReT = 7900 (lower due to the turbulence-induced uncertainties). The FI is further iteratively repeated to 40 kHz on the ReT = 7900 flames to facilitate pocket analysis by confidently linking their origin of formation, thus, enabling distinction from 3D tunnels, and improving statistical characterization of their consumption speeds. Given that the object features do not exhibit highly turbulent motions with regard to the initial time step, the proposed FI method is shown to be highly accurate and useful to analyzing finite-resolution experimental image sets including, but not restricted to, CH-PLIF