Effect of Diluents on the Autoignition of Propane Mixtures Using a

Subscriber access provided by WEBSTER UNIV

Combustion

Effect of Diluents on the Autoignition of Propane Mixtures Using a Rapid Compression Machine Omid Samimi-Abianeh, Joshua A. Piehl, Antowan Zyada, Mustafa Al-Sadoon, and Luis Bravo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04100 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Effect of Diluents on the Autoignition of Propane Mixtures Using a Rapid Compression

2

Machine

3

O. Samimi-Abianeh1*, J. A. Piehl1, A. Zyada1, M. AL-Sadoon1, L. Bravo2 1 Mechanical

4

Engineering Department of Wayne State University, Detroit, Michigan 2 Army

5

Research Laboratory, Adelphi, Maryland

6

Abstract

7

The autoignition process of different propane mixtures was studied to determine how the diluent

8

choice (helium and argon/nitrogen) affects the ignition delay, the production of excited radicals

9

and the overall combustion process. An optically accessible rapid compression machine (RCM),

10

high speed camera, flame spectrometer, and numerical model were used to investigate the

11

combustion process. The ignition delay times were measured at a wide range of compressed gas

12

temperatures from 860 to 950 K, a compressed gas pressure of approximately 34 bar, and two

13

equivalence ratios of 1.0 and 1.5. Chemiluminescence at wavelengths of 309, 555, 590, and 623

14

nm from the decay of excited radicals were detected during the autoignition process, and their

15

respective intensities were measured and compared at various test conditions. A new kinetic model

16

was built by combining reaction rates of excited radicals and species from literature. The new

17

model was validated against the measured ignition delay data of the current work. The ignition

18

delay and some of the excited radicals show strong dependency on the diluent choice. A new

19

numerical model was used to include the effect of heat transfer and diluent choice in the

1*

Corresponding author, [email protected]

1 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

temperature calculation and ignition delay modeling and measurements. By using the new

21

numerical model, the measured ignition delays are not dependent on the diluent choice.

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 2 ACS Paragon Plus Environment

Page 2 of 34

Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

39

Introduction

40

Propane combustion processes have been studied numerically and experimentally by many

41

researchers as briefly reviewed in [1]. Propane ignition delay measurements were conducted at

42

high gas temperatures using shock tubes [2-3, 5, 7-10, and 14], and at low to intermediate

43

temperatures using rapid compression machines (RCMs) [1, 3 and 6]. In addition, the oxidation

44

and time-resolved species concentrations were investigated using flow and jet stirred reactors [4,

45

11-13, and 15]. These experiments and corresponding test conditions have summarized in Table

46

1.

47

Table 1. Summary of the propane experiments in the literature. Acronyms ST, RCM, FR and JSR

48

correspond to shock tube, rapid compression machine, flow reactor and jet stirred reactor,

49

respectively. Facility

Pressure

Temperature (K)

Equivalence ratio

Diluent

Reference

ST

6, 24, 60 atm

980 to 1400

0.5

argon

[2]

ST

2 to 15 atm

1200 to 1700

0.125 to 2.0

argon

[5]

ST

2 to 10 atm

1250 to 1600

0.125 to 2.0

argon

[7]

ST

1 to 18 bar

1200 to 1700

0.5 to 2.0

argon

[8]

ST

5 to 40 bar

> 850

0.5

argon, nitrogen

[9]

ST

1.3 to 5 atm

1073 to 2211

-

argon, nitrogen

[10]

ST

0.75 to 1.57 bar

1350 to 1800

0.5 to 2.0

argon

[14]

ST, RCM

10, 20, 30 atm

740 to 1550

0.3 to 3.0

argon, nitrogen

[3]

JSR

1 to 10 atm

900 to 1200

0.15 to 4.0

nitrogen

[12]

FR

1 to 6 bar

1000

0.05 to 25

nitrogen

[13]

FR

7 to 15 atm

785 to 935

0.4 and 0.6

nitrogen

[11]

FR

3.6 and 10 atm

850 and 900

0.3

nitrogen

[15]

FR

1 atm

850 to 1250

0.03 to 0.8

nitrogen

[4]

RCM

21, 27, 37 atm

680 to 970

0.5, 1.0, 2.0

Argon, nitrogen

[6]

RCM

21 and 30 bar

860 to 1050

0.5 to 1.2

Argon, nitrogen

[1]

50


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

51

These experiments have provided a very detailed understanding of the autoignition characteristics

52

of propane at various combustion conditions. Furthermore, these experiments have assisted in the

53

development of kinetic models, e.g., [3, 16], due to the integral role of propane kinetics in the

54

reaction pathways used by heavier hydrocarbons. The most recent kinetic model was developed

55

by the NUI Galway combustion group and can mimic the ignition delay quite well [3].

56

The effect of the diluent mixture (especially argon, nitrogen and helium) on the ignition delay has

57

been rarely investigated. Würmel et al. [17] studied the effect of argon and nitrogen on the ignition

58

delay of 2,3-dimethylpentane. The study used a RCM and a shock tube to investigate the impact

59

of the diluents on the ignition delay and concluded that the diluents had the opposite effect in the

60

different environments. Within the RCM, a greater thermal conductivity of the diluent increases

61

the heat transfer from the hot gas to the combustion chamber wall. Hence, the ignition delay times

62

were longer as autoignition occurred at lower temperatures and pressures. However, in shock

63

tubes, reactions occur in an adiabatic process and the higher heat capacity of the diatomic gas had

64

an opposite effect. Helium has a higher specific heat with respect to nitrogen and argon as shown

65

in Table 2. The higher specific heat of helium can be used to reduce the combustion pressure (the

66

maximum pressure due to the energy release of the fuel) within the test apparatus when compared

67

to the other two inert gases. The combustion pressure limits the initial gas pressure of the fuel

68

mixture as the pressure sensors are mostly limited to 200 to 300 bar pressure range. In addition,

69

the safety of the facility leads to the use of highly diluted test conditions for combustion

70

experiments (e.g., 90% nitrogen by volume in the mixture) to control the combustion pressure.

71

Helium has a high thermal conductivity with respect to other insert gases as shown in Table 2;

72

hence, the pressure and temperature decrease considerably during the ignition delay period within

73

the RCM due to an increase in the heat transfer rate to the chamber wall. 4 ACS Paragon Plus Environment

Page 4 of 34

Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

74

Table 2. Physical properties of the gasses at a temperature of 273 K and pressure of 1 bar. Data

75

are from NIST website. Species

ρ (kg/m3)

cp (J/(g k))

k (W/(m K)

α (m2/s)

He

0.176

5.193

0.1461

1.597×10-4

Ar

1.761

0.521

0.0163

1.776×10-5

N2

1.234

1.041

0.0240

1.868×10-5

C3H8

1.984

1.585

0.0156

4.960×10-6

O2

1.411

0.916

0.0244

1.887×10-5

76 77

To the best of the authors’ knowledge, this is the first investigation to provide a detailed

78

understanding of the effect of helium on the combustion characteristics within an RCM. The

79

ignition delay of propane fuel is studied using pure helium and a mixture of argon and nitrogen as

80

diluents. Nitrogen and argon are used quite commonly for combustion testing as demonstrated in

81

Table 1, and are used as the comparison for helium. Helium has promising characteristics for

82

combustion studies, such as: the reduction of the combustion pressure (due to its heat capacity),

83

and high specific heat ratio with respect to nitrogen (which means high compressed gas

84

temperature at the end of compression). The concentration of nitrogen with respect to argon in the

85

diluent mixture is small as shown in Table 3; hence, the mixture is referred to as the argon mixture

86

for simplicity. Nitrogen was added to the argon mixture to reduce the final compressed gas

87

temperature, so that a wide range of gas temperatures could be investigated. The effect of diluent

88

choice on the combustion process is investigated using a high speed camera and flame

89

spectrometer, as will be discussed in the latter part of the paper. The measured ignition delay shows

90

strong dependency on the diluent choice. A new numerical methodology was used to include the

91

effect of heat transfer or diluent choice in the temperature calculation and ignition delay modeling


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 34

92

and measurements. By using the new numerical model, the measured ignition delays are

93

independent of the diluent used. The experimental setup is described briefly in the next section. Table 3. Mixture compositions

94 Mix. number

Name

Φ

Partial pressure (bar) C3H8

O2

N2

AR

HE

1

Argon mix.

0.9919

0.01940

0.09773

0.11969

0.76316

-

2

Helium mix.

1.0033

0.01976

0.09840

-

-

0.88184

3

Helium mix.

1.5295

0.02966

009699

-

-

0.87334

4

Argon inert mix.

-

0.01940

-

0.21742

0.76316

-

5

Helium inert mix.

-

0.01940

-

0.09840

-

0.88184

95 96

Experimental setup

97

A rapid compression machine is a device that simulates the compression stroke of a single piston

98

engine to study the autoignition process of a fuel in a controllable environment. The mixture of

99

the fuel and oxidizer is compressed in a short time, e.g., 30 ms, to reach a high compressed gas

100

pressure and temperature. The Wayne State University Combustion Physics Laboratory’s (CPL)

101

RCM is briefly described in our previous work [1] which can be referred to for more details about

102

the instrument. A creviced piston is used in the RCM to minimize the flow vortices formed during

103

compression and is based on an optimized design of [28]. The desired compressed gas conditions

104

can be reached by changing the initial gas pressure, the initial gas temperature, and the

105

compression ratio of the RCM. For this work, a compression ratio of approximately 9.7 was used

106

while the wall temperature of the test chamber ranged from 297 to 327 K depending on the desired

107

compressed gas temperature.


Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

108

The test mixtures, as shown previously in Table 3, were made from ultra-pure and research grade

109

gases and mixed in an external 5-gallon stainless steel vessel. The mixtures were prepared and left

110

overnight before use, allowing for an average of 10 hours of homogeneous mixing to occur. The

111

diluents (argon, helium, and nitrogen) consisted of approximately 88% of the mixture composition

112

in this work, as shown in Table 3. Argon (research plus grade with purity of 99.9999%), nitrogen

113

(ultra-high purity, 99.999%), helium (research grade with purity of 99.9999%), oxygen (ultra-high

114

purity, 99.994%), and propane (research grade, 99.99%) were supplied by Airgas for this study.

115

The relevant physical properties of the gases are shown previously in Table 2. An Agilent vacuum

116

pump (DS 202) was used to empty the mixing vessel and RCM of residual gases. For this work, a

117

pressure range of approximately 0.5 to 1 mbar was obtained to remove such residuals before

118

introducing the new mixture into the experimental apparatuses.

119

A sapphire optical window, approximately 2-inch in diameter with a thickness of 0.75-inch, was

120

placed at the end wall of the RCM to allow the entire test volume to be imaged using the high-

121

speed camera and flame spectrometer. A Phantom VEO 410 high-speed camera with a fast 50-mm

122

lens (f/0.95, Navitar) is used to record the ignition events in the test section by viewing along the

123

axis of the test section. The camera uses three filters (green, blue and red channels) and the working

124

wavelengths have been shown in appendix 2 of [1].

125

The chemiluminescence intensity of the excited radicals are collected using a collimating lens (84-

126

UV-25, Ocean Optics) with a working wavelength of 200 to 2000 nm. The collected

127

chemiluminescence is focused onto a premium optical fiber (a single leg fiber bundle, LG-455-

128

020-01, that contains a single column of 19 fibers, each 200 µm in diameter), after which the light

129

enters the spectrometer (IsoPlane 160, Princeton Instrument), passing through the slit (with an

130

opening width of 10 micron) and reaching the grating. The focusing mirror receives the light 7 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

131

reflected from the grating and focuses the light onto the Electron-Multiplying Charge-Coupled

132

Device (EMCCD, ProEM-HS 1024BX3, Princeton Instrument), which converts the optical signal

133

to a digital signal. The electron multiplying (EM) system can multiply the received photons by

134

thousands (depending on the application and setting) to ensure the colliding photons are detectable

135

by the camera sensor. An EM gain range from 1 to 1000 was used at low and high temperatures to

136

detect the chemiluminescence produced by the excited radicals during autoignition. A grating

137

groove density of 150 gr/mm with a blaze wavelength of 300 nm was used in this work. The

138

spectrometer system (EMCCD, grating, lens, fiber) excluding the sapphire window was calibrated

139

together using the spectral calibration system (IntelliCal, Princeton Instrument). The spectrometer

140

wavelength calibration was performed using the mercury source of the aforementioned calibration

141

system. The slit width is 10 micron for all of the studied cases. The schematic of the setup is shown

142

in Fig. 1.

143 144

Fig. 1. Experimental setup of the RCM with the spectrometer

145

Data measurement and processing

146

Pressure-time data were measured using a piezoelectric pressure transducer (Kistler 6045A)

147

coupled with a charge amplifier (Kistler 5018) with a total relative uncertainty of 1.9%. Initial gas

148

pressures inside the RCM chamber and mixing vessel were measured using a static pressure

149

transducer (Omega PX409) with 0.7% relative uncertainty. Initial gas and wall temperatures were 8 ACS Paragon Plus Environment

Page 8 of 34

Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

150

measured using several thermocouples (Omega KMQSS-125G-6) with 1.5 K absolute uncertainty.

151

The data acquisition system used National Instruments LabVIEW program to record the data. In

152

this work, the ignition delay time is defined as the time from the end of compression to the

153

maximum peak of pressure rise with respect to time (dp/dt) as shown in Fig. 2. This definition

154

imposes 5% relative uncertainty in the ignition delay time since autoignition could be defined

155

anywhere between the start of the pressure rise to the peak pressure after the pressure rise due to

156

combustion.

157 158

Fig. 2. Pressure and gradient of pressure history inside the RCM combustion chamber

159

The use of a creviced piston in this work mitigates the formation of vortices in front of the piston,

160

resulting in an adiabatic core of gas to be formed during compression as discussed by [28]. The

161

gas temperature at the end of compression (i.e. the compressed gas temperature, Tc) is calculated

162

using the measured initial gas temperature, Ti, initial gas pressure, Pi, specific heat ratio of the

163

mixture, and the gas pressure at the end of compression (i.e. the compressed gas pressure, Pc) by

164

applying the isentropic compression equation:


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

165

()

𝑙𝑛

𝑃𝑐 𝑃𝑖

=

∫

Page 10 of 34

𝑇𝑐

𝛾(𝑇) 𝑑𝑇 𝑇𝑖 𝛾(𝑇) ― 1 𝑇

(1)

166

The equation uses a variable specific heat ratio (γ) as a function of mixture composition and

167

temperature, and assumes the chemistry to be frozen during the compression stroke. Furthermore,

168

this equation assumes that the gas behaves like an ideal gas. This assumption was questioned as

169

the critical pressure and temperature of helium are 2.26 atm and 5.19° K, respectively. Helium is

170

at a supercritical condition and does not follow the ideal gas law at the studied compressed gas

171

pressure of approximately 34 bar. This further implies that its enthalpy and internal energy are a

172

function of both pressure and temperature at the studied conditions. However, the change of

173

helium’s enthalpy is less than 0.1% at pressure range of 1 to 40 bar and a fixed temperature of

174

1000 K. Therefore, it was concluded that Eq. 1 can still be applied to calculate the compressed gas

175

temperature. The total uncertainty of Eq. (1) due to the uncertainties of the pressure sensors and

176

thermocouples is estimated to be between 9 to 13 K.

177

The compressed gas pressure and temperature decrease due to the heat transfer from the hot gas

178

mixture to the combustion chamber wall during the ignition delay period, as shown in Fig. 2.

179

During this period, the mixture experiences various temperature and pressure conditions, which

180

are lower than the compressed gas pressure and temperature. There are several methodologies to

181

include heat transfer in simulations that can be used to evaluate and develop kinetic models using

182

RCM data as described in [1]. One method is to use the effective compressed gas pressure and

183

average compressed gas as discussed by [1] and [29]. In this model, the effective compressed

184

pressure, Peff, is calculated as the time-averaged pressure from the compressed gas pressure, Pc, to

185

the minimum gas pressure, Pmin, before the pressure rise resulting from combustion. The


Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

186

compressed gas pressure is replaced by effective compressed gas pressure in Eq. 1 to calculate the

187

average compressed gas temperature, Tavg:

( )

188

𝑙𝑛

𝑃𝑒𝑓𝑓 𝑃𝑖

=

∫

𝑇𝑎𝑣𝑔 𝑇𝑖

𝛾(𝑇) 𝑑𝑇 𝛾(𝑇) ― 1 𝑇

(2)

189

The calculated average gas temperature will be used in this work for kinetic model simulations to

190

account for the heat transfer. The calculated temperatures using the effective volume method [26]

191

and the average gas temperature method are within the acceptable uncertainty range since both

192

methods assume the compression and post-compression are isentropic processes. It should be

193

noted that these methods may not be valid for calculating the gas temperature during post-

194

compression when the ignition delay is long as shown by [27].

195

Kinetic model development

196

The kinetic model of [3] is used to simulate the autoignition of the propane mixture. This model

197

is the latest version of the Natural Gas III mechanism developed by the NUI Galway combustion

198

group and includes hydrocarbon species up to C5. The model does not include the excited radicals’

199

reactions. OH* and CH* reactions were added to the base mechanism of [3] to model the

200

concentrations of the excited radicals. Eleven OH* reactions from [18] and seventeen CH*

201

reactions from [18-19] were added to the base mechanism. In addition, six C2H reactions from [18

202

and 20] and GRI-Mech 3.0 were added to the base mechanism to develop the connection between

203

C2H species and CH*. While CH* has not been studied in this work, the connection between CH,

204

CH* and OH* by the reaction CH+O2CO+OH* mandated the inclusion of CH and CH* into

205

the kinetic model. The sub-mechanism of CH was also updated from the measured data in literature

206

[18, 23-25], GRI-Mech 3.0 and Aramco 2.0 mechanism [30]. Only two reactions were modified

207

and added based on the results of this study as shown in Table 4. The third body collider efficiency 11 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 34

208

of helium was added to the first reaction in Table 4 based on the results of this work, as will be

209

discussed, since the helium and argon affect OH* very similarly. Due to the same reason, the

210

reaction OH* + He OH + He was added to the mechanism and its reaction rates are assumed

211

to be the same as the reaction rates of OH* + Ar OH + Ar as developed by [18]. These sub-

212

mechanisms (excluding the modification in Table 4) are also available in the Aramco 2.0

213

mechanism. However, the Aramco 2.0 mechanism made poor predictions with respect to the

214

measured ignition delay and onset of OH* production and was not used for modeling. The

215

comparisons between Aramco 2.0, mechanism model of [3], and measured data are shown in

216

Appendix 3. The new model was built by combining the base model of [3] with the supplementary

217

reaction pathways as explained. The final model was validated against the measured ignition delay

218

data produced in the current work, as discussed later in the paper. The final model is reported in

219

Appendix 2 of the paper.

220

Table 4. The modified and added OH* reaction rates in Arrhenius format. The final mechanism is

221

reported in Appendix 2. Reaction

Pre-exponential factor

Temperature coefficient

Activation energy

H + O + M OH* + M

1.500E+13

0.000E+00

5.975E+03

Third body collider efficiencies: H2 / 1.000 / H2O / 6.500 / O2 / 0.400 / N2 / 0.400 / AR / 0.350 / HE / 0.350 / Note: Helium third body collider efficiency (HE / 0.350 /) was added to the above reaction rates of [20] OH* + He OH + He

1.690E+12

0.000E+00

4.135E+03

Note: Above new reaction is based on OH*+ArOH+Ar with the same rate coefficient as developed by [18]

222 223

Chemkin-Pro software was used for ignition delay and gas temperature modeling. A closed

224

homogenous batch reactor model was utilized to simulate the ignition delays. The compressed and

225

average gas temperatures were calculated using the homogenous-charge compression-ignition


Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

226

(HCCI) engine model available in the software to perform the integration of Eqs. (1) and (2) using

227

the variable specific heat ratio of the mixture.

228

Results and Discussion

229

In this section, the pressure history of the two inert mixtures are discussed initially. Following that

230

discussion, the ignition delays of the reacting cases are determined. Next, the ignition delay and

231

excited radicals’ histories are discussed at various temperature and equivalence ratios. The section

232

is concluded by investigating the effect of temperature on OH* intensity. For easy identification

233

and comprehension, different colors are used to represent the different mixtures in the figures.

234

Gray is used for the helium mixtures, whereas black is used for the argon mixtures for most of the

235

pictures.

236

The pressure history of mixtures 4 and 5 of Table 3 are shown in Fig. 3. The two tests using the

237

two mixtures were performed at the same initial pressure and temperature. These two mixtures are

238

equivalent to mixtures 1 and 2 but were made inert by replacing the oxygen with nitrogen. This

239

does not impact the heat transfer as nitrogen and oxygen have a very similar thermal conductivity

240

and specific heat. This allows for the effect of the diluent on the heat transfer to be investigated

241

during the compression and post-compression periods. As shown in Fig. 3, the peak pressure of

242

the helium inert mixture is slightly lower than that of the argon inert mixture. Furthermore, the

243

pressure drop of the helium inert mixture is greater than that of the argon inert mixture. These

244

observations are due to the greater thermal diffusivity of the helium inert mixture compared to that

245

of the argon inert mixture. As shown in Table 2, the thermal diffusivity of helium is 10 times

246

greater than argon’s, which produces a more uniform temperature field inside the combustion

247

chamber. In addition, helium’s greater thermal diffusivity increases the rate of heat transfer to the


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

248

wall. Hence, the energy within the combustion chamber dissipates more rapidly with the helium

249

mixture, resulting a lower peak pressure and greater pressure drop, as observed experimentally.

250 251

Fig. 3. Pressure histories of mixtures 4 and 5 of Table 3. The initial pressure and temperature are

252

approximately 1.44 bar and 289 K for both mixtures.

253

Shifting focus to the reacting cases, the ignition delay of mixtures 1 and 2 as a function of

254

compressed gas temperature at an equivalence ratio of 1 are shown in Fig. 4. The ignition delay of

255

the helium mixture is longer than that of the argon mixture at the same compressed gas

256

temperature. The longer ignition delay of the helium mixture is due to the greater drops in pressure

257

and temperature during post-compression, as previously discussed and shown in Fig. 3. In addition

258

to the ignition delay measurements, the onset of OH* measured by the flame-spectrometer is

259

shown in Fig. 4. The timings of OH* match well with the ignition delay defined by the peak

260

pressure rise with respect to time (i.e. max dp/dt).

261


Page 14 of 34

Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

262 263

Fig. 4. Measured ignition delay and onset of OH* of argon mixture (mixture 1 of Table 3) and

264

helium mixture (mixture 2 of Table 3) at equivalence ratio of 1 as a function of compressed gas

265

temperature. The initial pressure is approximately 1.44 bar for both mixtures. The average

266

compressed gas pressure of argon and helium mixtures are 35.12 bar and 33.08 bar, respectively.

267

The lines are trend-lines of the measured data, which plotted using the same color as the measured

268

data point.

269

The detected onsets of autoignition using the camera, flame spectrometer and pressure sensor using

270

the helium mixture are shown in Fig 5. The onset of OH*, timing of the maximum pressure rise

271

with respect to time, and the onset of the full-circle blue light (when the blue light appears in the

272

entire chamber) using the high speed camera are matched excellently as shown in Fig. 5. The

273

camera and spectrometer results will be discussed in more detail later in the section.

274 275


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

276 277

Fig. 5. Measured onsets of autoignition using various combustion diagnostic tools (camera, flame

278

spectrometer and pressure sensor) as a function of compressed gas temperature using the helium

279

mixture (mixture 2 of Table 3) at an equivalence ratio of 1. The initial pressure is approximately

280

1.44 bar and the average compressed gas pressure is 33.08 bar. The lines are trend-lines of the

281

measured data, which are plotted using the same color as their respective measured data points.

282

The ignition delays of the helium mixture at equivalence ratios of 1 and 1.5 are shown in Fig. 6.

283

The ignition delay of the mixture decreases by increasing the equivalence ratio. In addition, the

284

onset of OH* matches well with the timing of pressure rise for both equivalence ratios.

285


Page 16 of 34

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

286 287

Fig. 6. Measured ignition delay and onset of OH* at equivalence ratios of 1 and 1.5 as a function

288

of compressed gas temperature. Helium mixtures 2 and 3 are used in these tests. The initial pressure

289

is approximately 1.44 bar and 1.58 bar, with the average compressed gas pressures of 33.08 bar

290

and 34.97 bar for mixtures 2 and 3, respectively. The lines are trend-lines of the measured data,

291

which are plotted using the same color as their respective measured data points.

292 293

The measured ignition delay of the fuel using an RCM is a function of several parameters such as

294

fuel chemical structure, fuel and oxygen concentration, diluents used and machine geometry (e.g.,

295

piston shape). To eliminate the effect of the diluent choice on the ignition delay measurements and

296

reporting, the authors propose utilizing the average temperature (Eq. 2) as discussed in the previous

297

section. As shown in Fig. 7, the ignition delay of both mixtures of helium and argon (1 and 2 in

298

Table 3) match very well if reported as a function of average compressed gas temperature. Hence,

299

the average compressed gas temperature can be used instead of the compressed gas temperature

300

(defined by Eq. 1) to eliminate the effect of diluent on ignition delay measurement and reporting.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

301

Average compressed gas temperature includes the effect of heat transfer (which is a function of

302

the diluents) and can be used easily by kinetic model developers to validate their simulated data

303

without the knowledge of the diluent composition. The simulated ignition delays for the helium

304

and argon mixtures using the kinetic model are also shown in Fig. 7. The ignition delay predictions

305

matched the measured data very well; however, at lower average compressed gas temperatures the

306

modeled ignition delay slightly under predicted the measured data, and at higher gas temperatures

307

the modeled ignition delay slightly over predicted the measured data. The under prediction of the

308

ignition delay at low temperatures could be due to the inaccuracy of the isentropic assumption at

309

long ignition delay durations (e.g., 80 ms). For long ignition delays, the cold gas mixture near the

310

boundary layer and flow vortices have enough time to mix with the hot gas mixture at the core of

311

the chamber. Hence, the mixing ensures that the core gas temperature does not follow an isentropic

312

process. The over prediction of the ignition delay at high gas temperatures could be due to the

313

average temperature model or errors in the mechanism at high gas temperatures, which is out of

314

the scope of this paper. Measured gas temperatures are needed to develop a more comprehensive

315

gas temperature model which is not available at the time of writing this paper.

316


Page 18 of 34

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

317

Fig. 7. Measured and modeled ignition delay of the argon mixture (mixture 1 of Table 3) and the

318

helium mixture (mixture 2 of Table 3) at an equivalence ratio of 1 as a function of average

319

compressed gas temperature. The initial pressure is approximately 1.44 bar for both mixtures. The

320

average compressed gas pressures of the mixtures are 35.12 bar and 33.08 bar for argon and

321

helium, respectively. The average effective compressed gas pressures of the mixtures are 31.76

322

bar and 28.94 bar for argon and helium, respectively. The ignition delays were modeled using the

323

average gas temperature of the respective mixtures. The ignition delay modeling was performed

324

at a pressure of 30.3 bar. The maximum rate of pressure rise was used to determine the onset of

325

the modeled and measured ignition delays.

326 327

The measured and modeled pressure histories and the OH* concentration and intensity are shown

328

in Fig. 8. The uncertainty associated with measuring the timing of the OH* intensity is

329

approximately -1.5 ms and +0.5 ms. This uncertainty is due to the triggering mechanism, and

330

compression time calculation. The pressure rise and OH* concentration were simulated by using

331

the average temperature and effective pressure model. The measured onset of OH* intensity

332

matches very well with the timing of the measured pressure rise for all of the studied conditions,

333

as shown in the Fig. 8. The onset of the modeled concentration of OH* matches the modeled

334

pressure rise very well. Hence, the reaction model can accurately predict the OH* onset at various

335

gas temperatures. As discussed previously, the model under predicts the ignition delay at low gas

336

temperatures and over predicts the ignition delay at high gas temperatures. It is difficult to

337

determine if this behavior is due to the kinetic model or the temperature model in inefficiencies.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

338

339

340 341

Fig. 8. Measured and modeled pressure rises, OH* concentrations and intensities at several

342

compressed gas temperatures. The argon mixture (mixture 1 of Table 3) is at an equivalence ratio

343

of 1 for the measurements and modeling. The uncertainty in the measured timing of OH* intensity

344

is estimated to be between -1.5 ms and +0.5 ms. The pressure rise and OH* concentration were

345

simulated by using the average temperature and effective pressure model.

346


Page 20 of 34

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

347

The measured pressure rises and time-resolved intensity of various excited radicals for the

348

mixtures of argon and helium at the same equivalence ratio and similar compressed gas

349

temperature are shown in Figs. 9 to 12. There are several new findings and observations as follows:

350

-

Five excited radicals with a sharp intensity peak are detectable at wavelengths of

351

approximately 309 (OH*), 431 (CH*), 555, 590, and 623 nm. A bell shape intensity rise

352

between 290 nm to 550 nm is due to the continuum band emission of CO2*. This continuum

353

intensity is clearly visible for the helium mixture in Fig. 12. At lower temperatures, the bell

354

shape intensity is also visible for the argon mixture (not shown in the paper). This

355

continuum emission emits a visible blue light that was used to determine the ignition delay

356

reported previously as a full-circle blue light in the legend of Fig. 5. The onset of the blue

357

continuum occurs simultaneously as the onset of OH*, as shown in Fig. 12.

358

-

As shown in Fig. 10, there are three detectable excited radicals within the wavelength range

359

of 500 to 650 nm using the argon mixture. The excited radical at 590 nm is always

360

detectable for all of the studied temperatures using this mixture. The other two excited

361

radicals at 555 nm and 623 nm are detectable at compressed gas temperatures higher than

362

approximately 888 K. The presence of these two excited radicals at 555 and 623 nm occur

363

after the timing of the ignition delay, when high pressure combustion pressure occurs.

364

These wavelengths are especially prevalent when the pressure is higher than 100 bar. As

365

an example, by increasing the compressed gas temperature from 886 K to 930 K, the

366

combustion pressure increases from 100 bar to 175 bar as shown in Fig. 13 and the

367

intensities of these excited radicals increases significantly.

368 369

-

The intensity peak at 590 nm is not always detectable, especially at low gas temperatures using the helium mixture. The intensities at wavelengths of 555, 590, and 623 nm are 21 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

370

significant when the argon mixture is used and is probably associated with the higher

371

combustion temperature and pressure. At higher compressed gas temperatures (e.g., a

372

compressed gas temperature of 942 K), the emitted intensity increases significantly for the

373

helium mixture as well due to the increase in combustion pressure and temperature.

374

-

emission.

375 376

-

The onset of OH* (309 nm) and the excited radical at 590 nm match very well with the timing of the pressure rise, as shown in Figs. 9 and 11.

377 378

The emissions at 555 nm and 623 nm disappear before the cessation of the 590 nm

-

As shown in Figs. 9 and 11, the combustion pressure (maximum pressure at time of the

379

ignition delay) is approximately two times higher for the argon mixture than the helium

380

mixture at a similar compressed gas temperature. For example, the combustion pressure

381

reaches about 160 bar using the argon mixture and 80 bar using the helium mixture. The

382

specific heat of argon is about 10 times smaller than helium; thus, with a fixed energy

383

release, the final combustion pressure and temperature will be higher using the argon

384

mixture.

385


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

386

Fig. 9. Pressure rise and chemiluminescence intensity from the decay of excited radicals at

387

wavelengths of 309 nm (OH*) and 590 nm versus time. The ignition delay is 17.28 ms. The argon

388

mixture (mixture 1 of Table 3) is used at an equivalence ratio of approximately 1. Initial pressure

389

and initial temperature are 1.44 bar and 318 K. Compressed gas pressure and compressed gas

390

temperature are 35.16 bar and 922 K. Effective compressed gas pressure and average compressed

391

gas temperature are 32.90 bar and 903 K. An exposure time of 0.05 ms and an EM gain of 10 are

392

used for the intensity measurements.

393

394 395

Fig. 10. Chemiluminescence intensity from the decay of excited radicals at various timings. The

396

non-dimensional timing in the picture (e.g., 0.91) is the measured timing from the end of

397

compression divided by the timing of the ignition delay. The ignition delay is 17.28 ms. An

398

exposure time of 0.05 ms and an EM gain of 10 are used for the intensity measurements. The test

399

conditions are approximately the same as those reported in Fig. 9.

400


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

401 402

Fig. 11. Pressure rise and chemiluminescence intensity from the decay of excited radicals at

403

wavelengths of 309 nm (OH*) and 590 nm versus time. The ignition delay is 17.8 ms. The helium

404

mixture (mixture 2 of Table 3) is used at an equivalence ratio of approximately 1. Initial pressure

405

and initial temperature are 1.44 bar and 312 K. Compressed gas pressure and compressed gas

406

temperature are 32.98 bar and 932 K. Effective compressed pressure and average compressed gas

407

temperature are 30.11 bar and 902 K. An exposure time of 0.1 ms and an EM gain of 10 are used

408

for the intensity measurements.

409


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

410

Fig. 12. Chemiluminescence intensity from the decay of excited radicals at various timings. The

411

non-dimensional timing in the picture (e.g., 0.89) is the measured timing from the end of

412

compression divided by the timing of the ignition delay. The ignition delay is 17.8 ms. An exposure

413

time of 0.1 ms and an EM gain of 10 are used for the intensity measurements. The test conditions

414

are approximately the same as those reported in Fig. 11.

415

416 417

Fig. 13. Pressure rises at three different compressed gas temperatures. The argon mixture (mixture

418

2 of Table 3) is used at an equivalence ratio of approximately 1.

419 420

The pressure histories and images of combustion progression for the helium mixture at an

421

equivalence ratio of 1 and at two compressed gas temperatures of 910 and 944 K are shown in

422

Figs. 14 and 15, respectively. As shown in Fig. 14, by increasing the compressed gas temperature,

423

the maximum combustion pressure increases from approximately 75 bar to 100 bar. High

424

frequency oscillating pressure waves at the time of combustion (at end the of the ignition delay

425

duration) were observed at the 100 bar combustion pressure for this mixture but were absent at the 25 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

426

lower combustion pressure of 75 bar. All studied conditions using the argon mixtures exhibited

427

these oscillating pressure waves at the time of combustion. It is theorized that the origin of these

428

oscillating waves is due to autoignition of the trapped mass inside the crevice, as will be explained

429

shortly. As shown in Fig. 15A, the autoignition starts with a uniform blue light inside the chamber,

430

followed by a high-intensity white light and concluding with a blue light again. The high-speed

431

camera’s blue light wavelength response curve is between 350 to 550 nm, indicating that the blue

432

light corresponds to CO2* as discussed previously. The origin of the intense white light in Fig.

433

15A is not well understood, but it could be caused by the excited radicals emitted at 590 nm due

434

to its color, timing, and dependency on pressure and temperature at the time of combustion.

435

Autoignition at a higher compressed gas temperature is slightly different than at a lower

436

compressed gas temperature, as shown in Fig. 15B. At a higher compressed gas temperature,

437

autoignition starts with a blue light and develops into a white light (similar to the low temperature

438

autoignition). Next, very intense white light (saturating the camera chip) starts from the periphery

439

of the chamber until it fills the combustion chamber. Finally, combustion ends as the light fades

440

into a dim yellow light. The yellow light indicates the formation of soot [22]. As shown in the third

441

picture of Fig. 15B, the high intensity white light appears near the periphery of the chamber which

442

could be due to combustion of the trapped mass inside the crevice. It is hard to differentiate if it is

443

due to autoignition of the trapped mass inside the crevice or due to the autoignition of the trapped

444

mass which flows back into the chamber from the crevice during the autoignition. The gas pressure

445

after ignition is significantly higher (e.g., 100 bar as shown in Fig. 14) than the compressed gas

446

pressure (e.g., 34 bar as shown in Fig. 14); hence, the trapped mixture inside the crevice is at a

447

lower gas temperature (with respect to the compressed temperature of the core gas) which may

448

ignite and produce the pulsating waves inside the chamber at the time of combustion. The


Page 26 of 34

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

449

autoignition of the trapped mass is the most likely reason for the chemiluminescence intensities

450

observed at 555 and 625 nm since there no excited radicals that emit photons at these wavelengths

451

and the high intensity white light occurs at the periphery of the chamber when the gas temperature

452

is lower, as shown in Fig. 15A. In addition, the onset of the emissions at 555 and 625 nm appear

453

after some delay with respect to the onset of the emission at 590 nm or the onset of the ignition

454

delay. This phenomenon will be studied in a future investigation using a flat piston without a

455

crevice to better understand the physics and origin of these emissions.

456 457

Fig. 14. The pressure rise at two compressed gas temperatures of 910 K and 944 K. The helium

458

mixture (mixture 2 of Table 3) is used at an equivalence ratio of approximately 1. The timings of

459

the images processed are highlighted by gray shadow. The combustion images are shown in Fig.

460

15.

461 462


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

2

3

4

Page 28 of 34

5

6

7

463 464

A: (Timing/Ignition-delay): (1):0.963, (2):0.972, (3):0.993, (4):1.062, (5):1.107, (6):1.204,

465

(7):1.725 1

2

3

4

5

6

7

466 467

B: (Timing/Ignition-delay): (1):0.915, (2):0.923, (3):0.926, (4):0.932, (5):1.104, (6):1.263,

468

(7):2.311

469

Fig. 15. Combustion images at two compressed gas temperatures of A: 910 K and B: 944 K. The

470

helium mixture (mixture 2 of Table 3) is used at equivalence ratio of approximately 1. The pressure

471

rise of the two cases is shown in Fig. 14. Exposure time of 44 μs with a speed of 20,000 fps were

472

used for Fig. A, and exposure time of 17 μs with a speed of 57,000 fps were used for Fig. B.

473 474

Conclusions

475

The autoignition processes of propane fuel using two different diluents were studied using an

476

optically accessible rapid compression machine. The main objective of this research was to

477

understand how the diluent choice affects the ignition delay, excited radicals produced and the

478

combustion process using various combustion diagnostics tools and a numerical model. To

479

investigate the combustion process, a high speed camera, flame spectrometer, and kinetic model

480

were utilized. The ignition delay times were measured at wide range of compressed gas 28 ACS Paragon Plus Environment

Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

481

temperatures, two equivalence ratios (approximately 1 and 1.5), and a compressed gas pressure of

482

approximately 34 bar. Two mixtures were utilized, one made of argon/nitrogen/propane/oxygen

483

and the other helium/propane/oxygen.

484

Ignition determined by the timing of the onset of full blue light (detected by the high-speed

485

camera), onset of OH* (detected by the flame spectrometer), and pressure rise (measured by the

486

pressure sensor) match very well at studied conditions. The chemiluminescence from the decay of

487

several excited radicals were measured at wavelengths of 309, 555, 590, and 623 nm during

488

autoignition. Their intensity was measured and compared at the various test conditions. The

489

chemiluminescence intensity of the excited radical at 309 nm (OH*) is a strong function of

490

equivalence ratio and compressed gas temperature, but it does not show dependency to diluent

491

choice. In contrast, the chemiluminescence intensity of the excited radicals produced at 555, 590,

492

and 623 nm show a strong dependency on combustion pressure, temperature and diluent choice.

493

The ignition delay shows a strong dependency on the diluent choice. The average compressed gas

494

temperature and effective compressed gas pressure model were used to include the effect of heat

495

transfer in modeling and reporting the ignition delay. The ignition delay shows independency of

496

the diluent choice when it is reported using the average compressed gas temperature and effective

497

compressed gas pressure.

498

The kinetic model under predicts the ignition delay at low gas temperatures (lower than 840 K).

499

The measured onset of OH* concentration occurs at the same timing as the pressure rise, which

500

was predicted very well by the kinetic model.

501

Conflicts of interest

502

There is no conflict of interest. 29 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

503 504

Appendix

505

The measured ignition delay is reported in appendix 1. The mechanism and thermodynamic files

506

are reported in appendix 2. The comparisons between the two mechanisms of [3] and Aramco 2.0

507

[30] versus measured data are reported in Appendix 3.

508

Acknowledgment

509

Research was sponsored partially by the Army Research Laboratory and was accomplished under

510

Cooperative Agreement Number W911NF-18-2-0042. The views and conclusions contained in

511

this document are those of the authors and should not be interpreted as representing the official

512

policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government.

513

The U.S. Government is authorized to reproduce and distribute reprints for Government purposes

514

notwithstanding any copyright notation herein.

515

References

516

1. Goyal, T, Trivedi, D, Samimi-Abianeh, O, Autoignition and flame spectroscopy of propane

517

mixture in a rapid compression machine, Fuel, 2018; 233: 56-67.

518

2. Lam K, Hong Z, Davidson D, Hanson R, Shock tube ignition delay time measurements in

519

propane/O2/argon mixtures at near-constant-volume conditions. Proceedings of the Combustion

520

Institute 2011; 33(1):251-8.

521

3. Healy D, Curran H, Dooley S, Simmie J, Kalitan D, Petersen E, et. al., Methane/propane mixture

522

oxidation at high pressures and at high, intermediate and low temperatures. Combustion and Flame

523

2008; 155(3):451-61. 30 ACS Paragon Plus Environment

Page 30 of 34

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

524

4. Sabia P, Joannon M, Lavadera M, Giudicianni P, Ragucci R, Auto ignition delay times of

525

propane mixtures under MILD conditions at atmospheric pressure. Combustion and Flame 2014;

526

161(12):3022-30

527

5. Dagaut P, Cathonnet M, Boettner J, Kinetic Modeling of Propane Oxidation and Pyrolysis.

528

International Journal of Chemical Kinetics 1992; 24(9):813-37.

529

6. Gallagher S, Curran H, Metcalfe W, Healy D, Simmie J, Bourque G, A rapid compression

530

machine study of the oxidation of propane in the negative temperature coefficient regime.

531

Combustion and Flame 2008; 153(1-2):316–33.

532

7. Burcat A, Lifshitz A, Scheller K, Skinner G, Shock-tube investigation of ignition in propane-

533

oxygen-argon mixtures. Symposium (International) on Combustion 1971; 13(1): 745-55.

534

8. Lamoureux N, Paillard C, Vaslier V, Low hydrocarbon mixtures ignition delay times

535

investigation behind reflected shock waves. Shock Waves 2002; 11(4):309-22.

536

9. Cadman P, Thomas G, Butler P, The auto-ignition of propane at intermediate temperatures and

537

high pressures. Phys. Chem. Chem. Phys 2000; 2(2):5411–9.

538

10. Brown C, Thomas G, Experimental Studies of Shock-Induced Ignition and Transition to

539

Detonation in Ethylene and Propane Mixtures. Combustion and Flame 1999; 117(4):861–70.

540

11. Beerer, D., and McDonell, V. An experimental and kinetic study of alkane auto ignition at high

541

pressures and intermediate temperatures. Proceedings of the Combustion Institute 2011, 33(1),

542

301-307.

543

12. Dagaut P, Cathonnet M, Boettner J, Gaillard F, Kinetic Modeling of Propane Oxidation.

544

Combustion Science and Technology 1987; 56(1-3):23-63. 31 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

545

13. Cathonnet M, Boettner J, James H, Experimental study and numerical modeling of high

546

temperature oxidation of propane and n-butane. Symposium (International) on Combustion 1981;

547

18(1):903-13.

548

14. Kim, K., and Shin, K., Shock Tube and Modeling Study of the Ignition of Propane. Bulletin of

549

the Korean Chemical Society 2001: 22(3), 303-307.

550

15. Hoffman J, Lee W, Litzinger T, Santavicca D, Pitz W, Oxidation of Propane at Elevated

551

Pressures: Experiments and Modelling. Combustion Science and Technology 1991; 77(1-3):95-

552

125.

553

16. Li Y, Zhou CW, Somers KP, Zhang K, Curran, HJ, The Oxidation of 2-Butene: A High

554

Pressure Ignition Delay, Kinetic Modeling Study and Reactivity Comparison with Isobutene and

555

1-Butene. Proceedings of the Combustion Institute 2017; 36(1):403-411.

556

17. Würmel J, Silke EJ, Curran HJ, Ó Conaire MS, Simmie JM, The effect of diluent gases on

557

ignition delay times in the shock tube and in the rapid compression machine. Combustion and

558

Flame 2007; 151:289-302.

559

18. Kathrotia1 T, Fikri M, Bozkurt M, Hartmann M, Riedel U, Schulz C, Study of the H+O+M

560

reaction forming OH*: Kinetics of OH* chemiluminescence in hydrogen combustion systems.

561

Combustion and Flame 2010; 157:1261-1273.

562

19. Hwang SM, Gardiner WC, Frenklach M., Hidaka Y, Induction zone exothermicity of acetylene

563

ignition. Combustion and Flame 1987; 67(1): 65-75.


Page 32 of 34

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

564

20. Devriendt K., Van Look H, Ceursters B, Peeters J, Kinetics of formation of hemiluminescent

565

CH(A 2Δ) by the elementary reactions of C 2H(X 2Σ+) with O( 3P) and O 2(X 3Σg-): A pulse laser

566

photolysis study. Chemical Physics Letters 1996: 261(4) 450-456.

567

21. Coates J, Interpretation of infrared spectra, a practical approach, Encyclopedia of Analytical

568

Chemistry: Applications, Theory and Instrumentation. John Wiley & Sons Ltd, 2006.

569

22. Böhm H, Hesse D, Jander H, Lüers B, Pietscher J., Wagner HGG, Weiss M, The influence of

570

pressure and temperature on soot formation in premixed flames. Symposium (International) on

571

Combustion 1989, 22(1) 403-411.

572

23. Messing I, Filseth SV, Sadowski CM, and Carrington T, Absolute rate constants for the

573

reactions of CH with O and N atoms. The Journal of Chemical Physics 74, 3874 (1998).

574

24. Bergeat, A, Moisan, S, Méreau, R, Loison, J-C, Kinetics and mechanisms of the reaction of

575

CH with H2O. Chemical Physics Letters, 480(1–3),21-25, 2009.

576

25. Glarborg P, Miller JA, Kee RJ, Kinetic modeling and sensitivity analysis of nitrogen oxide

577

formation in well-stirred reactors. Combustion and Flame 1986: 65(2) 177-202.

578

26. Mittal G., Sung, C.-J., A Rapid compression machine for chemical kinetics studies at elevated

579

pressures and temperatures. Combustion Science and Technology, 2007: 179 497–530.

580

27. Desgroux, L., Sochet, L. R., Instantaneous temperature measurement in a rapid compression

581

machine using laser Rayleigh scattering. Applied Physics B, 1995: 61, 69-72

582

28. Mittal, G., Sung, C.-J., Aerodynamics inside a rapid compression machine. Combustion and

583

Flame 2006: 145, 160–180


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

584

29. He, X., Donovan, M. T., Zigler, B. T., Palmer, T. R., Walton, S. M., Wooldridge, M. S., Atreya,

585

A., An experimental and modeling study of iso-octane ignition delay times under homogeneous

586

charge compression ignition conditions. Combustion and Flame 2005: 142, 266-275

587

30. Li, Y., Zhou, C-W., Somers, K. P., Zhang, K., Curran, H. J., The Oxidation of 2-Butene: A

588

High Pressure Ignition Delay, Kinetic Modeling Study and Reactivity Comparison with Isobutene

589

and 1-Butene Proceedings of the Combustion Institute 2017: 36(1) 403–411


Page 34 of 34

Effect of Diluents on the Autoignition of Propane Mixtures Using a

Recommend Documents