Recent Submissions

  • Toward Flex-Fuel High Engine Efficiency Via Multiple Injection Strategies

    Aljohani, Bassam S. E.; Ben Houidi, Moez; AlRamadan, Abdullah; Cenker, Emre; Dyuisenakhmetov, Aibolat; Turner, James W. G.; Roberts, William L. (Elsevier BV, 2023-09-25) [Preprint]
    Multiple injection strategies enable control of heat release to attain constant-pressure combustion in compression ignition (CI) engines. A constant-pressure combustion allows to overcome the trade-off between improved thermal engine efficiency and heat transfer losses, particularly in high-pressure combustion. The primary objective of this study is to determine the optimal multiple injection strategies required to achieve isobaric combustion, considering fuels with varying properties in a heavy-duty CI engine. The implementation of optimized multiple injection strategies, adapted to fuel-specific and engine loads, leads to consistent improvement in the thermal engine efficiency with gasoline. In the case of diesel fuel, isobaric combustion is initiated with the first injection, regardless of the engine load. Conversely, fuels such as gasoline and isooctane require the assistance of spray-assisted jets to auto-ignite the fuel-air mixture. At lower engine loads, the first and second injections are strategically employed to establish a stratified in-cylinder condition. This condition promotes partially premixed combustion (PPC) and subsequently triggers the ignition of the subsequent injections, resulting in the achievement of isobaric combustion. At intermediate engine loads, ignition is triggered by the second injection, while at high engine loads, ignition is initiated with the first injection. At high engine loads, through the implementation of multiple injections, a mixing-controlled ignition regime can be achieved for all fuels, thereby ensuring engine operation flexibility in the face of various auto-ignition chemistry. Furthermore, this study analyzes the impact of the number of injections on different fuel reactivities, providing insights on the improvement of the isobaric combustion concept when using gasoline-like fuels as opposed to diesel fuel. The analysis demonstrates that gasoline-like fuels enhance the characteristics of the isobaric combustion concept and can achieve higher engine efficiency compared to conventional diesel fuel across all engine loads. Moreover, a high-pressure test rig was utilized to examine the rate of injection, to better understand the development of the rate of heat release and pollutant emissions based on the injection sequence. Overall, optimizing multiple injection strategies and employing fuels with properties similar to gasoline can enhance the isobaric combustion concept, leading to superior engine efficiency across a wide range of engine loads.
  • Optical Study of Active Narrow Throat Pre-Chamber Assisted Internal Combustion Engine at Lean Limit

    Sharma, Priybrat; Tang, Qinglong; Sampath, Ramgopal; Hlaing, Ponnya; Marquez, Manuel Echeverri; Cenker, Emre; Magnotti, Gaetano (Applications in Energy and Combustion Science, Elsevier BV, 2023-09-25) [Article]
    The pre-chamber assisted ignition offers several much-needed upgrades to gas engines, ranging from improved combustion efficiency and stability to extended lean limits. The concept has received intermittent attention from researchers over the past century, and the concept's fundamental understanding remains segmented. This study investigates pre-chamber assisted combustion (PCC) in a heavy-duty gas engine fueled by methane at the lean limits. The engine is operated at two lean limits at intake pressures of 1.2 and 1.4 bar. At lower intake pressure, global excess air ratio (λglobal) is 2.4, while at higher intake pressure λglobal is 2.6. The comparison of two lean limits through experimental data and GT-Power 1D model reveals the underlying ignition and combustion. Using a combination of acetone PLIF (N-PLIF) and OH* chemiluminescence imaging allows visualization of both the reacting and non-reacting part of the pre-chamber jet. The results suggest that pressure differential across the pre-chamber and main chamber controls the reacting jet growth speed. The combustion chamber boundaries affect the main combustion through the wall jet part of the impinging pre-chamber jets as higher OH* concentrations are observable at stagnation points of the jet. In addition, the study reports the appearance of post-combustion jets and dispersed OH* pockets as the combustion dwindles. The narrow throat pre-chamber shows a spectral pressure signature reminiscent of the Helmholtz oscillator, and circumferential resonant modes dominate the main chamber combustion. Although the PCC offers great ignitibility, the main chamber mixture cannot sustain prolonged combustion at a lean limit lambda value.
  • Effect of Gas Additives on Soot Formation in Flat Laminar Premixed Ethylene/Air Flames Under Low Pressure

    Algoraini, Safa; Zhao, Wanxia; Sun, Zhiwei; Dally, Bassam; Alwahabi, Zeyad (Elsevier BV, 2023-09-25) [Preprint]
    This study reports on the effect of the additive gases, namely N2, CO2, and Ar, on the soot formation in laminar premixed C2H4/air flames at low pressure of 40 kPa. The flames blended with N2, CO2, or Ar were experimentally investigated, where the emission spectra of CH* and C2*, qualitative concentration of PAHs, and soot volume fraction (fv) were measured using laser and optical diagnostic methods. The flame temperature was measured using a thermocouple. The results reveal that additive gases significantly influence flame height above the burner and fuel combustion as well as reduce soot formation and flame temperature. With respect to N2 and CO2, Ar proves the most effective in reducing soot volume fraction, achieving a 100-fold reduction compared to the reference flame. Moreover, the additive gases were found to delay the ignition, leading to a 5 mm downstream shift in soot inception. Despite the difference in the properties of three gas additives, it was found the first incepted soot particles were detected at a common temperature, Tinception, of 1515±70 K. Three regimes related to the soot appearance rate were identified, which are the fast increasing range from the soot inception location, the plateau region, and the decreasing range to the flame front of 20 mm. According to the Lagrangian time-derivative of soot volume fraction (dfv/dt) as a function of fv, the constant of surface growth (kSG) was determined to be 170 s-1 in the flame with additives of CO2 and Ar, measured at 40 kPa. The identified turning point between the plateau region and the negative soot increase regions can be used as an indicator of the transition from higher to lower soot formation rates in soot modelling.
  • Characteristics of ammonia-hydrogen nonpremixed bluff-body-stabilized flames

    Alfazazi, Adamu; Elbaz, Ayman M.; Li, Jiajun; Abdelwahid, Suliman; Im, Hong G.; Dally, Bassam (Combustion and Flame, Elsevier BV, 2023-09-23) [Article]
    The combustion of ammonia (NH3) has received much attention over the last few years due to challenges associated with its low reactivity and the emission of nitric oxides. One way to improve the reactivity of NH3 is to blend it with (H2/N2) mixture as the product of its dissociation, before introducing it into energy systems. In this study, experimental measurements were carried out on nonpremixed bluff-body stabilized flames to better understand the flame and emission characteristics of NH3/H2/N2 flames. Four fuel mixtures at different NH3 and (H2/N2) ratios were investigated to represent different levels of ammonia cracking. Photography, planar laser-induced fluorescence of OH, thermocouples, and gas analysis techniques were used to understand flame features, reaction zone characteristics, NO, and NH3 concentration within the flame and at the exhaust. It was observed that a decrease in NH3 ratio in the mixtures resulted in longer and more stable flame with reduced thermal radiation as compared to NH3-rich fuel blends. For the highest NH3 blend studied, the flames exhibit extinction and re-ignition in the neck zone, as evidenced by OH-planar images and temperature profiles. As the H2/N2 ratio in the fuel mixture is increased, while keeping the Re constant, the momentum flux ratio (jet/co-flow) also increased resulting in a fuel-lean recirculation zone (RZ), and a shift in the maximum temperature and OH region from the outer shear layer to the inner layer next to the central jet. At levels of NH3 in the fuel mixture above 50% by volume, unburned ammonia slips through the flame and into the exhaust, and the subsequent reburn mechanism resulted in reduced NO emission. CFD simulations using Reynolds-averaged Navier-Stokes (RANS) and the flamelet–progress-variable submodel were conducted and compared with the experimental results. The CFD results helped to qualitatively describe and further explain what was observed in the experiment including the flame appearance, mixing field, and the reaction zone location in the tested flames.
  • Oil Spill Risk Analysis For The NEOM Shoreline

    Mittal, HVR; Hammoud, Mohamad Abed ElRahman; Carrasco, Ana K.; Hoteit, Ibrahim; Knio, Omar (2023-09-21) [Preprint]
    A risk analysis is conducted considering an array of release sources located around the NEOM shoreline. The sources are selected close to the coast and in neighboring regions of high marine traffic. The evolution of oil spills released by these sources is simulated using the MOHID model, driven by validated, high-resolution met-ocean fields of the Red Sea. For each source, simulations are conducted over a 4-week period, starting from first, tenth and twentieth days of each month, covering five consecutive years. A total of 48 simulations are thus conducted for each source location, adequately reflecting the variability of met-ocean conditions in the region. The risk associated with each source is described in terms of amount of oil beached, and by the elapsed time required for the spilled oil to reach the NEOM coast, extending from the Gulf of Aqaba in the North to Duba in the South. To further characterize the impact of individual sources, a finer analysis is performed by segmenting the NEOM shoreline, based on important coastal development and installation sites. For each subregion, source and release event considered, a histogram of the amount of volume beached is generated, also classifying individual events in terms of the corresponding arrival times. In addition, for each subregion considered, an inverse analysis is conducted to identify regions of dependence of the cumulative risk, estimated using the collection of all sources and events considered. The transport of oil around the NEOM shorelines is promoted by chaotic circulations and northwest winds in summer, and a dominant cyclonic eddy in winter. Hence, spills originating from release sources located close to the NEOM shorelines are characterized by large monthly variations in arrival times, ranging from less than a week to more than two weeks. Similarly, large variations in the volume fraction of beached oil, ranging from less then 50\% to more than 80\% are reported. The results of this study provide key information regarding the location of dominant oil spill risk sources, the severity of the potential release events, as well as the time frames within which mitigation actions may need to deployed.
  • Simultaneous planar laser-induced fluorescence of nitric oxide and hydroxyl radical with a single dye laser in hydrogen flames

    Chatelain, Karl P.; Wang, Guoqing; Guiberti, Thibault (International Journal of Hydrogen Energy, Elsevier BV, 2023-09-20) [Article]
    Simultaneous, planar laser-induced fluorescence (PLIF) of nitric oxide (NO) and hydroxyl radical (OH) is evidenced with a single dye laser and two intensified CCD (ICCD) cameras. The technique is demonstrated on three premixed turbulent hydrogen-air flames, at atmospheric conditions, with several laser excitation wavelengths within the A2Σ+ − X2Π (0,0) and A2Σ+ − X2Π (1,0) vibrational bands of NO and OH, respectively. Via adjustments of the grating angle of the dye laser, the output wavelengths can be optimized so that both LIF signals are obtained simultaneously, or it can be tuned to reach the most common excitation wavelengths for OH-PLIF (i.e., near 283 or 284 nm) or NO-PLIF (i.e., 225 and 226 nm) alternatively. The flame visualization results, obtained with the different excitation strategies, are consistent for the three flame conditions and enable to characterize both the reaction zones and the nitric oxide formation. With appropriate selection of the wavelength pair, the technique shows its great potential to enhance the ability of existing lasers to simultaneously visualize and quantify distributions of two key species in reacting flows with only a single laser.
  • Effects of applied AC electric fields on flame spread over inclined polyethylene-insulated electrical wire with Cu-core

    Li, Zhisheng; Kim, Juhan; Kang, Minsung; Zhang, Yuchun; Park, Jeong; Chung, Suk Ho (Fire Safety Journal, Elsevier BV, 2023-09-19) [Article]
    Effects of applied electric fields on downward and upward flame spreads (DFS and UFS) over polyethylene (PE) insulated electrical wires with Cu-core are investigated by varying the applied voltage (VAC) and frequency (fAC) at various inclination angles (θ). With AC electric fields, FSRs are influenced appreciably by complex dynamic behaviors of molten-PE and flame. For the DFSs, the FSR with θ exhibits increasing, decreasing, and increasing trends. For the UFSs, the FSR with θ shows an increasing tendency, along with dependencies on VAC and fAC. Further analyses are conducted at θ = ±70°. For the DFSs, the globular molten-PE size, the distance between flame front and globular molten PE, and electrospray influence FSR. For the UFSs, downward flow of molten-PE in some cases increases FSR. While the strong downward flow causes a flame detachment from the main body of the flame and/or a merging phenomenon between molten-PE drops, reducing FSR. A serious of bulged flames due to electrospray increases FSR. These FSR behaviors for DFS and UFS are well characterized with their related physical parameters. Flame extinction occurs only for DFSs with NiCr- and Cu-core. The critical extinction frequency for DFSs is characterized with log (fAC,ext) = a × VAC + b.
  • Solution-processable poly(ether-ether-ketone) membranes for organic solvent nanofiltration: from dye separation to pharmaceutical purification

    Alqadhi, Nawader; Abdellah, Mohamed; Nematulloev, Sarvarkhodzha; Mohammed, Omar F.; Abdulhamid, Mahmoud A.; Szekely, Gyorgy (Separation and Purification Technology, Elsevier BV, 2023-09-15) [Article]
    Through polymer engineering, the membrane properties can be considerably changed and its performance can be improved. Organic solvent nanofiltration (OSN) membranes require polymers with good solution processability to facilitate membrane preparation. However, the resultant membranes should have excellent solvent resistance. Poly(ether-ether-ketone) (PEEK) is a potential polymer for OSN applications because of its high thermal stability and excellent solvent resistance. However, commercial PEEK has limited solution processability, and its fabrication requires a harsh acidic environment. Herein, two customized PEEKs were synthesized by incorporating methyl (–CH3) and sulfonyl (SO2) groups into the polymer backbone. The membranes were prepared by phase inversion using N-methyl-2-pyrrolidone (NMP) and TamiSolve as a green alternative. The effects of the polymer structure, green solvent, and crosslinking on the membrane morphology, chemical and mechanical stability, as well as separation performance have been examined. The molecular interaction between organic solvents and PEEKs were investigated through molecular dynamic simulations and density functional theory. The molecular weight cutoff (MWCO) values of the membranes were 540–768 g mol−1, with a high corresponding permeance of 8.2–20 L m−2 h−1 bar−1 in acetone. The long-term stability of membranes was successfully demonstrated over two weeks through a continuous crossflow filtration using acetone under a pressure of 30 bar. The membranes demonstrated excellent active pharmaceutical ingredient purification through the removal a 2-methoxyethoxymethyl chloride (125 g mol−1) carcinogenic impurity from roxithromycin (837 g mol−1).
  • Enhanced linearity through high-order antisymmetric vibration for MEMS DC power sensor

    zou, xuecui; Jaber, Nizar; Bu Khamsin, Abdullah; Yaqoob, Usman; Salama, Khaled N.; Fariborzi, Hossein (Applied Physics Letters, AIP Publishing, 2023-09-13) [Article]
    We present an electric power meter that capitalizes on the interaction of electrothermal strain and mechanical vibration in a micro-electro-mechanical systems (MEMS) beam undergoing the antisymmetric mode of vibration. This is achieved by using a resonant bridge driven with an electrothermal modulation technique. The change in electrical power is monitored through the alteration in the mechanical stiffness of the structure, which is tracked electrostatically. The observed deflection profile of the beam under the influence of electrothermal effects shows that the deflection geometry due to buckling exhibits similar trends as the first symmetric vibrational mode, in contrast to the antisymmetric mode. Therefore, we compare two distinct vibrational modes, converting the compressive thermal stress generated by the input electrical power via Joule heating into a shift in the resonance frequency. By employing antisymmetric vibrational mode, the output of our device is consistently monotonic to the input electrical power, even when the microbeam is experiencing buckling deflections. In addition, the sensing operation based on antisymmetric modes yields only a 1.5% nonlinear error in the response curve, which is ten times lower than that of symmetric modes. The observed deformation shape of the resonator agrees with the results obtained from multi-physics finite simulations. Finally, this approach has the potential to be extended to other frequency-shift-based sensors, allowing for higher linearity.
  • Downscaling using CDAnet under Observational and Model Noises: The Rayleigh-Benard Paradigm

    Hammoud, Mohamad Abed ElRahman; Titi, Edriss S; Hoteit, Ibrahim; Knio, Omar (2023-09-13) [Preprint]
    Efficient downscaling of large ensembles of coarse-scale information is crucial in several applications, such as oceanic and atmospheric modeling. The determining form map is a theoretical lifting function from the low-resolution solution trajectories of an infinite-dimensional dissipative dynamical system to their corresponding fine-scale counterparts. Recently, Hammoud et al. (2022b) introduced CDAnet a physics-informed deep neural network as a surrogate of the determining form map for efficient downscaling. CDAnet was demonstrated to efficiently downscale noise-free coarse-scale data in a deterministic setting. Herein, the performance of well-trained CDAnet models is analyzed in a stochastic setting involving (i) observational noise, (ii) model noise, and (iii) a combination of observational and model noises. The analysis is performed employing the Rayleigh-Bénard convection paradigm, under three training conditions, namely, training with perfect, noisy, or downscaled data. The effects of observational and model noise on the CDAnet downscaled solutions are analyzed. Furthermore, the effects of the Rayleigh number and the spatial and temporal resolutions of the coarse-scale information on the downscaled fields are examined. The results suggest that the expected l 2 -error of CDAnet behaves quadratically in terms of the standard deviations of the observational and model noises. The results also suggest that CDAnet responds to uncertainties similar to CDA with an additional error overhead due to CDAnet being a surrogate model of the determining form map.
  • Performance studies of Pt, Pd and PtPd supported on SBA-15 for wet CO and hydrocarbon oxidation

    Khan, Hassnain Abbas; Abou Daher, Mohamad; de Freitas, Ana Luiza S.; Subburaj, Janardhanraj; Tall, Omar EI; Farooq, Aamir (Catalysis Today, Elsevier BV, 2023-09-12) [Article]
    Climate change is one of the most pressing issues confronting modern society due to the greenhouse gas emissions. Reducing anthropogenic greenhouse gas emissions is critical to preventing further global warming. This work explores the oxidation of methane, propane and ethylene in the presence of carbon monoxide (CO) and water vapors over Pt and Pd mono and bimetallic catalysts supported on SBA-15. Pt and Pd catalysts with uniform distribution are prepared on high surface area SBA-15 support using different precursors and solvents. The performance of catalysts is evaluated as a function of temperature and time. The inhibitory effect of water vapors and co-feeding of carbon monoxide is studied on the light-off temperature and stability of Pt, Pd and bimetallic catalysts. We performed in-depth characterization using XRD, BET, XPS, TEM, CO chemisorption and ICP-OES techniques to correlate the properties of catalysts with oxidation results. In terms of their oxidation activity, Pd exhibited higher activity towards alkanes (methane and propane) at low temperatures, while Pt is more active for ethylene. Carbon monoxide undergoes oxidation at the lowest temperature over both Pt and Pd, followed by the oxidation of all other components in the feed. It is confirmed that PdO remains highly active when reductants are only alkanes. However, the coexistence of M/MOx ionic species (Pt0, Pt2+, Pd0, Pd2+) appears be to more active when mixed feed is used. Optimal alloying compositions have the potential to minimize the decrease in conversion activity in the presence of CO and water vapors. This work is expected to advance the rational synthesis and application of multicomponent and multifunctional catalytic materials.
  • Shock Tube/Laser Absorption Measurements of the High-Temperature Spectra and Decomposition of Propyl Ethers.

    Adil, Mohammad; Farooq, Aamir (The journal of physical chemistry. A, 2023-09-10) [Article]
    This work presents measurements of temperature-dependent absorption spectra and thermal decomposition rates of propyl ethers, specifically di-n-propyl ether (DnPE) and diisopropyl ether (DiPE), which are two renewable fuel candidates. We employed a broadband rapid-tuning MIRcat-QT laser, operating in the scan/fixed-wavelength mode in combination with a shock tube. Spectral measurements were performed over the wavelength range of 8.4–11 μm (909.1–1190.5 cm–1), covering the strongest infrared absorption bands of the studied ethers, at temperatures of 559–853 K and pressure near 1 bar. These high-temperature spectra help in selecting the optimum wavelength for sensitive and selective measurements of the target ethers. Based on the criteria of high sensitivity, minimum interference, and insensitivity to temperature and pressure variations, we selected a wavelength of 1121.82 cm–1 for high-temperature diagnostics of DnPE and DiPE. Absorption cross sections at the selected wavelength of 1121.82 cm–1 were measured over 550–1500 K, and pressures ranging from 0.3–1.4 bar. This diagnostic was then applied to study the high-temperature pyrolysis of these ethers by measuring their time histories behind the reflected shock waves. Our experimentally measured overall decomposition rate coefficients for DnPE and DiPE are given as (unit of s–1) kDnPE = 1.25 × 1027 × T–3.483 × exp(−37620 K/T) and kDiPE = 5.26 × 1023 × T–2.857 × exp(−32360 K/T).
  • An updated functional-group-based approach to modeling the vacuum residue oil gasification kinetics

    Zhang, Xiaoyuan; Khandavilli, Muralikrishna; Gautam, Ribhu; AlAbbad, Mohammed A.; Li, Yang; Chatakonda, Obulesu; Kloosterman, Jeffrey W.; Middaugh, Joshua; Sarathy, Mani (Fuel, Elsevier BV, 2023-09-09) [Article]
    Gasification of heavy petroleum residues can convert low-value feedstock to hydrogen-rich syngas, which can further be used for power generation and/or chemical production. The present work proposed an updated functional-group-based approach (FGMech) to modeling the gasification of a vacuum residue oil (VRO), which is helpful for better understanding the detailed kinetics of the gasification and improving gasifier performance. Elemental, average molecular weight (AMW) and nuclear magnetic resonance (NMR) analyses were conducted experimentally to characterize VRO, including elemental composition, average molecular formula and the functional group distribution, which were further used for model construction. A lumped mechanism for VRO devolatilization was constructed based on the updated FGMech approach: stoichiometric parameters of stable gases, tars and char were obtained from experiments, while those of radicals were based on the multiple linear regression (MLR) correlations; thermodynamic and kinetic parameters were derived from Benson group additivity method and rate rules, respectively. A merged detailed model was adopted for describing the conversion of gases and tars, and a global model was used for char. To test the reliability of the present model approach, Orimulsion gasification experiments from literature were simulated using an integrated perfectly stirred reactor (PSR) and plug flow reactor (PFR) model. It shows that the present model can reasonably predict measured results under various equivalence ratios, and has better performance on the prediction of CH4 compared with most literature models. Based on model analyses, syngas comes from the conversion of C2H4, H2S, CH4 and char in different gasification stages. Benzene, toluene, naphthalene and 1-methylnaphthalene are initial tar species considered in the devolatilization of the VRO. They can undergo hydrogen abstraction acetylene addition (HACA) and C3H3/C5H5 addition reaction pathways to produce large PAHs.
  • Experimental estimation of turbulence modulation in droplet-laden two-phase jet

    Wu, Hao; Zhang, Zhenyu; Zhang, Fujun; Wu, Kun; Roberts, William L. (Physical Review Fluids, American Physical Society (APS), 2023-09-05) [Article]
    The effect of liquid droplets generated from air-assisted atomization on gas flow characteristics was studied experimentally. A phase/Doppler particle analyzer was used to measure velocity and size distributions of continuous and dispersed phases in the droplet-laden two-phase flow. A comparison of mean gas velocity with and without droplets indicates the expected influence of dispersed phase on the carrier phase, i.e., two-way coupling. The flow characterization result shows the presence of liquid droplets contributes to the increase of gas-phase flow velocity in the spray field. The effect of liquid droplets on gas-phase turbulence is manifested in three ways. First, the presence of droplets leads to the increase in fluctuation velocity of gas-phase flow. Subsequently, it is observed that the range of fluctuation velocities in the gas phase is expanded in two-phase flow compared with single-phase flow. In the region characterized by a steep velocity gradient, the initial gas fluctuation velocities in two-phase flow demonstrate a notable enhancement of 20% compared with single-phase flow. Furthermore, the presence of droplets induces axial stretching within the shear region of the gas phase, and this stretching effect is particularly pronounced in cases of higher fuel-injection durations, primarily due to the influence of droplet gravity. The data obtained from the analysis of velocity gradient and fluctuation velocity within the two-phase flow field reveal a distinct segmental linear relationship, deviating from previous findings reported in the literature and highlighting a deeper understanding of the underlying mechanisms in current two-phase flow systems.
  • Lagrangian Analysis of Droplet Dynamics Using Computational Singular Perturbation

    Angelilli, Lorenzo; Malpica Galassi, Riccardo; Ciottoli, Pietro Paolo; Hernandez, Francisco; Valorani, Mauro; Im, Hong G. (Elsevier BV, 2023-09-05) [Preprint]
    Computational singular perturbation (CSP) has been successfully used in the analysis of complex chemically reacting flows by systematically identifying the intrinsic timescales and slow invariant manifolds that capture the essential subprocesses driving the dynamics of the system. In this article, the analytical and computational framework is applied for the first time to analyze the Lagrangian droplets undergoing evaporation and dispersion in the surrounding gases. First, a rigorous mathematical formulation is derived to adapt the CSP tools into the droplet dynamics equations, including the formal definition of the tangential stretching rate (TSR) that represents the explosive/dissipative nature of the system. Canonical case studies are then conducted to demonstrate the utility of the CSP methodology in identifying various physical mechanisms driving the evolution of the system, such as the distinction of thermal-driven and mass-driven regimes. Various definitions of the importance indices are also examined to provide in-depth analysis of different subprocesses and their interactions in modifying the droplet dynamics.
  • Effects of Ammonia Substitution in the Fuel Stream and Exhaust Gas Recirculation on Extinction Limits of Non-premixed Methane– and Ethylene–Air Counterflow Flames

    Chu, Carson Noel; Scialabba, Gandolfo; Liu, Peng; Serrano-Bayona, Raul; Aydin, Faruk; Pitsch, Heinz; Roberts, William L. (Energy & Fuels, American Chemical Society (ACS), 2023-09-04) [Article]
    The global extinction limits of non-premixed nitrogen/ammonia-substituted methane– and ethylene–air counterflow flames were experimentally evaluated. In comparison to nitrogen substitution, ammonia substitution reduced the extinction strain rates more. Measurements of OH* chemiluminescence, of which the intensity correlates with extinction limits, suggest that ammonia substitution reduces OH* production. The effects of transport, thermal and chemical properties on flame extinction of the ammonia-substituted flames were assessed, and it was found that their lower extinction limits were due to reactions that consume radicals, which hinder the chain-branching reactions. To mimic the effect of exhaust gas recirculation on the extinction limits of ammonia-substituted flames, carbon dioxide was added to the oxidizer stream. Lower extinction limits were observed with carbon dioxide addition as a result of thermal and chemical effects. Carbon dioxide addition lowered flame temperatures and, like ammonia substitution, introduced reactions that consume radicals. Nitric oxide (NO) production was quantitatively analyzed by simulations. It was found that, for ammonia flames, NO production was promoted by ammonia oxidation with OH, whereas for carbon dioxide addition, NO production was suppressed by the reduction of OH production.
  • Mini Review of Ammonia for Power and Propulsion: Advances and Perspectives

    Guiberti, Thibault; Pezzella, Giuseppe; Hayakawa, Akihiro; Sarathy, Mani (Energy & Fuels, American Chemical Society (ACS), 2023-09-01) [Article]
    Ammonia is a molecule that has been essential to human activities for centuries. It is widely used as a feedstock for fertilizers, industrial chemicals, and emissions after-treatment systems. The properties of ammonia have led to its interest as a carrier for hydrogen in energy applications. The combustion of ammonia for power and propulsion offers direct applications of this molecule in energy and transportation applications. However, there are significant challenges related to ammonia combustion, including low flammability and potentially high emissions. Blending of ammonia with hydrogen or hydrocarbons offers opportunities to improve combustibility. This mini review discusses challenges related to ammonia combustion and current state-of-the-art approaches to overcoming these challenges with research into chemical kinetics, laminar and turbulent flames, and engine and turbine systems. This paper seeks to introduce and summarize recent results on ammonia combustion by highlighting pertinent aspects of this rich and rapidly increasing body of information.
  • Synergistic effect of non-thermal plasma and CH4 addition on turbulent NH3/air premixed flames in a swirl combustor

    Kim, Gyeong Taek; Park, Jeong; Chung, Suk Ho; Yoo, Chun Sang (International Journal of Hydrogen Energy, Elsevier BV, 2023-09-01) [Article]
    The synergistic effect of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) and CH4 addition on turbulent swirl-stabilized NH3/air premixed flames in a laboratory-scale gas turbine combustor is experimentally investigated by varying the mixture equivalence ratio, φ, the mixt velocity, U0, and the mole fraction of CH4 in the fuel, Xf,CH4. It is found that the streamer intensity is significantly increased by adding CH4 to NH3/air flames compared with that by adding H2. This is because positive ions generated by CH4 addition play a critical role in generating streamers. Such streamers intensified by CH4 addition enhance the ammonia combustion more together with CH4, and hence, the lean blowout (LBO) limits of NH3/CH4/air flames are significantly extended compared with those without applying NTP. The maximum streamer intensity is found to be linearly proportional to φ⋅Xf,CH4⋅U0 in wide ranges of φ, Xf,CH4, and U0. NTP is also found to significantly reduce the amount of NOx and CO emissions simultaneously. All of the results suggest that NTP can be used more effectively with CH4 addition to stabilize turbulent premixed NH3/air flames and reduce NOx/CO emissions, which is attributed to their synergistic effect on the ammonia combustion.
  • Learning to Read Analog Gauges from Synthetic Data

    Leon-Alcazar, Juan; Alnumay, Yazeed; Zheng, Cheng; Trigui, Hassane Ahmed; Patel, Sahejad; Ghanem, Bernard (arXiv, 2023-08-28) [Preprint]
    Manually reading and logging gauge data is time inefficient, and the effort increases according to the number of gauges available. We present a computer vision pipeline that automates the reading of analog gauges. We propose a two-stage CNN pipeline that identifies the key structural components of an analog gauge and outputs an angular reading. To facilitate the training of our approach, a synthetic dataset is generated thus obtaining a set of realistic analog gauges with their corresponding annotation. To validate our proposal, an additional real-world dataset was collected with 4.813 manually curated images. When compared against state-of-the-art methodologies, our method shows a significant improvement of 4.55 in the average error, which is a 52% relative improvement.
  • Effects of Engine Speed on Prechamber-Assisted Combustion

    Palombi, Lucia; Sharma, Priybrat; Cenker, Emre; Magnotti, Gaetano (SAE International, 2023-08-28) [Conference Paper]
    Lean combustion technologies show promise for improving engine efficiency and reducing emissions. Among these technologies, prechamber-assisted combustion (PCC) is established as a reliable option for achieving lean or ultra-lean combustion. In this study, the effect of engine speed on PCC was investigated in a naturally aspirated heavy-duty optical engine: a comparison has been made between analytical performances and optical flame behavior. Bottom view natural flame luminosity (NFL) imaging was used to observe the combustion process. The prechamber was fueled with methane, while the main chamber was fueled with methanol. The engine speed was varied at 1000, 1100, and 1200 revolutions per minute (rpm). The combustion in the prechamber is not affected by changes in engine speed. However, the heat release rate (HRR) in the main chamber changed from two distinct stages with a faster first stage to more gradual and merged stages as the engine speed increased. NFL imaging revealed that lower mean piston speed allowed for longer free jet propagation inside the combustion chamber resulting in faster and stronger HRR stages. At higher speeds, the jet-piston interaction started earlier and was dispersed in radial directions, resulting in a relatively prolonged HRR. Finally, the study emphasizes the importance of prechamber jet and piston interaction in shaping HRR.

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