Ignition characteristics of heptane–hydrogen and heptane–methane fuel blends at elevated pressures

Abstract

There is significant interest in using hydrogen and natural gas for enhancing the performance of diesel engines. We report herein a numerical investigation on the ignition of n-C 7 H 16 /H 2 and n-C 7 H 16 /CH 4 fuel blends. The CHEMKIN 4.1 software is used to model ignition in a closed homogenous reactor under conditions relevant to diesel/HCCI engines. Three reaction mechanisms used are (i) NIST mechanism involving 203 species and 1463 reactions, (ii) Dryer mechanism with 116 species and 754 reactions, and (iii) a reduced mechanism (Chalmers) with 42 species and 168 reactions. The parameters include pressures of 30atm and 55atm, equivalence ratios of =0.5, 1.0 and 2.0, temperature range of 800-1400K, and mole fractions of H 2 or CH 4 in the blend between 0-100%. For n-C 7 H 16 /air mixtures, the Chalmers mechanism not only provides closer agreement with measurements compared to the other two mechanisms, but also reproduces the negative temperature coefficient regime. Consequently, this mechanism is used to characterize the effects of H 2 or CH 4 on the ignition of n-C 7 H 16 .Results indicate that H 2 or CH 4 addition has a relatively small effect on the ignition of n-C 7 H 16 /air mixtures, while the n-C 7 H 16 addition even in small amount modifies the ignition of H 2 /air and CH 4 /air mixtures significantly. The n-C 7 H 16 addition decreases and increases the ignition delays of H 2 /air mixtures at low and high temperatures, respectively, while its addition to CH 4 /air mixtures decreases ignition delays at all temperatures. The sensitivity analysis indicates that ignition characteristics of these fuel blends are dominated by the pyrolysis/oxidation chemistry of n-heptane, with heptyl (C 7 H 16 -2) and hydoxyl (OH) radicals being the two most important species.

The effects of hydrogen addition on the auto-ignition delay of homogeneous primary reference fuel/air mixtures in a rapid compression machine

“…Many researchers have also studied the effects of adding hydrogen to hydrocarbon fuels via sensitivity analyses [47,48]. Fig.…”

“…It has been widely reported [32,48] that the increase in the ignition delay at low temperatures is primarily caused by the following reaction:…”

3D CFD simulation of a natural gas fueled HCCI engine with employing a reduced mechanism

“…For instance, by utilizing the proposed approach, the size of the comprehensive mechanism of n-heptane (561 species and 2539 reactions) reduced to only 140 species and 491 reactions. Aggarwal et al [19] investigated the ignition characteristics of n-heptane/ methane fuel blends at HCCI engines condition in a closed homogenous reactor. They showed that the Chalmers reduced mechanism [18], consisting of 42 species and 168 reactions, has good agreement with the shock tube data for the mixtures of pure n-heptane/air and also for the mixture of pure methane/air.…”

Kinetic and dynamic analysis of hydrogen-enrichment mixtures in combustor systems – A review paper

“…The fundamental and practical aspects of the usage of hydrogen-hydrocarbon mixtures on both transportation and power generation (in terms of their mechanical and chemical properties) in different systems, such as engines [11][12][13][14][15][16][17][18][19][20][21][22][23][24], burners [25][26][27][28][29][30][31][32][33][34], combustion bombs [35][36][37][38][39][40][41], vessels [8,9,[42][43][44][45][46] and chambers [5,10,[47][48][49][50][51][52][53][54], have been broadly investigated. The parameters affecting hydrogen-hydrocarbon mixtures have been studied, including flame configurations, e.g., laminar premixed …”

“…Moreover, for the diffusion of combustion, hydrogen-enrichments can suppress the formation of soot particles [72,90] and reduce ignition delay [6,26,53,91].…”