Droplet size and morphology characterization for diesel sprays under atmospheric operating conditions

The shape of microscopic fuel droplets may differ from the perfect sphere, affecting their external surface area and thus the heat transfer with the surrounding gas. Hence there is a need for the characterization of droplet shapes, and the estimation of external surface area, in order to enable the development of physically accurate mathematical models for the heating and evaporation of diesel fuel sprays. We present ongoing work to automat-ically identify and reconstruct the morphology of fuel droplets, primarily focusing in this study on irregularly-shaped, partially-deformed and oscillating droplets under atmospheric conditions. We used direct imaging tech-niques based on long-working distance microscopy and ultra-high-speed video to conduct a detailed temporal investigation of droplet morphology. We applied purpose-built algorithms to extract droplet size, velocity, vol-ume and external surface area from the microscopic ultra-high-speed video frames. High resolution images of oscillating droplets and a formation of a droplet form ligament, sphericity factors, volume as well as external surface area are presented for 500 bar injection pressure in the near nozzle region (up to 0.7 mm from nozzle exit) under atmospheric conditions. We observed a range of different liquid structures, including perfectly spher-ical, non-spherical droplets and stretched ligaments. We found that large droplets and ligaments exceeding the size of the nozzle hole could be found at the end of injection. In order to estimate droplet volume and external surface area from two-dimensional droplet information, a discrete revolution of the droplet silhouette about its major centroidal axis was used. Special attention was paid to the estimation of actual errors in the prediction of volume and surface characteristics from a droplet silhouette. In addition to the estimation of droplet volume and external surface area, the actual shape reconstruction in 3D coordinates from a droplet silhouette was performed in order to enable future numerical modelling studies of real droplets

Rayleigh scattering temperature measurements in a swirl stabilized burner

Rayleigh scattering temperature measurements were obtained in a turbulent reactive swirling coaxial jet discharged from a swirl-stabilized burner along the jet-flame centerline. They are reported up to 10 fuel nozzle diameters downstream of the burner exit at a Reynolds number of 29000. The effect of swirl numbers (S=0.3, 0.58, 1.07) on the temperature fields, the power spectral density of temperature fluctuations and on the probability density functions of the temperature fluctuations was determined

Experimental Assessment of ‘subgrid’ scale Probability Density Function Models for Large Eddy Simulation

Filtered density functions (FDFs) of mixture fraction are quantified by analyzing experimental data obtained from two-dimensional planar laser-induced fluorescence scalar measurements in the isothermal swirling flow of a combustor operating at a Reynolds number of 28,662 for three different swirl numbers (0.3, 0.58 and 1.07). Two-dimensional filtering using a box filter was performed on the measured scalar to obtain the filtered variables used for presumed FDF for Large Eddy Simulations (LES). A dependant variable from the measured scalar, which was a pre-computed temperature, was integrated over the experimentally obtained FDF as well as over the presumed beta or top-hat FDFs and a relative error in temperature prediction was calculated. The experimentally measured FDFs depended on swirl numbers and axial and radial positions in the flow. The FDFs were unimodal in the regions of low variance and bimodal in the regions of high variance. The influence of the filter spatial dimension on the measured FDF was evaluated and consequences for subgrid modeling for LES discussed

Experimental assessment of presumed filtered density function models

Measured filtered density functions (FDFs) as well as assumed beta distribution model of mixture fraction and “subgrid” scale (SGS) scalar variance, used typically in large eddy simulations, were studied by analysing experimental data, obtained from two-dimensional planar, laser induced fluorescence measurements in isothermal swirling turbulent flows at a constant Reynolds number of 29 000 for different swirl numbers (0.3, 0.58, and 1.07)

Simulation and Measurement of Transient Fluid Phenomena within Diesel Injection

Rail pressures of modern diesel fuel injection systems have increased significantly over recent years, greatly improving atomisation of the main fuel injection event and air utilisation of the combustion process. Continued improvement in controlling the process of introducing fuel into the cylinder has led to focussing on fluid phenomena related to transient response. High-speed microscopy has been employed to visualise the detailed fluid dynamics around the near nozzle region of an automotive diesel fuel injector, during the opening, closing and post injection events. Complementary computational fluid dynamic (CFD) simulations have been undertaken to elucidate the interaction of the liquid and gas phases during these highly transient events, including an assessment of close-coupled injections. Microscopic imaging shows the development of a plug flow in the initial stages of injection, with rapid transition into a primary breakup regime, transitioning to a finely atomised spray and subsequent vaporisation of the fuel. During closuring of the injector the spray collapses, with evidence of swirling breakup structures together with unstable ligaments of fuel breaking into large slow-moving droplets. This leads to sub-optimal combustion in the developing flame fronts established by the earlier, more fully-developed spray. The simulation results predict these observed phenomena, including injector surface wetting as a result of large slow-moving droplets and post-injection discharge of liquid fuel. This work suggests that post-injection discharges of fuel play a part in the mechanism of the initial formation, and subsequent accumulation of deposits on the exterior surface of the injector. For multiple injections, opening events are influenced by the dynamics of the previous injection closure; these phenomena have been investigated within the simulations

Direct imaging of primary atomisation in the near-nozzle region of diesel sprays

The spray formation and breakup of n-dodecane was investigated experimentally on a common rail diesel injector using a long working distance microscope. The objectives were to further the fundamental understanding of the processes involved in the initial stage of diesel spray formation under engine-like operating conditions, i.e. high ambient pressure and temperature. Present measurements show that the end of injection is dependent on injection pressure for low injection pressure of 50 MPa and independent for 100-150 MPa pressure range. The end of injection was characterized by large ligaments and deformed droplets along with spherical droplets. It was noted that formation of large droplets during end of injection was not related to injection pressure. The large droplets were found to be in the range of up to 50 μm, which were moving with relatively low velocity. Typical velocity range for large droplets (30-50 μm) was between 1.5 to 5 m/s. The trajectory of individual droplets appeared to be random from injection to injection. It was particularly emphasized that the real fuel injector under engine-like operating conditions can produce a fuel spray, which can be a mix of liquid and vapour at the start of injection. In this publication we report on progress made with ongoing experimental investigations of the atomisation of n-dodecane by using microscopic imaging and high-speed video using ECN ‘Spray A’ injector. A long-distance microscopy was used to study near-nozzle region (1.025x0.906 mmm). Our study focuses on the primary atomisation during the start, the steady-state and the end of the injection process

MANAGEMENT PRIORITIES OF LAND RESOURCES USE IN THE CONTEXT OF THE AIRPORTS AND THEIR INFRASTRUCTURE OPERATION

The place and significance of land resources in the air transport functioning are researched. It is shown that in the questions of land use it is expedient to apply management approaches, which forms the preconditions for a broader understanding of the exploitation issues. The purpose and functions that are realized in the context of the aviation transport land use (operation) are investigated. Functions might be differentiated into general and specific ones that are oriented by criteria-characteristics.The complex of processes influencing of the land resources structure of the transport industry is investigated

Experimental Study of flow and Scalar Mixing in Swirl-stabilised burners

Instantaneous measurements of scalar, velocity and temperature in a swirl stabilized burner are presented. The scalar mixing is discussed in terms of mixture fraction distribution as well as the rate of scalar dissipation. The later quantity, occurring at the micro-scale of turbulent flow, characterizes the level of mixing between a fuel and an oxidizer. This quantity is regarded as one of the most important parameters that affect the combustion process. The scalar dissipation rate was computed from the measured mixture fraction, which was measured by laser-induced fluorescence of acetone. The measurements have been made in a turbulent non-reactive swirling coaxial jet discharged from a swirl-stabilized burner along the jet centerline at different downstream distances from the burner exit. They are reported up to 7 fuel nozzle diameters downstream of the burner exit at a Reynolds number of 29000 for three swirl numbers, namely 0.3, 0.58 and 1.07. The influence of the swirl number on the scalar mixing, unconditional and conditional scalar dissipation statistics was investigated. The purpose of obtaining these measurements is to assess experimentally the validity of the scalar variance models and the associated so-called 'filter density functions' that are being developed in the context of Large Eddy Simulation (LES). Instantaneous (as well as mean) temperature measurements were also obtained in the same swirl-stabilized burner under the same operating conditions with reaction. Temperature was measured by Rayleigh thermometry. Careful selection of the fuel allowed nearly constant Rayleigh scattering cross section across the flame and led to temperature measurements with typical accuracy of around 5%. The temperature measurements quantify the effect of different degrees of swirl on flame stability and on the mechanism of flame stabilization. The temperature statistics, temperature power spectra and thermal dissipation rates are presented. Finally, flow velocity measurements were obtained in the non-reacting and reacting cases by employing particle image velocimetry (PIV) technique. The purpose was to review the occurrence of the processing vortex core (PVC) and its role on flame stabilization. The effect of swirl number on a processing vortex core and a recirculation zone is shown and discussed in association with observations for the scalar measurements

Scalar dissipation rate statistics in turbulent swirling jets

The scalar dissipation rate statistics were measured in an isothermal flow formed by discharging a central jet in an annular stream of swirling air flow. This is a typical geometry used in swirl-stabilised burners, where the central jet is the fuel. The flow Reynolds number was 29 000, based on the area-averaged velocity of 8.46 m/s at the exit and the diameter of 50.8 mm. The scalar dissipation rate and its statistics were computed from two-dimensional imaging of the mixture fraction fields obtained with planar laser induced fluorescence of acetone. Three swirl numbers, S, of 0.3, 0.58, and 1.07 of the annular swirling stream were considered. The influence of the swirl number on scalar mixing, unconditional, and conditional scalar dissipation rate statistics were quantified. A procedure, based on a Wiener filter approach, was used to de-noise the raw mixture fraction images. The filtering errors on the scalar dissipation rate measurements were up to 15%, depending on downstream positions from the burner exit. The maximum of instantaneous scalar dissipation rate was found to be up to 35 s−1, while the mean dissipation rate was 10 times smaller. The probability density functions of the logarithm of the scalar dissipation rate fluctuations were found to be slightly negatively skewed at low swirl numbers and almost symmetrical when the swirl number increased. The assumption of statistical independence between the scalar and its dissipation rate was valid for higher swirl numbers at locations with low scalar fluctuations and less valid for low swirl numbers. The deviations from the assumption of statistical independence were quantified. The conditional mean of the scalar dissipation rate, the standard deviation of the scalar dissipation rate fluctuations, the weighted probability of occurrence of the mean conditional scalar dissipation rate, and the conditional probability are reported