An experimental study on turbulent lifted flames of methane in coflow jets at elevated temperatures

Abstract

An experimental study was conducted on the effects of initial temperature variation on the stabilization characteristics of turbulent nonpremixed flames in coflow jets of methane fuel diluted by nitrogen. The typical behavior seen in the study showed that the liftoff height increased linearly with the jet velocity regardless of the initial temperature in the turbulent regime. Two models were investigated for predicting liftoff heights in the methane jets: the premixed flame model and the large-scale mixing model. For the premixed flame model, the liftoff heights in the methane jets were accurately predicted using the thermal diffusivity of the unburned gas temperature α_st,0, instead of that of the burned gas temperature α_st,b. For the large-scale mixing model, however, the prediction of liftoff heights differed slightly for the various fuel mole fractions. However, when considering the initial fuel mass fraction Y_F,0, the liftoff heights were successfully predicted. This result implies that the characteristics of the unburned fuel–air mixture play a crucial role for flame stabilization in coflow jets for a variety of initial conditions. In the turbulent regime, the blowout velocity and the liftoff height at blowout could be accurately predicted by the two models based on a consideration of the physical properties and the buoyancy effect of the initial temperature variation.

Introduction

The liftoff height of a turbulent lifted flame increases linearly as the jet velocity increases. This is typical behavior for the stabilization of such a flame. The turbulent lifted flame is distinguished from a laminar lifted flame, for which the liftoff height increases nonlinearly with jet velocity [1], [2]. In particular, there are several competing theories on the mechanism of flame stabilization in a turbulent nonpremixed jet [1], [2]. Among these, the premixed flame model [3], [4] and the large-scale mixing model [5], [6], [7] are valid for predicting turbulent flame stabilization and blowout.

Vanquickenborne and van Tiggelen proposed a premixed flame model in which the liftoff height was based on the balance between the mean turbulent propagation speed and the mean flow velocity when the base of a turbulent lifted flame had a premixed fuel and air mixture [3]. In addition, Kalghatgi investigated the characteristics of various fuels such as hydrogen, methane, propane, and ethylene, based on the hypothesis that a turbulent lifted flame is stabilized at the location of the mean flow velocity and local propagation speed of the flame [4].

Broadwell et al. and Dahm and Dibble predicted the blowout velocity from the ratio of mixing time to chemical reaction time occurring because of the mixing of reentrained burned gas into unburned reactants. However, their model did not successfully predict linear liftoff height of a turbulent flame [5], [6]. Therefore, Miake-Lye and Hammer could successfully predict the liftoff height of a turbulent flame based on the jet velocity, using a large-scale mixing model that adopts large-scale dynamics and supports the existence of a flame until the strain reaches a critical one imposed on the stoichiometric contour line of the fuel and air mixture [7].

Recently, the current authors and their colleagues conducted several experimental studies on the stabilization of a laminar lifted flame containing various hydrocarbon fuels in nonpremixed coflow jets, by varying the initial temperature [8], [9], [10], [11]. When the initial temperature was relatively low, the stabilization mechanisms for lifted flames were influenced by the propagation speed of the lifted flame edge [8], [9], [10], which frequently had a tribrachial structure. Especially in laminar jets, the Schmidt number of the fuel, Sc, influenced the existence of lifted flames.

When the initial temperature was increased above a certain autoignition temperature, the autoignited lifted flames of CH₄, C₂H₄, C₂H₆, C₃H₈, n-C₄H₁₀, and a mixture of CH₄/H₂ were stabilized regardless of Sc, and the ignition delay time played an important role in the stabilization [9], [10], [11]. According to the experimental study results for fuel jets with the mixture of CH₄/H₂ in coflow air with an elevated temperature, differential diffusion among fuels could play an important role in autoignition in a nonpremixed jet [11].

For the turbulent lifted flames, the experiment in heated coflow jets was mainly focused on the reduction of NOx emission and the flame stability [12], [13], [14]. Systematic studies on the stabilization mechanism of turbulent lifted flames are rather limited, however, especially at elevated temperatures. In one such study, the authors conducted experimental investigations of turbulent lifted flames of propane in nonpremixed coflow jets with initial temperature variation [15]. The behavior of the turbulent lifted flames was analyzed using the premixed flame and large-scale mixing models. In addition, the prediction of the liftoff height and blowout velocity for such flames was improved by considering the physical properties for the initial temperature variation [15].

The purpose of the present study is to extend the generality of the previous experiment to methane fuel based on the theory of a turbulent jet. The study will focus on methane because it is widely used in various combustion facilities as a clean fuel. In addition, the study will report on the characteristics of a turbulent lifted flame of methane fuel diluted by nitrogen for nonpremixed coflow jets with initial temperature variation.