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The paper considers the mixing patterns of the flow behind a two-stage swirler in a flame tube in order to determine the optimal vane angle by numerical simulation. [ABSTRACT FROM AUTHOR]

In recent studies, it has been established that improvements in the injection rate results in improved mixing, combustion efficiency, and reduced pollutant emissions. Varying injection rates have been observed to find out the optimization injection rate. In this study, split injection with different injection rates was used as the flexible injection to investigate the diesel spray mixing efficiency. Large eddy simulation (LES) was used to investigate the complex diesel mixing processes of unsteady turbulence. The split injection was combined with different fuel injection rates to approximate the ramping-down rate shape. The simulation results of flexible injection rates showed the formation of a highly unstable aerosol vapor structure with the turbulence structure that produces the vortex in the spray area. The powerful injection rates provide a very strong spray turbulence structure and vortex formation. The backflow and vortex are evident when the dwell time of injection is setup. The flexible injection rates have a huge influence on the mixing efficiency of the fuel spray. Basic mixing efficiency predictions revealed that the dwell time of the split injection and inverse injection pressure affect the turbulence structure. Another prediction is that the initial pressure of the injection rate has a significant impact on evaporation and mixing efficiency, even using the same fuel quantity and same condition. The double-rectangular split injection has a slight effect on the spray radial extension, and the spray area grows rapidly while the equivalence ratio is low. The double-drop split injection with a high pressure difference has a great influence on the spray radial extension and spray length. Additionally, the higher injection pressure results in lower equivalence ratios and more efficient mixing.

• Methanol spray and combustion within the narrow-confined pre-chambers were fully transparent investigated for the first time. • The effects of injection strategy was studied in an actual-geometry optical pre-chamber to fill the lack of test data in turbulence jet ignition mechanism. • Multiple spray-wall impingements and spray tumble were experimentally conformed in pre-chamber. • Flame propagation through complex walls in confined space and hot jets formation along with Mach rings were discussed. Methanol spray impingement, mixing, flame propagation and jet evolution within the narrow-confined active pre-chamber (APC) with complex walls were experimentally observed by optical method for the first time. The effects of injection pressure and duration was investigated on an actual-geometry optical APC assembled in constant volume combustion chamber (CVC). Exploration on APC internal processes within static environments forms the basis for optimization of APC and analysis of jet formation. Mie scattering coupled with flame natural luminosity imaging were used to record spray mixing and combustion. The results show that in the APC with narrow-confined space, methanol spray undergoes three wall impingements, inducing a tumble that promotes mixing. High injection pressure enhances rapid spray tip penetration, while long injection duration slows methanol diffusion. In longer injection duration cases, the spray tip after the third impingement interferes with subsequent sprays. For higher injection pressure, this interference occurs in the APC cone section due to a larger spray cone angle, whereas for lower pressure, it happens in the APC duct section. Higher injection pressure also causes more spray to impact the APC bottom after the first wall impingement, hindering flame acceleration in the duct section. Irregular flame propagation is observed in the APC cone section due to wider space and uneven mixture conditions, while the convergent geometry from the APC throat to the duct section significantly accelerates flame propagation. The APC jet appears due to the pressure difference between the APC and the main chamber, with the brightness and length of the visible flame jet increasing with APC flame luminosity and pressure difference. A Mach ring structure is observed initially, but the jet velocity decreases as the Mach ring disappears during the APC inner flame extinguishing process. Through this study, the actual spraying and combustion process in APC is experimentally captured and revealed, which are an important contribution for APC optimization and development. [ABSTRACT FROM AUTHOR]

In diesel engines, fuel mixing is an important process in determining the combustion efficiency and emissions level. One of the measures used to achieve fuel mixing is controlling the nature and behavior of the fuel spray by shaping the injection rate. The mechanism underlying the behavior of the spray with varying injection rates before the start of combustion is not fully understood. Therefore, in this research, the fuel injection rate shape is investigated to assess the spraying and mixing behavior. Diesel sprays with different ambient temperatures and injection pressures are modeled using the CONVERGE-CFD software. The validation is performed based on experimental data from an Engine Combustion Network (ECN). The verified models are then used to analyze the characteristics of the diesel spray before and after the end-of-injection (EOI) with four fuel injection rate shapes, including a rectangular injection rate shape (RECT), a quick increase gradual decrease injection rate shape (QIGD), a gradual increase gradual decrease injection rate shape (GIGD), and a gradual increase quick decrease injection rate shape (GIQD). The spray vapor penetrations, liquid lengths, evaporation ratios, Sauter mean diameter (SMDs), distributions of turbulence kinetic energy, temperatures, and equivalence ratios were compared under different injection rate shapes. The results show that the QIGD injection rate shape can enhance mixing during injection, while the GIQD injection rate shape can achieve better mixing after the EOI. [ABSTRACT FROM AUTHOR]

In diesel engines, fuel mixing is an important process in determining the combustion efficiency and emissions level. One of the measures used to achieve fuel mixing is controlling the nature and behavior of the fuel spray by shaping the injection rate. The mechanism underlying the behavior of the spray with varying injection rates before the start of combustion is not fully understood. Therefore, in this research, the fuel injection rate shape is investigated to assess the spraying and mixing behavior. Diesel sprays with different ambient temperatures and injection pressures are modeled using the CONVERGE-CFD software. The validation is performed based on experimental data from an Engine Combustion Network (ECN). The verified models are then used to analyze the characteristics of the diesel spray before and after the end-of-injection (EOI) with four fuel injection rate shapes, including a rectangular injection rate shape (RECT), a quick increase gradual decrease injection rate shape (QIGD), a gradual increase gradual decrease injection rate shape (GIGD), and a gradual increase quick decrease injection rate shape (GIQD). The spray vapor penetrations, liquid lengths, evaporation ratios, Sauter mean diameter (SMDs), distributions of turbulence kinetic energy, temperatures, and equivalence ratios were compared under different injection rate shapes. The results show that the QIGD injection rate shape can enhance mixing during injection, while the GIQD injection rate shape can achieve better mixing after the EOI.

The effects of injection timing, injector type, injection pressure and engine flowfield on fuel spray mixing in a direct injection engine were investigated using planar laser-induced fluorescence. The fluorescence images had sufficient resolution and quality to permit, for the first time in an engine, the calculation of the scalar dissipation rate. The probability density function of the scalar dissipation scaled by the mean showed excellent agreement with turbulent jet and shear layer data for late injection conditions, indicating that the same fundamental mixing process existed in the different flows. The effect of shot noise limited such comparisons for the more homogeneous early injection conditions. A dual-metric method was developed to characterize the degree of mixedness. The two metrics employed were the spatial variation, which describes the homogeneity of the scalar population, and the mean scalar dissipation, which describes the average magnitude of local scalar gradients and represents the rate of fine-scale mixing. Using this method, it was found that the presence of a strong bulk flowfield dominated the mixing rate in the test engine, while injector characteristics showed lesser effects. The data set averaged results of the two metrics for a wide range of conditions were found to define a single, unique curve that was accurately described by a quadratic relationship. This curve defines the path that turbulent mixing follows from an initial segregated state to the fully mixed limit. [ABSTRACT FROM AUTHOR]