Shamit Bakshi

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 - ε 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. Copyright © 2005 by ASME.

It is known from recent studies that evaporation induces flow around a droplet at atmospheric conditions. This flow is visible even for slowly evaporating liquids like water. In the present study, we investigate the influence of the ambient gas on the evaporating droplet. We observe from the experiments that the rate of evaporation at atmospheric temperature and pressure decreases in a heavier ambient gas. The evaporation-induced flow in these gases for different liquids is measured using particle image velocimetry and found to be very different from each other. However, the width of the disturbed zone around the droplet is seen to be independent of the evaporating liquid and the size of the needle (for the range of needle diameters studied), and only depends on the ambient gas used.

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.

Common rail direct injection (CRDI) system is a modern variant of direct injection diesel engine featuring higher fuel injection pressure and flexible injection scheduling which involves two or more pulses. Unlike a conventional diesel engine, the CRDI engine provides simultaneous reduction of oxides of nitrogen and smoke with an injection schedule that has optimized start of injection, fuel quantity in each injection pulse, and dwell periods between them. In this paper, the development of a multizone phenomenological model used for predicting combustion and emission characteristics of multiple injection in CRDI diesel engine is presented. The multizone spray configuration with their temperature and composition histories predicted on phenomenological spray growth and mixing considerations helps accurate prediction of engine combustion and emission (nitric oxide and soot) characteristics. The model predictions of combustion and emissions for multiple injection are validated with measured values over a wide range of speed and load conditions. The multizone and the two-zone model are compared and the reasons for better comparisons for the multizone model with experimental data are also explored.

An experimental investigation of cavitating structures in the near-wake region of a cylinder is presented. From high-speed imaging of this subcritical flow (Reynolds number of 64,000), it is found that inception of cavities occurs in the shear layer. At the developed cavitation condition, the cavities in the separated zone and the free shear layer merge. A distinct spanwise variation in cavitation activity is observed. The non-dimensionalized correlation length at inception varies from close to a non-cavitating value of about 3.5 to about 1 at developed cavitation. The non-dimensionalized length of formation, characterized by crossover of the free shear layer and the wake axis, increases from 1 to 1.8 as the cavitation number is reduced from 85% to 50% of the inception value. A frequency analysis of the cavity dynamics indicates that although the vortex shedding frequency is dominant in the shear layer, there are peaks corresponding to other frequencies in other flow regions. The presence of a sharp peak at 125 Hz, corresponding to a Strouhal number of 0.2, along with a range of frequencies, is also verified independently through measurement of fluctuating pressure at the cylinder surface.

In gasoline direct injection engines, fuel injector nozzle is one of the vital components that determine the efficiency of fuel atomization which controls combustion and emission. There is an increased focus on developing better nozzles by studying the internal flow behavior especially during the needle transient phase and at the end of injection phase. Multiphase flow characteristics involving cavitation and hydraulic flip are observed inside the nozzle during its operation. At the end of injection, fuel inside the nozzle sac and nozzle holes is purged with the gas from the engine cylinder. The efficiency of this purging process is critical to prevent the carbon deposit formation on the wall of nozzle holes and on the surface of the nozzle tip. In this article, a numerical method is presented to predict the internal nozzle flow during the needle movement and during the end of injection to predict the purging capacity of a gasoline direct injection nozzle. Needle motion is accomplished using a moving mesh method via a user-defined function. The numerical model is compared with the X-ray measurements from the literature. Based on the validated model, the study is extended to analyze various parameters like injection pressure, nozzle hole location, number of nozzle holes and the inlet rounding of the nozzle hole which affects the effectiveness of nozzle purging. Fuel wetting at the nozzle tip after the end of injection is also numerically modeled to evaluate the thickness profile of the thin liquid film.

A Gasoline Direct Injection (GDI) engine typically operates on multiple fuel-preparation modes. In general, at higher loads a homogeneous mixture is favoured whereas a stratified mixture is preferred at part and low load conditions. This is usually achieved by altering the injection timing with respect to load and speed. In this paper the effect of injector configuration on the mixing process has been studied systematically. Two different injector configurations are considered, one with a central-hole injection and other with a 6-hole injection. The objective is to investigate the effect of initial fuel distribution inside the engine cylinder on charge preparation at the onset of ignition. This study also aims to explore a better solution for mixing in GDI engines by optimizing the GDI injector for both stratified and homogeneous mode of operations. An engine with a pentroof combustion chamber with centrally mounted injector and upright straight intake port and flat piston is selected. The computation begins from the start of the induction process and continued till the point of ignition. The dynamics of the mixing process is studied by grouping the in-cylinder charge in different bins in terms of the equivalence ratio. The temporal variation of the fraction of the mixture in different bins is studied as a function of time to understand the dynamics of the mixing process. Results from the parametric study indicate the possibility of switching the modes of mixing with respect to the operating conditions.

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-offtime 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-offtime. 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-offtime. 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-offtime. At the end of this work, we propose a ridge to ridge separation that effectively reduces the lift-offtimes for impacts both at the tip of the ridge and offset from it.

In this paper, noniterative interface reconstruction algorithms for volume of fluid (VOF) method are presented. Analytical expressions are derived to determine the coefficients of the curve to achieve a noniterative reconstruction procedure. Static interface reconstruction tests of standard geometries like circle and ellipse are performed. The noniterative algorithm based on Piecewise Linear Interface Calculation from an earlier work is found to reconstruct the VOF value within machine tolerance. Two piecewise continuous algorithms are also proposed in this work, which ensures C0 continuity. The first algorithm, namely, Piecewise Continuous Linear Interface Calculation (PCLIC), uses piecewise continuous linear segments to approximate the interface. The second algorithm uses piecewise continuous quadratic curve and is named as Piecewise Continuous Quadratic Interface Calculation. PCLIC is seen to achieve machine tolerance in reconstructing the VOF value. Piecewise Continuous Quadratic Interface Calculation is found to give a more accurate estimate of the length of the interface than Piecewise Linear Interface Calculation and PCLIC. © 2013 John Wiley & Sons, Ltd.

Experiments are carried out on an evaporating pendant droplet and the flow induced around the droplet due to evaporation is studied for different ambient gases. A pendant drop of ethanol is formed on a steel needle in a closed chamber maintained at room temperature (301 K). The chamber is filled with a pure gas (nitrogen, oxygen, argon or carbon dioxide). The evaporating droplet causes a flow due to thermal buoyancy (due to the temperature difference caused by evaporative cooling) and solutal buoyancy (due to the difference in molecular weight between the evaporating liquid and surrounding gas). This flow around the evaporating droplet is characterized by using PIV (particle image velocimetry) technique. From these experiments, it is observed that the flow characteristics such as the velocity and penetration length of the flow changes depending upon the molecular weight of the ambient gas. It is observed that the flow penetrates significantly in case of a lower molecular weight gas (e.g. nitrogen) and the penetration decreases with increasing molecular. This, in turn, affects the evaporation rate of the suspended droplet.