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.

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.

The combustion characteristics and emissions in IC engines are influenced by the fuel injection rate. The study of fuel injection rate profile thus becomes essential to improve engine performance. In the present study, the injection rate of GDI injectors was measured in terms of the instantaneous mass flow rate of fuel. A setup was fabricated to obtain the injection rate based on Bosch tube method. Multi-hole GDI injectors of two different manufacturers were used in this study and their performances were measured up to 175 bar of injection pressure. The injector energizing current, wave pressure and rail pressure data were acquired at the rate of 500 kHz on a common time scale. The injected mass value per pulse, obtained by integrating the discharge rate profile was compared with the average values of mass injected measured using a mass balance for both the injectors and their performance were compared. The hydraulic delay measured during injector closing was found to be more than that during injector opening for both the injectors. Injection rate values for any given energizing time was found to be higher for one of the injectors by both the rate profile method and the mass balance averaging method.

A simple computational model was created to predict the vaporization rates of droplets of urea-water-solution (UWS) evaporating in a hot air stream. A single-component evaporation model, which was based on Abramzon-Sirignano's vaporization model, was adapted to handle UWS droplets, and the governing equations were solved numerically in MATLAB®. The temperature and species concentrations within the droplet were assumed to be uniform, and various methods available in literature were employed to estimate important thermophysical properties such as saturation vapor pressures, partial pressures, vaporization enthalpy, and vapor diffusivities. The suitability and limitations of the computational model were assessed by comparing the results with experimental data on UWS droplets evaporating under forced convective conditions. The temperatures considered in this study ranged from 373 K to 673 K, and the corresponding relative air velocities were between 1.5 m/s and 4.3 m/s. The model was found to be able to capture the two-stage vaporization behavior of the UWS droplet (except crystallization, puffing and micro-explosion) with reasonable accuracy. While the first-stage vaporization rates, predicted by the model, were accurate to within 1.8% to 17.7%, the accuracies of the second-stage vaporization rates were significantly dependent on the methods used to estimate fluid properties such as the vapor pressures and the vaporization enthalpy.

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−2 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. EF/proceedings.

This experimental work investigates the evaporation rates of liquids (water, ethanol) and the flow induced thereby for both pendant and sessile droplets in high density ambient gas (sulfur hexafluoride). The flow induced around an evaporating droplet is studied at ambient pressure, in a sealed glass chamber. A steel needle and aluminium substrate are used for the pendant and sessile droplet study, respectively. The evaporation rates of the droplets are measured using shadow imaging technique. Particle image velocimetry (PIV) technique is used to measure the flow induced around the evaporating droplet. This flow is observed for both water and ethanol droplets. The flow moves upward for both pendant and sessile droplets of ethanol. In case of water, the pendant droplet induces a flow which is initially upward, but with time, its direction changes downward. The evaporation-induced flow for a sessile droplet of water is however always in the upward direction. This indicates the relative role of thermal and solutal buoyancy in the genesis of the flow. Droplet surface temperatures are also measured and discussed in this paper.

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.

The behavior of droplets of urea-water-solution (UWS) evaporating under the influence of a hot stream of air was investigated experimentally, under temperatures ranging from 100°C to 400°C. The droplets were suspended on a glass microfiber to minimize the influence of heat conduction, through the fiber, on the evaporation rate of the droplet. The flow rate of air, under all experimental conditions, was measured and these data were used to estimate the average velocity of air around the droplet. Experiments were also conducted on droplets of pure water and the results were compared. The initial mass fraction of urea, in the solution, did not appear to have a significant effect on the evaporation constant, but it did affect a few essential aspects of the evaporation behavior. The evaporation of water droplets was in accordance with the d2 law at all temperatures, whereas the evaporation of UWS droplets was ambient-temperature dependent.

A new air-assisted injection system-based two-stroke engine has been developed at the Indian Institute of Science, Bangalore over the past few years and shows the potential to meet emission norms such as Euro II and Euro III and yet deliver satisfactory performance. The 70 cc engine works on the direct mixture injection principle with scavenging performed by air alone instead of an air-fuel mixture. A small 20 cc pump driven off the engine is used for mixture preparation prior to in-cylinder injection of the mixture. In the present study, the mixture injection and subsequent charge stratification process inside the engine cylinder is modelled. A three-dimensional compressible flow code with a standard k-ε turbulence model with wall functions is developed and used for this modelling. 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 modelling 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, the sensitivity of two critical parameters in the mixing and stratification is investigated. The suggested parameters substantially enhance the flammable proportion at the onset of combustion. The predicted pressure-crank angle history from a combustion simulation supports this recommendation. © IMechE 2007.