In this study, based on the extinction characteristics of nonpremixed flames, global single-step kinetics parameters are estimated for ethanol oxidation in air. Using a quasi-steady heterogeneous experimental setup, the air velocity at which an envelope flame surrounding the sphere transits to a wake flame that burns in the rear region of the sphere, termed as the transition or the extinction velocity, were obtained in a previous study. These extinction velocities for different sphere sizes are employed to estimate the single-step kinetics parameters. The estimated global single-step kinetics has been employed in a numerical model and extinction characteristics of opposed-flow ethanol-air diffusion flames have been predicted and validated against the available experimental results in literature.

Vasudevan Raghavan

Department of Mechanical Engineering

Karnam Bhadraiah

AIR OXIDATION

AIR VELOCITIES

Diffusion Flame

ENVELOPE FLAME

ETHANOL OXIDATION

Experimental setup

EXTINCTION CHARACTERISTICS

FLAME EXTINCTION

KINETICS PARAMETER

NON-PREMIXED FLAME

Numerical models

POROUS-SPHERE EXPERIMENTS

QUASI-STEADY

Single-step

Diffusion

Ethanol

Experiments

oxidation

Parameter estimation

Spheres

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IIT Madras

On the Estimation and Validation of Global Single-Step Kinetics Parameters of Ethanol-Air Oxidation Using Diffusion Flame Extinction Data

Combustion Science and Technology

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 Damkhler number is also presented to show the cases where combustion or pure evaporation would occur. © 2011 American Society of Mechanical Engineers.

Fulltext

Journal of Heat Transfer

Numerical modeling of steady burning characteristics of spherical ethanol particles in a spray environment

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 velocities, and its movement towards and away from the leading edge of the fuel surface, as the air velocity is increased. This is studied by defining and evaluating local or cell based chemical and flow time scales and their ratios. Copyright © Taylor & Francis Group, LLC.

A numerical study of leading edge anchoring characteristics of ethanol sourced laminar boundary layer diffusion flames

A fundamental experimental study to determine the burning rates of ethanol and ethanol-blended fossil fuels is presented. Pure liquid ethanol or its blends with liquid fossil fuels such as gasoline or diesel, has been transpired to the surface a porous sphere using an infusion pump. Burning of the fuel takes place on the surface of the porous sphere, which is placed in an air stream blowing upwards with a uniform velocity at atmospheric pressure and temperature under normal gravity conditions. At low air velocities, when ignited, a flame envelopes the sphere. For each sphere size, air stream velocity and fuel type, the fuel feed rate will vary and the same is recorded as the burning rate for that configuration. The flame stand-off distances from the sphere surface are measured by post-processing the digital image of the flame photograph using suitable imaging software. The transition velocity at which the flame moves and establishes itself at the wake region of the sphere has been determined for different diameters and fuel types. Correlations of these parameters are also presented. © 2008 The Combustion Institute.

Combustion and Flame

Experimental investigation of burning rates of pure ethanol and ethanol blended fuels

In this work, experimental and numerical investigations of spheres burning in a convective environment have been carried out. In the numerical simulations, transient axi-symmetric Navier-Stokes equations along with species and energy conservation equations are solved using a finite volume technique based on non-orthogonal semi-collocated grids. A global single step reaction involving two reactants, two products and one inert species together with an Arrhenius rate equation has been used to model kinetics. For the sake of comparison, an infinite rate chemistry model has also been attempted. The density of the mixture has been evaluated from ideal gas equation of state. Thermo-physical properties like thermal conductivity and viscosity have been evaluated using the Chapman-Enskog description of binary gas mixtures. Specific heats and species enthalpies have been evaluated using piece-wise polynomials of temperature. The burning of isolated spherical particles in a mixed convective environment at atmospheric pressure has been simulated for various particle sizes, free-stream velocities and ambient temperatures. The numerical predictions have been compared with experimental results obtained using the porous sphere technique and the agreement is found to be good. Correlations have been developed for the critical Reynolds number at which transition from the envelope to wake flame occurs and also for the mass burning rates at sub-critical or super-critical Reynolds number regimes. It is observed that, at higher ambient temperatures, transition to wake flame is delayed to a higher critical Reynolds number value. The infinite rate chemistry model predicts flame shapes and mass burning rates with reasonable accuracy in the sub-critical Reynolds number regime, but it fails to predict transition to wake flame shape. For analyzing transition phenomena, a finite rate chemistry model is required. © 2005 Elsevier Ltd. All rights reserved.

International Journal of Heat and Mass Transfer

Flame shapes and burning rates of spherical fuel particles in a mixed convective environment

The combustion of a fuel droplet in a mixed convective environment has been simulated both theoretically and experimentally. In the theoretical model, the flow and transport equations have been solved subject to the assumptions of constant properties (except for density) and infinite rate kinetics, using the finite element method. The gas density has been treated as a variable, only to determine the buoyancy force contribution, and it has been evaluated assuming an ideal gas mixture. A porous-sphere facility has been employed for simulating the burning characteristics of a fuel droplet experimentally. The effects of airflow rate and droplet size have been studied for both upward and downward airflow configurations. Theoretical predictions for the mass burning rate and flame shape are in excellent agreement with the experimental results of the present study and also those reported in the literature. For upward airflow configuration, the mass burning rate is under-predicted by 15 to 20 percent when buoyancy effects are neglected. For downward airflow configuration, the flame shape is similar to the case of upward airflow for low air velocities. For higher downward air velocities, a flattened cylindrical flame front is obtained due to the opposing natural convection and forced convection flow fields, which has been predicted successfully. A detailed parametric study involving the variation of Reynolds number, ambient temperature and ambient oxygen concentration has also been carried out.

Combustion of a fuel droplet in a mixed convective environment

Progress in Energy and Combustion Science

Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines

Proceedings of the Combustion Institute

Numerical and experimental studies of ethanol flames

On the estimation and validation of global single-step kinetics parameters of ethanol-air oxidation using diffusion flame extinction data

Journal	Combustion Science and Technology
ISSN	00102202
Open Access	No