Abstract

The combustion of 2,4,4-trimethyl-1-pentene (diisobutylene, C8H16), which is a biofuel and a component of surrogate fuels, is examined in this work. Carbon monoxide time–histories and ignition delay times are collected behind reflected shock waves utilizing a shock tube and mid-infrared laser absorption spectroscopy. Measurements were obtained near 10 atm pressure during stoichiometric oxidation of 0.15%C8H16/O2/Ar. Simulated results from chemical kinetic models are provided, and sensitivity analyses are used to discuss differences between models for both ignition delay times and carbon monoxide formation. In addition, laminar burning speeds are obtained at 1 atm, 428 K, and equivalence ratios, phi, between 0.91 and 1.52 inside a spherical chamber facility. Measured burning speeds are found to be less than that of ethanol over the equivalence ratio span. Burning speed measurements are compared to predictions of chemical kinetic mechanisms and are in agreement for the richest conditions; however, at lean conditions, the model predicts a far slower-burning speed. The maximum burning speed occurs at an equivalence ratio of 1.08 with a magnitude of 0.70 m/s. The current work provides the crucial experimental data needed for assessing the feasibility of this biofuel and for the development of future combustion chemical kinetics models.

1 Introduction

Real fuels such as gasoline, diesel, and aviation fuels can consist of hundreds of individual components whose compositions change based on the origin of the harvested crude oil, the refinery process, and even the season in which the oil was refined. For gasoline, the primary chemical classes are n-alkanes, iso-alkanes, aromatics, naphthenes, and olefins, which can be represented by n-heptane, isooctane, toluene, cyclohexane, and diisobutylene, respectively [1]. Because of concerns about climate change and CO2 emissions, many researchers, including the present authors, have recently helped the development of alternate energy sources, fuels, and power cycles [214]. Diisobutylene (DIB) is also a candidate fuel under the Department of Energy’s Co-Optimization of Fuels and Engines (Co-Optima) initiative 2,3, aiming to optimize new fuels and engine designs simultaneously [11,12]. One of the objectives of this initiative is to present advanced biofuels for high-efficiency spark and compression ignition engines. There have been recent studies of the combustion behavior of several of these Co-Optima fuels using a variety of laboratory techniques [1518]. DIB is composed of three parts 2,4,4-trimethyl-1-pentene (DIB1) and 1 part 2,4,4-trimethyl-2-pentene (DIB2) and is preferred to represent the olefin category because its branched structure is more representative of the branched olefins typically found in gasoline, and it has a similar carbon length to isooctane. However, since DIB is primarily DIB1, this is often times the only species considered in reduced surrogate mechanisms to represent the olefin class [19,20].

The literature on neat DIB kinetics is rather scarce though several studies exist for lower alkenes (e.g., ethylene, propene, butene) [2128]. Metcalfe et al. [29] examined the ignition delay times (IDTs) of DIB1, DIB2, and DIB between 1200 and 1550 K, at 1 and 4 atm, and equivalence ratios from 0.25 to 1.0. It was found that DIB2 is more reactive than DIB1 under the conditions investigated, and DIB responds in a linear fashion to the ratio that the constituent isomers are combined. A chemical kinetic model was developed and was successful at reproducing ignition delay times for the conditions studied with slight underprediction of times at higher temperatures. The authors also found that DIB1 decomposition is highly dependent on isobutene. Mittal and Sung [30] used a rapid compression machine to investigate the ignition characteristics of binary blends of DIB1, toluene, and isooctane. It was found that within the range of 740–1060 K, 15–45 bar, and at an equivalence ratio of 0.75, DIB1 has a higher reactivity among the fuels at temperatures greater than 820 K and lies between isooctane and toluene for lower temperatures. The addition of DIB1 or isooctane to toluene resulted in a much greater reactivity than just neat toluene at similar conditions, with the effect of DIB1 addition being much greater than that of isooctane. Hu et al. [31] measured ignition delay times of DIB1 and extended the previously measured range of Metcalfe et al. [29] to higher pressures and equivalence ratios. Ignition delay times were measured from 2 to 10 atm and at equivalence ratios of 0.5–2.0. Realizing a higher reactivity from the mechanism presented by Metcalfe et al. to the experimental IDT times, the authors performed rate adjustments to reactions sensitive to the IDT, yielding great agreement to the collected data. Zhang et al. [32] measured the laminar burning speed (LBS) of DIB1 with the constant pressure method at 400 K and 450 K. The constant pressure method needs extrapolation to calculate unstretched laminar burning speed, and the authors observed that the nonlinear method performed better than the linear method to calculate the laminar burning speed.

As it is clear, most of the mentioned literature is the only one concerned with the ignition characteristics of neat DIB or one of its isomers; the majority of the literature available focuses on surrogate mechanisms containing some portion of DIB [1,19,33,34]. A recent publication from Li et al. [1] compares experimental IDTs of three DIB-containing surrogates (0.0465–1.0072 mol%) to a quinary surrogate mechanism with a modified DIB submechanism from Metcalfe et al. [29]. The newly constructed model demonstrated superior performance over the previous DIB-containing surrogate mechanisms at conditions of 10, 15, and 20 bar, equivalence ratios of 0.5–2.0, temperatures of 950–1300 K, and exhaust gas recirculation (EGR) loadings ranging from 0% to 60%.

To this point, all chemical kinetic measurements in the literature for DIB or its isomers consist only of IDTs or LBS; however, many other parameters such as turbulent burning speeds, species time–histories, and heat release rate are essential for model validation. Therefore, in the present study, we also obtained measurements of carbon monoxide time–histories using laser absorption spectroscopy in the mid-infrared during the oxidation of DIB1 for model validation and improvement. Laminar burning speeds were measured at 1 atm and 428 K between equivalence ratios of 0.91–1.52 with the constant volume method. Time–histories of CO and ignition delay times were collected on a stoichiometric mixture of DIB1 between 8.5 and 10.9 atm and temperatures of 1186–1414 K.

2 Experiments

2.1 Shock Tube.

Ignition delay times and CO speciation data were collected in the University of Central Florida (UCF) shock tube facility (details in Refs. [17,3538]). Data were collected behind reflected shockwaves 2.00 cm from the end wall with temperatures ranging from 1186 to 1414 K (max uncertainty 0.81%) and pressures from 8.5 to 10.9 atm (max uncertainty 1.50%). Absorbance time–histories of CO were collected using a DBF QCL from Alpes lasers (TO3-L-50) centered at 2046.30 cm−1. CO concentrations were then obtained through the Beer–Lambert relation, Eq. (1), where L (m) is the path length of the absorbing species, R (J/mol*K) is the universal gas constant, σ(λ, T, P) is the absorption cross section of the absorbing molecule, α is the measured absorbance, and T (K) and P (Pa) are the temperature and pressure of the absorbing species, respectively. Cross sections of CO were extracted through the correlation given in Ref. [39], with an uncertainty of 2.60%. Emission of OH* was monitored using a PDA25K GaP detector from Thorlabs with a 310 nm CWL, 10 nm full width at half maximum (FWHM) bandpass filter from Edmund Optics (67-819). Ignition delay times are reported from time zero, the time of minimum signal of the laser due to passage of the reflected shock wave, to the time of maximum OH* emission.
XCO=αRTLPσ(λ,T,P)
(1)

Mixtures were prepared manometrically with 100 and 10,000 Torr capacitance manometers (MKS E27, error 0.1% of reading and MKS 628D, error 0.25% of reading, respectively). Argon and oxygen gases were supplied by Nexair with purities of 99.999%.

2.2 Spherical Combustion Rig.

Laminar burning speeds were collected in UCF’s constant volume, spherical combustion rig, Fig. 1. Our earlier works can provide specific details about the subsections of the combustion chamber, such as the ignition circuit and mixture preparation process [3941].

Fig. 1
Spherical rig setup for laminar burning speed measurement
Fig. 1
Spherical rig setup for laminar burning speed measurement
Close modal

In this study, synthetic air was used as the oxidant and was created with 21.0 ± 0.01% mole of O2 (99.999%, Praxair) and 79.0 ± 0.01% mole N2 (99.999+%, Air Liquide) in the mixing tank. The prepared synthetic air was held for a minimum of 2 hr to achieve uniformity before conducting experiments. DIB1 from Acros Organics (Lot# A0370437, 99% pure) was used for both LBS and shock tube measurements. Table 1 presents detailed information on the studied compound.

Table 1

Physical and chemical properties of 2,4,4-trimethyl-1-pentene [42]

Formulaρ (g/mL)Mw (g/mol)Tboil (K)Pvapor (mmHg)Flash point (K)
(CH3)3CCH2(CH3)=CH20.715112.2137444.7268
Formulaρ (g/mL)Mw (g/mol)Tboil (K)Pvapor (mmHg)Flash point (K)
(CH3)3CCH2(CH3)=CH20.715112.2137444.7268

In a vacuumed tank of less than 0.20 Torr, synthetic air was created to a total pressure of 19,760 Torr. Before injecting fuel, a furnace, which houses the combustion chamber, was set and maintained at 428 K to retain the fuel in a vapor phase. To check the condensation status of injected fuel, the vapor pressure of the fuel and the mass of injected fuel were compared. There was a linear relationship between the injected mass and vapor pressure, which indicates no condensation in the chamber. After checking the condensation status, synthetic air was introduced into the chamber until the desired initial experimental pressure was reached. The equivalence ratio (ϕ) was calculated by partial pressures of the fuel and synthetic air. After obtaining a stable and homogenous mixture in the chamber, the mixture was ignited by a modified NGK-BR4HS spark plug. A ceramic tube was installed on the modified spark plug to ensure the spark started at the top of the electrode. Schlieren imaging was used to capture flame propagation with a high-speed camera (v12.1, Phantom) with a resolution of 512 × 512 pixels at 20,000 fps. The pressure trace during the combustion event was recorded by a dynamic pressure transducer (603B1, Kistler) and charge amplifier (Type 5010, Kistler).

3 Results and Discussion

3.1 Shock Tube Data.

The UCF shock tube was used to measure temporally resolved speciation measurements of CO and ignition delay times of a 0.15%DIB1/1.8%O2/Ar mixture over a temperature range of 1186–1414 K and pressures of 8.5–10.9 atm. Experimental and model ignition delay times are given in Fig. 2. The mechanisms demonstrated by Hu et al. [31] and Metcalfe et al. [29] are in great agreement at lower temperatures; however, a higher reactivity at elevated temperatures is predicted by Metcalfe et al.’s model. The rate constant adjustments to H-atom abstraction reactions of isobutene and DIB1, performed by Hu on Metcalfe’s mechanism, adjusted the reactivity at higher temperatures, improving ignition delay time predictions. The quinary, DIB1 containing gasoline surrogate mechanism from Li et al. [1] predicts a much longer ignition delay time over all temperatures in the displayed region. However, as shown in the study by Li et al. [1], there is good agreement with ignition data for a 0.75% DIB1 mixture performed at various equivalence ratios (0.5–1.0) and pressures (1–4 atm) throughout the entire temperature range (1200–1600 K). In addition, with an increase in pressure from 1 to 4 atm, the data also show an improvement in agreement. To gain insights into the reaction chemistry of this fuel, the discrepancy in modeled IDT compared to the current study’s data (10 atm) will be further discussed, along with a comparison to the mechanism of Hu. Emphasis will be placed on the mechanism of Li et al. due to the practicality of a five-component surrogate model, and if properly validated, it can be immensely beneficial in the design of advanced combustion technologies.

Fig. 2
Ignition delay times compared to various literature mechanisms, Metcalfe et al. [29], Li et al. [1], and Hu et al. [31]. Fuel loading of 0.15%, balanced in oxygen and argon, ϕ = 1.
Fig. 2
Ignition delay times compared to various literature mechanisms, Metcalfe et al. [29], Li et al. [1], and Hu et al. [31]. Fuel loading of 0.15%, balanced in oxygen and argon, ϕ = 1.
Close modal

Measured CO profiles, compared to mechanisms several literature, are shown in Fig. 3. Speciation profiles are truncated at either the end of the test time when a compression or expansion wave is forced into the test section by the interaction of the reflected shock wave and the contact surface or at the time of ignition, where the pressure and temperature rise are significant. Nevertheless, there are often several milliseconds to compare model predictions against experimental data. Figure 3(a) shows excellent agreement with the mechanism presented by Hu et al. [31]. Because the mechanism presented Li et al. [1] overpredicts the ignition delay time, it underpredicts the rate of formation of CO. As the temperature is increased, specifically from Figs. 3(b) and 3(c), the predictive capabilities of the mechanism presented by Hu et al. deteriorate slightly, while the mechanism presented by Li et al. improves mildly, indicating a shift in the dominant chemistry for both models as temperature shifts.

Fig. 3
Comparison of CO time–histories with the studies by Metcalfe et al. [29], Hu et al. [31], and Li et al. [1] at various conditions: (a) T = 1186 K, P = 9.57 atm, (b) T = 1224 K, P = 8.60 atm, and (c) T = 1413 K, P = 10.91 atm
Fig. 3
Comparison of CO time–histories with the studies by Metcalfe et al. [29], Hu et al. [31], and Li et al. [1] at various conditions: (a) T = 1186 K, P = 9.57 atm, (b) T = 1224 K, P = 8.60 atm, and (c) T = 1413 K, P = 10.91 atm
Close modal
First, a brute force sensitivity analysis was conducted to gain an understanding of the reactions governing overall reactivity during DIB1 combustion. This analysis compares reactions from the mechanisms of Li et al. and Hu et al.; since the predictions from the latter are in very good agreement, this gives a good starting point in identifying pertinent reactions and pathways. Figure 4 displays the sensitivity coefficient for reactions highly sensitive to IDT (hydroxide radical, OH). The sensitivity coefficient (S) is calculated through Eq. (2), where (ki) is the reaction rate of reaction i, and τ is the IDT calculated at three multiplicative modifications of ki. A negative sensitivity coefficient indicates that the reaction promotes ignition, while a positive value indicates that a reaction hinders ignition. In Fig. 4, oxidation of the radical isobutenyl, iC4H7, given by iC4H7­ + O2 = aC3H4 + CH2O + OH, is highly sensitive to overall reactivity and is a source of aC3H4, allene. Another sensitive reaction to ignition is the H-atom abstraction of allene by the hydroperoxyl radical, HO2, promoting the formation of the radical 1-propenyl, C3H3, and hydrogen peroxide, H2O2, given by aC3H4 + HO2 = C3H3 + H2O2. There also exists a significant discrepancy in the sensitivities of these reactions between the mechanisms of Hu et al. [31] and Li et al. [1] The mechanisms presented by Li et al. lack significant sensitivity to these very critical elementary reactions for ignition and overall reactivity, which are inevitably connected with CO chemistry. By using this as a starting point, a foundational pathway from iC4H7 to CO and OH* was constructed, as shown in Fig. 5.
Si=(τ(2.0ki)τ(0.5ki)1.5τ(ki))
(2)

From this pathway analysis, the relationship between allene and the critical radicals isobutenyl and 1-propenyl can be used to understand the governing chemistry responsible for the discrepancies observed with the mechanism of Li et al. [1] Concentration time–histories for isobutenyl and allene were output using the mechanisms presented by Hu et al. and Li et al. to understand their limitation or availability. Figures 6(a) and 6(b) show that there is a vast disparity between the predictions, with the mechanism presented by Hu et al. forming nearly four times more isobutenyl radicals and three times more allene. It should be noted that the differences in the overall trend/shape of allene formation indicate that allene chemistry in the mechanism by Li et al. is drastically different and likely a key factor. To better understand the limited formation of iC4H7 and aC3H4 in the mechanism by Li et al., pathway analyses were performed at times where the maximum formation of each intermediate was reached.

Fig. 4
Brute force sensitivity analysis for IDT from the mechanisms of Li et al. [1] and Hu et al. [31] at T = 1250 K and P = 9.5 atm
Fig. 4
Brute force sensitivity analysis for IDT from the mechanisms of Li et al. [1] and Hu et al. [31] at T = 1250 K and P = 9.5 atm
Close modal
Fig. 5
A simplified pathway analysis showing the overarching chemistry of ignition and CO formation
Fig. 5
A simplified pathway analysis showing the overarching chemistry of ignition and CO formation
Close modal
Fig. 6
Concentration time–history predictions of (a) radical iC4H7 and (b) aC3H4 (allene) during oxidation of DIB1 at 1250 K and 9.5 atm
Fig. 6
Concentration time–history predictions of (a) radical iC4H7 and (b) aC3H4 (allene) during oxidation of DIB1 at 1250 K and 9.5 atm
Close modal

Figure 7 shows a pathway analysis of allene formation using both the mechanisms presented by Hu et al. and Li et al., with the directional rate of production (ROP) starting from DIB1. The ROPs (percentage) for outputs of the studied by Hu et al. and Li et al. are marked in black and blue, respectively. From this, one can observe a glaring difference in the amount of fuel decomposition between the two mechanisms, with 68.8% and 85.7% in the studies by in Li et al. and Hu et al. This difference in fuel decomposition may partially explain the lack of iC4H7 radicals that, in turn, limit the radical involvement via critical pathways (e.g., iC4H7 + O2 = aC3H4 + CH2O + OH). By limiting iC4H7 radical concentration, aC3H4 formation is hindered, which is necessary for reaction pathways to proceed for ignition and CO formation. In addition, the relationship between iC4H8 and iC4H7 from the mechanism by Li et al. shows discrepancies in ROP; moreover, the brute force sensitivity analysis shows the opposite effect on overall reactivity when compared to the Hu mechanism, specifically the reactions iC4H8 + H → iC4H7 + H2 and iC4H7­ + H → iC4H8. From Fig. 5, it is also clear that consumption of aC3H4 resulting in the C3H3 radical is critical in CO formation. To better understand this chemistry, the pathway analysis of aC3H4 consumption leading to important intermediates necessary for CO production was performed, shown in Fig. 8. These pathway analyses for each mechanism were performed at the time near the max formation of aC3H4, and as shown in Fig. 6(b), this time is rather early in the mechanism by Li et al.

Fig. 7
Pathway analysis of aC3H4 formation for the mechanisms presented by Hu et al. [31] (black, top) and Li et al. [1] (blue, bottom) at the time of max formation of iC4H7, near 12 µs in the study by Li et al. [1] and near 39 µs in the study by Hu et al. [31]
Fig. 7
Pathway analysis of aC3H4 formation for the mechanisms presented by Hu et al. [31] (black, top) and Li et al. [1] (blue, bottom) at the time of max formation of iC4H7, near 12 µs in the study by Li et al. [1] and near 39 µs in the study by Hu et al. [31]
Close modal
Fig. 8
Pathway analysis of C3H3 formation for the mechanisms by Hu et al. [31] (black, top) and Li et al. [1] (blue, bottom) at the time of max formation of allene, near 220 µs in the study by Li et al. [1] and near 2.4 µs in the study by Hu et al. [31]
Fig. 8
Pathway analysis of C3H3 formation for the mechanisms by Hu et al. [31] (black, top) and Li et al. [1] (blue, bottom) at the time of max formation of allene, near 220 µs in the study by Li et al. [1] and near 2.4 µs in the study by Hu et al. [31]
Close modal

It is clear that a reduced iC4H7 radical pool, when compared to the mechanism of Hu et al., ultimately results in insufficient aC3H4 formation that is required to participate in the reactions presented in Fig. 8. The ROP of C3H3 in reactions of aC3H4 + OH → H2O + C3H3 and aC3H4 + HO2 → H2O2 + C3H3 shows a major difference between the mechanisms presented by Hu et al. and Li et al.; the former reaction also being sensitive to promoting IDT (from Fig. 4). This discrepancy in C3H3 and aC3H4 ROP between the two mechanisms explains the insufficient sensitivity to IDT as shown from the brute force analysis and ultimately describes how this chemistry is connected to the formation of CO.

As was discussed from the pathway analysis shown in Fig. 7, the limited fuel decomposition is speculated to initiate much of the subsequent discrepancies in the chemistry when comparing the two mechanisms. Although plotting the fuel decomposition time–history predictions, there is a minimal difference; however, the rate of fuel decomposition is slightly less in the mechanism presented by Hu et al. Max formation of the iC4H7 radical occurs slightly sooner in the mechanism by Li et al. when compared to that by Hu et al., and so temporally, less fuel has been consumed according to the mechanism presented by Li et al. In the mechanism presented by Li et al., multiple reaction rates of DIB1, iC4H8, and iC4H7 were updated such that Arrhenius parameters (reaction coefficients) matched those found in the mechanism by Hu et al. It should be noted that the iC4H8 and iC4H7 reactions updated were those mentioned previously found through the pathway analysis in Fig. 7 and from the brute force sensitivity analysis in Fig. 4. With these updates, the performance of mechanism by Li et al. was then assessed by comparing the predictions of CO time–histories, as shown in Fig. 9. Although the updates show an improvement, this indicates that the overall discrepancies observed in predicted IDTs and CO time–histories are not completely dependent on fuel decomposition chemistry. Thus, the subsequent C3H3 and aC3H4 chemistries in the mechanism by Li et al. were investigated and updated such that corresponding reaction rates matched those found in the mechanism presented by Hu et al. During this process, many of the reactions found in the mechanism presented by Li et al. did not have reverse reaction rates present, although some were present and commented out in the file. It should be noted that two reactions were added, which were not originally present in the mechanism by Li et al. These include the following:
aC3H4+M=C3H3+H+M
pC3H4+M=C3H3+H+M

Shown in Fig. 10 are CO time–history comparisons of the updated mechanism presented by Li et al. with the experimental data gathered in the current study. Very good agreement is achieved at the lower temperature condition of 1186 K; however, as the temperature increases, the predictability of the updated mechanism deteriorates, with vast disparity at the higher temperature of 1413 K. Although the poor agreement exists at elevated temperatures, the sensitive early low-temperature chemistry is well captured. For example, in Fig. 10(a), there seems to be very early CO chemistry occurring in the experimental profile (i.e., the early “hump”), which the updated mechanism captures with good agreement. Interestingly, this early CO chemistry is not prominent at elevated temperatures, although it is still mildly present. The early stages of CO formation are in very good agreement throughout the temperature spread; however, as time proceeds, this rate of formation is expedited, which is indicative of increased global reactivity (IDT) and is observed in data presented in Fig. 11. As stated previously, several of the changed reactions required input for reverse reaction rates. Even still, many (hundreds) reactions do not include reverse reaction rates, which may be a major contributing factor to the significant disagreement in the updated mechanism at the later stages of CO formation. Moreover, a multitude of isobutene and isobutenyl radical reactions are likely missing and/or do not match the most up-to-date rates found in the mechanism presented by Hu et al. Implementing these changes will require extensive modifications to Li et al.’s current quinary surrogate mechanism.

Fig. 9
A model comparison of CO time–history predictions at 1186 K and 9.5 atm with updates to DIB1, iC4H8, and iC4H7 reactions
Fig. 9
A model comparison of CO time–history predictions at 1186 K and 9.5 atm with updates to DIB1, iC4H8, and iC4H7 reactions
Close modal
Fig. 10
Comparison of CO time–histories with experimental data, and original and updated mechanisms by Li et al. [1] at (a) T = 1186 K, P = 9.57 atm, (b) T = 1224 K, P = 8.60 atm, and (c) T = 1413 K, P = 10.91 atm
Fig. 10
Comparison of CO time–histories with experimental data, and original and updated mechanisms by Li et al. [1] at (a) T = 1186 K, P = 9.57 atm, (b) T = 1224 K, P = 8.60 atm, and (c) T = 1413 K, P = 10.91 atm
Close modal
Fig. 11
Experimental IDTs compared with predictions from the updated mechanism presented by Li et al. [1]
Fig. 11
Experimental IDTs compared with predictions from the updated mechanism presented by Li et al. [1]
Close modal

Furthermore, from the comparison shown in Fig. 3, the mechanisms of Li et al. and Hu et al. exhibit a shift in chemistry at elevated temperatures, where the predictions from Li et al. are in better agreement, and predictions from Hu et al. indicate increased reactivity. In general, all the mechanism predictions (with the original mechanism of Li et al.) in Fig. 3 show overreactivity at elevated temperatures. With sensitive allene and 1-propenyl chemistry matching the Hu et al.’s mechanism in the update, this seems to further support deficiencies in the isobutene submechanism as a limiting factor. It is hypothesized that the isobutene chemistry (C0—C4 kinetic mechanism) incorporated in the mechanisms compared in this study is not accurately described, resulting in poor predictions of experimental targets for DIB1 oxidation. The consumption and formation captured in isobutene chemistry are critical in accurately predicting these various experimental combustion targets of higher-order hydrocarbons, and continued efforts dedicated to elucidating these issues are necessary.

Uncertainty in reported CO mole fractions is calculated as a time-dependent quantity via the root-mean-square (RMS) of the uncertainties present in the parameters of Beer’s law, similar to the method presented in Ref. [39] except reported concentrations are not carried through the ignition event due to large changes in temperature and pressure in the current study. Root-mean-square uncertainties calculated in this manner for CO profiles range between 11.5% and 19.6% and are displayed as transparent regions in Fig. 3. Uncertainties in ignition delay times were calculated via Hu et al.’s mechanism [31] and Chemkin-PRO [43]. This was done by calculating the IDTs with the mechanism at T5 and P5 conditions and then calculating the IDTs at T5 + UT5, P5 + UP5, and T5-UT5, P5-UP5, where UT5 (0.75–0.81% of T5) and UP5 (1.47–1.50% or P5) are the uncertainties in temperature and pressure of state 5 conditions calculated from normal shock relations. By this means, the RMS uncertainty in IDT is found to be 12%.

3.2 Laminar Burning Speeds.

The constant volume method with a multizone model was used to calculate LBS. For a more detailed explanation of this multizone model, the reader is directed to our earlier works [41,44,45]; this article provides only a brief explanation of the model. Two zones, burned and unburned, are considered in the multizone model. Equilibrium of thermodynamic properties in the burned zone was calculated for many discrete regions within the zone, [46], while the unburned mixture was considered as a single region. The cantera software [47], with Metcalfe et al.’s mechanism [29], was used to calculate the thermodynamic properties to obtain the burned mass fraction of species in each time-step.

O’Donovan and Rallis [48] and Bradley and Morley [49] derived an expression for the flame radius (Eq. (3)). Equation (3) is a function of the vessel radius (Ro), mass fraction (X), initial pressure (Pi), and instantaneous pressure (P) from a pressure sensor. Assumptions for this equation include: the flame is smooth and has a spherical shape, all gases are ideal, and the buoyancy of the fluids is negligible during the combustion process. Hill and Hung [50] and Takizawa et al. [51] derived Eq. (4), which is a theoretical expression of LBS with Ro, P, and Pi, flame radius (Rf), and burned mass fraction rate (dx/dt).

Laminar burning speed measurements in a constant volume chamber typically need to consider the effect of flame stretch. However, LBS measurements were made in the stretch-free region, allowing this effect to be dropped from the calculations [52,53]. Chen et al. [52] observed that the LBS stretch effect could be negligible when P/Pi is greater than 1.5. Since LBS was calculated with linear fit from LBS value of larger than P/Pi = 1.5 to before flame instability, calculated LBS represents unstretched burning speed.
Rf=RO(1(1X)(PiP)1γ)13
(3)
Su=RO33Rf2(PiP)1γdXdt
(4)
The combined standard uncertainty for LBS measurements can be calculated by Eq. (5) [54]:
uc(Su)=i=1N(dfdxi)2u2(xi)
(5)
where uc(Su) is the combined standard uncertainty of LBS, df/dxi is the sensitivity coefficient, and xi (P, T, Φ, Model) is the concentration. The sensitivity coefficient is considered as unity. For pressure factor, a dynamic Kistler 603B pressure transducer with ±1.00% accuracy, a Kistler charger meter type 5015A with ±0.50% accuracy, and the human error in filling to initial pressure with ±0.55% were considered. Two K-type thermocouples on the exterior of the combustion chamber have ±0.75% standard accuracy, where the discrepancy from the target temperature is less than ±0.5%. Equivalence ratio (Φ) is calculated by partial pressures using an MKS 628F Baratron and MKS E27 Baratron, which have an uncertainty of ±0.25% of reading and 0.12%, respectively. The number of zones (±30 zones) and fitting range (±4 K) of LBS calculations were considered for model accuracy. The accuracy quantity for the model consists of the number of zones and the fitting range of LBS calculations, which yields uncertainties of ±0.60% and ±1.00%, respectively. Considering the uncertainty in the individual parameters, the LBS combined standard uncertainty is calculated to be 1.95%.

Figure 12 shows the DIB1 flame propagation image for an equivalence ratio of 1.1, which showed the highest LBS as captured with the high-speed camera. In Fig. 12, the left, center, and right images represent times at 2 ms, 4 ms, and 6 ms, respectively. The flame shape maintains its smooth and spherical shape as a laminar flame until it is no longer within the experimental viewing window on the combustion chamber.

Fig. 12
Flame propagation of DIB1, equivalence ratio of 1.1, T = 428 K, P = 1 atm
Fig. 12
Flame propagation of DIB1, equivalence ratio of 1.1, T = 428 K, P = 1 atm
Close modal

Figure 13 presents LBS of DIB1 at 1 atm and 428 K in different equivalence ratios. The maximum LBS of DIB, 0.70 m/s, occurred at an equivalence ratio of 1.08. The present DIB1 LBS was compared with that presented in the study by Zheng et al. [32], which was conducted at an initial temperature and pressure of 450 K and 1 atm, respectively. Since higher unburned temperature results in higher adiabatic temperature, higher unburned temperature results in higher LBS [55]. Even though the initial temperatures were different, the present study and the previous findings were in good agreement, notably up to an equivalence ratio of 1.3. The present work was also compared with burning speed calculations of ethanol under the same initial conditions since ethanol is one of the standards of biofuels. DIB1 LBS is slower than ethanol LBS in the whole equivalence ratio range. For the simulations, the mechanisms presented by Metcalfe et al. [29], Hu et al. [31], and Li et al. [1] were used in Chemkin-PRO [43]. In the fuel-rich region, the present work was in the best agreement with the mechanism by Hu et al., whereas in the lean region, the present data were in best agreement with the mechanism by Metcalfe et al.

Fig. 13
The laminar burning speed of DIB as a function of equivalence ratio at 428 K and 1 atm. The present work was compared with the studied by Zheng et al. [32] and burning speed simulation from the studied by Metcalfe et al. [29] and Hu et al. [31].
Fig. 13
The laminar burning speed of DIB as a function of equivalence ratio at 428 K and 1 atm. The present work was compared with the studied by Zheng et al. [32] and burning speed simulation from the studied by Metcalfe et al. [29] and Hu et al. [31].
Close modal
A sensitivity analysis was conducted to deepen the understanding of the burning speed reactions during DIB1 combustion at 1 atm and 428 K with the mechanism by Hu et al. [31]. Equation (6) was used to calculate the normalized sensitivities (si) of the laminar burning speed (Su) for the reaction rate constants (ki) [47]. Figures 14(a)14(c) show the top 15 highest burning speed sensitivity coefficients at equivalence ratios of 0.8, 1.1, and 1.5. Figure 14(d) shows a comparison of the top 15 sensitivity coefficients for an equivalence ratio of 1.0 with equivalence ratios of 0.8, 1.1, and 1.5. Figure 14 indicates that the reaction of H + O2 → O + OH is the highest positive sensitivity in all equivalence ratios. The second highest sensitivity is CO + OH → CO2 + H for all equivalence ratios excluding 1.5. The second highest sensitivity, at an equivalence ratio of 1.5, is HCO + M → CO + H + M. Products of DIB1 can explain the reason. The production of CO2 decreases because of the competition for oxygen molecule with the equivalence ratio increasing. Isobutylene (iC4H8) formation is present in all cases with a negative sensitivity coefficient. There is no reverse of the sensitivity coefficient, but the values of sensitivity reaction and orders are different in all cases.
si=kiSudSudki
(6)
Fig. 14
(a)–(d) Burning speed sensitivity analysis in different equivalence ratio at 1 atm and 428 K
Fig. 14
(a)–(d) Burning speed sensitivity analysis in different equivalence ratio at 1 atm and 428 K
Close modal

4 Conclusions

Carbon monoxide time–histories and ignition delay times were collected behind reflected shock waves during the combustion of 2,4,4-trimethyl-1-pentene (diisobutylene). Measurements were compared to the predictions of several chemical kinetic models in the literature, and the model of Hu et al. was found to perform very well over most conditions investigated. Discrepancies between model and experiment are present in the mechanism by Li et al. throughout the temperature range, although a better agreement is apparent at higher temperatures. Laminar burning speeds of 2,4,4-trimethyl-1-pentene were collected in a constant volume, spherical combustion chamber over a range of equivalence ratios, and a burning speed sensitivity analysis was conducted at different equivalence ratios. Burning speeds were found to be slightly less than ethanol velocities over the same conditions. The maximum laminar burning speed was recorded at an equivalence ratio of 1.08 with a velocity of 0.7 m/s. A burning speed sensitivity analysis was also conducted at different equivalence ratios.

Sensitivity and pathway analyses were used to discuss the differences in CO formation and overall reactivity (IDT) between the mechanisms presented by Hu et al., and Li et al. The sensitive chemistry of the quinary gasoline surrogate mechanism by Li et al. was compared to that by Hu et al. The disparity in chemistry involving isobutene and allene, in addition to isobutenyl and 1-propenyl radicals, was found. From this, sensitive reactions to IDT and CO formation were targeted and changed in the mechanism by Li et al. to match with the mechanism by Hu et al. This showed improved CO predictions during low-temperature chemistry, as well as early formation stages. However, the updated mechanism showed poor predictions of overall CO formation at higher temperatures. Deficiencies in the isobutene kinetics are likely to present in the mechanisms, which will need to be further investigated and updated in the future work. These effects will become exacerbated as the temperature increases. The multitude of reactions should be compared between the mechanisms presented by Hu et al. and Li et al. to elucidate the complexities of the quinary surrogate mechanism. As modifications are necessary to the surrogate mechanism, an emphasis on capturing low-temperature chemistry is paramount for its practical application in simulating the combustion of a real fuel.

Acknowledgment

This contribution was identified by Gaurav Agrawal (Exxon Mobil) as the Best Presentation in the session Developments in Alternative Fuels and Enabling Technologies I of the 2019 AIChE Annual Meeting in Orlando. The authors acknowledge help from Owen Pryor (presently at Southwest Research Institute, San Antonio, TX) for his assistance with experiments. This material is mainly based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Co-Optima initiative (Award Numbers DE-EE0007982, DE-EE0007984).

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Conflict of Interest

There are no conflicts of interest.

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