Recent Submissions

  • Investigation of soot sensitivity to strain rate in ethylene counterflow soot formation oxidation flames

    Quadarella, Erica; Li, Zepeng; Guo, Junjun; Roberts, William L.; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2022-11-29) [Article]
    Soot sensitivity to strain rate is mainly responsible for soot formation intermittence in practical combustion devices. This work provides a fundamental study on soot formation in Soot Formation Oxidation (SFO) counterflow flames at varying strain rates. While the problem has been extensively studied in Soot Formation (SF) configurations, where the dominant process is nucleation, investigations remain scarce in the corresponding SFO cases. In the latter, the high temperatures and strong oxidative environments make the surface reactions prevail over nucleation. The work provides a new dataset for ethylene SFO flames in a wide range of strain rates and sheds light on the main processes concurring in determining soot strain rate sensitivity in such conditions. In particular, the peak of soot volume fraction (SVF) is primarily controlled by surface growth and oxidation. The latter becomes progressively more dominant on the side of the SVF distribution toward the oxidizer nozzle, where the presence of oxidizing agents is significant. The soot mechanism adopted predicts a SVF distribution and sensitivity to strain rate in agreement with experimental data. The latter is found similar to corresponding SF cases, although soot loads in the two configurations differ by almost an order magnitude, and the SVF sensitivity is known to be more accentuated for lower soot loads. A deeper investigation revealed that the nucleation process through dimerizations primarily controls the SVF sensitivity, providing the onset of soot necessary for further growth. Then, the latter tends to reduce SVF sensitivity depending on its impact. PAH sensitivities mostly agree with theoretical observation even though further validations on the kinetic mechanism are needed to improve its predictions in lean conditions. The simplistic yet effective model based on the hybrid method of moments and the employment of a reduced kinetic mechanism makes the approach amenable for turbulent computational fluid dynamic (CFD) simulations.
  • Comparative Study of Spark-Ignited and Pre-Chamber Hydrogen-Fueled Engine: A Computational Approach

    Aljabri, Hammam H.; Silva, Mickael Messias; Ben Houidi, Moez; Liu, Xinlei; Al-lehaibi, Moaz; Almatrafi, Fahad A.; AlRamadan, Abdullah S.; Mohan, Balaji; Cenker, Emre; Im, Hong G. (Energies, MDPI AG, 2022-11-26) [Article]
    Hydrogen is a promising future fuel to enable the transition of transportation sector toward carbon neutrality. The direct utilization of H2 in internal combustion engines (ICEs) faces three major challenges: high NOx emissions, severe pressure rise rates, and pre-ignition at mid to high loads. In this study, the potential of H2 combustion in a truck-size engine operated in spark ignition (SI) and pre-chamber (PC) mode was investigated. To mitigate the high pressure rise rate with the SI configuration, the effects of three primary parameters on the engine combustion performance and NOx emissions were evaluated, including the compression ratio (CR), the air–fuel ratio, and the spark timing. In the simulations, the severity of the pressure rise was evaluated based on the maximum pressure rise rate (MPRR). Lower compression ratios were assessed as a means to mitigate the auto-ignition while enabling a wider range of engine operation. The study showed that by lowering CR from 16.5:1 to 12.5:1, an indicated thermal efficiency of 47.5% can be achieved at 9.4 bar indicated mean effective pressure (IMEP) conditions. Aiming to restrain the auto-ignition while maintaining good efficiency, growth in λ was examined under different CRs. The simulated data suggested that higher CRs require a higher λ, and due to practical limitations of the boosting system, λ at 4.0 was set as the limit. At a fixed spark timing, using a CR of 13.5 combined with λ at 3.33 resulted in an indicated thermal efficiency of 48.6%. It was found that under such lean conditions, the exhaust losses were high. Thus, advancing the spark time was assessed as a possible solution. The results demonstrated the advantages of advancing the spark time where an indicated thermal efficiency exceeding 50% was achieved while maintaining a very low NOx level. Finally, the optimized case in the SI mode was used to investigate the effect of using the PC. For the current design of the PC, the results indicated that even though the mixture is lean, the flame speed of H2 is sufficiently high to burn the lean charge without using a PC. In addition, the PC design used in the current work induced a high MPRR inside the PC and MC, leading to an increased tendency to engine knock. The operation with PC also increased the heat transfer losses in the MC, leading to lower thermal efficiency compared to the SI mode. Consequently, the PC combustion mode needs further optimizations to be employed in hydrogen engine applications.
  • A Computational Assessment of Combustion Submodels for Predictive Simulations of Pre-Chamber Combustion Engines

    Silva, Mickael; Liu, Xinlei; Hlaing, Ponnya; Cenker, Emre; Turner, James W. G.; Im, Hong G. (American Society of Mechanical Engineers, 2022-11-23) [Conference Paper]
    Pre-chamber combustion (PCC) modeling has been progressing in recent years, while there are lingering questions on fundamental modeling aspects, whether a flame-based or an ignition-based model predicts the combustion with higher fidelity. This mode of ignition concept is known to enable a stable engine operation at ultralean conditions with a short combustion duration, thus enhancing engine efficiency. The current work utilizes computational fluid dynamics to assess well-known combustion models: multi-zone well-stirred reactor (MZ-WSR) and G-Equation. The former models combustion as an ignition-based phenomenon while the latter as a flame propagation type of combustion. A pre-chamber containing twelve nozzles divided into two layers on a narrow throat was chosen. The jets from the two layers of nozzles and the local thermodynamic conditions differ substantially, which makes it a suitable configuration for assessing the predictive capabilities of distinct combustion models. The fuel utilized was methane and the global air-fuel ratio (λ) was varied, ranging from global-λ of 1.6, 1.8, and 2.0, and the total fuel injected through the pre-chamber was varied for one of the cases (3%, 7%, and 13%). The results suggest that both combustion models can potentially match experimental engine performance data upon appropriate calibration; however, fundamental differences in jet topology arise since the G-Equation formulation accounts for turbulence-chemistry interaction, while MZ-WSR does not.
  • Direct Injection Strategy to Extend the Lean Limit of a Passive Pre-Chamber

    Almatrafi, Fahad A.; Uddeen, Kalim; Ben Houidi, Moez; Cenker, Emre; Turner, James W. G. (American Society of Mechanical Engineers, 2022-11-23) [Conference Paper]
    Lean operation increases the efficiency of the Otto-cycle internal combustion engine and decreases its emissions. However, increasing the air-fuel ratio beyond stoichiometry requires higher ignition energy to maintain the stable operation of the engine. The pre-chamber emerges as one of the promising enablers of lean operation, providing much larger energy into the main combustion chamber than simple a spark plug at multiple sites to increase combustion stability. Pre-chambers are classified into two categories based on their fuel input; active pre-chambers, with a dedicated fuel injection system, and passive pre-chambers, which are solely charged with the main chamber air-fuel mixture through nozzle holes. Therefore, the passive pre-chamber type is favorable for existing engines because of its compact design and limited modification requirements. Nevertheless, passive pre-chambers have issues with igniting very lean mixtures. In this study, a single-cylinder light-duty engine is used to study the possibility of extending the lean limit of the passive pre-chamber using a split direct injection (DI) strategy and indirect enrichment of the pre-chamber mixture. The results of the split injection method were then compared to port fuel injection (PFI) measurements. Also, another set of experiments was performed with a standard spark plug using PFI and split DI for comparison. The results showed an increase in the lean limit of passive pre-chamber operation when using the split DI strategy compared to PFI, from λ = 1.5 to 1.7. However, increased soot production was observed when using the split injection strategy.
  • Counterflow flame extinction of ammonia and its blends with hydrogen and C1-C3 hydrocarbons

    Alfazazi, Adamu; Es-sebbar, Et-touhami; Zhang, Xiaoyuan; Dally, Bassam; Abdullah, Marwan; Younes, Mourad; Sarathy, Mani (Applications in Energy and Combustion Science, Elsevier BV, 2022-11-19) [Article]
    Ammonia as a fuel offers the potential to avoid carbon emissions, but its combustion is hindered by low reactivity. Here, the extinction limits of NH3 and NH3 plus reactivity enhancers were measured in the counterflow laminar non-premixed flames. A stable NH3-N2 flame was established with an oxygen-enriched oxidizer stream, and when the fuel was blended with CH4, C2H6, C3H8, and H2. For blended mixtures, results showed that CH4 has the least potential to enhance the stability of NH3 flames compared to the other additives. The extinction limits of C2H6 and C3H8 blended NH3 flames are nearly identical. At low percentage addition, H2-blended flames extinguish earlier than those blended with C1-C3 hydrocarbons, but this trend is reversed at higher H2 blends. Experimental conditions were simulated using Okafor et al. 2018 model and an extended Zhang et al 2021 model developed here. The models captured the measured trends, including the crossover between NH3-H2 and NH3-C2/C3 hydrocarbon fuels. Quantitatively, both models under-predicted the extinction limits of NH3-N2/enriched oxidizer flame. Better quantitative agreement is observed for the blended fuels using the model developed here. Discrepancies have been observed in the reported rates for reactions involving HNO (+OH, H), and if addressed, could improve models' capability in predicting extinction behavior in non-premixed flames. Numerical analyses were carried out to understand the kinetic coupling between NH3 and H2/C2-C3 in counter-flow flames. Extinction limits of NH3-C2-C3/H2 flames are shown to be affected by H abstraction and NH3 related chain termination reactions, heat producing reactions, and chain branching reactions. It has also been observed that at high blending ratios, C2H6/C3H8 addition in NH3 flames reduced the peak H and OH concentration via recombination and termination reactions, which compete with branching pathways. H2-blended flames are mostly influenced by reactions producing active radicals.
  • An experimental and modeling study of tetramethyl ethylene pyrolysis with polycyclic aromatic hydrocarbon formation

    Nagaraja, Shashank S.; Liang, Jinhu; Liu, Bingzhi; Xu, Qiang; Shao, Can; Kukkadapu, Goutham; Lu, Haitao; Wang, Zhandong; Pitz, Willam J.; Sarathy, Mani; Curran, Henry J. (Proceedings of the Combustion Institute, Elsevier BV, 2022-11-16) [Article]
    Iso-olefins, in the C5–C8 range can potentially be blended with renewable gasoline fuels to increase their research octane number (RON) and octane sensitivity (S). RON and S increase with the degree of branching in iso-olefins and this is a desirable fuel anti-knock quality in modern spark-ignited direct-injection engines. However, these iso-olefins tend to form larger concentrations of aromatic species leading to the formation of polycyclic aromatic hydrocarbons (PAHs). Thus, it is important to understand the pyrolysis chemistry of these iso-olefins. In this study, a new detailed chemical kinetic mechanism is developed to describe the pyrolysis of tetramethyl ethylene (TME), a symmetric iso-olefin. The mechanism, which includes the formation of PAHs, is validated against species versus temperature (700–1160 K) measurements in a jet-stirred reactor at atmospheric pressure and in a single-pulse shock tube at a pressure of 5 bar in the temperature range 1150–1600 K. Synchrotron vacuum ultraviolet photoionization mass spectrometer (SVUV-PIMS) and gas chromatography (GC) systems were used to quantify the species in the jet-stirred reactor and in the single-pulse shock tube, respectively. The mechanism derives its base and PAH chemistry from the LLNL PAH sub-mechanism. The predictions are accurate for most of the species measured in both facilities. However, there is scope for mechanism improvement by understanding the consumption pathways for some of the intermediate species such as isoprene. The formation of 1, 2, and 3-ring aromatic species such as benzene, toluene, naphthalene and phenanthrene measured experimentally is analyzed using the chemical kinetic mechanism. It is found that the PAH formation chemistry for TME under pyrolysis conditions is driven by both propargyl addition reactions and the HACA mechanism.
  • Turbulent flame speed of NH3/CH4/H2/H2O/air-mixtures: Effects of elevated pressure and Lewis number

    Wang, Shixing; Elbaz, Ayman M.; Wang, Guoqing; Wang, Zhihua; Roberts, William L. (Combustion and Flame, Elsevier BV, 2022-11-13) [Article]
    This study investigated the turbulent flame speed (ST) of NH3/CH4/H2/air mixtures subjected to differential-diffusion effect characterized by sub-unity Lewis number (Le) at elevated pressures (1 and 5 atm), temperature (373 K), lean equivalence ratios (ϕ = 0.6–0.91) with and without 10% volumetric H2O dilution. The experiments were conducted in a fan-stirred constant volume combustion vessel, with a homogeneous turbulence intensity of u′ = 0 - 2.34 m/s. Schlieren imaging was used to derive the instantaneous turbulent flame speed. Morphology analysis shows that at atmospheric-pressure conditions, the laminar diffusional-thermal unstable flame surface is subjected to cellular wrinkling, this instability is increased at elevated-pressure conditions. Whereas, external turbulence will aggravate the wrinkling but the increase in turbulence intensity suppressed differential-diffusion effects on the flame structure. Flame speed development at sub-unity Le cases show differential diffusion effect and propagates faster than unity Le cases even though they have similar laminar flame speed. Then, the turbulent spherical flame speeds are scaled with a power-law fitting of flame Reynolds number, ReT,flame. The normalized turbulent propagation speed (ST/SL) is much higher for the sub-unity Le flames but it decreases as hydrogen mole fraction (XH2) increases in the fuel mixture. This was explained by the synergistic effect of differential diffusion and turbulence. Water dilution and elevated pressure via increase of Karlovitz number (Ka) and turbulent ReT,flow affect the ST/SL increasing. At sufficiently low Damköhler numbers, Da, a ratio of normalized turbulent flame speed ST/SL is mainly controlled by ReT and scales as a half power law, in line with the classical hypothesis by Damköhler. Additional turbulent stretch modification with Ka or Da can correlate with different hydrogen content conditions, this is because the independent pair of scaling parameters from a set of (u′/SL, lT/lf, Da, Ka, ReT) is considered. The scaling parameter ((u′/SL)α(lT/lf)β(Le)γ) is insensitive to Lewis number, because of the dominance of relative turbulent intensity and turbulent length scale.
  • Experimental and modelling study of hydrogen ignition in CO2 bath gas

    Harman-Thomas, James M.; Kashif, Touqeer Anwar; Hughes, Kevin J.; Pourkashanian, Mohamed; Farooq, Aamir (Fuel, Elsevier BV, 2022-11-13) [Article]
    Direct-fired supercritical CO2 power cycles, operating on natural gas or syngas, have been proposed as future energy technologies with 100 % carbon capture at a price competitive with existing fossil fuel technologies. Likewise, blue or green hydrogen may be used for power generation to counter the intermittency of renewable power technologies. In this work, ignition delay times (IDTs) of hydrogen were measured in a high concentration of CO2 bath gas over 1050 – 1300 K and pressures between 20 and 40 bar. Measured datasets were compared with chemical kinetic simulations using AramcoMech 2.0 and the University of Sheffield supercritical CO2 (UoS sCO2 2.0) chemical kinetic mechanisms. The UoS sCO2 2.0 mechanism was recently developed to model IDTs of methane, hydrogen, and syngas in CO2 bath gas. Sensitivity analyses were used to identify important reactions and to illustrate the trends observed among various datasets. The performance of both mechanisms was evaluated quantitatively by comparing the average absolute error between the predicted and experimental IDTs, which showed UoS sCO2 2.0 as the superior mechanism for modelling hydrogen IDTs in CO2 bath gas. The importance of OH time-histories is identified as the most appropriate next step in further validation of the kinetic mechanism.
  • Prediction of the developing detonation regime in a NTC-fuel/air mixture with temperature inhomogeneities under engine conditions

    Luong, Minh Bau; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2022-11-12) [Article]
    The developing detonation process resulting in superknock development under realistic engine conditions in the presence of low-temperature chemistry (LTC) is investigated using two-dimensional direct numerical simulations (DNS). A new model is proposed to predict the developing detonation regime under engine conditions. The prediction is validated against the DNS results. Dimethyl ether (DME) exhibiting a typical negative temperature coefficient (NTC) behavior is used as a fuel. In the presence of the NTC regime, the mean distance of dissipation elements of the ignition delay field, lDE, is considered as the characteristic length scale of hot spots to improve the predictive accuracy of the model. The model also accounts for the multi-dimensional effect resulting from the interaction and collision of multiple ignition kernels that is found to promote the onset of detonation and reduce the runup distance of detonation initiation as compared to that of 1D laminar detonation cases. The trasient mixture state is also incorporated in the model development. The model is demonstrated to accurately capture the developing detonation boundary that is consistent with the knock intensity level obtained from the statistical analysis of DNS dataset.
  • Review of life cycle assessments (LCA) for mobility powertrains

    Sarathy, Mani; Nagaraja, Shashank S.; Singh, Eshan; Cenker, Emre; Amer, Amer (Transportation Engineering, Elsevier BV, 2022-11-10) [Article]
    Continued economic growth is accelerating the demand for transport energy in the road, aviation, and marine sectors. The environmental impacts of technologies across these sectors need significant attention if society is to meet its climate change mitigation targets. Current policies focusing entirely on electrification are shown to be insufficient in mitigating environmental impacts. This is because powertrains equipped with combustion engines will still be operational 2040 onwards, especially in the developing countries and currently underdeveloped countries. In addition, the environmental impacts of electrified systems are not negligible, and lifecycle environmental impacts need attention. This paper presents a detailed summary of fuel-powertrain options that society will be dependent on, for next several decades. Environmental impacts using lifecycle assessment (LCA) are presented for various technologies. This review highlights the current inadequacies in application of LCA methods for transport systems, and the need to improve LCA methodologies to drive effective policy and decision making.
  • Evaporation and clustering of ammonia droplets in a hot environment

    Angelilli, Lorenzo; Hernandez Perez, Francisco; Im, Hong G.; Ciottoli, Pietro P.; Valorani, Mauro (Physical Review Fluids, American Physical Society (APS), 2022-11-09) [Article]
    Recent developments in the transition to zero-carbon fuels show that ammonia is a valid candidate for combustion. However, liquid ammonia combustion is difficult to stabilize due to a large latent heat of evaporation, which generates a strong cooling effect that adversely affects the flame stabilization and combustion efficiency. In addition, the slow burning rate of ammonia enhances the undesired production of NOx and N2O. To increase the flame speed, ammonia must be blended with a gaseous fuel having a high burning rate. In this context, a deeper understanding of the droplet dynamics is required to optimize the combustor design. To provide reliable physical insights into diluted ammonia sprays blended with gaseous methane, direct numerical simulations are employed. Three numerical experiments were performed with cold, standard, and hot ambient in nonreactive conditions. The droplet radius and velocity distribution, as well as the mass and heat coupling source terms are compared to study the effects on the evaporation. Since the cooling effect is stronger than the heat convection between the droplet and the environment in each case, ammonia droplets do not experience boiling. On the other hand, the entrainment of dry air into the ammonia-methane mixture moves the saturation level beyond 100% and droplets condense. The aforementioned phenomena are found to strongly affect the droplet evolution. Finally, a three-dimensional Voronoi analysis is performed to characterize the dispersive or clustering behavior of droplets by means of the definition of a clustering index.
  • Quantitative laser-induced fluorescence of NO in ammonia-hydrogen-nitrogen turbulent jet flames at elevated pressure

    Wang, Guoqing; Tang, Hao; Yang, Chaobo; Magnotti, Gaetano; Roberts, William L.; Guiberti, Thibault (Proceedings of the Combustion Institute, Elsevier BV, 2022-11-09) [Article]
    Ammonia is a very promising carbon-free fuel, but its combustion is prone to generate a large amount of harmful nitric oxide (NO). Designing NO reduction strategies for ammonia flames requires computational fluid dynamics and accurate kinetic mechanisms. However, there are currently no available experimental data that can be used to validate models describing chemistry-turbulence interactions in ammonia flames. This study introduces two non-premixed turbulent jet flames that emulate some features of the cracked ammonia combustion at 5 bar, relevant to micro gas turbines. These ammonia-hydrogen-nitrogen jet flames feature well-controlled boundary conditions and are particularly amenable to modeling. A one-dimensional NO laser-induced fluorescence (NO-LIF) method was implemented and combined with 1-D Raman spectroscopy to measure the NO mole fraction quantitatively. To avoid laser absorption by ammonia, excitation in the A2 + − X 2 (0–1) band near 236 nm was chosen instead of the more conventional (0–0) band near 226 nm. Due to the unsteady nature of turbulent jet flames, offline NO-LIF measurements are not useful, and undesirable interferences from Rayleigh scattering and oxygen LIF were instead quantified and removed using the temperature and major species mole fractions measured with Raman spectroscopy. NO quantification algorithms were optimized and validated first with a laminar NH3–H2–N2 counterflow flame, and then applied to the turbulent jet flames. Results show that a large amount of NO (∼1500 ppm) is produced in the NH3–H2–N2 jet flames. Increasing the ammonia cracking ratio from 14% to 28% reduces the NO concentration in laminar and turbulent NH3–H2–N2 flames. Data also shows that turbulence smear effects of differential diffusion. To the best of our knowledge, this is the first available database featuring quantitative measurements of spatially-resolved NO mole fraction in turbulent NH3–H2–N2 flames at a practically-relevant pressure. This unique database can be used in the future to validate turbulent combustion models for such flames.
  • Effects of oxygen partial premixing on soot formation in ethylene counterflow flames with oscillating strain rates

    Zhou, Mengxiang; Xu, Lei; Yan, Fuwu; Chung, Suk Ho; Wang, Yu (Combustion and Flame, Elsevier BV, 2022-11-04) [Article]
    The sooting characteristics of counterflow partially-premixed flames subject to monochromatic oscillation of strain rates were investigated. Strain rate oscillations with well controlled frequencies and amplitudes were imposed on a series of ethylene counterflow partially-premixed flames. Flow fields and soot volume fractions were measured non-intrusively in a spatially and temporally resolved manner. Numerical simulations were also performed to support the experimental observations. The results showed that oxygen partial premixing in the ethylene fuel stream notably increased soot production due to the enhanced temperature and oxidative pyrolysis that led to increased formation of benzene precursors. More importantly, despite these enhancing effects on soot loading, we demonstrated for the first time that partial premixing does not alter the sensitivity of soot formation to strain rate in steady counterflow flames. In addition, when subject to oscillating strain rates, the unsteady responses of partially-premixed flames were still seen to be independent of partial premixing. This indicates that our previous conclusion—the sensitivity of soot formation to strain rate in steady counterflow diffusion non-premixed flames determines its response under unsteady condition—still holds for partially-premixed flames. The results could be useful for the development of advanced flamelet model to quantitatively account for unsteady effects on soot formation in practical turbulent flames that frequently encounters partial premixing.
  • Flame detachment of jet fires at windward and leeward sides in crossflow: Experiment and a Damköhler number based model

    Hu, Longhua; Shang, Fengju; Chen, Yuhang; Chung, Suk Ho (Proceedings of the Combustion Institute, Elsevier BV, 2022-11-02) [Article]
    Flame detachment of jet fires are important in practical application such as elevated flares, where flame detachment is used to minimize damage on nozzle and the attendant fire safety problem. In this work, detachment behavior of a jet flame under crossflow was studied, especially focused on the difference in critical crossflow speeds for flame detachments at the windward and leeward sides of the nozzle exit, such problem has not been quantified previously. Experiments were conducted in a wind tunnel using propane as the fuel by adopting jet nozzles with inner diameters of 3, 5, 8, and 10 mm and an outer diameter of 15 mm; together with inner diameters of 13 and 15 mm and an outer diameter of 20 mm. The results showed that the critical crossflow speeds for flame detachments both at the windward and leeward sides decreased as the jet velocity increased. The critical crossflow speed for flame detachment at the windward side was weakly dependent on nozzle diameter, while that at the leeward side increased with the increase in nozzle diameter. A model was proposed to characterize this difference on the basis of a Damköhler number, which is defined as the ratio of the characteristic mixing time to chemical reaction time. A satisfactory correlation was achieved for the difference in critical crossflow speeds for flame detachments at the windward and leeward sides as a function of fuel jet velocity and nozzle dimensions of inner diameter and wall thickness.
  • Microwave-Assisted Solvent Deasphalting of Heavy Fuel Oil: Process Parameters Optimization With RSM and ANN

    Saha, Biswajit; Vedachalam, Sundaramurthy; Paul, Atanu Kumar; Dalai, Ajay K.; Saxena, Saumitra; Roberts, William L.; Dryer, Frederick L. (Elsevier BV, 2022-11-02) [Preprint]
    As petroleum recovery has progressed historically, the portion of heavier crudes and bottom of the barrel residues from the refining process has increased. These crudes are challenging to process, leaving vacuum residues with large fractions of ash and refractory sulfur due to high asphaltene content. Asphaltenes are known to form coke in catalytic upgraders and deactivate refining catalysts. Asphaltenes, which are present in significant amounts in heavy crudes, are the cause of reduction of combustion efficiency, clogging of refinery pipes, and particulate matter emissions. Asphaltenes can be removed from heavy crudes by solvent deasphalting. But the requirement of a high solvent to oil ratio limits its commercial viability. To lower the requirement of solvent, this study investigates deasphalting of heavy fuel oil (HFO) with n-heptane, n-hexane, and n-pentane by microwave-assisted, ultrasound-assisted, and supercritical solvent deasphalting methods. Among the different methods investigated, the microwave-assisted method removed 88 and 80 wt% of asphaltenes from HFO using heptane and hexane, respectively. Microwave irradiation selectively heats asphaltenes in microwave transparent non-polar solvents and increases the degree of collision of asphaltenes for aggregation and thus precipitation. Besides, resins are readily solubilized by the solvent under microwave heating and thus they are unable to act as peptizing agents of asphaltenes. The optimization of process parameters such as solvent to HFO ratio, microwave power, and holding time was investigated for microwave-assisted deasphalting using Central Composite Design (CCD) and artificial neural network (ANN). The optimum removal of asphaltene was observed when the solvent to HFO ratio, microwave power, and holding time were kept at 3, 150W, and 20 min, respectively. Deasphalting also significantly improved the quality of HFO by dropping the viscosity along with the sulfur and nitrogen contents of HFO. The outcomes of this study are significant for the petrochemical industry as potentially improved crude oil processing with lower solvent to HFO ratios can be achieved in a more effective, economical manner using microwave assistance.
  • Effects of Water Vapor Addition on Downstream Interaction in Co/O2 Counterflow Premixed Flames

    Kim, Gyeong Taek; Park, Jeong; Chung, Suk Ho; Yoo, Chun Sang (Elsevier BV, 2022-11-01) [Preprint]
    The effects of H2O on downstream interaction in counterflow premixed CO/O2 flames are investigated by varying the global strain rate (ag) and CO mole fractions (XCO,L, XCO,U) in the lower and upper nozzles, respectively. For interacting premixed CO/O2 flames, the flammable region is very narrow such that the flames cannot be sustained for ag > 11.75 s−1 [[EQUATION]] When 1.0% vol H2O is added to O2/CO2 mixtures, the flammable region is appreciably extended. At low ag, the lean-lean and rich-rich extinction boundaries show strong and weak interaction modes similar to those observed previously in hydrocarbon fuels. The flammable lean-lean and rich-rich regions gradually shrink with the increase of ag. When XCO,U is small for asymmetric lean double flames at low ag, the extinction boundary exhibits weak interaction behavior, where the weaker flame is parasitic to the stronger flame by XCO,L. The stronger flame experiences heat loss to the weaker flame. As ag increases, the reaction cannot be completed due to the reduction in the flow time. The thermal energy loss by incomplete reaction leads to the flame extinction. This effect changes the qualitative nature of extinction boundary as ag increases, resulting in the extinction boundary having only the strong interaction mode having near constant (XCO,L+XCO,U) and bending toward larger XCO,L, and eventually forming an island shape at higher strain rate. The local equilibrium temperature (LET) concept is introduced to explain these flame extinction mechanisms. Local temperature behaviors are well explained by investigating major reaction contributions to heat release rate. In all cases, LET decreases by the effect of preferential diffusion because of the Lewis number of deficient reactant being larger than unity. For asymmetric double flames, conductive heat transfer (CHT) from the stronger to weaker flame reduces the LET of the stronger flame. Flame extinction mechanism can be explained by introducing a loss ratio. For CO/O2 flames, the effects of incomplete reaction as well as preferential diffusion and CHT lead to flame extinction. For (CO/O2+1.0% H2O) flames with the increase of ag, thermal energy loss by incomplete reaction becomes appreciable, as compared with the effects of preferential diffusion and CHT.
  • Visualization of the combustion process using a narrow throat pre-chamber geometry for a heavy-duty engine

    Marquez, Manuel Alejandro Echeverri (2022-11) [Dissertation]
    Advisor: Turner, James W. G.
    Committee members: Magnotti, Gaetano; Finkbeiner, Thomas; Armas, Octavio
    Lean combustion is one of the most applied methods to increase engine efficiency and maintain a good trade-off with engine emissions. The pre-chamber combustion (PCC) is one of the most promising combustion concepts to extend the lean operating limits of the engine. The Narrow throat pre-chamber has shown long lean limit extension than other ignition sources. The pre-chamber combustion and main-chamber combustion were studied in a Heavy-Duty optical engine using methane fuel to determine the generalities of the combustion process in the two volumes: pre- and main chamber. The combustion process was recorded using three collection systems: (a) Natural Flame Luminosity (NFL), (b) OH* Chemiluminescence, and (c) CH* Chemiluminescence. Additionally, the effect of three pre-chamber geometrical parameters, volume, nozzle area and throat diameter, on the pre-chamber combustion was also addressed in this research. The generalities of the pre-chamber combustion inside the pre-chamber exhibited a flame propagation nature for the combustion process, with high propagation velocities inside the throat. The main chamber process for the reference narrow throat pre-chamber exhibited defined jets from six of the twelve jets corresponding to the bottom row of nozzles for the reference pre-chamber. Regarding the geometrical parameters, the throat area to nozzle area ratio determines the propagation mode for the main chamber, evolving from only six jets and ultra-low throat intensity for ultra-low ratios to, twelves jethe ts with same penetration and high throat intensity for ratios above one.
  • Solar-driven ultrafast lithium extraction from low-grade brine using microfluidics-mediated vortex in scalable electrochemical reactors

    Zhang, Xianyun; Li, Zhen; Liu, Jiang; Xu, Fuzong; Zheng, Leiliang; De Wolf, Stefaan; Lai, Zhiping; Lu, Xu (Chemical Engineering Journal, Elsevier BV, 2022-11-01) [Article]
    Electrochemical lithium (Li) extraction from low-grade salt lake brine, when powered by off-grid renewables, represents a potential approach to meeting the substantially increasing demand for battery-grade Li2CO3. However, this technology has been drastically challenged by the low extraction rate and high production cost, largely due to the lack of research on reactor engineering and system scale-out. Herein, we rationally designed a scalable spiral-microstructured electrochemical reactor (SMER) to accomplish ultrafast and economical Li extraction under harsh brine conditions by virtue of significantly accelerated mass transfer. We showcased that the SMER was stably operated at a Li extraction rate over 5.6 times as much as that of state-of-art devices, and could be up-scaled for commercial production of battery-grade Li2CO3 driven by solar cells. This work lays the ground for sustainable Li extraction from remote low-grade salt lake brine and can be readily applied to more minable Li reserves/resources.
  • Optimized Escape Path Planning for Commercial Aircraft Formations

    Saber, Safa I.; Feron, Eric (IEEE, 2022-10-31) [Conference Paper]
    There is growing interest in commercial aircraft formation flight as a means of reducing both airspace congestion and the carbon footprint of air transportation. Wake vortex surfing has been researched extensively and proven to have significant fuel-saving benefits, however, commercial air transportation has yet to take advantage of these formation benefits due to understandable safety concerns. Formation contingency scenarios are much more complex than those of individual aircraft and have not yet been studied in depth. This work investigates the utility of mixed-integer linear programming and optimization in generating aircraft escape paths for formation contingency planning. Two high-altitude commercial aircraft formation scenarios are presented; formation join and formation escape. Pilot expertise is used to evaluate the optimized paths. The linear programming formulation results compare well with pilot intuition and confirm viability of pilot-generated plans from previous work. The model proves useful both in presenting solutions not previously considered and in evaluating separation requirements for improvement of escape path planning.
  • Drop-impact Singular Jets, Acoustic Sound and Bouncing with Filaments

    Yang, Zi Qiang (2022-10-30) [Dissertation]
    Advisor: Thoroddsen, Sigurdur T
    Committee members: Lacoste, Deanna; Mishra, Himanshu; Castrejon-Pita, Jose Rafael
    This dissertation talks about the dynamics of the drop impact in two parts, the impact of the drop on the deep liquid pool with singular jet and sound emission, and the bouncing drop with filaments on the superhydrophoic solid surface. First, we use experiments and simulations to study drop impacts on a deep liquid pool, with a focus on fine vertical jetting and underwater sound emission from entrapped bubbles, during the rebounding of the hemispherical crater. The much larger parametric complexity introduced by the use of two immiscible liquids, compared to that for the same liquid, leads to an extended variety of compound-dimple shapes. The fastest jet occurs from the rebounding of a telescope dimple shape without bubble pinch-off, at around 45 m/s, which leaves a toroidal micro-bubbles from the air-cusp at the base of the dimple. The finest jets have diameter of only 12 µm. A new focusing mechanism for singular jetting from collapsing drop-impact craters is then proposed based on high-resolution numerical simulations. The fastest jet is confined in a converging conical channel with the entrained air sheet providing a free-slip outer boundary condition. Sound can be emitted from the oscillation of the entrapped dimple-bubble, while the tiny bubble from the initial impact is induced to oscillate with the entrapped bubble, triggering the double crest of the acoustic signal. We track the compression of the bubble volume from the high-speed imaging and relate it to the hydrophone signal. In the second part, we investigate the impact of a polymeric drop on a superhydrophobic solid substrate with micropillar structure. The drop spreads on the substrate, wets the tops of the pillars, and rebounds out of the superhydrophobic soild surface. Numerous liquid filaments are stretched from the liquid drop to the attached adjacent pillars, and minuscule threads would be left on the top of the pillars using the inclined superhydrophobic solid surface. The well-organized exposed polymer threads are left on the top of the pillars after solvent evaporation. The thickness of the deposition of filament bundles using the bouncing method are thinner than those formed by drop evaporation or drop rolling from SEM (scanning electron microscope) observation.

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