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(2001). a Study of the High-Temperature Autoignition And A STUDY OF THE HIGH-TEMPERATURE AUTOIGNITION AND THERMAL DECOMPOSITION OF HYDROCARBONS By David Charles Horning Report No. TSD-135 JUNE 2001 A STUDY OF THE HIGH-TEMPERATURE AUTOIGNITION AND THERMAL DECOMPOSITION OF HYDROCARBONS by David Charles Horning Report No. TSD-135 Work supported by TDA Research, Inc., Air Force Office of Scientific Research, Office of Naval Research, and Army Research Office June 2001 High-Temperature Gasdynamics Laboratory Department of Mechanical Engineering Stanford University ii Stanford, California 94305-3032 Abstract The autoignition of propane, n-butane, n-heptane, n-decane, and ethylene has been studied in the reflected shock region of a shock tube over the range of 1250-1750 K, 1-6 atm, and mixture compositions of 2-20% oxygen with an equivalence ratio of 0.5 to 2.0. The time of ignition was determined from CH emission (431 nm) traces measured at the shock tube endwall. Ignition time correlations for n-butane and n-heptane have been developed to assess the sensitivity of the ignition delay to a variety of parameters (e.g., temperature, pressure, and mixture composition) and to facilitate comparisons of the current data to previous studies and the predictions of detailed models. The correlations presented herein reveal that the ignition time sensitivities of both n-butane and n-heptane to key parameters are markedly similar, leading to the presentation of a unique correlation form in which the stoichiometric ignition time data for propane, n-butane, n-heptane, and n-decane are correlated into a single expression. Direct comparisons to previous ignition time studies further validate the correlations presented. Also, ignition time measurements for ethylene are presented, which reveal that the ignition time behavior of this fuel is quite dissimilar from that of the n-alkanes. In addition, the thermal decomposition of propane, n-butane, n-heptane, and n-decane has been studied at a pressure of nominally 1 atm over the temperature range of 1500-2100 K. The temporal variation of ethylene was measured during the pyrolysis of these fuels using a new ultraviolet absorption diagnostic (174 nm). All four n-alkanes were found to rapidly decompose into ethylene, with the ethylene yield increasing with the molecular size of the fuel. An analysis of detailed mechanisms was performed to elucidate the main pathways by which the n-alkanes decompose, and to determine the key reactions that control the predicted ethylene time-history. For the pyrolysis of propane and n-butane, it was found that modifications to the initial fuel decomposition rates greatly improved the agreement between the predicted and measured ethylene time-histories. Also, a comparison of three detailed n-heptane mechanisms iii revealed marked differences among these models regarding their predicted time-histories of key species. Table of Contents ABSTRACT .....................................................................................................................III TABLE OF CONTENTS .....................................................................................................IV LIST OF FIGURES ...........................................................................................................VII LIST OF TABLES ..............................................................................................................X CHAPTER 1 - INTRODUCTION ...................................................................................1 1.1 MOTIVATION ...........................................................................................................2 1.2 CURRENT STUDY .....................................................................................................3 CHAPTER 2 - BACKGROUND......................................................................................5 2.1 PREVIOUS EXPERIMENTAL METHODS .......................................................................5 2.2 IGNITION TIME CORRELATIONS ..............................................................................10 2.3 UNCERTAINTIES IN SHOCK TUBE STUDIES ..............................................................12 2.3.1 Parametric Uncertainties .................................................................................12 2.3.2 Measurement Uncertainties.............................................................................14 CHAPTER 3 - EXPERIMENTAL METHOD..............................................................16 3.1 SHOCK TUBE FACILITY...........................................................................................16 3.2 FUEL MEASUREMENT.............................................................................................17 3.2.1 Infrared Absorption........................................................................................18 3.2.2 Gas Chromatography......................................................................................24 3.3 SPECIES MEASUREMENT ........................................................................................27 3.3.1 CH Emission Diagnostics................................................................................27 3.3.2 Ethylene Absorption Diagnostic.......................................................................28 3.4 IGNITION TIME DEFINITION ....................................................................................31 3.4.1 Endwall Ignition Time Definition......................................................................32 3.4.2 Sidewall Ignition Time Definition......................................................................34 3.4.3 Comparison of Sidewall and Endwall Measurements .......................................35 CHAPTER 4 - IGNITION TIME MEASUREMENTS ...............................................39 4.1 PARAMETRIC STUDY OF N-BUTANE AND N-HEPTANE IGNITION .............................39 4.1.2 n-Butane and n-Heptane Ignition Time Correlations ........................................44 4.2 COMPARISON OF IGNITION DELAYS AMONG N-ALKANES .......................................50 4.2.2 n-Alkane Ignition Time Correlation.................................................................52 4.3 PARAMETRIC STUDY OF ETHYLENE IGNITION .........................................................54 iv CHAPTER 5 - COMPARISON TO PAST IGNITION TIME STUDIES...................57 5.1 METHOD OF COMPARISON.....................................................................................57 5.2 PREVIOUS N-ALKANE STUDIES ...............................................................................58 5.2.1 Propane .........................................................................................................58 5.2.2 n-Butane ........................................................................................................61 5.2.3 n-Heptane......................................................................................................63 5.2.4 n-Decane .......................................................................................................69 5.2.5 n-Alkane Series..............................................................................................72 5.3 PREVIOUS ETHYLENE STUDIES ................................................................................74 CHAPTER 6 - DETAILED MODELING OF IGNITION TIMES.............................76 6.1 DETERMINATION OF PREDICTED IGNITION TIMES ...................................................76 6.2 N-ALKANE MECHANISMS.......................................................................................78 6.2.1 Propane .........................................................................................................78 6.2.2 n-Butane ........................................................................................................79 6.2.3 n-Heptane......................................................................................................81 6.2.4 n-Decane .......................................................................................................86 6.3 ETHYLENE MECHANISM .........................................................................................87 CHAPTER 7 - THERMAL DECOMPOSITION MEASUREMENTS ......................90 7.1 CORRECTIONS TO ABSORPTION MEASUREMENTS ...................................................90 7.2 THERMAL DECOMPOSITION OF N-ALKANES ...........................................................91 7.2.1 Propane .........................................................................................................91 7.2.2 n-Butane ........................................................................................................93 7.2.3 n-Heptane......................................................................................................96 7.2.4 n-Decane .......................................................................................................98 7.2.5 Ethylene Yields of n-Alkanes........................................................................100 7.3 PREDICTED ETHYLENE TRACE FOR AN OXIDIZING REACTION................................101 CHAPTER 8 - ANALYSIS OF N-ALKANE MECHANISMS..................................104 8.1 PROPANE.............................................................................................................104 8.1.1 Analysis of Mechanisms................................................................................104 8.1.2 Modifications to Mechanisms........................................................................107
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