Showing papers by "Vasudevan Raghavan published in 2011"

A numerical study of evaporation characteristics of spherical n-dodecane droplets in high pressure nitrogen environment

Abstract: Evaporation of stagnant (zero relative velocity) as well as moving spherical droplets of n-dodecane in a zero-gravity and high pressure nitrogen environment is modeled. The non-ideal effects, solubility of ambient gas into the liquid-phase, variable thermo-physical properties, and gas- and liquid-phase transients are included in the model. The model is quantitatively validated using published experimental data. Numerical predictions show that, for stagnant droplets at sub-critical ambient temperatures, the droplet lifetime continuously increases with pressure, while at critical temperature, the lifetime initially increases and thereafter remains almost constant. At super-critical temperatures, the lifetime decreases continuously with increasing ambient pressure and the average evaporation constant shows a local maximum at a particular ambient pressure. In the case of moving droplets, at super-critical ambient temperature, the rate of increase of average evaporation constant with ambient pressure becomes significant as the initial droplet relative velocity increases. For low initial velocities (<1 m/s), the average evaporation constant gradually increases with ambient pressure and subsequently levels-off with further increase in ambient pressure. The droplet lifetime decreases with increase in ambient pressure or initial velocity. Penetration distance of the moving droplets decreases with ambient pressure and increases with initial droplet relative velocity. The mechanisms influencing the differences in evaporation under varying conditions of pressure and temperature are discussed.

Numerical investigation of laminar diffusion flames established on a horizontal flat plate in a parallel air stream

Abstract: Numerical investigation of laminar diffusion flames established on a flat plate in a parallel air stream is presented. A numerical model with a multi-step chemical kinetics mechanism, variable thermo-physical properties, multi-component species diffusion and a radiation sub-model is employed for this purpose. Both upward and downward injection of fuel has been considered in a normal gravity environment. The thermal and aerodynamic structure of the flame has been explained with the help of temperature and species contours, net reaction rate of fuel and streamlines. Flame characteristics and stability aspects for several air and fuel velocity combinations have been studied. An important characteristic of a laminar boundary layer diffusion flame with upward injection of fuel is the velocity overshoot that occurs near the flame zone. This is not observed when the fuel is injected in the downward direction. The flame standoff distance is slightly higher for the downward injection of fuel due to increase in dis...

Numerical Modeling of Steady Burning Characteristics of Spherical Ethanol Particles in a Spray Environment

Abstract: A numerical study of steady burning of spherical ethanol particles in a spray environment is presented. A spray environment is modeled as a high temperature oxidizer stream where the major products of combustion such as carbon dioxide and water vapor will be present along with reduced amounts of oxygen and nitrogen. The numerical model, which employs variable thermophysical properties, a global single-step reaction mechanism, and an optically thin radiation model, has been first validated against published experimental results for quasi-steady combustion of spherical ethanol particles. The validated model has been employed to predict the burning behavior of the ethanol particle in high temperature modified oxidizer environment. Results show that based on the amount of oxygen present in the oxidizer the burning rate constant is affected. The ambient temperature affects the burning rate constant only after a sufficient decrease in the oxygen content occurs. In pure air stream, ambient temperature variation does not affect the evaporation constant. Results in terms of burning rates, maximum temperature around the particle, and the evaporation rate constants are presented for all the cases. The variation of normalized Damkohler number is also presented to show the cases where combustion or pure evaporation would occur.

A Parametric Study of the Choice of Binary Interaction Parameter and Equation of State for High Pressure Vapor-Liquid Equilibrium of Nitrogen – n-Dodecane Binary System

Abstract: A parametric study of the effects of binary interaction parameter and real-gas equations of state on the high pressure vapor-liquid equilibrium of nitrogen-n-dodecane system was carried out Different values of the binary interaction parameter reported in literature, including one which depends on temperature, were employed in different equations of state to predict the vapor-liquid equilibrium as a function of ambient pressure and temperature The findings were compared against the available experimental values reported in literature Constant values of binary interaction parameter, estimated based on temperature dependent values, are demonstrated to predict the experimentally observed vapor-liquid equilibrium values accurately The Peng-Robinson equation of state and an average binary interaction parameter were demonstrated to predict the vapor-liquid equilibrium over a wide range of temperature and pressures for nitrogen-n-dodecane binary system

Investigation of the temperature and species distributions in co-flow methane?oxygen laminar diffusion flames

Abstract: Investigation of the structure of co-flow methane?oxygen diffusion flame is presented. A numerical model with a 43-steps chemical kinetics mechanism and an optically thin radiation sub-model is employed in simulations. Through simple co-flow diffusion flame experimental arrangement, visible flame extents have been measured. The methane oxygen flames have four distinct visible regions; a dark inner zone, a reddish zone, a highly luminous zone and a blue or bluish-white zone. The variation of the extents of these zones with increasing oxygen flow rate is discussed. The effect of oxygen flow rate on the species and temperature distribution is studied in detail.

A numerical study of the effect of radial confinement on the characteristics of laminar co-flow methane–oxygen diffusion flames:

Abstract: A numerical investigation of the characteristics of laminar co-flow methane–oxygen diffusion flames has been carried out. The temperature and nitric oxide (NO) distributions in unconfined and partly confined flames are studied in detail. Radial confinements of different diameters and with a length of 150 times the fuel jet diameter have been considered to allow atmospheric nitrogen entry only from the top. A numerical model with a 43-step chemical kinetics mechanism and an optically thin radiation sub-model is employed to carry out simulations. The numerical model has been validated using the experimental data available in the literature. The effect of oxygen flowrate on temperature distributions is studied thoroughly. Confined flame extents are compared with the corresponding unconfined flame extents with the help of OH contours. The effect of confinement diameter on temperature and NO distributions is analysed in detail. At low oxygen flowrates, the extents of confined flames are higher than those of an...

The Effect of Upstream Annular Cavities in Circular Jet Burners on Stability Characteristics of Jet Diffusion Flames

Abstract: The characteristics of diffusion flames from circular jet burners in the presence of upstream annular cavities are investigated experimentally. The influences of cavity dimensions such as cavity diameter and cavity length, and postcavity length, on methane flame characteristics are investigated. Experimental investigation employing 18 circular jet burners with an upstream cavity of different dimensions and a burner without cavity has been carried out. For burners with cavity, smaller flame lengths are obtained in the laminar region when compared to the burner without cavity due to enhanced mixing. The transition from laminar to turbulent regime occurs at higher flow rates for burners with cavity. Liftoff velocity, the fuel velocity at which the flame lifts off from the burner exit, and blowoff velocity, at which the flame blows off, are higher with the inclusion of cavity. This suggests that improved flame stability and increased operating range can be attained in burners with cavity as compared to that w...

A Numerical Study of Leading Edge Anchoring Characteristics of Ethanol Sourced Laminar Boundary Layer Diffusion Flames

Abstract: A numerical study of leading edge anchoring characteristics of diffusion flames established over a liquid ethanol film in a confined environment at atmospheric pressure under normal gravity with a forced air flow parallel to its surface is presented. A numerical model, which solves the transient, two-dimensional, gas-phase governing conservation equations with proper interface coupling conditions, is employed. The model uses a global single-step reaction for ethanol–air oxidation to model the finite rate chemical kinetics and an optically thin radiation model to account for thermal radiation losses by absorbing species in a nonluminous flame. Validation of the numerical model is carried out against the available experimental data in terms of temperature profiles. The effect of free stream air velocity on the fuel mass burning rate and the movement of the flame anchoring point is further investigated. The emphasis is on investigating flame anchoring point, located upstream of the leading edge at low air ve...

Numerical Analysis of Auto-ignition of Ethanol

Abstract: Renewable and alternative fuels such as ethanol find application in several combustion devices. Fundamental characteristics of these fuels in terms of ignition and burning rate are to be understood in order to use them in these applications. In this study, a numerical analysis of auto ignition characteristics of ethanol is presented. Opposed flow configuration, in which fuel and nitrogen emerges from the bottom duct and hot air flows down from the top duct, has been employed. Commercial CFD software FLUENT is used in the simulations. Global single step mechanism is used to model kinetics. For various mass fractions of fuel, the air temperature at which auto-ignition occurs has been recorded. The numerical results are compared with the experimental data available in literature.

An estimation of modelling parameters for condensed multi-component fuels

Abstract: A case study to evaluate the thermo-physical properties and chemical kinetics parameters, which are employed to model the gas-phase combustion taking place over the surface of a condensed fuel, has been presented. This procedure relies on accurate experimental measurements of gaseous pyrolysis products. The pyrolysis gases can be treated as a virtual fuel, namely CxHyOz, where x, y, and z are estimated by performing atom balances for C, H, and O atoms in all the pyrolysis species. Thermo-physical properties of this virtual fuel are calculated based on the temperature and species concentration of the pyrolysis mixture. Similarly, based on the rate constant values of the reactant species present in the pyrolysis mixture, Arrhenius parameters such as pre-exponential factor and activation energy have been calculated. Polymethylmethacrylate (PMMA) is used as an example fuel in this study, and a step-by-step procedure for the estimation of the thermo-physical properties and global reaction kinetics parameters o...

Numerical Modeling of Steady BurningCharacteristics of Spherical EthanolParticles in a Spray Environment

Abstract: A numerical study of steady burning of spherical ethanol particles in a spray environment is presented. A spray environment is modeled as a high temperature oxidizer stream where the major products of combustion such as carbon dioxide and water vapor will be present along with reduced amounts of oxygen and nitrogen. The numerical model, which employs variable thermophysical properties, a global single-step reaction mechanism, and an optically thin radiation model, has been first validated against published experimental results for quasi-steady combustion of spherical ethanol particles. The validated model has been employed to predict the burning behavior of the ethanol particle in high temperature modified oxidizer environment. Results show that based on the amount of oxygen present in the oxidizer the burning rate constant is affected. The ambient temperature affects the burning rate constant only after a sufficient decrease in the oxygen content occurs. In pure air stream, ambient temperature variation does not affect the evaporation constant. Results in terms of burning rates, maximum temperature around the particle, and the evaporation rate constants are presented for all the cases. The variation of normalized Damkohler number is also presented to show the cases where combustion or pure evaporation would occur.