Shamit Bakshi

Investigations on the impact of a drop onto a small spherical target

This paper reports on experimental and theoretical investigations of the impact of a droplet onto a spherical target. Spatial and temporal variation of film thickness on the target surface is measured. Three distinct temporal phases of the film dynamics are clearly visible from the experimental results, namely the initial drop deformation phase, the inertia dominated phase, and the viscosity dominated phase. Experiments are also conducted to study the effect of droplet Reynolds number and target-to-drop size ratio on the dynamics of the film flow on the surface of the target. It is observed that for a given target-to-drop size ratio, the nondimensional temporal variation of film thickness collapses onto a single curve in the first and second phases. The transition to the viscosity dominated regime occurs earlier for the low Reynolds number cases and residual thickness is also larger. A simplified quasi-one-dimensional approach has been used to model the flow on the spherical target. The theory accounts fo...

Evaporation-induced flow around a pendant droplet and its influence on evaporation

Studies on the evaporation of suspended microlitre droplets under atmospheric conditions have observed faster evaporation rates than the theoretical diffusion-driven rate, especially for rapidly evaporating droplets such as ethanol. Convective flow inside rapidly evaporating droplets has also been reported in the literature. The surrounding gas around the evaporating droplet has, however, been considered to be quiescent in many studies, the validity of which can be questioned. In the present work, we try to answer this question by direct experimental observation of the flow. The possible causes of such a flow are also explored.

Evidence of oscillatory convection inside an evaporating multicomponent droplet in a closed chamber

Visualization of an evaporating binary (ethanol-water) droplet reveals presence of oscillatory internal circulation. The visualization is done by using a laser scattering technique. The oscillatory circulation possibly results from the opposing effect of solutal and thermal Marangoni convection as proposed in some earlier theoretical works. The frequency of this oscillation is measured and the variation of this frequency with the initial concentration of the volatile component (ethanol) is reported.

Phenomenological modeling of combustion and emissions for multiple-injection common rail direct injection engines

The high-pressure multiple injections in common rail direct injection diesel engines offer a possibility of simultaneous reduction of exhaust smoke and oxides of nitrogen. The purpose of the present work is to develop a phenomenological model to enable parametric understanding of the combustion and emission characteristics of multiple-injection common rail direct injection engines. The model is based on a two-zone formulation comprising of fuel–air spray and the surrounding air. The model predictions for combustion and emissions are validated with measured results of different multiple-injection schedules available in the published literature. The effect of parametric variations of multiple-injection scheduling on emission characteristics are predicted using the proposed model. It is observed that the simultaneous reduction of oxides of nitrogen and smoke is possible with an optimized pilot fuel quantity and dwell between the injection pulses.

Morphology of drop impact on a superhydrophobic surface with macro-structures

Drop-surface interaction is predominant in nature as well as in many industrial applications. Superhydrophobic surfaces show potential for various applications as they show complete drop rebound. In a recent work, it has been reported that the drop lift-off time on a superhydrophobic substrate could be further reduced by introducing a macro-ridge. The macro-ridge introduces asymmetry on the morphology of drop spreading and retraction on the surface. This changes the hydrodynamics of drop retraction and reduces the lift-off time. Keeping practical applications in view, we decorate the surface with multiple ridges. The morphology of the hydrodynamic asymmetry is completely different for the drops impacting onto the tip of the ridges from those impacting onto the middle of the valley between the ridges. We show that the morphology forms the key to the lift-off time. We also show that the outward flow from the ridge triggers a Laplace pressure driven de-wetting on the tip of the ridge, thus aiding the lift-of...

Predicting Mixing Rates in Multiple Injection CRDI Engines

The possibility of multiple-injection in Common Rail Direct Injection (CRDI) engine allows achieving improved combination of oxides of nitrogen (NOx) and smoke emissions. In CRDI engines, the turbulent kinetic energy due to high pressure fuel injection is primarily responsible for fuel air mixing and hence the in-cylinder mixture formation. The air fuel mixing characteristics in the case of multiple-injection are quite different from that of single injection schedule. In this work a zero-dimensional model is proposed for mixing rate calculations with multiple-injection scheduling. The model considers generation and dissipation of in-cylinder turbulence through processes namely fuel injection, air swirl and combustion. The model constants are fine tuned with respect to the data available in existing literature. The model predictions are validated with the available data for the cylinder pressure and heat release rate histories on known single and multiple-injection schedules. These comparisons show good agreement to establish the role of mixing rate variations with multiple-injection. A single set of constants were found to match the cylinder pressure and heat release rate histories for single and multiple-injection from different sources in the literature. Further, the mixing rate and peak temperature predictions of the model are found to relate with the possible effect of specific injection scheduling on emission reductions reported in CRDI engine investigations.© 2009 ASME

Triggering of flow asymmetry by anisotropic deflection of lamella during the impact of a drop onto superhydrophobic surfaces

A water drop impacting a superhydrophobic surface (SHS) rebounds completely with remarkable elasticity. For a given drop size, the time of contact on a flat SHS remains constant. However, recent studies show that the contact time can be reduced further by triggering an asymmetry in the hydrodynamics of impact. This can be achieved in different ways; an example being the impact on a cylindrical SHS with a curvature comparable to the drop. Here, the anisotropic flow generated from the tangential momentum and elliptical footprint of the drop before the crash leads to the formation of lobes. In the present work, we perform drop impact experiments on a bathtub-like SHS and show that the radial anisotropy can be triggered even in the absence of both the tangential momentum and non-circular footprint. This is shown to be a consequence of lamella deflection during the drop spreading. The reduction in contact time is quite clearly evident in this experimental regime.

Droplet ski-jumping on an inclined macro-textured superhydrophobic surface

Rapid shedding of impinging water drops is crucial in a cold habitat for diverse reasons spanning from self-cleaning to thermal regulation in most plants, animals, and industrial applications as well. It was shown recently that deploying linear millimetric ridges on a superhydrophobic surface can reduce the contact time (for drops crashing normally) up to 50% compared to a flat surface. However, the contact time rises for drops impacting at an increasing offset to the structure. Counter-intuitively, we demonstrate a ski-jumping mechanism occurring only over a range of offsets from the macro-structure with a remarkable reduction in contact time ( ∼65%) during oblique impacts. Theoretically, the reduction can be as high as 80%. The flow hydrodynamics is very similar to the oblique impacts on a flat surface. However, the architecture of ridge allows the drop to rapidly fly away from the surface. This work provides new insight which can be useful for the design of surfaces with high water repellency.Rapid shedding of impinging water drops is crucial in a cold habitat for diverse reasons spanning from self-cleaning to thermal regulation in most plants, animals, and industrial applications as well. It was shown recently that deploying linear millimetric ridges on a superhydrophobic surface can reduce the contact time (for drops crashing normally) up to 50% compared to a flat surface. However, the contact time rises for drops impacting at an increasing offset to the structure. Counter-intuitively, we demonstrate a ski-jumping mechanism occurring only over a range of offsets from the macro-structure with a remarkable reduction in contact time ( ∼65%) during oblique impacts. Theoretically, the reduction can be as high as 80%. The flow hydrodynamics is very similar to the oblique impacts on a flat surface. However, the architecture of ridge allows the drop to rapidly fly away from the surface. This work provides new insight which can be useful for the design of surfaces with high water repellency.

A new injector concept for multimode operation in gasoline direct injection engines

A gasoline direct injection engine typically operates in multiple fuel preparation modes. In general, at higher loads, a homogeneous mixture is favored, whereas a stratified mixture is preferred at part- and low-load conditions. This is usually achieved by altering the injection parameters and injection strategy with respect to load and speed and through appropriate cylinder and piston geometries. In this article, a new injector concept has been proposed to aid attaining multiple modes in a gasoline direct injection engine. This is achieved by shaping the spray structure to suit the required mixture preparation. Detailed simulations are performed to assess the mixing process in a gasoline direct injection engine using the new injector. The charge preparation at the onset of ignition is studied for different injection modes of the same injector. The results indicate a significant improvement in the mixing process for different modes of operation.

Computational Studies on Charge Stratification and Fuel-Air Mixing in a New Two-Stroke Engine

In this paper, detailed computational study is presented which helps to understand and improve the fuel-air mixing in a new direct-mixture-injection two-stroke engine. This new air-assisted injection system-based two-stroke engine is being developed at the Indian Institute of Science, Bangalore over the past few years. It shows the potential to meet emission norms such as EURO-II and EURO-III and also deliver satisfactory performance. This work proposes a comprehensive strategy to study the air-fuel mixing process in this engine and shows that this strategy can be potentially used to improve the engine performance. A three-dimensional compressible flow code with standard k–e turbulence model with wall functions is developed and used for this modeling. To account for the moving boundary or piston in the engine cylinder domain, a non-stationary and deforming grid is used in this region with stationary cells in the ports and connecting ducts. A flux conservation scheme is used in the domain interface to allow the in-cylinder moving mesh to slide past the fixed port meshes. The initial conditions for flow parameters are taken from the output of a three-dimensional scavenging simulation. The state of the inlet charge is obtained from a separate modeling of the air-assisted injection system of this engine. The simulation results show that a large, near-stoichiometric region is present at most operating conditions in the cylinder head plane. The state of the in-cylinder charge at the onset of ignition is studied leading to a good understanding of the mixing process. In addition, sensitivity of two critical parameters on the mixing and stratification is investigated. The suggested parameters substantially enhance the flammable proportion at the onset of combustion. The predicted P–θ from a combustion simulation supports this recommendation.© 2005 ASME