Proceedings of the Combustion Institute Proceedings of the Combustion Institute 31 (2007) 1215–1222 www.elsevier.com/locate/proci High temperature ignition and combustion enhancement by dimethyl ether addition to methane–air mixtures q Zheng Chen, Xiao Qin, Yiguang Ju *, Zhenwei Zhao, Marcos Chaos, Frederick L. Dryer Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA Abstract The effects of dimethyl ether (DME) addition on the high temperature ignition and burning properties of methane–air mixtures were studied experimentally and numerically. The results showed that for a homo- geneous system, a small amount of DME addition to methane resulted in a significant reduction in the high temperature ignition delay. The ignition enhancement effect by DME addition was found to exceed that possible with equivalent amounts of hydrogen addition, and it was investigated by using radical pool growth and computational singular perturbation analysis. For a non-premixed methane–air system, it was found that two different ignition enhancement regimes exist: a kinetic limited regime and a transport limited regime. In contrast to the dramatic ignition enhancement in the kinetic limited regime, the ignition enhancement in the transport limited regime was significantly less effective. Furthermore, laminar flame speeds as well as Markstein lengths were experimentally measured for methane–air flames with DME addi- tion. The results showed that the flame speed increases almost linearly with DME addition. However, the Markstein length and the Lewis number of the binary fuel change dramatically at small DME concentra- tions. Moreover, comparison between experiments and numerical simulations showed that only the most recent DME mechanism well reproduced the flame speeds of both DME–air and CH4–air flames. Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Dimethyl ether; Ignition enhancement; Laminar flame speed; Markstein length 1. Introduction and can be mass produced from natural gas, coal, or biomass [1–5], is emerging as a substitute for Environmental regulations and energy diversi- liquefied petroleum gas (LPG), diesel fuels, and ty produce an urgent need to develop new clean liquid natural gas (LNG). In addition, DME can fuels. Dimethyl ether (DME), which has low soot also be used as an ignition enhancer in propulsion emission, no air or ground-water pollution effects, systems and internal combustion engines [6]. Recently, the study of DME combustion has received significant attention [1–6]. In kinetic q Supplementary data for this article can be accessed research, several detailed chemical mechanisms online. See Appendix A. for low and high temperature DME oxidation * Corresponding author. Fax: +1 609 258 6233. [7–11] have been developed and validated against E-mail address: [email protected] (Y. Ju). burner stabilized flames [12,13], non-premixed 1540-7489/$ - see front matter Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2006.07.177 1216 Z. Chen et al. / Proceedings of the Combustion Institute 31 (2007) 1215–1222 counterflow flame ignition [14], and laminar flame coupling on the Markstein length and Lewis speeds [15–17]. More recently, DME jet diffusion number was studied. flames were also studied [18]. It was shown that due to the existence of oxygen atoms in DME, lift-off characteristics are distinctly different from 2. Experimental/computational methods and kinetic other hydrocarbon fuels. model selection DME has shown promise as an additive and/or fuel extender. Recently, Yao and Qin [19] have The laminar flame speed and Markstein length undertaken studies on DME addition to methane of DME/CH4–air premixed flames were measured for homogeneous charge compression ignition by using outwardly propagating spherical flames (HCCI) engines. In such cases, the coupling of in a dual-chambered, pressure-release-type, and DME kinetics with those of methane involves high pressure combustion facility [17]. Pre-mix- the low temperature kinetics of DME. On the tures were prepared by using the partial pressure other hand, DME/methane utilization in burners method from pure methane and DME com- and in gas turbine applications is expected to pressed gas sources. The purities of the DME involve principally high temperature kinetic cou- and CH4 were 99.8% and 99.9%, respectively. pling effects. For example, the effects of DME Experiments were conducted for DME/CH4–air on the high temperature ignition of methane have mixtures: {aCH3OCH3 +(1À a)CH4} + air, with been experimentally studied by Amano and Dryer values of the volume fraction, a, ranging from [6], who showed that DME was an effective pro- zero to one. The combustible mixture was spark- moter of high temperature methane ignition. ignited at the center of the chamber with the min- However, the underlying kinetic coupling between imum ignition energy so as to preclude significant DME and CH4 responsible for the observed igni- ignition disturbances. The flame propagation tion enhancement was not explored in any detail. sequence was imaged by using Schlieren photog- In addition, kinetic coupling effects on flame raphy. A high-speed digital video camera operat- properties and auto-ignition in non-premixed sys- ing at 8000 frames per second was used to tems have not been studied. Moreover, the exist- record the propagating flame images. To avoid ing kinetic mechanisms have not been validated possible effects caused by the initial spark distur- against the flame speeds of DME/CH4 mixtures. bance and wall interference, data reduction was It is well known that flame properties such as performed only for flame radii between 1.5 and burning rate and flame stability depend on the 2.5 cm. The pressure rise in the combustion cham- overall activation energy and the Lewis number ber was monitored using a pressure sensor. At the (Le) [20]. Kinetic coupling may result in a dramat- small flame radii studied in this work, the pressure ic change in the overall activation energy with a rise is about 2%, resulting in a nearly constant small amount of DME addition to methane. pressure flame propagation condition. DME has a molecular weight larger than air so The stretched flame speed was first obtained that the Le is larger than unity for lean DME/ from the flame history and then was linearly air mixtures in comparison to methane/air mix- extrapolated to zero stretch rate to obtain the tures. On the other hand, DME is expected to unstretched flame speed [17,22]. The results pre- react more quickly in the preheating zone decom- sented here are the averaged value of at least posing to form lighter molecules. Therefore, it is two tests at each experimental condition. The esti- of interest to investigate how the effective mixture mated experimental error for flame speed determi- Lewis number depends on the DME content in nations is approximately 5%. All experiments binary fuels with disparate molecular weights, were performed at an initial temperature of such as DME/CH4 mixtures. 298 ± 3 K and at atmospheric pressure. In order The objective of the present study was to to examine the available kinetic mechanisms, the investigate kinetic coupling effects of DME addi- measured flame speeds were compared with the tion on the high temperature ignition and burn- numerical results obtained using PREMIX [23]. ing properties of methane–air mixtures. The Markstein length and the effective Lewis Experimentally measured laminar flame speeds number of the binary fuel mixtures were then of DME–air and CH4–air mixtures were com- extracted from the measured flame speed data. pared with predictions by existing DME mecha- The effect of adding DME on ignition enhance- nisms including a recently developed model by ment of methane was investigated numerically in Zhao et al. [21]. The latter mechanism is then two different systems, a homogeneous flame con- used to study the effect of DME addition on figuration to examine the kinetic ignition enhance- the ignition enhancement in both homogeneous ment, and a non-premixed counterflow and non-homogeneous systems. Finally, the configuration to examine the effect of transport. flame speeds of DME/CH4–air mixtures were The ignition time of homogeneous mixtures at measured by using outwardly propagating constant pressure and enthalpy was calculated spherical flames. The results were compared by using SENKIN [24]. For the non-premixed with model predictions and the effect of kinetic simulations, the quasi-steady temperature and Z. Chen et al. / Proceedings of the Combustion Institute 31 (2007) 1215–1222 1217 species distributions of counterflowing DME/CH4 (298 K at the boundary) and hot air jets (1400 K at the boundary) were determined under a frozen flow constraint. At time zero, chemical reactions were allowed in the pre-calculated frozen flow field. Ignition time was recorded when the first increase in the temperature field exceeded 400 K, indicating thermal runaway. Simulations were conducted using an unsteady potential counter- flow flame code described by Ju et al. [25,26].To further examine the effect of flow residence time on ignition enhancement, the stretch rate in the frozen flow configuration was varied from low stretch to that near the ignition limit. In order to properly model and interpret the present work, the chemical kinetic model used in the calculations must be capable of predicting Fig. 2. Laminar flame speeds of CH4–air mixtures as a the pure fuel–air laminar flame speed and high function of equivalence ratio at 298 K, atmospheric temperature shock tube ignition properties. One pressure. might expect that comprehensively developed detailed mechanisms for DME oxidation would also be capable of predicting high temperature in the above models. Because decomposition and kinetic properties for methane oxidation. The abstraction pathways are coupled during both measured laminar flame speeds of pure DME– pyrolysis and oxidation, considerable re-assess- air and CH4–air mixtures at atmospheric pressure ment, and updating of other parameters impor- and room temperature are shown in Figs.