Hyung Sub Sim

Korea Advanced Institute of Science and Technology, Aerospace Engineering, Faculty Member

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Modeling consolidation of wax deposition for progressive cavity pump using computational fluid dynamics

Engineering Science and Technology, an International Journal

Considerations for the temperature stratification in a pre-burn constant-volume combustion chamber

Experimental Thermal and Fluid Science

In recent years, the Engine Combustion Network (ECN) has developed as a worldwide reference for u... more In recent years, the Engine Combustion Network (ECN) has developed as a worldwide reference for understanding and describing engine combustion processes, successfully bringing together experimental and numerical efforts. Since experiments and numerical simulations both target the same boundary conditions, an accurate characterization of the stratified environment that is inevitably present in experimental facilities is required. The difference between the core-, and pressure-derived bulk-temperature of pre-burn combustion vessels has been addressed in various previous publications. Additionally, thermocouple measurements have provided initial data on the boundary layer close to the injector nozzle, showing a transition to reduced ambient temperatures. The conditions at the start of fuel injection influence physicochemical properties of a fuel spray, including near nozzle mixing, heat release computations, and combustion parameters. To address the temperature stratification in more detail, thermocouple measurements at larger distances from the spray axis have been conducted. Both the temperature field prior to the pre-combustion event that preconditions the high-temperature, high-pressure ambient, as well as the stratification at the moment of fuel injection were studied. To reveal the cold boundary layer near the injector with a better spatial resolution, Rayleigh scattering experiments and thermocouple measurements at various distances close to the nozzle have been carried out. The impact of the boundary layers and temperature stratification are illustrated and quantified using numerical simulations at Spray A conditions. Next to a reference simulation with a uniform temperature field, six different stratified temperature distributions have been generated. These distributions were based on the mean experimental temperature superimposed by a randomized variance, again derived from the experiments. The results showed that an asymmetric flame structure arises in the computed results when the temperature stratification input is used. In these predictions, first-stage ignition is advanced by 24 μs, while second-stage ignition is delayed by 11 μs. At the same time a lift-off length difference between the top and the bottom of up to 1.1 mm is observed. Furthermore, the lift-off length is less stable over time. Given the shown dependency, the temperature data is made available along with the vessel geometry data as a recommended basis for future numerical simulations.

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The role of cool-flame fluctuations in high-pressure spray flames, studied using high-speed optical diagnostics and Large-Eddy Simulations

Proceedings of the Combustion Institute

Soot and PAH formation in high pressure spray pyrolysis of gasoline and diesel fuels

Combustion and Flame, 2022

Time-resolved soot and PAH formation from gasoline and diesel spray pyrolysis are visualized and ... more Time-resolved soot and PAH formation from gasoline and diesel spray pyrolysis are visualized and quantified using diffuse back illumination (DBI) and laser induced fluorescence (LIF) at 355 nm, respectively, in a constantvolume vessel at 60 bar from 1400 to 1700 K for up to 30 ms. The delay, maximum formation rate, and yield of soot and PAHs are compared across fuels and temperatures and correlated with the yield sooting indices on either the mass or mole basis. The delays generally decrease with increasing temperature, and the formation rates of both PAHs and soot generally increase with temperature. The apparent PAH-LIF yield may decrease with temperature due to PAH growth and conversion into larger species, signal trapping, and thermal quneching. Soot yield generally increases with temperature. The mass-based YSI correlates reasonably well with soot delay, but YSI does not correlate well with soot yield. The mass-based YSI is a more appropriate predictor of sooting propensity than the mole-based YSI.

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Periodic Partial Extinction Regime in Acoustically Coupled Fuel Droplet Combustion

Bulletin of the American Physical Society, 2016

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Understanding the role of multifunctional nanoengineered particulate additives on supercritical pyrolysis and combustion of hydrocarbon fuels/propellants

Numerical Investigation on Nonequilibrium Energy Transfer in Thin Metal Film Structures during the Irradiation of Femtosecond Pulse Laser

The Transactions of the Korean Institute of Electrical Engineers, 2007

The present study investigates numerically nonequilibrium energy transfer between electrons and p... more The present study investigates numerically nonequilibrium energy transfer between electrons and phonons in metal thin films irradiated by ultrashort pulse lasers and it also provides the temporal and spatial variations of electron and phonon temperatures using the well-established two-temperature model(TTM) on the basis of the Boltzmann transport equation(BTE). This article predicts the crater shapes in gold film structures, and compares the results by using two-dimensional energy transport equation. From the results, it is found that nonequilibrium energy transfer between electrons and phonons takes place, and the equilibrium time increases with the increase of laser fluence. On the other hand, above threshold fluence the ablation time doesn't change nearly with increasing fluences. Compared with one-dimensional TTM, it also reveals that the temporal distributions of electron and phonon temperatures at the top surface estimated by using two-dimensional TTM have a similar tenden...

Large-Eddy Simulation of Laser-Ignited Direct Injection Gasoline Spray for Emission Control

Energies, 2021

Large-Eddy Simulations (LES) of a gasoline spray, where the mixture was ignited rapidly during or... more Large-Eddy Simulations (LES) of a gasoline spray, where the mixture was ignited rapidly during or after injection, were performed in comparison to a previous experimental study with quantitative flame motion and soot formation data [SAE 2020-01-0291] and an accompanying Reynolds-Averaged Navier–Stokes (RANS) simulation at the same conditions. The present study reveals major shortcomings in common RANS combustion modeling practices that are significantly improved using LES at the conditions of the study, specifically for the phenomenon of rapid ignition in the highly turbulent, stratified mixture. At different ignition timings, benchmarks for the study include spray mixing and evaporation, flame propagation after ignition, and soot formation in rich mixtures. A comparison of the simulations and the experiments showed that the LES with Dynamic Structure turbulence were able to capture correctly the liquid penetration length, and to some extent, spray collapse demonstrated in the exper...

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Simultaneous High-Speed Formaldehyde PLIF and Schlieren Imaging of Multiple Injections From an ECN Spray D Injector

The interaction of multiple injections in a diesel engine facilitates a complex interplay between... more The interaction of multiple injections in a diesel engine facilitates a complex interplay between freshly introduced fuel, previous combustion products, and overall combustion. To improve understanding of the relevant processes, high-speed Planar Laser-Induced Fluorescence (PLIF) with 355-nm excitation of formaldehyde and Polycyclic Aromatic Hydrocarbon (PAH) soot precursors is applied to multiple injections of n-dodecane from Engine Combustion Network Spray D, characterized by a converging 189-μm nozzle. High-speed schlieren imaging is applied simultaneously with 50-kHz PLIF excitation to visualize the spray structures, jet penetration, and ignition processes. For the first injection, formaldehyde (as an indicator of low-temperature chemistry) is first found in the jet periphery, after which it quickly propagates through the center of the jet, towards the jet head prior to high-temperature ignition. At second-stage ignition, downstream formaldehyde is consumed rapidly and upstream formaldehyde develops into a quasi-steady structure for as long as the momentum flux from the injector continues. Since the first injection in this work is relatively short, differences to a single long injection are readily observed, ultimately resulting in high-temperature combustion and PAH structures appearing farther upstream after the end of injection. For the second injection in this work, the first formaldehyde signal is significantly advanced because of the entrained high-temperature combustion products, and an obvious premixed burn event does not occur. The propensity for combustion recession after the end of the first injection changes significantly with ambient temperature, thereby affecting the level of interaction between the first- and second injection.

Functionalized graphene sheet as a dispersible fuel additive for catalytic decomposition of methylcyclohexane

Combustion and Flame, 2020

The decomposition of methylcyclohexane (MCH) containing suspended functionalized graphene sheets ... more The decomposition of methylcyclohexane (MCH) containing suspended functionalized graphene sheets (FGS) was studied using a high-pressure flow reactor, under supercritical conditions over the temperature range of 780 -825 K at a constant pressure of 4.72 MPa. Experiments showed both fuel conversion rates and C 1 -C 2 product yields were increased by 43.3% and 62.1%, respectively, with the addition of 50 ppmw of FGS at 820 K. The reaction mechanisms between hydrocarbon and FGS were computationally investigated using reactive molecular dynamics (MD) with ReaxFF force fields at the temperatures of 1700, 180 0, and 190 0 K. The MD simulations revealed that oxygen-containing functional groups attached to the FGS plays an important role in catalyzing the decomposition of the fuel. Heterogeneous dehydrogenation of MCH into a C 7 H 13 radical intermediate led to formation of secondary and tertiary radicals, such as H, CH 3 , and C 2 H 5 , during early extents of reaction promoting additional fuel-consuming steps, including H-abstraction and hydrogenation.

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Enhanced Fuel Decomposition in the Presence of Colloidal Functionalized Graphene Sheet-Supported Platinum Nanoparticles

ACS Applied Energy Materials, 2020

Experiments and simulations were used to demonstrate that decorating functionalized graphene shee... more Experiments and simulations were used to demonstrate that decorating functionalized graphene sheets (FGSs) with platinum nanoparticles (Pt@FGS) stabilized these particles. Addition of these particles to liquid hydrocarbon fuels was observed to significantly affect decomposition under supercritical conditions at a pressure of 4.75 MPa and temperatures from 753 to 803 K. The suspension of only 50 ppmw Pt@FGS in the fuel (equivalent to adding 10 ppmw Pt) enhanced fuel conversion rates (by up to 24%) with a major effect on specific product yields. The production of lowmolecular-weight species increased in the pyrolysis products (with the hydrogen yield increasing by a factor of 12.5). ReaxFF molecular dynamics (MD) simulations supported a mechanism in which synergy between Pt and FGS catalyzed dehydrogenation during n-C 12 H 26 pyrolysis. The highest conversion rates and greatest yields of hydrogen and low-molecular-weight species were observed for fuels containing Pt@FGS particles rather than those containing either FGSs or Pt-clusters alone. Analysis of the platinum decorated FGSs post reaction indicated no deterioration of the composite particles.

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Multifunctional Graphene-Based Additives for Enhanced Combustion of Cracked Hydrocarbon Fuels under Supercritical Conditions

Combustion Science and Technology, 2020

As a particulate fuel additive, functionalized graphene sheets (FGS), with and without the decora... more As a particulate fuel additive, functionalized graphene sheets (FGS), with and without the decoration of nanoparticles, provide a means to form stable colloids with liquid hydrocarbons, act as in situ catalysts, and prevent attached nanoparticles from agglomerating or sintering during heating. Recent pyrolysis experiments and simulations have shown the synergetic effect of Pt and FGS structures accelerating fuel conversion and hydrogen formation. In this paper, the role of graphene-based additives (with and without platinum (Pt) nanoparticles) on enhancing the ignition and combustion characteristics of cracked n-dodecane hydrocarbon fuels under supercritical conditions is examined. Supercritical ignition and combustion experiments were conducted using a high pressure and high temperature windowed combustion chamber coupled to a supercritical fuel pyrolysis reactor. The combustion experiments indicated that the presence of small amounts of particulate additives (100 ppmw) to the fuel can reduce ignition times and increase subsequent combustion rates. In particular, the addition of FGS-supported Pt nanoparticles reduced ignition delay times by nearly a factor of 3 (12.4 to 4.2 ms), increased spray spreading angles by approximately 32.0% (15.4 to 20.3°), reduced the flame liftoff length by 54.0% (1.74 to 0.8 mm), and demonstrated an increase in fuel conversion for a fixed reaction time of 35.0% relative to the pure fuel baseline. These results support the notion that nanostructured fuel additives have the potential to enhance both the heat sink capacity and combustion performance of liquid hydrocarbon fuels for their use in advanced high speed propulsion systems.

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Laminar Microjet Diffusion Flame Response to Transverse Acoustic Excitation

Combustion Science and Technology, 2020

The present experiments focused on the response of burning gaseous fuel jets to prescribed transv... more The present experiments focused on the response of burning gaseous fuel jets to prescribed transverse acoustic excitation as a means of exploring the coupling of reactive, acoustic, and flow processes relevant to combustion instabilities. Single reactive methane microjets were exposed to transverse resonant standing wave disturbances within an acoustic waveguide for which a range of amplitudes of excitation were applied. Temporal flame response to acoustic excitation was studied via simultaneous phase-locked OH* chemiluminescence and visible imaging, and additionally via high speed, temporally resolved visible imaging. Quantification of flame dynamics was achieved for phase-locked data via the Rayleigh and temporal flame distortion measurements, and for time-resolved data via proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). Characteristic signatures associated with various types of flame response were identified, including weakly oscillatory combustion, full-scale flame coupling and lock-in to excitation, and multi-mode flame dynamics preceding flame extinction. It was found that modal decomposition of flame dynamics via POD and DMD analysis revealed important additional dynamical features and signatures in metrics characteristic of transitions in the flame-acoustic coupling.

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Effects of Inert and Energetic Nanoparticles on Burning Liquid Ethanol Droplets

Combustion Science and Technology, 2018

The effect of nanoscale energetic aluminum (nAl) and inert silicon dioxide (nSiO 2 ) particulate ... more The effect of nanoscale energetic aluminum (nAl) and inert silicon dioxide (nSiO 2 ) particulate additives on ethanol droplet combustion was studied under atmospheric conditions. Three different types of droplet experiments were performed to study the influence of the experiment itself on combustion behavior. Simultaneous visible and intensified ultraviolet (UV) images were taken to determine the burning rate constant (K) as well as flame dynamics via OH* chemiluminescence imaging. The addition of nAl appeared to yield a systematic increase in K, by up to 13%, and increasing loading concentrations led to changes in droplet combustion dynamics. Flow instabilities, including liquid jetting and altered droplet deformation, were observed, creating unsteady combustion when the nAl-laden droplet was continuously fed via a quartz capillary. In contrast, the addition of nSiO 2 showed relatively small changes in K, possibly only as large an increase as 5%, with a lack of consistent trends for increasing nSiO 2 concentration for different fuel delivery methods, in part due to the formation of large residual shell-like structures in the later stages of combustion. A simple droplet combustion model suggests that possible enhancement mechanisms for K are related to alterations in thermal conductivity as well as flame temperature with the nAl additive. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of particulate residue revealed further differences in morphology and residue constituents after combustion.

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Periodic partial extinction in acoustically coupled fuel droplet combustion

Combustion and Flame, 2018

This experimental study explored the response of burning liquid fuel droplets to one-dimensional ... more This experimental study explored the response of burning liquid fuel droplets to one-dimensional acoustic standing waves created within a closed, atmospheric waveguide. Building upon prior droplet combustion studies quantifying mean and temporal flame response of several alternative fuels to moderate acoustic excitation (Sevilla-Esparza, et al., Combustion and Flame, 161(6): 1604-1619, the present work focused on higher amplitude acoustic forcing observed to create periodic partial extinction and reignition (PPER) of flames enveloping the droplet. Detailed examination of ethanol droplets exposed to a range of acoustic forcing conditions (frequencies and amplitudes in the vicinity of a pressure node) yielded several different combustion regimes: one with sustained oscillatory flames, one with PPER, and then full extinction at very high excitation amplitudes. Phase-locked OH* chemiluminescence imaging and local temporal pressure measurements allowed quantification of the combustion-acoustic coupling through the local Rayleigh index. Similar behavior was observed for JP-8 and liquid synthetic fuel derived via the Fischer-Tropsch process, but with quantitative differences based on different reaction time scales. Estimates of the mean and oscillatory strain rates experienced by the flames during excitation assisted with interpreting specific relationships among acoustic, chemical, and fluid mechanical/ straining time scales that can lead to a greater understanding of PPER.

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Flow Characteristics and Filling Time Estimation for Underfill Process

Journal of the Korean Welding and Joining Society, 2007

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Femtosecond Laser Pulse Train Effects on Optical Characteristics and Nonequilibrium Energy Transport in Metal Thin Films Considering Quantum Effects

First International Conference on Integration and Commercialization of Micro and Nanosystems, Parts A and B, 2007

ABSTRACT

Femtosecond pulse laser interactions with thin silicon films and crater formation considering optical phonons and wave interference

Microsystem Technologies, 2008

The extensive numerical study is conducted to examine interaction of femtosecond pulse laser with... more The extensive numerical study is conducted to examine interaction of femtosecond pulse laser with thin silicon film structure by using the two-temperature model (2TM) and the three-temperature model (3TM), on the basis of the relaxation time approximation to Boltzmann’s transport equation. The present study investigates mainly transient behaviors for carrier number density, electron temperatures, and phonon temperatures, and it compares

Three Temperature Model for Nonequilibrium Energy Transfer in Semiconductor Films Irradiated with Short Pulse Lasers

MATERIALS TRANSACTIONS, 2006

This article investigates numerically carrier-phonon interaction and nonequilibrium energy transf... more This article investigates numerically carrier-phonon interaction and nonequilibrium energy transfer in direct and indirect bandgap semiconductors during sub-picosecond pulse laser irradiation and also examines the recombination effects on energy transport from the microscopic viewpoint. In addition, the influence of laser fluence and pulse duration is studied by using the self-consistent three-temperature model, which involves carriers, longitudinal optical phonons, and acoustic phonons. It is found that a substantial non-equilibrium state exists between carriers and phonons during short pulse laser irradiation because of time scale difference between the relaxation time and the pulse duration. It is clear that the two-peak structure in carrier temperature exists and it depends mainly on laser pulses, fluences, and recombination processes. During laser irradiation, in particular, the Auger recombination for Si becomes dominant due to the increase in the carrier number density, whereas for GaAs, the Auger recombination process can be ignored due to an abrupt increase in SRH recombination rates at the initial stages of laser exposure.

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Femtosecond Laser Pulse Train Effect on Optical Characteristics and Nonequilibrium Heat Transfer in Thin Metal Films

MATERIALS TRANSACTIONS, 2010

The objective of this study is to numerically investigate the electron-phonon interactions and th... more The objective of this study is to numerically investigate the electron-phonon interactions and the nonequilibrium energy transfer in metal thin films irradiated by ultrashort pulse train lasers. In particular, the temporal and spatial variations in the optical properties during laser irradiation are discussed; the influence of the number of pulses per train and the pulse separation time are also examined. The present study uses the well-established two-temperature model to describe laser-solid interactions and the quantum approach to determine various properties such as electron heat capacity, electron thermal conductivity, collision frequencies, reflectivity, and absorption rates. It is found that as the number of pulses per pulse train increases, the nonequilibrium state between electrons and phonons gradually disappears because of the energy relaxation and the low electron thermal conductivity. The results show that the electron-electron and electron-phonon collision frequencies vary significantly with the number of pulses per train and the separation time per pulse, and that they considerably affect the reflectivity and absorption rate, in turn leading to a change in the ablation mechanism of thin metal films for pulse train laser heating.

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