Fuel Effects in Turbulent Premixed Pre-vaporised Alcohol/Air Jet Flames

J Trabold,D Geyer,A Dreizler,A Johchi,S Hartl,S Walther

doi:10.1007/s10494-020-00166-6

J Trabold, D Geyer + Show 4 more

Open Access

https://doi.org/10.1007/s10494-020-00166-6

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Abstract

To study combustion fundamentals of complex fuels under well-defined boundary conditions, a novel Temperature Controlled Jet Burner (TCJB) system is designed that can stabilise both gaseous or pre-vaporised liquid fuels. In a first experimental exploratory study, piloted turbulent jet flames of pre-vaporised methanol, ethanol, 2-propanol and 2-butanol mixtures are compared to methane/air as a reference fuel. Complementary one-dimensional laminar flame calculations are used to provide flame parameters for comparison. Blow-off and flame length as global flame characteristics are measured over a wide range of equivalence ratios. For fuel rich conditions, blow-off limits correlate well with extinction strain rate calculations. Differing flame lengths from lean to rich conditions are explained partly by different flame wrinkling that is assessed using planar laser-induced fluorescence imaging of the hydroxyl radical (OH-PLIF). A study of Lewis-number effects indicates that they have substantial influence on flame wrinkling. Lean alcohol/air flames, opposed to methane/air, have a Lewis-number greater than unity. This impedes curvature development, which promotes relatively large flame lengths. In contrast, across stoichiometric conditions, all alcohol/air mixture Lewis-numbers decrease significantly. At such conditions, alcohol/air flames show alike or even larger wrinkling compared to methane/air flames. However, quantitatively, the differences in flame length and wrinkling observed among the flames can neither be explained alone by Lewis-number differences, nor other global mixture parameters available from 1D laminar flame calculations. This study shall therefore emphasise the need for more detailed experimental analyses of the full thermochemical state of laminar and turbulent flames fuelled with complex fuels.

Highlights

Improving combustion technologies within short innovation cycles requires predictive mathematical models
The results showed that from lean over stoichiometric to rich conditions, the mean turbulent flame stretch factor, flame length and thickness, flame surface density and fuel consumption rate varied differently for each fuel
The magnitude of sL for a specific fuel/air mixture depends on the molecular structure of the fuel and intermediate products formed in the reaction zone (Veloo et al 2010)

Summary

Introduction

Improving combustion technologies within short innovation cycles requires predictive mathematical models. For an improved phenomenological understanding and the validation of predictive models, comprehensive experimental investigations are needed, in particular for fuel flexible systems that operate on renewable fuels. Renewable fuels including biofuels have the potential for reducing greenhouse gas emissions. Despite a growing market of electrical vehicles, biofuels are forecasted to make up about 90% of the total renewables in the transport sector in 2023, a share that even increased since 2017 (OECD 2018). Ethanol contributes by approximately two thirds of the grown biofuels and biodiesel, whereas hydro-treated vegetable oil (HVO) provides the remainder (OECD 2018). A more detailed understanding of the relevant processes within the combustion of renewable fuels, e.g. combustion chemistry and the turbulence-chemistry interaction, is required.

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