A Shock-Tube Autoignition Study of Jet, Rocket, and Diesel Fuels

Mar 12, 2019 - Ignition delay times were measured for gas-phase jet fuel (Jet-A), rocket propellant (RP-1), and diesel fuel (DF-2) in a heated, high-p...
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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

A Shock-Tube Autoignition Study of Jet, Rocket, and Diesel Fuels Sulaiman A. Alturaifi,*,† Rachel L. Rebagay,† Olivier Mathieu,† Bing Guo,‡ and Eric L. Petersen† †

Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States Department of Mechanical Engineering, Texas A&M University at Qatar, PO Box 23874, Doha , Qatar

‡

Energy Fuels Downloaded from pubs.acs.org by UNIV OF CAMBRIDGE on 03/19/19. For personal use only.

S Supporting Information *

ABSTRACT: Ignition delay times were measured for gas-phase jet fuel (Jet-A), rocket propellant (RP-1), and diesel fuel (DF2) in a heated, high-pressure shock tube. The measurements were performed behind reflected shock waves for each fuel in air over a temperature range of 785−1293 K, a pressure range of 7−26 atm, and at two equivalence ratios, ϕ = 0.5 and 1.0. Ignition delay time was determined using the pressure and OH* chemiluminescence (∼307 nm) signals at the endwall location. Measured ignition delay times for Jet-A are in excellent agreement with the available historical data from the literature. Overall, the results showed few differences in ignition delay times between any of the three fuels over the range of temperatures studied. High-temperature correlations were developed to accurately predict the ignition delay times of the three fuels. The experimental measurements for Jet-A and DF-2 were modeled using several chemical kinetics mechanisms utilizing different surrogate mixtures. To the authors’ knowledge, this study presents the first gas-phase ignition delay time measurements for RP-1. In addition, the data presented herein expand the archival data of Jet-A and DF-2 to a broader range of conditions.

1. INTRODUCTION While the consumption of liquid fuels, essentially derived from fossil fuels, is expected to rise from 95 million barrels per day in 2015 to 113 million barrels per day in 2040,1 there are increasing concerns about CO2 and pollutant emissions associated with the combustion of these fuels. The majority of liquid fuels are consumed in the transportation sector, and improving the combustion efficiency of liquid fuels will therefore lead to several economic and environmental benefits. Improving the design of the different types of engines used in transportation relies on many factors such as heat transfer, gas dynamics, multiphase flows, and flow turbulence. Nevertheless, the predominant factor on the overall efficiency of these engines is controlled by combustion chemistry. Usually, combustion engine designers rely on numerical simulation software packages that utilize chemical kinetics models to predict the combustion chemistry. Therefore, developing accurate models is vital to the advancement of combustion engines. It is worth mentioning that liquid petroleum-based fuels, such as kerosene and diesel, are blends of thousands of different hydrocarbon molecules. This fact makes the detailed kinetics mechanisms of liquid fuels too complicated to be fully developed because there are many intermediate species and numerous possible reactions. The method currently used to tackle this problem consists of simplifying these mechanisms by using surrogate models. These surrogate models contain a much smaller number of hydrocarbons, but formulated in such a way that the chemical and physical properties of the parent liquid fuel are mimicked.2−4 To validate their models, developers of kinetics mechanisms typically utilize fundamental combustion parameters, such as ignition delay time, as targets. One commonly used device to measure ignition delay times of a gas-phase fuel/oxidizer mixture is a shock tube. However, liquid fuels typically have low vapor pressures, which complicate gas-phase shock-tube © XXXX American Chemical Society

experiments. One successful method to test a low-vaporpressure liquid fuel is to heat the shock tube, thus increasing the vapor pressure of the liquid fuel which allows gas-phase shock-tube experiments.5−8 Another method is to use an aerosol shock tube where the liquid fuel is introduced into the shock tube in micron-sized droplets.9 Several research groups have been active in performing shock-tube ignition delay time measurements of liquid fuels such as those of interest herein. Dean et al.5 were the first to test kerosene-based fuels in a heated shock tube, where they measured the ignition delay times of Jet-A/air mixtures at fuelto-air equivalence ratios of ϕ = 0.5, 1.0, and 2.0 at reflectedshock pressures of 8.5 atm and a temperature range of 1000− 1700 K. Later, Vasu et al.,6 Wang and Oehlschlaeger,7 and Zhukov et al.8 used heated shock tubes to further study the ignition delay times of several types of jet fuels. Haylett et al.9 used an aerosol shock tube to measure the ignition delay time of diesel fuel (DF-2) and O2 mixtures diluted in argon. Zhang et al.10 investigated the ignition behavior of a rocket propellant fuel (RP-3) and air mixture. More recently, Davidson et al.,11 De Toni et al.,12 and Burden et al.13 conducted studies on ignition delay times of jet, diesel, and rocket fuels using aerosol and heated shock tubes. Table 1 shows a summary of the recent ignition delay time studies conducted on conventional, petroleum-based liquid fuels. The current study aims to improve the understanding of liquid-fuel combustion by providing new ignition delay times of three liquid fuels, namely Jet-A, RP-1, and DF-2. The liquid fuels were mixed with air, and the measurements were made at elevated temperatures and pressures to mimic practical engine conditions. This study provides ignition delay time data at new conditions for both Jet-A and DF-2. Moreover, this study Received: December 11, 2018 Revised: February 19, 2019

A

DOI: 10.1021/acs.energyfuels.8b04290 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Recent Autoignition Studies on Conventional, Petroleum-Based Liquid Fuels5−15 authors

year

device

fuels

P [atm]

T [K]

Dean et al. Vasu et al. Haylett et al. Kumar et al. Wang et al. Zhukov et al. Zhang et al. Zhu et al. Davidson et al. De Toni et al. Burden et al. current study

2007 2008 2009 2010 2012 2014 2014 2015 2017 2017 2018 2019

heated ST heated ST aerosol ST RCM heated ST heated ST heated ST heated ST aerosol & heated ST RCM, heated ST heated ST, CVSCC heated ST

Jet-A Jet-A, JP-8 DF-2 Jet-A, JP-8 Jet-A, JP-8 Jet-A RP-3 JP-8 Jet-A, JP-8, JP-5, RP-2, DF-2, gasoline Jet-A-1 Jet A (A-1, A-2, A-3) Jet A, RP-1, DF-2

8.5 17−51 2.3−8.0 7, 15, 30 8−39 10, 20 1−20 2−44 6−60 7−30 10−80 7−26

1000−1700 715−1229 900−1300 650−1100 651−1381 1040−1380 650−1500 1047−1520 1000−1400 670−1200 620−1310 785−1293

mixture ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ

= = = = = = = = = = = =

0.5, 1.0, 2.0 in air 0.5, 1.0 in air 0.3−1.35 (21% O2 in Ar) 0.42−2.26 in air 0.25−1.5 in air 0.5, 1.0, 2.0 in air 0.2, 1.0, 2.0 in air 0.25−2.2 in air and in Ar 0.85−1.15 in air 0.3−1.3 in air 1.0 in air 0.5, 1.0 in air

Table 2. Properties of the Three Studied Fuels22,23 composition by volume fuel

military POSF#

MWavg (g/mol)

H content (% wt)

average formula

aromatics (%)

iso-paraffins (%)

n-paraffins (%)

cycloparaffins (%)

Jet-A RP-1 DF-2

10325 5235 12758

159 168 182

14.5 14.4 13.3

C11.4H22.1 C12.0H24.1 C13.1H24.0

16.5 0.2 27.6

31.5 37.2 23.5

21.2 0.5 14.1

30.8 62.1 34.8

presents the first ignition delay time measurements for RP-1 fuel. These measurements add to the available data on liquid fuels in a global effort to create a benchmark for researchers to validate liquid fuel chemical kinetics mechanisms. In this paper, a description of the experimental apparatus along with a sample measurement is given, followed by the ignition delay time results for the three fuels. The results are then compared with the previous measurements from other groups whenever available. Then, high-temperature Arrhenius correlations are developed for each fuel. Finally, the predictions from six kerosene-based kinetics models are obtained and compared with the experimental measurements.

recommended by Pei et al.,20 which is composed of 77% n-dodecane and 23% m-xylene, was utilized. To perform a high-pressure test, two types of diaphragms were used, polycarbonate and aluminum, which produced post-reflectedshock pressures of around 10 and 20 atm, respectively. Helium was typically used as the driver gas, although a mixture of 5−20% mol of N2 in He was used whenever necessary to achieve longer test times, as per the method of Amadio et al.21 2.2. Heating System. To accommodate gas-phase testing of lowvapor pressure fuels, the shock tube has been recently fitted with custom-made heating jackets composed of five independently controlled elements. The jackets were supplied by BriskHeat and can reach temperatures up to 200 °C. Additional heating elements and fiberglass insulation were applied on the mixing tank and manifold to prevent any possible fuel condensation. Temperature measurements along the axial direction of the driven section were collected by placing several thermocouples along the driven section. The thermocouples were extended to approximately measure the air temperature in the center of the shock tube. Uniform temperature distribution along the last 3 m of the driven section is shown in Figure 1 for several set-points. The temperature variation, within the

2. EXPERIMENTAL SETUP 2.1. Description of the Apparatus. Ignition delay times for JetA, RP-1, and DF-2 (the properties of each fuel are shown in Table 2) in air were measured in the High-Pressure Shock Tube at Texas A&M University. The stainless steel shock tube has a 4.72 m long driven section with an inner diameter of 15.24 cm and a 2.42 m driver section with an inner diameter of 7.62 cm. A detailed description of the shock tube can be found in Aul et al.16 The pressure at the test section was observed via two piezoelectric pressure transducers, a Kistler 603B1 located at the sidewall (1.6 cm from the endwall) and a PCB P113A located at the endwall. To measure the shock wave speed, the shock tube is equipped with six, fast-response piezoelectric pressure transducers (PCB P113A), equally spaced, located along the last 2 m of the driven section. The signal from each transducer was recorded using a digitizer/ oscilloscope data acquisition board (GaGe model CSE8382), and they were used to determine the velocity of the incident shock as it propagated through the driven section. A linear fit was then used to extrapolate the incident shock speed at the endwall position. The initial temperature of the gas (prior to the shock) in the driven section was obtained via a thermocouple placed at the endwall. Post-shock conditions (T5 and P5) were determined using the extrapolated incident shock speed, the initial gas temperature, one-dimensional shock wave relations, and thermodynamic data for each fuel. For JetA, thermodynamic data were taken from the Burcat and Ruscic database.17 For RP-1, thermodynamic data were taken from the recommended values of MacDonald et al.,18 which is based on the surrogate mixture of Huber et al.19 For DF-2, the surrogate

Figure 1. Temperature distribution along the last 3 m of the driven section. Solid lines are 50, 75, 100, and 150 °C set-points. Dashed lines are low-temperature variation limits of ±1 °C, and dotted lines are high-temperature variation limits of ±2 °C. B

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Energy & Fuels thermocouple uncertainty, was limited to ±1 °C for the set-points of 50, 75, and 100 °C, while a limit of ±2 °C was observed for the 150 °C set-point. 2.3. Mixture Preparation. Preparing accurate mixtures for a lowvapor pressure fuel requires knowledge of the fuel’s vapor pressure. Thus, a simple experiment was conducted by injecting various amounts of liquid fuel into the mixing tank and observing the resulting pressure. Figure 2 shows these measurements for the three fuels used in this study at a mixing tank temperature of 120 °C. The three fuels exhibit similar behavior such that a linear region, where the pressure is directly proportional to the volume of fuel injected, for low volumes. This region is followed by a nonlinear region where the injection of additional fuel leads to only a slight increase in pressure. This nonlinear region could be explained by the nonvaporization of the heaviest components of the liquid fuel (saturating vapor pressure limit). Hence, the mixtures’ preparation should be limited to the linear region of the curves in Figure 2. This limit in the amount of fuel that can be vaporized makes high-pressure shock-tube experiments often impractical, particularly for DF-2 and RP-1, which display lower vapor pressure limits. This impracticality is due to the inability to make fuel/air mixtures in the mixing tank with a high enough pressure to fill the shock tube to the desired levels. To overcome this limitation, the fuel/air mixtures were prepared directly in the driven section of the shock tube for the present study. To perform high-pressure shock-tube experiments, an air mixture, defined herein as 21% O2 and 79% N2 by volume, was prepared in a separate, 74 L heated stainless steel mixing tank. The fuel/air mixture was prepared directly in the driven section of the shock tube by first pumping down this section to a pressure of ∼1 × 10−5 Torr using a combination of a rotary vane pump (Varian DS402) and a turbomolecular pump. Then, the liquid fuel was injected into the evacuated, heated driven section via a septum/needle system, and the fuel was allowed to evaporate for 10 min. The amount of fuel injected corresponded to a pressure of fuel within the linear section of Figure 2. The air mixture from the mixing tank was then introduced into the driven section through two separate locations along the driven section to create turbulence and to increase mixing for a homogeneous mixture. To reach the desired equivalence ratio, the fuel and air were controlled manometrically via two, heated pressure capacitance manometers, 0−100 and 0−1000 Torr (MKS 631D), and a hightemperature pressure transducer (ESI HI2300). Fuel and air stoichiometry was defined using the average molecular formulas given in Table 2. The mixture was then kept anywhere between 10 and 20 min before running the experiment to allow for mixing. The leak rate for the tube was measured regularly, and a rate of 3 mTorr/ min was achieved. This leak rate coupled with the mixing time induced an uncertainty in the equivalence ratio of the mixtures that is well within the total uncertainty of the ignition delay time. To illustrate this estimated uncertainty, the worst case would be the

mixture with the highest equivalence ratio (ϕ = 1.0) and the lowest total pressure (P1 = 164.8 Torr) with a mixing time of 30 min. This worst-case combination would lead to an equivalence ratio of ϕ = 0.99 instead of ϕ = 1.0. For this study, the mixing tank, manifold, and tube temperatures were kept anywhere between 90 and 145 °C. Ignition delay times obtained using this method were compared with ignition delay times obtained by making the fuel/air mixture directly in the mixing tank for Jet-A and showed good agreement (discussed later). Other groups have been successful using this method of preparing mixtures directly in the shock tube.24 A schematic of the experimental setup is shown in Figure 3. 2.4. Optical Diagnostic. In this study, ignition delay time was defined as the time from the arrival of the incident shock wave at the endwall to the time of the steepest rise of OH* extrapolated to the baseline. Ignition delay times were measured using the pressure transducer and OH* chemiluminescence near 307 nm at the endwall location. The pressure transducer was used to determine time zero of the ignition delay time by monitoring the sharp increase in signal corresponding to the arrival of the incident shock wave at the endwall. Additionally, a photomultiplier tube (Hamamatsu 1P21) and a UV bandpass filter (310 nm center and 10 nm full width at half maximum) located at the endwall were used to record OH* time histories. The extrapolation of the steepest slope to the baseline of the OH* signal is the time corresponding to the ignition of the fuel/ oxidizer mixture. Figure 4 shows a representation of an ignition delay time experiment. The overall, estimated uncertainty in the ignition delay time is around ±20%. The main factors contributing to this uncertainty are the uncertainty in determining the incident-shock velocity (which translates to an uncertainty in the postshock temperature) and the uncertainty in preparing mixtures from liquid fuels.

3. RESULTS Ignition delay time (τign) measurements for Jet-A, RP-1, and DF-2 in air were conducted at two pressures of ∼10 and ∼20 atm for two equivalence ratios of ϕ = 0.5 and 1.0. All of the ignition delay times measured and their associated conditions behind reflected shock waves are provided in the Supporting Information. 3.1. Validation of the Mixture Preparation Method. To validate the mixture preparation method utilized in this study, τign measurements of Jet-A/air at 10 atm and an equivalence ratio of ϕ = 1.0 were used as a target. The measurements were obtained via two mixture preparation methods; the method explained above (denoted as tube mixture) and a second, classical, method where mixtures are prepared in a separate mixing tank (denoted as tank mixture). In the tank mixture preparation method, the liquid fuel was injected into the evacuated, heated mixing tank via a septum/ needle system. The fuel was allowed to evaporate for 10 min before the addition of air to reach the final desired mixture. Then, the mixtures were left anywhere between 1 and 4 h to allow mixing before conduction the experimental measurements. Figure 5 shows a comparison between the τign of Jet-A obtained via the two methods. To allow a direct comparison, the measurements have been scaled using the appropriate scaling laws of pressure (τign ∝ P−1).6 The results showed that the ignition delay times obtained from both methods are indistinguishable from each other. This favorable comparison shows no significant influence of the mixing procedure on τign. That is, both methods produced suitably well-mixed conditions. In addition, the uncertainty in the equivalence ratio via the tube mixture method is negligible (as explained earlier). Therefore, all the other ignition delay time experi-

Figure 2. Measured tank pressure for a range of liquid fuel injected volume at a mixing tank temperature of 120 °C. C

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Figure 3. Schematic of the shock-tube setup.

Figure 6. Ignition delay time results of Jet-A (black symbols), RP-1 (blue symbols), and DF-2 (red symbols) in air at a pressure of 10 atm and equivalence ratios of ϕ = 0.5 (open symbols) and ϕ = 1.0 (closed symbols). All data are scaled using the pressure dependence factor in Table 3.

Figure 4. Method of determination of the ignition delay time.

Figure 5. Comparison of ignition delay time measurements using two different methods of preparing mixtures. Figure 7. Ignition delay time results of Jet-A (black symbols), RP-1 (blue symbols), and DF-2 (red symbols) in air at a pressure of 20 atm and equivalence ratios of ϕ = 0.5 (open symbols) and ϕ = 1.0 (closed symbols). All data are scaled using the pressure dependence factor in Table 3.

ments reported in this study were carried out using mixtures that were directly prepared in the shock tube (tube mixture). 3.2. Ignition Delay Time Results. The ignition delay times for Jet-A, RP-1, and DF-2 in air at equivalence ratios of ϕ = 0.5 and 1.0, pressures ranging from 7 to 26 atm, and a wide range of temperatures are shown in Figures 6 and 7. The ignition delay time measurements were classified into two sets: measurements close to 10 atm (7.6−12.3 atm) and measurements close to 20 atm (16.0−26.4 am). Ignition delay times of both sets have been scaled to their respective common pressures via τign ∝ P−B, where B is the pressure coefficient in Table 3 (discussed later). Figure 6 shows the ignition delay times of the three fuels in air at 10 atm for two equivalence ratios of ϕ = 0.5 and 1.0. These measurements mainly span the high-temperature regime

Table 3. Constants and Standard Errors for Ignition Delay Time Correlations (Eq 1)

D

fuel

A

B

C

E [kcal/mol]

standard error (%)

Jet-A RP-1 DF-2

0.123 0.206 0.217

1.078 0.996 0.959

0.365 0.335 0.183

24.41 22.78 22.57

13 11 13

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Energy & Fuels (above 1000 K), where τign decreases as the temperature increases for all the cases. For all the three fuels, there is no effect of the equivalence ratio at temperatures above 1150 K as all the measurements collapse on top of each other, while slight differences in ignition are observed at lower temperatures with the fuel-lean case producing longer ignition delay times. Figure 7 shows the ignition delay times of the three fuels in air at around 20 atm for two equivalence ratios of ϕ = 0.5 and 1.0. At this pressure, the effect of equivalence ratio is evident with the fuel-lean condition showing longer ignition delay times. At ϕ = 0.5 and 20 atm, the measurements were extended to reach the negative-temperature-coefficient (NTC) region, which is visible for temperatures lower than 900 K. At both pressures, 10 and 20 atm, the autoignition results for Jet-A, RP1, and DF-2 in air showed similar results at all equivalence ratio conditions. 3.3. Comparison with Previous Measurements. 3.3.1. Jet Fuel. Ignition delay time measurements of different blends of Jet-A have been reported in the literature. Davidson et al.11 and Burden et al.13 measured τign for Jet-A in air at 10 and 20 atm, respectively. They tested the same blend of jet fuel that has been used in the present study (POSF 10325). When compared with their measurements, τign measurements are in a very good agreement with both studies as shown in Figure 8. All the ignition delay times have been normalized using τign ∝ P−1. A representation of the uncertainty in the τign is shown for the two extremes of temperature investigated herein. Other research groups have conducted ignition delay time tests on different blends of Jet-A. For example, Wang et al. and Vasu et al. measured the ignition delay time of a different blend of Jet-A (POSF 4658). A comparison between the ignition delay time measurements from the two studies is shown in Figure 9 at ϕ = 0.5 and 20 atm (scaled using τign ∝ P−1). Both blends of Jet-A produced similar ignition delay times, as the reported data of the two studies are in a good agreement with the current study within the experimental uncertainty. 3.3.2. Diesel Fuel. There are several ignition delay time studies for DF-2 reported in the literature. However, only a few studies have been conducted using shock tubes for hightemperature combustion. Haylett et al.25 measured τign of an unknown blend of DF-2 using an aerosol shock tube at a pressure of around 7 atm and an equivalence ratio close to 0.5. The ignition delay times have been scaled to 10 atm using τign

∝ P−0.82 which was provided in the Haylett et al. study. In addition, Haylett et al. provided an equivalence ratio scaling factor of τign ∝ ϕ−0.7, which was used to scale their data to an equivalence ratio of 0.5. This adjustment allows direct comparison with the current measurements as shown in Figure 10. The τign measurements are in reasonable agreement for the low-temperature region (below ∼1150 K). However, there is considerable scatter in the measurements reported by Haylett at higher temperatures, and only the top portion data (longest ignition delay times) are in good agreement with our data below 1150 K. 3.3.3. Rocket Fuel. There are only a few ignition delay time studies for rocket fuels in the literature. Davidson et al.26 measured τign for two different blends of rocket fuel (RP-2). These blends are POSF 7688 and POSF 5433, which have average molecular formulas of C12.0H24.1 and C12.6H25.6, respectively. The measurements were conducted over a wide range of temperature and at a pressure of ∼13 atm. A simple pressure dependence factor of τign ∝ P−1 was used to scale the ignition delay times to 10 atm to allow direct comparison. Figure 11 shows that the difference in τign of RP-1 and RP-2 is not large. 3.4. Ignition Delay Time Correlations. Multiple linear regression analysis is utilized to develop correlations to predict ignition delay times at the high-temperature “linear” region for each fuel. The linear regression of the dependent variable

Figure 8. Ignition delay time comparison of the same blend of Jet-A (POSF10325) in air at ϕ = 1.0 with Davidson et al.11 and Burden et al.13

Figure 10. Ignition delay time comparison for DF-2 in air with Haylett et al.25 at a normalized pressure of 10 atm and an equivalence ratio of ϕ = 0.5.

Figure 9. Ignition delay time comparison with Vasu et al.6 and Wang et al.7 for different blends of Jet-A in air at 20 atm and ϕ = 0.5.

E

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where τign is the ignition delay time in μs, P is the pressure in atm, ϕ is the equivalence ratio, E is the activation energy in kcal/mol, T is the temperature in K, and R is the universal gas constant (1.987 × 10−3 kcal/mol K). The mean activation energy E, the coefficients A, B, and C, and the standard error for each fuel are given in Table 3. These correlations cover a range of temperature of 950−1400 K, a pressure range of 7−26 atm, and an equivalence ratio range of ϕ = 0.5−1.0. The correlations are plotted against high-temperature ignition delay times at 10 and 20 atm and ϕ = 0.5 and 1.0 for each fuel in Figure 12a−c. As the ignition delay times of the three fuels were in close agreement, multiple linear regression analysis of all the data collected for the three fuels was used to develop a correlation that predicts the high-temperature ignition of the three fuels. Equation 2 shows the correlation of jet, rocket, and diesel fuel ignition in air for high temperatures (above 950 K), an equivalence ratio between 0.5 and 1.0, and a pressure range of 7−26 atm. All of the ignition delay time measurements of the three fuels are scaled to 20 atm and ϕ = 1.0 using the factors provided in this correlation and plotted against the correlation in Figure 12d. The standard error of this correlation is 15%, which is higher than the standard errors of the individual fuel correlations. ÅÄÅ 23.48 ÑÉÑ ÑÑ τign = 0.1705 P−1.037ϕ−0.292expÅÅÅÅ Ñ ÅÇ RT ÑÑÖ (2)

Figure 11. Ignition delay time comparison of different rocket fuel types in air at an equivalence ratio of ϕ = 1.0 and a pressure of 10 atm.26

(logarithmic of the ignition delay time) versus the three independent variables (logarithmic of the pressure, logarithmic of the equivalence ratio, and the inverse of temperature) was used to determine a correlation for each fuel. The effect of all three independent variables was equally weighted. All three fuels, Jet-A, RP-1, and DF-2, could be correlated for highertemperature ignition above (∼950 K) using an Arrhenius expression of the form ÅÄÅ E ÑÉÑ ÑÑ τign = A [P ]−B [ϕ]−C expÅÅÅÅ Ñ ÅÇ RT ÑÑÖ (1)

3.5. Kinetic Mechanisms Comparisons. The kinetic modeling of petroleum-based liquid fuels was performed using

Figure 12. Ignition delay time measurements (symbols) and correlations (lines) for (a) Jet-A, (b) RP-1, and (c) DF-2. (d) Ignition delay time measurements for all three liquid fuels/air normalized to 20 atm and ϕ = 1.0 using the pressure and equivalence ratio dependence factors of eq 2. The correlation of eq 2 is shown as a solid line. F

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Energy & Fuels Table 4. Kinetics Models and Surrogate Mixture Compositions target fuel

kinetics mechanism

jet fuel

Honnet et al.27

jet fuel

Malewicki et al.28

jet fuel

Narayanaswamy et al.29

diesel fuel

Pei et al.20

diesel fuel

Frassoldati et al.30

diesel fuel

Yao et al.31

surrogate composition (mol %)

details

77.17% n-decane 22.83% 1,2,4-trimethylbenzene 40.41% n-dodecane 29.48% iso-octane 22.83% n-propylbenzene 7.28% 1,3,5-trimethylbezene 30.3% n-dodecane 48.5% methylcyclohexane 21.2% m-xylene 77% of n-dodecane 23% m-xylene 63.1% n-decane 36.9% methyl-naphthelene 100% n-dodecane

122 spp 900 rxn 2080 spp 8310 rxn

369 spp 2691 rxn 163 spp 887 rxn 123 spp 1017 rxn 54 spp 269 rxn

Figure 13. Prediction from the kinetics model of (a) Honnet et al.,27 (b) Malewicki et al.,28 and (c) Narayanaswamy et al.29 for jet fuel in air. (d) Comparison of the three models against the experimental measurements is done at 20 atm and ϕ = 0.5.

The ignition delay time predictions from these kinetics mechanisms were compared with the experimental shock-tube measurements. These predictions were carried out using a closed, homogenous batch reactor via CHEMKIN 19.0 by constraining the volume and solving the energy equation. None of the kinetics mechanisms accounted for OH* reactions; therefore, the mechanism of Hall and Petersen32 was added to allow prediction of OH* time histories. Ignition delay time predictions were obtained by extrapolating the steepest slope of OH* from the model simulation to zero. 3.5.1. Jet Fuel Models. Figure 13a−c shows a comparison (for the high-temperature region, above 950 K) of Jet-A/air ignition delay times and the predictions from Honnet et al.,27

surrogate models. These surrogate models contain few numbers of hydrocarbons to mimic the physical and chemical properties of the targeted fuel. The ignition delay times obtained in this study provide validation targets for kinetics mechanisms. Several groups have been active in developing kinetics mechanisms and surrogate models for jet and diesel fuels. However, there is a lack of kinetics mechanisms specifically assembled for rocket fuels. In this study, the predictions from six kinetics mechanisms for kerosene-based fuels are considered. Table 4 shows that these kinetics mechanisms along with the surrogate composition used to represent the real fuel. G

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Figure 14. Predictions from the kinetics models of (a) Pei et al.,20 (b) Frassoldati et al.,30 and (c) Yao et al.31 for diesel fuel in air. (d) Comparison of the three models against the experimental measurements at 20 atm and ϕ = 0.5.

Malewicki et al.,28 and Narayanaswamy et al.29 The Honnet et al.27 mechanism (shown in Figure 13a) predicts autoignition within the experimental uncertainty (±20%) at the highpressure conditions (20 atm) while under predicts the autoignition at the low-pressure condition (10 atm) of about a factor of 2. In Figure 13b, the Malewicki et al.28 model shows good predictions at the 10 atm condition. At 20 atm, the autoignition prediction from this model is well within the uncertainty of the measurements for temperatures above 1100 K, but overpredicts autoignition (approximately a factor of 2) at lower temperatures. In Figure 13c, the predictions from the model of Narayanaswamy et al.29 showed good agreement at both 10 and 20 atm over most of the temperature range, but it slightly overpredicts (a factor of less than 2) autoignition at 20 atm for temperatures less than 1050 K. Figure 13d shows a direct comparison of the predictions from three models of jet fuel. The comparison is made against the experimental measurements of Jet-A in air at 20 atm and ϕ = 0.5 which covers both the high-temperature and the NTC regions. The model of Honnet et al.27 accurately captured the transition from the high-temperature to the NTC region, while the model of Malewicki et al.28 showed slight overprediction of the transition region. Nevertheless, both models predict the autoignition in the NTC region well within the uncertainty of the experimental measurements. On the other hand, the model of Narayanaswamy et al.29 overpredicts autoignition at the NTC region (approximately a factor of 2). 3.5.2. Diesel Fuel Models. In Figure 14a−c, the autoignition measurements of DF-2 in air are compared with the predictions from the models of Pei et al.,20 Frassoldati et al.,30 and Yao et al.31 at the high-temperature regime. Figure

14a shows that the model of Pei et al.20 produced excellent autoignition predictions over all of the high-temperature experimental conditions. The model prediction of Frassoldati et al.30 (shown in Figure 14b) overpredicts (approximately a factor of 2) the autoignition in all the presented condition with the exception of the autoignition at 10 atm and ϕ = 1.0, which is within the experimental uncertainty. In Figure 14c, the model prediction of Yao et al.31 at 10 atm shows good prediction (within the experimental error) but slightly under predicted the autoignition at temperatures above 1200 K. In addition, at 20 atm and ϕ = 1.0, the model showed good prediction of the autoignition within the experimental error. Figure 14d presents a comparison at the high-temperature and NTC regions of the three model predictions for diesel fuel in air at 20 atm and ϕ = 0.5. The predictions from the Pei et al.20 model captured both the transition and the NTC regions well within the uncertainty of the measurements while the model of Frassoldati et al.30 overpredicts (factor of 2) both regions. Finally, the model of Yao et al.31 overpredicts (by approximately a factor of 2) the transition from the hightemperature to the NTC region, while capturing the autoignition at the NTC regime within the measurement uncertainty.

4. DISCUSSION The development of detailed chemical kinetics mechanisms is vital to accurately model the combustion of real, complex fuels such as kerosene and diesel. Due to the complex nature of these real fuels, their modeling is performed using surrogate mixtures. However, ignition delay time data of the real fuel are used to validate the kinetics mechanisms and the selected H

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Energy & Fuels

correlations were obtained via multiple linear regression analysis of the experimental data with three independent variables of pressure, equivalence ratio, and temperature. Pressure scaling factors were provided for each fuel and performed well to scale ignition delay times from other groups. The predictions from several chemical kinetics mechanisms of jet and diesel fuels were obtained and compared to the experimental measurements. The current study presents new ignition delay time measurements for the rocket fuel (RP-1) and expands the available literature data for both jet fuel (JetA) and diesel fuel (DF-2).

surrogate mixture. In recent years, few research groups have performed experimental measurements of ignition delay times of real fuels. These measurements are mainly conducted in shock tubes and rapid compression machines. However, most of the reported experimental studies were performed on jet fuels with little attention to other types of fuels. In addition, most of these studies were performed at moderate pressures ranging from 10 to 30 atm, while some engines (especially gas turbine engines) running on similar fuels operate at higher pressure. Autoignition studies covering wider ranges of pressure and various fuel/air equivalence ratios would be very valuable to verify earlier studies and provide additional validation targets for the kinetics mechanisms. Moreover, current shock-tube techniques allow for species time history measurements. Experimental measurements for selected radical species such as OH and CH3 would provide important validation targets for kinetics mechanism developers. In almost all of the previous studies, heating systems are used to allow gas-phase testing of the low-vapor pressure fuels. This method relies on testing the fuel in the gas-phase, and there is limitation associated with this method on the amount of fuel concentrations. Recently, new experimental techniques have emerged to overcome this limitation and allow testing of the low-vapor pressure fuels without the use of a heating system. One promising method is the use of aerosol shock tubes, where the low-vapor-pressure fuel is introduced to the shock tube in micron-sized particles using a nebulizer. Previous studies showed that the liquid fuel particles can be uniformly distributed along the driven section of the shock tube. Yet, the passing of the incident shock wave evaporates the liquid fuel particles and allows testing of the fuel in the gas phase. However, this method is in the early stages of development, and there are only handful of studies reported in the literature. To assist in the development of this method, the experimental measurements obtained via heated shock tubes (such as the present study) can be used as baselines for comparison with similar low-vapor-pressure fuels using the aerosol shock-tube method.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b04290.

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Ignition delay time experimental data for Jet-A, RP-1, and DF-2 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: Sulaiman.turaifi@tamu.edu. ORCID

Sulaiman A. Alturaifi: 0000-0002-2514-3900 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported primarily by the Qatar National Research Fund NPRP award 8-1358-2-579. The authors would like to thank Dr. T. Edwards from the Air Force Research Laboratory for providing the fuels used in this work.

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REFERENCES

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5. CONCLUSIONS Ignition delay time measurements for Jet-A, RP-1, and DF-2 in air were obtained using a high-pressure shock-tube facility. These liquid fuels have low-vapor pressures, and a new heating system was utilized to allow for shock-tube, gas-phase experiments. The measurements were conducted at elevated temperatures and pressures to mimic practical engine conditions. The experiments were performed over a pressure range of 7−26 atm and a temperature range of 785−1293 K for two equivalence ratios, ϕ = 0.5 and 1.0. Endwall pressure and OH* emission signals allowed accurate definition of ignition delay times. The ignition delay times had low scatter and were in excellent agreement with previously published data when comparisons were possible. Such good agreement validates both the present data and the accuracy and repeatability of the data that exist in the prior literature. Longer test times (above 2500 μs) were obtained via a driver gas-tailoring method to explore the NTC regime for the three fuels. The ignition delay times for the three fuels were found to be similar at all the tested conditions. At 20 atm and ϕ = 0.5, a clear NTC regime was observed for the three fuels at temperatures below about 900 K. Correlations to accurately predict the high-temperature ignition delay times of the three fuels were presented. The I

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