Analysis of the Spray Characteristics of Water and Water/Glycerin Mixtures using an Interferometric Laser Imaging for Droplet Sizing Technique

Fuel injection; ILIDS; Spray characteristic; Water-glycerin mixture

Various visualization techniques have been used in the literature to study the blending of biofuels. Patel et al. (2016) used phase Doppler interferometry to investigate droplet size and velocity distributions for Jatropha biodiesel blends up to 20%. The results from Patel showed that with a higher concentration of Jatropha biodiesel, brake-specific carbon monoxide emissions reduced at higher engine speeds. In a more recent study that focused on the engine performance of biodiesel in a diesel engine, blends of avocado seed oil may substitute biodiesel mixtures from palm oil mill effluent blends with some reduction in power output (Sutrisno et al., 2019). Said et al. (2018) discovered that palm oil mill effluent blended with diesel fuel decreased CO₂, CO, and hydrocarbon emissions in various engine loads. However, the results also showed an increase in NO_X emissions up to 5.9%.

Atomization of liquids with high viscosity has been a challenge in various engineering applications when high speed and small diameter sprays are required. Garcia et al. (2017) analyzed Venturi-vortex twin-fluid swirl nozzle application for pure glycerin at room temperature. At 10 liter/hour flow rate, 26% of glycerin volume was atomized to droplets with a diameter of 20 ?m. Liquid ligaments were present at the nozzle exit and tended to vanish moving away from the nozzle. However, the size distribution variations could not be identified by Garcia et al. (2017), who used laser diffractometry.

Although the potential usage of biofuels has been extensively investigated, these studies tend to focus solely on engine performance. However, combustion efficiency for ICEs is dependent on the injection characteristics, which tend to change with the different types of fuel used. To assess and control the combustion process more effectively, it is crucial to know the mechanism and the relationship behind the injection conditions and the fuel. In fact, new generation bio-fuels are highly viscous and multicomponent in nature. For this reason, the spray characteristics of these fuel blends need to be investigated as they present different physical properties, such as viscosity and surface tension. Typical methods for investigating spray characteristics involve particle image velocimetry (Adrian, 1991) and phase doppler interferometry (Kobashi et al., 1991). Unfortunately, these methods cannot simultaneously measure particle size, velocity, and spatial distributions, which are vital to understanding spray behavior. By contrast, the interferometric laser imaging for droplet sizing (ILIDS) method provides a clear spatial correlation between the particles and allows simultaneous measurement of particle size and velocity with improved accuracy. There have been various improvements in the ILIDS method, including the reduction of the overlap of interference images (Kawaguchi et al., 2002), higher resolution of the spray particles (Dehaeck and Van Beeck, 2008), and extended measurable particle size range (Hardalupas et al., 2010). In addition to these advanced techniques, Shigeta et al. (2012) improved the accuracy of the particle position detection, which enabled a simultaneous spatial mass flux evaluation.

The aim of this study was to explore the influence of viscosity and injection pressure for different ambient temperatures on droplet size and distribution to optimize the injection mechanisms. To this end, in this study, the ILIDS method was chosen as an effective method to evaluate and measure the characteristics of water-glycerin mixtures. This work focused on port injection conditions for naturally aspirated and supercharged engines. The working fluid was a water-glycerin mixture, which is easier to work with compared to fuel oils, as the composition of fuel oils is harder to control and analyze. Upon the validation of the experimental setup, various biofuel blends will be utilized to lower conventional ICEs CO₂ emissions.

ILIDS technique was used to investigate the effects of temperature, injection pressure, and different viscosities on the spray behavior in ambient and pressurized conditions. The findings of this study are summarized as follows: (1) For distilled water in ambient conditions, SMD and AMD were reduced due to the effect of the heated plate. For all particles, the average velocity increased with plate temperature; (2) As the plate temperature increased, the ratio of small-sized particles increased. The average velocity of the droplets increased with a higher plate temperature. However, SMD and AMD decreased with increasing temperature. These observations were all related to the evaporation effect on the spray; (3) As the injection pressure increased, the ratio of small-sized particles increased. SMD and AMD were both decreased with increasing injection pressure due to the nozzle shear effect. On average, droplet velocities increased as the injection pressure increased; and (4) When the viscosity of the working fluid was changed in ambient conditions, SMD and AMD were slightly increased. At the nozzle exit, the standard deviation increased as the variation in particle size increased. Additionally, for the higher viscous fluids where the glycerin ratio was increased, the growing rate of the droplet diameter became smaller as the radial distance increased. This phenomenon is related to the presence of liquid ligaments that are fragmented as the droplets moved away from the nozzle.

Measurement range for the observation area was 12×10 mm² and measurable spray droplets ranged between 14–450 µm. As the next step of this study, current experimental apparatus with various biofuel blends are aimed to be analyzed in order to understand the spray particle distribution and reduce exhaust emissions.

This work was supported by the Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research (No.19K04244).

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