This thesis focuses on the effect of initial ambient turbulence on the spray characteristics of fuel injected into a constant volume. The investigation is performed experimentally and computationally. The spray axial penetration length and radial penetration width, area and velocity are used as key parameters to characterise the spray in the experimental work together with vapour fuel mass fraction, mean droplet diameters and number of droplets in the computational work. In the experimental study, the liquid fuel (iso-octane) is injected, using a high pressure swirl atomiser, into ambient nitrogen with different prescribed nearly isotropic turbulence levels, characterised by the root mean square (RMS) turbulence velocity. Mie scattering laser sheet and Schlieren techniques are combined with a high speed camera to capture images of the vapour and liquid phases simultaneously as a function of injection pressure and ambient turbulence. These indicate that the latter has a significant influence on tip penetration length, penetration width, cross sectional area and velocity. Increased initial ambient turbulence levels lead to reductions in the penetration length in the axial direction and increases in the penetration width in the radial direction; it is also shown to improve fuel evaporation and mixing. In the computational investigation, the commercial Computational Fluid Dynamics code Fluent is used to explore the effect of the RMS turbulence velocity in a constant volume vessel on the spray characteristics of liquid fuel injected into it. The theoretical results are compared with corresponding experimental data obtained for the case of iso- octane fuel injected into nitrogen; the main features of sprays are successfully predicted. The results show how increased ambient turbulence level in the gas into which fuel is injected influences spray atomisation, break-up and evaporation and leads to reduced vapour penetration length and sauter mean droplet diameter, together with increasing vapour penetration width and number of fuel droplets. Additionally, the effect of injection pressure, ambient density and ambient temperature on spray characteristics is investigated at quiescent and turbulent ambient conditions; in which case increasing the injection pressure leads to increases in both the penetration length and the number of droplets, and a corresponding reduction in the sauter mean diameter.

In-cylinder flows and CAI combustion were investigated in a single cylinder, air-assisted gasoline direct injection engine. CAI was promoted and controlled by internal exhaust gas recirculation, achieved by employing short duration camshafts and early exhaust valve closure. The effects of valve and injection timing and engine speed on exhaust emissions, fuel consumption, combustion phasing and operating region were investigated. The results show that valve timing mainly affects engine load and CAI combustion phasing through changes in trapped residual levels and stratification of fresh and residual gases respectively. Injection of fuel into residual gases during the recompression process was found to increase the operating region and reduce uHC emissions though charge cooling effects and increased fuel ignitability via internal fuel reformation. The increased ignitability of the mixture also advanced ignition timing, resulting in increased in-cylinder temperatures and NOx concentrations. It was found that, compared to SI combustion in the same engine, CAI operation reduced NOx emissions by between 98% and 80% while fuel consumption was reduced by between 9% and 17%. The in-cylinder flows of intake air and fuel droplets from the air-assisted injection system and cylinder head were investigated using the PIV technique. No significant large-scale flow structures were found in the in-cylinder airflow and the fuel spray appeared unaffected by the in-cylinder air motion. In addition, the in-cylinder fuel distribution from the air-assisted injection system was measured using laser induced exiplex fluorescence. A combination of naphthalene and DMA in isooctane was used to form an exiplex and simultaneous qualitative images of the liquid and vapour fuel phases were obtained. When using a late injection strategy, a well stratified mixture was formed at the end of the compression stroke, while injection during the intake stroke left a well mixed homogenous charge.

Developments m Gasoline Direct Injection, GDI, technology have enhanced the viability of long tenn SI engine development. Many automotive manufacturers are developing and offer production cars with first generation GDI engines. GDI fuel injection strategies provide power and effi ciency improvements, due to superior fuel metering, incylinder mixture preparation and the ability to run throttle-less under di fferent combustion modes depending on engine load. Although significant improvements in perfonnance and economy have been demonstrated, work is still required to optimise the GDI strategies fo r varying engine loads and emissions. Matching liquid fuel sheet break up and atomisation timescales to those of the charge motion occurring in the engine cylinder is essential. Many fundamental studies have investigated the mechanisms of liquid sheet break up, however, most have concentrated on steady state low pressure conditions. It is felt that little can be applied from these studies to analyse high pressure GDI sprays which produce an initial liquid sheet annulus then a complex hollow cone spray, transient in nature due to the cyclic behaviour of an SI engine. This experimental study assesses the liquid fuel sheet break up mechanism of a GDI pressure-swirl injector in the pressure range 10-50 bar. The fundamental study simplifies the problems associated with a 3-dimensional spray by considering a 2-dimensional transient liquid sheet and characterising the sheet wave structure and break up process. A unique rotary valve has been specifically designed and manufactured to allow the break up of a transient flat liquid sheet to be studi ed under an injection pressure range of 10-50 bar. A precursor to liquid sheet break up is the appearance of perforations in the sheet. The onset of perforations in the fl at sheet were measured as a function of distance downstream from the nozzle for a range of sheet velocities 12-36m/s; i.e. Reynolds number range 800 - 2400. This highlighted a peak in the perforation onset length between 20 and 25 bar injection pressure; i.e. sheet velocity of approximately 25m/s. Subsequent increases of sheet velocity lead to a reduction in the perforation onset length, strongly indicating that above 25rn/s, aerodynamic forces dominated the sheet break up process. Spreading the liquid laterally, introduced sheet stretching, which affected the position of the perfo ration onset by as much as 30% at higher injection pressures. Estimations of sheet thickness at the perforation location were calculated to be in the range 0.05-0. llrrun. Particle Image Velocimetry, PIV, and Laser Doppler Anemometry, LDA, was used to assess the liquid sheet velocity flow field, which indicated the presence of large velocity gradients in both the axial direction and across the sheet respectively.

This thesis presents a numerical study of the break-up and atomisation of gasoline fuel sprays injected into atmospheric flow conditions and environments related to combustion chamber conditions. Calculations of the fuel break-up process were achieved by four different models: Taylor Analogy Break-up (TAB), the wave instability theory (WAVE), the Hybrid Sheet-TAB and the Hybrid WAVE-FIPA models. The TAB model relates the break-up process to the droplet oscillations; whereas the WAVE models calculate the fuel break-up from the unstable waves on the droplet surface. The modified version of the TAB model, called the Hybrid Sheet-TAB model delays the start of the break-up further downstream from the nozzle tip. A new hybrid model, the WAVE-FIPA model, divides the spray atomisation processes into a primary stage, where the WAVE model is used, and a secondary stage, which is simulated using experimental correlations to calculate the break-up time for the low Weber number droplets.

Laser-Induced Incandescence (LII) occurs when a high-energy pulsed laser beam encounters graphitic particulate matter particles like soot or carbon black. The particles absorb laser energy from the beam and see an increase in their internal energy, resulting in an increase of temperature. At the same time, the particles loose energy through heat transfer mechanisms. If the energy absorption rate is sufficiently high, particle temperature will rise to levels where significant incandescence (blackbody emission) can occur .Typically, Laser-Induced Incandescence produces 50ns to 1μs long light pulses at atmospheric pressure. So far, LII measurements had been restrained to conduction-dominated conditions, whereby signals are short-lived (less than one microsecond) and require sensitive nanosecond resolution instrumentation. This thesis introduces a novel LII – based measurement method performed under high vacuum conditions. The novelty of LII under vacuum resided in the fact that heat conduction away from the soot particle becomes negligible below 10-2 mbar and this constituted a step away from the typical situation, whereby laser absorption is followed by heat conduction from the particles to the surrounding medium. Instead, sublimation and radiative heat transfer would follow laser absorption. The consequence was the obtention of long-lived LII signals (up to 100 microseconds) and a large gain of photons (ranging between 50 to 300) emitted per primary soot particle during LII temperature decays. Furthermore, the refractive index function E(m) value could be determined directly from measured radiative temperature decays, with potentially an uncertainty of circa 7%, which outperformed current soot extinction measurements. In addition, for laser fluences below 0.06 J/cm2, a regime where only laser energy absorption and radiative heat transfer apply would be reached and LII signals became independent of particle size. Throughout this project, Laser-Induced Incandescence under vacuum was applied to a sample of carbon powder (agglomerated soot particles) sealed in a glass vessel and held below 10-3 mbar. Initial spectral measurements performed at a laser fluence of 0.3 J/cm2 confirmed the obtention of long-lived (60 microseconds long) blackbody spectra, which confirmed the feasibility of the technique and yielded an E(m) measurement of between 0.35 and 0.45. A second study was performed with a dualwavelength pyrometric system specifically designed for recording live LII temperatures and signal intensities coupled to an absolute light intensity calibrated intensified imaging system. Experimental results unveiled the thermo-physical behaviours of agglomerates enduring LII. The most remarkable outcomes of the results concerning carbon nanoparticles agglomerates were that: clusterous particulate matter absorbs and radiates light in a very similarly to single isolated carbon nanoparticles and therefore obey largely the Rayleigh limit; soot agglomerates also dissociate during LII in an explosive mode and ejecta were measured to reach up to 400 m/s following chain dissociations; complete agglomerate dissociations can be obtained and measurements performed on individual aggregates of primary soot nanoparticles. In parallel, LII measurements revealed that optical shielding is largely present within agglomerates, and therefore clusters dissociations exposed large quantities of particulate matter and increased greatly LII signal levels. Overall, radiative heat transfer measurements yielded E(m) = 0.4 to 0.6 and time-integrated ICCD measurements resolved signal levels as low as groups of 6 carbon nanoparticles. This sensitivity clearly was the highest recorded to date for Laser Induced Incandescence and the sensitivity boundary of the technique was increased to nearly resolving single nanoparticles. Further measurements were performed in collaboration with the National Research Council (NRC) of Ottawa, Canada, at the Combustion Research Group facility. The results obtained validated the obtention of repeatable temperature profiles for Laser- Induced Incandescence under vacuum. In addition, comparison between results obtained on a controlled source of agglomerates at atmospheric pressure established that the increase for LII signals with laser fluence for both atmospheric and vacuum conditions could be directly associated with agglomerates dissociations. Indeed, net diminutions in optical shielding were measured in both conditions and could be coupled with diminutions in thermal shielding at atmospheric pressures. Highresolution temperature measurements established that laser absorption, annealing, sublimation and radiative heat transfer rates could be unprecedently and directly measured by laser-induced incandescence under vacuum. Annealing and sublimation of soot primary particles could also reasonably be assumed to be the phenomena at the heart of agglomerate dissociations. It was also established that agglomerate dissociation was dependent not only on laser fluence but also on the instantaneous laser power absorbed by the carbon agglomerates: indeed measurements performed at NRC were effected with a instantaneous laser powers four times lower than previously and radiative heat transfer measurements attested incomplete agglomerate dissociations with E(m) values measured between 0.8 and 1. Overall, the present work introduces LII under vacuum as a high sensitivity measurement method for particulate matter. The sensitivities obtained approached nanoparticles resolution and constitutes one of the most sensitive particulate matter measurement technique to date with real-time measurements capability. Because of the sample studied, agglomerate dynamics during LII were unveiled for the first time and explained the increase of LII signals with laser fluence as a diminution of both thermal and optical shielding. The LII under vacuum technique also proved its ability to resolve and isolate some of the key phenomena occurring during LII: laser absorption, annealing, sublimation and heat radiation.

A burner for investigating lean stratified premixed flames propagating in intense isotropic turbulence has been developed. Lean pre-mixtures of methane at different equivalence ratios are divided between two concentric co-flows to obtain annular stratification. Turbulence generators are used to control the level of turbulence intensity in the oncoming flow. A third annular weakly swirling air flow provides the flame stabilization mechanism. A fundamental characteristic is that flame stabilization does not rely on flow recirculation. The flames are maintained at a position where the local mass flux balances the burning rate, the result is a freely propagating turbulent flame front. The absence of physical surfaces in the vicinity of the flame provides free access for laser diagnostics. Stereoscopic Planar Image Velocimetry (SPIV) has been applied to obtain the three components of the instantaneous velocity vectors on a vertical plane above the burner outlet where the flames propagate. The instantaneous temperature fields have been determined through Laser Induced Rayleigh (LIRay) scattering. Planar Laser Induced Fluorescence (PLIF) on acetone has been used to calculate the average equivalence ratio distributions. Instantaneous turbulent burning velocities have been extracted from SPIV results, while flame curvature and flame thermal thickness values have been calculated using the instantaneous temperature fields. The probability distributions of these quantities have been compared considering the separate influence of equivalence ratio stratification and turbulence. It has been observed that increased levels of turbulence determine higher turbulent burning velocities and flame front wrinkling. Flames characterized by stronger fuel stratification showed higher values in turbulent burning velocities. From the curvature analysis emerged that increased fuel concentration gradients favour flame wrinkling, especially when associated with positive small radius of curvature. This determines an increased surface area available for reaction that promotes a faster propagation of the flame front in the oncoming combustible mixtures.

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