Recent Submissions

  • Detailed investigation of the mixing field and stability of natural gas and propane in highly turbulent planar flames

    Elbaz, Ayman M.; Mansour, Mohy S.; Akoush, Bassem M.; Juddoo, Mrinal; Khedr, Alaa M.; Al-Bulqini, Hazem M.; Zayed, Mohamed F.; Ahmed, Mahmoud M.A.; Roberts, William L.; Masri, Assaad R. (Fuel, Elsevier BV, 2021-10-18) [Article]
    In most practical combustion devices, the actual combustion process occurs within different mixture inhomogeneity levels. Investigating the mixture fraction field upstream of the reaction zones of these flames is an essential step toward understanding their structure, stability, and emission formation. In this study, the mixture fraction fields were measured for turbulent non-reacting inhomogeneous mixtures immediately downstream from the slot burner exit, using Rayleigh scattering imaging. The slot burner had two concentric slots. The inner air slot can be recessed at distances upstream from the exit of the outer fuel slot, allowing various degrees of mixture inhomogeneity. Mixture fraction field statistics and the two-dimensional gradient were utilized to characterize the impact of the air-to-fuel velocity ratio, global equivalence ratio, fuel composition, Reynolds number, and the premixing length on the mixture mixing field, and thus flame stability. These impacts were evaluated by tracking the normalized mean mixture fraction and mixture fraction fluctuation transition across the regime diagram for partially premixed flames. The results showed that the air-to-fuel velocity ratio was the critical parameter affecting the mixture fraction field for the short premixing length. Stability results showed that the level of mixture inhomogeneity mainly influenced the flame stability. High flame stability is achieved if a large portion of the inhomogeneous mixture fraction is within the fuel flammability limits.
  • On the Formation of Hydrogen Peroxide in Water Microdroplets

    Jr., Adair Gallo; Musskopf, Nayara H.; Liu, Xinlei; Yang, Zi Qiang; Petry, Jeferson; Zhang, Peng; Thoroddsen, Sigurdur T; Im, Hong G.; Mishra, Himanshu (arXiv, 2021-10-14) [Preprint]
    Recent reports on the formation of hydrogen peroxide (H2O2) in water microdroplets produced via pneumatic spraying or capillary condensation have garnered significant attention. How covalent bonds in water could break under such conditions challenges our textbook understanding of physical chemistry and the water substance. While there is no definitive answer, it has been speculated that ultrahigh electric fields at the air-water interface are responsible for this chemical transformation. Here, we resolve this mystery via a comprehensive experimental investigation of H2O2 formation in (i) water microdroplets sprayed over a range of liquid flowrates, the (shearing) air flow rates, and the air composition (ii) water microdroplets condensed on hydrophobic substrates formed via hot water or humidifier under controlled air composition. Specifically, we assessed the contributions of the evaporative concentration and shock waves in sprays and the effects of trace O3(g) on the H2O2 formation. Glovebox experiments revealed that the H2O2 formation in water microdroplets was most sensitive to the air-borne ozone (O3) concentration. In the absence of O3(g), we could not detect H2O2(aq) in sprays or condensates (detection limit ≥250 nM). In contrast, microdroplets exposed to atmospherically relevant O3(g) concentration (10–100 ppb) formed 2–30 μM H2O2(aq); increasing the gas–liquid surface area, mixing, and contact duration increased H2O2(aq) concentration. Thus, the mystery is resolved –the water surface facilitates the O3(g) mass transfer, which is followed by the chemical transformation of O3(aq) into H2O2(aq). These findings should also help us understand the implications of this chemistry in natural and applied contexts.
  • Impact and post-impact response of lightweight CFRP/wood sandwich composites

    Basha, Muhammad; Wagih, A.; Melaibari, A.; Lubineau, Gilles; Abdraboh, A. M.; Eltaher, M. A. (Composite Structures, Elsevier BV, 2021-10-07) [Article]
    Lightweight CFRP/wood sandwich composites receive considerable attention because they can be tailored to satisfy many specifications. Therefore, the damage modes during low velocity impact and compression after impact (CAI) of CFRP/wood sandwich laminate with different wood core types are investigated in this article. The effect of wood fiber orientation on the specific energy dissipation and CAI strength are studied. Results demonstrated that the laminate with balsa core wood, where the wood fibers are perpendicular to the CFRP face plies, shows higher impact load and dissipated energy due to the ability of wood cells to deform during impact and hence dissipate more energy. However, the laminate with birch core, where the wood fibers are parallel to the CFRP surface plies, reveals slightly larger CAI strength due to the role of wood fibers in sharing the load during compression. The damage initiated in the balsa core sandwich laminates at the impacted face and propagate to the unimpacted CFRP surface, while the opposite damage behavior occurs for the birch core sandwich laminate. The balsa core laminate showed larger specific impact load (load/density), dissipated energy (dissipated energy/density), and CAI strength (strength/density), reaching 2.21-, 2.24-, and 1.41-fold times compared to birch core laminates.
  • Ignition delay time and laminar flame speed measurements of ammonia blended with dimethyl ether: A promising low carbon fuel blend

    Issayev, Gani; Giri, Binod; Elbaz, Ayman M.; Shrestha, Krishna P.; Mauss, Fabian; Roberts, William L.; Farooq, Aamir (Renewable Energy, Elsevier BV, 2021-10-04) [Article]
    Ammonia (NH3) has recently received much attention as a promising future fuel for mobility and power generation. The use of ammonia as a fueling vector can help curb global warming by cutting CO2 emissions because it is a carbon-free fuel and a hydrogen carrier with a high percentage of hydrogen atoms per unit volume. Liquid ammonia contains a higher volumetric density of hydrogen than liquid hydrogen. The low reactivity of ammonia, however, hinders its direct usage as a combustible fuel. One feasible way to boost the reactivity of ammonia is to target a dual-fuel system comprising of ammonia and a suitable combustion promoter. In this work, combustion properties of ammonia were investigated by blending it with various proportions of dimethyl ether (DME) using a rapid compression machine (RCM) and a constant volume spherical reactor (CVSR) over a wide range of experimental conditions. DME is a highly reactive fuel that may be produced in a sustainable carbon cycle with a net zero-carbon emission. Ignition delay times (IDTs) of NH3/DME blends were measured over a temperature (T) range of 649–950 K, pressures (P) of 20 and 40 bar, equivalence ratios (Φ) of 0.5 and 1 for a range of DME mole fractions (χDME) of 0.05–0.5 in the blends. In addition, the laminar burning velocities of NH3/DME blends were measured at P = 1, 3 and 5 bar, Φ = 0.8–1.3 and T = 300 K for χDME ranging from 0.18 to 0.47. Our results suggest that DME is a good ignition promoter, resulting in a significant shortening of IDTs and an increase of flame speeds of NH3. A detailed chemical model has been developed and validated against the experimental data. Overall, our kinetic model offered reasonable predictive capabilities capturing the experimental trends over a wide range of conditions. In the worst-case scenario, our model underpredicted IDTs by a factor of ∼2.5 while overpredicting laminar flame speed by ∼20%.
  • Temperature dependent Raman spectra of ammonia ranging from 3150 cm−1 to 3810 cm−1 for combustion applications

    Yang, Chaobo; Ezendeeva, Diana; Yu, Tao; Magnotti, Gaetano (Optics Express, The Optical Society, 2021-09-29) [Article]
    Applying in combustion research, Raman scattering technique can provide high accuracy and high precision measurements of temperature and major species concentrations. However detailed knowledge of the temperature dependent Raman spectra of the probed species is a precondition to realise the potential of high precision and accuracy of the technique. As a carbon free novel fuel, the knowledge of high temperature Raman spectra of ammonia is rarely reported. We measured the Raman spectra of ammonia ranging from 299 K to 760 K. The high resolution Raman spectra are excited with a continuous wave 532 nm laser and detected with an low aberration Schmidt-Czerny-Turner spectrometer. The temperature of probe volume was determined by the fitting of N2 Raman spectra. The Raman spectra of ammonia under different temperatures were quantitatively normalized to the same number density to research the temperature behavior of spectra. Within the Raman shift region from 3150 cm−1 to 3810 cm−1, the Raman intensity and the polarization anisotropy of vibrational modes ν1, ν3, and 2ν4 were reported. The relative intensity between ν1, ν3, and 2ν4 modes were also analyzed under different temperatures.
  • Symmetric Ethers as Bioderived Fuels: Reactivity with OH Radicals

    Belmekki, Myriam; Giri, Binod; Liu, Dapeng; Farooq, Aamir (Energy & Fuels, American Chemical Society (ACS), 2021-09-27) [Article]
    Environmental pollution and greenhouse gas emissions are major challenges faced by our society. One possible way to mitigate global warming is to cut CO2 emissions by taking a shift from conventional fuels to renewable fuels for future sustainability. Carbon neutral fuels produced in a sustainable carbon cycle can close the carbon cycle and reach net zero-carbon emission. To this end, ethers are promising renewable fuels and/or additives for future advanced combustion engines. Therefore, understanding the oxidation behavior of ethers under engine-relevant conditions is of utmost importance. In this work, the reaction kinetics of hydroxyl radicals with dimethyl ether (DME), diethyl ether (DEE), di-n-propyl ether (DPE), and di-n-butyl ether (DBE) were investigated behind reflected shock waves over the temperature range of 865–1381 K and the pressure range of 0.96–5.56 bar using a shock tube and a UV laser diagnostic technique. Hydroxyl radicals were monitored near 306.7 nm to follow the reaction kinetics. These reactions did not exhibit discernible pressure effects. The temperature dependence of the measured rate coefficients can be expressed by the following modified Arrhenius equations in units of cm3 mol–1 s–1: k1(DME+OH) = 1.19 × 1014 exp(−2469.8/T), k2(DEE+OH) = 1.27 × 107T2 exp(327.8/T), k3(DPE+OH) = 1.64 × 107T2 exp(368.4/T), k4(DBE+OH) = 9.12 × 1011T0.65 exp(−843.5/T). Our measured rate data were analyzed to obtain site-specific rates and branching ratios. Our results are compared with the available literature data wherever applicable. Furthermore, the ability of Atkinson’s structure–activity relationship (SAR) to predict the kinetic behavior of the reactions of dialkyl ethers with OH radicals was examined.
  • Investigations into the Effects of Spark Plug Location on Knock Initiation by using Multiple Pressure Transducers

    Uddeen, Kalim; Shi, Hao; Tang, Qinglong; Turner, James William (SAE International, 2021-09-21) [Conference Paper]
    Despite a long history of development, modern spark-ignition (SI) engines are still restricted in obtaining higher thermal efficiency and better performance by knock. Knocking combustion is an abnormal combustion phenomenon caused by the autoignition of unburned air-fuel mixture ahead of the propagating flame front. This work describes investigations into the significance of spark plug location (with respect to inlet and exhaust valve position) on the knock formation mechanism. To facilitate the investigation, four spark plugs were installed in a specialized liner at four equispaced distinct locations to propagate flames from those locations, which provoked a distinct flame propagation from each and thus individual autoignition profiles. Six pressure transducers were arranged to precisely record the pressure oscillations, knock intensities, and combustion characteristics. Four of the six transducers were mounted on the circumference of the liner (each next to one of the spark plugs), one was placed at the center of the cylinder head, and one at a slight offset from the center of cylinder head. The results showed that the spark plug which was close to the exhaust valves triggered higher knock intensity along with earlier CA50, but the spark plug near the inlet valves caused weaker knock intensities for the same operating conditions. In addition, the study also covered the effect of swirl direction to suppress knock. A band pass filtering analysis was applied to estimate the pressure oscillations with respect to the spark plug locations, using data from the multiple pressure sensors. Furthermore, Fast Fourier Transform (FFT) analyses were implemented to estimate the frequency of the pressure oscillation resulting from knock. It was found that firing the spark plugs, near the inlet and between the inlet and exhaust valves promoted the (1, 0) acoustic mode effectively, while the spark plug near the exhaust valves caused the (1, 0) mode along with the (2, 0) acoustic mode for the same operating conditions, indicating that the autoignition was initiated near the cylinder walls.
  • Chemical speciation and soot measurements in laminar counterflow diffusion flames of ethylene and ammonia mixtures

    Zhou, Mengxiang; Yan, Fuwu; Ma, Liuhao; Jiang, Peng; Wang, Yu; Chung, Suk Ho (Fuel, Elsevier BV, 2021-09-17) [Article]
    Ammonia is considered as one of the most promising alternative fuels due to its carbon neutrality and the existing infrastructure for its mass production and delivery. However, burning neat ammonia has the issue of poor flame stability and high NOx emissions, making co-firing ammonia with conventional fuel a more feasible approach. The present work investigated the sooting characteristics of counterflow diffusion flames of ethylene/ammonia mixtures. Experimentally, soot volume fraction (SVF) and average soot particle diameter in the neat ethylene, ammonia- and nitrogen- doped flames were non-intrusively measured. Both SVF and average soot particles diameter were found to decrease with the addition of ammonia. Flame temperature were measured with tunable diode laser absorption spectroscopy and the results suggested that the inhibiting effect of ammonia on soot formation was chemical instead of thermal. For further kinetic insights, numerical simulation with newly-constructed reaction mechanisms were performed and the results were compared against chemical speciation data from gas chromatography (GC) measurements; the results showed that ammonia doping would lead to more significant reduction of benzene concentration than nitrogen doping. Kinetic pathways of the chemical suppressing effect of ammonia addition on soot and its precursor formation were then explained based on numerical results. The major contribution of the present work can be summarized in the following aspects: 1) New comprehensive experimental data on sooting characteristics, important intermediate species concentrations of diffusion counterflow flames of ethylene/ammonia mixtures were provided for model validation; 2) One coupled mechanism with detailed hydrocarbon-nitrogen interactions was established to predict PAHs formation and soot formation; 3) Detailed chemical kinetics insight of ammonia effect on soot formation was presented in the counterflow flame.
  • Evaluation of Thermoacoustic Applications Using Waste Heat to Reduce Carbon Footprint

    Spoor, Philip; Prabhudharwadkar, Deoras Mukund; Somu, Srinath; Saxena, Saumitra; Lacoste, Deanna; Roberts, William L. (American Society of Mechanical Engineers, 2021-09-16) [Conference Paper]
    Abstract Thermoacoustics (TA) engines and refrigerators typically run on the Stirling cycle with acoustic networks and resonators replacing the physical pistons. Without moving parts, these TA machines achieve a reasonable fraction of Carnot’s efficiency. They are also scalable, from fractions of a Watt up to kW of cooling. Despite their apparent promise, TA devices are not in widespread use, because outside of a few niche applications, their advantages are not quite compelling enough to dislodge established technology. In the present study, the authors have evaluated a selected group of applications that appear suitable for utilization of industrial waste heat using TA devices and have arrived at a ranked order. The principal thought is to appraise whether thermoacoustics can be a viable path, from both an economic and energy standpoint, for carbon mitigation in those applications. The applications considered include cryogenic carbon capture for power plant exhaust gases, waste-heat powered air conditioning/water chilling for factories and office buildings, hydrogen liquefaction, and zero-boiloff liquid hydrogen (LH2) storage. Although the criteria used for evaluating the applications are somewhat subjective, the overall approach has been consistent, with the same set of criteria applied to each of them. Thermoeconomic analysis is performed to evaluate the system viability, together with overall consideration of a thermoacoustic device’s general nature, advantages, and limitations. Our study convincingly demonstrates that the most promising application is zero-boiloff liquid hydrogen storage, which is physically well-suited to thermoacoustic refrigeration and requires cooling at a temperature and magnitude not ideal for standard refrigeration methods. Waste-heat powered air conditioning ranks next in its potential to be a viable commercial application. The rest of the applications have been found to have relatively lower potentials to enter the existing commercial space.
  • A numerical method for self-similar solutions of the ideal magnetohydrodynamics

    Chen, Fang; Samtaney, Ravi (Journal of Computational Physics, Elsevier BV, 2021-09-10) [Article]
    We present a numerical method to obtain self-similar solutions of the ideal magnetohydrodynamics (MHD) equations. Under a self-similar transformation, the initial value problem (IVP) is converted into a boundary value prob1 lem (BVP) by eliminating time and transforming the system to self-similar coordinates (ξ ≡ x/t, η ≡ y/t). The ideal MHD system of equations is augmented by a generalized Lagrange multiplier (GLM) to maintain the solenoidal condition on the magnetic field. The self-similar solution to the BVP is solved using an iterative method, and implemented using the p4est adaptive mesh refinement (AMR) framework. Existing Riemann solvers (e.g., Roe, HLLD etc.) can be modified in a relatively straightforward manner and used in the present method. Numerical tests numerical tests illustrate that the present self-similar solution to the BVP exhibits sharper discontinuities than the corresponding one solved by the IVP. We compare and contrast the IVP and BVP solutions in several one dimensional shock-tube test problem and two dimensional test cases include shock wave refraction at a contact discontinuity, reflection at a solid wall, and shock wave diffraction over a right angle corner.
  • Effects of ammonia and hydrogen on the sooting characteristics of laminar coflow flames of ethylene and methane

    Steinmetz, S.A.; Ahmed, H.A.; Boyette, Wesley; Dunn, M.J.; Roberts, William L.; Masri, A.R. (Fuel, Elsevier BV, 2021-09-10) [Article]
    Hydrogen and its derivatives, including ammonia, are gaining increasing attention as carbon-neutral fuel alternatives. An intermediate step in the transition to hydrogen and ammonia is the blending of these fuels with hydrocarbons, introducing the challenge of soot formation. The impact of ammonia on soot formation has recently been the focus of several studies, but a complete understanding of its chemical effects is lacking. Hydrogen, by comparison, has received significant attention from the soot community. However, controversy remains with regards to hydrogen’s chemical impact, and the dependence of this impact on fuel and flame configuration. This work investigates the effect of both hydrogen and ammonia on soot formation in laminar coflow flames of both ethylene and methane. Hydrogen or ammonia are introduced either by addition or substitution, with parallel studies of helium and argon, in order to isolate their chemical effects. Time- and spectrally-resolved laser-induced emissions from UV and IR excitation are used to quantify differences in soot and soot precursor formation. Additionally, chemical kinetics calculations and analyses are used to elucidate the effects of ammonia introduction to ethylene flames. Ammonia is found to chemically inhibit soot when mixed with either ethylene or methane, with increasing effects on larger precursors. Calculations suggest that this suppression is due to carbon consumption in the formation of HCN and CN. Hydrogen is found to chemically enhance soot formation in both ethylene and methane flames.
  • Flame edge dynamics in counterflow nonpremixed flames of CH4/He versus air at low strain rates: An experimental and numerical study

    Jung, Ki Sung; Kwon, Soon Hyeong; Chung, Suk Ho; Park, Jeong; Yoo, Chun Sang (Combustion and Flame, Elsevier BV, 2021-09-09) [Article]
    The characteristics of the flame structure, stabilization, and extinction of counterflow nonpremixed flames of CH4/He versus air at low strain rates are investigated by performing a series of experiments and twodimensional (2-D) numerical simulations. By adopting an experimental methodology using He curtain flow, we can locate the flames near the center of the counterflow burner and measure the critical He mole fraction in the fuel stream, XHe,cr, for flame extinction at very-low strain rates. XHe,cr obtained from 2-D numerical simulations in normal and zero gravity show a good agreement with those from the experiments, which substantiates that the experimental methodology can effectively reduce the buoyancy effect at low strain rates. It is found from various steady and unsteady 2-D numerical simulations that the dynamics of flame edge plays a critical role in determining the flame stabilization and extinction, and the edge flame is stabilized at a location where negative edge flame propagation speed, Se, balances positive local flow velocity, Ue. The transport budget analysis reveals that despite the negative Se by the diffusive loss of heat and radicals, the edge flame can survive by the help of the convective gain of heat and radicals from the trailing diffusion flame. It is also found that the counterflow flame can survive the increase of He mole fraction in the fuel stream, XHe, by shrinking its flame length since the local chemical reaction at the flame edge is enhanced with decreasing the flame length. However, as XHe exceeds XHe,cr, a slight inward movement of the edge flame induces a large magnitude of negative Se compared to positive Ue such that the counterflow flame is totally extinguished by the shrinkage of the outer edge flame toward the flame center.
  • Computational comparison of the conventional diesel and hydrogen direct-injection compression-ignition combustion engines

    Babayev, Rafig; Andersson, Arne; Serra Dalmau, Albert; Im, Hong G.; Johansson, Bengt (Fuel, Elsevier BV, 2021-09-09) [Article]
    Most research and development on hydrogen (H2) internal combustion engines focus on premixed-charge spark ignition (SI) or diesel-hydrogen dual-fuel technologies. Premixed charge limits the engine efficiency, power density, and safety, while diesel injections give rise to CO2 and particulate emissions. This paper demonstrates a non-premixed compression-ignition (CI) neat H2 engine concept that uses H2 pilots for ignition. It compares the CI H2 engine to an equivalent diesel engine to draw fundamental insights about the mixing and combustion processes. The Converge computational fluid dynamics solver is used for all simulations. The results show that the brake thermal efficiency of the CI H2 engine is comparable or higher than diesel, and the molar expansion with H2 injections at TDC constitutes 5–10 % of the total useful work. Fuel-air mixing in the free-jet phase of combustion is substantially higher with H2 due to hydrogen's gaseous state, low density, high injection velocity, and transient vortices, which contribute to the 3 times higher air entrainment into the quasi-steady-state jet regions. However, the H2 jet momentum is up to 4 times lower than for diesel, which leads to not only ineffective momentum-driven global mixing but also reduced heat transfer losses with H2. The short H2 flame quenching distance may also be inconsequential for heat transfer in CI engines. Finally, this research enables future improvements in CI H2 engine efficiency by hypothesizing a new optimization path, which maximizes the free-jet phase of combustion, hence is totally different from that for conventional diesel engines.
  • Synthesis of Silicon and Silicon Carbide Nanoparticles by Pulsed Electrical Discharges in Dielectric Liquids

    Hamdan, Ahmad; Abiad, Douaa El; Cha, Min Suk (Plasma Chemistry and Plasma Processing, Springer Science and Business Media LLC, 2021-09-07) [Article]
    Silicon carbide (SiC) has been widely used in many applications, which require high mechanical endurance or high electrical resistance. It also serves as a basic material for light emitting diodes. Here, we present an in-liquid plasma method to produce SiC nanoparticles. A sustained spark-discharge in a dielectric liquid, which is energized by a nanosecond pulsed power supply, is established for the synthesis. To provide Si and C, we employed graphite and silicon as electrodes and cyclohexane (CHX) and tetramethylsilane (TMS) as dielectric liquids. For a reasonable comparison, we tested various combinations of electrode and liquid, namely Si-to-C in CHX, Si-to-Si in CHX, and C-to-C in TMS. We found that discharges in CHX produce Si particles encapsulated in C-shell and Si nanoparticles in C-matrix. Meanwhile, discharges in TMS consistently produce SiC nanoparticles with an average size of ~ 10 nm, regardless of the electrode material.
  • Effects of Fuel Composition on Auto-Ignition and Detonation Development in Boosted Spark-Ignited Engines

    Gorbatenko, Inna; Singh, Eshan; Sarathy, Mani; Nicolle, Andre (SAE International, 2021-09-05) [Conference Paper]
    The development of highly boosted and high compression spark-ignition engines with enhanced thermal efficiencies is primarily limited by knock and super-knock. Super-knock is an excessively high intensity knock which has been related to a developing detonation process. This study investigates the knocking tendency of different gasoline surrogate fuels with varying research octane numbers (RON), octane sensitivity (S) and composition. The ξ/ɛ diagram with an enclosed detonation peninsula is used to assess the knocking tendency of different fuels. The diagram plots ξ, the ratio of acoustic to auto-ignitive velocity, against ɛ, the ratio of the transit time of an acoustic wave through a hot spot, to the heat release time (τe). Constant volume simulations of auto-ignition delay times (τi) and excitation times (τe) obtained from chemical kinetic calculations, enable calculations of ξ and ɛ. Their location for different fuels and operating conditions on the ξ/ɛ diagram, relative to the detonation peninsula, defines their mode of reaction propagation and the severity of a detonation. It was shown that excitation times are not affected by RON and S of the fuel. However, they are strongly dependent on the mixture composition. Fuels exhibiting a strong negative temperature chemistry (NTC) region are found to enter detonation development and explosion region, and are more likely to result in super-knock events in boosted spark-ignition engines.
  • Computational Investigation of the Effects of Injection Strategy and Rail Pressure on Isobaric Combustion in an Optical Compression Ignition Engine

    Aljabri, Hammam H.; Liu, Xinlei; Allehaibi, Moaz; AlRamadan, Abdullah S.; Badra, Jihad; Ben Houidi, Moez; Johansson, Bengt; Im, Hong G. (SAE International, 2021-09-05) [Conference Paper]
    The high-pressure isobaric combustion has been proposed as the most suitable combustion mode for the double compre4ssion expansion engine (DCEE) concept. Previous experimental and simulation studies have demonstrated an improved efficiency compared to the conventional diesel combustion (CDC) engine. In the current study, isobaric combustion was achieved using a single injector with multiple injections. Since this concept involves complex phenomena such as spray to spray interactions, the computational models were extensively validated against the optical engine experiment data, to ensure high-fidelity simulations. The considered optical diagnostic techniques are Mie-scattering, fuel tracer planar laser-induced fluorescence (PLIF), and natural flame luminosity imaging. Overall, a good agreement between the numerical and experimental results was obtained. Upon validation, the optimized models have been used to conduct a comparative study between the conventional diesel combustion (CDC) and the isobaric combustion cases with different pressure levels, in terms of engine performance and emissions. Compared to the CDC case, the isobaric combustion cases led to a lower NOx emission but higher sooting tendency due to the increased diffusion combustion feature, although most of the soot was oxidized in the later engine cycle. To further reduce soot emission, the effects of various rail pressures and injector holes number were evaluated. The results indicated that the higher injection pressure was more effective in soot reduction for the isobaric combustion case but it deteriorated the thermal efficiency. It was also found that increasing the number of injector holes from the reference six to ten led to the lowest soot emission without significantly affecting the efficiency.
  • Investigation of the Engine Combustion Network Spray C Characteristics at High Temperature and High-Pressure Conditions Using Eulerian Model

    Al-lehaibi, Moaz; Liu, Xinlei; Aljabri, Hammam H.; Ben Houidi, Moez; Mohan, Balaji; Im, Hong G. (SAE International, 2021-09-05) [Conference Paper]
    The morphology of the internal flow of Spray C was numerically investigated using an Eulerian volume-of-fluid (VOF) method in the finite-volume framework. The injector geometry available in the Engine Combustion Network (ECN) was employed, and the simulations were performed under the ambient condition at 900 K and 60 bar. The simulation data were analyzed for three important events: the initial nozzle opening, steady injection, and nozzle closing. First, projected densities on XY and XZ planes are computed radially at four axial locations. Projected density at 2 mm is compared with available experimental results, which show similar results. Then, the mass flow rate is found to match the reported experimental results and the virtually generated values from CMT using an appropriate discharge coefficient. An investigation on the appropriate discharge coefficient is performed and found that Cd = 0.63 ± 0.02 is acceptable for Spray C. A grid-convergent study revealed that the predicted cavitation formation process was significantly affected by various grid sizes. Based on this study, a minimum mesh size of 7.8 µm was needed to reproduce the experimental observation properly. The predicted flow characteristics at the sharper edge at 0° and the smooth edge at 180° degrees showed comparable trends to the experimental data at the same injection pressure but different ambient pressure.
  • Flow-Field Analysis of Isobaric Combustion Using Multiple Injectors in an Optical Accessible Diesel Engine

    Panthi, Niraj; Goyal, Harsh; Ben Houidi, Moez; AlRamadan, Abdullah; Badra, Jihad; Magnotti, Gaetano (SAE International, 2021-09-05) [Conference Paper]
    Isobaric combustion has shown the potential of improving engine efficiency by lowering the heat transfer losses. Previous studies have achieved isobaric combustion through multiple injections from a single central injector, controlling injection timing and duration of the injection. In this study, we employed three injectors, i.e. one centrally mounted (C) on the cylinder head and two side-injectors (S), slant-mounted on cylinder head protruding their nozzle tip near piston-bowl to achieve the isobaric combustion. This study visualized the flame development of isobaric combustion, linking flow-field details to the observed trends in engine efficiency and soot emissions. The experiments were conducted in an optically accessible single-cylinder heavy-duty diesel engine using n-heptane as fuel. Isobaric combustion, with a 50 bar peak pressure, was achieved with three different injection strategies, i.e. (C+S), (S+C), and (S+S). Bottom-view high-speed soot luminosity images were recorded at a frame rate of 20 kHz for all cases, together with pressure traces. Flame image velocimetry (FIV) analysis was performed on the high-speed soot luminosity images to obtain a qualitative description of the flow-field obtained for the three injection strategies. Distinctive vortex structures were evident from the FIV analysis and that can be attributed to strong flame-wall and flame-flame interactions. For the C+S and S+S injection strategies, the distinct large vortex structures were found near the bowl-wall while for the S+C case, vortex structures are less prominent. The large vortex structures close to the cylinder walls contribute to lower gross indicated efficiency and higher soot level intensity of the C+S and S+S cases, compared to the S+C configuration.
  • On the Relevance of Octane Sensitivity in Heavily Downsized Spark-Ignited Engines

    Singh, Eshan; Mohammed, Abdulrahman; Gorbatenko, Inna; Sarathy, Mani (SAE International, 2021-09-05) [Conference Paper]
    Over the years, spark-ignition engine operation has changed significantly, driven by many factors including changes in operating conditions. The variation in operating conditions impacts the state of the end-gas, and therefore, its auto-ignition. This can be quantified in terms of K-factor, which weighs the relative contribution of Research Octane Number (RON) and Motor Octane Number (MON) to knocking tendency at any operating condition. The current study investigates the fuel requirements when operating an engine at increasing intake air pressures. A model engine was operated at varying intake air pressure in GT-Power software, from naturally aspirated intake air to heavily boosted intake air pressure of 4 bar absolute. The pressure-temperature information from the GT-Power model was used to calculate ignition delay times of the unburnt end-gas composed of a sensitive and a non-sensitive fuel in ChemKin software. The results show that high octane sensitivity is desired at negative K values (operating at high intake air pressures). In contrast, zero octane sensitivity fuel performed best at low load operation (positive K). Interestingly, the maximum benefit for using a sensitive fuel was achieved at an intake air pressure of 1.75 bar with diminishing returns at higher intake air pressure for 1000 rpm and at lower intake pressures, as engine speed increased. The pressure effect on auto-ignition tendency was also investigated over existing HCCI data. The auto-ignition tendency was found to be sensitive to octane index in a region of low K value (K~0). This region lies in the negative temperature coefficient (NTC) region, where Primary Reference Fuels (PRFs) shown an increased sensitivity to pressure variation.
  • Performance Analysis and In-Cylinder Visualization of Conventional Diesel and Isobaric Combustion in an Optical Diesel Engine

    Goyal, Harsh; Panthi, Niraj; Ben Houidi, Moez; AlRamadan, Abdullah S.; Badra, Jihad; Magnotti, Gaetano (SAE International, 2021-09-05) [Conference Paper]
    Compared to conventional diesel combustion (CDC), isobaric combustion can achieve a similar or higher indicated efficiency, lower heat transfer losses, reduced nitrogen oxides (NOx) emissions; however, with a penalty of soot emissions. While the engine performance and exhaust emissions of isobaric combustion are well known, the overall flame development, in particular, the flow-field details within the flames are unclear. In this study, the performance analysis of CDC and two isobaric combustion cases was conducted, followed by high-speed imaging of Mie-scattering and soot luminosity in an optically accessible, single-cylinder heavy-duty diesel engine. From the soot luminosity imaging, qualitative flow-fields were obtained using flame image velocimetry (FIV). The peak motoring pressure (PMP) and peak cylinder pressure (PCP) of CDC are kept fixed at 50 and 70 bar, respectively. The two isobaric combustion cases, achieved using multiple injections, are maintained at the CDC PMP level of 50 bar for the low-pressure case (IsoL) and CDC PCP level of 70 bar for the high-pressure case (IsoH). For each operating condition, soot luminosity signals are captured at a frame rate of 20 kHz, and a semi-quantitative velocity flow-field is obtained from FIV post-processing. Consistent with previous metal engine experiments, isobaric combustion - in particular IsoH, resulted in similar gross indicated efficiency, lower heat losses but higher exhaust losses, compared to CDC. The soot luminosity images of CDC show initial signals originated close to the bowl-wall for certain jets while for the isobaric combustion, the flames corresponding to each jet are clearly distinguished during the earlier flame development process. The vector field distribution within the flames shows the transition of flame-wall impingement to flame-flame interaction regions between the neighboring jets for each combustion mode. Furthermore, higher flame-flame interaction regions and uniform distribution of signals around the combustion chamber for isobaric combustion, justifying higher soot formation and lower heat transfer losses, respectively, compared to CDC.

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