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Ignition and combustion characteristics of jet fuel liquid film containing graphene powders at meso-scale
Fuel 177 (2016) 113–122
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Ignition and combustion characteristics of jet fuel liquid film containing graphene powders at meso-scale Xuefeng Huang a, Shengji Li b,⇑ a b
School of Science, Hangzhou Dianzi University, Hangzhou 310018, China College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Jet fuel suspensions containing
graphene powders were prepared. Graphene was trapped and ignited by
optical tweezers prior to jet fuel. Ignition and combustion
characteristics of jet fuel liquid film were identified. A schematic physical model was proposed to understand the combustion mechanism.
a r t i c l e
i n f o
Article history: Received 30 October 2015 Received in revised form 21 January 2016 Accepted 3 March 2016 Available online 10 March 2016 Keywords: Meso-scale combustion Liquid film Jet fuel Graphene Optical tweezers
a b s t r a c t At meso-scale, ignition and combustion characteristics of jet fuel liquid film containing graphene powders were investigated. Jet fuel/graphene suspensions were prepared, and sprayed to produce the liquid film. Liquid film was ignited by optical tweezers, five distinctive stages including graphene trap, ignition and combustion of graphene, bubble formation, jet fuel vaporization and bubble growth, bubble rupture and combustion of liquid film were identified. Ignition of graphene is prior to jet fuel. The combustion heat of graphene serves a heat source to accelerate the vaporization of jet fuel. The graphene serves as a nucleation point to form a bubble. Expansion of both combustion products and jet fuel vapor result in the bubble growth. The thickness of bubble boundary layer depends on graphene concentration. As the bubble escaped, liquid film ruptured and micro-explosion occurred. Jet fuel was then ignited, and combusted sustainably till burnt out. During combustion, the flame front fluctuated slightly, indicating good flame stability. Finally, a schematic physical model was presented to analyze the inductive mechanism of graphene for ignition and combustion of jet fuel liquid film by optical tweezers. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction As the space of energy system decreases from macro-scale to meso-scale and even till micro-scale, ignition and combustion sta⇑ Corresponding author. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.fuel.2016.03.004 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
bility of liquid fuels are becoming significant challenges due to large heat loss and short residence time [1–5]. To sustain the combustion of liquid fuels in the meso-scale combustor, Sirignano et al. [6,7] proposed a liquid film combustor. The liquid film was generated by using swirling air on the inner combustor wall, which can impede heat loss by creating a surface where the flame’s heat vaporized the liquid rather than transferred directly to the walls.
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At macro-scale, recent advances in using colloidal suspension additives for improving ignition and combustion performance of liquid fuels have been shown. Tyagi et al. [8] demonstrated that the ignition probability for the diesel fuel mixtures containing nanoparticles was significantly higher than that of pure diesel. Javed et al. [9] reported that kerosene droplets with aluminum nanoparticles can automatically ignite. Nachmoni and Natan [10] found that organic metallic gellant gels can increase the heat of vaporization with increasing the gellant content in the liquid JP5. Gan and Qiao [11] demonstrated several distinctive burning stages for liquid fuel with aluminum particles in comparison with pure liquid fuel. Ohkura et al. [12,13] and Conner and Dlott [14] reported that Al nanoparticles can be ignited by a camera flash. These studies have shown the promise of using nanoscale and micro-scale additives to enhance the ignition and combustion performance of liquid fuels. Nano-structured additives offer distinct advantages over larger scale particles due to their high surface to volume ratio and increased density of surface functionalities. Moreover, nanoscale materials also exhibit optical properties favorable to radiative heat transfer that could aid in combustion [15–18]. However, at meso-scale, ignition and burning characteristics of liquid fuels containing colloidal suspension additives have not been reported. In the present work, we investigated ignition and combustion behaviors of jet fuel at meso-scale. To upgrade ignition and burning performances, nanometer and micron-sized graphene powders were proposed to add into jet fuel. Graphene is a flat monolayer of carbon atoms tightly packed into a twodimensional (2D) honeycomb lattice, and is a basic building bulk for graphitic materials of all other dimensionalities. It can be wrapped up into zero-dimensional fullerenes, rolled into onedimensional nanotubes or stacked into three-dimensional graphite [19]. Graphene can be considered as an emerging high energetic material to apply widely into many fields including combustion because of its prominent intrinsic properties, such as ultrahigh thermal conductivity coefficient of 5000 W/m K at room temperature and high surface to volume ratio of 2630 m2/g for nano singlelayer sheet [20–22]. Graphene can be also considered as a fuel additive to assist ignition and as a catalyst to enhance combustion rate and efficiency. Sabourin et al. [23] demonstrated that the combustion rate was increased by 47% at a concentration of functionalized graphene sheet additive as low as 0.075%, in comparison with pure nitromethane. Gilje et al. [24] reported that photothermal deoxygenation of graphene oxide might be a promising additive as an ignition promoter for the distribution ignition of fuels. In our work, an optical tweezers (OT) tool was used. It is not only considered as an igniter, but is also used to manipulate graphene suspended into jet fuel. The OT can exert extremely small forces via a highly focused laser beam, which is capable of manipulating nanometer and micron-sized dielectric particles. Since first demonstration of the OT appeared by Ashkin et al. [25], it has been considered as an effective tool to trap and manipulate particles in all kinds of surrounding ambiences, and applied in many fields like atmosphere, physics, micro-fluidics, medicine, bio, and life science etc. [26]. Moreover, the OT can heat the particles due to high energy density. Therefore, the OT can not only ignite the fuel, but trap the fuel particles. It is different from the spark, hot wire, camera flash and common laser igniter. Our previous studies [27,28] have already demonstrated that micron-sized active carbon particles can be effectively trapped and ignited by the OT. Generally, at micro-scale and meso-scale, gas phase hydrogen or hydrocarbon fuels were used, and their igniter is an electric spark or hot wire. These ignition sources are installed inside the combustor and immovable, and thus the ignition point cannot be optionally changed. In this study, the OT ignition system is our first effort for solving in situ ignition of a microflame outside the combustor and
stabilizing the ignition at the desired location within the combustor at small scale. The objectives of present study will be involving: (1) to investigate the dispersion performance of nanometer and micron-sized graphene in jet fuel, (2) to testify the feasibility of the OT igniter for the ignition application of liquid fuel at meso-scale, and (3) to explore ignition and combustion behaviors of jet fuel liquid film containing graphene powders at meso-scale. Moreover, a schematic physical model was built to analyze the ignition and combustion stages of jet fuel liquid film containing graphene powders.
2. Experimental 2.1. Preparation of jet fuel/graphene suspensions Graphene powders (purchased from XFNANO Tech. Co., Ltd, Nanjing, China) typically consist of single layer graphene nanosheets. The graphene was prepared by reducing the graphene oxide, and reduced graphene oxide had many oxygen-containing groups. The oxygen content of graphene reached 7.0–7.5 at.%, indicating a large amount of defects and a more active chemical property. This tends to make the graphene sheet much easier to be oxidized than pure graphene. Graphene samples have average surface area of 700–800 m2/g. The size of graphene powders ranges from 500 nm to 5.0 lm. Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines, consisting of very complex hydrocarbon mixtures [29]. Additionally, jet fuel has been identified as a potential jet propellant for various applications [30]. Due to low oxygen content and reactivity, jet fuel behaves as a fuel at ordinary pressure, requiring additional oxidizers to sustain combustion. In this work, pure jet fuel (no any oxidizers or additives) was used to prepare the suspensions. The physical properties of pure jet fuel (purchased from Fd-wzsh Co., Ltd, Hangzhou, China) were listed into Table 1. The preparation of jet fuel/graphene suspensions is a key step. Special handling is needed to achieve homogeneous, long-term, stable suspensions and a low level of particle agglomeration. Many studies have demonstrated that sonication can reduce the coagulation of nanoparticles in liquid [31]. As the nanofluid is exposed to ultrasound, the ultrasound waves that propagate into it result in alternating high pressure and low pressure cycles. The applied mechanical stress can separate the nanoparticles from one another and reduce the agglomeration. The criteria for the selection of graphene to fuel ratio should be able to significantly enhance the evaporation rate of jet fuel, and minimize the cost of fuel additive as far as possible. Considering two aspects of evaporation rate and the cost, the graphene to fuel ratio of 0.1 mg/mL was selected in this study. Graphene powders (1 mg) were added into jet fuel (10 mL), and thus graphene concentration was 0.1 mg/mL (0.013 wt.%). After bath ultrasonication about 1 h, the suspension was then allowed to settle for a couple of days. Fig. 1 shows the pictures of the suspensions after 0.5 h (left) and after 5 days (right). With sonication, suspensions of jet fuel/graphene typically can remain stable for about 0.5 h, beyond which the particles will start
Table 1 Physical properties of pure jet fuel. Items
Values
Items
Values
Density (kg/m3) Flashing point (°C) Freezing point (°C) Smoking point (mm) Specific heat (kJ/(kg K))
794.5 46.5 56.0 25 2.01
Net calorific value (MJ/kg) Boiling temperature (°C) Existent gum (mg/100 mL) Kinematic viscosity (mm2/s)
43.30 156 <1.0 4.063
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Fig. 1. Digital pictures of as-prepared jet fuel/graphene suspensions through bath ultrasonication about 1 h. Left: after 0.5 h. Right: after 5 days.
to settle. After 5 days, most of micron-sized graphene particles and the aggregate have settled on the bottom of the test bottle.
2.2. Experimental setup The schematic of experimental setup is shown in Fig. 2. The beam launched from near infra-red (NIR) Nd: YVO4 laser (New Industries Optoelectronics Tech. Co. Ltd., Changchun, China) goes through a shutter, beam expander and the dichroic, and arrives at the microscope objective. The laser beam possesses TEM00 mode and wavelength of 1064 nm. The output power can be adjusted in the range from 0 to 600 mW by controlling the driving current. A shutter was configured in front of the laser to control the on/off status of beam. The beam expander can enlarge the beam diameter three times than original one. A reflecting film was coated on the dichroic to reflect completely the laser beam, whereas, the film cannot obstacle the visible light of illuminator and the flame through the dichroic. The microscope objective (Olympus, Tokyo, Japan), with numerical aperture of 0.67 and magnification of 40, can highly focus the laser beam. The waist diameter of
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focusing beam is about 3.0 lm. The highly focused laser beam, called as the OT, will exert extremely small forces on fuel particles. The OT traps the fuel particles, and positions them at the light axis near the beam waist. As the power density increases, the OT will heat the particles and ignite them. An illuminator and a condenser supply uniform bright illumination. The transmitted light travelling through the objective and the dichroic is collected by a lens, and then enters into the CCD by filtering. The filter located in front of the CCD minimizes the interference of laser beam. Simultaneously, the flame in the combustor can be also picked up and recorded by the CCD. The CCD camera (Microshot technology Co. Ltd., Guangzhou, China) has full pixels of 2592 1944 with 25 frames per second. The meso-scale combustor is a cuboid, which was made up of by two layers of materials. The bottom layer material is transparent flat glass, and the top layer one is cover glass. The inner dimensions of the combustor are 20 mm (length) 8 mm (width) 170 lm (depth). Both ends of meso-scale combustor were open in the stagnation air at atmospheric pressure, i.e., the combustor was a static, open volume one. The oxygen diffused by natural convention. The meso-scale combustor was fixed on the stage with a 3axis (X, Y, and Z) translation that can be adjusted along with three axial directions with a solution of 10 lm. The prepared jet fuel/graphene suspensions or pure jet fuel were sprayed into the combustor by a micropump, and liquid film formed on the bottom wall of combustor. In this study, the top layer wall of meso-scale combustor is a transparent plane glass, and thus the beam highly focused by the microscope objective goes through the plane glass with little attenuation. It will not result in the movement of the OT focal point. If the top layer wall of combustor is a convexity or a concavity, the top layer glass should be considered as an equivalent lens, i.e., a convex lens or a concave lens. This indicates that the OT beam will be more convergent or more divergent, which tends toward make the OT focal point move upward or downward, respectively. The focal point of the beam that goes through the convex lens or the concave lens can be calculated by the lens imaging formula. Therefore, the OT igniter beam will be significantly affected by the curvature of combustor wall, which leads to the change of ignition point and ignition delay. The larger the curvature of combustor wall is, i.e., the smaller the radius of combustor wall is, the more serious the impact on the ignition location and delay will become.
Fig. 2. The schematic of experimental setup.
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3. Results 3.1. Ignition and combustion of pure jet fuel liquid film Pure jet fuel with the mass of 10 mg was injected into the mesoscale combustor by the micropump. Jet fuel quickly extended and formed liquid film on the bottom wall. As the OT with the power of 420 mW (power density of 5.9 1010 W/m2) was turned on and last at least one minute, the input power is at least 25.2 J, which is much higher than the vaporization energy of 2.81 J for the whole jet fuel liquid film. However, it is astonishingly found that the liquid film did not be ignited at all. It can be observed that the boundary of liquid film will alternatively expand or shrink at the corresponding on/off status of the OT, as shown in Fig. 3. In each picture, the dark color represents the boundary of jet fuel liquid film. The arrow represents movement direction of the jet fuel boundary (i.e., the interface between the air and jet fuel), and the sign ‘‘v” means that jet fuel boundary moves with certain a velocity. Fig. 3(a)–(d) demonstrated the expansion of liquid film to lower left corner of these images as the OT turned on. On the contrary, turning off the OT, the liquid film contracted to top right corner of these images (Fig. 3(e)–(h)). The expansion of liquid film increased heat transfer area, and enhanced the heat loss of combustor wall. It has been proved in Refs. [3–7] that the heat loss of combustor wall is large due to high surface to volume ratio at small scale. Significant heat loss of combustor wall should be the obvious reason for non-ignition of pure jet fuel–air mixture. In addition, low Reynolds number due to natural convection and insufficient supply of oxygen should also be not negligible reasons. 3.2. Ignition and combustion of graphene Ignition and combustion experiments of graphene were carried out to obtain the threshold ignition power and ignition delay time. In meso-scale combustor, graphene in the stagnant air at atmospheric pressure was ignited with the delay time below 6 ms by the OT. The minimum ignition power is only 3.2 mW. The size of graphene scarcely influences the minimum ignition power. As the minimum ignition power inputted, the graphene took flameless combustion (Fig. 4(a)) and decreased in size (Fig. 4(b)). As the OT power increased till 15.0 mW, the graphene combusted heterogeneously with dazzling flame (Fig. 4(c) and (d)). 3.3. Ignition and combustion of jet fuel liquid film containing graphene The jet fuel/graphene suspension was pumped into meso-scale combustor, extended and formed liquid film on the bottom wall.
The boundary of liquid film was clearly observed. Inside liquid film, graphene Brownian motion and collisions among them were also clearly identified. The OT was positioned near the upper surface of liquid film, the graphene particles dispersed into jet fuel moved toward the OT focal point, and was quickly trapped (Fig. 5(a)). The white spot represents the OT focal point, and the black spots are graphene particles. The graphene particles were then ignited by the OT with ignition delay time below 40 ms, which is over six times than that in air (6 ms) as stated above. Since the input ignition power of 420 mW is over 130 times than the minimum ignition power of graphene (3.2 mW), the graphene combusted with a dazzling flame (Fig. 5(b)). In Fig. 5(c), the bubble formed, and the graphene around the OT focal point gathered together and combusted. At that moment, a large size graphene particle was dragged and affixed to the outer boundary of bubble (Fig. 5(d)). Turning off the OT or moving the bubble far from the OT transiently, the bubble still stably settled inside liquid film. However, the bubble disappeared if the liquid film was ruptured. To testify the particle size effect on ignition, we choose a micron-sized graphene inside liquid film to ignite. Fig. 6 showed the similar burning behavior of the micron-sized graphene dispersed into liquid film. The micron-sized graphene was firstly trapped by the OT, but the trap efficiency was low. The ignition delay (below 40 ms) of micron-sized graphene dispersed in the liquid film kept the same as that of nano-graphene. The combustion flame and the bubble were clearly observed. The burning process of jet fuel liquid film containing graphene powders was illustrated in Fig. 7. During the combustion of graphene shown in Fig. 7(a), jet fuel was ignited (see supplementary material S1). The flame front suddenly diffused toward the air, and then microexplosion occurred. The main flame stabilized at the center, and the smaller flame located at lower right corner and gradually extinguished, as shown in Fig. 7(b)–(e). In the following stage, the main flame front fluctuated slightly, and then extinguished gradually with the consumption of jet fuel (see supplementary material S2). During the process, graphene outside the focal point moved toward the center of flame, and combusted after jet fuel was mostly vaporized. This indicates that the combination of dispersive graphene powders and OT igniter is conducive to stabilize the flame of jet fuel at meso-scale. The whole combustion process sustained 3 min till jet fuel was completely consumed. Since the combustion temperature exceeds the glass fusion one, the meso-scale combustor was ablated. Many pits and a black cave emerged on the bottom wall (Fig. 7(f)).
Fig. 3. The expansion or shrinkage of pure jet fuel liquid film at the corresponding on/off status of the OT, (a) 0 s; (b) 0.04 s; (c) 0.08 s; (d) 0.2 s; (e) 11.76 s; (f) 11.88 s; (g) 11.96 s; (h) 12.2 s.
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Fig. 4. Ignition and burning behavior of the graphene in air, (a) and (b) flameless combustion in an input power of 3.2 mW at 0 s and 0.162 s; (c) and (d) bright combustion flame in an input power of 15.0 mW at 0 s and 0.1 s.
Fig. 5. An ignition and burning sequence of the stabilized jet fuel liquid film containing graphene in an input power of 420 mW, (a) 0 s; (b) 0.04 s; (c) 0.28 s; (d) 0.52 s.
4. Discussion The distinctive stages can be identified for different additives in liquid fuel. Takahashi et al. [32] proposed a three-step mechanism based on experimental observations, including the d2-law combustion, shell formation and disruption stages. Byun et al. [33] pro-
posed a stage in addition to the pressure buildup stage. Gan and Qiao [11] identified five distinctive stages for an n-decane/nanoAl droplet with the surfactants. In this work, the distinctive stages and mechanism on ignition and combustion of jet fuel liquid film containing graphene powders will be discussed as following.
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Fig. 6. Another ignition and burning sequence of the stabilized jet fuel liquid film with addition of micron-sized graphene in an input power of 420 mW, (a) 0 s; (b) 0.04 s; (c) 0.28 s.
Fig. 7. The burning sequence of jet fuel liquid film containing graphene powders, (a) 0 s; (b) 3.72 s; (c) 7 s; (d) 9.6 s; (e) 45.92 s; (f) picture of combustor ablation.
The results showed that jet fuel can be ignited and sustainably combusted with the aid of graphene. This indicates that graphene is conducive to stabilize the flame of jet fuel at meso-scale. On one hand, the graphene serves as a nucleus of boiling for jet fuel vaporization. On the other hand, it is of great importance that graphene reaction heat serves as another heat source for ignition of jet fuel. On the basis of experimental observations, a schematic physical model for ignition and combustion was established and illustrated in Fig. 8. Detailed procedures of ignition and combustion of jet fuel liquid film containing graphene may be separated into five steps: Step I: Graphene is trapped by the OT In Fig. 8(a), the OT is positioned near the upper surface of liquid film. Aggregated graphene particle suffers from a resultant force including radiation pressure, thermophoretic force, photophoretic force, Brownian force and drag force. Graphene particle will be then pulled by the OT to the upper surface of liquid film. During step I, both graphene and liquid film will be preheated. As the resultant force is directed to the OT focal point, graphene particle dispersed in the jet fuel will be efficiently trapped by the OT. The event has been demonstrated in Figs. 5(a) and 6(a). Maragó et al. [34] also showed the trap and manipulation of nanographene in another base fluid. However, another event can also occur. For strong absorbing particles, the resultant force can inversely direct toward the OT
focal point since photophoretic force dominates [35], which makes graphene particle move far from the OT focal point. The phenomenon was observed as graphene is dispersed in the nitromethane in our experiments. Therefore, the trap of graphene is one of key steps for ignition and combustion of jet fuel. To guarantee effective trap, some recommendations can be made, like adding surfactant [36] and using an optical bottle beam [37]. Step II: Graphene is firstly ignited, and combusts In Fig. 8(b), the air fills with the upper space of mesocombustor. Near the end of step I, the graphene particle is ignited by the OT. Since graphene has ultra low minimum ignition power and short ignition delay time in comparison with jet fuel, graphene will be ignited by the concentrated OT igniter prior to jet fuel. The OT power is higher than the minimum ignition power of graphene, and thus the dazzling flame was observed (Figs. 5(b) and 6(b)). The reaction between the graphene and oxygen includes the following four cases:
C þ O2 ¼ CO2 393:5 kJ
ð1Þ
2C þ O2 ¼ 2CO 221:0 kJ 2CO þ O2 ¼ 2CO2 566:0 kJ
ð2Þ ð3Þ
C þ CO2 ¼ 2CO þ 172:5 kJ
ð4Þ
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Fig. 8. A schematic physical model for ignition and combustion of jet fuel liquid film containing graphene, (a) graphene trap; (b) ignition and combustion of graphene; (c) nucleus of boiling and bubble formation; (d) jet fuel vaporization and bubble growth; (e) bubble rupture and combustion of liquid film.
The first and second reactions represent the complete combustion and partial combustion of graphene. The third reaction shows the delayed combustion. The surface of graphene is only oxidized as CO, and then CO will react with diffusive oxygen to generate CO2. The fourth reaction demonstrates that CO2 is reduced by the graphene. The exothermic heat of per unit mole graphene is between 110.5 and 393.5 kJ, which assists the vaporization of jet fuel and benefits the flame stability of jet fuel by compensating the heat loss of meso-combustor wall. Step III: Graphene serves as nucleus of boiling to form the bubble as the graphene is ignited In Fig. 8(c), the ignited graphene can be considered as a nucleus of boiling. Gas phase products are sealed by jet fuel, leading to form a bubble. Graphene located inside the bubble sustainably combusts (Figs. 5(c) and 6(c)). The burning time of graphene is dependent of the graphene size. Step IV: Vaporization of jet fuel is accelerated and the bubble grows In Fig. 8(d), the vaporization of jet fuel is accelerated by the exothermic heat of graphene particle combustion and the heat of OT. The blend of jet fuel vapor and gas products diffuse quickly and make the bubble expand. Supposing graphene particle is spherical and its diameter and density is 2 lm and 0.2 103 kg/
m3 respectively, the heat release of complete reaction for single graphene particle is 107 J. As thousands of graphene particles gather together and combust, the heat release will be up to 104–103 J. The heat of OT per millisecond is 4.2 104 J. At the same time of graphene combustion, it is likely that jet fuel is also being oxidized. There exists an oxidized competition between graphene and jet fuel as the oxygen is insufficiently supplied due to low natural convection at meso-scale. Experimental observation shown in Figs. 5 and 6 demonstrated, in early ignition stage of jet fuel, the oxidizing combustion of graphene is prior to the jet fuel, and thus oxidation of jet fuel is weak. Experimental results of bubble expansion were demonstrated in Fig. 9. Expansion displacement and expansion velocity of the bubble were extracted and plotted in Fig. 10(a) and (b). The relationship between the expansion displacement and time can be exponentially fitted as s = 232.202–163.315 e0.016 t with R2 of 0.993. The expansion velocity can be expressed as v = 10.525 t0.447 by allometric function fitting with R2 of 0.997. Here, s and v represent expansion displacement and expansion velocity of bubble, respectively. In Fig. 10(a) and (b), the uncertainties of expansion displacement and expansion velocity of the bubble are dependent of the systematic error resulted from the calibration rule and resolution of images, and random error owing to repetitive measurement. By calculation, the uncertainties of expansion displacement and velocity of bubble are less than 0.5 lm and 0.1 lm/s.
Fig. 9. Expanding and growing sequence of the bubble, (a) 0 s; (b) 6.08 s; (c) 23.52 s; (d) 76.44 s; (e) 120.04 s; (f) 175.36 s.
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Fig. 10. The curves of bubble growth, (a) expansion displacement; (b) expansion velocity; (c) bubble boundary thickness.
Fig. 11. Enlargement of the bubble boundary thickness, (a) 0 s; (b) 1.76 s; (c) 9.16 s; (d) 10.92 s.
In Fig. 9, the bubble boundary thickness almost kept constant, since graphene was sparsely dispersed in jet fuel. As graphene is densely dispersed in jet fuel, the boundary thickness will also enlarge, as shown in Fig. 11. The bubble expanded, and simultaneously the bubble boundary thickness increased. Owing to the density and temperature gradients, quantities of graphene dispersed in jet fuel gathered around the bubble boundary. Large size graphene
stayed static outside the bubble, however, small graphene particles moved and rearranged. The enlargement of bubble boundary thickness limited the expansion of bubble. The bubble boundary thickness versus time was plotted in Fig. 10(c). Since the preliminary distribution of graphene in jet fuel complies with Boltzmann distribution law, the accumulation of graphene on the bubble boundary will be followed by Boltzmann
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function (d = 34.099–31.456/(1 + e(t4.288)/0.670)) with R2 of 0.964. The uncertainty of bubble boundary thickness is less than 0.3 lm. Here d is the bubble boundary thickness, and t is time. The bubble boundary thickness is dependent of the graphene concentration in jet fuel. The growing velocity of the thickness relates to the OT power and viscosity of jet fuel.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.03.004.
References Step V: Bubble ruptures and liquid film sustainably combusts In Fig. 8(e), the bubble ruptures, jet fuel vapor escapes from the upper surface of liquid film. The vapor is ignited, and the liquid film combusts till burns out (Fig. 7). During combustion, graphene outside the focal point cannot be directly heated by the OT, and only depends on the heat transfer of jet fuel. Therefore, it combusts after jet fuel is mostly vaporized, like water coal slurry. Graphene enhances the burning rate of liquid film due to high thermal conductivity and radiation effect.
5. Conclusions Ignition and burning characteristics of jet fuel liquid film containing nanometer and micron-sized graphene powders were investigated at meso-scale. The emphasis was to explore the differences of burning behaviors between pure jet fuel and jet fuel/graphene suspension, and to understand the combustion mechanism of jet fuel liquid film containing graphene. The following conclusions can be drawn. (1) Nanometer and micron-sized graphene is dispersed in jet fuel. Nanometer graphene in suspensions remains stable for much longer time than micron-sized graphene. The dispersion stability of the suspensions is not good, but it can be improved by adding surfactant or selecting smaller sized graphene. (2) Pure jet fuel liquid film cannot be ignited in such a short time. However, jet fuel liquid film containing graphene powders can be inductively ignited with short delay time. (3) The combustion of graphene is prior to the ignition of vaporized jet fuel. The combination of dispersive graphene particles and OT igniter can stabilize the flame of jet fuel at meso-scale, since the graphene serves as a nucleation point for accelerating the vaporization of jet fuel, and its combustion heat serves as heat source to assist the ignition and combustion of jet fuel. Jet fuel combusts sustainably till burns out with the support of graphene additive. (4) Five distinctive stages were identified. Particle size scarcely influences the distinctive stage, but micron-sized graphene requires higher trap power and longer burning time than nanometer graphene. Graphene concentration is crucial to the bubble growth. In addition, the OT ignition system has been proved as a novel igniter for in situ ignition of a microflame outside the combustor and stabilizing the ignition at the desired location within the combustor at small scale.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51276053 and 51506041), Natural Science Foundation of Zhejiang Province (LY14E060002 and Y15E060027), Young and middle-aged leading academic climbing project of Zhejiang Province (pd2013158).
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Ignition and combustion characteristics of jet fuel liquid film containing graphene powders at meso-scale Xuefeng Huang a, Shengji Li b,⇑ a b
School of Science, Hangzhou Dianzi University, Hangzhou 310018, China College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Jet fuel suspensions containing
graphene powders were prepared. Graphene was trapped and ignited by
optical tweezers prior to jet fuel. Ignition and combustion
characteristics of jet fuel liquid film were identified. A schematic physical model was proposed to understand the combustion mechanism.
a r t i c l e
i n f o
Article history: Received 30 October 2015 Received in revised form 21 January 2016 Accepted 3 March 2016 Available online 10 March 2016 Keywords: Meso-scale combustion Liquid film Jet fuel Graphene Optical tweezers
a b s t r a c t At meso-scale, ignition and combustion characteristics of jet fuel liquid film containing graphene powders were investigated. Jet fuel/graphene suspensions were prepared, and sprayed to produce the liquid film. Liquid film was ignited by optical tweezers, five distinctive stages including graphene trap, ignition and combustion of graphene, bubble formation, jet fuel vaporization and bubble growth, bubble rupture and combustion of liquid film were identified. Ignition of graphene is prior to jet fuel. The combustion heat of graphene serves a heat source to accelerate the vaporization of jet fuel. The graphene serves as a nucleation point to form a bubble. Expansion of both combustion products and jet fuel vapor result in the bubble growth. The thickness of bubble boundary layer depends on graphene concentration. As the bubble escaped, liquid film ruptured and micro-explosion occurred. Jet fuel was then ignited, and combusted sustainably till burnt out. During combustion, the flame front fluctuated slightly, indicating good flame stability. Finally, a schematic physical model was presented to analyze the inductive mechanism of graphene for ignition and combustion of jet fuel liquid film by optical tweezers. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction As the space of energy system decreases from macro-scale to meso-scale and even till micro-scale, ignition and combustion sta⇑ Corresponding author. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.fuel.2016.03.004 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
bility of liquid fuels are becoming significant challenges due to large heat loss and short residence time [1–5]. To sustain the combustion of liquid fuels in the meso-scale combustor, Sirignano et al. [6,7] proposed a liquid film combustor. The liquid film was generated by using swirling air on the inner combustor wall, which can impede heat loss by creating a surface where the flame’s heat vaporized the liquid rather than transferred directly to the walls.
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At macro-scale, recent advances in using colloidal suspension additives for improving ignition and combustion performance of liquid fuels have been shown. Tyagi et al. [8] demonstrated that the ignition probability for the diesel fuel mixtures containing nanoparticles was significantly higher than that of pure diesel. Javed et al. [9] reported that kerosene droplets with aluminum nanoparticles can automatically ignite. Nachmoni and Natan [10] found that organic metallic gellant gels can increase the heat of vaporization with increasing the gellant content in the liquid JP5. Gan and Qiao [11] demonstrated several distinctive burning stages for liquid fuel with aluminum particles in comparison with pure liquid fuel. Ohkura et al. [12,13] and Conner and Dlott [14] reported that Al nanoparticles can be ignited by a camera flash. These studies have shown the promise of using nanoscale and micro-scale additives to enhance the ignition and combustion performance of liquid fuels. Nano-structured additives offer distinct advantages over larger scale particles due to their high surface to volume ratio and increased density of surface functionalities. Moreover, nanoscale materials also exhibit optical properties favorable to radiative heat transfer that could aid in combustion [15–18]. However, at meso-scale, ignition and burning characteristics of liquid fuels containing colloidal suspension additives have not been reported. In the present work, we investigated ignition and combustion behaviors of jet fuel at meso-scale. To upgrade ignition and burning performances, nanometer and micron-sized graphene powders were proposed to add into jet fuel. Graphene is a flat monolayer of carbon atoms tightly packed into a twodimensional (2D) honeycomb lattice, and is a basic building bulk for graphitic materials of all other dimensionalities. It can be wrapped up into zero-dimensional fullerenes, rolled into onedimensional nanotubes or stacked into three-dimensional graphite [19]. Graphene can be considered as an emerging high energetic material to apply widely into many fields including combustion because of its prominent intrinsic properties, such as ultrahigh thermal conductivity coefficient of 5000 W/m K at room temperature and high surface to volume ratio of 2630 m2/g for nano singlelayer sheet [20–22]. Graphene can be also considered as a fuel additive to assist ignition and as a catalyst to enhance combustion rate and efficiency. Sabourin et al. [23] demonstrated that the combustion rate was increased by 47% at a concentration of functionalized graphene sheet additive as low as 0.075%, in comparison with pure nitromethane. Gilje et al. [24] reported that photothermal deoxygenation of graphene oxide might be a promising additive as an ignition promoter for the distribution ignition of fuels. In our work, an optical tweezers (OT) tool was used. It is not only considered as an igniter, but is also used to manipulate graphene suspended into jet fuel. The OT can exert extremely small forces via a highly focused laser beam, which is capable of manipulating nanometer and micron-sized dielectric particles. Since first demonstration of the OT appeared by Ashkin et al. [25], it has been considered as an effective tool to trap and manipulate particles in all kinds of surrounding ambiences, and applied in many fields like atmosphere, physics, micro-fluidics, medicine, bio, and life science etc. [26]. Moreover, the OT can heat the particles due to high energy density. Therefore, the OT can not only ignite the fuel, but trap the fuel particles. It is different from the spark, hot wire, camera flash and common laser igniter. Our previous studies [27,28] have already demonstrated that micron-sized active carbon particles can be effectively trapped and ignited by the OT. Generally, at micro-scale and meso-scale, gas phase hydrogen or hydrocarbon fuels were used, and their igniter is an electric spark or hot wire. These ignition sources are installed inside the combustor and immovable, and thus the ignition point cannot be optionally changed. In this study, the OT ignition system is our first effort for solving in situ ignition of a microflame outside the combustor and
stabilizing the ignition at the desired location within the combustor at small scale. The objectives of present study will be involving: (1) to investigate the dispersion performance of nanometer and micron-sized graphene in jet fuel, (2) to testify the feasibility of the OT igniter for the ignition application of liquid fuel at meso-scale, and (3) to explore ignition and combustion behaviors of jet fuel liquid film containing graphene powders at meso-scale. Moreover, a schematic physical model was built to analyze the ignition and combustion stages of jet fuel liquid film containing graphene powders.
2. Experimental 2.1. Preparation of jet fuel/graphene suspensions Graphene powders (purchased from XFNANO Tech. Co., Ltd, Nanjing, China) typically consist of single layer graphene nanosheets. The graphene was prepared by reducing the graphene oxide, and reduced graphene oxide had many oxygen-containing groups. The oxygen content of graphene reached 7.0–7.5 at.%, indicating a large amount of defects and a more active chemical property. This tends to make the graphene sheet much easier to be oxidized than pure graphene. Graphene samples have average surface area of 700–800 m2/g. The size of graphene powders ranges from 500 nm to 5.0 lm. Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines, consisting of very complex hydrocarbon mixtures [29]. Additionally, jet fuel has been identified as a potential jet propellant for various applications [30]. Due to low oxygen content and reactivity, jet fuel behaves as a fuel at ordinary pressure, requiring additional oxidizers to sustain combustion. In this work, pure jet fuel (no any oxidizers or additives) was used to prepare the suspensions. The physical properties of pure jet fuel (purchased from Fd-wzsh Co., Ltd, Hangzhou, China) were listed into Table 1. The preparation of jet fuel/graphene suspensions is a key step. Special handling is needed to achieve homogeneous, long-term, stable suspensions and a low level of particle agglomeration. Many studies have demonstrated that sonication can reduce the coagulation of nanoparticles in liquid [31]. As the nanofluid is exposed to ultrasound, the ultrasound waves that propagate into it result in alternating high pressure and low pressure cycles. The applied mechanical stress can separate the nanoparticles from one another and reduce the agglomeration. The criteria for the selection of graphene to fuel ratio should be able to significantly enhance the evaporation rate of jet fuel, and minimize the cost of fuel additive as far as possible. Considering two aspects of evaporation rate and the cost, the graphene to fuel ratio of 0.1 mg/mL was selected in this study. Graphene powders (1 mg) were added into jet fuel (10 mL), and thus graphene concentration was 0.1 mg/mL (0.013 wt.%). After bath ultrasonication about 1 h, the suspension was then allowed to settle for a couple of days. Fig. 1 shows the pictures of the suspensions after 0.5 h (left) and after 5 days (right). With sonication, suspensions of jet fuel/graphene typically can remain stable for about 0.5 h, beyond which the particles will start
Table 1 Physical properties of pure jet fuel. Items
Values
Items
Values
Density (kg/m3) Flashing point (°C) Freezing point (°C) Smoking point (mm) Specific heat (kJ/(kg K))
794.5 46.5 56.0 25 2.01
Net calorific value (MJ/kg) Boiling temperature (°C) Existent gum (mg/100 mL) Kinematic viscosity (mm2/s)
43.30 156 <1.0 4.063
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Fig. 1. Digital pictures of as-prepared jet fuel/graphene suspensions through bath ultrasonication about 1 h. Left: after 0.5 h. Right: after 5 days.
to settle. After 5 days, most of micron-sized graphene particles and the aggregate have settled on the bottom of the test bottle.
2.2. Experimental setup The schematic of experimental setup is shown in Fig. 2. The beam launched from near infra-red (NIR) Nd: YVO4 laser (New Industries Optoelectronics Tech. Co. Ltd., Changchun, China) goes through a shutter, beam expander and the dichroic, and arrives at the microscope objective. The laser beam possesses TEM00 mode and wavelength of 1064 nm. The output power can be adjusted in the range from 0 to 600 mW by controlling the driving current. A shutter was configured in front of the laser to control the on/off status of beam. The beam expander can enlarge the beam diameter three times than original one. A reflecting film was coated on the dichroic to reflect completely the laser beam, whereas, the film cannot obstacle the visible light of illuminator and the flame through the dichroic. The microscope objective (Olympus, Tokyo, Japan), with numerical aperture of 0.67 and magnification of 40, can highly focus the laser beam. The waist diameter of
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focusing beam is about 3.0 lm. The highly focused laser beam, called as the OT, will exert extremely small forces on fuel particles. The OT traps the fuel particles, and positions them at the light axis near the beam waist. As the power density increases, the OT will heat the particles and ignite them. An illuminator and a condenser supply uniform bright illumination. The transmitted light travelling through the objective and the dichroic is collected by a lens, and then enters into the CCD by filtering. The filter located in front of the CCD minimizes the interference of laser beam. Simultaneously, the flame in the combustor can be also picked up and recorded by the CCD. The CCD camera (Microshot technology Co. Ltd., Guangzhou, China) has full pixels of 2592 1944 with 25 frames per second. The meso-scale combustor is a cuboid, which was made up of by two layers of materials. The bottom layer material is transparent flat glass, and the top layer one is cover glass. The inner dimensions of the combustor are 20 mm (length) 8 mm (width) 170 lm (depth). Both ends of meso-scale combustor were open in the stagnation air at atmospheric pressure, i.e., the combustor was a static, open volume one. The oxygen diffused by natural convention. The meso-scale combustor was fixed on the stage with a 3axis (X, Y, and Z) translation that can be adjusted along with three axial directions with a solution of 10 lm. The prepared jet fuel/graphene suspensions or pure jet fuel were sprayed into the combustor by a micropump, and liquid film formed on the bottom wall of combustor. In this study, the top layer wall of meso-scale combustor is a transparent plane glass, and thus the beam highly focused by the microscope objective goes through the plane glass with little attenuation. It will not result in the movement of the OT focal point. If the top layer wall of combustor is a convexity or a concavity, the top layer glass should be considered as an equivalent lens, i.e., a convex lens or a concave lens. This indicates that the OT beam will be more convergent or more divergent, which tends toward make the OT focal point move upward or downward, respectively. The focal point of the beam that goes through the convex lens or the concave lens can be calculated by the lens imaging formula. Therefore, the OT igniter beam will be significantly affected by the curvature of combustor wall, which leads to the change of ignition point and ignition delay. The larger the curvature of combustor wall is, i.e., the smaller the radius of combustor wall is, the more serious the impact on the ignition location and delay will become.
Fig. 2. The schematic of experimental setup.
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3. Results 3.1. Ignition and combustion of pure jet fuel liquid film Pure jet fuel with the mass of 10 mg was injected into the mesoscale combustor by the micropump. Jet fuel quickly extended and formed liquid film on the bottom wall. As the OT with the power of 420 mW (power density of 5.9 1010 W/m2) was turned on and last at least one minute, the input power is at least 25.2 J, which is much higher than the vaporization energy of 2.81 J for the whole jet fuel liquid film. However, it is astonishingly found that the liquid film did not be ignited at all. It can be observed that the boundary of liquid film will alternatively expand or shrink at the corresponding on/off status of the OT, as shown in Fig. 3. In each picture, the dark color represents the boundary of jet fuel liquid film. The arrow represents movement direction of the jet fuel boundary (i.e., the interface between the air and jet fuel), and the sign ‘‘v” means that jet fuel boundary moves with certain a velocity. Fig. 3(a)–(d) demonstrated the expansion of liquid film to lower left corner of these images as the OT turned on. On the contrary, turning off the OT, the liquid film contracted to top right corner of these images (Fig. 3(e)–(h)). The expansion of liquid film increased heat transfer area, and enhanced the heat loss of combustor wall. It has been proved in Refs. [3–7] that the heat loss of combustor wall is large due to high surface to volume ratio at small scale. Significant heat loss of combustor wall should be the obvious reason for non-ignition of pure jet fuel–air mixture. In addition, low Reynolds number due to natural convection and insufficient supply of oxygen should also be not negligible reasons. 3.2. Ignition and combustion of graphene Ignition and combustion experiments of graphene were carried out to obtain the threshold ignition power and ignition delay time. In meso-scale combustor, graphene in the stagnant air at atmospheric pressure was ignited with the delay time below 6 ms by the OT. The minimum ignition power is only 3.2 mW. The size of graphene scarcely influences the minimum ignition power. As the minimum ignition power inputted, the graphene took flameless combustion (Fig. 4(a)) and decreased in size (Fig. 4(b)). As the OT power increased till 15.0 mW, the graphene combusted heterogeneously with dazzling flame (Fig. 4(c) and (d)). 3.3. Ignition and combustion of jet fuel liquid film containing graphene The jet fuel/graphene suspension was pumped into meso-scale combustor, extended and formed liquid film on the bottom wall.
The boundary of liquid film was clearly observed. Inside liquid film, graphene Brownian motion and collisions among them were also clearly identified. The OT was positioned near the upper surface of liquid film, the graphene particles dispersed into jet fuel moved toward the OT focal point, and was quickly trapped (Fig. 5(a)). The white spot represents the OT focal point, and the black spots are graphene particles. The graphene particles were then ignited by the OT with ignition delay time below 40 ms, which is over six times than that in air (6 ms) as stated above. Since the input ignition power of 420 mW is over 130 times than the minimum ignition power of graphene (3.2 mW), the graphene combusted with a dazzling flame (Fig. 5(b)). In Fig. 5(c), the bubble formed, and the graphene around the OT focal point gathered together and combusted. At that moment, a large size graphene particle was dragged and affixed to the outer boundary of bubble (Fig. 5(d)). Turning off the OT or moving the bubble far from the OT transiently, the bubble still stably settled inside liquid film. However, the bubble disappeared if the liquid film was ruptured. To testify the particle size effect on ignition, we choose a micron-sized graphene inside liquid film to ignite. Fig. 6 showed the similar burning behavior of the micron-sized graphene dispersed into liquid film. The micron-sized graphene was firstly trapped by the OT, but the trap efficiency was low. The ignition delay (below 40 ms) of micron-sized graphene dispersed in the liquid film kept the same as that of nano-graphene. The combustion flame and the bubble were clearly observed. The burning process of jet fuel liquid film containing graphene powders was illustrated in Fig. 7. During the combustion of graphene shown in Fig. 7(a), jet fuel was ignited (see supplementary material S1). The flame front suddenly diffused toward the air, and then microexplosion occurred. The main flame stabilized at the center, and the smaller flame located at lower right corner and gradually extinguished, as shown in Fig. 7(b)–(e). In the following stage, the main flame front fluctuated slightly, and then extinguished gradually with the consumption of jet fuel (see supplementary material S2). During the process, graphene outside the focal point moved toward the center of flame, and combusted after jet fuel was mostly vaporized. This indicates that the combination of dispersive graphene powders and OT igniter is conducive to stabilize the flame of jet fuel at meso-scale. The whole combustion process sustained 3 min till jet fuel was completely consumed. Since the combustion temperature exceeds the glass fusion one, the meso-scale combustor was ablated. Many pits and a black cave emerged on the bottom wall (Fig. 7(f)).
Fig. 3. The expansion or shrinkage of pure jet fuel liquid film at the corresponding on/off status of the OT, (a) 0 s; (b) 0.04 s; (c) 0.08 s; (d) 0.2 s; (e) 11.76 s; (f) 11.88 s; (g) 11.96 s; (h) 12.2 s.
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Fig. 4. Ignition and burning behavior of the graphene in air, (a) and (b) flameless combustion in an input power of 3.2 mW at 0 s and 0.162 s; (c) and (d) bright combustion flame in an input power of 15.0 mW at 0 s and 0.1 s.
Fig. 5. An ignition and burning sequence of the stabilized jet fuel liquid film containing graphene in an input power of 420 mW, (a) 0 s; (b) 0.04 s; (c) 0.28 s; (d) 0.52 s.
4. Discussion The distinctive stages can be identified for different additives in liquid fuel. Takahashi et al. [32] proposed a three-step mechanism based on experimental observations, including the d2-law combustion, shell formation and disruption stages. Byun et al. [33] pro-
posed a stage in addition to the pressure buildup stage. Gan and Qiao [11] identified five distinctive stages for an n-decane/nanoAl droplet with the surfactants. In this work, the distinctive stages and mechanism on ignition and combustion of jet fuel liquid film containing graphene powders will be discussed as following.
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Fig. 6. Another ignition and burning sequence of the stabilized jet fuel liquid film with addition of micron-sized graphene in an input power of 420 mW, (a) 0 s; (b) 0.04 s; (c) 0.28 s.
Fig. 7. The burning sequence of jet fuel liquid film containing graphene powders, (a) 0 s; (b) 3.72 s; (c) 7 s; (d) 9.6 s; (e) 45.92 s; (f) picture of combustor ablation.
The results showed that jet fuel can be ignited and sustainably combusted with the aid of graphene. This indicates that graphene is conducive to stabilize the flame of jet fuel at meso-scale. On one hand, the graphene serves as a nucleus of boiling for jet fuel vaporization. On the other hand, it is of great importance that graphene reaction heat serves as another heat source for ignition of jet fuel. On the basis of experimental observations, a schematic physical model for ignition and combustion was established and illustrated in Fig. 8. Detailed procedures of ignition and combustion of jet fuel liquid film containing graphene may be separated into five steps: Step I: Graphene is trapped by the OT In Fig. 8(a), the OT is positioned near the upper surface of liquid film. Aggregated graphene particle suffers from a resultant force including radiation pressure, thermophoretic force, photophoretic force, Brownian force and drag force. Graphene particle will be then pulled by the OT to the upper surface of liquid film. During step I, both graphene and liquid film will be preheated. As the resultant force is directed to the OT focal point, graphene particle dispersed in the jet fuel will be efficiently trapped by the OT. The event has been demonstrated in Figs. 5(a) and 6(a). Maragó et al. [34] also showed the trap and manipulation of nanographene in another base fluid. However, another event can also occur. For strong absorbing particles, the resultant force can inversely direct toward the OT
focal point since photophoretic force dominates [35], which makes graphene particle move far from the OT focal point. The phenomenon was observed as graphene is dispersed in the nitromethane in our experiments. Therefore, the trap of graphene is one of key steps for ignition and combustion of jet fuel. To guarantee effective trap, some recommendations can be made, like adding surfactant [36] and using an optical bottle beam [37]. Step II: Graphene is firstly ignited, and combusts In Fig. 8(b), the air fills with the upper space of mesocombustor. Near the end of step I, the graphene particle is ignited by the OT. Since graphene has ultra low minimum ignition power and short ignition delay time in comparison with jet fuel, graphene will be ignited by the concentrated OT igniter prior to jet fuel. The OT power is higher than the minimum ignition power of graphene, and thus the dazzling flame was observed (Figs. 5(b) and 6(b)). The reaction between the graphene and oxygen includes the following four cases:
C þ O2 ¼ CO2 393:5 kJ
ð1Þ
2C þ O2 ¼ 2CO 221:0 kJ 2CO þ O2 ¼ 2CO2 566:0 kJ
ð2Þ ð3Þ
C þ CO2 ¼ 2CO þ 172:5 kJ
ð4Þ
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Fig. 8. A schematic physical model for ignition and combustion of jet fuel liquid film containing graphene, (a) graphene trap; (b) ignition and combustion of graphene; (c) nucleus of boiling and bubble formation; (d) jet fuel vaporization and bubble growth; (e) bubble rupture and combustion of liquid film.
The first and second reactions represent the complete combustion and partial combustion of graphene. The third reaction shows the delayed combustion. The surface of graphene is only oxidized as CO, and then CO will react with diffusive oxygen to generate CO2. The fourth reaction demonstrates that CO2 is reduced by the graphene. The exothermic heat of per unit mole graphene is between 110.5 and 393.5 kJ, which assists the vaporization of jet fuel and benefits the flame stability of jet fuel by compensating the heat loss of meso-combustor wall. Step III: Graphene serves as nucleus of boiling to form the bubble as the graphene is ignited In Fig. 8(c), the ignited graphene can be considered as a nucleus of boiling. Gas phase products are sealed by jet fuel, leading to form a bubble. Graphene located inside the bubble sustainably combusts (Figs. 5(c) and 6(c)). The burning time of graphene is dependent of the graphene size. Step IV: Vaporization of jet fuel is accelerated and the bubble grows In Fig. 8(d), the vaporization of jet fuel is accelerated by the exothermic heat of graphene particle combustion and the heat of OT. The blend of jet fuel vapor and gas products diffuse quickly and make the bubble expand. Supposing graphene particle is spherical and its diameter and density is 2 lm and 0.2 103 kg/
m3 respectively, the heat release of complete reaction for single graphene particle is 107 J. As thousands of graphene particles gather together and combust, the heat release will be up to 104–103 J. The heat of OT per millisecond is 4.2 104 J. At the same time of graphene combustion, it is likely that jet fuel is also being oxidized. There exists an oxidized competition between graphene and jet fuel as the oxygen is insufficiently supplied due to low natural convection at meso-scale. Experimental observation shown in Figs. 5 and 6 demonstrated, in early ignition stage of jet fuel, the oxidizing combustion of graphene is prior to the jet fuel, and thus oxidation of jet fuel is weak. Experimental results of bubble expansion were demonstrated in Fig. 9. Expansion displacement and expansion velocity of the bubble were extracted and plotted in Fig. 10(a) and (b). The relationship between the expansion displacement and time can be exponentially fitted as s = 232.202–163.315 e0.016 t with R2 of 0.993. The expansion velocity can be expressed as v = 10.525 t0.447 by allometric function fitting with R2 of 0.997. Here, s and v represent expansion displacement and expansion velocity of bubble, respectively. In Fig. 10(a) and (b), the uncertainties of expansion displacement and expansion velocity of the bubble are dependent of the systematic error resulted from the calibration rule and resolution of images, and random error owing to repetitive measurement. By calculation, the uncertainties of expansion displacement and velocity of bubble are less than 0.5 lm and 0.1 lm/s.
Fig. 9. Expanding and growing sequence of the bubble, (a) 0 s; (b) 6.08 s; (c) 23.52 s; (d) 76.44 s; (e) 120.04 s; (f) 175.36 s.
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Fig. 10. The curves of bubble growth, (a) expansion displacement; (b) expansion velocity; (c) bubble boundary thickness.
Fig. 11. Enlargement of the bubble boundary thickness, (a) 0 s; (b) 1.76 s; (c) 9.16 s; (d) 10.92 s.
In Fig. 9, the bubble boundary thickness almost kept constant, since graphene was sparsely dispersed in jet fuel. As graphene is densely dispersed in jet fuel, the boundary thickness will also enlarge, as shown in Fig. 11. The bubble expanded, and simultaneously the bubble boundary thickness increased. Owing to the density and temperature gradients, quantities of graphene dispersed in jet fuel gathered around the bubble boundary. Large size graphene
stayed static outside the bubble, however, small graphene particles moved and rearranged. The enlargement of bubble boundary thickness limited the expansion of bubble. The bubble boundary thickness versus time was plotted in Fig. 10(c). Since the preliminary distribution of graphene in jet fuel complies with Boltzmann distribution law, the accumulation of graphene on the bubble boundary will be followed by Boltzmann
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function (d = 34.099–31.456/(1 + e(t4.288)/0.670)) with R2 of 0.964. The uncertainty of bubble boundary thickness is less than 0.3 lm. Here d is the bubble boundary thickness, and t is time. The bubble boundary thickness is dependent of the graphene concentration in jet fuel. The growing velocity of the thickness relates to the OT power and viscosity of jet fuel.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.03.004.
References Step V: Bubble ruptures and liquid film sustainably combusts In Fig. 8(e), the bubble ruptures, jet fuel vapor escapes from the upper surface of liquid film. The vapor is ignited, and the liquid film combusts till burns out (Fig. 7). During combustion, graphene outside the focal point cannot be directly heated by the OT, and only depends on the heat transfer of jet fuel. Therefore, it combusts after jet fuel is mostly vaporized, like water coal slurry. Graphene enhances the burning rate of liquid film due to high thermal conductivity and radiation effect.
5. Conclusions Ignition and burning characteristics of jet fuel liquid film containing nanometer and micron-sized graphene powders were investigated at meso-scale. The emphasis was to explore the differences of burning behaviors between pure jet fuel and jet fuel/graphene suspension, and to understand the combustion mechanism of jet fuel liquid film containing graphene. The following conclusions can be drawn. (1) Nanometer and micron-sized graphene is dispersed in jet fuel. Nanometer graphene in suspensions remains stable for much longer time than micron-sized graphene. The dispersion stability of the suspensions is not good, but it can be improved by adding surfactant or selecting smaller sized graphene. (2) Pure jet fuel liquid film cannot be ignited in such a short time. However, jet fuel liquid film containing graphene powders can be inductively ignited with short delay time. (3) The combustion of graphene is prior to the ignition of vaporized jet fuel. The combination of dispersive graphene particles and OT igniter can stabilize the flame of jet fuel at meso-scale, since the graphene serves as a nucleation point for accelerating the vaporization of jet fuel, and its combustion heat serves as heat source to assist the ignition and combustion of jet fuel. Jet fuel combusts sustainably till burns out with the support of graphene additive. (4) Five distinctive stages were identified. Particle size scarcely influences the distinctive stage, but micron-sized graphene requires higher trap power and longer burning time than nanometer graphene. Graphene concentration is crucial to the bubble growth. In addition, the OT ignition system has been proved as a novel igniter for in situ ignition of a microflame outside the combustor and stabilizing the ignition at the desired location within the combustor at small scale.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51276053 and 51506041), Natural Science Foundation of Zhejiang Province (LY14E060002 and Y15E060027), Young and middle-aged leading academic climbing project of Zhejiang Province (pd2013158).
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