Differential diffusion effect on the stabilization characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air

Abstract

The characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air are numerically investigated using laminarSMOKE code with a 57-species detailed methane/air chemical kinetic mechanism. Detailed numerical simulations are performed for various fuel jet velocities, U 0 , with different hydrogen ratio of the fuel jet, R H , and the inlet temperature, T 0 . Based on the flame characteristics, the autoignited laminar lifted jet flames can be categorized into three regimes of combustion mode: the tribrachial edge flame regime, the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime, and the transition regime in between. Under relatively low temperature and high hydrogen ratio (LTHH) conditions, an unusual decreasing liftoff height, H L , behavior with increasing U 0 is observed, qualitatively similar to those of previous experimental observations. From additional simulations with modified hydrogen mass diffusivity, it is substantiated that the unusual decreasing H L behavior is primarily attributed to the high diffusive nature of hydrogen molecules. The species transport budget, autoignition index, and displacement speed analyses verify that the autoignited lifted jet flames are stabilized by autoignition-assisted flame propagation or autoignition depending on the combustion regime. Chemical explosive mode analysis (CEMA) identifies important variables and reaction steps for the MILD 1 combustion and tribrachial edge flame regimes.

A numerical study of the pyrolysis effect on autoignited laminar lifted dimethyl ether jet flames in heated coflow air

“…Unlike previous experimental results on liftoff height increasing monotonically with the increase in U 0 for single component fuels [6][7][8], they observed an unusual 'U'-shaped liftoff height behavior within the MILD combustion regime: H L decreases with increasing U 0 for relatively-low U 0 conditions while H L increases with increasing U 0 for relatively-high U 0 conditions. In fact, the decreasing H L behavior with increasing U 0 has been observed for autoignited laminar lifted jet flames of binary mixture fuels such as methane/hydrogen [8,11] and carbon monoxide/hydrogen fuels [21]. The differential diffusion effect of two fuel components is found to lead the decreasing H L behavior [11].…”

“…Note that the residence time for the fuel jet passing through the fuel tube is 0.125 ∼ 0.536 s for U 0 = 1.4 ∼ 6 m/s. Identical to our previous numerical study of autoignited lifted methane/hydrogen jet flames [11], we first obtain a stationary autoignited laminar lifted DME jet flame of X F,0 = 0.08, T 0 = 980 K, and U 0 = 5 m/s without applying any external ignition source (i.e., literally autoignition). Then, we obtain steady solutions for other lifted flames at different…”

“…In fact, the decreasing H L behavior with increasing U 0 has been observed for autoignited laminar lifted jet flames of binary mixture fuels such as methane/hydrogen [8,11] and carbon monoxide/hydrogen fuels [21]. The differential diffusion effect of two fuel components is found to lead the decreasing H L behavior [11]. Since DME can be easily pyrolyzed into small species such as methane, hydrogen, and formaldehyde at high temperature condition [22,23], we may partially explain the U-shaped H L behavior of the autoignited lifted DME jet flames by the differential diffusion effect of fuel components decomposed from DME.…”

“…Prior to examining the details of numerical results, we need to clarify several definitions, which will be used through the following sections. First, we determine that autoignition occurs within the computational domain if the homogeneous ignition delay of the stoichiometric mixture evaluated from the fuel mixture at the fuel jet exit is less than one-jet flow-through time of the coflow air, L z /U co , as in our previous study [11]. Thus, the autoignition temperature, T ig , can be defined as the minimum temperature of which τ ig,st is less than L z /U co .…”

“…Laminar lifted flames under autoignitive conditions have also been recently investigated because they can play a fundamental role in elucidating the liftoff and stabilization characteristics of corresponding turbulent lifted flames [6][7][8][9][10][11][12][13][14][15]. Under non-autoignitive conditions, a laminar lifted flame usually exhibits a tribrachial structure which consists of two rich/lean premixed flame branches and a trailing diffusion flame branch [16].…”