N-heptane micro pilot assisted methane combustion in a Rapid Compression Expansion Machine

N-heptane micro pilot assisted methane combustion in a Rapid Compression Expansion Machine The autoignition and combustion behaviour of an n-heptane spray in a premixed methane/air charge was investigated. A Rapid Compression Expansion Machine (RCEM) with a free floating piston was employed to reach engine relevant conditions at Start of Injection (SOI) of the micro n-heptane pilot. The methane content in the ambient gas mixture was varied by injecting different amounts of methane directly into the combustion chamber; the ambient equivalence ratio for the methane content ranged from 0.0 to 0.99. Filtered OH∗ chemiluminescence images of the combustion were taken with a UV intensified camera at a rate of 20kHz through the optical access in the piston and schlieren imaging was performed at 20kHz for the detection of density gradients arising from pilot injection, ignition and flame propagation in the unburned mixture. Filtered photomultiplier signals of the total emitted light for OH∗, CH∗ and C2∗ radicals as well as the pressure signal were simultaneously recorded and the effect of methane content, charge temperature and ambient oxygen concentration on ignition locations, ignition timing and combustion behaviour was analysed. It was found that increasing methane contents in the ambient mixture considerably prolongs the ignition delays of the pilot spray due to inhibiting effects of the premixed methane on the reactions leading to high temperature ignition triggered by n-heptane. Longer ignition delays due to lower ambient temperatures or dilution of the ambient oxidizer cause higher heat release during the combustion of the pilot spray, since more methane/air is entrained and mixed with the pilot spray and oxidized simultaneously. This behaviour is reversed with increasing ignition delays arising from increased methane addition. There, lower heat release rates during pilot combustion were observed due to decreased reactivity in the n-heptane spray area. Comparison of different strategies with respect to the determination of high temperature ignition delay highlighted shortcomings as well as the potential in the usage of spectrally filtered chemiluminescence data, since changes in the spectrum of the emitted chemiluminescence are expected to change over time in the case of pilot ignition due to the transition between combustion modes. An assessment of the methane content influence on the ignition reactions by means of homogeneous, adiabatic Perfectly Stirred Reactor (PSR) simulations with a suitable reaction mechanism is further presented which allows for separation of influences and supports the experimental findings. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Fuel Elsevier

N-heptane micro pilot assisted methane combustion in a Rapid Compression Expansion Machine

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Publisher
Elsevier
Copyright
Copyright © 2016 Elsevier Ltd
ISSN
0016-2361
D.O.I.
10.1016/j.fuel.2016.03.006
Publisher site
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Abstract

The autoignition and combustion behaviour of an n-heptane spray in a premixed methane/air charge was investigated. A Rapid Compression Expansion Machine (RCEM) with a free floating piston was employed to reach engine relevant conditions at Start of Injection (SOI) of the micro n-heptane pilot. The methane content in the ambient gas mixture was varied by injecting different amounts of methane directly into the combustion chamber; the ambient equivalence ratio for the methane content ranged from 0.0 to 0.99. Filtered OH∗ chemiluminescence images of the combustion were taken with a UV intensified camera at a rate of 20kHz through the optical access in the piston and schlieren imaging was performed at 20kHz for the detection of density gradients arising from pilot injection, ignition and flame propagation in the unburned mixture. Filtered photomultiplier signals of the total emitted light for OH∗, CH∗ and C2∗ radicals as well as the pressure signal were simultaneously recorded and the effect of methane content, charge temperature and ambient oxygen concentration on ignition locations, ignition timing and combustion behaviour was analysed. It was found that increasing methane contents in the ambient mixture considerably prolongs the ignition delays of the pilot spray due to inhibiting effects of the premixed methane on the reactions leading to high temperature ignition triggered by n-heptane. Longer ignition delays due to lower ambient temperatures or dilution of the ambient oxidizer cause higher heat release during the combustion of the pilot spray, since more methane/air is entrained and mixed with the pilot spray and oxidized simultaneously. This behaviour is reversed with increasing ignition delays arising from increased methane addition. There, lower heat release rates during pilot combustion were observed due to decreased reactivity in the n-heptane spray area. Comparison of different strategies with respect to the determination of high temperature ignition delay highlighted shortcomings as well as the potential in the usage of spectrally filtered chemiluminescence data, since changes in the spectrum of the emitted chemiluminescence are expected to change over time in the case of pilot ignition due to the transition between combustion modes. An assessment of the methane content influence on the ignition reactions by means of homogeneous, adiabatic Perfectly Stirred Reactor (PSR) simulations with a suitable reaction mechanism is further presented which allows for separation of influences and supports the experimental findings.

Journal

FuelElsevier

Published: Sep 1, 2016

References

  • Effect of engine parameters and type of gaseous fuel on the performance of dual-fuel gas diesel engines–a critical review
    Sahoo, B.; Sahoo, N.; Saha, U.
  • Experimental characterization of diesel ignition and lift-off length using a single-hole ECN injector
    Benajes, J.; Payri, R.; Bardi, M.; Martí-Aldaraví, P.
  • Liquid length and vapor penetration of conventional, Fischer–Tropsch, coal-derived, and surrogate fuel sprays at high-temperature and high-pressure ambient conditions
    Kook, S.; Pickett, L.M.
  • An experimental study on the effect of pressure and strain rate on CH chemiluminescence of premixed fuel-lean methane/air flames
    Higgins, B.; McQuay, M.; Lacas, F.; Candel, S.
  • Two-stage ignition and NTC phenomenon in diesel engines
    Fu, X.; Aggarwal, S.K.

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