Effects of hydrogen enrichment and turbulence intensity on the combustion mode in locally stratified dual-fuel mixtures of n-dodecane/methane

Abstract

In dual-fuel (DF) engines, a high-reactivity fuel (HRF) ignites a low-reactivity fuel (LRF) premixed with air. Identification of the combustion mode after autoignition in DF engines with different fuel combinations and under various mixture stratification levels is of crucial importance since the combustion mode may affect the engine performance and emissions levels. However, to avoid expensive computational or experimental tests for different fuel combinations under various initial conditions, an analytical diagnostics tool can help identify the modes of combustion. Recently, we developed an a-priori tool (β-curve), which separates between the deflagration and spontaneous combustion modes in transient problems based on two parameters: HRF stratification amplitude and wavelength. However, the developed analysis was only evaluated for a single fuel combination and under 2D turbulent conditions. The present study extends our previous work by (1) evaluating the 1D β-curve analysis for more common fuel combinations including hydrogen, and (2) extending the combustion mode analysis to 3D turbulent combustion. First, using 1D numerical simulations, the combustion mode for a new DF combination of n-dodecane (HRF) and methane (LRF) with and without hydrogen enrichment in the LRF blend is investigated. Although hydrogen enrichment shortens the combustion duration, its role in changing the combustion mode remains subtle. Therefore, only n-dodecane/methane (no hydrogen enrichment) is considered in the 3D simulations to avoid highly expensive 3D numerical simulations with hydrogen enrichment. Second, using flame-resolved 3D numerical simulations, variations of the combustion mode for DF n-dodecane/methane mixtures at three different turbulence intensities are studied. It is observed that high turbulence levels can switch the combustion mode from deflagration to autoignition. For the first time, the diagnostic tool is evaluated compared to 3D turbulent numerical simulations and a correction factor is proposed in the β-curve equation.