Srikrishna Sahu

In this paper, novel designs of air swirlers having variable aspect ratio (VAR) with variation in hub/tip diameter from swirler inlet to exit are proposed and the performance of VAR swirlers with respect to spray atomization and dispersion is experimentally investigated. A laser Doppler velocimeter is used to track the air velocity field, and high-speed imaging and phase Doppler interferometer are used to measure the spray characteristics. Airflow rate is varied from 0 to 2400 lpm, and liquid injection pressure is varied between 0.1 bar to 3 bar (gauge). The reduction in aspect ratio and flow area in the flow direction leads to acceleration of airflow toward the exit. In addition, higher magnitudes of tangential and radial air velocities are induced near the orifice, which improves spray atomization. The size of the air recirculation zone is also larger for VAR swirlers. At very low liquid injection pressures, with a conventional air swirler, the spray does not open up; also, the radial dispersion of the spray is poor and the variation of average droplet size is very significant. However, a fully developed open spray with finer atomization, better dispersion, and milder spatial variation of droplet size is observed when VAR air swirlers are employed.

The demand for higher volumetric calorific value of fuels for modern high performance aircraft has led to development of nanofuels that opens a promising future for enhancing the energy density of liquid fuels. This paper intends to study the effect of addition of boron nano-particles (NPs) on the burning characteristics of JP10 fuel droplets for various particle mass loadings (2%, 5%, 7% and 10%). Stable suspension of the NPs was achieved by adding Span80 and Tween80 as surfactant followed by ultra-sonication of the mixture. The fuel droplet was suspended on a horizontal quartz fiber. All experiments were conducted under atmospheric conditions. The droplet burning process was recorded using a high speed camera. The burning of pure JP10 and JP10 plus surfactant (without NPs) were also studied. In case of burning of pure JP10 fuel, the linear trend in time evolution of normalized droplet surface area suggested that the droplet burning follows the classical d2 − law and formation of bubbles inside the droplet was observed. However, these bubbles did not lead to puffing, which though occurred for JP10 plus surfactant (no NPs). This means that addition of surfactants, which has low burning rate, modifies the droplet burning process. In case of nanofuel droplet, burning

Post wall-impingement characteristics of pulsatile and split injection sprays in multi-hole Gasoline Direct Injection (GDI) are investigated. A combined experimental and numerical approach is adopted, to examine the influence of wall temperature as well as gas temperature and pressure on spray evolution. For fuel injection condition, split proportion and wall stand-off distance are not varied. The single injection case is also studied for comparison. The experiments were conducted under atmospheric conditions but for different wall temperatures, and the fuel spray was visualized using high-speed shadowgraph technique. The three-dimensional Eulerian–Lagrangian spray simulations were validated against the measured post-impingement plume width and height. Simulations were carried out to analyse spray–wall interaction process under elevated ambient temperature and pressure conditions also. Considerably smaller radial and vertical spread of the spray plume is obtained for split injection, highlighting its benefits over the single injection case. Both plume and wall film characteristics are strongly influenced by the ambient gas density and temperature rather than the wall temperature. However, time evolutions of the mass, thickness and temperature of the film are similar for both single and split cases at different operating conditions, exhibiting some interesting trends. For split injection, the vapour plumes from the two injection pulses interact. At high ambient temperatures, the peak equivalence ratio values are lower for split injection due to lower injected mass per pulse, in comparison to the single injection strategy.

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 investigates flapping instability in liquid jets in the presence of annular swirling air flow. The jet flapping is a large-scale instability, which may lead to vigorous lateral oscillation of the tail end of the jet core region, and, as a consequece, may lead to spatio-temporal fluctuations spray droplet characteristics further downstream. Though such instability has been reported in earlier works, the influence of the air swirl and nozzle geoemtry are yet unknown. The reported experiments in the present paper consider two coaxial air-blast atomizers with the same annular gas jet diameter but different central liquid jet diameter. Experiments were performed over a wide range of aerodynamic Weber number, Weg (81-1200) and air to liquid momentum flux ratio, M (0.4-33). For each pair of air-liquid velocities, experiments were conducted in presence and absence of air swirl. The primary breakup of the liquid jet was imaged using High Speed Shadowgraphic technique. The liquid jet flapping was characterized by temporal tracking of the liquid-air interface close to the jet breakup point. The flapping frequency was found to be higher than the shear driven Kelvin-Helmholtz instability for lower range of M. It was also found that the air swirl does not influence the jet breakup mode, however, it enhances the amplitude and the frequency of the jet flapping. Also, the nozzle geometry has a strong influence on flapping characteristics.

The current research work focuses on spray characterization in a model Lean Premixed and Pre-vaporized (LPP) atomizer that aims to minimize NOx emission in gas turbine combustors. The injector allows cross-stream atomization of two radially injected liquid jets from a central hub due to the crossflowing air within a surrounding annular region. Both non-swirling and swirling air flows were considered. Optical measurement of spray characteristics is reported for a range of aerodynamic Weber number, (Weg ≈ 40–140) and momentum flux ratio, (MFR ≈ 3–7) that ensured that the jet–wall interactions leading to liquid films were avoided prior to completion of the jet breakup. Planar laser sheet imaging of the spray illustrated significantly wider dispersion of the spray droplets and modification in the overall spray structure due to introduction of air swirl upstream of the liquid jets within the atomizer. The droplet size and all three velocity components were measured using a Phase Doppler Particle Analyzer (PDPA) technique. The SMD was found to significantly reduce with increase in Weg for swirling as well as non-swirling crossflows. The tangential and radial velocity of droplets increased under the presence of swirling air as compared to non-swirling crossflow. The proper orthogonal decomposition (POD) analysis of the spray images was done which revealed the dominant structures in spray. The second and third POD modes were found to depict the alternate feature of region with concentrated droplet number density. Spray fluctuations due to unsteady jet breakup become dominant feature over flapping of spray structure for higher Weg.

The aim of this paper is to characterize large-scale instabilities during the primary breakup process in liquid centered coaxial air-water jets. The interest here is to investigate the role of annular air swirl on such instabilities. A coaxial airblast atomizer that incorporates an axial swirler is considered for this purpose. The atomizer was operated in a wide range of the Weber number, Weg(80-958), momentum flux ratio, M(1-26), and air swirl strength, S(0-1.6). High-speed shadowgraphic images of the primary jet breakup process were recorded. Proper orthogonal decomposition (POD) analysis of the time-resolved images was performed for each operating condition. The 2nd and 3rd POD modes depicted some universal spatial features which refer to large scale instabilities. Three different dominant large scale instabilities were identified, viz., jet flapping, wavy breakup, and explosive breakup, for the entire range of the injector operating condition either in the presence or absence of air swirl. It was found that jet flapping (referred to as the lateral oscillation of the tail end of the jet) is the dominant mode of jet instability for a lower range of M, while explosive jet breakup (referred to as the radial expansion of the jet) governs jet breakup unsteadiness for a higher range of M. The wavy or sinuous mode of breakup is a secondary mechanism relevant under low M conditions. The mechanisms of large scale instabilities and the role of air swirl in that context are explained based on the Fourier analysis of the temporal coefficients of the corresponding POD modes.

Understanding the physics of primary liquid breakup process and its correlation with the evolution of spray characteristics in a rotary slinger atomizer is the goal of the present research. Experiments were conducted in a high-speed slinger test rig that houses a static liquid delivery manifold to uniformly supply the liquid to the rotating slinger disc that contains a single row of orifices carved on its peripheral surface for liquid injection.The atomizer was operated for a wide range of conditions by varying the rotational speed and liquid feed rate. The liquid breakup structure at the exit of the slinger orifices was visualized using front light illumination technique, while the droplet size was measured at different radial stations away from the slinger surface by application of the Interferometric Laser Imaging for Droplet Sizing (ILIDS) technique. The visualization images highlighted strong influence of Coriolis force as the liquid tends to accumulate on one side of the channel (that is opposite to the rotational direction) for all cases. It was observed that while the liquid thickness is smaller for higher rotational speed, it does not vary much with liquid feed rate at the same speed and, instead, in such case the span of the liquid is wider. A theoretical analysis was developed to describe the in-channel liquid behaviour that accounts for the effect of Coriolis and surface tension forces. Interestingly, the theory could explain the above observations. The differences in the predictions in comparison to the analysis by Dahm et al. (2006a) was attributed to the assumption of annular film flow in the latter. The liquid breakup mode (stream, sheet or transition mode) could be described by Coriolis Bond number (Bo) that refers to the ratio of Coriolis to surface tension forces and proportional to the spread parameter (span to thickness ratio), while the liquid breakup regimes were identified on a Bo−q plot, where q is the liquid to air momentum flux ratio. The variation of characteristic droplet sizes with both rotational speed and feed rate were examined, and again some interesting trends are identified. The correlation between liquid breakup mode/regime with the measured droplet size was established using the above non-dimensional numbers.

The present study numerically investigates the influence of introducing a spin-type mixer and different angular orientations of the mixer blades on the spray-wall interaction and mixing, following cross-stream injection of a pulsed spray into airflow in a circular duct. This is relevant to the Selective Catalytic Reduction system in diesel engines for exhaust gas after-treatment. The spin-type static mixer is located downstream of the injector and generates a swirling airflow in the duct. All simulations were carried out using ANSYS Fluent V18.0. The standard k-ω model is used to simulate the turbulent continuous phase flow, while the discrete phase model is employed to track the spray droplets. The Taylor Analogy Breakup and Kuhnke wall film models are adopted to model droplet breakup and wall-film formation, respectively. First, the swirling airflow characteristics without spray injection are validated against in-house particle image velocimetry measurements. Second, the spray computations are compared with the experiment. Overall, good agreement between simulation and experiment is achieved. Furthermore, the choice of water and urea water solution injection liquid on the in-channel spray characteristics is also studied. The main focus of the present work is on the study of the influence of spin mixer clocking on the post-impingement spray evolution, droplet redistribution and mixing, and wall-film characteristics. The results show that the choice of the angular orientation of the mixer governs the extent of droplet deposition and splashing on the mixer blades and, as a result, strongly influences the spatial uniformity of droplets and ammonia species at the channel exit.

Evaporative sprays are encountered in a wide range of engineering applications. Since clustering of droplets in sprays leads to strong inhomogeneity in the spatial distribution of droplet concentration that impacts mass, momentum, and energy exchange between the spray and the surrounding flow, a detailed investigation of droplet clustering in evaporating sprays is important. In the current research work, we experimentally investigate the spatial evolution of droplet cluster characteristics in an evaporating acetone spray injected from an air-assist atomizer. The droplet size and velocity are measured using Interferometric Laser Imaging for Droplet Sizing technique. In detail, characterization of the droplet clusters is achieved by the application of Voronoi analysis to particle image velocimetry images of the spray droplets. This approach not only identifies the droplet clusters but also provides area, length scale, and local droplet number density within the clusters. The identified droplet clusters are multi-scale and could be classified into either large- or small-scale clusters, which scale with spray half-width and Kolmogorov length scale, respectively. Experiments are also conducted in water spray under the same operating conditions. Despite the similarity in the droplet clustering process between the two sprays at small scales of air turbulence, some distinct trends are observed for the large-scale clusters in the acetone spray. This is attributed to the higher evaporation rate of acetone droplets, which promotes preferential accumulation of droplets.