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.

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.

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.

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.

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.

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.

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.

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.

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.

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.