Predictive modeling of pre-chamber combustion engines relies primarily on the correct prediction of laminar and turbulent flame speeds. While the latter has been rigorously derived, the former correlations are mostly semi-empirical and valid for a limited range of operating conditions. The current work aims at highlighting the fundamental significance of correct laminar flame speed prediction on numerical modeling of ultralean prechamber engine combustion. Gulder's empirical correlation for laminar flame speed was chosen for the current work. It was modified for ranges beyond what it was originally derived for. It was initially observed that the numerical results that utilize Gulder's correlation for the laminar flame speed underperform compared to the one computed from the skeletal GRI3.0 by Lu and Law. In all cases, Peters' turbulent flame speed correlation was used, which evidences that any potential difference comes from the laminar flame speed. Using Lu and Law's chemical mechanism as a reference for laminar flame speed calculations, the values of the empirical constants α, η, and ξ in Gulder's correlation were optimized to yield more accurate flame speeds at ultralean engine conditions. The updated Gulder's correlation for methane was implemented in CONVERGE, a three-dimensional computational fluid dynamics (CFD) solver, and validated against the experimental engine results from KAUST. The flame topology was also explored to correlate the observed behaviors in the pressure predictions among all tested cases. Finally, the Borghi-Peters diagram provides insightful information on combustion regimes encountered in pre-chamber combustion engines.

Electric field can be a viable method in controlling various combustion properties. Comparing to traditional actuators, an application of electric field requires very small power consumption. Especially, alternating current (AC) has received attention recently, since it could modulate flames appreciably even for the cases when direct current (DC) has minimal effects. In this study, the effect of AC electric fields on small coflow diffusion flames is focused with applications of various laser diagnostic techniques. Flow characteristics of baseline diffusion flames, which corresponds to stationary small coflow diffusion flames when electric field is not applied, were firstly investigated with a particular focus on the flow field in near-nozzle region with the buoyancy force exerted on fuels due to density differences among fuel, ambient air, and burnt gas. The result showed that the buoyancy force exerted on the fuel as well as on burnt gas significantly distorted the near-nozzle flow-fields. In the fuels with densities heavier than air, recirculation zones were formed very close to the nozzle exit. Nozzle heating effect influenced this near-nozzle flow-field particularly among lighter fuels. Numerical simulations were also conducted and the results showed that a fuel inlet boundary condition with a fully developed velocity profile for cases with long fuel tubes should be specified inside the fuel tube to obtain satisfactory agreement in both the flow and temperature fields with those from experiment. With sub-critical AC applied to the baseline flames, particle image velocimetry (PIV), light scattering, laser-induced incandescence (LII), and laser-induced fluores- cence (LIF) techniques were adopted to identify the flow field and the structures of OH, polycyclic aromatic hydrocarbons (PAHs), soot zone. Under certain AC condi- tions of applied voltage and frequency, the distribution of PAHs and the flow field near the nozzle exit were drastically altered from the baseline case, leading to the formation of toroidal vortices. Increased residence time and heat recirculation inside the vortex resulted in appreciable formation of PAHs and soot near the nozzle exit. Decreased residence time along the jet axis through flow acceleration by the vortex led to a reduction in the soot volume fraction in the downstream sooting zone. Electromagnetic force generated by AC was proposed as a viable mechanism for the formation of the toroidal vortex. By varying applied AC in a wide range of frequency and voltage, several insta- bility modes were observed, including flicking flames, partial pinch-off of flames, and spinning flames. High speed imaging together with Mie scattering techniques were combined to reveal the flame dynamics as well as the flow structure inside the flames. Original steady toroidal vortices triggered by AC were noted to exhibit axisymmetric axial instability in the flicking and partial pinch-off modes and non-axisymmetric azimuthal instability in the spinning mode. Electrical measurements were also conducted simultaneously to identify the voltage, current, and electrical power responses. Integrated power was noted to be sensitive to indicate subtle variation of flames properties and to the occurrence of axial instability. Under low frequency AC forcing with electrical conditions not generating toroidal vortices, responses of flames were further investigated. Several nonlinear flame responses, including frequency doubling and tripling phenomena, were identified. Spectral analysis revealed that such nonlinear responses were attributed to the combined effects of triggering buoyancy-induced oscillation of the flame as well as the Lorenz force generated by applying AC. Phase delay behaviors between the applied voltage and the heat release rate (or flame size) were also studied to explore the potential of applying AC in controlling flame instability. It was found that the phase delay had large variations for AC frequency smaller than 80 Hz and became saturated at over 80 Hz, which has been explained based on the interaction between the buoyancy and ionic wind. Electrical measurement showed the power consumed by the AC was smaller than 0.01% of the heat release rate from the flame. To improve the understanding on the electric current resulting from applying electric field on flames, a simplified one-dimensional model was developed in that the reaction zone was modeled as a thin ionized layer. Model governing equations were derived from species equations by implementing mobility differences depending on the type of charged particles, especially between ions and electrons. The result showed that the sub-saturated current along with field intensity was significantly influenced by the polarity of DC due to the combined effect of non-equal mobility of charged particles as well as the position of the ionized layer in a gap relative to two electrodes. Experiments with quasi-one-dimensional flames under DC were conducted to substantiate the model and measured currents agreed qualitatively well with the model predictions.

Nanofluids are homogeneous mixture of dispersed solid particles in base fluids. These solid particles are usually smaller than 100nm. Suspended nanoparticles modify the properties of based fluids. It is claimed, in some literature, for nanofluids to have greater than expected heat transfer performance. Due to this, nanofluids have gained great attention from both research and development and industries active in cooling systems. This thesis reports several measurements of convective heat transfer coefficient in a horizontal open micro-tube test section under laminar flow regime. The test section has an inner diameter of 0.5mm made of stainless steel and it has a length of 30cm. Two different test sections have been built. The first one has 13 thermocouples attached on the wall and the second one has 10. These thermocouples are used to measure the wall temperature distribution along the tube. In addition, two more thermocouples are used inside the micro-tube, at the inlet and outlet, to measure the bulk temperature of the nanofluids. A syringe pump is used for injecting the nanofluids through the micro-tube. A DC power supply provides constant heat flux along the test section and a differential pressure transducer measures the pressure drop of the test section. Aqueous based Al2O3 (9 wt %), ZrO2 (9 wt %), TiO2(9 wt %), CeO2 (9wt %), CNT (0.15 wt %), and diamond (1 wt %) have been tested in this thesis. Local Shah’s correlation predicts very well the behaviour of these nanofluids. The results are compared with water in six different ways: heat transfer forconstant Reynolds numbers, volume and mass flow rates, pressure drops andpumping powers. Enhancement in heat transfer is recognisable only in thegraphs of Nu numbers for constant Reynolds numbers. This can be attributed to the higher viscosity for nanofluids. Moreover, friction factor for constant Reynolds numbers has been compared. All the nanofluids with the exception of Al2O3 and diamond suit quite well with Darcy-Weisbach correlation.

Emissions of soot and polycyclic aromatic hydrocarbons (PAHs) from incomplete burning of hydrocarbon fuels pose a great threat to the environment and human health. To reduce such emissions, a comprehensive understanding of their evolution process is essential. In this work, a series of research studies were conducted to evaluate sooting tendencies and to experimentally and theoretically develop PAH mechanisms. The sooting tendencies of oxygenated fuels were quantitively investigated in counterflow diffusion flames. Sooting limits are described by critical fuel and oxygen mole fractions, measured with a laser scattering technique. The addition of dimethyl ether displays non-monotonic behavior on sooting tendencies at elevated pressures, which is attributed to the chemical effect from kinetic simulations. The tendency of incipient soot formation of other oxygenated fuels (e.g., alcohol, acid, ether, ketone, and carbonate ester) was also assessed, using a similar approach. As the precursor of soot, PAH measurement using laser induced fluoresecnce was implemented to track the evolution processes from PAHs to incipient soot. Developing a PAH mechanism is essential to the understanding of soot formation; however, PAH formation and its growth process are not well understood. Based on previous research, PAHs with 5-membered rings are abundant in flames. Therefore, the growth of PAHs with 5-membered rings was investigated, using acenaphthylene (A2R5) as the example. The density functional theory (DFT) and the transition state theory (TST) were adopted to calculate potential energy surfaces and reaction rate coefficients. The existence of 5-membered rings appreciably impacts PAH production by facilitating the formation of planar PAHs with C2H substitution, thereby improving existing PAH mechanisms. In PAH mechanisms, the thermochemistry properties are not all calculated, but are hypothesized to be equal to those of a similar structure. The simulation accuracy of the hypothesis is explored here by discussing the sensitivity of the thermochemistry parameters in flame simulations. The group additivity method utilizing THERM codes is used to calculate thermochemistry properties. PAH loading affects the sensitivity of thermochemistry properties to both flame temperature and product yields. These results show that either accurate thermochemistry properties, or reverse reaction rates should be provided in the mechanism to improve simulation accuracy.

The thesis comprises effects of electric fields on a fundamental study of spheri­cal premixed flame propagation.Outwardly-propagating spherical laminar premixed flames have been investigated in a constant volume combustion vessel by applying au uni-directional electric potential.Direct photography and schlieren techniques have been adopted and captured images were analyzed through image processing. Unstretched laminar burning velocities under the influence of electric fields and their associated Markstein length scales have been determined from outwardly prop­agating spherical flame at a constant pressure. Methane and propane fuels have been tested to assess the effect of electric fields on the differential diffusion of the two fuels.The effects of varying equivalence ratios and applied voltages have been in­vestigated, while the frequency of AC was fixed at 1 KHz. Directional propagating characteristics were analyzed to identify the electric filed effect. The flame morphology varied appreciably under the influence of electric fields which in turn affected the burning rate of mixtures.The flame front was found to propagate much faster toward to the electrode at which the electric fields were supplied while the flame speeds in the other direction were minimally influenced. When the voltage was above 7 KV the combustion is markedly enhanced in the downward direction since intense turbulence is generated and as a result the mixing process or rather the heat and mass transfer within the flame front will be enhanced.The com­bustion pressure for the cases with electric fields increased rapidly during the initial stage of combustion and was relatively higher since the flame front was lengthened in the downward direction.

No description available.

Laminar flat flame combustion has been studied with molecular-beam mass-spectrometry (MBMS) for a fuel-rich cyclohexane (Ф = 2.003) flame, a fuel-lean toluene (Ф = 0.895), and a fuel-rich toluene (Ф = 1.497) flame. Different hydrocarbon species in these flames were identified, and their mole fraction profiles were measured. The information can be used to propose reaction mechanisms for the different hydrocarbon flames. One MBMS apparatus located at Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory was used to identify and measure the mole-fraction profiles of different species in these flames. The MBMS apparatus located at University of Massachusetts Amherst was used to measure the temperature profile of the cyclohexane flame. The temperature profile of two different fuel-rich toluene flames (Ф= 2.02 , Ф = 3.94) and a fuel-lean (Ф=0.452) methane flame were also measured with the UMass apparatus.

<p> An implicit finite difference technique has been developed for the solution of the steady two dimensional boundary layer equations. </p> <p> The numerical method is free of stability limitations and similarity assumptions. Use has been made of Wu-type starting profiles which enable one to start the calculation from the leading edge. </p> <p> Attractive features of the technique are its simplicity, flexibility and applicability to a wide range of boundary layer problems. In addition, results obtained from several case studies indicate that the numerical procedure is both accurate and fast. </p> / Thesis / Master of Engineering (MEngr)

In an effort to understand the behavior of the laminar separation bubbles on NACA 0012 and Liebeck LA2573a airfoils at different Reynolds numbers and angles of attack, the boundary layers on the solid airfoils were investigated by measuring the mean and fluctuating components of the velocity profiles over the upper surfaces of the airfoils. Surface pressure measurements were carried out to complete the mapping of the laminar separation bubble and to calculate the lift generated by the airfoils. The experiments were carried out at Reynolds numbers of 150,000 and 250,000. The locations of separation, transition and reattachment were determined as functions of angle of attack and Reynolds number for the two airfoils. The drag was estimated from wake pressure measurements and was based on the momentum deficit generated by the airfoil. The size and location of the laminar separation bubble did not show significant changes with Reynolds number and angle of attack for values of the angle of attack between 0 and 6 d grees. The baseline results of the size and location of the laminar separation bubble on the LA2573a airfoil were used to design a suction distribution. This suction distribution was designed based on Thwaites’ criterion of separation. The effects of applying suction on the size and location of the laminar separation bubble were investigated. The results showed that the suction distribution designed in this work was effective in controlling the size of the laminar separation bubble, maintaining an un-separated laminar boundary layer to the transition point, and controlling the location of transition. The effects of different suction rates and distributions on the drag were investigated. Drag reductions of 14-24% were achieved. A figure of merit was defined as drag reductions divided by the equivalent suction drag to assess the worthiness of the utilizing suction on low Reynolds number flows. The values of the figure of merit were around 4.0 which proved that the penalty of using suction was significantly less than the gain obtained in reducing the drag.

No description available.