On the microexplosion mechanisms of burning droplets blended with biodiesel and alcohol

Combustion and Flame 205 (2019) 397–406

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On the microexplosion mechanisms of burning droplets blended with biodiesel and alcohol Chi-Yao Chao, Hsuan-Wei Tsai, Kuo-Long Pan∗, Chih-Wei Hsieh Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 22 November 2018 Revised 5 April 2019 Accepted 6 April 2019 Available online 3 May 2019 Keywords: Droplet combustion Biodiesel Alcohol Microexplosion Heterogeneous

a b s t r a c t Microexplosions are reported to occur frequently in droplet combustion of biodiesel mixed with alcohol, which are nominally miscible. We found, however, that they could be prevented if the standing time of fuel samples before burning was sufficiently long. To realize the microexplosion mechanisms, experiments based on suspending droplets and free-falling droplets were conducted. It was found that, before ignition, heterogeneous sites, which appeared similar to small drops, emerged from the droplet surface and subsequently merged into a bigger one, which then deposited at the bottom of the droplet. The diameter of this heterogeneous drop enlarged with increasing ambient humidity. At the same humidity, by using methanol, ethanol and isopropanol, respectively, the experiments showed that the merged heterogeneous drop of biodiesel/methanol was the largest and the smallest one was generated by that mixed with isopropanol. These heterogeneous sites were likely caused by the hygroscopic characteristics of alcohol fuels, which were supposed to mix well with biodiesel fuel and treated as a miscible blend in the first place. The miscibility, however, was destroyed by the absorbed water and phase separation occurred, leading to microexplosions when the droplet was burned. On the other hand, the interior heterogeneous drop vanished after sufficiently long time of standing prior to burning, and consequently no microexplosion was observed during the whole combustion process. For the mixture fuel of alcohol, methanol had higher propensity to absorb water and needed longer standing time for dissolution of the heterogeneous drop, as compared to ethanol and isopropanol, which had lower hygroscopicity. From the results, the heterogeneous sites inside the blended droplet were concluded unambiguously to be the primary cause of microexplosions during the combustion process, leading to the highest burning rate at equi-volume proportion of mixed fuels. In contrast, elimination of the heterogeneous sites in the burning droplets could prohibit microexplosions and yielded the highest burning rate at 25% fraction of alcohol. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Biodiesel is considered to be an attractive alternative to the petroleum-based fuels due to its renewable nature, lower emissions of pollutants, various sources of production, and mitigation of greenhouse effects. As compared to petroleum diesel, it is also attractive for the biodegradability, higher flash point and lubricity. Alcohol is another possible alternative. It can be produced from a wide range of sources, either from biological or organic origins, and is featured with reduced formation of soot during combustion due to its inherent non-sooting nature. By considering both, the mixtures of biodiesel and alcohol possess great potential to become an energy source that would meet the present needs for immediate and near future utilization, thus drawing our attention.

∗

Corresponding author. E-mail address: [email protected] (K.-L. Pan).

They can be mixed further with diesel, owing to its miscibility with biodiesel which can serve as a co-solvent and provide a mixing interface with alcohol, to mitigate soot formation [1,2] and produce synthetic fuels of sufficiently high energy density. Droplet mixtures consisting of a high-volatility component and a low-volatility component have a high potential of creating secondary atomization following explosive boiling inside the superheated liquid. The droplet fragmentation with disruptive burning is known as a microexplosion. The related phenomena have been investigated in terms of different blended fuels and environment [3–5]. Droplet combustion of biodiesel and alcohol mixtures were usually observed to show microexplosive behaviors, e.g., in the experiments using the technique of free-falling droplets [2] and that using the suspending fiber method [6,7]. When an microexplosion occurs, the overall burning rate is generically much higher than the cases without microexplosions, thus rendering completely different results. Burning with such instant disruption and hence rapid dispersion of fuel masses into the gas medium would yield

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substantial reduction of soot, CO and NOx formation and thus is favorable in many situations of liquid-fueled combustion. Occurrence of microexplosions as reported in the literature, however, was usually stochastic. Larger propensity and intensity were reported by Botero et al. [2] for near equi-volume mixtures, thus leading to a prominently higher consumption rate of the blended fuels. In their study, the technique of free-falling droplets was used to investigate the combustion characteristics of premixed fuels of biodiesel, diesel and ethanol. Due to homogeneous nucleation resulted from the large differences in the boiling points, causing superheating of the more volatile components inside the droplets, microexplosions were observed in the processes of combustion. Similar results were also observed in experiments using the suspended droplet method. For instance, Hoxie et al. [6] used two ceramic fibers with a nominal diameter of 11 μm for suspending a droplet and studied the combustion characteristics of soybean oil/butanol blended droplets. They observed a three-staged burn during which a microexplosion was generated for all mixed droplets. The microexplosion occurred as a result of the more volatile component trapped within the droplet superheating at flame shrinkage, which exhibited the greatest intensity for equivolume mixtures that were suggested to be the most favorable blends. Avulapati et al. [7] used a thermocouple to suspend a droplet, ranging from 1 to 1.5 mm in diameter, at the tip, to study the combustion characteristics of diesel-biodiesel-ethanol blended droplets. The size of the thermocouple bead was about one tenth of the droplet diameter and it was claimed not to initiate nucleation, puffing or microexplosions. The experiment showed that microexplosions occurred in the combustion process, with initial ethanol percentages ranging between 10% and 40%. Extensive studies following similar tracks have been reported and showed essential formation of microexplosions in combustion of alcohol/biodiesel blended fuels [8,9]. On the other hand, complete burning of diesel/biodiesel droplets mixed with alcohols including ethanol, without microexplosions, was observed and investigated in our previous work [1]. By using two ultra-thin ceramic fibers of 2.5 μm in diameter, a droplet of ∼500 μm, made of blended biodiesel, diesel and various proportions of alcohols, was suspended on the cross point and burned in a microgravity condition created in a drop tower designed for limited lab space. The result indicated a larger burning rate with increasing proportion of alcohol, whereby a nonlinear trend was speculated to exist at higher proportions. Large uncertainty of experiment, however, was yielded by the low repeatability observed particularly with a high fraction of alcohol in the blended fuel and hence large variation in the droplet burning rate. By performing a series of tests, microexplosion phenomena were observed sometimes while the occurrence rate could hardly be assessed. In [1], the unknown factors causing microexplosions were conjectured to relate with heterogeneous sites inside the droplet or internal inhomogeneity such as inherent impurities of diesel/biodiesel fuels or bubbles generated by the fibers. Due to the unpredictable nature, as stated in the article, microexplosions were not investigated in more detail and only complete burning cases were focused, with discussion of the burning rate, preheating delay, and soot formation which were related to the physiochemical properties of the blended fuels. Most of previous studies have attributed the cause of microexplosions of biodiesel-alcohol blends to the general superheating mechanism of homogeneous nucleation, which leads to internal gasification and disruptive burning. The microexplosive phenomena are usually created in a stochastic manner and can hardly be predicted accurately, particularly considering variations of the experimental techniques and conditions of droplets. In the present study, however, we have found that the occurrence of

microexplosions of the blended fuels could be controlled and interpreted in a systematic way. We examined the combustion process of a single droplet mixed with biodiesel and various alcohols (methanol, ethanol, and isopropanol), and investigated its microexplosive behaviors. Recognizing the constraints and merits of different approaches for controlling burning droplets, two different state-of-the-art techniques, i.e., the methods of free-falling droplets and suspending droplets, have been adopted. They were used to identify the effects of various factors and verify the results at desired experimental conditions. Specifically, to understand the causes of microexplosions, the intrusive effects of suspending fibers, gravity and buoyancy, relative humidity of the ambience, and the miscibility between the ingredients have been considered. Moreover, to distinguish the burning behaviors, both of the cases with and without microexplosions have been investigated. This study thus provides more comprehensive elucidation of microexplosions, which shall help further understanding and utilization of the combustion characteristics of blended biodiesel and alcohol droplets. 2. Experimental setup The experimental investigations are based on the approaches of suspended droplets and free-falling droplets. For the suspended droplet method, a fuel droplet was hung by two crossed fibers and could be clearly monitored throughout the combustion process in both conditions of normal-gravity and microgravity. For the freefalling droplet setup, a stream of droplets were generated steadily and heated to burn while moving downward without being constrained. The general descriptions are given in the following. More details specifically regarding the uncertainties and operational processes can be found in [1,10,11]. 2.1. Suspended droplet method The experimental configuration of apparatus is shown in Fig. 1(a) and (b). The drop tower is ∼3.5 m in height, as shown in Fig. 1(a). The burning experiment is performed in the combustion chamber, which is located inside the drop package as shown in Fig. 2. The package was enclosed by a drag shield used to reduce the gravity (g-) level up to ∼10−2 to 10−4 . Thus the Grashof number, Gr = gD3 T/(ν 2 T), was estimated to be around 10−4 , leading to an environment with negligible effects of natural convection. A fuel droplet was produced by a piezoelectric generator. The droplet size, with an initial diameter of 500−560 μm, was mainly controlled by altering the size of the glass nozzle. One or several droplets were injected and stabilized on the crossing point of two ceramic fibers (2.5 μm in diameter), and then placed into the

Fig. 1. Schematic plots of the experimental setup: (a) the drop-tower facility containing (1) foam cushion, (2) drag shield, (3) drop package, (4) electromagnet, and (5) lifter; (b) the freely-falling droplet facility containing (1) gas control panel, (2) strobe-light, (3) flat-flame burner, (4) quartz tube, (5) droplet generator, (6) CCD camera, and (7) Nikon digital camera.

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Table 1 Properties of fuels. Methanolb

Ethanolb

2-Propanolb

Biodiesela

Molecular formula

CH4 O

C2 H 6 O

C3 H 8 O3

Density at 20 °C (g/cm3 ) Boiling point (°C) Vapor pressure (kPa at 20 °C) Flash point (°C)

0.792 65 13.2 11–12

0.789 78.4 5.95 13–14

0.786 82.5 5.33 ∼12

RCOOCH3 , R=C15 H31 , C17 H35 , C17 H31 , C17 H33 0.882 204 <0.133 167

a b

Fig. 2. Schematic plots of the drop package: (a) side view, (b) front view, (c) top view, showing (1) foam cushion, (2) high speed camera (color), (3) high speed camera (black/white), (4) reflection mirror, (5) LED lamp, (6) droplet carrier, (7) push– pull solenoid, (8) hot wire, (9) droplet, and (10) suspension fibers.

combustion chamber. The whole package was then hung by an electromagnet at the top of the tower. After the trigger via an electronic signal, the package was released, undergoing a free-falling process of ∼0.68 s. The droplet was heated during the fall by a pair of hot wires, providing a total energy of 90 W in ∼370 ms. In order to minimize the excess heat generated by the hot wires, which would raise the environmental temperature, they were set to retreat by a pair of push–pull solenoids after ignition. In general, the droplet would be ignited after 0.1 s from the retraction of the electromagnet so as to avoid experimental uncertainty caused by initial vibrations. After a free-falling process, the drop package landed on the ground, and a single test was completed. Without falling, this setup was also used to study droplet combustion in normal gravity. During the freely falling process of the whole package, the images of the droplet and flame were recorded respectively by a black/white camera (EPIX Silicon Video 642M, 200 fps in 640 × 480 resolution) and a color camera (EPIX Silicon Video 9M0 01C, 20 0 fps in 640 × 240 resolution), as shown in Fig. 2. The recording path of the black/white camera was oriented through a mirror on top so as to provide a top view of the suspended droplet, while that of the color camera was directly through the transparent window on the wall of the chamber, thereby giving a side view for the burning process. The timing and delay of all operating facilities including the heating wire, push–pull solenoid, electromagnet, and droplet generator were controlled electronically via a computer. 2.2. Free-falling droplet method Figure 1(b) shows a schematic of the experimental setup for freely falling droplets. The droplets were created by a glass nozzle connected to a piezoelectric droplet generator, whereby the test liquid was supplied from a reservoir and squeezed out by the vibration of a piezoelectric plate. The droplet size was mainly controlled by altering the size of the glass nozzle. In the experiments, the initial diameters of droplets were about 360 μm. The freefalling droplets were then adjusted to pass through a hole with 5 mm diameter and enter a chamber of burner-generated, hightemperature and uniform-flowing flue gases, in which the droplets were ignited and burned. The chamber inlet was made of porous bronze metal for sustaining a flat flame used to ignite droplets. The droplets entered the combustion chamber with a speed ∼70 cm/s, hence with the maximum Reynolds number of O(1). The

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environment had a temperature ∼1030 °C and residual oxygen concentration of 21 vol%. After the flat-flame burner, the temperature declined at a rate ∼18 °C/cm in the quartz tube [12]. The droplets were generated at a frequency of 10 Hz and so they were separated by over several hundred of diameters to minimize the effects of droplet interactions. To facilitate observation, photography and assembly, a quartz tube was used to confine the flue gases. During the free-falling process, the images of droplets and the flame were recorded respectively by a CCD (MINTRON 64V1N 800 × 400) with a long focus microscopic lens (Baush & Lomb Mono-zoom-7) and a color camera (NIKON D7100). A stroboscope having variable phase lag was synchronized with the droplet generator for exposure such that the camera could monitor the moving droplets at any instant of interest during their lifetime. The uncertainty in the determination of droplet size was estimated to be about 2%, mainly from reading the boundary of the droplet image. The droplet burning process was visualized via the flame streak and microphotography of the droplet images. 2.3. Fuel preparation In the experiment, the binary-component fuels were made of biodiesel plus methanol, ethanol, and isopropanol, respectively. The alcohol used was purchased from the commercial market, which had different purities of 95% and 99.5%. To minimize the influence of impurities specifically of water, 99.5% alcohol was used as the base fuel. Furthermore, to assure stability and consistency, the mixtures which appeared well-mixed after being stirred up by an ultrasonicator for 15 min were stored for 24 h before tests. The issues regarding the solubility and stability have been discussed in [13,14]. The biodiesel used was also purchased from the commercial market, which had properties as that listed typically for the material. The properties of biodiesel are summarized in Table 1 [1]. While its properties may change slightly due to variations of the constituents, the major outcomes considered in the study regarding microexplosions would not be affected essentially due to the distinct natures of the tested fuels as to be discussed. Specifically, as tested for pure biodiesel, no heterogeneities and microexplosions (ME) were observed. Different commercial biodiesels may have somewhat different fractions of liquid hydrocarbons, but they do not change the mechanisms of ME as investigated herein. 3. Results and discussion 3.1. Combustion characteristics of biodiesel/alcohol blended droplets Figure 3 shows the burning processes of biodiesel droplets doped with various proportions of ethanol in the free-falling experiment. In the long exposure of image recording process, the streaks present the traces of the burning droplets throughout their lifetime. Microexplosions always occur in combustion of the binary

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Fig. 5. Visualization of a suspended B75E25 droplet before ignition showing (a) a top view, (b) a top view focused on the interior drop, and (c) a side view.

Fig. 6. Transient images of a freely-falling B75E25 droplet before passing the flatflame burner (∼4–5 cm after the nozzle exit of the droplet generator).

Fig. 3. The burning process of freely-falling droplets composed of (a) biodiesel only, (b) 75% biodiesel and 25% ethanol, (c) 50% biodiesel and 50% ethanol, (d) 25% biodiesel and 75% ethanol, and (e) ethanol only.

fuels, as demonstrated by the fireballs created in the mean time before the droplet is completely burned out, but not of single fuel (Fig. 3(a) and (e)) which renders gradual shrinking of the streak till the end. Moreover, as seen in the end of the flame streaks, the shortest streak shown in Fig. 3(c) indicates the earliest generation of prominent disruption and hence largest overall burning rate. Figure 4 shows the burning process of a suspended droplet composed of 75% biodiesel and 25% ethanol (designated as B75E25) in the conditions of normal and reduced gravity, under which a spherically symmetric flame is formed (as shown in

Fig. 4(b) with microexplosion and Fig. 4(c) without microexplosion). Microexplosions were clearly observed in the burning processes (Fig. 4(a) and (b)) but not all the time. These microexplosive phenomena were dramatically disruptive, essentially distinct from those exhibiting much weaker ruptures usually known as puffing [7,8,15]. Furthermore, changing the droplet size in the available range appeared not to compromise the essential pattern of abrupt disruption, as demonstrated in the Supplementary Material (Fig. S1). Scrutinizing the droplet interior, a prominent inhomogeneity resembling an inner drop can be identified in the suspended droplet before ignition (Fig. 5). Due to the interior heterogeneity, microexplosions always occurred during combustion. The same consequences were observed for the binary fuels of biodiesel mixed with methanol and isopropanol, respectively. Similar formation of heterogeneous sites is also observed in the freefalling droplet experiment where the droplet is not fixed by fibers, as shown in Fig. 6. In the figure, it is observed that non-uniform sites appear during the free fall (∼30 ms), in contrast to the uniform appearance observed at the beginning (1). Whenever such inhomogeneities were formed, microexplosions were found to occur all the time. 3.2. Analysis of internal inhomogeneity in biodiesel/alcohol droplets

Fig. 4. The burning process of suspended droplets composed of B75E25 in (a) normal gravity, (b) microgravity without standing time, and (c) microgravity with standing time.

3.2.1. Effect of droplet generation on internal inhomogeneity of a blended droplet To understand the causes of inhomogeneities inside the droplet before ignition, we first check the generation of droplets, specifically on the miscibility between alcohol and biodiesel. Before a droplet was generated, the premixed fuels were filled inside the droplet generator (Fig. 7(a)). It is seen in Fig. 7(b) that, originally, the binary fuels appear miscible with each other, which can be justified by the transparency inside the nozzle. When the piezoelectric diaphragm starts vibrating, however, the liquid becomes opaque specifically near the head, which looks like an emulsion, as shown in Fig. 7(c). During the process, the liquid was squeezed through a nozzle and violently vibrated at the contraction region of nozzle. This might cause formation of inhomogeneity and internal nucleation inside the generated droplet, due to breaking of the diffusion equilibrium between two different fluids [16,17]. To further identify the mechanism, we varied the location of liquid–air

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Fig. 8. Droplet images of (a) glycerol+water (b) using a backlit LED and of (c) water+biodiesel and (d) biodiesel+methanol.

of alcohols. They caused the partly-miscible fuels of biodiesel and alcohol to separate temporarily, likely due to breaking of the diffusion equilibrium between two different fluids [16,17]. The separation gradually relaxed and the droplet could return to its original state due to diffusion between the two fuels, which allowed the mixture to become miscible again. This scenario will be further manifested with more demonstrations to be discussed. Based on the realization, removal of the internal inhomogeneity could be achieved through a settling process, after which a well-mixing condition is recovered and combustion without microexplosion can be obtained.

Fig. 7. Droplet generation process of blended fuels through a nozzle with an orifice diameter Dn = 0.62 mm, showing (a) the configuration and images of (b) initially clean liquid, (c) inhomogeneity formed with vibration, (d) clear liquid with an interface against air in the middle region, (e) opaque liquid with an interface at the exit, (f) non-uniform liquid with larger perturbation in the middle region, (g) turbid liquid filled in the nozzle with overflow on the outer surface, and (h) clean liquid under smaller perturbation with Dn = 0.13 mm, assisted by an enlarged view near the nozzle exit.

interface from the exit to the middle region of the nozzle. As shown in Fig. 7(d) with a liquid surface being at the middle, the liquid is clean and no turbidity is generated owing to the perturbations. With an interface near the exit, however, the liquid underneath the perturbed surface becomes opaque, as shown in Fig. 7(e). In this situation inhomogeneities were generated where larger perturbations were rendered due to area contraction and the interface was in contact with the ambient air. In contrast to Fig. 7(d), furthermore, with the interface located in the same region at the middle of the nozzle but given a larger perturbation, such opaqueness is observed as well, as shown in Fig. 7(f). Moreover, Fig. 7(g) shows that when the liquid flows over the exterior surface of the nozzle due to the vibration, even weaker, turbidity is clearly created both outside and inside the nozzle. When a smaller nozzle is used, without flow over the exterior surface, and weaker perturbation is given, the liquid appears clear, as shown in Fig. 7(h). To further demonstrate the phenomena, we have tested the mixtures of glycerol+water and water+biodiesel, which are completely miscible and immiscible, respectively, and compared the results to the mixing phenomena of biodiesel+methanol. As shown in Fig. 8(a), a glycerol+water droplet appears transparent, which is further identified by the image manifested by a backlit LED with higher intensity in Fig. 8(b). For a water+biodiesel droplet, however, multiple bubble-like sites are observed inside the droplet due to the immiscibility, as shown in Fig. 8(c). This shares a similar appearance with that of a biodiesel+methanol droplet which has undergone a standing process, as shown in Fig. 8(d). From these results, it is conjectured that the internal nucleation and inhomogeneity were resulted from the perturbations during the droplet generating process, along with the hygroscopic features

3.2.2. Fiber effect It was analyzed in our prior studies [1,10] that heat transfer through the ultrathin fibers was negligible. Furthermore, in contrast to usage of only one fiber that would cause significant deformation of the suspended droplet [15], two fibers were used for suspension in this work, which would minimally distort the spherical shape of a droplet. Following the earlier discussion on the droplet generation process through a nozzle, we further investigate the next process of injecting a droplet onto the cross point of fibers. It was observed that, if the two fibers were not in close attachment, small bubbles could be generated inside the droplet, particularly after some relative movement of the fibers with gaps in between. As exemplified by the injection process of a droplet onto the crossing point of two fibers shown in the supplementary material (Fig. S2), which were not firmly attached in the first place, a bubble was formed. The size, location and pattern, however, are ostensibly different from that observed in Fig. 5(c) which shows a large drop at the bottom inside the droplet. Such small bubbles, however, were not formed if the fibers were tightly attached to each other (as performed in the experiment) and no interstice was yielded; they can thus be easily avoided in the experiments. To further identify the effect of fibers, various fibers with distinct materials and sizes have been tested (made of carbon with 7 μm, SiC with 14 μm, and ceramics with 2.5 μm in diameter). It was found that a large heterogeneous drop always appeared at the bottom of the droplet and its size did not change appreciably with the variation of suspending fibers. Interior inhomogeneities have been observed by using the suspended droplet method in both conditions of normal and reduced gravity, as well as the free-falling droplet method. From the observations, it is thus concluded that the fibers were not responsible for the formation of internal heterogeneous drops, which could cause microexplosions when the droplets were burned. 3.2.3. Hygroscopic characteristics of alcohol fuels It was concluded in [13] that, via tests at different temperatures, the mixtures of biodiesel and ethanol with purity ≥ 99.5% still appeared miscible after seven days. The consequence was also repeated in our preparation for the blended fuels when the bottle was covered by a lid (Fig. 9(a)). After removing the lid, however, a heterogeneous drop was formed from the liquid–air interface (Fig. 9(b)) for a B75E25 mixture. The heterogeneous drop then sank to the bottom (Fig. 9(c)); with a larger bottle, a few heterogeneous

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Fig. 9. The fuel blended of 75% biodiesel / 25% ethanol was stored in (a) an airtight bottle and (b) an open bottle, respectively. The latter showed formation of a heterogeneous drop at the air–liquid interface, which then sank onto the bottom (c).

drops were observed to sink at the bottom. These phenomena occurred also for methanol and isopropanol, and were more readily observable for methanol. Eventually, the heterogeneous drops disappeared after one month. To identify the compositions of the heterogeneous drops, the samples were analyzed by the Karl Fischer titration method, which can measure the percentage of water. The heterogeneous drops (0.02 g) were extracted by a needle from the bottom of the open bottle, generated in the mixture (7 g) of 75% biodiesel and 25% methanol. The measurement indicated that the sample was constituted of 60% water, while the other composition could not be identified. Since the mixture appears transparent, as shown in Fig. 10, it is surmised reasonably to consist of alcohol, in addition to water. Therefore, the heterogeneous drops should be resulted from the absorption of water by alcohol which then separated from the bulk biodiesel. The inhomogeneity is closely related to the heterogeneous drop inside the droplet as seen in Fig. 5, which is critical for occurrence of microexplosion. The cause can be interpreted by water absorption on the droplet surface. It is known that water from the air can be absorbed by low-C alcohol due to its hydrogen bond, rendering it a nature of deliquescence. This could introduce biodiesel/water interfaces due to immiscibility between ester and

Fig. 10. The liquid of heterogeneous drops extracted from the mixture of 75% biodiesel/25% methanol.

Fig. 11. Evolution of a B75E25 droplet sitting on the crossing fibers (d0 = 525 μm, t = 1.5 s).

water [18]. The hydroxyl group is hydrophilic and can enhance the solubility of alcohol in water. Methanol, ethanol, n-propanol, isopropanol, and tert-butanol are all miscible with water [19,20]. Alcohols with higher molecular weights are less water-soluble because the hydrocarbon part of the molecule, making it hydrophobic, is larger with increasing molecular weight. The hygroscopic characteristics of alcohol fuels may cause separation of the originally mixed ingredients, particularly when they are perturbed, as discussed in Section 3.2.1. To identify the scenario, Fig. 11 shows the evolving sequence of a biodiesel/ethanol droplet resting on the fibers. Homogeneity was seen at the initial state, whereas heterogeneous nucleation was observed after some time (∼3 s). Viewing from the top, the drop-like heterogeneous sites, emerged from the droplet surface and subsequently merged into a bigger one and settled to the bottom of the droplet, as seen from the side view (not shown but similar to Fig. 5(c)). Inhomogeneities were also observed in the free-falling droplet experiment (c.f. Fig. 6) though not as prominent as those seen in the suspended-droplet experiment where the droplets can be anchored on the fibers so as to facilitate visualization. Figure 12 shows different results of biodiesel blended with ethanol and butanol, respectively. When biodiesel mixed with ethanol having higher hygroscopicity, the experiment showed that

Fig. 12. Images of biodiesel droplets doped with (a) ethanol and (b) butanol, with a view focused on the interior drop as marked by the arrow.

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Fig. 15. The evolution sequence (side view) when a biodiesel droplet initially resting on the crossed fibers merges with a droplet of pure ethanol injected from the down side.

Fig. 13. The evolution sequence of a droplet composed of 75% ethanol/25% dodecanol sitting on the crossed fibers (d0 = 525 μm, t = 1.5 s).

a heterogeneous drop was created inside the droplet, but none was formed when biodiesel mixed with butanol having lower hygroscopicity. The same consequence was observed also for mixtures of 50% and 75% butanol, respectively. Concerned by the nominal miscibility between alcohol and biodiesel, to further identify the mechanism, the base fuel was changed from biodiesel to dodecanol. Analogous to biodiesel, it is immiscible with water but completely miscible with the alcohols as tested. The sequence of an ethanol/dodecanol droplet suspended on the fibers is presented in Fig. 13. Similar to that with biodiesel (Fig. 11), small heterogeneous drops were gradually generated and merged into a big one inside the droplet which was initially homogeneous. Therefore, phase separation occurred as well, demonstrating strong correlation to the hygroscopic features of ethanol. Formation of the heterogeneous drops inside the droplet due to hygroscopicity of alcohol after being injected onto the fibers can be further demonstrated with varied ambient humidity. Figure 14 shows the final size of the heterogeneous drop generated inside the biodiesel droplet blended with methanol, ethanol, and isopropanol, respectively, in an environment with different relative humidity (∼55% in Fig. 14(a) and ∼76% in Fig. 14(b)). The results showed that higher environmental humidity led to larger size of

Fig. 14. Images of heterogeneous drops inside the blended fuels of methanol, ethanol, and isopropanol, respectively, at different ambient humidity: (a) 55%; (b) 76%.

a heterogeneous drop. Moreover, liquid with higher hygroscopicity as methanol yielded a larger heterogeneous drop. To understand the scenarios and mechanisms creating the heterogeneous drops, the mixing process was investigated by injecting the fuels onto the fibers in different ways. First, a biodiesel droplet was settled at the cross point of the fibers. An ethanol droplet was then injected onto and merged with the biodiesel droplet. As seen in Fig. 15, they appear to well mix at the beginning, but soon inhomogeneities form and merge into a big drop adhering to the fibers. By giving a small knock on the fiber, the heterogeneous drop detached and sank to the bottom, showing the same phenomena as the cases discussed earlier (c.f. Fig. 5). It is thus proved to be a liquid drop but not a bubble, which has density higher than biodiesel. Second, a droplet of pure ethanol is suspended at the cross point of fibers for 15 s, as shown in Fig. 16. Then a biodiesel droplet was made to slide along the fiber and merge with the ethanol droplet. The experiment showed that, unlike the first case, the two components appeared to separate right after coalescence and a heterogeneous drop was formed immediately. By knocking slightly on the fibers, it fell down to the bottom, similar to the previous case. These observations demonstrated that, since the ethanol droplet had time to absorb water from the ambience, the nominal miscibility with biodiesel was destroyed and phase separation immediately occurred. Similar to that seen in Figs. 5(c), 9(b) and (c), ethanol absorbed water due to the hygroscopic nature and inhomogeneities formed from the liquid–air interface. They then merged into a large heterogeneous drop due to high surface tension of water and had higher density, which could deposit at the bottom of the droplet.

Fig. 16. The merging sequence of a biodiesel droplet sliding toward a pure ethanol droplet initially resting on the cross point of fibers for 15 s. (a) Top view and (b) side view (t = 3.33 s except the last image).

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Fig. 17. Vanishing of the interior heterogeneous drop after a sufficient standing time (d0 = 525 μm, t = 15 s). Fig. 18. Evolution of the heterogeneous drop diameter (dhd ) inside a biodiesel droplet blended with methanol, ethanol, and isopropanol, respectively.

3.2.4. Regaining of miscibility of blended biodiesel/alcohol fuels The internal heterogeneous drop may vanish after sufficient time of standing and the whole droplet becomes transparent again, as shown in Fig. 17. Since the separated components would mix again gradually due to diffusion between the two fuels, the droplet could return to its original state showing a well-mixing condition. The process can be interpreted by the equilibrium of a ternary system of blended fuels. The blended fuel of biodieselalcohol–water mostly exhibited a micro-emulsion state characterized by immiscibility among the components [21]. However, the blended fuel could reach a miscible condition when the components follow the equilibrium curves or binodal curves of methanol/biodiesel/water, at which the proportion of water increased from 0 to 3.99% as biodiesel decreased from 100% to 6.77%, in addition to the composition of methanol [22,23]. Therefore, at the beginning, a heterogeneous drop could be formed from the liquid–air interface whereby absorbed water broke the equilibrium state of the originally mixed liquid. Then excessive water migrated from ethanol–water phase into the biodiesel-ethanol phase until it was distributed uniformly between the two phases, corresponding to the maximum dispersion of water. Consequently, the ternary system could return to an equilibrium state and the heterogeneous drop vanished in the blended fuel. Based on this mechanism, the internal inhomogeneity could be removed through a settling process, which would lead to combustion without microexplosion. Figure 18 shows vanishing time of an interior heterogeneous drop in a biodiesel droplet blended with methanol, ethanol and isopropanol, respectively, in different fractions. It is seen that for alcohol which had higher hygroscopicity, specifically methanol, more time was needed for the heterogeneous drop to dissolve. Furthermore, with higher fraction of alcohol, the vanishing time was longer. Compared to the liquid pool in an open bottle as discussed for Fig. 9, the heterogeneous drop formed in a droplet took much less time to dissolve. This difference should be related to its relatively larger area of interfacial contact with the air. Via the liquid–air interface, alcohol and water contained in the heterogeneous drops evaporated more readily [24],

thus facilitating disappearance of a heterogeneous drop in the suspending droplet, which was formed at the bottom. In contrast to the vanishing time, the time needed to form a heterogeneous drop at beginning was much shorter, which was about 12 s and changed slightly with ambient humidity, and the proportion and type of alcohol. In passing it is noted that, while alcohol would vaporize due to its high volatility and hence change the compositions of mixtures, the amount was not large enough to affect the essential results and overturn the conclusions presented as follows. For instance, it was measured that the volume of a blended biodiesel droplet (526 μm in diameter) with 25% ethanol reduced ∼2.82% in 90 s, and that of a droplet (438 μm in diameter) with 50% ethanol reduced ∼4.71%. Therefore, according to the measurements and a comparison with the vaporization rate of alcohol reported in the literature [19] as well, the amount of evaporation in the test time was much smaller than the bulk liquid and could not affect significantly the resulted characteristics regarding the occurrence of ME as to be discussed. 3.3. Microexplosions of biodiesel/alcohol droplets To demonstrate the cause of microexplosion by burning blended fuels, the test results for various blends of binary fuels are summarized in Table 2. Being consistent with earlier discussion, for combustion of B75E25, microexplosions always occurred when a heterogeneous drop was formed inside the droplet. A typical sequence of the suspended droplet experiment is shown in Fig. 19. When the heterogeneous drop dissolved after sufficient standing time, however, no microexplosion was observed and the burning could be completed (Fig. 19(a)). Similar results were also found by replacing ethanol by methanol and isopropanol, with various proportions as well. As shown in Table 2, identical consequences were observed in both experiments of free-falling and suspended droplets, and in both normal and reduced gravity while using the method of suspending droplets.

Table 2 The statistics of four blended fuels using the free-falling droplet and suspended droplet methods in normal and microgravity conditions. PME designates the probability of microexplosion and N is the number of tests. Fuel

Boiling point difference

Heterogeneous drop

Freely-falling droplets method

Suspended droplet method Normal-gravity

75% 75% 75% 75%

biodiesel/25%ethanol biodiesel/25%butanol n-hexadecane/25% n-heptane ethanol/25% dodecanol

125.6 °C 86.3 °C 188.4 °C 180.6 °C

Yes No No yes

Microgravity

N

PME (%)

N

PME (%)

N

>50 >50 >50 >50

100 0 0 100

10 7 7 8

100 0 0 100

3 5 4 4

PME (%) 100 0 0 100

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Fig. 19. The burning process of a suspended droplet composed of 75% biodiesel/25% ethanol (a) without interior heterogeneous drop and microexplosion, and (b) with an interior heterogeneous drop and microexplosion.

It is known that, preferential vaporization is closely related to the difference in boiling points of the mixture components, whereby microexplosion can be created [25]. To assess the significance of volatility difference of the mixed fuels, as emphasized in previous studies, three different compositions were tested by using the free-falling and suspended droplet techniques. As shown in Table 2 (with images referred to Figs. 20 and 21), with very little hygroscopicity of 25% butanol mixed with 75% biodiesel, no heterogeneous drop is observed and complete burning is resulted. Even with a much larger difference in the boiling point (188.4 °C) of nhexadecane mixed with n-heptane, the same outcome was yielded. This difference is obviously larger than that between the boiling points of biodiesel (∼204 °C) and methanol (64.7 °C), ethanol (78.4 °C) and isopropanol (82.6 °C), respectively. Therefore, these experiments showed that a large difference in the volatility of blend fuels, and hence the superheat limit that generally dominated homogeneous nucleation, could not be the primary factor of microexplosions as observed largely. For blended fuels with high hygroscopicity, however, heterogeneous nucleation

Fig. 20. The burning process of free-falling droplets composed of (a) 75% biodiesel/25% butanol (T = 86.3 °C), (b) 75% n-hexadecane/25% n-heptane (T = 188.4 °C), and (c) 75% ethanol/25% dodecanol (T = 180.6 °C).

405

Fig. 21. The burning process of a suspended droplet composed of (a) 75% biodiesel/25% butanol (T = 86.3 °C), (b) 75% n-hexadecane/25% n-heptane (T = 188.4 °C), and (c) 75% ethanol/25% dodecanol (T = 180.6 °C).

could be formed (Figs. 20(c) and 21(c)) and lead to microexplosions. 3.4. Burning rate of a blended droplet 3.4.1. Combustion with microexplosion Due to disruptive burning characterized by microexplosions of the binary droplets, the burning rate with microexplosions (KME ) was much higher than those without microexplosions (K). As shown in Fig. 22, the averaged burning rate increases nonlinearly with added portion of ethanol in biodiesel, indicating occurrence of the highest KME at the composition of 50%. Since microexplosions were directly related to the internal inhomogeneity, which was introduced by the nonuniform mixing, the strongest microexplosion would be yielded at the fraction when both ingredients had equal volumes. The results were consistent with the findings in [2,6] and were the same as that using methanol and propanol as tested in the present study. It is also seen in Fig. 22 that, as compared to the free-falling droplet method, the suspended droplet experiment exhibits higher burning rate. This is most likely due to different states of inhomogeneities inside the droplets prepared in the two methods. As implied by comparing Figs. 5 and 6, respectively for the two methods, since there is more time for the formation of heterogeneous sites in a suspended droplet, they can generally merge into bigger heterogeneous drops. Consequently, the burning rate could be

Fig. 22. The average burning rate without (K) and with microexplosion (KME ) of various ethanol proportions mixed with biodiesel in the experiments of suspended droplets and free-falling droplets, with the former being tested in the microgravity condition as well.

406

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enhanced by increased heterogeneous nucleation and stronger microexplosions, while showing the same nonlinear dependence on the alcohol fraction.

burning rate with added proportion of alcohol, showing a peak at 25% fraction. Acknowledgment

3.4.2. Combustion without microexplosion As discussed, the interior state of a droplet may change with time and heterogeneous sites can be created due to temporary separation of the ingredients. By using the suspended droplet method, when the standing time was long enough for the heterogeneous drop to dissolve and for the droplet to regain a miscible state, burning could be completed without microexplosions. In the lack of standing time, therefore, microexplosions always happened in free-falling droplet experiment but could be prevented in the suspended droplet experiment. Without disruptive burning, the complete burn of blended droplets would yield a burning rate that was more subject to the physiochemical properties of the materials but not simply to the difference in boiling points of the mixed fuels, as discussed usually for homogeneous nucleation that would need higher temperature to reach a superheat limit and achieve microexplosions. Complete burn of a blended droplet was studied when the internal heterogeneous drop vanished after a standing period while the composition of ingredients did not vary much as mentioned. As shown in Fig. 22, K increases nonlinearly with the ethanol fraction and attains the maximum at ∼25%; this is different from the results of binary fuels exhibiting microexplosions. Generally, the trend would be a monotonic decrease with increasing alcohol fraction owing to its lower value of K than that of biodiesel, being affected also by the larger heat capacity and absorption of water on the droplet surface that hampered burning and even caused extinction [20]. By adding a small amount of ethanol, however, the binary fuels exhibited a higher burning rate, with a peak at the proportion of 25% alcohol; this was possibly due to enhanced evaporation and reduced preheating delay as discussed in [1]. This trend is particularly clear in the suspended droplet experiment performed in microgravity, due to elimination of natural convection, as shown in Fig. 22. Specifically, with increase of ethanol fraction, soot formation was delayed and reduced, rendering a higher burning rate when the impeding effects of mass and heat transfer induced by the soot shell were mitigated. The two competing propensities of adding alcohol in biodiesel thus led to the highest burning rate at the proportion of ∼25% alcohol, including methanol, ethanol and 2-propanol as tested.

4. Concluding remarks In the present study, the free-falling and suspended droplet methods were used to investigate the combustion characteristics of biodiesel/alcohol blended fuels, for both complete burn and disruptive burning with microexplosions. Specifically, heterogeneous nucleation was observed inside the blended fuel droplet before ignition. A heterogeneous drop was formed inside the droplet due to the hygroscopic characteristics of alcohol fuels after the droplets were generated. The interior heterogeneous drop was mainly constituted of water and alcohol, as measured in a sample to contain water up to 60%, whose size was affected by humidity of the environment. The heterogeneous drops led to microexplosions all the time during the combustion process and yielded the highest burning rate at equi-volume proportion of alcohol and biodiesel. This internal inhomogeneity, however, could vanish after a sufficient standing time by using the suspended droplet method. Consequently, complete burning without any microexplosion was achieved for these binary fuels, leading to a nonlinear increase of

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