Laminar Burning Characteristics of 2-Methylfuran Compared with 2,5

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Laminar burning characteristics of 2-methylfuran compared with 2, 5-dimethylfuran and isooctane Xiao Ma, Changzhao Jiang, Hongming Xu, Shijin Shuai, and Haichun Ding Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef401181g • Publication Date (Web): 19 Aug 2013 Downloaded from http://pubs.acs.org on August 29, 2013

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Laminar burning characteristics of 2-methylfuran compared with 2, 5-dimethylfuran and isooctane 1

Xiao Ma1, Changzhao Jiang , Hongming Xu1,2*, Shijin Shuai2 , Haichun Ding1 1. Mechanical Engineering Department, University of Birmingham, Birmingham, B15 2TT, UK; 2. Department of Automotive Engineering, State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, 100084, China; Abstract The gasoline alternative 2-methylfuran (MF), has attracted the attention of fuel researchers due to a breakthrough in its production technology. Both 2, 5-dimethylfuran (DMF) and 2-methylfuran can be produced by dehydration and hydrogenolysis of fructose. Little is known about MF’s laminar burning characteristics though its combustion performance in spark ignition (SI) engines has been proved to be attractive by previous studies. Using high speed schlieren photography, this work examines the laminar burning characteristics of MF-air mixtures with varying temperatures (60oC, 90oC and 120 oC) and equivalence ratios (0.6-1.1) at 0.1MPa initial pressure in a constant volume vessel. The stretched flame speeds are determined by the outwardly spherical flame method. The unstretched flame speed, Markstein length, laminar burning velocity, flame thickness, density ratio, Markstein number and laminar burning flux of MF combustion at different equivalence ratios and temperatures are then deduced and compared to those of DMF and isooctane. The results show that the stretched flame propagation speed of MF decreases as the stretch rate increases. The unstretched flame speed of MF is 10%-30% faster than that of DMF and 10%-50% faster than that of isooctane. The maximum unstretched flame speed of MF under the three Page 1 ACS Paragon Plus Environment

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temperatures tested occurs at an equivalence ratio (Φ) of 1.1, whereas the maximum unstretched flame speeds of DMF and isooctane occur at equivalence ratios between 1.1 and 1.2. The Markstein lengths for the three fuels decrease with the increase of equivalence ratio and the Markstein length of MF is generally smaller than those of DMF and isooctane. The burning velocity of MF is the fastest amongst the three fuels at all temperatures and equivalence ratios tested. The flame thickness of MF is the smallest at all test conditions compared to DMF and isooctane. The laminar burning flux of MF increases with the increase of the initial temperature and exhibits peak values between equivalence ratios of 1.1 and 1.2.

Keywords: 2-methylfuran, Markstein length, laminar burning velocity, Marksetin number, flame instability

1. Introduction With the increasing demand to reduce the dependence on fossil fuels, great efforts have been made by researchers concerning renewable biomass derived alternative fuels in both improving their production methods and understanding their physicochemical properties. For many years, bio-ethanol has been the market leading gasoline alternative1-4 due to its renewable nature and mature production methods, and numerous reports on investigations of the use of bio-ethanol in SI and CI engines are available.5-13 However, bio-ethanol has its limitations mainly associated with its low energy density and high energy consumption in the production process.

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The gasoline alternative 2-methylfuran (MF) is a heterocyclic compound derivative of furan and a breakthrough in the research of a new production method of MF was made in 2009, by Román and Luque, and Zhao et al. independently.14-16 There are generally two main oxygen removal steps to convert fructose into MF: firstly, 5-hydroxymethylfurfural (HMF) is produced by removing three oxygen atoms through dehydration; secondly, two oxygen atoms in HMF are removed through hydrogenolysis; thus, MF and DMF are produced in the same process.17 MF is considered as a sustainable bio-fuel candidate because fructose is abundant in bio-mass.

The properties of a bio-fuel determine its combustion performance in engines, and for MF, they are very attractive as shown by the comparison in Table 1 with those of DMF, isooctane and gasoline. The initial boiling point of MF is 64.7ºC and the density is 913.2 kg/m3 at 20ºC. The octane number of MF (103, RON) is higher than gasoline (96.8, RON) and this means that MF can be used at high engine compression ratios for high thermal efficiencies. Its latent heat of vaporization (358.4kJ/kg) is close to that of gasoline (373kJ/kg). More importantly, its energy density (28.5MJ/L) is very close to that of gasoline (31.9MJ/L) and this means that with the same volume of fuel, MF contains 34% more energy compared to the market leading bio-fuel ethanol.

DMF is another promising biofuel candidate which has previously received worldwide attention. The author’s group is the first to study the performance of DMF as an alternative fuel in the engine and is one of the first few groups that studied the properties of MF. Zhong Page 3 ACS Paragon Plus Environment

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et al19, Daniel et al.23 and Wu et al.24 investigated the engine performance of DMF compared with gasoline and ethanol in a single cylinder direct injection spark ignition (DISI) engine. Tian et al.20 studied the spray characteristics of DMF using Phase Doppler Particle Analyzer (PDPA). Their work showed that DMF is very similar to gasoline compared with ethanol. Several studies of DMF by other groups are also reported.26, 27

As for investigation of MF, fewer publications are available. Wang et al.25 examined the engine performance, regulated emissions, particulate matters and unregulated emissions of MF compared to DMF, ethanol and gasoline. It was found that the combustion characteristics of MF are significantly different from DMF even though they have similar chemical structure. The impact of MF on mixture formation and combustion was examined in a DISI engine by Thewes et al.28. The auto-ignition characteristics of MF in a SI engine were studied by Ohtomo et al.29. A detailed chemical kinetic modeling study of MF oxidation was established and compared to an experiment by Somers et al.30

Laminar flame propagation characteristics are important fundamental physicochemical properties for a fuel; they are the basic data required for combustion modeling and also can be used in validating the chemical reaction mechanisms of the fuel.31 A large number of publications can be found on laminar flame investigation of various fuels or fuel alternatives. 32-35

Wu et al.22 and Tian et al.21 studied stretched laminar flame speeds, Markstein lengths,

unstretched laminar flame speeds and laminar burning velocities of DMF-air mixtures and the latter also compared the results with ethanol and gasoline. The laminar propagation speeds of Page 4 ACS Paragon Plus Environment

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DMF and gasoline were found to be quite close but lower than that of ethanol. Until now, there is no report on the laminar flame propagation characteristics of MF, although the combustion and emissions of MF in the engine have been investigated. In this work, the schlieren photography method was used for laminar flame visualization whereby the laminar flame speed, Markstein length and laminar burning velocity were calculated from an in-house image processing MATLAB program. The results of MF were compared with those of DMF and isooctane under the same test conditions.

2. Experiment set up and procedures The schlieren experimental setup has been described by Tian et al.21 as shown in Figure 1. A constant volume vessel, with two circular quartz windows (100mm in diameter) at each side and eight heating units at each corner of the vessel, was used for the experiment.

The

temperatures of the air/fuel mixtures were controlled by a close-loop temperature controller, which can control the 8 heating units. A Bosch AJ133 injector, controlled by a Bosch ECU, was mounted on the top of the vessel to inject the fuel into the vessel chamber. The injection system was calibrated before the test and the calibration data was saved. A pair of electrodes was installed in the middle of the vessel to initiate the spark. For safety reasons, a pressure release valve was used and the release limitation was set to 0.7MPa.

A 500W xenon lamp was coupled with a lens group and a pin hole was placed in front of them to generate a point light source for the test. The point light was then guided by the first concave mirror to produce parallel lights which passed through the vessel and formed the test Page 5 ACS Paragon Plus Environment

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field. At the other side of the vessel, another concave mirror was used to integrate the parallel lights. The light path was then cut by a knife edge at the focus for the schlieren effects. The images were recorded by a Phantom V710 high-speed camera at a sample rate of 10 kHz and a resolution of 800x800 pixels. The camera was synchronized with the spark timing.

For each test, compressed air was used to scavenge the burned mixtures, and then the vessel chamber was connected to the ambient air until the air temperature inside the vessel rose to the set point. When the temperature was settled after sealing the vessel by closing the valves, the fuel was injected via the injector into the vessel to form homogenous air/fuel mixtures. The amount of fuel injected was accurately controlled by the injection duration and the number of injections using the calibration data. After the injection, the chamber was left undisturbed for 5 minutes to ensure the homogeneousness of the mixtures and the attainment of an approximate quiescent condition. The mixtures were then ignited by an electrode discharge and the signal also triggered the high-speed camera. After combustion, the chamber was flushed with fresh compressed air and the test could start over again. The tests were performed at temperatures elevated from 60°C to 120°C and equivalence ratios varied from 0.6 to 1.4 with under 0.1MPa initial pressure (pressure before the injection of fuel). Each point of the experiments was repeated at least three times and the average data was used in the analysis.

3. Laminar flame speed and Markstein length The schlieren images were processed by an in-house developed MATLAB code to Page 6 ACS Paragon Plus Environment

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characterize the laminar flame quantitatively. In order to obtain accurate results and to avoid the impact of flame quenching from the electrodes, all the original images were turned by 45° and the flame radii were calculated by detecting the density change between the burned and unburned gas in four directions (see Figure 2). The averaged results from four directions were used in order to produce accurate results.

In order to avoid the effect of spark ignition disturbance36-38, pressure increase during combustion39, and space confinement40, only the images with flame radii between 6mm and 18mm were used to determine the stretched laminar flame speed, Sn, via the following formula: Sn=dru/dt

(1)

where ru is the flame radius and t is the time after ignition. By knowing the stretched laminar flame speed, the stretch rate (α) could be deduced as36, 41: α=2Sn/ru

(2)

At the early stage of flame propagation, there is a linear relationship between the stretch rate and flame speed which can be expressed as36, 41: Sn=Ss-Lb*α

(3)

where Ss is laminar flame speed, and Lb is Markstein length. Ss is determined by extrapolating Sn to zero stretch rate. Lb is the negative value of the gradient of the flame propagation speed against the stretch rate curve. The laminar burning velocity (ul) can be obtained from the equation 36, 41: µl =Ss*ρb/ρu

(4) Page 7

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where ρb and ρu are burned and unburned mixture densities, respectively. Assuming the pressure is constant, the burned (ρb ) and unburned gas densities (ρu ) can be found from the conservation of mass equation: ρb/ρu =Vu/Vb=nuTu/nbTb where nu and Tu

and

(5)

nb are mole numbers of reactants and products Tb are initial and adiabatic flame temperatures

The adiabatic flame temperatures are calculated in this study using HPFLAME42, which incorporates the Olikara & Borman equilibrium routines43. The flame thickness is calculated by the ratio of kinematic viscosity (v) to laminar flame velocity via36: δl=v/ µl

(7)

The Markstein number is calculated as the ratio of Markstein length and the flame thickness: Ma= Lb/ δl

(8)

The laminar burning flux, which is the eigenvalue of the flame propagation, is calculated by 36

: f= µl *ρu

(9)

4. Results and discussion

4.1. Result validation In order to validate the system setup and procedures used, laminar burning velocities of isooctane-air mixtures at 0.1MPa initial pressure and elevated temperatures were calculated and compared with the data from the literature in Figure 3. It is shown that the current Page 8 ACS Paragon Plus Environment

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measurement has a good agreement with the widely accepted result of Bradley et al.36, and in addition, and Hasse et al.45 This validates the present experimental setup and methodology.

4.2. Flame morphology The schlieren images for MF, DMF and isooctane at stoichiometric conditions and the initial temperature of 90°C are shown in Figure 4. MF flame propagation is the fastest and isooctane flame propagation speed is the slowest. Due to the quenching effect from the electrodes the flame propagation speed is always slower along the direction of the electrodes than the vertical direction thus the flame is not perfectly spherical. Small wrinkling also appears near the electrodes, but it does not affect the overall shape of the flame.

Based on the repeated schlieren imaging, it was found that the early stage of the flame is greatly affected by the spark energy. While as the flame approaches the vessel boundary, the shape of the flame becomes distorted with the flatter surface at the vertical sides due to the influence of the internal geometry. 40

4.3 Flame propagation and Markstein length 4.3.1 Stretched flame propagation speed Figure 5 shows the stretched flame propagation speed versus stretch rate for the three fuels at different equivalence ratios and 120oC initial temperature. Both the stretched propagation speed and the stretch rate are calculated by detecting the flame front from four directions, as shown in Figure 2. With respect to time, the flame expands in the vessel; the flame stretch Page 9 ACS Paragon Plus Environment

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rate becomes smaller due to the inverse relationship between the flame stretch rate and flame radius. The close-to-linear relationship between the flame stretch rate and the flame radius can be observed at a large stretch rate. For instance, in Figure 5a, it can be seen that except for Φ=0.6, all the results show a good linearity at large stretch rates. The result for Φ=0.6 is non-linear at the large stretch rate and this might be because the combustion for this lean condition at the early stage is unstable. At a relatively smaller stretch rate, the acceleration of the stretched flame speed decreased or even stopped, thus the trends become non-linear. This demonstrates that the geometry of the vessel affects the flame propagation.

4.3.2 Unstretched flame propagation speed and Markstein Length The unstretched flame propagation speed was obtained by extrapolating the unstretched flame propagation speed to zero stretch rate and the Markstein length was obtained by calculating the gradient of the stretched flame propagation speed to stretched rate slope. Figure 6 shows the unstretched flame speed of the three fuels at different temperatures and equivalence ratios. For all the temperatures (60°C, 90°C and 120°C), MF has the fastest unstretched flame propagation speed at all the equivalence ratios. For instance, at 120°C and under the same equivalence ratio, the unstretched flame propagation speed of MF is about 15% faster than that of DMF and about 20%-50% faster than that of isooctane. The unstretched flame propagation speeds of the three fuels increase significantly with the increase of initial temperatures, as expected. The maximum unstretched flame propagation speed of MF occurs at slightly rich region when Φ=1.1 for the three temperatures tested, although the unstretched flame propagation speed for Φ=1.2 is only slightly slower than that at Φ=1.1. However for Page 10 ACS Paragon Plus Environment

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DMF and isooctane, the maximum unstretched flame propagation speeds occur between the equivalence ratios of 1.1 and 1.2.

Markstein length indicates the effect of stretch rate on flame propagation speed, and characterizes the diffusion-thermal instability31, 47 of the fuel. It depends on the Lewis number of the fuel in lean mixtures or oxidizer in rich mixtures.48 For heavy hydrocarbon–air mixtures, Markstein length decreases with the increase of equivalence ratio; while for light hydrocarbon–air mixtures it increases with the increase of the equivalence ratio.49 Figure 7 shows that the Markstein lengths of MF and the other two fuels at tested temperatures all decrease with the increase of the equivalence ratios, and this observation is in agreement with the above theory since these three fuels are all heavy hydrocarbon fuels. The results also show that the Markstein length of MF is significantly smaller than those of DMF and isooctane at equivalence ratios lower than 1.1. However, at equivalence ratios higher than 1.1, the differences between MF and the other two fuels are within the error range. So, it can be concluded that the diffusion-thermal instability of MF is higher than DMF and isooctane at low equivalence ratios and it is nearly the same compared to DMF and isooctane at equivalence ratios higher than 1.1.

With respect to initial temperature under the same equivalence ratio conditions ， the differences between the Markstein lengths of MF for 90°C and 120°C at different equivalence ratios are very small. At 60°C, MF has bigger Markstein lengths compared to the cases under the other two temperatures (90°C and 120°C) at low equivalence ratios Page 11 ACS Paragon Plus Environment

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(0.7-1.0), but the differences become very small at high equivalence ratios (1.1-1.4).

4.4 Adiabatic flame temperatures and laminar burning velocities The adiabatic flame temperatures for MF, DMF and isooctane under 120°C initial temperature at varying equivalence ratios are shown in Figure 8. MF has the highest adiabatic flame temperature, followed by DMF and then isooctane. The adiabatic flame temperatures of MF and DMF reach their peaks at the equivalence ratio of 1.1. The same trend can be observed for isooctane except that the peak occurs under the condition closer to a stoichiometric ratio of 1.0. For hydrocarbon-air mixtures the adiabatic flame temperature peaks at the rich mixture side due to the product dissociation and reduced amount of heat release.46

The laminar burning velocity is a strong function of the equivalence ratio and initial temperature of the reactants.50 It is the speed at which the flame is advancing into the unburned mixture. Figure 9 shows the laminar burning velocities versus the equivalence ratios at different initial temperatures. The laminar burning velocities of MF under varying initial temperatures reach their peaks in the equivalence ratio range of 1.1 to 1.2 and this is correlated to the state of the adiabatic flame temperatures. For the other two fuels, a similar trend can be observed. Compared with DMF and isooctane, MF has the highest laminar burning velocities at all conditions. For instance, at 90 °C initial temperature and the equivalence ratio of 1.2, the laminar burning velocities of MF, DMF and isooctane are 69cm/s, 61cm/s and 56cm/s, respectively. Based on Tabaczynski’s theory described in the literature 50, Page 12 ACS Paragon Plus Environment

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the flame burning rate in a spark ignition engine is largely affected by the laminar burning velocity and turbulence intensity in the combustion chamber. With the same engine configuration and operation conditions, the fuel which possesses higher laminar burning velocities burns faster in the cylinder. The work of Wang et al.25 proved that the combustion duration of MF is much shorter than that of DMF and gasoline in a DISI engine, which leads to higher indicated thermal efficiency than the other two fuels. In addition, the laminar burning velocities for all the three fuels increase with the initial temperature. For MF, the laminar burning velocity at 120 °C is about 16%-18% faster than that at 90 °C and 34%-40% faster than that at 60 °C within the range of equivalence radio 0.7-1.1.

4.5 Flame instability and the affecting factors In order to understand the effect of the influencing factors on the flame front instability, it is necessary to give a short review on the theory. According to the literature31, 51, there are mainly two types of flame surface instabilities acting on the flame front when the laminar burning velocity is relatively high: the diffusion-thermal instability and the hydrodynamic instability. The diffusion-thermal instability is a result of diffusion in the flame front while the hydrodynamic instability is a result of thermal expansion of the gas upon crossing the flame. The diffusion-thermal instability is characterized by the Markstein length and the Markstein number. The hydrodynamic instability of the flame front, which is induced by the density transition across the flame front, is characterized by the flame thickness and the density ratio. The increase of the density ratio or the decrease of the flame thickness indicates the promotion of this kind of instability. Page 13 ACS Paragon Plus Environment

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4.5.1 Flame thickness and density ratio Figure 10 shows the flame thickness and density ratio versus equivalence ratio for MF at different initial temperatures and the three fuels at an initial temperature of 90oC. For MF, the flame thickness is not sensitive to variation of initial temperature whereas the density ratio decreases with the increase of the initial temperature, as shown in Figure 10a. It is indicated in Figure 10b that MF has the smallest flame thickness at all equivalence ratios compared to DMF and isooctane. Also, it can be seen that the density ratio of MF and DMF are similar and both of them are slightly higher than that of isooctane. Therefore, the hydrodynamic instability of MF is the highest amongst the three fuels followed by DMF and then isooctane and it decreases with the increase of the initial temperature.

4.5.2 Markstein number and burning flux Markstein number, which characterizes the effect of local heat release on the flame morphology and the flame front curvature, quantifies the response of a laminar flame to the stretch and can be used to indicate the diffusion-thermal stability of laminar and turbulent flame front. Figure 11 shows the Markstein number versus equivalence ratio for MF at different initial temperatures and the three fuels at an initial temperature of 60oC.

The

change of the Markstein numbers of MF does not show a very clear trend in regard to the temperature. This means that the initial temperature is not the main parameter affecting the Markstein number of MF.

Also, it should be noted that the flame thicknesses of MF at three

initial temperatures are nearly the same, as shown in Figure 10a. So the trend of the Page 14 ACS Paragon Plus Environment

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Markstein numbers of MF under different initial temperatures is mainly determined by the Markstein lengths and the differences among three temperatures are due to the error accumulated in the calculation process using combined parameters.

As shown in Figure 11b, even though the three fuels are all gasoline alternatives, they show noticeable differences in their Markstein numbers. Isooctane has the biggest Markstein number at equivalence ratios lower than 1.2, followed by DMF and then MF. However, at equivalence ratios higher than 1.2, the Markstein number of the three fuels are nearly the same. From Figure 10b, it can be seen that at all equivalence ratios the flame thickness of isooctane is the highest and the flame thickness of MF is the smallest. So, both the Markstein length and the flame thickness are affecting the Markstein number. However, regardless of the higher flame thickness of isooctane and DMF compared to MF, the Markstein number of MF is still the smallest amongst the three fuels at equivalence ratios lower than 1.2. This indicates that the Markstein length is playing the main role and the flame thickness is playing the secondary role on influencing the Markstein numbers of the three fuels.

Figure 12 shows the burning flux versus equivalence ratio for MF at different initial temperatures and the three fuels at an initial temperature of 120oC. The laminar burning flux of MF increases with the increase of the initial temperature. With increasing initial temperature, the increase of the laminar burning velocity of MF is the main effect on the laminar burning flux compared to the decreasing density of the MF-air mixture. The peak values of the laminar burning flux of MF at three temperatures appear between equivalence ratio 1.1 and 1.2. Figure 12b shows that the burning flux of MF is the highest amongst the Page 15 ACS Paragon Plus Environment

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three fuels followed by DMF and then isooctane. The laminar burning flux of MF and DMF exhibit peak values between equivalence ratios of 1.1 and 1.2 whereas the laminar burning flux of isooctane exhibits peak values between equivalence ratios of 1.0 and 1.1.

5. Conclusions An experimental investigation of the laminar combustion characteristics of 2-methylfuran (MF) was conducted using high speed schlieren photography in a constant volume vessel at elevated temperatures (60°C, 90°C and 120°C) and varied equivalence ratios (Φ=0.6-1.4) under 0.1MPa initial pressure. The characteristics of MF laminar flame were compared to those of DMF and isooctane. The conclusions drawn from the study for the tested conditions are as follows:

1. The unstretched flame speed of MF is up to 30% faster than that of DMF and up to 50% faster than that of isooctane. The highest unstretched flame speed of MF under all temperatures occurs at the equivalence ratio of 1.1, whereas for DMF and isooctane, the highest unstretched flame speed occurs at equivalence ratios between 1.1 and 1.2. 2. At equivalence ratios lower than 1.1, MF flame is less stable than DMF and isooctane and the Markstein lengths of MF are smaller than those of DMF and isooctane.

At the

equivalence ratios higher than 1.1, the Markstein length differences between MF and the other two fuels are very small. 3. The laminar burning velocity of MF is the highest amongst the three fuels under all the conditions tested, and it reaches its maximum of 70.4cm/s at an initial temperature of Page 16 ACS Paragon Plus Environment

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120°C and an equivalence ratio of 1.1. 4. MF has the smallest flame thickness at all the conditions tested compared to DMF and isooctane. For MF, the density ratio is significantly affected by the initial temperature whereas the flame thickness is very insensitive to initial temperature change. 5. The laminar burning flux of MF increases with the increase of initial temperature and exhibits peak values between the equivalence ratios of 1.1 and 1.2. Compared to DMF and isooctane, the burning flux of MF is the highest at all the conditions tested.

Acknowledgement: This research was conducted in the Future Engines and Fuels Lab at the University of Birmingham and sponsored by EPSRC under the Grant EP/F061692/1. The authors would like to acknowledge the support from Jaguar Land Rover, Advantage West Midlands and Birmingham Science City. They also wish to extend their appreciation to colleagues Asish Sarangi, Cen Zhang, Chongming Wang, Peter Thornton and Carl Hingley who provided great help during the experiment. A special thank is due to Professor C.K. Law of Princeton University for helpful discussions and advice about the work reported in the paper.


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Performance and

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Prospects.chapter

"DMF-A

New

Biofuel

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Candidate,"

Available

from:

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components.

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(U.S.

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NREL/TP-5400-50791. (19) Zhong, S.; Daniel, R.; Xu, H.; Zhang, J.; Turner, D.; Wyszynski, M. L.; Richards, P. Combustion and Emissions of 2,5-Dimethylfuran in a Direct-Injection Spark-Ignition Engine. Energy & Fuels. 2010, 24 (5), 2891-2899. (20) Tian, G.; Li, H.; Xu, H.; Li, Y.; Raj, S. M. Spray Characteristics Study of DMF Using Phase Doppler Particle Analyzer. SAE Technical Paper. 2010, SAE2010-01-1505. (21) Tian, G.; Daniel, R.; Li, H.; Xu, H.; Shuai, S.; Richards, P.

Laminar Burning

Velocities of 2,5-Dimethylfuran Compared with Ethanol and Gasoline. Energy & Fuels. 2010, 24 (7), 3898-3905. (22) Wu, X.; Li, Q.; Fu, J.; Tang, C.; Huang, Z.; Daniel, R.; Tian, G.; Xu, H. Laminar burning characteristics of 2,5-dimethylfuran and iso-octane blend at elevated temperatures and pressures. Fuel. 2012, 95, 234–240. (23) Daniel, R.; Tian, G.; Xu, H.; Wyszynski, M. L.; Wu, X.; Huang, Z. Effect of spark timing and load on a DISI engine fuelled with 2,5-dimethylfuran. Fuel. 2011, 90, 449–458. (24) Wu, X.; Daniel, R.; Tian, G.; Xu, H.; Huang, Z.; Richardson, D. Dual-injection: The flexible, bi-fuel concept for spark-ignition engines fuelled with various gasoline and biofuel blends. Applied Energy. 2011, 88, 2305–2314. Page 20 ACS Paragon Plus Environment

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Energy & Fuels

(25) Wang, C.; Xu, H.; Daniel. R.; Ghafourian, A.; Herreros, J. M.; Shuai, S.; Ma, X. Combustion characteristics and emissions of 2-methylfuran comparedto 2,5-dimethylfuran, gasoline and ethanol in a DISI engine. Fuel. 2013, 103, 200–211. (26) Rothamer, D. A.; Jennings, J. H. Study of the knocking propensity of 2,5-dimethylfuran–gasoline and ethanol–gasoline blends. Fuel. 2012, 98, 203–212. (27) Zhang, Q.; Chen, G.; Zheng, Z.; Liu, H.; Xu, J.; Yao, M. Combustion and emissions of 2,5-dimethylfuran addition on a diesel engine with low temperature combustion. Fuel. 2013, 103, 730–735. (28) Thewes, M.; Muether, M.; Pischinger, S.; Budde, M.; Brunn, A.; Sehr, A.; Adomeit, P.; Klankermayer, J. Analysis of the Impact of 2-Methylfuran on Mixture Formation and Combustion in a Direct-Injection Spark-Ignition Engine. Energy Fuels. 2011, 25 (12), 5549–5561. (29) Ohtomo, M.; Nishikawa, K.; Suzuoki, T.; Miyagawa, H. Auto-ignition Characteristics of Biofuel Blends for SI Engines. SAE Technical Paper. 2011, SAE2011-01-1989. (30) Somers, K. P.; Simmie, J. M.; Gillespie, F.; Burke, U.; Connolly, J.; Metcalfe, W. K.; Battin-Leclerc, F.; Dirrenberger, P.; Herbinet, O.; Glaude, P. A.; Curran, H. J. A high temperature and atmospheric pressure experimental and detailed chemical kinetic modelling study of 2-methyl furan oxidation. Proceeding of the Combustion Institute. 2013, 347, 225-232. (31) Law, C. K.; Sung, C. J. Structure, aerodynamics and geometry of premixed flamelets. Progress in Energy and Combustion Science. 2000, 26, 459–505. (32) Gu, X.; Li, Q.; Huang, Z.; Zhang, N. Measurement of laminar flame speeds and flame Page 21 ACS Paragon Plus Environment

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stability analysis of tert-butanol–air mixtures at elevated pressures. Energy Conversion and Management. 2011, 52, 3137 3146. (33) Wu, X.; Huang, Z.; Yuan,Kuiwen, Z.; Wei, L. Identification of combustion intermediates in a low-pressure premixed laminar 2,5-dimethylfuran/oxygen/argon flame with tunable synchrotron photoionization. Combustion and Flame. 2009, 156, 1365-1376. (34) Hu, E.; Huang, Z.; He, J.; Zheng, J.; Miao, H. Measurements of laminar burning velocities and onset of cellular instabilities of methane–hydrogen–air flames at elevated pressures and temperatures. International Journal of Hydrogen Energy. 2009, 34, 5574-5584. (35) Leylegian, J. C.; Sun, H. Y.; Law, C. K. Laminar flame speeds and kinetic modeling of hydrogen/chlorine combustion. Combustion and Flame. 2005, 143, 199-210. (36) Bradley, D.; Hicks, R. A.; Lawes, M.; Sheppard, C. G. W.; Woolley, R. The measurement of laminar burning velocities and Markstein numbers for iso-octane–air and isooctane isooctane- n-heptane–air mixtures at elevated temperatures and pressures in an explosion bomb. Combustion and Flame. 1998, 115, 126–44. (37) Chen, Z.; Burke, M. P.; Ju, Y. Effects of Lewis number and ignition energy on the determination of laminar flame speed using propagating spherical flames. Proceeding of the Combustion Institute. 2009, 32, 1253–1260. (38) Huang, Z.; Zhang, Y.; Zeng, K.; Liu, B.; Wang, Q.; Jiang, D. Measurements of laminar burning velocities for natural gas–hydrogen–air mixtures. Combustion and Flame. 2006, 146, 302–311. (39) Zhang, Z.; Huang, Z.; Wang, X.; Xiang, J.; Wang, X.; Miao, H. Measurements of laminar burning velocities and Markstein lengths for methanol–air–nitrogen mixtures at Page 22 ACS Paragon Plus Environment

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Energy & Fuels

elevated pressures and temperatures. Combustion and Flame. 2008, 155, 358–368. (40) Burke, M. P.; Chen, Z.; Ju, Y.; Dryer, F. L. Effect of cylindrical confinement on the determination of laminar flame speeds using outwardly propagating flames. Combustion and Flame. 2009, 156, 771–779. (41) Gu, X. J.; Haq, M. Z.; Lawes, M.; Woolley, R.; Laminar Burning Velocity and Markstein Lengths of Methane–Air Mixtures. Combustion and Flame. 2000, 121, 41–58. (42) Turns, S. R. An Introduction to Combustion. New York: McGraw-Hill. 1996; pp 55-56. (43) Olikara, C.; Borman, G. L. A Computer Program for Calculating Properties of Equilibrium Combustion Products with Some Applications to I. C. Engines. SAE Technical Paper. 1975, SAE750468. (44) Kumar, K. Global combustion response of practical hydrocarbon fuels: n-heptane, iso-octane, n-decane, n-dodecane and ethylene. 2007, Ph.D. Case Western Reserve University. USA. (45) Hasse, C.; Bollig, M.; Peters, N.; Dwyer, H. A. Quenching of Laminar iso-Octane Flames at Cold Walls. Combustion and Flame. 2000, 122(1-2), 117-129. (46) Law, C. K.; Makino, A.; Lu, T. F. On the off-stoichiometric peaking of adiabatic flame temperature. Combustion and Flame. 2006, 145, 808–819. (47) Karlin, V.; Sivashinsky, G. Asymptotic modelling of self-acceleration of spherical flames. Proceedings of the Combustion Institute. 2007, 31, 1023-1030. (48) Matalon, M.; Matkowsky, B. J. Flames as gas dynamic discontinuities. Fluid Mechanics. 1982, 124, 239–59. (49) Bechtold, J. K.; Matalon, M. The dependence of the Markstein length on stoichiometry. Page 23 ACS Paragon Plus Environment

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Combustion and Flame. 2001, 127, 1906–1913. (50) Stone, R. Introduction to internal combustion engine, laminar burning velocity, 3rd Edition. London. 1999; pp 363-365. (51) Matalon, M. Intrinsic flame instabilities in premixed and nonpremixed combustion. Annu. Rev. Fluid Mech. 2007, 39, 163-191


Page 25 of 39

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Energy & Fuels

Nomenclature MF

2-methylfuran

Sn

stretched laminar flame speed

DMF

2, 5-dimethylfuran

ru

instantaneous flame radius

ISO

isooctane

α

stretched rate

HMF

5-hydroxymethylfurfural

Ss

laminar flame speed

Φ

equivalence ratio

Lb

Markstein length

RON

research octane number

µl

laminar burning velocity

MON

motor octane number

ρb

burned mixture densities

LHV

lower heating value

ρu

unburned mixture densities

PDPA

phase doppler particle analyzer

Tb

adiabatic flame temperatures

MBT

maximum break torque

Tu

initial temperatures

DISI

direct injection spark ignition

nu

mole numbers of reactants

THC

hydrocarbon emissions

nb

mole numbers of products

δl

flame thickness

Ma

Markstein number

f

laminar burning flux


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 26 of 39

List of tables Table 1: Properties of the test fuels compared to gasoline


Page 27 of 39

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

Table 1 : Properties of the test fuels compared to gasoline 2-Methylfuran

DMF

ISO

Chemical formula H/C ratio O/C ratio Gravimetric oxygen content (%) Density @ 20 C (kg/m3) Research Octane Number (RON) Motor Octane Number (MON) Stoichiometric air–fuel ratio LHV (MJ/kg) LHV (MJ/L) Heat of vaporization (kJ/kg) Initial boiling point (oC)

Gasoline C2-C14

1.2 0.2

1.333 0.167

2.25 0

1.795 0

19.51

16.67

0

0

913.2b

889.7a

691.9

744.6

103b

101.3c

100

96.8

86b

88.1c

100

85.7

10.05

10.72

15.13

14.46

31.2b 28.5b

32.89b 29.3a

44.3 30.66

42.9 31.9

358.4b

332

307.63

373

64.7

92

99

32.8

a Measured at the University of Birmingham, 2010. b NREL/TP-5400-50791. 18 c API Research Project 45, 1956.


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 28 of 39

List of Figures Figure 1: Schlieren experiment set up. Figure 2: Laminar flame radius detection. Figure 3: Laminar burning velocities of isooctane-air mixtures versus equivalence ratios at 0.1MPa pressure and elevated initial temperature Figure 4: Chronological schlieren images of stoichoimetric fuel-air mixtures at initial temperature of 90°C. Figure 5: Stretched flame speed of the test fuels at 120°C initial temperature at different equivalence ratios and stretch rates. Figure 6: Unstretched flame speed of the test fuels at different temperatures and equivalence ratios Figure 7: Markstein length of test fuels at different temperatures and equivalence ratios. Figure 8: Adiabatic flame temperatures for three fuels at 120 oC with varying equivalence ratios. Figure 9: Laminar burning velocities of test fuels at different temperatures and equivalence ratios. Figure 10: Flame thickness (solid line) and density ratio (dot line) versus equivalence ratio for (a) MF under three temperatures (b) three fuels under 90oC. Figure 11: Markstein number versus equivalence ratio for (a) MF under three temperatures (b) three fuels under 60oC. Figure 12: Burning flux versus equivalence ratio for (a) MF under three temperatures (b) three fuels under 120oC.


Page 29 of 39

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

Figure 1: Schlieren experiment set up.

（a）Original image

(b) Rotated image Figure 2: Laminar flame radius detection.


Energy & Fuels

60 50 40

ul(cm/s)

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 30 of 39

30

Tu=90oC, Current work Tu=85oC, Bradley et al.

20 10

Tu=87oC, Hasse et al.

ISO 0

0.8

1.0

1.2

1.4

Equivalence Ratio Φ Figure 3: Laminar burning velocities of isooctane-air mixtures versus equivalence ratios at 0.1MPa pressure and elevated initial temperature


Page 31 of 39

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

Time elapsed

MF

DMF

ISO

1ms

2.5ms

4.0ms

5.5ms

7.0ms

8.5ms

10ms

11.5ms

Figure 4: Chronological schlieren images of stoichoimetric fuel-air mixtures at initial temperature of 90°C. Page 31 ACS Paragon Plus Environment

Energy & Fuels

5

Sn(m/s)

4

3

Φ =0.6 Φ =0.7 Φ =0.8 Φ =0.9 Φ =1.0 Φ =1.1 Φ =1.2 Φ =1.4

2

1 MF 0

0

200

400

600

800

1000

α(s-1) 4

Sn(m/s)

3

2 Φ=0.8 Φ=0.9 Φ=1.0 Φ=1.1 Φ=1.2

1 DMF 0

0

200

400

600

800

1000

α(s-1)

3

Sn(m/s)

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 32 of 39

2

1

Φ=0.9 Φ=1.0 Φ=1.1 Φ=1.2

Isooctane 0

0

200

400

600

800

1000

α(s-1)

Figure 5: Stretched flame speed of the test fuels at 120°C initial temperature at different equivalence ratios and stretch rates. Page 32 ACS Paragon Plus Environment

Page 33 of 39

4

Ss(m/s)

MF DMF Isooctane 3

2

60oC 0.6

0.8

1.0

1.2

1.4

Equivalence ratio Φ MF DMF Isooctane

Ss(m/s)

4

3

2

90oC 0.6

0.8

1.0

1.2

1.4

Equivalence ratio Φ MF DMF Isooctane

4

Ss(m/s)

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

3

2

120oC 0.6

0.8

1.0

1.2

1.4

Equivalence ratio Φ Figure 6: Unstretched flame speed of the test fuels at different temperatures and equivalence ratios


Energy & Fuels

3

MF DMF Isooctane

Lb(mm)

2

1

0

60oC -1 0.6

0.8

1.0

1.2

1.4

Equivalence ratio Φ 3

MF DMF Isooctane

Lb(mm)

2

1

0

90oC -1 0.6

0.8

1.0

1.2

1.4


MF DMF Isooctane

2

Lb(mm)

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 34 of 39

1

0

-1 0.6

120oC 0.8

1.0

1.2

1.4

Equivalence ratio Φ Figure 7: Markstein length of test fuels at different temperatures and equivalence ratios.


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

Adiabatic flame temperature (K)

Page 35 of 39

MF DMF Isooctane

2400 2300 2200 2100 2000 1900 1800 1700

120oC 0.6

0.8

1.0

1.2

1.4

1.6

Equivalence ratio Φ Figure 8: Adiabatic flame temperatures for three fuels at 120 oC with varying equivalence ratios.


Energy & Fuels

ul(cm/s)

50

MF DMF Isooctane

40

30

60oC 0.6

0.8

1.0

1.2

1.4


MF DMF Isooctane

ul(cm/s)

50

40

30

90oC

0.6

0.8

1.0

1.2

1.4


MF DMF Isooctane

60

ul(cm/s)

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 36 of 39

50

40

30 0.6

120oC 0.8

1.0

1.2

1.4

Equivalence ratio Φ Figure 9: Laminar burning velocities of test fuels at different temperatures and equivalence ratios.


Page 37 of 39

Flame thickness (mm)

0.12 0.10

6

0.08 4

0.06 0.04 0.02

2

Density ratio σ

10 o T=60 C o T=90 C o T=120 C 8

0.14

MF 0.6

0.8

1.0

1.2

1.4

0 1.6

Equivalence ratios Φ (a)

0.12 0.10

6

0.08 4

0.06 0.04

2

Density ratio σ

10 MF DMF Isooctane 8

0.14

Flame thickness (mm)

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

o

0.02 90 C 0.6

0.8

1.0

1.2

1.4

0 1.6

Equivalence ratios Φ （b）

Figure 10: Flame thickness (solid line) and density ratio (dot line) versus equivalence ratio for (a) MF under three temperatures (b) three fuels under 90oC.


Energy & Fuels

60

o

T=60 C o T=90 C o T=120 C

Markstein Number

50 40 30 20 10 0 MF -10 0.6

0.8

1.0

1.2

1.4

Equivalence ratios Φ (a) 60 MF DMF Isooctane

50

Markstein Number

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 38 of 39

40 30 20 10 0 o

60 C -10 0.6

0.8

1.0

1.2

1.4

Equivalence ratio Φ (b)

Figure 11: Markstein number versus equivalence ratio for (a) MF under three temperatures (b) three fuels under 60oC.


Page 39 of 39

0.75

0.65

2

Burning flux(kg/m .s)

0.70

0.60 0.55 0.50 0.45

o

T=60 C o T=90 C o T=120 C

0.40 0.35

MF

0.30 0.6

0.8

1.0

1.2

1.4

Equivalence ratio Φ (a) 0.75

MF DMF Isooctane

0.70 2

Burning flux(kg/m .s)

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

0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.6

o

120 C 0.8

1.0

1.2

1.4

Equivalence ratios (b)

Figure 12: Burning flux versus equivalence ratio for (a) MF under three temperatures (b) three fuels under 120oC.


Laminar Burning Characteristics of 2-Methylfuran Compared with 2,5

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