Transcritical real-fluid effects on dual-fuel combustion of methane and n-dodecane

Abstract

A comprehensive set of accurate physical models and numerical simulation methods for transcritical dual fuel combustion systems is presented. The method combines multiphase real-fluid physical properties modeling, flamelet-based chemistry reduction, and large-eddy simulation (LES). It allows a fully integrated simulation of the multiphase region and the combustion zones in the turbulent flames. A central challenge in dual-fuel combustion applications is the successful ignition of low-reactivity ambient fuel by the high-reactivity pilot fuel jet. The interaction of the radical pools associated with both fuels is known to be an important characteristic. This study captures the evolution of this interaction across both the low-temperature combustion (LTC) and high-temperature combustion (HTC) regimes. Three cases are investigated, each involving the injection of an n-dodecane jet into a 6 MPa, 1000 K ambient. The cases differ only in ambient composition: (1) a single-fuel (SF) baseline case with a mixture of air and combustion products, and two dual-fuel (DF) cases with methane and a 90:10 (by volume) methane/ethane blend added to the ambient. Insights on ignition and flame structure are gained from homogeneous reactor simulations, transcritical counterflow diffusion flames, and LES. Combustion modes are defined by threshold levels of key species and temperature. The flame volume is subdivided according to the modes and for each mode the ignition delay and the temporal evolution of heat release rate (HRR) are obtained. The differences in the radical pools of SF and DF combustion are demonstrated. LTC ignition is shown to start in the multiphase region near the base of the jet. The high accuracy of the proposed methods provides a firm basis for modeling prospective dual-fuel systems using alternative fuels.