Recent Submissions

  • Alignment statistics of pressure Hessian with strain rate tensor and reactive scalar gradient in turbulent premixed flames

    Chakraborty, Nilanjan; Ahmed, Umair; Klein, M.; Im, Hong G. (Physics of Fluids, AIP Publishing, 2022-05-24) [Article]
    The relative alignment of the eigenvectors of pressure Hessian with reactive scalar gradient and strain rate eigenvectors in turbulent premixed flames have been analysed for Karlovitz number values ranging from 0.75 to 126 using a detailed chemistry three-dimensional Direct Numerical Simulations (DNS) database of H2-air premixed flames. The reactive scalar gradient preferentially aligns with the most extensive strain rate eigendirection for large Damköhler number and small Karlovitz number values, whereas a preferential collinear alignment between the reactive scalar gradient with the most compressive strain rate eigendirection is observed in flames with small Damköhler number and large Karlovitz number. By contrast, the eigenvectors of pressure Hessian do not perfectly align with the reactive scalar gradient, and the net contribution of the pressure Hessian to the evolution of the normal strain rate contribution to the scalar dissipation rate transport acts to reduce the scalar gradient in the zone of high dilatation rate. The eigenvectors of pressure Hessian and strain rate are aligned in such a manner that the contribution of pressure Hessian to the evolution of principal strain rates tends to augment the most extensive principal strain rate for small and moderate values of Karlovitz numbers, whereas this contribution plays an important role for the evolution of the intermediate principal strain rate for large values of Karlovitz number. As the reactive scalar gradient does not align with the intermediate strain rate eigenvector, the influence of pressure Hessian contributions to the scalar-turbulence interaction remains weak for large values of Karlovitz number.
  • Kinetic study of plasma assisted oxidation of H2 for an undiluted lean mixture

    Snoeckx, Ramses; Jun, Daeyoung; Lee, Bok Jik; Cha, Min Suk (Combustion and Flame, Elsevier BV, 2022-05-23) [Article]
    For the past couple of decades, electrical discharges (or plasmas) have been widely investigated in pursuit of the advancement in combustion and fuel reforming. Particularly, nonthermal plasma has attracted researchers’ attention to improve ignition characteristics, promote flame stability, and reform hydrocarbons. Nevertheless, due to the nonthermal plasma's complex physicochemical nature, most of the experimental findings have not been fully explained yet. Recently, plasma-chemical kinetic studies have been initiated to address the important roles of plasma chemistry in hydrocarbon chemistry including combustion phenomena. However, we still have a long way to go to fully understand the underlying mechanisms and predict experimental outcomes. Here, we present a kinetic study of plasma-assisted low-temperature oxidation of H2 for an undiluted H2/O2 mixture. The aim of this study is to establish a foundation for low-temperature plasma assisted combustion as well as high-temperature plasma assisted reforming processes. We employed a plasma-chemical reaction mechanism and plasma-chemical kinetic modeling platform (KAUSTKin) and a temperature controlled dielectric barrier discharge reactor to study the plasma assisted oxidation of H2. As a result of systematically varying the gas temperature and discharge power, we found a non-linear oxidation behavior highlighting a Negative Temperature Coefficient (NTC)-like trend in a temperature range of 600–750 K. We investigated the effects of both the reduced electric field and the temperature on the plasma assisted oxidation chemistry. We found that (i) the oxidation is initiated by the electron impact dissociation of O2, which is governed by the reduced electric field and controls the oxidation degree, (ii) HO2 is the key intermediate for the full oxidation to H2O, and (iii) O3 and H2O2 production negatively affect the oxidation for temperatures below 400 K and over 600 K, respectively. We believe that these findings will further contribute to a better description and deeper understanding of the plasma chemistry with hydrocarbons as well as other H2 mixtures.
  • Analysis of Pyrolysis Index and Reaction Mechanism in Microwave-Assisted ex-situ Catalytic Co-Pyrolysis of Agro-residual and Plastic Wastes.

    Suriapparao, Dadi V; Gautam, Ribhu; Rao Jeeru, Lakshmana (Bioresource technology, Elsevier BV, 2022-05-21) [Article]
    Catalytic and non-catalytic microwave-assisted co-pyrolysis of biomass with plastics was performed to understand the interactions. An ex-situ configuration was adopted for performing catalytic co-pyrolysis experiments with ZSM-5 as a catalyst. Co-pyrolysis promoted cracking of vapors resulting in enhanced gas yields. ZSM-5 further enhanced the secondary cracking which resulted in low oil yields. The oil fraction collected from the pyrolysis of plastics was rich in hydrocarbons, whereas biomass pyrolysis led to the formation of oxygenated compounds in the oil. A plausible reaction mechanism scheme is proposed to understand the formation of major pyrolysis products via different pathways during different pyrolysis processes investigated. Also, a new parameter, the pyrolysis index is introduced to understand the pyrolysis intensity by utilizing the feedstock conversion, pyrolysis time, heating value, mass of feedstock, and energy consumption. The value of the pyrolysis index was found to be higher for plastics pyrolysis than biomass pyrolysis. Co-pyrolysis further increased the pyrolysis index due to the synergistic interactions.
  • Low temperature oxidation of toluene in an n-heptane/toluene mixture

    Chen, Bingjie; Liu, Peng; Xu, Qiang; Wang, Zhandong; Roberts, William L.; Pitsch, Heinz (Combustion and Flame, Elsevier BV, 2022-05-19) [Article]
    As an important component of transportation fuels, toluene has little reactivity in the low temperature regime. However, the low temperature reactivity of toluene may be enhanced by the reaction of other reactive components (e.g., n-heptane) in fuel mixtures. This work examines low temperature oxidation of toluene in jet stirred reactor oxidation of an n-heptane/toluene mixture (1:1 in mole, 500–800 K, ϕ=0.5, τ=2.0 s, p = 1 bar). Two measurement techniques, time of flight molecular beam mass spectrometry using synchrotron vacuum ultraviolet radiation as the photon ionization source and gas chromatography mass spectrometry, were applied to identify and measure 32 species, including four polycyclic aromatic hydrocarbons (PAH) and oxygenated PAH (OPAH). Numerical simulations using the latest kinetic model from Lawrence Livermore National Laboratory predicted the mole fractions of fuel molecules and small intermediates well, but under-predicted the mole fractions of oxygenated aromatics (phenol, benzyl alcohol, and cresol). The identification of benzyl peroxide–an important intermediate–supported the proposed formation pathways for the identified aromatics. Model analysis highlighted the influence of H-atom abstraction, OH/H radical ipso substitution, and OH addition reactions of toluene on the formation of phenol, benzyl alcohol, and cresol, which may further grow to OPAH molecules by the addition of benzyl radical from H-atom abstraction of toluene.
  • Revisiting low temperature oxidation chemistry of n-heptane

    Xie, Cheng; Lailliau, Maxence; Issayev, Gani; Xu, Qiang; Chen, Weiye; Dagaut, Philippe; Farooq, Aamir; Sarathy, Mani; Wei, Lixia; Wang, Zhandong (Combustion and Flame, Elsevier BV, 2022-05-17) [Article]
    Benefitting from the rapid development of instrumental analysis methods, intermediate products that were difficult to probe in the past can now be measured and quantified in complex reaction systems. To understand low temperature reactions of interest for combustion applications, and reduce the deviations between model predictions and experimental measurements, constant advancement in understanding low temperature oxidation process is necessary. This work examines the oxidation of n-heptane in jet-stirred reactors at atmospheric pressure, with an initial n-heptane mole fraction of 0.005, equivalence ratio of 0.5, a residence time of 1s, and over a temperature range of 500-800 K. Reaction products were analyzed using synchrotron ultra-violet photoionization mass spectrometry, gas chromatography, and Fourier-transform infrared spectroscopy. Ignition delay times of n-heptane/O2/CO2 mixture were measured in a rapid compression machine at 20 and 40 bar over a 600-673 K temperature range. Based on the experimental results, a comprehensive kinetic model of n-heptane low temperature oxidation was developed by considering the sub-mechanisms of keto-hydroperoxide, cyclic ether, heptene isomers, and the third O2 addition reaction, and by updating the rate constants of keto-hydroperoxide decomposition and second oxygen addition reactions. The combination of reaction mechanism development and evaluation of the rate constants of key reactions enabled the model to effectively predict the species concentrations and ignition delay times of n-heptane low temperature oxidation, providing additional insight into alkane low temperature oxidation chemistry.
  • The impact of gasoline formulation on turbulent jet ignition

    Gorbatenko, Inna; Nicolle, Andre; Silva, Mickael; Im, Hong G.; Sarathy, Mani (Elsevier BV, 2022-05-13) [Article]
    Turbulent jet ignition (TJI) is a promising technology for burning ultra-lean mixtures; the process is comprised of hot reactive jets issuing from a pre-chamber (PC) and initiating combustion in the main chamber (MC). The present study employs a simplified zero-dimensional (0D) partially stirred reactor (PaSR) model to describe the complex mixing and reaction progress within the PC and its subsequent impact on the MC combustion in terms of combustion efficiency and pollutant formation characteristics. Full three-dimensional (3D) computational fluid dynamics (CFD) data are used to calibrate the PC model, which is subsequently linked to predict the MC combustion behavior. We propose a model to predict the effects of the fuel formulations with varying research octane number (RON) and octane sensitivities (OS) on the TJI performance. After a careful parametric study, a dedicated merit function for identifying the optimal TJI operating conditions was proposed to assess multiple fuel properties and their influence on MC combustion. The model properly accounts for micro-mixing effects in the early jet expansion phase, and represents the effects of a PC jet on enhancing flammability and pollutant mitigation. It was demonstrated that aromatic content affects not only the progress of the thermokinetic runaway, but also the importance of NO formation paths in MC (N2O vs NNH routes), and the effect of the PC jet on MC flammability limits. Among the jet active species, OH and NO exhibited the greatest chemical impact on MC reactivity, while the chemical effects of CO2 and H2O remained limited. The overall fuel TJI merit function showed optimum performance for fuels with 2 < OS < 6 and high RON, similar to the requirements for spark-ignited engine operation beyond motor octane number (MON) conditions, fuel lean advanced compression ignition operation, and spark-induced compression ignition.
  • The effect of hydrogen bonding on the reactivity of OH radicals with prenol and isoprenol: a shock tube and multi-structural torsional variational transition state theory study

    Mohamed, Samah; Monge Palacios, Manuel; Giri, Binod; KHALED, Fethi; Liu, Dapeng; Farooq, Aamir; Sarathy, Mani (Physical Chemistry Chemical Physics, Royal Society of Chemistry (RSC), 2022-05-11) [Article]
    The presence of two functional groups (OH and double bond) in C5 methyl-substituted enols (i.e., isopentenols), such as 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol), makes them excellent biofuel candidates as fuel additives. As OH radicals are abundant in both combustion and atmospheric environments, OH-initiated oxidation of these isopentenols over wide ranges of temperatures and pressures needs to be investigated. In alkenes, OH addition to the double bond is prominent at low temperatures (i.e., below ∼700 K), and H-atom abstraction dominates at higher temperatures. However, we find that the OH-initiated oxidation of prenol and isoprenol displays a larger role for OH addition at higher temperatures. In this work, the reaction kinetics of prenol and isoprenol with OH radicals was investigated over the temperature range of 900–1290 K and pressure of 1–5 atm by utilizing a shock tube and OH laser diagnostic. To rationalize these chemical systems, variational transition state theory calculations with multi-structural torsional anharmonicity and small curvature tunneling corrections were run using a potential energy surface characterized at the UCCSD(T)/jun-cc-pVQZ//M06-2X/6-311++G(2df,2pd) level of theory. A good agreement was observed between the experiment and theory, with both predicting a non-Arrhenius behavior and negligible pressure effects. OH additions to the double bond of prenol and isoprenol were found to be important, with at least 50% contribution to the total rate constants even at temperatures as high as 700 and 2000 K, respectively. This behavior was attributed to the stabilizing effect induced by hydrogen bonding between the reacting OH radical and the OH functional group of isopentenols at the saddle points. These stabilizing intermolecular interactions help mitigate the entropic effects that hinder association reactions as temperature increases, thus extending the prominent role of addition pathways to high temperatures. The site-specific rate constants were also found to be slower than their analogous reactions of OH + n-alkenes.
  • Computational assessment of effects of throat diameter on combustion and turbulence characteristics in a pre-chamber engine

    Silva, Mickael Messias; Liu, Xinlei; Hlaing, Ponnya; Sanal, Sangeeth; Cenker, Emre; Chang, Junseok; Johansson, Bengt; Im, Hong G. (Applied Thermal Engineering, Elsevier BV, 2022-05-06) [Article]
    Towards fundamental investigation of key physical aspects of pre-chamber combustion, the current work utilizes computational fluid dynamics to comprehend the effect of the throat diameter in an engine operated with methane. Previous studies showed that this parameter is dominant in pressure build-up and flow pattern inside the pre-chamber, suggesting that a detailed characterization is necessary. This pre-chamber type is composed of an upper conical part that lodges the spark plug and fuel injector, followed by a straight and tubular region called throat, which tip accommodates the nozzles responsible for the charge exchange between pre and main chambers. Two types of pre-chamber having distinct throat diameters are investigated, while utilizing consistent experimental data for validation of the model. The combustion process is modeled with the G-Equation model; the laminar flame speed was tabulated from a methane oxidation mechanism reduced from the GRI 3.0; the turbulent flame speed was computed using Peters' relation. The simulations were run for a full cycle, starting at exhaust valve opening. A homogeneous charge of methane is considered at the intake port, maintaining a global λ = 1.8, while 3% of total energy fuel is added through the pre-chamber. The results show that the throat changes the flow field inside the pre-chamber, impacts the air-fuel ratio, stratification, turbulence, jet dynamics, and ultimately the pre and main chambers combustion processes and heat fluxes. The combustion regime according to the Borghi-Peters diagram were found to lay in the thin reaction zone and in the flamelet regime.
  • A New Method to Measure the Spatial Distribution of Pressure Oscillations in Engine Knock Using Optical Diagnostics

    Shi, Hao; Tang, Qinglong; Uddeen, Kalim; Magnotti, Gaetano; Turner, James W. G. (Elsevier BV, 2022-05-02) [Preprint]
    Engine knock is one of the major obstacles limiting the thermal efficiency of spark-ignition (SI) engines. The in-cylinder spatial distribution of the pressure oscillation is of great importance to investigate the knock initiation and development. Using multiple pressure sensors to detect the local pressure oscillations is expensive and has a low spatial resolution. This study proposes a new method to measure the pressure oscillation distribution by monitoring the fluctuations of the local natural flame luminosity (NFL) during engine knocking combustion. To validate this method, simultaneous six-point pressure measurements and high-speed NFL imaging are implemented on an optical engine. The results indicate that end-gas auto-ignition leads to local fluctuations of both pressure and natural flame intensity. The local NFL oscillation phasing is 0.5 crank angle degrees earlier than those of the pressure signals, possibly due to the different response delays of high-speed imaging and pressure data acquisition systems. After applying a time offset and amplitude normalization, the flame luminosity oscillations could reproduce the pressure oscillations with very similar phase and frequency spectra. Based on the six channels of pressure signals, the distributions of pressure oscillation amplitude in the cylinder are exhibited by a fitted two-dimensional contour; the pressure oscillation distribution can be well predicted by the six flame monitors at the same locations. The prediction accuracy is affected by the flame monitor size and the optimal radius is between 5 and 8 mm. More flame detectors can present more details of the knock-induced pressure oscillations. A proper monitor number can maintain the measurement accuracy while keeping an appropriate computation load. The proposed new method provides a non-intrusive way to measure the spatial distribution of the knock-induced pressure oscillations, which could be directly applied in optical engines or metal engines through optical fibers.
  • Experimental investigations on coherent flow structures in acoustically excited swirling flames using temporally-separated dual-plane Stereo-PIV

    ZHENG, Jianyi; Wang, Sirui; Yang, Zifeng; Li, Lei; Wang, Guoqing; Gao, Yi; Liu, Xunchen; Qi, Fei (Experimental Thermal and Fluid Science, Elsevier BV, 2022-04-30) [Article]
    A temporally-separated dual-plane stereoscopic particle image velocimetry (PIV) technique is utilized to investigate the dynamics of periodically-excited vortices in a swirling flame under four experimental conditions. The time-averaged results under the unexcited condition are analyzed to verify the accuracy of the method. For the excited conditions, Q criteria is used to extract and classify the vortex structures within the swirling flame. It is found that under different excitation frequencies of the same amplitude, the outer vortex rings (OVRs) and the inner vortex rings (IVRs) show opposite rotational characteristics. While the excitation frequency has little influence on the central vortices induced by the swirler, the amplitude does have a great influence on the development trajectory of the central vortices. Furthermore, the amplitude and phase of oscillation velocities in three directions are successfully decomposed by using sine curve fittings at each point. A periodic acceleration and deceleration process is observed only in the axial direction. Besides the acoustic wave, it is found that the vortex rings (both OVRs and IVRs) also contribute to the axial velocity oscillations.
  • Effects of non-thermal plasma on turbulent premixed flames of ammonia/air in a swirl combustor

    Kim, Gyeong Taek; Park, Jeong; Chung, Suk Ho; Yoo, Chun Sang (Fuel, Elsevier BV, 2022-04-29) [Article]
    The effects of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) reactor on the stability of turbulent lean and rich premixed ammonia/air flames and the NOx emission characteristics in a modified model gas turbine combustor are experimentally investigated by varying the applied AC voltage, , frequency, , and the mean mixture velocity, . Applying NTP to ammonia/air flames augments the flame stability such that the stable flame regime is extended to lower equivalence ratio, , under fuel-lean conditions or higher under fuel-rich conditions. NTP is found to significantly reduce the amount of NOx emission for both lean and rich premixed ammonia/air flames and the amount of NOx emission is well correlated with . The reduction of NOx emission through NH2 reactions is also identified by measuring NH chemiluminescence.
  • Accurate thermochemistry prediction of extensive Polycyclic aromatic hydrocarbons (PAHs) and relevant radicals

    Li, Yang; Wang, Tairan; Yalamanchi, Kiran K.; Kukkadapu, Goutham; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2022-04-26) [Article]
    Polycyclic aromatic hydrocarbons (PAHs) are important intermediates to soot formation in combustion. A reliable database of their thermochemistry is required for the development of chemical kinetic models describing the gas-phase chemistry of hydrocarbon fuels. In this study, temperature-dependent thermodynamic properties are consistently determined using high-accuracy quantum chemistry calculations for an extensive set of PAH compounds. The developed database comprehensively consists of 125 C6-C18 PAH molecules and radicals, which are important and commonly included in chemical mechanisms studies. At the M06-2X/6-311++G(d,p) level of theory, geometry optimizations, vibrational frequency calculations, and dihedral angle scans are performed for all PAH species. The G3 method, together with the atomization reaction approach, is selected to derive the average atomization formation enthalpy. This method produces the most accurate thermochemistry quantities for PAHs, as demonstrated in a previous study. The entropy and heat capacity values are calculated using statistical thermodynamics in MultiWell. These results exhibit good agreement with the databases in literature. To examine the application of the group additivity (GA) method for PAHs, the Bland−Altman plot, a statistical analysis approach, is employed to visualize the agreement between the results from the quantum chemical calculations and GA methods. Two GA methods are examined and significant differences are found, which indicates that GA values of relevant groups need to be further updated. The database of thermodynamic quantities developed in this study are of particular value in modeling studies and important for exploring mechanisms of the PAH growth.
  • On the Mechanism Responsible for Extreme Turbulence Intensity Generation in the Hi-Pilot Burner

    Boxx, Isaac G.; Skiba, Aaron W.; Carter, Campbell D.; Ceschin, Alberto; Pérez, Francisco E. Hernández; Im, Hong G. (Flow, Turbulence and Combustion, Springer Science and Business Media LLC, 2022-04-21) [Article]
    In this study, we apply particle image velocimetry (PIV), hot-wire anemometry (HWA), and large-eddy simulation (LES) to identify and characterize a key mechanism by which high-intensity turbulence measured in the “Hi-Pilot” burner is generated. Large-scale oscillation of the high-velocity jet core about its own mean axial centerline is identified as a dominant feature of the turbulent flow field produced by this piloted Bunsen burner. This oscillation is linked to unsteady flow separation along the expanding section of the reactant nozzle and appears stochastic in nature. It occurs over a range of frequencies (100–300 Hz) well below where the turbulent kinetic energy (TKE) spectrum begins to follow a – 5/3 power law and results in a flow with significant scale separation in the TKE spectrum. Although scale separation and intermittency are not unusual in turbulent flows, this insight should inform analysis and interpretation of previous, and future studies of this unique test case
  • A consistent soot nucleation model for improved prediction of strain rate sensitivity in ethylene/air counterflow flames

    Quadarella, Erica; Guo, Junjun; Im, Hong G. (Aerosol Science and Technology, Informa UK Limited, 2022-04-12) [Article]
    An improved consistent soot nucleation model was proposed and tested on ethylene counterflow flames at different strain rates. The main objective of the proposed model is to capture the correct strain rate sensitivity and broaden the applicability of the aerosol part of the soot model with different gas-phase kinetic mechanisms. Due to the uncertainties associated with experimental measurements of quantitative soot volume fraction (SVF), the approach’s effectiveness is mainly investigated on qualitative behavior in terms of strain rate sensitivity. Starting from a dimer-based nucleation model available in literature, soot inception is described as heterogeneous collisions of polycyclic aromatic hydrocarbons (PAHs) forming an intermediate dimer. Such a model justifies the inclusion of small precursors that conciliate a satisfactory reproduction of SVF sensitivity to strain rate, while retaining the quantitative accuracy of SVF prediction. The nucleation and condensation rates sensitivities are found to be regulated by the presence of the dimer to maintain the right balance with the upstream dimerization process. The heterogeneous collision model helps generalize the procedure and makes the model more adaptable to different kinetic mechanisms. Details about the inclusion of temperature-dependent sticking coefficients are also provided and found to be pivotal for a correct synergistic prediction of SVF trends and PAHs sensitivities to strain rate. The integration of important features in the soot nucleation model allows a generalized soot model free of empirical corrective factors, capturing the correct sensitivity to strain rates. Its ease of implementation and low computational cost make it suitable for turbulent flame simulations.
  • Double compression-expansion engine (DCEE) fueled with hydrogen: Preliminary computational assessment

    Babayev, Rafig; Andersson, Arne; Dalmau, Albert Serra; Im, Hong G.; Johansson, Bengt (Transportation Engineering, Elsevier BV, 2022-04-08) [Article]
    Hydrogen (H2) is currently a highly attractive fuel for internal combustion engines (ICEs) owing to the prospects of potentially near-zero emissions. However, the production emissions and cost of H2 fuel necessitate substantial improvements in ICE thermal efficiency. This work aims to investigate a potential implementation of H2 combustion in a highly efficient double compression-expansion engine (DCEE). DICI nonpremixed H2 combustion mode is used for its superior characteristics, as concluded in previous studies. The analysis is performed using a 1D GT-Power software package, where different variants of the DICI H2 and diesel combustion cycles, obtained experimentally and numerically (3D CFD) are imposed in the combustion cylinder of the DCEE. The results show that the low jet momentum, free jet mixing dominated variants of the DICI H2 combustion concept are preferred, owing to the lower heat transfer losses and relaxed requirements on the fuel injection system. Insulation of the expander and removal of the intercooling improve the engine efficiency by 1.3 and 0.5%-points, respectively, but the latter leads to elevated temperatures in the high-pressure tank, which makes the selection of its materials harder but allows the use of cheaper oxidation catalysts. The results also show that the DCEE performance is insensitive to combustion cylinder temperatures, making it potentially suitable for other high-octane fuels, such as methane, methanol, ammonia, etc. Finally, a brake thermal efficiency of 56% is achieved with H2 combustion, around 1%-point higher than with diesel. Further efficiency improvements are also possible with a fully optimized H2 combustion system.
  • Effects of DC Electric Fields on Flickering and Acoustic Oscillations of an M-shape Premixed Flame

    Xiong, Yuan; Lacoste, Deanna; Chung, Suk Ho; Cha, Min Suk (Flow, Turbulence and Combustion, Springer Science and Business Media LLC, 2022-04-07) [Article]
    This paper reports on the effect of DC electric fields on the dynamics of a premixed methane-air laminar flame, in a buoyant environment. DC electric fields can be capable of affecting both the buoyancy-driven flickering oscillation of the flame and the response of the flame to acoustic modulation of the flow. We conduct fast visualization of the emission of excited methylidyne radicals (CH*), representing the heat release rate of the flame. Such visualizations are also synchronized with electric current and voltage measurements. We notice that the suppression of buoyancy-driven flickering oscillations can be obtained by applying sub-critical negative DC voltages. Moreover, the current measured in the inter-electrodes area is analyzed for positive and negative DC applied voltages and we find that this quantity cannot be used as a tracer of heat release rate in a configuration where the flame location in the inter-electrodes gap varies with sub-critical electric fields. In addition, the effect of DC electric fields on the flame transfer function for acoustic modulation of the flow is reported and discussed.
  • Laser sensors for energy systems and process industries: Perspectives and directions

    Farooq, Aamir; Alquaity, Awad; Raza, Mohsin; Nasir, Ehson Fawad; Yao, Shunchun; Ren, Wei (Progress in Energy and Combustion Science, Elsevier BV, 2022-04-07) [Article]
    Sensors are perhaps the most important and integral components of our modern society. With global warming and environmental pollution garnering ever-increasing attention, as well as solutions for sustainabile and smart cities, the optimized performance of current and future energy systems and process industries is paramount. The accurate sensing and quantification of key parameters of such systems are essential for monitoring, controlling, and optimization efforts. In situ laser-based optical sensors are most suitable for achieving the desired characteristics of accuracy, sensitivity, selectivity, portability, speed, safety, and intelligence. In recent decades, significant progress has been made in the development and deployment of laser-based sensing solutions, although new challenges and opportunities continue to emerge rapidly. In this review paper, we focus on laser absorption spectroscopy (LAS)-based sensors owing to their simple architecture, easy implementation, and market penetration. We detail recent advancements made in LAS variants using new laser sources and techniques. A brief discussion on other laser-based sensing techniques, namely, photoacoustic spectroscopy, laser-induced fluorescence, coherent anti-Stokes Raman spectroscopy, and laser-induced breakdown spectroscopy, is provided to compare these strategies with LAS. The applications of laser-based sensors in various energy systemsincluding engines, turbines, power plants, furnaces, and boilers—as well as process industriessuch as petrochemical, semiconductor, natural gas leak detection, and corrosion detectionare presented, illustrating their many benefits and possible uses. A distinguishing aspect of this review paper is that we present the comparison of previous studies in tabular formats, making it easy to appreciate the recent progress in laser-based sensing solutions. Finally, suggestions on future directions and emerging technologies to pursue for the further enhancement, development, and deployment of laser-based sensors are proposed.
  • Large-Scale Thermochemistry Calculations for Combustion Models

    Yalamanchi, Kiran; Li, Yang; Wang, Tairan; Monge Palacios, Manuel; Sarathy, Mani (Elsevier BV, 2022-04-05) [Preprint]
    Accurate thermochemical properties for chemical species are of vital importance in combustion research. Empirical group additivity approaches are extensively used to generate thermochemistry data used in chemical kinetic models, but the accuracy is limited. In this work, we performed electronic structure calculations to determine reliable thermochemistry data for an extensive set of molecules that were taken from a large and well-established chemical kinetic model. The developed database consists of 1340 species that contain up to 18 and 5 carbon and oxygen atoms, respectively. The M06-2X/aug-cc-pVTZ level of theory was used for the geometry optimizations, vibrational frequency calculations, and dihedral angle scans. The potential energy of the different species was further refined with different composite methods, and the G3 method, together with the atomization reaction approach, was selected to calculate the enthalpy of formation at 0 K. This information was then used in statistical thermodynamics to calculate standard enthalpies of formation and entropy, as well as heat capacities at different temperatures. Our thermochemistry data exhibit good agreement with existing values in the literature, verifying the accuracy of our approach. The group additivity (GA) method is also examined based on the calculated values and significant differences are found, which indicates that GA values of relevant functional groups need to be updated. The database of thermochemistry quantities developed in this study is of particular interest not only for the update of GA values, but also to develop machine learning models for predicting the data of new species, which can assist in the development of combustion models. The impact of the developed dataset is illustrated by examining the variation in ignition delay times with the updated thermochemistry values.
  • Flexible, Air-Stable, High-Performance Heaters Based on Nanoscale-Thick Graphite Films

    Deokar, Geetanjali; Reguig, Abdeldjalil; Tripathi, Manoj; Buttner, Ulrich; Fina, Alberto; Dalton, Alan B.; Da Costa, Pedro M. F. J. (ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2022-03-31) [Article]
    Graphite sheets are known to exhibit remarkable performance in applications such as heating panels and critical elements of thermal management systems. Industrial-scale production of graphite films relies on high-temperature treatment of polymers or calendering of graphite flakes; however, these methods are limited to obtaining micrometer-scale thicknesses. Herein, we report the fabrication of a flexible and power-efficient cm$^{2}$-scaled heater based on a polycrystalline nanoscale-thick graphite film (NGF, ∼100 nm thick) grown by chemical vapor deposition. The stability of these NGF heaters (operational in air over the range 30-300 °C) is demonstrated by a 12-day continuous heating test, at 215 °C. The NGF exhibits a fast switching response and attains a steady peak temperature of 300 °C at a driving bias of 7.8 V (power density of 1.1 W/cm$^{2}$). This excellent heating performance is attributed to the structural characteristics of the NGF, which contains well-distributed wrinkles and micrometer-wide few-layer graphene domains (characterized using conductive imaging and finite element methods, respectively). The efficiency and flexibility of the NGF device are exemplified by externally heating a 2000 μm-thick Pyrex glass vial and bringing 5 mL of water to a temperature of 96 °C (at 2.4 W/cm$^{2}$). Overall, the NGF could become an excellent active material for ultrathin, flexible, and sustainable heating panels that operate at low power.
  • A Numerical Study on the Effect of a Pre-Chamber Initiated Turbulent Jet on Main Chamber Combustion

    Sanal, Sangeeth; Echeverri Marquez, Manuel Alejandro; Silva, Mickael; Cenker, Emre; Im, Hong G. (SAE International, 2022-03-29) [Conference Paper]
    To elucidate the complex characteristics of pre-chamber combustion engines, the interaction of the hot gas jets initiated by an active narrow throated pre-chamber with lean premixed CH4/air in a heavy-duty engine was studied computationally. A twelve-hole KAUST proprietary pre-chamber geometry was investigated using CONVERGE software. The KAUST pre-chamber has an upper conical part with the spark plug, and fuel injector, followed by a straight narrow region called the throat and nozzles connecting the chambers. The simulations were run for an entire cycle, starting at the previous cycle's exhaust valve opening (EVO). The SAGE combustion model was used with the chemistry modeled using a reduced methane oxidation mechanism based on GRI Mech 3.0, which was validated against in-house OH chemiluminescence data from the optical engine experiments. Two different piston geometries, a flat piston geometry, and a more realistic bowl piston geometry were studied to understand the influence of jet on main chamber combustion. Varying the piston geometries results in different free jet times and hence main chamber combustion characteristics. Pre-chamber fuel ratio (PCFR) 6#x00025; of the total amount of fuel was investigated while keeping the global excess air ratios (?) condition a constant value of 2.0. Both piston cases resulted in similar pre-chamber pressurization, with almost the same pre-chamber discharge and the equal pressure difference between pre-and main-chamber ( "P) at the start of jet ejection. Different combustion behaviors were observed on analysis of the heat release rate in the main chamber. The importance of turbulence generated by the pre-chamber-initiated jets was further studied. It was observed that free jet time is a critical factor in developing turbulence in the main chamber. This increase in turbulence helps in increasing the burning velocity causing faster combustion. The influence of the jet-piston interaction is also analyzed as that determines the combustion behavior in the later CAD.

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