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

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.

Experimental investigation of unconfined turbulent premixed bluff-body stabilized flames operated with vapourised liquid fuels

“…Hence, there is still value in measuring the contribution of flame surface wrinkling to the local consumption speed of the present flames and assessing the impact of fuel properties on them. That is, here, S T,LC values are merely utilized as a convenient means of assessing the influence fuel type has on the local level of wrinkling exhibited by the flames, which is inline with the assessment of Trabold et al [35]. The results presented here, like those in Ref.…”

“…In the near field region (up to 20 mm) no significant difference in S T,LC /S L was observed between all four fuels at both conditions. Trabold et al [35] observed a similar value of integrated Σ 2D in the near field for alcohol and methane flames. They hypothesized that in near field regions the fame surface is mostly influenced by turbulent shear flow.…”

“…They speculated that the lower Le of lean methane flame, results in higher reactivity, rendering such flames more resilient to quenching than flames with higher Le. The effect of Le was also studied by Trabold et al [35] in turbulent premixed vaporised jet-flames with alcohol fuels with C 1 to C 4 . They found that the stability of alcohol flames increases with decreasing molecular weight of the fuels.…”

Lean Blow-Off Scaling of Turbulent Premixed Bluff-Body Flames of Vaporized Liquid Fuels

“…While these prior studies have provided significant insight into the LBO limits and structure of gaseous fuels, recent studies reported that flames with heavy hydrocarbon or complex fuels show a substantial difference in terms of LBO and flame structure [13,15,17]. Tambold et al [20] showed that alcohol flames blow-off at higher equivalence ratios than methane. Carbone et al [21] observed changes in global properties of the flame with complex fuels and suggested re-examining their scaling parameters.…”

“…Additionally, it was observed in Refs. [20,21] that flames with a high Lewis number ( ) are more susceptible to extinction than those with a low .…”