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Numerical Analysis of Auto-ignition of Ethanol

Vaibhav Kumar Sahu1, Shrikrishna Deshpande1, Vasudevan Raghavan1,*

1Department of Mechanical Engineering
Indian Institute of Technology Madras, Chennai 600036, INDIA.

Received 28 June 2011; accepted 5 August 2011, available online 24 August 2011

1. Introduction
Ethanol is the most effective bio-fuel. Its easier production from agricultural feed-stocks, sugar cane and Fischer-Tropsch method makes it dominant among other bio-derived fuels. It can be employed directly for engine application, both from the point of view of developing renewable fuels for energy needs in future and to address the environmental issues such as exhaust emissions and global warming. For these relatively new bio-fuels, fundamental studies in terms of burning characteristics are done in several configurations [1]. The conditions at which bio-fuel auto-ignites should be understood thoroughly to reveal the combustion performance with better efficiency. Numerical simulation including global single-step chemistry in oxidative environment is useful where the study in detailed chemistry is expensive. The chemical kinetics of gas phase oxidation of ethanol has been reported over last five decades. Data have been reported from nonflow reactors, flow reactors, diffusion flames, and laminar premixed flames experiments by several researchers [2-5]. Detailed chemical kinetics models to describe the gas-phase oxidation of ethanol in air are available in literature [6]. Apart from that a global single step mechanism for ethanol oxidation is also available [7]. Experimental studies have been reported on extinction and autoignition of methanol and ethanol flames in laminar, non uniform flows for both non premixed and premixed flames in counter-flow configuration by Seiser et al. [8]. Numerical investigations in this paper are performed for non-premixed laminar counter-flow flame using global single step kinetics as reported in Dubey et al. [9]. The focus of present numerical study is to further validate the single-step reaction kinetics using the available experimental data. Numerical investigation has been done for the critical conditions of auto-ignition against various strain rates and mass fractions. 2. Numerical Model

In the present study, a comprehensive combustion model has been developed using commercial CFD software, FLUENT 6.3. The salient features of the numerical combustion model include temperature and concentration dependent thermo-physical properties such as density, specific heat, thermal conductivity and viscosity, multi-component mass diffusion, single-step chemical kinetics mechanism. Appropriate model parameters available in FLUENT has been chosen. The effect of normal gravity has also been included in the model. The mass, momentum and species conservation equations are solved along with energy equation. The energy equation for two dimensional, steady, axisymmetric and laminar reactive flows can be written as follows:

[pic]
[pic] (1)

In equation (1), T is the temperature; k, cp and ρ are mixture thermal conductivity, specific heat and density, respectively, and Vi, ωi and hi, are the diffusion velocity, net rate of production and absolute enthalpy of ith species, respectively. A Finite Volume Method based approach along with suitable upwind scheme for convection terms and SIMPLE type algorithms for velocity pressure coupling, is employed for discretizing the above governing equations. Appropriate second order accurate solution techniques have been employed for solving the discretized equations.

. Numerical Domain
In the axisymmetric model, the computational domain is as shown in Fig. 1. A domain with extents of 12 mm in the axial direction and 25 mm in the radial direction is chosen. A commercially available meshing software called GAMBIT 2.0 has been employed to create the computational domain and for generating grid.

1 Boundary...
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