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.

The present work reports an experimental investigation on pulsed water spray impingement into cross-stream air flow in an optically accessible channel with circular cross section. Such flow configuration refers to a complex fluid dynamics system due to interaction of polydispersed droplets with the channel wall. The current study aims at gaining some insight into post-impact droplet characteristics and the in-channel mixing process following spray interaction. The spatial distribution of spray droplets across the channel cross section was visualized by application of the laser sheet imaging (LSI) technique at different axial locations. A uniformity index parameter was measured to quantify the degree of mixing. Measurement of spray droplet size distribution post-impingement and at the channel exit was achieved by the interferometric laser imaging for droplet sizing (ILIDS) technique. The impingement of the spray droplets on the channel wall was visualized by high-speed shadowgraphic imaging technique. The effect of introducing a swirler-type mixer in the channel was examined. Experiments were performed by varying the flow and injection parameters. It was found that efficient mixing of droplets after impingement is not very sensitive to moderate variations in the injection parameters though careful swirler design is important.

The present research aims to investigate the liquid atomization process in a slinger atomizer test rig that houses a high-speed motor which allows high rotational speed of the slinger disc. Instead of delivering the liquid directly on the slinger disc, which is commonly reported in the literature, a stationary manifold was designed that receives the liquid from the pump and supply multiple liquid jets that impinge on the rotating slinger disc. The liquid jet breakup process was visualized using front light illumination technique. All experiments were performed using water as the working fluid and under atmospheric conditions. Four different water flow rates, ranging from 0.2 lpm up to 0.8 lpm were considered. The rotational speed of the slinger was varied from 5000 rpm up to 30000 rpm, which has been rarely reported in the past. The paper reports a comprehensive study on the differences in the liquid breakup modes due to higher liquid flow rate for the same rotational speed and vice-versa. Mostly the liquid was found to attach to the side of the slinger holes that is opposite to the direction of rotation indicating the strong influence of Coriolis forces on the liquid flow within the slinger and hence the atomization process. The droplet size in the spray was measured using the Interferometric Laser Imaging for Droplet Sizing (ILIDS) technique.

In this paper, the goal is to understand the influence of spray combustion on the evolution of droplet clusters and collective gasification processes of droplet groups in a fuel spray. Experiments were conducted in an acetone spray from a pressure swirl injector under both non-reacting and reacting conditions for the same fuel flow rate through the injector. The PIV technique was employed to capture Mie-scattering images of the spray and to measure droplet velocity, while droplet sizing was achieved by application of the ILIDS technique. Voronoi analysis of the PIV images facilitated identification of droplet clusters. Accordingly, the cluster length scale and local droplet number density within clusters could be obtained. In addition, the Group evaporation/combustion number (G) for individual clusters was also evaluated. Either in the presence or absence of spray combustion, wide range of cluster size exists in the spray which highlights multi-scale clustering of droplets. Accordingly, G varies in a broad range that indicates multi-mode evaporation/combustion of the droplet clusters. Distinct behavior of droplet clusters is identified depending on the cluster size relative to the mean cluster area. While the evolution of small clusters is not sensitive to the presence of the spray flame, the length scale of the large-scale clusters and gasification process of such groups of droplets are modified due to spray combustion compared to the non-reacting condition.

The clustering of fuel droplets and group combustion of droplet clusters are experimentally investigated in a spray burner. The burner was operated in the presence and absence of coflowing air around a pressure swirl injector which generates a kerosene spray. The Mie scattering images of the spray droplets were recorded by PIV technique, while planar measurement of droplet size was achieved by application of the ILIDS technique. The photographs of the spray flame and droplets gave evidence of distributed flame structure with the inner reaction zone containing several small diffusion flames indicating group burning of droplet clusters. The invariance of average droplet size downstream of the injector exit further supported the above observation and suggested significant reduction of gasification rate of droplets within the clusters. Application of Voronoi analysis to focused images of the spray droplets facilitated characterization of clusters of droplets. Accordingly, the Group combustion number was evaluated for each droplet cluster based on the measured cluster size and inter-droplet distance. The results reveal multi-scale droplet clustering and multi-mode combustion of the clusters during the spray burning process.

This paper reports an experimental study of spray characteristics in rotary slinger atomizers using different laser diagnostic tools. The main objective of the present study is to investigate the effect of size of the slinger orifices on liquid breakup dynamics and droplet size distribution over a wide range of rotational speed (5000–45,000 rpm) and liquid flow rates (0.2–2 lpm). Three different slinger discs having the same number of orifices but different sizes (d0 = 1, 1.5, and 2 mm) are considered. The primary liquid breakup visualization is achieved using volumetric laser-induced fluorescence (VLIF) technique; whereas, the droplet size is measured by interferometric laser imaging for droplet sizing (ILIDS) technique. The results demonstrate critical role of orifice size on liquid breakup mode and droplet size up to rotational speed about 30,000 rpm, beyond which the aerodynamic force dominates the atomization process such that neither the orifice size nor rate of rotation has strong influence.