Now showing items 21-40 of 2371

    • Enhanced Open-Hole Strength and Toughness of Sandwich Carbon-Kevlar Woven Composite Laminates

      Khan, Mohammad K. A.; Junaedi, Harri; Alshahrani, Hassan; Wagih, Ahmed; Lubineau, Gilles; Sebaey, Tamer A. (Polymers, MDPI AG, 2023-05-11) [Article]
      Fiber-reinforced plastic composites are sensitive to holes, as they cut the main load-carrying member in the composite (fibers) and they induce out-of-plane stresses. In this study, we demonstrated notch sensitivity enhancement in a hybrid carbon/epoxy (CFRP) composite with a Kevlar core sandwich compared to monotonic CFRP and Kevlar composites. Open-hole tensile samples were cut using waterjet cutting at different width to diameter ratios and tested under tensile loading. We performed an open-hole tension (OHT) test to characterize the notch sensitivity of the composites via the comparison of the open-hole tensile strength and strain as well as the damage propagation (as monitored via CT scan). The results showed that hybrid laminate has lower notch sensitivity than CFRP and KFRP laminates because the strength reduction rate with hole size was lower. Moreover, this laminate showed no reduction in the failure strain by increasing the hole size up to 12 mm. At w/d = 6, the lowest drop in strength showed by the hybrid laminate was 65.4%, followed by the CFRP and KFRP laminates with 63.5% and 56.1%, respectively. For the specific strength, the hybrid laminate showed a 7% and 9% higher value as compared with CFRP and KFRP laminates, respectively. The enhancement in notch sensitivity was due to its progressive damage mode, which was initiated via delamination at the Kevlar–carbon interface, followed by matrix cracking and fiber breakage in the core layers. Finally, matrix cracking and fiber breakage occurred in the CFRP face sheet layers. The specific strength (normalized strength and strain to density) and strain were larger for the hybrid than the CFRP and KFRP laminates due to the lower density of Kevlar fibers and the progressive damage modes which delayed the final failure of the hybrid composite.
    • Electric fields in a counterflow nonpremixed flame: measurement and simulation

      Park, Jin; Son, Jinwoo; Butterworth, Thomas D.; Cha, Min Suk (Scientific Reports, Springer Science and Business Media LLC, 2023-05-10) [Article]
      In electric field modified flames, the electric body force on fluid elements can play a role in modifying the flow field, affecting flame characteristics by this modified flow motion. Numerical studies have developed ion kinetic mechanisms and appropriate transport models for charged species, validating them with a voltage-current trend in 1D premixed flames. Recent experimental approaches have measured the electric field by adopting the Electric Field Induced Second Harmonic generation (EFISH) technique. However, the quantification has turned out very challenging due to the inherent distortion in the EFISH signal, as well as inhomogeneous temperature and concentration fields in the combustion field. Here, we propose measurement and calibration schemes to quantify the EFISH signal in a laminar counterflow nonpremixed flame and present comparison with numerical results using an in-house multi-physics CFD (Computational Fluid Dynamics) code. Overall, the quantified electric fields agreed well with those from numerical simulation, specifically capturing null electric fields near the flame in the sub-saturated regime due to the electric field screening effect. In the saturated regime, notable discrepancy was found in a fuel stream when electrons moved through it: experiment indicated a significant number of negative ions in the fuel stream, whereas numerical results predicted negligible negative ions, due to the implemented ion-mechanism. This suggested that the experimentally obtained electric fields may serve as validation data for modeling studies to improve transport models and ion-mechanism. In-situ measurement of charged species in the presence of external electric fields should be a future work.
    • Impacts of NO on low-temperature oxidation of n-heptane in a jet-stirred reactor

      Zhai, Yitong; Xu, Qiang; Feng, Beibei; Shao, Can; Wang, Zhandong; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2023-05-10) [Article]
      Low-temperature (low-T) oxidation experiments of n-heptane –with and without NO addition– were experimentally and numerically investigated at stoichiometric conditions in a jet-stirred reactor. Experiments were performed at atmospheric pressure over a temperature range of 500–800 K. Reactants, intermediates, and products, were measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A detailed kinetic model was developed to gain insight into the chemical effect of NO on low-T oxidation chemistry of n-heptane. Taking 650 K as the transition temperature, the results revealed that NO addition exhibited an inhibiting effect on fuel reactivity below 650 K and a promoting effect above 650 K. The reactions of ROO + NO = RO + NO2 and HO2 + NO = OH + NO2 at different temperature regions were responsible for the inhibition and promotion effects, respectively. Evidence gathered from both experimental measurements and kinetic model predictions indicated that NO addition had a significant inhibitory effect on the formation of cool flame species during the low-T oxidation process. NO suppressed low-T oxidation via the reaction of ROO + NO = RO + NO2, which impeded the subsequent isomerization, O2 addition, OH-, and HO2-elimination reaction, and influenced product distribution of the cool flame species. The experimental observations provided detailed information about these reactive intermediates, which offered new insights into low-T oxidation phenomena and clarified the importance of NO reactions which prevent the formation of cool flame products during low-T oxidation.
    • Computational Design of a Vertical Wind Tunnel for Stable Droplet Levitation

      Nawaz, Muneebullah (2023-05-10) [Thesis]
      Advisor: Truscott, T. T.
      Committee members: Thoroddsen, Sigurdur T; Grande, Carlos A.
      The efficient study of liquid droplets ranging from micrometers to a few centimeters by levitation is usually hindered by conventional design limitations. This is due to continuous droplet deformation in the test section. This research discusses the development of a robust design methodology for large droplet-stabilization (d > Capillary Number (Ca)) vertical wind tunnels. A modeling and simulation design environment has been developed that involves component sizing and integration at a central ANSYS-Fluent platform, followed by design optimization. The work inculcates numerical analysis of guide vanes to minimize the viscous losses and, subsequently, the wind tunnel dimensions. The process is followed by the design of honeycomb and wire screens and their analyses for a given geometry. A multi-variable design optimization problem has been optimized with response surface approximations. Statistical modeling of the expensive functions obtained from the solution of Navier-stokes equations has been accomplished in order to deal with non-linear and discontinuous behavior. Numerical optimization of the meta-model can help to find the most feasible wind tunnel design with computational efficiency. A non-conventional design with varying test area cross-sections has been introduced to investigate the droplet stability in constantly changing velocity profiles. Longitudinal as well as lateral velocity variations in the test section, creating velocity buckets with minimum turbulence intensity, has been introduced and analyzed using novel concept designs. The research highlights a systematic design methodology and an alternate configuration for liquid droplet wind tunnels while focusing on stable droplet levitation.
    • Reduced chemical kinetic model for CH4-air non-premixed flames including excited and charged species

      López-Cámara, Claudia-F.; Saggese, Chiara; Pitz, William J.; Shao, Xiao; Im, Hong G.; Dunn-Rankin, Derek (Combustion and Flame, Elsevier BV, 2023-05-09) [Article]
      Electric fields can impact small laminar flames by changing their shape and overall behavior by acting on charged species produced in combustion. However, no reduced chemical kinetic model has been developed considering both major species and minor species related to flame characterization and flame behavior in the presence of an electric field. This study presents a reduced chemical kinetic model for methane-air combustion which includes minor excited species (CH∗ and OH∗) and charged species (H3O+,HCO+, C2H3O+, CH5 O+, O−2 , OH−, e− , CO−3 , CHO−2 , O−, CHO−3 ). The results employing the reduced chemistry model have been validated for a two-dimensional flame geometry by comparison with (i) detailed chemistry simulation results for species location and peak values, and (ii) experimental CH∗ chemiluminescence location, considering the self-repulsion of charges yet without externally applied electric field to the flame. This reduced chemical kinetic model, with 45 species and 216 reactions, shows a computational demand one-third that of employing its equivalent detailed chemistry (83 species and 394 reactions). The reduction is modest but significant considering that high fidelity is needed to capture the behavior of the chemi-ion and chemiluminescent species. Future works will involve the use of this model for simulations predicting flame behavior with applied electric field (i.e., field strength = 0 kV/cm).
    • Computational Optimization of the Performance of a Heavy-Duty Natural Gas Pre-Chamber Engine

      Liu, Xinlei; Silva, Mickael; Mohan, Balaji; AlRamadan, Abdullah; Cenker, Emre; Im, Hong G. (Elsevier BV, 2023-05-08) [Preprint]
      Optimization of the performance of a heavy-duty natural gas active pre-chamber (PC) engine was conducted using computational fluid dynamics (CFD) simulations. Seven different piston geometries were evaluated for their impact on engine performance. Unlike the narrow-throat PC, various piston geometries yielded a significant impact on the PC flow motion and fuel-air mixing process using the large-throat PC. Compared to flat-bottom pistons, w-bottom pistons promoted the reverse flow from the MC and induced a broader lean-mixture distribution in the PC. For various w-bottom pistons, enlarging the inner piston radius resulted in slower flame propagation within the PC, delaying the jet issuing and slowing down the turbulent flame propagation within the MC. The flat piston effectively reduced jet flame-piston interaction and promoted turbulent flame propagation within the squish region during the late combustion stage, but it generated higher wall heat transfer loss from the cylinder liner. A composite of w- and flat-shaped pistons maintained the benefits of relatively high flame propagation speed within the squish region but low wall heat transfer loss, yielding the highest thermal efficiency of seven piston designs. The PC fueling ratio (PCFR) was found to be a critical factor in influencing the jet issuing process. A lower PCFR to establish a near-stoichiometric mixture within the PC promoted flame propagation and led to a faster jet ejection. An increase in compression ratio yielded higher engine efficiency due to the advanced combustion phasing and larger expansion ratio, it also resulted in higher wall heat transfer loss and nitric oxide (NOx) emission. The introduction of exhaust gas recirculation was able to effectively inhibit NOx formation, while also resulting in higher engine efficiency due to the lower wall heat transfer loss.
    • C7 reaction mechanism and its self-imitation in the kinetic modeling of PAH formation

      Jin, Hanfeng; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2023-05-08) [Article]
      Polycyclic aromatic hydrocarbons (PAHs), serving as critical soot precursors, are formed via complicated chemical processes, such as the first aromatic ring formation and the aromatic ring growth, from small molecules. Benzene/phenyl (“C6”) is commonly considered as the critical first aromatic ring formed, which has attracted many studies on its formation and its further mass growth. Reactions of cyclopentadienyl (“C5”) are also recognized as PAH formation pathways without involving benzene. Compared to “C5” and “C6”, less attention has been paid to “C7” aromatics, such as vinyl-cyclopentadienyl, tropyl and fulvenallenyl, for their roles in PAH formation. Previous PAH models included very few reactions of “C7” sub-mechanism, and somewhat ignored the self-consistency of the reaction networks of aromatic species with characteristic molecular structures. In this study, we upgraded “C7” sub-mechanism and revealed the self-imitation between the reaction network of monocyclic and polycyclic aromatic hydrocarbons. Model validation with literature experimental data shows that: a) benzyl, vinyl-cyclopentadienyl, tropyl, and fulvenallenyl are crucial intermediates in “C7” chemistry; b) “C7” resonance stabilized radicals (RSRs) are mainly contributed by different entrance channels from small aromatic precursors; c) the reactions of “C7” species provide unique contribution to PAH formation according to their specific molecular moieties. This study also reveals the basic aromatic classes, “C5”, “C6”, “C7”, and “fC7”, for the hierarchical reaction network, and the self-imitation between the hierarchical reaction networks of monocyclic and polycyclic aromatic hydrocarbons. Future work will build a kinetic model up to very large PAHs, close to soot nanoparticle, using the self-imitation of the hierarchical reaction network proposed in this study.
    • Methane and n-hexane ignition in a newly developed diaphragmless shock tube

      Subburaj, Janardhanraj; Kashif, Touqeer Anwar; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2023-05-03) [Article]
      Shock tubes have been routinely used to generate reliable chemical kinetic data for gas-phase chemistry. The conventional diaphragm-rupture mode for shock tube operation presents many challenges that may ultimately affect the quality of chemical kinetics data. Numerous diaphragmless concepts have been developed to overcome the drawbacks of using diaphragms. Most of these diaphragmless designs require significant alterations in the driver section of the shock tube and, in some cases, fail to match the performance of the diaphragm-mode of operation. In the present work, an existing diaphragm-type shock tube is retrofitted with a fast-acting valve, and the performance of the diaphragmless shock tube is evaluated for investigating the ignition of methane and n-hexane. The diaphragmless shock tube reported here presents many advantages, such as eliminating the use of diaphragms, avoiding substantial manual effort during experiments, automating the shock tube facility, having good control over driver conditions, and obtaining good repeatability for reliable gas-phase chemical kinetic studies. Ignition delay time measurements have been performed in the diaphragmless shock tube for three methane mixtures and two n-hexane mixtures at P5 = 10–20 bar and T5 = 738–1537 K. The results obtained for fuel-rich, fuel-lean, and oxygen-rich (undiluted) mixtures show very good agreement with previously reported experimental data and literature kinetic models (AramcoMech 3.0 [1] for methane and Zhang et al. mechanism [2] for n-hexane). The study presents an easy and simple method to upgrade conventional shock tubes to a diaphragmless mode of operation and opens new possibilities for reliable chemical kinetics investigations.
    • Coupling physics-informed neural networks and constitutive relation error concept to solve a parameter identification problem

      Wei, Y.; Serra, Q.; Lubineau, Gilles; Florentin, E. (Computers and Structures, Elsevier BV, 2023-05-03) [Article]
      Identification of material model parameters using full-field measurement is a common process both in industry and research. The constitutive equation gap method (CEGM) is a very powerful strategy for developing dedicated inverse methods, but suffers from the difficulty of building the admissible stress field. In this work, we present a new technique based on physics-informed neural networks (PINNs) to implement a CEGM optimization process. The main interest is to easily construct the admissible stress thanks to automatic differentiation (AD) associated with PINNs. This new method combines the high quality of the CEGM with the numerical effectivity of the PINNs and realizes the identification of material properties in a more concise way. We compare two variants of the developed method with the classical identification strategies on simple two-dimensional (2D) cases and illustrate its effectiveness in three-dimensional (3D) problems, which is of interest when dealing with tomographic images. The results indicate that the proposed method has good performance while avoiding complex calculation procedures, showing its great potential for practical applications.
    • Detonation peninsula of different stoichiometric ammonia/hydrogen/air mixtures under engine-relevant conditions

      Zhang, Jiabo; Luong, Minh Bau; Im, Hong G. (Combustion and Flame, Elsevier BV, 2023-05-01) [Article]
      This study numerically investigates the detonation development of carbon-free fuels, namely ammonia and hydrogen (NH3 and H2), using one-dimensional (1D) simulations under the end-gas autoignitive conditions relevant to internal combustion (IC) engines. Five stoichiometric NH3/H2/air mixtures with different NH3/H2 blending ratios are studied. A 1D hot spot with varied lengths and temperature gradients is used to induce different ignition modes. The detonation peninsulas are quantitatively identified by two non-dimensional parameters, namely the resonance parameter, ξ, and the reactivity parameter, ε. Increasing the H2 blending ratio up to 80% results in a unique horn-shaped detonation peninsula, i.e., the magnitude of the upper and lower ξ limits, ξu,l, near the leftmost boundaries of the detonation peninsula of the rich-H2 mixtures becomes larger by an order of magnitude as compared to those of the lean-H2 mixtures. Such behavior is attributed primarily to the large heat diffusion of hydrogen, leading to rapid heat dissipation of the hot spot and the significantly decreased transient ξ over time, thus promoting detonation development. The analysis reveals that the characterization of detonation propensity in the rich-H2 mixtures needs to account for the fast heat diffusion of the initial hot spot, in which the initial magnitude of ξ is not representative of its detonability. As such, a correction factor, β, weighted by the ignition Damköhler number, is proposed to resolve the discrepancy of the ξu,l limits between different NH3/H2/air mixtures. With this correction, the transient magnitudes of ξ, ξt, prior to the main ignition are well predicted such that a unified shape of the detonation peninsula for different NH3/H2/air mixture compositions is achieved.
    • Investigation of Formic Acid Chemistry and Ignition

      Alsewailem, Ahmad (2023-05) [Thesis]
      Advisor: Farooq, Aamir
      Committee members: Roberts, William L.; Hoteit, Hussein
      This thesis investigates the oxidation chemistry and ignition properties of formic acid (FA). The study reports experimental measurements of ignition delay time (IDT) and CO/CO2 time histories during FA oxidation in a shock tube. The initial concentration of FA was measured with a laser to minimize uncertainties arising from its low vapor pressure and tendency to form dimers. Shock tube experiments were carried out at two pressures, around 1.7 and 3.5 bar, and temperatures ranging from 1194 to 1658 K, with two equivalence ratios, 0.72 and 1.47. The results show a noticeable dependence of IDTs on temperature and pressure, while there was insignificant dependence on equivalence ratio. Six kinetic models for FA oxidation available in the literature were tested against the obtained data to evaluate their accuracy and suggest potential improvements. We found that 4 models performed well in predicting IDTs and CO/CO2 profiles with some overprediction at certain conditions. Sensitivity analysis revealed that the IDTs of FA are governed by unimolecular decomposition, H abstraction, and radical consumption (HOCO) reactions. The concentration of HO2 is higher at low temperatures, which is favorable for the system’s reactivity as it makes IDTs more sensitive to the reaction HOCHO + HO2 = H2O2 + HOCO. CO formation is controlled by two reactions: CO + OH = HOCO and HOCHO (+M) = CO + H2O, while the second reaction is more pronounced at high temperatures. Moreover, the dissociation of HOCO is faster at higher pressures, leading to higher initial CO concentrations. The formation of CO2 is determined by CO + OH = CO2 + H, while at higher temperatures, HOCHO (+M) = CO2 + H2 (+M) becomes more important, resulting in higher initial CO2 concentrations.
    • A comprehensive experimental and kinetic modeling study of laminar flame propagation of ammonia blended with propene

      Elbaz, Ayman M.; Giri, Binod; Shrestha, Krishna P.; Arab, O. Z.; Farooq, Aamir; Mauss, Fabian; Roberts, William L. (Combustion and Flame, Elsevier BV, 2023-04-27) [Article]
      Enhancement of ammonia reactivity is crucial for potential applications of ammonia as an engine and gas turbine fuel. A common strategy for improving ammonia's poor reactivity is blending it with more reactive fuels like hydrogen and methane. However, fundamental studies of ammonia combustion with higher hydrocarbons and key intermediate oxidation species of higher alkanes such as propene do not exist. Thus, this work presents an effort to study the laminar flame propagation of ammonia blended with propene. Laminar burning velocity (SL) of NH3/C3H6/air mixtures was measured at 298 K, pressures up to 5 bar, equivalence ratios of 0.7 to 1.3, and various propene to ammonia ratios (i.e.,% propene to ammonia mole fraction, xC3H6 = 10 to 50) in a high-pressure spherical propagating flame vessel. A kinetic model was developed based on our previous work to characterize the combustion behavior of NH3/C3H6/air mixture. The model reasonably agrees with the experimental data and follows the observed trends very well. The results showed that blending NH3 with C3H6 positively enhanced SL of NH3 by promoting the formation of key radicals e.g., O, OH, and H. Relative to a neat ammonia/air mixture, co-firing ammonia with propene leads to a reduced pressure dependence of the laminar burning velocity. However, the reaction H + O2(+M)=HO2(+M) leads to strong pressure dependency of lean NH3/C3H6 mixtures compared to rich mixtures. The model reveals that besides fuel-NO coming from NH3, prompt NO also actively contributes to NO formation. It is seen that N2O formation is significantly suppressed with increasing pressure or increasing C3H6 content in the fuel blend. In contrast to NO and N2O, NO2 concentration increases slightly with an increase in pressure. The reported experimental data and model will be useful in understanding the interaction between NH3 and alkenes.
    • Large eddy simulations of ammonia-hydrogen jet flames at elevated pressure using principal component analysis and deep neural networks

      Abdelwahid, Suliman; Malik, Mohammad Rafi; Al Kader Hammoud, Hasan Abed; Hernandez Perez, Francisco; Ghanem, Bernard; Im, Hong G. (Combustion and Flame, Elsevier BV, 2023-04-24) [Article]
      The combustion of ammonia/hydrogen is currently gaining importance in the power generation sector as an alternative for hydrocarbon fuels and improved fundamental insights will facilitate its application. To investigate the complex interactions between turbulence and chemistry for ammonia-hydrogen jet flames under high-pressure conditions, large eddy simulation (LES) computations are conducted using the PC-transport model, which is based on Principal Component Analysis (PCA), coupled with nonlinear regression that utilizes deep neural networks (DNN) to enhance the size-reduction potential of PCA. Classical statistics-based nonlinear regression methods are inefficient at fitting highly nonlinear manifolds and when large data sets are involved. These two drawbacks can be overcome by utilizing DNN tools. Several DNN architectures composed of fully connected layers of different depths and widths, batch normalization, and various activation functions coupled with various loss functions (mean squared error, absolute error, and R2) are explored to find an optimal fit to the thermo-chemical state-space manifold. The ability to achieve highly accurate mapping through DNN-based nonlinear regression with an R2-score >0.99 is shown by employing a single graphical processing unit (Nvidia RTX 3090). Furthermore, the proposed PC-DNN approach is extended to include differential diffusion based on a rotation technique and utilization of the mixture-averaged transport model for the training data set. To demonstrate the potential of the PC-DNN approach in modeling turbulent non-premixed combustion, LES results are compared with the recent Raman/Rayleigh scattering measurements that were obtained at the KAUST high-pressure combustion duct (HPCD). Results show that the PC-DNN approach is able to capture key flame characteristics with reasonable accuracy using only two principal components. The inclusion of differential diffusion leads to improved predictions, although some discrepancies are observed in fuel-lean regions.
    • A Highly Sensitive and Selective Laser-Based BTEX Sensor for Occupational and Environmental Monitoring

      Mhanna, Mhanna; Sy, Mohamed; Arfaj, Ayman; Llamas, Jose; Farooq, Aamir (Copernicus GmbH, 2023-04-24) [Preprint]
      A mid-infrared laser-based sensor is designed and demonstrated for trace detection of benzene, toluene, ethylbenzene, and xylene isomers at ambient conditions. The sensor is based on a distributed feedback inter-band cascade laser emitting near 3.29 μm and an off-axis cavity-enhanced absorption spectroscopy configuration with an optical gain of ~2800. Wavelength tuning and a deep neural networks (DNN) model were employed to enable simultaneous and selective BTEX measurements. The sensor performance was demonstrated by measuring BTEX mole fractions in various mixtures. At an integration time of 10 seconds, minimum detection limits of 11.4, 9.7, 9.1, 10, 15.6, and 12.9 ppb were achieved for benzene, toluene, ethylbenzene, m-xylene, o-xylene, and p-xylene, respectively. The sensor can be used to detect tiny BTEX leaks in petrochemical facilities and to monitor air quality in residential and industrial areas for workplace pollution.
    • Gamma-valerolactone (GVL) as a biofuel: Investigation of GVL thermal decomposition and GVL + OH reaction

      Liu, Dapeng; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2023-04-20) [Article]
      Recent works have highlighted the potential of lignocellulosic-derived γ-valerolactone (GVL) as a promising biofuel or a fuel additive. Its detailed chemistry is, however, not yet well-understood. In this work, we present the first high-temperature measurements of GVL + OH reaction behind reflected shock waves over 990–1294 K and pressures ∼ 1.2 bar. Around 990 K, our rate coefficients are ∼3 times faster than the average room-temperature determination. Our measured OH + GVL rate coefficients may be given as (unit: cm3molecule−1s−1): [Formula presented] We also conducted the first direct measurements of GVL thermal decomposition over 1214–1427 K and pressures ∼ 1.4 bar. GVL decomposition rate coefficients exhibit a similar slope to its acyclic counterpart, ethyl propionate, but GVL decomposes ∼80–120 times slower than ethyl propionate. Our determined GVL thermal decomposition rate coefficients may be expressed as (unit of s−1): [Formula presented] Our experimental work represents the first quantitative measurements of the reactions of the lactone family. Both reactions of the lactone family studied here proceeded slower than the normal-chain counterparts which affirms that ring constraints must be considered in rate coefficient analogies between normal-chain and cyclic molecules.
    • Optical study of knocking phenomenon in a spark-ignition engine by using high-speed OH* chemiluminescence imaging: A multiple ignition sites approach

      Uddeen, Kalim; Shi, Hao; Tang, Qinglong; Magnotti, Gaetano; Turner, James W. G. (International Journal of Engine Research, SAGE Publications, 2023-04-19) [Article]
      Knock in a spark-ignition (SI) engine is a complicated combustion phenomenon that stimulates high-pressure oscillations inside the combustion chamber and restricts engine performance. This study presents a high-speed OH* chemiluminescence imaging technique to investigate the knock mechanism resulting from firing multiple spark plugs. The experiment was performed using a customized liner having three spark plugs installed at equal spacing, and to compare the results with conventional SI conditions, in which one spark plug was mounted at the center of the cylinder head. In addition, multiple pressure transducers were used at various locations to record the frequencies induced by the pressure oscillations inside the cylinder during knocking events. The results showed that firing a single central spark plug generated mild knock with late combustion phasing and lower power output. However, adding more spark plugs could advance the initiation of autoignition and produce higher knock intensity along with lower combustion duration for the same operating conditions. Additionally, a weak OH* chemiluminescence intensity oscillation was monitored before the autoignition of the unburned charge occurred. The crank angle location of peak OH* intensity and peak HRR showed a good linear curve fit with a positive slope. Furthermore, the larger amount of unburned mass fraction produced stronger pressure waves due to multiple autoignition sites, and the unburned mass fraction exhibited a good linear relationship with unburned temperature and end-gas area at the knock onset point. Moreover, the frequency spectrum recorded by the multiple pressure sensors illustrated that in the case of a single central spark plug only circumferential acoustic waves were formed with low power intensity. However, multiple ignition sites promoted both circumferential and radial pressure waves inside the combustion chamber because of multiple autoignitions occurring both near the center and cylinder wall.
    • Towards decoupling chemical and mechanical adhesion at the electroplated metal/polymer interface via precision surface texturing

      Melentiev, Ruslan; Tao, Ran; Fatta, Lujain; Tevtia, Amit K.; VERGHESE, Nikhil; Lubineau, Gilles (Surfaces and Interfaces, Elsevier BV, 2023-04-18) [Article]
      Electroplating is the most widely used method for coating metals on polymers, the adhesion of which is ensured by chemical and mechanical interactions of their surface. Unfortunately, the relative contributions of these factors to the overall adhesion was debated over the past century with no quantitative arguments. This study decouples the contribution of polymer chemistry from other adhesion factors by using non-interlocking micro-textures with variated actual surface area. We used state-of-the-art photolithography to micro-texture nine molds with 2, 4 and 6 µm wide and 2, 4 and 6 µm tall pillars (1) hot embossing to replicate the pillars on the ABS polymer (2), electroplating to deposit copper on the textured and etched ABS polymer (3), and peeling test to measure the adhesion strength (4). In the absence of mechanical interlocking, the measured adhesion can be fully ascribed to the surface-amplified chemical adhesion between the metal and the polymer. In result, the chemical adhesion appeared linearly proportional to the actual interfacial area, yet it contributed only a minor fraction (1/7) to the overall adhesion strength in electroplating with classical chromic acid etching. Evidently, chemical adhesion is not the dominant factor in polymer electroplating.
    • Dynamic response amplification of resonant microelectromechanical structures utilizing multi-mode excitation

      Zhao, Wen; Rocha, Rodrigo Tumolin; Alcheikh, Nouha; I.Younis, Mohammad (Mechanical Systems and Signal Processing, Elsevier BV, 2023-04-11) [Article]
      This work presents an analytical and experimental study to enhance the dynamic response of the higher-order vibration modes of resonant microstructures. The detection of higher order vibration modes is usually very difficult due to their low amplitude response, which gets buried in noise. Higher input voltages can be used to enhance the response; however, it is highly energy demanding, can result in electronics problems, and may lead to pull-in of the lower participating modes. In this paper, we present a multi-mode excitation (MME) technique to enhance the vibration response of the higher order modes of an electrostatically actuated micro cantilever beam. First, we derive the dynamic equations of motion of the micro resonator accounting for two electrostatic excitation sources. Then, we apply the Galerkin method to analyze the dynamics of the system analytically. We investigate the effect of using various dynamic AC combinations on the involved modes to yield optimal conditions of the amplitude magnification. Then, an experimental investigation is conducted based on a micro resonator made of polyimide. The results showed that, compared to the single mode excitation (SME), the MME can significantly raise the level of the response above noise and amplify the displacement amplitude to yield a higher signal-to-noise ratio. The maximum displacement amplitude can be amplified multiple times as determined by the AC loads. Hence, the improved displacement amplitude with the proposed technique makes it promising for future signal processing technologies and for realizing high-sensitivity sensors.
    • Multiple Spark Ignition Approach to Burn Ammonia in a Spark-Ignition Engine: An Optical Study

      Uddeen, Kalim; Tang, Qinglong; Shi, Hao; Magnotti, Gaetano; Turner, James W. G. (SAE International, 2023-04-11) [Conference Paper]
      The future of the internal combustion (IC) engine relies on carbon-free fuels to mitigate climate change. Ammonia (NH3) is a promising carbon-free fuel, which can be used as an energy carrier for hydrogen (H2) and directly as a combustible fuel inside the engines. However, burning pure ammonia fuel is difficult due to its low flammability, burning velocity, and consequently large cycle-to-cycle variation. This study used a multiple-spark-plug approach to burn pure ammonia gas with reduced combustion duration and higher engine power output. The natural flame luminosity (NFL) imaging method was used to capture the multiple flames initiated by various ignition sites. In order to perform the experiment a customized liner having four spark plugs installed at equal spacing to each other, and to compare the results with conventional spark-ignition (SI) conditions, one spark plug was mounted at the center of the cylinder head. The results show that firing the single central spark plug generated lower in-cylinder pressure and heat release rate (HRR) along with higher combustion duration due to the low flame speed. However, adding more spark plugs increased the cylinder pressure generation and HRR along with creating shorter combustion duration for the same operating conditions. In addition, multiple flames produced by multiple plugs increased the engine power output and reduced the cyclic variation significantly due to higher-pressure generation. Additionally, NFL imaging was used to evaluate the flame intensity and flame area proportion for various ignition cases, and it was found that multiple spark plugs burned the air-fuel mixture more quickly with faster flame area proportion along with higher flame intensity. Furthermore, firing multiple spark plugs produced higher NOx emissions than the single spark plug case due to higher in-cylinder temperatures generated by multiple flame kernels.
    • Experimental Investigations of Methane-Hydrogen Blended Combustion in a Heavy-Duty Optical Diesel Engine Converted to Spark Ignition Operation

      Panthi, Niraj; Chang, Junseok; AlRamadan, Abdullah; Magnotti, Gaetano (SAE International, 2023-04-11) [Conference Paper]
      The global need for de-carbonization and stringent emission regulations are pushing the current engine research toward alternative fuels. Previous studies have shown that the uHC, CO, and CO2 emissions are greatly reduced and brake thermal efficiency increases with an increase in hydrogen concentration in methane-hydrogen blends for the richer mixture compositions. However, the combustion suffers from high NOx emissions. While these trends are well established, there is limited information on a detailed optical study on the effect of air-excess ratio for different methane-hydrogen mixtures. In the present study, experimental investigations of different methane-hydrogen blends between 0 and 100% hydrogen concentration by volume for the air-excess ratio of 1, 1.4, 1.8, and 2.2 were conducted in a heavy-duty optical diesel engine converted to spark-ignition operation. The engine was equipped with a flat-shaped optical piston to allow bottom-view imaging of the combustion chamber. High-speed natural combustion luminosity images were recorded at a frame rate of 7.2 kHz for all cases, together with in-cylinder pressure measurements. Results showed that the increase in hydrogen concentration has shifted the CA50 towards TDC thus increasing the peak combustion pressure. Methane combustion shows the lean limit at lambda 1.4 and extension of the lean limit requires at least 20% of hydrogen addition while maintaining the COV of IMEP below 5%. However, at lambda 1.8 case, 60% of hydrogen enhancement was needed to achieve stable combustion. Overall, with higher hydrogen concentration, there is an improvement in the combustion stability irrespective of the air-excess ratio. Image analysis was performed on the high-speed natural combustion luminosity images to obtain quantitative information on the flame front propagation speed for the tested methane-hydrogen blends. Hydrogen addition results in an increase in flame front propagation speed. When the hydrogen concentration in methane-hydrogen blends is about 50% by volume and more, the flame kernel propagates rapidly at the onset of combustion and decreases, resulting in a shorter combustion duration.