Now showing items 1-20 of 2313

    • On the Products from the Pyrolysis of Heavy Fuel and Vacuum Residue Oil

      Gautam, Ribhu; AlAbbad, Mohammed A.; Guevara, Edwin; Sarathy, Mani (Elsevier BV, 2023-03-12) [Preprint]
      Incorporating unconventional fuel sources into the global energy mix is necessary to meet increasing energy demand. One attractive option is the gasification of residual fuels, such as heavy fuels, which produce clean combustible gases. Pyrolysis is the first step in the gasification process, so its understanding can help in the development of the gasification process. The information on the pyrolysis products can serve the development of chemical kinetics mechanisms for gasification of heavy fuels for their efficient utilization. In this study, pyrolysis of heavy fuel oil (HFO) and vacuum residue oil (VRO) is reported. Prior to the pyrolysis experiments, these fuels were characterized for proximate analysis, elemental and trace metal composition. The pyrolysis experiments were conducted in a wide temperature range of 400–1000 °C in a customized tubular furnace-based reactor. A two-stage condensation system was used to collect the condensable fraction evolved from the pyrolysis of HFO and VRO and non-condensable gases were collected in Tedlar® bags. Both liquid and gaseous products were characterized using a gas chromatography (GC)-mass spectrometry (MS)-flame ionization detector (FID) and Fourier transform–ion cyclotron resonance–mass spectrometer (FT-ICR-MS) to understand the presence of different types of compounds. The identified compounds were classified as benzene derivatives, naphthalene derivatives, polycyclic aromatic hydrocarbons, saturated and unsaturated hydrocarbons and sulfur-containing compounds. In order to better understand the liquid fraction, the heavy fraction (with high molecular weight) which could not be analyzed and identified in GC/MS-FID was analyzed in FT-ICR-MS. This shed significant insights on the deconstruction of HFO and VRO. Twenty lighter hydrocarbons ranging from C1-C5 were identified and quantified in the pyrolysis vapors of both fuels.
    • On the pursuit of emissions-free clean mobility - Electric vehicles versus e-fuels.

      Ravi, Sai Sudharshan; Brace, Chris; Larkin, Charles; Aziz, Muhammad; Leach, Felix; Turner, James W. G. (The Science of the total environment, Elsevier BV, 2023-03-10) [Article]
      With the passing of every second we get closer to a society that is more cognizant of the effect carbon dioxide emissions are having on our planet, and that is more willing to take part in sustainable efforts to combat this and ever more interested in investing in cleaner technologies like electric vehicles (EVs). EVs are marching strongly into a market that is currently dominated by internal combustion engine vehicles, the current main fuel of which is a known contributor to most of the emission related climate problems that we now find ourselves in. Moving ahead, it is important that any move from internal combustion engines to more nascent technologies like EVs is sustainable and not detrimental to the environment. There is an ongoing debate between proponents of so-called e-fuels (being synthetic fuels made from atmospheric carbon dioxide, water, and renewable energy) and EVs wherein e-fuels are largely accused of being a half-measure while EVs are thought to contribute more in terms of brake and tire emissions than the ICE vehicles. This raises the question of whether there should even be a complete replacement of the combustion engine vehicle fleet or that should there be a 'mobility mix' similar to how we currently refer to an energy mix with power grids. This article offers some perspectives by critically analyzing and diving deeper into these pressing concerns to answer some of these questions.
    • On-chip photoacoustic transducer based on monolithic integration of piezoelectric micromachined ultrasonic transducers and metasurface lenses

      Zhai, Yanfen; Sasaki, Takashi; Moridi, Mohssen; Lin, Ronghui; Alnakhli, Zahrah; Shamim, Atif; Li, Xiaohang; Younis, Mohammad I.; Hane, Kazuhiro; Wu, Lixiang (SPIE, 2023-03-09) [Conference Paper, Poster]
      An on-chip photoacoustic transducer is proposed by monolithically integrating piezoelectric micromachined ultrasonic transducers (PMUTs) on metasurface lenses for applications such as single-cell metabolic photoacoustic microscopy (SCM-PAM)1 . As shown in Figure 1a, every PMUT cell has a ring-shaped top electrode, and the membrane center is transparent without piezoelectric and electrode materials. The laser beam, therefore, can travel through a PMUT cell after being focused by a metasurface lens bonded on the backside of the PMUT (see Figure.3). The on-chip photoacoustic transducer fully leverages current PMUT and metasurface technologies and does not rely on transparent piezoelectric and electrode materials like typical transparent ultrasonic transducers2 . Moreover, the on-chip photoacoustic transducer has a monolithic integrated achromatic metasurface lens (see Figure 3), which can easily and efficiently focus the visible light (wavelength range: 400-700 nm) at the same focus point. Design and process this and preliminarily test the performance of PMUT and metasurface.
    • On flame speed enhancement in turbulent premixed hydrogen-air flames during local flame-flame interaction

      Yuvraj,; Ardebili, Yazdan Naderzadeh; Song, Wonsik; Im, Hong G.; Law, Chung K.; Chaudhuri, Swetaprovo (arXiv, 2023-03-02) [Preprint]
      Given the need to develop zero-carbon combustors for power and aircraft engine applications, Sd of a turbulent premixed flame, especially for H2-air, is of immediate interest. The present study investigates 3D DNS cases of premixed H2-air turbulent flames at varied pressures for different Ret and Ka with detailed chemistry to theoretically model Sd at negative curvatures. Prior studies at atmospheric pressure showed Sd˜ to be enhanced significantly over SL at large negative κ due to flame-flame interactions. 1D simulations of an imploding cylindrical H2-air laminar premixed flame used to represent the local flame surfaces undergoing flame-flame interaction in a turbulent flame at the corresponding pressure conditions are performed to understand the interaction dynamics. These simulations emphasized the transient nature of the flame structure during flame-flame interactions and enabled analytical modeling of Sd˜ at these regions of extreme negative κ of the 3D DNS. The JPDF of Sd˜ and κ and the corresponding conditional averages from 3D DNS showed a negative correlation between Sd˜ and κ. The model successfully predicts the variation of ⟨Sd˜|κ⟩ with κ for the regions on the flame surface with κδL≪−1 at all pressures, with good accuracy. This shows the aforementioned configuration to be fruitful in representing local flame-flame interaction in 3D turbulent flames. Moreover, at κ=0, on average Sd˜ can deviate from SL, manifested by the internal flame structure, controlled by turbulence transport in the large Ka regime. Thus, the correlation of ⟨Sd˜⟩/SL with ⟨|∇cˆ|c0⟩ at κ=0 is explored.
    • Conical focusing: mechanism for singular jetting from collapsing drop-impact craters

      Tian, Yuansi; Yang, Zi Qiang; Thoroddsen, Sigurdur T (Journal of Fluid Mechanics, Cambridge University Press (CUP), 2023-02-27) [Article]
      Fast microjets can emerge out of liquid pools from the rebounding of drop-impact craters, or when a bubble bursts at its surface. The fastest jets are the narrowest and are a source of aerosols both from the ocean and from a glass of champagne, of importance to climate and the olfactory senses. The most singular jets, which we observe experimentally at a maximum velocity of 137±4 m s−1 and a diameter of 12 μm, under reduced ambient pressure, are produced when a small dimple forms at the crater bottom and rebounds without pinching off a small bubble. The radial collapse and rebounding of this dimple is purely inertial, but highly sensitive to initial conditions. High-resolution numerical simulations reveal a new focusing mechanism, which drives the fastest jet within a converging conical channel, where an entrained air sheet provides effective slip at the outer boundary of the conically converging flow into the jet. This configuration bypasses any viscous cutoff of the jetting speed and explains the extreme sensitivity to initial conditions observed in detailed experiments of the phenomenon.
    • Effects of water vapor addition on downstream interaction in CO/O2 counterflow premixed flames

      Taek Kim, Gyeong; Park, Jeong; Chung, Suk Ho; Sang Yoo, Chun (Fuel, Elsevier BV, 2023-02-27) [Article]
      The effects of H2O addition 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. When 1.0% vol H2O is added to O2/CO2 mixtures, the flammable region is appreciably extended. At low strain rate, the lean-lean and rich-rich extinction boundaries show strong and weak interactions 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 strain rate, 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 the strain 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 strain rate 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 strain rate, thermal energy loss by incomplete reaction becomes appreciable, as compared with the effects of preferential diffusion and CHT.
    • The effect of ash, water vapor, and heterogeneous chemistry on the evolution of a Pinatubo-size volcanic cloud

      Abdelkader, Mohamed; Stenchikov, Georgiy L.; Pozzer, Andrea; Tost, Holger; Lelieveld, Jos (Copernicus GmbH, 2023-02-26) [Poster]
      We employ the ECHAM5/MESSy2 atmospheric chemistry general circulation model (EMAC)that incorporates calculations of gas-phase and heterogeneous chemistry coupled with the ozone cycle and aerosol formation, transport, and microphysics to calculate the 1991 Pinatubo volcanic cloud. We considered simultaneous injections of SO2, volcanic ash, and water vapor. We conducted multiple ensemble simulations with different injection configurations to test the evolution of SO2, SO4, ash masses, stratospheric aerosol optical depth, surface area density (SAD), and 24the stratospheric temperature response against available observations. We found that the volcanic cloud evolution is sensitive to the altitude where volcanic debris is initially injected and the initial concentrations of the eruption products that affect radiative heating and lofting of the volcanic cloud. The numerical experiments with the injection of 12 Mt SO2, 75 Mt of volcanic ash, and 150 Mt of water vapor at 20 km show the best agreement with the observation of aerosol optical depth and stratospheric temperature response. Volcanic water injected by eruptive jets and/or intruding through the tropopause accelerates SO2 oxidation. But the mass of volcanic water retained in the stratosphere is controlled by the stratospheric temperature at the injection level. For example, volcanic materials are released in the cold point above the tropical tropopause, and most of the injected water freezes and sediments as ice crystals. The water vapor directly injected into the volcanic cloud increases the SO2− mass and stratospheric aerosol optical depth by about 5%. The coarse 4ash comprises 98% of the ash injected mass. It sediments within a few days, but aged submicron ash could stay in the stratosphere for a few months providing SAD for heterogeneous chemistry. The presence of ash accelerates SO2 oxidation by 10%–20% due to heterogeneous chemistry, radiative heating, lofting, and faster dispersion of volcanic debris. Ash aging affects its lifetime and optical properties, almost doubling the radiative ash heating. The 2.5-year simulations show that the stratospheric temperature anomalies forced by radiative heating of volcanic debris in our experiments with the 20 km injection height agree well with observations and reanalysis data. This indicates that the model captures the long-term evolution and climate effect of the Pinatubo volcanic cloud. The volcanic cloud’s initial lofting, facilitated by ash particles’ radiative heating, controls the oxidation rate of SO2. Ash accelerates the formation of the sulfate layer in the first 2 months after the eruption. We also found that the interactive calculations of OH and heterogeneous chemistry increase the volcanic cloud sensitivity to water vapor and ash injections. All those factors must be accounted for in modeling the impact of large-scale volcanic injections on climate and stratospheric chemistry.
    • A laser-based sensor for selective detection of benzene, acetylene, and carbon dioxide in the fingerprint region

      Mhanna, Mhanna; Sy, Mohamed; Elkhazraji, Ali; Farooq, Aamir (Research Square Platform LLC, 2023-02-24) [Preprint]
      A mid-infrared laser-based sensor is designed and demonstrated for trace detection of benzene, acetylene, and carbon dioxide at ambient conditions. The sensor is based on a distributed feedback quantum cascade laser (DFB-QCL) emitting near 14.84 µm. Tunable diode laser absorption spectroscopy (TDLAS) and a multidimensional linear regression algorithm were employed to enable interference-free measurements of the target species. The laser wavelength was tuned over 673.8–675.1 cm-1 by a sine-wave injection current at 1 kHz repetition rate. Minimum detection limits of 0.22, 5.92, and 8.32 ppm were achieved for benzene, acetylene, and carbon dioxide, respectively. The developed sensor is insensitive to interference from overlapping absorbance spectra, and its performance was demonstrated by measuring the target species in known mixture samples. The sensor can be used to detect tiny leaks of the target species in petrochemical facilities and to monitor air quality in residential and industrial areas.
    • Experimental and modelling study of syngas combustion in CO2 bath gas

      Harman-Thomas, James M.; Kashif, Touqeer Anwar; Hughes, Kevin J.; Pourkashanian, Mohamed; Farooq, Aamir (Fuel, Elsevier BV, 2023-02-24) [Article]
      Syngas produced from coal and biomass gasification has been proposed as a potential fuel for direct-fired supercritical power cycles. For instance, the Allam-Fetvedt cycle can offer price-competitive electricity production with 100 % inherent carbon capture while utilizing CO2 dilution of about 96 %. In this work, ignition delay times (IDTs) of syngas have been measured in CO2 diluted conditions using a high-pressure shock tube at two pressures (20 and 40 bar) over a temperature range of 1100 – 1300 K. Syngas mixtures in this study were varied in equivalence ratio and H2:CO ratios. The datasets were compared against the predictions of AramcoMech 2.0 and the University of Sheffield supercritical CO2 2.0 (UoS sCO2 2.0) kinetic models. Quantitative comparative analysis showed that the UoS sCO2 2.0 was superior in its ability to predict the experimental IDTs of syngas combustion. We found that the reaction of CO2 and H to form CO and OH caused the separation of H2 and CO ignition in two events, which increased the complexity of determining the IDTs. We investigated this phenomenon and proposed a method to determine simulated IDTs for an effective comparison against the experimental IDTs. The chemical kinetics of syngas combustion in a CO2 and N2 bath gas are contrasted by sensitivity and rate-of-production analyses. By altering the ratio of H2 and CO as well as mixture equivalence ratio, this work provides vital IDT data in CO2 bath gas for further development and validation of relevant kinetics mechanisms.
    • Electro osmotically flow of fourth-grade fluid in complex channel with porous medium and lubricated walls: applications in petroleum industry

      Nazeer, Mubbashar; Pasha, Amjad A.; Hussain, Farooq; ADIL, MOHAMMAD (Waves in Random and Complex Media, Informa UK Limited, 2023-02-21) [Article]
      This article offers a distinct survey of a multiphase flow through a uniform channel. The electro-osmotic phenomenon is the main source of the flow. Fourth-grade fluid model is used as the base liquid which is suspended with metallic particles to form a bi-phase viscous suspension. Skin friction effects are tackled by employing lubrication effects on the opposite walls of the channel. Porous media is governed by the modified Darcy’s law and the Poisson differential equation is considered for electric double-layer effects. An approximate solution is achieved for the nonlinear fluid flow. The obtained findings are also compared with the existing literature for the limiting case. It is inferred that the shear thickness effects cause the momentum of the bi-phase flow to decline, gradually. However, the motion of the metallic particles is supported by the lubrication effects on the walls of the channel.
    • A species-weighted flamelet/progress variable model with differential diffusion effects for oxy-fuel jet flames

      Jiang, Xudong; Guo, Junjun; Wei, Zhengyun; Quadarella, Erica; Im, Hong G.; Liu, Zhaohui (Combustion and Flame, Elsevier BV, 2023-02-21) [Article]
      A flamelet/progress variable (FPV) model accounting for the differential diffusion (DD) effects is proposed and applied to large eddy simulation (LES) of turbulent oxy-fuel flames (Sevault et al. 2012). Based on tabulations with the detailed molecular diffusion and the equal diffusion (ED) assumption, a species-weighted flamelet (SWF) model is developed to represent the DD effects. The influences of combustion progress, mixture fraction, and turbulence on variable Lewis numbers are incorporated into the model. The model assessments are first conducted on a laminar coflow flame, showing that the effect of DD on temperature and species is accurately captured, in that the importance of DD is seen in the laminar flame. Subsequently the model is implemented in a fully coupled LES of oxy-fuel jet flames. The LES results show that DD plays an important role in the reaction zone and regions near the fuel nozzle, and its effect decreases farther downstream, consistent with the experimental observations. The LES with the SWF model yields good predictions on the mean temperature and major species at the fuel-rich side while slight deviation (less than 6%) at the fuel-lean side. In comparison, LES with the original unity Lewis numbers flamelet model (Pierce et al. 2004) predicts a full extinction because of neglecting the DD characteristics, while the variable Lewis number flamelet model provides the maximum deviation of 26% because of ignoring the ED characteristics produced by turbulent disturbance. The trend and location of localized extinction of oxy-fuel flame are also well predicted using the SWF model.
    • Selectively converting CO2 to HCOOH on Cu-alloys integrated in hematite-driven artificial photosynthetic cells

      Zhao, Jiwu; Huang, Liang; Xue, Lan; Niu, Zhenjie; Zhang, Zizhong; Ding, Zhengxin; Yuan, Rusheng; Lu, Xu; Long, Jinlin (Journal of Energy Chemistry, Elsevier BV, 2023-02-16) [Article]
      The integration of electrochemical CO2 reduction (CO2RR) and photoelectrochemical water oxidation offers a sustainable access to valuable chemicals and fuels. Here, we develop a rapidly annealed hematite photoanode with a photocurrent density of 2.83 mA cm−2 at 1.7 VRHE to drive the full-reaction. We also present Cu-alloys electrocatalysis extended from CuInSnS4, which are superior in both activity and selectivity for CO2RR. Specifically, the screened CuInSn achieves a CO2 to HCOOH Faradaic efficiency of 93% at a cell voltage of −2.0 V by assembling into artificial photosynthesis cell. The stability test of IT exhibits less than 3% degradation over 24 h. Furthermore, in-situ Raman spectroscopy reveals that both CO32- and CO2 are involved in CO2RR as reactants. The preferential affinity of C for H in the *HCO2 intermediate enables an improved HCOOH-selectivity, highlighting the role of multifunctional Cu in reducing the cell voltage and enhancing the photocurrent density.
    • Energy Loss for Droplets Bouncing Off Superhydrophobic Surfaces

      Thenarianto, Calvin; Koh, Xue Qi; Lin, Marcus; Jokinen, Ville; Daniel, Dan (Langmuir, American Chemical Society (ACS), 2023-02-16) [Article]
      A water droplet can bounce off superhydrophobic surfaces multiple times before coming to a stop. The energy loss for such droplet rebounds can be quantified by the ratio of the rebound speed UR and the initial impact speed UI; i.e., its restitution coefficient e = UR/UI. Despite much work in this area, a mechanistic explanation for the energy loss for rebounding droplets is still lacking. Here, we measured e for submillimeter- and millimeter-sized droplets impacting two different superhydrophobic surfaces over a wide range of UI (4–700 cm s–1). We proposed simple scaling laws to explain the observed nonmonotonic dependence of e on UI. In the limit of low UI, energy loss is dominated by contact-line pinning and e is sensitive to the surface wetting properties, in particular to contact angle hysteresis Δ cos θ of the surface. In contrast, e is dominated by inertial-capillary effects and does not depend on Δ cos θ in the limit of high UI.
    • Mid-IR absorption spectra of C1-C4 alkyl acetates at high temperatures

      Adil, Mohammad; Giri, Binod; Elkhazraji, Ali; Farooq, Aamir (Journal of Quantitative Spectroscopy and Radiative Transfer, Elsevier BV, 2023-02-15) [Article]
      We present measurements of the temperature-dependent absorption cross-section of C1-C4 alkyl acetates (methyl acetate, ethyl acetate, n-propyl acetate, n‑butyl acetate) covering their strongest infrared absorption band near 1250 cm−1. The methodology employed here consists of a broadband rapid-tuning MIRcat-QT™ laser in conjunction with a shock tube. The spectral measurements were performed over the wavelength range of 7.52 – 8.62 μm (1160 – 1330 cm−1), temperatures of 580 – 980 K, and pressures near 1.3 bar. The reported IR spectra are the first experimental measurements of the high-temperature absorption spectra of these species, and the measured spectra showed strong temperature dependence. We provide herein absorption cross-section correlations for a wide range of spectra and a wide range of temperatures. Rapid measurements of alkyl acetate in transient environments may be performed in a fixed-wavelength mode by choosing a suitable wavelength. Based on our analysis, we propose a wavelength of 1221.27 cm−1 which exhibited minimal temperature and pressure dependence over the range of investigated conditions. We utilized this strategy for measuring methyl acetate concentration during its high-temperature pyrolysis.
    • Emerging technologies for catalytic gasification of petroleum residue derived fuels for sustainable and cleaner fuel production-An overview

      Jafarian, Mehdi; Haseli, Pegah; Saxena, Saumitra; Dally, Bassam (ENERGY REPORTS, Elsevier BV, 2023-02-15) [Article]
      Utilization of bottom-of-the-barrel petroleum products like heavy fuel oils (HFO) derived from vacuum residue oils (VRO) is essential for the sustainable use of crude oil. The high sulfur content and asphaltenes with metallic impurities of these products, such as vanadium and nickel, lead to the production of criteria pollutants and inorganic ash upon combustion, which has limited their widespread use in many industries and jurisdictions. Industrially mature thermo-chemical conversion technologies (e.g., cracking, pyrolysis, partial oxidation (POx), and gasification) have been utilized to upgrade and convert oil residues like HFO into cleaner fuels such as syngas and hydrogen. Among these processes, commercial POx and gasification typically proceed via catalyst free-operations. However, the novel variants of gasification processes that can improve the utilization of heavy oil residues can be benefited from using catalysts. That is because not only can catalysts facilitate the reactions within a smaller reactor, but also they can provide better control over the product composition. Nevertheless, the typical characteristics of high carbon, impurities, and sulfur content lead to the deactivation of the solid-state catalysts mainly due to the coking and poisoning. The use of molten salts, metal/metal oxide as catalysts or oxygen carriers, and hydrogen peroxide as a gasifying agent in the gasification of heavy hydrocarbons are identified to offer significant potential to overcome these challenges, albeit introducing alternative difficulties. This paper surveys and briefly discusses the state-of-the-art thermo- and catalytic chemical conversion technologies for petroleum residue derived fuels, focusing on the use of molten catalysts, oxygen carriers, and hydrogen peroxide as oxidizing agents. The paper also briefly reviews methane pyrolysis, dry reforming with molten metal catalysts, and liquid chemical looping gasification to find similarities and know-how that can be used for gasifying heavy oil residues with the molten metal/metal oxides being used as either catalyst or reaction medium.
    • Direct numerical simulation of particle-laden flow in an open channel at

      Gao, Wei; Samtaney, Ravi; Richter, David H. (Journal of Fluid Mechanics, Cambridge University Press (CUP), 2023-02-09) [Article]
      We perform two-way coupled direct numerical simulation of particle-laden flow in an open channel at a friction Reynolds number of 5186, which exhibits many characteristics of high-Reynolds-number wall-bounded turbulence, such as the distinct separation of scales in the inner and outer layers. Three representative cases, an unladen case and low- and high-Stokes-number particle-laden cases, are performed to investigate the turbulent modification by particles. To this end, we compare several statistical quantities to understand the particle effect on momentum exchange and interphasial energy transfer. The modulation of large-scale motions (LSMs) and very-large-scale motions (VLSMs) are analysed using spectral information, and we find that the LSMs and VLSMs are generally weakened in the inner and outer layers, which is qualitatively different from similar simulations at lower Reynolds numbers . The spatial structures are investigated with correlation analysis, and inclined VLSMs are observed in the near-wall region, with decreased inclination angles by particles. The particles tend to widen and shorten the spanwise and streamwise extent of coherent structures, respectively. Furthermore, we find that the vorticity vector displays a preferential alignment with the eigenvector corresponding to the intermediate eigenvalue of the strain-rate tensor, independent of the particle Stokes number.
    • Data-driven framework for input/output lookup tables reduction: Application to hypersonic flows in chemical nonequilibrium

      Scherding, Clément; Rigas, Georgios; Sipp, Denis; Schmid, Peter J.; Sayadi, T. (Physical Review Fluids, American Physical Society (APS), 2023-02-09) [Article]
      Hypersonic flows are of great interest in a wide range of aerospace applications and are a critical component of many technological advances. Accurate simulations of these flows in thermodynamic (non)equilibrium (accounting for high temperature effects) rely on detailed thermochemical gas models. While accurately capturing the underlying aerothermochemistry, these models dramatically increase the cost of such calculations. In this paper, we present a model-agnostic machine-learning technique to extract a reduced thermochemical model of a gas mixture from a library. A first simulation gathers all relevant thermodynamic states and the corresponding gas properties via a given model. The states are embedded in a low-dimensional space and clustered to identify regions with different levels of thermochemical (non)equilibrium. Then, a surrogate surface from the reduced cluster space to the output space is generated using radial-basis-function networks. The method is validated and benchmarked on simulations of a hypersonic flat-plate boundary layer and shock-wave boundary layer interaction with finite-rate chemistry. The gas properties of the reactive air mixture are initially modeled using the open-source Mutation++ library. Substituting Mutation++ with the lightweight, machine-learned alternative improves the performance of the solver by up to 70% while maintaining overall accuracy in both cases.
    • Analysis of energy flows and emission characteristics of conventional diesel and isobaric combustion in an optical engine with laser diagnostics

      Goyal, Harsh; Panthi, Niraj; AlRamadan, Abdullah; Cenker, Emre; Magnotti, Gaetano (Energy, Elsevier BV, 2023-02-04) [Article]
      In this study, the thermodynamic analysis of energy distribution, exhaust emissions, and particulate characterization was conducted in an optical engine with an all-metal configuration. Additionally, the line-of-sight integrated imaging of combustion luminosity, and OH* chemiluminescence along with planar laser induced fluorescence of formaldehyde (HCHO-PLIF), and planar laser induced incandescence of soot (soot-PLII) were applied in the optical configuration. The experiments were conducted with conventional diesel combustion at λ = 3 (i.e., CDC), isobaric combustion at λ = 3 (i.e., Iso3), and isobaric combustion at λ = 4.2 (i.e., Iso4.2) using n-heptane fuel. Compared to Iso3 and CDC, Iso4.2 yielded higher thermal efficiency and lower heat losses; whilst the exhaust losses were exacerbeted. Isobaric combustion also resulted in lower NOx but increased soot emissions. For all operating conditions, the combustion luminosity and OH* chemiluminescence imaging showed that the signal grows and develops from the jet-axis downstream of the nozzle to the jet-wall impingement point, followed by movement towards the squish region. HCHO-PLIF showed that isobaric combustion leads to a faster transition of low-to high-temperature reactions compared to CDC. Soot-PLII showed increased in-cylinder soot distribution for isobaric combustion due to lesser charge pre-mixing time and spray-flame interaction induced by close-coupled injections.
    • Computational assessment of the effects of pre-chamber and piston geometries on the combustion characteristics of an optical pre-chamber engine

      Liu, Xinlei; Echeverri Marquez, Manuel Alejandro; Sanal, Sangeeth; Silva, Mickael Messias; AlRamadan, Abdullah S.; Cenker, Emre; Sharma, Priybrat; Magnotti, Gaetano; Turner, James W. G.; Im, Hong G. (Fuel, Elsevier BV, 2023-02-04) [Article]
      Pre-chamber combustion (PCC) has the potential to extend the lean-burn limit in spark-ignition engines, which can promote engine efficiency and relieve the concern of emissions of nitrogen oxides. This work assessed the effects of pre-chamber (PC) and piston geometries on the combustion characteristics of an optical methane PCC engine with both experimental and computational approaches. Five active-type PCs with different volumes and nozzle diameters (12 nozzles distributed evenly in two layers) and two pistons (bowl and flat) were tested under lean-burn conditions. Multi-cycle pressure and heat release profiles, natural flame luminosity images, and OH* chemiluminescence images were measured and employed for CFD modeling validations. The PCs with the smaller nozzle diameter yielded more intensely-reacting jets from the upper layer of nozzles compared to the other PCs, attributed to the stronger gas choke there, which dramatically affected the flow fields. A larger PC volume allowed more air–fuel mixture to be trapped within the PC whose combustion then resulted in faster pressure buildup, which, however, led to higher heat transfer loss. Compared to the bowl piston, the flat piston generated a higher heat release rate during the late combustion period owing to the relatively longer jet propagation within the squish region.
    • On the combined effects of compositional inhomogeneity and ammonia addition to turbulent flames of ethylene

      Boyette, Wesley; Macfarlane, Andrew R.W.; Steinmetz, Scott; Dunn, Matt J.; Roberts, William L.; Masri, Assaad R. (Proceedings of the Combustion Institute, Elsevier BV, 2023-02-04) [Article]
      This paper is part of a broader program aimed at investigating the effects of co-firing clean fuels such as ammonia or hydrogen with hydrocarbons. The focus is on soot formation as well as flame stability in turbulent mixed-mode combustion, which is highly relevant in practical combustors. Ammonia substitution for nitrogen results in reduced flame stability, and this is correlated to differences in flame speed and extinction strain rate. While it is known that the addition of ammonia suppresses soot, visual inspection of compositionally inhomogeneous flames of ethylene-ammonia indicates a reduction in ammonia's ability to suppress soot formation. Measurements of soot volume fraction and laser-induced fluorescence in selected UV and visible bands are made along the centreline in selected flames to test this hypothesis. Experimental results are then compared to simulations in laminar diffusion flames, stratified counterflow flames, and partially premixed flames. All results confirm the soot-inhibiting ability of ammonia. Increasing inhomogeneity, leading to higher centreline mixture fractions, enhances soot formation, and the level of enhancement is greater for flames with ammonia than without. Moreover, it is found that partial premixing is ultimately responsible for determining the amount of soot formed as opposed to stratification of fuel mixtures near the pilot.