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1

Numerical study of laminar non-premixed biogas-air flames behind backward facing steps

Harish Alagani, Raghavan Vasudevan

Archive of Mechanical Engineering

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2022

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LXIX, nr 2

221–-244

EN

Biogas, a renewable fuel, has low operational stability range in burners due to its inherent carbon-dioxide content. In cross-flow configuration, biogas is injected from a horizontal injector and air is supplied in an orthogonal direction to the fuel flow. To increase the stable operating regime, backward facing steps are used. Systematic numerical simulations of these flames are reported here. The comprehensive numer- ical model incorporates a chemical kinetic mechanism having 25 species and 121 elementary reactions, multicomponent diffusion, variable thermo-physical properties, and optically thin approximation based volumetric radiation model. The model is able to predict different stable flame types formed behind the step under different air and fuel flow rates, comparable to experimental predictions. Predicted flow, species, and temperature fields in the flames within the stable operating regime, revealing their anchoring positions relative to the rear face of the backward facing step, which are difficult to be measured experimentally, have been presented in detail. Resultant flow field behind a backward facing step under chemically reactive condition is compared against the flow fields under isothermal and non-reactive conditions to reveal the sig- nificant change the chemical reaction produces. Effects of step height and step location relative to the fuel injector are also presented.

2

Numerical analysis of structure, stability and entropy generation in biogas coflow diffusion flames

Kumar R. Nivethana, Kumaran S. Muthu, Raghavan Vasudevan

Archive of Mechanical Engineering

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2022

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LXIX, nr 1

99--128

EN

Biogas is a gaseous biofuel predominantly composed of methane and carbon-dioxide. Stability of biogas flames strongly depend upon the amount of carbon-dioxide present in biogas, which varies with the source of biomass and reactor. In this paper, a comprehensive study on the stability and flame characteristics of coflow biogas diffusion flames is reported. Numerical simulations are carried out using reactive flow module in OpenFOAM, incorporated with variable thermophysical properties, Fick’s and Soret diffusion, and short chemical kinetics mechanism. Effects of carbon-dioxide content in the biogas, temperatures of the fuel or coflowing air streams (preheated reactant) and hydrogen addition to fuel or air streams are analyzed. Entropy generation in these flames is also predicted. Results show that the flame temperature increases with the degree of preheat of reactants and the flames show better stability with the preheated air stream. Preheating the air contributes to increased flame stability and also to a significant decrease in entropy generation. Hydrogen addition, contributing to the same power rating, is seen to be relatively more effective in increasing the flame stability when added to the fuel stream. Results in terms of flow, temperature, species and entropy fields, are used to describe the stability and flame characteristics.

3

Prediction of vapor-liquid equilibrium of ternary system at high pressures

Ray Saroj, Harsha V. V. Sree, Raghavan Vasudevan

Archives of Thermodynamics

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2019

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Vol. 40, no 2

137--149

EN

The paper presents a numerical model for analyzing vaporliquid equilibrium of ternary (three-component) system at high pressures. The gas-phase non-idealities and solubility of gas in liquid are considered in the numerical model. The model is useful for studies involving evaporation of liquid at different pressure and temperature conditions, where the interface liquid and vapor compositions are required. At high ambient pressures, ambient gases dissolve into the liquid. Thus, even a single component liquid fuel evaporating in a high pressure ambient gas, effectively behaves like a two-component liquid system. This study considers a ternary system. The numerical model has been validated against the experimental data available in literature. The validated model is used to study the solubility of ambient gas in a binary liquid mixture at high pressures. The effects of pressure, temperature and liquid phase composition on the solubility of gas in liquid have been studied systematically.