Srikrishna Sahu

The clustering of droplets in air-assisted water sprays operating under ambient atmospheric conditions is experimentally studied with the aim to characterize the droplet clusters and study the consequence of clustering on local turbulent mass flux of droplets. Planar measurements of droplet number density and velocity were achieved by application of the PIV technique, while the ILIDS technique was used for sizing individual droplets. Experiments were performed for four different injector operating conditions corresponding to different liquid mass fractions at the radial measurement stations far downstream of the injector exit. The droplet clusters were statistically characterized by the measurement of the D parameter. The clustering of droplets occurs over a range of length scales, however, the largest length scale of droplet clusters (Lc) was found to scale with large eddies of the turbulent air flow around droplets. For higher local liquid mass fraction, the D parameter was also higher, while Lc was smaller, indicating intense clustering. The local turbulent number flux of droplets, which is essentially the correlation between fluctuations of the droplet number density and the droplet velocity (nu‾), was found to be non-negligible relative to the steady flux especially towards the edge of the spray, where the tendency of the droplets for clustering was found to be higher. Also, the correlation nu‾ was always negative suggesting that locally higher droplet number density due to passage of the clusters of droplets leads to smaller droplet velocity fluctuations.

Understanding the influence of fuel composition and air turbulence on evaporation of bi-component fuel droplet is the aim of the current investigation. Experiments were carried out with a single fuel droplet (a mixture of n-heptane and ethanol) suspended using the cross-wire technique within a box of turbulence facility that is capable of generating near zero-mean homogeneous and isotropic turbulence at its center. The fuel composition (i.e. the volume fraction of ethanol) as well as turbulent intensity within the chamber were varied over a wide range. The images of the evaporating droplet were recorded using back-light illumination. The initial droplet size was about 30–40 times smaller than the integral length scale of air turbulence, while it was about 10 times larger than the Kolmogorov length scale. For each fuel composition, five different turbulent conditions were considered. While the effect of turbulence was found to enhance droplet evaporation rate for all cases, the extent of its influence depends on fuel vapor mass diffusivity in air, and, in addition, on the initial fuel composition of the binary mixtures. The evaporation constant for the first stage of evaporation is sensitive to both air turbulence level and composition, while that for the second stage of evaporation shows a specific trend with fuel composition for all cases of turbulent intensity. Finally, an experimental correlation for normalized evaporation constant is presented.

It is well established that spray characteristics from automotive injectors depend on, among other factors, whether cavitation arises in the injector nozzle. Bulk cavitation, which refers to the cavitation development distant from walls and thus far from the streamline curvature associated with salient points on a wall, has not been thoroughly investigated experimentally in injector nozzles. Consequently, it is not clear what is causing this phenomenon. The research objective of this study was to visualize cavitation in three different injector models (designated as Type A, Type B, and Type C) and quantify the liquid flow field in relation to the bulk cavitation phenomenon. In all models, bulk cavitation was present. We expected this bulk cavitation to be associated with a swirling flow with its axis parallel to that of the nozzle. However, liquid velocity measurements obtained through particle image velocimetry (PIV) demonstrated the absence of a swirling flow structure in the mean flow field just upstream of the nozzle exit, at a plane normal to the hypothetical axis of the injector. Consequently, we applied proper orthogonal decomposition (POD) to analyze the instantaneous liquid velocity data records in order to capture the dominant coherent structures potentially related to cavitation. It was found that the most energetic mode of the liquid flow field corresponded to the expected instantaneous swirling flow structure when bulk cavitation was present in the flow.