Differences of two-component droplets breakup at the high temperatures

Journal of the Energy Institute xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute

Differences of two-component droplets breakup at the high temperatures D.V. Antonov, G.V. Kuznetsov, P.A. Strizhak*, 1 National Research Tomsk Polytechnic University, 30, Lenin Avenue, Tomsk, 634050, Russia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 January 2019 Received in revised form 19 February 2019 Accepted 26 February 2019 Available online xxx

Heated liquid droplets may break up in the puffing and micro-explosion regimes. Many gas-vapor-droplet technologies can be improved significantly by using partial or full dispersion and explosive breakup of parent-droplets in a rational and controlled manner. Here we present the findings of experimental research into the fragmentation of boiling two-component droplets of different origin: emulsions, solutions, slurries, and immiscible two-liquid droplets. We consider three ways of energy supply to the droplet e conductive, convective, and radiative e typical of the current thermal liquid treatment technologies. We also identify the conditions that can provide monotonous evaporation, rapid fragmentation, and droplet aerosol. For the most interesting behavior e micro-explosion e the charts are obtained showing droplet heating times before breakup, number and size of child-droplets, and the ratio of the evaporation surface area before and after atomization. We show how much interference is brought by the main factors and processes e temperature, heat fluxes, component concentrations, dimensions or droplets, and their total evaporation surface areas. The scientific novelty of the research findings comes from the comparative dependences of fuel droplet fragmentation characteristics on the magnitude of the heat flux supplied at the heating temperatures ranging from 250 to 450
C. These functions can be used to predict the optimal schemes of combined heat exchange with several heating mechanisms. This way we can provide the fastest possible micro-explosive breakup and ignition with relatively low energy costs. The practical value of the experimental results is attached to the key characteristics of microexplosive breakup obtained for a wide range of promising fuels. These characteristics include the minimum heating times, the required threshold values of heat fluxes, and the necessary proportions of flammable and non-flammable components. The functions obtained are important for the testing and development of micro-explosion models, especially in order to update the characteristics of child-droplets. The number of child-droplets ranges from 2 to 3 to several hundreds or even thousands, and their size is several orders of magnitude smaller than that of the parent-droplet. These characteristics traditionally require updating during simulation by arranging more experiments. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Keywords: Two-component droplets Emulsions Slurries Solutions Micro-explosion

1. Introduction To develop brand new technologies for thermal treatment of wastewater (for example, as an atomized flow) and to improve the current ones, it is important to know the physics of processes occurring when droplets of water solutions, emulsions, and slurries move through high-temperature gases at over 500
C. Regrettably, a general theory of heat and mass transfer and phase transformations has not yet been developed for such conditions. However, the experimental results, e.g. Refs. [1e6], obtained over the recent years can become the premises for such a theory. As far as we know, no theoretical research findings on these processes have been published so far apart from several attempts at mathematical modeling, for instance, those analyzed in Ref. [7]. What makes such research difficult is a large number of

* Corresponding author. E-mail address: [email protected] (P.A. Strizhak). 1 Website: http://hmtslab.tpu.ru. https://doi.org/10.1016/j.joei.2019.02.005 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

2

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

Nomenclature and units a Ca q qcond qconv qrad Rd Rd0 Rd1 Rdn S S0 S1 Sm Ta Td Ts Tsub t

thermal diffusivity, m2/s heat capacity of air, J/(kg$
C) heat flux, kW/m2 conductive heat flux, kW/m2 convective heat flux, kW/m2 radiative heat flux, kW/m2 droplet radius, mm initial two-component droplet radius, mm droplet radius before breakup, mm mean radius of droplets in a group, mm total area of the droplet evaporation surface after breakup, mm2 initial droplet surface area, mm2 droplet surface area before breakup, mm2 frontal cross-sectional area of a, mm2 gas flow temperature,
C temperature in a droplet,
C surface temperature of the droplet,
C copper substrate surface temperature,
C time, s;

Ua Vd

high-temperature gas flow velocity, m/s droplet volume, ml

Greek

a 3d 3a

h la ld na ra s te th

heat transfer coefficient, W/(m2$
C) emissivity factor of water droplet; emissivity factor of air flammable liquid (oil) concentration, vol% thermal conductivity coefficient of air, W/(m$
C) thermal conductivity coefficient of water droplet, W/ (m$
C) kinematic viscosity of air, m2/s air density, kg/m3 Stefan-Boltzmann constant, W/(m2$K4) times of droplet breakup, s; times of droplet heating and evaporation (with the droplet integrity being preserved), s

Dimensionless numbers Nu Nusselt number Pr Prandtl number Re Reynolds number

interfaces with highly nonlinear boundary conditions of rapid vaporization. Review paper [7] outlined these difficulties among the factors slowing down the development of models simulating the rapid heating and evaporation of droplets of fuels and fuel-based emulsions. Experimental results [8] established that the leading droplets in a flow significantly affect the heat exchange of the following droplets with the heated gas medium. According to a hypothesis from Ref. [8], due to rapid vaporization, the first droplets considerably decrease the gas temperature in the front of all the subsequent droplets. Thus, a thermal insulation of a sort is created for the following droplets. No experimental or theoretical findings of detailed research into such elements of thermal insulation of evaporating droplets have been obtained so far. It is important to obtain reliable experimental data and use them to develop adequate physical and mathematical models of heat and mass transfer. When analyzing the findings from Refs. [9e20], we concluded that optic techniques are most likely the only possible way to solve the formulated problem. We need reliable information on temperature distributions in droplets of water and water-based solutions, slurries and emulsions exposed to intense heating. At the same time, by analyzing the results from Ref. [21] we can come to a conclusion that the unsteady heating of a droplet has a considerable effect on its lifetime. Therefore, it is not safe to assume that an evaporating droplet has a constant temperature field under such conditions. What complicates the task even more is the relatively high temperatures of the gas medium (over 500
C) that this research requires. Vysokomornaya et al. [22] show that traditional evaporation models [9e20] provide good agreement between the theoretical research findings and experimental data at moderate gas medium temperatures under 500
C. Puffing and micro-explosion of emulsion and slurry droplets in a high-temperature gas medium were studied experimentally [1e6,23,24]. The authors determined the threshold temperatures, at which this effect emerges, for a group of solid and liquid organic additives. Their study revealed that following the explosive fragmentation of multi-component droplets, the evaporation surface area increases by a factor of 12e15. It is important to enhance the experimental database with the evaporation characteristics of typical wastewater compositions to improve thermal treatment technologies. Of special interest are studies seeking to optimize the atomization of emulsion droplets with various component compositions. The aim is to develop new fuel types as a replacement for diesel [25e30] and ultimately to improve the economic, environmental, and energy performance. This increases the demand for experiments determining threshold conditions and the corresponding characteristics of heating, evaporation, boiling, and breakup of droplets of typical emulsions, slurries, and solutions within the temperature range of thermal and flame liquid treatment as well as fuel and chemical technologies. It is important to analyze how much the concentration of solid particles and various additives affects droplet heating and evaporation. We need to determine how much thermal energy will provide dispersion and explosive breakup of two-component droplets of various origin: emulsions, solutions, slurries, two-liquid droplets without component mixing. In line with the present-day design of heating chambers, engines, and combustion chambers, we are to study two-component droplets exposed to conductive, convective, and radiative heating and determine threshold heat fluxes. The temperature ranges of choice should be wide enough for the said applications: 20
Ce500
C. The main reasons for choosing three heating schemes and the temperature ranges come from their popularity in actual industries [12e20]. For instance, they are used in heat and mass transfer technologies to intensify fuel burnout and minimize the anthropogenic emissions. Also, they are applied in firefighting to shorten the fire suppression time and reduce the necessary amount of water. Droplets exposed to conductive heating contact with the walls of the heating chambers, whereas radiative and convective heating is typically used for droplets in gas media. The components [25e30] of the liquids under study are chosen with the most promising applications in mind. These include the intensification of heating and ignition of fuel in the form of mixed and immiscible compositions as well as the evaporation and burning of impurities from heterogeneous droplets. Here it makes sense to consider droplets with varying volume concentration of flammable and non-flammable components in a wide range (from 1 to 3 vol% to 95e97 vol%). Flammable components include diesel, gasoline, kerosene, industrial and vegetable oils, petroleum, etc. The list of non-flammable components comprises tap water, distilled and gas-saturated water, and industrial waste liquids. Solid particles are often added to emulsified fuels to intensify their heating. Judging by earlier theoretical and experimental studies [12e30], the scientific Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

3

community shows significant interest in such droplet heating schemes and component compositions (emulsions, slurries, and immiscible liquids). The purpose of this research is to study experimentally the differences between behaviors and outcomes of evaporating two-component droplet fragmentation. 2. Experimental setup and procedure 2.1. Droplets under study The study involved the substances typical of fuel technologies as well as thermal and flame water treatment (Table 1): transformer oil, diesel, carbonaceous particles, and NaCl. Table 1 presents the main properties of liquid flammable components based on generalized information from Refs. [1e6]. These will be used for further analysis of possible changes in the results of experiments based on the corresponding liquid flammable components from various regions of the world. Transformer oil was used to produce two-component droplets (water and flammable liquid) and served as a component in saline solutions and slurries (with graphite particles). Oil concentration in the droplets under study varied from 3 vol% to 97 vol%. Diesel fuel was used as one of the emulsion components. Emulsions contained 30 vol% of water and 70 vol% of diesel. To stabilize invert W/D emulsions, we used a nonionic emulsifier of fatty acid monoethanolamides with a hydrophilic-lipophilic balance of 2. Fatty acid monoethanolamides are tall oil fatty acids and monoethanolamine condensation products. They are classified as alkylolamides. The stabilizer was obtained by direct vacuum amination of tall oil distillate by monoethanolamine at 150e160
С (acid value 11.5 mg KOH/g) [31]. This nonionic emulsifier was developed to make W/D emulsions more stable [31] as compared to W/D emulsions stabilized by commercial analogs, such as SPAN-80 and TWEEN 85 [31]. In addition to two-component droplets and emulsions, the experiments involved saline solutions (NaCl concentration varied from 2.5 wt % to 5 wt%) and slurries (graphite particles concentration ranged from 1 wt% to 3 wt%). The main properties of graphite particles and salt are presented in Refs. [32,33]. These components were chosen because carbonaceous particles are typical coal processing wastes, oil sludge, etc. [34e36]. NaCl solutions represent wastewater, lakes, and seas. We used two schemes [6] to obtain two-component droplets. In the first one, we suspended a water droplet on a holder using an electronic dispenser Finnpipette Novus with a volume increment of 0.1 ml. After that, another Finnpipette Novus dispenser was employed to insert a droplet of the combustible component. The high-speed video recording showed that the combustible additive usually enveloped the water droplet and created a 0.05e0.5 mm film on its surface. Water was located in the upper part of the droplet, touching the holder, whereas the flammable additive gravitated towards the bottom. The second scheme, on the contrary, implied placing a flammable liquid droplet on the holder and then adding water. In this scheme, a combustible additive gathered at the top of the droplet and water, at the bottom. In accordance with experimental results from Ref. [6], the first recording scheme was chosen for this study as the best one for the water and diesel location during emulsion heating. Emulsions were obtained using a high-speed GJe3S mixer. A sample of the emulsifier was dissolved in a dispensed volume of diesel. While mixing the resulting solution at 11,000 rpm, we gradually added the required volume of distilled water and then mixed it for 7 min in the same conditions. The resulting emulsion was degassed for at least 20 min under vacuum produced by an aspirator. The solutions and slurries were produced as follows: the necessary amount of NaCl (for the saline solution) and graphite particles (for the slurry) was mixed with the necessary amount of water for the required concentration. A hollow metal tube 0.3 mm in inner diameter and 0.6 mm in outer diameter was used as a droplet holder for two-component droplets of W/D emulsions, solutions, and slurries. The holder was made of steel X6CrNiMoTi 17-12-2, because it provided minimum interference with droplet heating and minimum contact area [37]. The initial volume Vd of two-component droplets, W/D emulsions, solutions, and slurries ranged from 10 to 25 ml. This corresponded to the 1.3e1.8 mm radius variation. Fig. 1 shows the schemes we used to produce twocomponent droplets, W/D emulsions, solutions, and slurries. 2.2. Convective/conductive/radiative droplet heating The heating methods used in the experiments were similar to those from Ref. [21]. Fig. 2 shows a schematic representation of recording the lifetimes of two-component droplets, W/D emulsions, solutions, and slurries under conductive heating. A substrate was placed inside the inductor and heated due to eddy currents. The temperature was measured and maintained using a high-speed National Instruments 9213 (DAQ) module and two fast chromel-alumel thermocouples TC1 and TC2 (temperature range 0e1200
C, accuracy ± 1
C, and response 0.1 s). A high-speed camera recorded fast processes of heating, evaporation, and breakup of two-component droplets of W/D emulsions, solutions, and slurries. The video frames were processed by the Phantom Camera Control and Tema Automotive software. The recording frequency in the experiments was 1000 frames per second. Fig. 3 shows a schematic representation of recording the lifetimes of two-component droplets, W/D emulsions, solutions, and slurries under convective heating. The heating system featured a Leister CH 6060 air blower (air velocity 0.5e5 m/s) and Leister LE 5000 HT air heater (temperature range 20e1000
C) generating the necessary parameters of a high-temperature gas flow (velocity Ua and temperature Ta). The Table 1 Main properties of liquid flammable components (based on data from Refs. [1e6]) similar to those used in the experiments in this study. Component

Density, kg/m3

Humidity, %

Ash, %

Flash point temperature,
С

Ignition temperature,
С

Calorific value, MJ/kg

Boiling temperature,
С

Vaporization heat, MJ/kg

Transformer oil Diesel

877 820

e 1

e 5

148 40e85

169 200

44.98 42.4

320e330 240e347

0.167e0.209 0.210

Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

4

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

Fig. 1. Schemes of producing two-component droplets of immiscible fluids (a), W/D emulsions (b), solutions and slurries (c).

Fig. 2. Recording scheme of two-component droplets, W/D emulsions, solutions, and slurries under conductive heating.

flow was formed in a follow transparent silica-glass cylinder (inner diameter 0.1 m, wall thickness 2 mm). The cylinder had three 10-mm openings for laser illumination of a droplet, its introduction into a high-temperature gas flow, and recording of the processes under study. The temperature of heated gases in a glass cylinder was measured using a system featuring a high-speed National Instruments 9213 data collection module and two fast chromel-alumel thermocouples (temperature range 0e1200
C, accuracy ± 1
C, and response 0.1 s). High-temperature gas flow velocities (Ua) were measured by the optical technique of Particle Image Velocimetry. The values of Ua were recorded before the main experiment, i.e., before placing a droplet into the cylindrical channel. The error of velocity Ua calculation did not exceed 2%. The two-component droplets of W/D emulsions, solutions, and slurries were placed on a small-size steel holder, which was introduced into a high-temperature gas flow by a motorized manipulator. The heating, evaporation, and breakup of droplets were recorded like in the experiments with conductive heating. In convective heating experiments, we also recorded temperatures in different droplet sections using the PLIF technique (see Refs. [6,37] for more detail). Fig. 4 shows a schematic representation of recording the lifetimes of two-component droplets, W/D emulsions, solutions, and slurries under radiative heating. The experiments were conducted similarly to those with convective heating but with a muffle furnace as the heating system: maximum temperature 1200
C, chamber volume 0.004 m3, permissible deviation ±1
С. Two-component droplets, W/D emulsions, solutions, and slurries were placed in the center of the heating system to observe the necessary experimental conditions [21]. We use a steel holder in experiments with convective and radiative heating because it has negligible interference with droplet heating based on the results from Ref. [38]. Antonov et al. [38] discovered that the disintegration times of two-component droplets are minimum when holders with a low thermal diffusivity are used (a <10 mm2/s), and maximum when thermal diffusivity is high (a >80 mm2/s). For a Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

5

Fig. 3. Recording scheme of two-component droplets, W/D emulsions, solutions, and slurries under convective heating.

steel holder, the thermal diffusivity is approximately 9.9 mm2/s. If we consider the methods involving falling droplets, convective heating plays a pivotal role in droplet heating, because the droplets are moving. In such experiments, even in a muffle furnace, it is hardly possible to determine the impact of radiative heat exchange, since convective heat flux exceeds the radiative one several-fold when droplets are falling in a heated area. Therefore, an effective way to evaluate the impact of the radiative heat flux could be by fixing a droplet on a steel holder (the contribution of the radiative heat flux reaches 95%). Table 2 shows the variation ranges of convective, conductive, and radiative heat exchange in experiments with falling droplets and with droplets fixed on a holder. In these experiments, we studied the impact of convective and radiative heat exchange. Clearly, the role of the holder is minor. 2.3. Methods for studying the boiling droplet breakup Using a high-speed video camera Phantom Miro M310 (with a frame rate 3260 fps at a resolution 1280х800 px), we recorded the heating, evaporation and breakup of two- and multi-component droplets. The resulting video frames were processed using the Tema Automotive and ActualFlow software packages for continuous tracking of moving objects [6]. The processing helped us determine the initial droplet radius Rd and the total liquid evaporation surface area S before and after breakup (Fig. 5). Volkov et al. [6] presented the stages of experimental data processing in detail. The droplet was assumed spherical and its frontal crosssectional area, a circle. Using the formula Rd¼(Sm/p)0.5, we calculated the mean radius Rd of the parent-droplet before breakup and radii Rdn of droplet fragments after breakup (i.e., in an aerosol cloud) known as child-droplets. The errors of the Rd calculation did not exceed 2.5%. After that, using the formula S ¼ 4pR2d, we calculated the total area of the droplet evaporation surface. These parameters allowed us to evaluate how effective is the so-called repeated atomization of multi-component droplets due to explosive breakup. 2.4. Main registered parameters and tolerances Table 3 shows the experimentally recorded parameters as well as systematic errors of the measurement tools. Maximum random errors of Ta measurement reached 20
C, and Td, no more than 3
C. We derived approximations in the form of exponential and power functions for the times of droplet heating and evaporation th (with the droplet integrity being preserved), and for the times of droplet breakup te. Moreover, confidence intervals were determined for each time curve to illustrate the dispersion of experimental values. 3. Results and discussion 3.1. Breakup behaviors of heated droplets Fig. 6 shows video frames illustrating the heating, evaporation, and breakup behaviors of two-component droplets (water and oil), slurries, emulsions, and solutions. Each component composition is notable for a number of specific features. For instance, solutions and slurries without a combustible component only show monotonous evaporation (Fig. 6a), whereas emulsions (30 vol% water and 70 vol% diesel) without an additional combustible component boil (Fig. 6b) and break up only under conductive heating. Two-component droplets (water and oil) as well as slurries, emulsions, and solutions with a combustible component (oil) show puffing and micro-explosion (Fig. 6c and Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

6

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

Fig. 4. Recording scheme of two-component droplets of W/D emulsions, solutions, and slurries under radiative heating.

Table 2 Mean values of radiative, conductive, and convective heat fluxes when heating heterogeneous droplets in the temperature range Ta ¼ 250e450 & Scy. Scheme

qconv, kW/m2

qrad, kW/m2

qcond, kW/m2

P q, kW/m2

On holder (in muffle furnace) On holder (in high-temperature gases) In free falling (in muffle furnace)

e 20e57 20e57

4e15 e 4e15

0.2e0.8 0.2e0.8 e

4.2e15.8 20.2e57.8 24e72

Fig. 5. Recording of heated droplet breakup and aerosol generation.

Table 3 Main registered parameters and tolerances. Parameter

Technique

Errors

Air temperature (Т a) Air velocity (Ua) Droplet volume (Vd) Droplet radius (Rd) Droplet temperature (Td) Full evaporation droplet time (th), Heating time before breakup (te)

Thermal converter (IT-8) Particle Image Velocimetry (PIV) Dispensers Finnpipette Novus Phantom Miro M310, Photron Fastcam SA1, Tema Automotive Planar Laser Induced Fluorescence (PLIF) Phantom Miro M310, Photron Fastcam SA1, Phantom V 411, Tema Automotive

±(0.2 þ 0.001Ta) ±2% ±0.05 ml 4% ±1.5e2
С 4%

fig6d). Explosive breakup is notable for a sharp popping sound and the formation of fine mist. Differences between the behaviors come from a number of reasons and factors described below. Puffing and micro-explosion were affected by the presence of NaCl additives (relative mass fraction 0e5 wt%) in a water droplet. Density, viscosity, surface tension, and other properties of NaCl-based solutions (with mass fractions of NaCl within the range corresponding to the experiments) do not differ from water without NaCl by more than 4e6 wt%. Therefore, the stabilization of breakup times depending on the temperature and concentration of a flammable liquid in saline solutions vs. water without NaCl cannot be explained by the effect of these properties only. The decisive role in the stabilization of the solution breakup times most likely belongs to molecular bonds [31]. The heating of two-component droplets containing graphite particles requires more energy. Therefore, adding graphite particles into two-component droplets stabilizes and prolongs the breakup times. Volkov et al. [24] show that solid particles in a droplet change the conditions of energy accumulation. Unlike a two-component droplet, an emulsion droplet with the same component composition does not break into fragments under convective and radiative heating, because there is no overheating at the water e combustible liquid interface. Under conductive heating, however, with a rapid change in temperature (and, consequently, high gradients) [21], a droplet breaks up in the micro-explosion regime. Two-component droplets usually undergo puffing and micro-explosion. The breakup of such droplets is triggered by the temperature at the inter-component interface exceeding the water boiling temperature (100e120
C) [5,6]. Here, the main impact came from the surface tension pressure of the droplet, which prevented the free release of vapor bubbles forming at the inter-component interface. When the vapor pressure in a droplet exceeded the threshold, the droplet exploded to form droplet aerosol, mist, and smog. Depending on the heat flux supplied and the type of droplet heating (radiative, convective, or conductive), droplets may explode, boil, or steadily evaporate. For example, in the case of conductive heating, a droplet was heated more on one side, so the heating was local and highly uneven in space. However, two-component droplets as well as slurry, solution, and emulsion droplets broke up to form an aerosol Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

7

Fig. 6. Typical snapshots showing droplet breakup behaviors: a e evaporation; b e boiling; c e puffing; d e micro-explosion.

(micro-explosion). During convective heating, two-component droplets, emulsions, solutions, or slurries were suspended on a holder, and the flux came from one side. When the explosion conditions were met, we recorded two kinds of outcomes in two-component droplets with and without solid particles: puffing or micro-explosion. However, unlike in the case of conductive heating, slurry, solution, and emulsion droplets without an additional flammable component did not break up into smaller fragments: they either evaporated steadily, or boiled locally. In the third, radiative heating scheme, a two-component droplet was heated evenly all around, unlike in the above-mentioned conductive and convective regimes. Therefore, a droplet broke up to form a fine aerosol and mist (micro-explosion) or evaporated steadily (when the radiative heat flux was low). Radiative heating never induced puffing. According to the processed experimental results, the common mechanism of puffing and micro-explosion is the overheating of the intercomponent interface above the water boiling temperature (100e120
C). Volkov et al. [6] draw similar conclusions from their experiments with two-component droplets under convective heating. Based on Table 4, we can compare our experimental findings with those of other authors, for instance Refs. [39e41], in terms of threshold temperatures and heat fluxes in three heating arrangements, component compositions, and parameters recorded. In all of these studies, the authors believe that the main reason for the micro-explosive breakup is the overheating of the inter-component boundary above the boiling temperature of the non-combustible component (water). Practical applications from Refs. [39e41] and this study are notable for wide Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

8

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

Table 4 Review of results of droplet micro-explosion. Articles

Review

This research

Liquids, sizes of droplets Scheme of heating Temperature ranges and heat fluxes of micro-explosion and puffing

Breakup modes Main registered parameters [38]

[39]

[40]

Liquids, sizes of droplets Initial droplet temperature ranges Breakup modes Main registered parameters Liquids, sizes of droplets Scheme of heating Temperature ranges Breakup modes Main registered parameters

Liquids, sizes of droplets Scheme of heating Initial droplet temperature ranges Breakup modes Main registered parameters

Slurries; solutions; emulsions; two-component droplets Rd0z1.53 mm Radiative, convective and conductive heating Radiative heating: Ta ¼ 250e450
C, qrad ¼ 4e15 kW/m2; Convective heating: Ta ¼ 250e450
C, qconv ¼ 20e57 kW/m2; Conductive heating: Tsub ¼ 150e250
C, qcond ¼ 30e90 kW/m2 Puffing; micro-explosion Times of droplet breakup: 1e40 s; Ratios of droplet evaporation areas: S/S0 ¼ 10e25 Emulsions; Rd0z200e500 mm Td0 ¼ 32e67
C Micro-explosion Times of droplet breakup: 50e600 ms Single droplets of water in pure diesel emulsion; Rd0z0.1e1.3 mm Conductive heating Tsub ¼ 500 ± 2
C Puffing; micro-explosion Times of droplet breakup: 1e2 s; Number of puffing: 1e80; Average diameter of child-droplets after micro-explosion: 0.030e0.053 mm N-dodecane, Rd0z25e100 mm Conductive heating Td0 ¼ 70e90
C Puffing; micro-explosion Time to puffing: 0.5e10 ms

ranges of temperatures and heat fluxes as well as initial droplet dimensions. Another parameter that is often called decisive is pressure in the heating chamber [39e41]. We have not varied this parameter yet due to setup limitations. Therefore, the heating times before breakup differed significantly from those presented in Refs. [39e41]. However, the variation trends of these values with an increase in the energy supplied as well as fragmentation regimes (puffing and micro-explosion) are the same. The video frames of the experiments show the common mechanism behind the surface transformation and breakup of fuel droplets.

3.2. Impact of key factors Fig. 7 shows the curves of droplet lifetimes, i.e., complete evaporation (without breakup), puffing or micro-explosion, of slurries (Fig. 7a), solutions (Fig. 7b), emulsions, and two-component droplets (Fig. 7c) versus the temperature of the gas under convective heating. The curves have a common nature for all the component compositions under study: with increasing gas medium temperature, the lifetimes decrease nonlinearly. The curves are described by approximations with exponential and power functions. However, such patterns were not observed for slurry droplets with 1 wt% and 3 wt% of graphite particles. At higher temperatures (over 350
С), slurry droplets last longer, because they stop evaporating monotonously but they accumulate internal stresses with rapid dispersion. Adding a combustible component (oil) to a two-component droplet (slurry or solution) reduces the droplet lifetime, because droplets undergo explosive breakup. From the curves in Fig. 7aec, we can conclude that adding a solid component to a two-component droplet stabilizes the temperature dependence of droplet lifetimes. Most probably, this is caused by the fact that a solid component contributes to even droplet heating before breakup. So, it accumulates heat at the first stage due to its heat capacity and releases it at the second stage. The experiments established that a flammable component (oil) in an emulsion droplet prolongs the droplet lifetimes throughout the temperature range. This happens because the droplet breakup behavior changes from dispersion (Fig. 6c) to micro-explosion (Fig. 6d). Based on the above curves, the micro-explosion mechanism is the boiling of a low-boiling component e water. Water is heated from the surface to the center. An uneven temperature field with its maximum on the surface changes into an even one. A rise in the air temperature Ta leads to an increase in the droplet temperature Td. WatereNaCl solutions are heated and evaporate more slowly but the curves are of the same nature. This happens because the molecular bond energy increases several-fold when Naþ and Cle ions are added. Consequently, both the breakup and evaporation in general and liquid molecular bonds in particular require more accumulated energy. Slurries are heated similarly to water but we recorded local temperature increases at the surface of inclusions over time. Due to an increase in the temperature (Ta or Tsub), the temperature in a slurry droplet Td went up as well. Also, with a temperature increase in the muffle, the air flow, or substrate, the share of energy accumulated at the surface of solid particles grew. Emulsions have higher thermal conductivity than water and lower total heat of vaporization. Moreover, the optical properties of a flammable component differ greatly from those of water. A flammable component has a higher level of blackness, and more heat energy is absorbed, so a droplet like that is heated faster. This is especially noticeable when water aggregates into a single drop in the center and the flammable component envelops it. Two-component droplets usually underwent puffing and micro-explosion at the temperatures under study, because they contained a flammable component (oil). Small droplets of oil were heated rapidly and evaporated to form a gas-vapor mixture with a high content of flammable gases. This, in turn, intensified the chemical reactions of fuel vapor interaction with air accompanied by the release of flue gases and the corresponding smell. Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

9

Fig. 7. Droplet lifetimes (Vd ¼ 15 ml) vs. temperature under convective heating: a e slurries (1 ewater, 1 wt% graphite particles; 2 e water, 1 wt% graphite particles, 3 vol% oil; 3 e water, 3 wt% graphite particles; 4 e water, 3 wt% graphite particles, 3 vol% oil); b e solutions (1 e water, 2.5 wt% NaCl; 2 ewater, 2.5 wt% NaCl, 3 vol% oil; 3 e water, 5 wt% NaCl; 4 e water, 5 wt% NaCl, 3 vol% oil); c e emulsions (1e30 vol% water, 70 vol% diesel; 2e27 vol% water, 70 vol% diesel, 3 vol% oil), two-component droplets (3e97 vol% water, 3 vol% oil).

Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

10

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

The optimal temperature, at which explosive droplet breakup occurred stably and rapidly under convective and radiative heating was 350
С. Below that, the breakup times increased significantly and above that, they remained practically the same. For conductive heating, no optimal heating temperature was observed. The droplet surface was transforming rapidly and the droplet often broke up due to the repeated contacts with the substrate. We obtained curves for radiative heating (Fig. 8) in the same way as those for convective heating (Fig. 7). The droplet lifetimes throughout the temperature range were higher or close to the lifetimes of droplets exposed to convective heating, because the heat flux was lower under radiative heating than under convective one [21]. The resulting curves (Fig. 8) for all the component compositions under study are of a similar nature. With an increase in the gas medium temperature, the droplet lifetimes decrease nonlinearly (the curves are described by approximations with exponential functions). This pattern only fails to work for emulsion droplets with a combustible component (oil), because the atomization behavior changes from dispersion to breakup at 350
С. The heat fluxes peak under conductive heating. Therefore, the curves obtained (Fig. 9) for all the component compositions have lower droplet lifetimes when the substrate temperature is varied. In the same way as shown in Figs. 7 and 8, adding oil to the slurries, emulsions and solutions reduces the droplet lifetimes. Quite complex functions of lifetimes of two-component droplets, W/D emulsions, solutions, and slurries under conductive heating stem from changes in the droplet breakup behaviors (Fig. 6). From temperature distributions in Figs. 7e9, we derived the heat fluxes to the droplet surface under convective, conductive, and radiative heating:

qconv ¼ aðTa  Ts Þ; qcond ¼ ld ðTsub  Ts Þ=Rd ; qrad ¼ εd ,s,ðT 4a  T 4s Þ þ εa ,s,T 4a at the a ¼ Nu,la =ð2Rd Þ; Nu ¼ 2 þ 0:6,Re1=2 ,Pr1=3 ; Re ¼ 2Rd ,Ua =na ; Pr ¼ na ,ra ,Ca =la ; εd ¼ 0:9; εa ¼ 0:1; s ¼ 5:68,108 ;  W ðm2 ,o C4 Þ In Fig. 10, the ranges of heat flux variation (in line with explosive droplet breakup) under convective and conductive heating were practically the same, and they significantly exceeded the heat flux under radiative heating. Therefore, the breakup times are the longest under radiative heating. Fig. 10 shows the calculated heat fluxes with specific values described above and illustrating the optical, thermal, and physical properties of the system components. In the actual practice, these properties may differ from those used for calculations by 10e15%. Consequently, the quantitative values of qconv, qrad, and qcond may vary within the corresponding limits but the appearance of the th(q) and te(q) functions and the relative curve position will not change throughout the actual range of parameter variation. Fig. 11 shows the breakup times under different types of heating and with different component compositions of droplets. The shortest breakup time corresponds to the conductive droplet heating. Fig. 12 shows the lifetimes of droplets of slurries, solutions, and emulsions with combustible liquids under convective heating as well as the lifetimes of two-component droplets vs. oil concentration. What all the component compositions have in common is that the longest breakup times are provided by the 50 vol% concentration of the flammable component in a droplet (similarly to the functions obtained in Refs. [5,6]). Fig. 12 also shows that a two-component (oil-water) droplet has the minimum breakup times. Adding a solid component increases the droplet lifetimes, because the outcomes of droplet atomization in the case of curves 1 and 2 are larger-scale (the evaporation surface area increases significantly). Volkov et al. [6] establish that the larger-scale the breakup outcomes are, the more time is required for the accumulation of stresses within droplets. As for oil-containing emulsion droplets (curve 3), the flammable component promotes droplet breakup into a fine aerosol and increases droplet lifetimes. Just as in the curves shown in Figs. 7e9, a solid component contributes to the stabilization of droplet lifetimes depending on the concentration of the flammable component (oil). In all the experiments, micro-explosions were observed. The concentration of combustible and non-combustible components has a considerable impact on the micro-explosive breakup of liquid droplets. Liquid combustible components are heated and evaporate faster because the heat of their phase transition is several times lower as compared to water, although water has much higher thermal diffusivity. This caused liquid combustible components to rapidly exceed the water boiling temperature. The droplet breakup lag largely depended on how fast water reached its boiling temperature. The lower the water fraction in a multi-component droplet, the sooner these processes ended. However, to provide the most massive multi-component droplet breakup (more child-droplets), the inter-component interface must have a large area. Therefore, the volume of water in a multicomponent droplet should not be reduced significantly relative to flammable components. It is rather difficult to heat the whole volume of the added water up to its boiling temperature due to the high heat capacity and vaporization heat of water. Therefore, the breakup of a droplet is based on the nucleation of bubbles at the inter-component boundary. The bubbles are filled with vapors of a more heated liquid (in our case, it is a liquid flammable component). Due to their growing in size, merging and moving, bubbles with increased pressure are formed in a two-liquid droplet. The droplet breaks up as soon as the pressure in these bubbles exceeds the pressure caused by the surface tension forces acting on the droplet. If the growth of bubble pressure in a droplet is considerable, it breaks up to form child-droplets. If pressure increases slowly, droplet fragmentation of different scales is observed. The more components a droplet contains (and the greater the number and total area of inter-phase and inter-component boundaries), the more intense its partial fragmentation. This reduces the vapor pressure in a droplet and prolongs the breakup time. Therefore, to intensify the explosive breakup of multi-component droplets, it is sensible to increase the heating temperature up to 400e500
C. At these temperatures, the breakup times of multi-component droplets become comparable. Another way is to add a component with high thermal diffusivity and low vaporization heat (hence high vaporization rate and low surface tension) in high concentration. These findings can help optimize the time and energy consumption needed to intensify the corresponding heat and mass transfer processes in promising practical applications. These conclusions explain the maximum droplet heating times with the comparable concentrations of liquid combustible and noncombustible components. The video frames show that with large proportions of water, the liquid combustible component overheats locally and intensifies bubble nucleation in a small area of the inter-component surface. This was enough to intensify the breakup of the whole droplet. If water concentration was minimal within the droplet, then, due to lower heat capacity and vaporization heat, the liquid combustible component heated up and evaporated quickly and the droplet size decreased. This intensified its heating. Water in these droplets boiled quickly, bubbles filled the droplet and disrupted it. But the longer it took a heterogeneous droplet to heat up to the microexplosion, the more small fragments it produced. Due to high-speed video recording, these patterns were observed stably for all the

Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

11

Fig. 8. Droplet lifetimes (Vd ¼ 15 ml) vs. temperature under radiative heating: a e slurries (1 e water, 1 wt% graphite particles; 2 e water, 1 wt% graphite particles, 3 vol% oil; 3 e water, 3 wt% graphite particles; 4 ewater, 3 wt% graphite particles, 3 vol% oil); b e solutions (1 e water, 2.5 wt% NaCl; 2 e water, 2.5 wt% NaCl, 3 vol% oil; 3 e water, 5 wt% NaCl; 4 e water, 5 wt% NaCl, 3 vol% oil); c e emulsions (1e30 vol% water, 70 vol% diesel; 2e27 vol% water, 70 vol% diesel, 3 vol% oil), two-component droplets (3e97 vol% water, 3 vol% oil).

Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

12

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

Fig. 9. Droplet lifetimes (Vd ¼ 15 ml) vs. temperature under conductive heating: a e slurries (1 e water, 1 wt% graphite particles; 2 e water, 1 wt% graphite particles, 3 vol% oil; 3 e water, 3 wt% graphite particles; 4 e water, 3 wt% graphite particles, 3 vol% oil); b e solutions (1 e water, 2.5 wt% NaCl; 2 ewater, 2.5 wt% NaCl, 3 vol% oil; 3 e water, 5 wt% NaCl; 4 e water, 5 wt% NaCl, 3 vol% oil); c e emulsions (1e30 vol% water, 70 vol% diesel; 2e27 vol% water, 70 vol% diesel, 3 vol% oil), two-component droplets (3e97 vol% water, 3 vol% oil).

Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

13

Fig. 10. Two-component droplet breakup times (Vd ¼ 15 ml, h ¼ 3 vol%) vs. heat fluxes under radiative (1), convective (2) and conductive (3) heating.

Fig. 11. Breakup times of two- and multi-component droplets under various heating conditions and droplet production schemes (Vd ¼ 15 ml): 1 e convective heating (Ta ¼ 350
C, Ua ¼ 3 m/s, qconv ¼ 37.89 kW/m2), water, 1 wt% graphite particles, 3 vol% oil; 2 e convective heating (Ta ¼ 350
C, Ua ¼ 3 m/s, qconv ¼ 37.89 kW/m2), water, 2.5 wt% NaCl, 3 vol% oil; 3 e convective heating (Ta ¼ 350
C, Ua ¼ 3 m/s, qconv ¼ 37.89 kW/m2), 27 vol% water, 70 vol% diesel, 3 vol% oil; 4 e convective heating (Ta ¼ 350
C, Ua ¼ 3 m/s, qconv ¼ 37.89 kW/m2), 97 vol% water, 3 vol% oil; 5 e radiative heating (Ta ¼ 350
C, qrad ¼ 8.18 kW/m2), water, 1 wt% graphite particles, 3 vol% oil; 6 e radiative heating (Ta ¼ 350
C, qrad ¼ 8.18 kW/m2), water, 2.5 wt% NaCl, 3 vol% oil; 7 e radiative heating (Ta ¼ 350
C, qrad ¼ 8.18 kW/m2), 27 vol% water, 70 vol% diesel, 3 vol% oil; 8 e radiative heating (Ta ¼ 350
C, qrad ¼ 8.18 kW/m2), 97 vol% water, 3 vol% oil; 9 e conductive heating (Tsub ¼ 150
C, qcond ¼ 51.16 kW/m2), water, 1 wt% graphite particles, 3 vol% oil; 10 e conductive heating (Tsub ¼ 150
C, qcond ¼ 51.16 kW/m2), water, 2.5 wt% NaCl, 3 vol% oil; 11 e conductive heating (Tsub ¼ 150
C, qcond ¼ 51.16 kW/m2), 27 vol% water, 70 vol% diesel, 3 vol% oil; 12 e conductive heating (Tsub ¼ 150
C, qcond ¼ 51.16 kW/m2), 97 vol% water, 3 vol% oil.

component compositions under study (Fig. 12). In our earlier studies [6,37], we used PLIF to record local centers of overheating of the noncombustible component e water e until vapor bubble nucleation. 3.3. Droplet breakup outcomes When analyzing the outcomes of the explosive breakup (Fig. 13) of an emulsion containing a flammable liquid (30 vol% water e 70 vol% diesel, 30 vol% water e 70 vol% oil), we ensured that a droplet aerosol was formed with the maximum quantity of small fragments. The liquid evaporation surface area increased by more than 25 times under such conditions. This result shows a common pattern for the 30e70 vol% oil concentrations. At the same time, the maximum increment of the liquid surface area during the parent-droplet breakup was in agreement with the experiments with comparable concentrations of the flammable and non-flammable component (Fig. 13). This most likely stems from higher temperatures of all the deep droplet layers due to a long droplet lifetime. Fig. 12 shows the longest droplet heating times until breakup with h z 50 vol%. However, the longer the heating time before breakup, the more small fragments we observe. Such trends are especially noticeable when fuel droplets are exposed to radiative heating. The experimental data, curves and approximations obtained in this study can help control the explosive breakup of droplets containing liquid and solid components with fundamentally different properties. The findings of this research can be applied not only to develop the theory of two-phase and multi-component flows. They also promote two sets of technologies [42e44]: thermal and flame liquid treatment as well as combustion or gasification of liquid and slurry fuels [45,46]. Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

14

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

Fig. 12. Droplet lifetimes (Vd ¼ 15 ml) vs. flammable liquid concentration under convective heating: 1 e slurries with flammable liquids (water e 1 wt% graphite particles e oil); 2 e solutions with flammable liquids (water e 2.5 wt% graphite particles e oil); 3 e emulsions with flammable liquids (water e 70 vol% diesel e oil); 4 e two-component droplets (water e oil).

Fig. 13. Ratios of droplet evaporation areas vs. flammable liquid (oil) concentration under convective heating: 1 e slurries with flammable liquids (water e 1 wt% graphite particles e oil); 2 e solutions with flammable liquids (water e 2.5 wt% graphite particles e oil); 3 e emulsions with flammable liquids (water e 70 vol% diesel e oil); 4 e two-component droplets (water e oil).

Based on the experimental data obtained, we made the following conclusion: the most favorable way of heating the droplets of slurries, emulsions, and solutions as well as two-component droplets is convective heating. In this case, the droplet breakup is triggered not only by the overheating of the inter-component interface above the water boiling temperature (100e125
C) [5,6] but also by strong convective currents forming in droplets [21]. However, such heating method does not always provide the minimum droplet breakup times and the largest-scale breakup outcomes. Minimum times were provided by the conductive heating due to greater heat fluxes as compared to convective and radiative heating [21]. The maximum increase in the evaporation surface area after breakup was observed in the case of radiative heating due to the even heating of the droplet [21].

4. Conclusions (i) The scientific novelty and practical value of the research findings primarily comes from the threshold values of the heat fluxes supplied using conductive, convective, and radiative heating to intensify puffing and micro-explosion. Using the functions of the key microexplosion parameters (heating time and number of resulting small fragments), we can forecast the optimal conditions to intensify such processes in actual heating chambers. The latter use combined heat exchange, that is, the contribution of conductive, convective, and radiative mechanisms is substantial. The research findings show that radiative heating can provide the longest droplet heating times until breakup but the smallest fragments, hence maximum S/S0. Conductive heating provides the shortest heating times until droplet disruption but the S/S0 values are lower than with convective and radiative heating. In actual technologies, due to combined heating mechanisms, average S/S0 values can be predicted relative to those presented in this study for the three heating schemes. Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

15

(ii) We ascertained the nature of the principally different breakup behaviors of droplets of slurries, emulsions, and solutions with and without oil. Two main droplet atomization behaviors were identified: dispersion and breakup. Depending on the combination of solid and liquid components in a droplet, the threshold temperatures, times, and outcomes of the breakup changed. The behavior patterns of slurry, emulsion, and solution droplets depend on the gas flow temperature and component composition. This makes it possible to use them in a wide range of industrial technologies (in thermal liquid treatment or fuel combustion chambers). By adjusting temperature and concentration conditions, we can vary the times th and te by a factor of 10e15. Depending on the heating conditions of slurry, emulsion, and suspension droplets, their behavior may differ. Under conductive heating and maximum heat fluxes, droplets of slurries, emulsions, and solutions disperse or break up into fine aerosol. Under convective and radiative heating, no such patterns have been observed. However, adding a small fraction of oil (up to 3 vol%) promotes droplet breakup and reduces droplet lifetimes th and te by a factor of 3e5. (iii) In addition to minimizing the heating and ignition times of heterogeneous liquid droplets, micro-explosive breakup can also provide a several-fold increase in the free liquid surface area. This parameter determines the conditions and characteristics of phase transformations. In the case of heterogeneous droplets and high concentration of the non-combustible component, an increase in the evaporation area may significantly reduce the ignition costs. The lower the heating temperatures and the higher the concentrations of non-combustible components, the more substantial the contribution of this effect. The droplet evaporation surface area reaches its maximum with the 50/50 oil-water volume concentration in the parent-droplet. In the case of oil-containing emulsions, the evaporation surface area may increase more than 25-fold. Acknowledgments Research was supported by National Research Tomsk Polytechnic University (project VIU-ISHFVP-60/2019). References [1] D. Tarlet, J. Bellettre, M. Tazerout, C. Rahmouni, Prediction of micro-explosion delay of emulsified fuel droplets, Int. J. Therm. Sci. 48 (2009) 449e460. https://doi.org/10. 1016/j.ijthermalsci.2008.05.005. [2] E. Mura, P. Massoli, C. Josset, K. Loubar, J. Bellettre, Study of the micro-explosion temperature of water in oil emulsion droplets during the Leidenfrost effect, Exp. Therm. Fluid Sci. 43 (2012) 63e70. https://doi.org/10.1016/j.expthermflusci.2012.03.027. [3] D. Tarlet, E. Mura, C. Josset, J. Bellettre, C. Allouis, P. Massoli, Distribution of thermal energy of child-droplets issued from an optimal micro-explosion, Int. J. Heat Mass Transf. 77 (2014) 1043e1054. https://doi.org/10.1016/j.ijheatmasstransfer.2014.06.054. [4] D. Tarlet, C. Josset, J. Bellettre, Comparison between unique and coalesced water drops in micro-explosions scanned by differential calorimetry, Int. J. Heat Mass Transf. 95 (2016) 689e692. https://doi.org/10.1016/j.ijheatmasstransfer.2015.12.054. [5] P.A. Strizhak, M.V. Piskunov, R.S. Volkov, J.C. Legros, Evaporation, boiling and explosive breakup of oilewater emulsion drops under intense radiant heating, Chem. Eng. Res. Des. 127 (2017) 72e80. https://doi.org/10.1016/j.cherd.2017.09.008. [6] R.S. Volkov, P.A. Strizhak, Using Planar Laser Induced Fluorescence to explore the mechanism of the explosive disintegration of water emulsion droplets exposed to intense heating, Int. J. Therm. Sci. 127 (2018) 126e141. https://doi.org/10.1016/j.ijthermalsci.2018.01.027. [7] S.S. Sazhin, Modelling of fuel droplet heating and evaporation: recent results and unsolved problems, Fuel 196 (2017) 69e101. https://doi.org/10.1016/j.fuel.2017.01.048. [8] D.B. Spalding, Some Fundamentals of Combustion, Butterworth’s, London, 1955. [9] N.A. Fuchs, Evaporation and Droplet Growth in Gaseous Media, Pergamon Press, London, 1959. [10] W.E. Ranz, W.R. Marshall, Evaporation from drops e I, Chem. Eng. Prog. 48 (1952) 141e146. [11] M.C. Yuen, L.W. Chen, Heat-transfer measurements of evaporating liquid droplets, Int. J. Heat Mass Transf. 21 (1978) 537e542. https://doi.org/10.1016/0017-9310(78) 90049-2. [12] M. Renksizbulut, M.C. Yuen, Numerical study of droplet evaporation in a high-temperature stream, J. Heat Transf. 105 (1983) 389e397. https://doi:10.1115/1.3245591. [13] A.K. Thokchom, A. Gupta, P.J. Jaijus, A. Singh, Analysis of fluid flow and particle transport in evaporating droplets exposed to infrared heating, Int. J. Heat Mass Transf. 68 (2014) 67e77. https://doi.org/10.1016/j.ijheatmasstransfer.2013.09.012. [14] S.S. Sazhin, A.E. Elwardany, P.A. Krutitskii, V. Depredurand, G. Castanet, F. Lemoine, E.M. Sazhina, M.R. Heikal, Multi-component droplet heating and evaporation: numerical simulation versus experimental data, Int. J. Therm. Sci. 50 (2011) 1164e1180. https://doi.org/10.1016/j.ijthermalsci.2011.02.020. [15] V.I. Terekhov, V.V. Terekhov, N.E. Shishkin, K.Ch Bi, Heat and mass transfer in disperse and porous media experimental and numerical investigations of nonstationary evaporation of liquid droplets, J. Eng. Phys. Thermophys. 83 (2010) 883e890. https://doi.org/10.1007/s10891-010-0410-7. [16] M.N. Nikitin, Impact of directed water injection in a heat generator on the pressure of the resulting gas-vapor mixture, Indep. Energy 6 (2010) 42e46. [17] A.A. Fedorets, I.V. Marchuk, O.A. Kabov, Coalescence of a droplet cluster suspended over a locally heated liquid layer, Interfacial Phenom. Heat Transf. 1 (2013) 51e62. https://doi.org/10.1615/InterfacPhenomHeatTransfer.2013007434. [18] A.Yu Varaksin, Fluid dynamics and thermal physics of two-phase flows: problems and achievements, High Temp. 51 (2013) 377e407. https://doi.org/10.1134/ S0018151X13030073. [19] V.I. Terekhov, M.A. Pakhomov, Flow Dynamics and Heat and Mass Transfer in a Gas-Droplets Flows: Monograph, NSTU Publisher, Novosibirsk, 2009. [20] G.V. Kuznetsov, P.A. Strizhak, Effect of the volume concentration of a set of water droplets moving through high-temperature gases on the temperature in the wake, J. Appl. Mech. Tech. Phys. 56 (2015) 558e568. https://doi.org/10.1134/S0021894415040033. [21] G.V. Kuznetsov, M.V. Piskunov, R.S. Volkov, P.A. Strizhak, Unsteady temperature fields of evaporating water droplets exposed to conductive, convective and radiative heating, Appl. Therm. Eng. 131 (2018) 340e355. https://doi.org/10.1016/j.applthermaleng.2017.12.021. [22] O.V. Vysokomornaya, G.V. Kuznetsov, P.A. Strizhak, Evaporation of water droplets in a high-temperature gaseous medium, J. Eng. Phys. Thermophys. 89 (2016) 141e151. https://doi.org/10.1007/s10891-016-1361-4. [23] G.V. Kuznetsov, M.V. Piskunov, P.A. Strizhak, How to improve efficiency of using water when extinguishing fires through the explosive breakup of drops in a flame: laboratory and field tests, Int. J. Therm. Sci. 121 (2017) 398e409. https://doi.org/10.1016/j.ijthermalsci.2017.08.004. [24] R.S. Volkov, M.V. Piskunov, G.V. Kuznetsov, P.A. Strizhak, Water droplet with carbon particles moving through high temperature gases, J. Heat Transf. 138 (2016) 1e5. https://doi.org/10.1016/j.ijthermalsci.2017.08.004. [25] E. Mura, R. Calabria, V. Califano, P. Massoli, J. Bellettre, Emulsion droplet micro-explosion: analysis of two experimental approaches, Exp. Therm. Fluid Sci. 56 (2013) 69e74. https://doi.org/10.1016/j.expthermflusci.2013.11.020. [26] L. Marchitto, R. Calabria, C. Tornatore, J. Bellettre, P. Massoli, A. Montillet, G. Valentino, Optical investigations in a CI engine fueled with water in diesel emulsion produced through microchannels, Exp. Therm. Fluid Sci. 95 (2018) 96e103. https://doi.org/10.1016/j.expthermflusci.2018.02.008. [27] D.L. Dietrich, R. Calabria, P. Massoli, V. Nayagam, F.A. Williams, Experimental observations of the low-temperature burning of decane/hexanol droplets in microgravity, Combust. Sci. Technol. 189 (2017) 520e554. https://doi.org/10.1080/00102202.2016.1225730. [28] K. Meng, Y. Wu, Q. Lin, F. Shan, W. Fu, K. Zhou, T. Liu, L. Song, F. Li, Microexplosion and ignition of biodiesel/ethanol blends droplets in oxygenated hot co-flow, J. Energy Inst. (2018) 1e10. https://doi.org/10.1016/j.joei.2018.07.021. [29] A.M. Ithnin, H. Noge, H.A. Kadir, W. Jazair, An overview of utilizing water-in-diesel emulsion fuel in diesel engine and its potential research study, J. Energy Inst. 87 (4) (2014) 273e288. https://doi.org/10.1016/j.joei.2014.04.002. [30] B.S. Bidita, A.R. Suraya, M.A. Shazed, A. Mohd Salleh, Preparation, characterization and engine performance of water in diesel nanoemulsions, J. Energy Inst. 89 (3) (2016) 354e365. https://doi.org/10.1016/j.joei.2015.03.004.

Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005

16

D.V. Antonov et al. / Journal of the Energy Institute xxx (xxxx) xxx

[31] K. Minaev, A. Epikhin, D. Novoseltsev, M. Andropov, V. Yanovsky, O. Ulyanova, Research of inverted emulsions properties on the base of new emulsifiers, IOP Conf. Ser. Earth Environ. Sci. 21 (2014) 1e6. https://doi.org/10.1088/1755-1315/21/1/012033. [32] R.S. Volkov, G.V. Kuznetsov, P.A. Strizhak, Transformation of solution and suspension Massifs during their free fall in air, Theor. Found. Chem. Eng. 51 (2017) 1055e1062. [33] G.V. Kuznetsov, P.A. Strizhak, R.S. Volkov, O.V. Vysokomornaya, Integral characteristics of water droplet evaporation in high temperature combustion products of typical flammable liquids using SP and IPI methods, Int. J. Therm. Sci. 108 (2016) 218e234. https://doi.org/10.1016/j.ijthermalsci.2016.05.019. [34] D.O. Glushkov, P.A. Strizhak, M.Yu Chernetskii, Organic coal-water fuel: problems and advances (Review), Therm. Eng. 63 (10) (2016) 707e717, https://doi.org/10.1134/ S0040601516100037. [35] M. Xu, J. Zhang, H. Liu, H. Zhao, W. Li, The resource utilization of oily sludge by co-gasification with coal, Fuel 126 (2014) 55e61. https://doi.org/10.1016/j.fuel.2014.02. 048. [36] X. Ma, Z. Zhang, X. Gao, X. Li, Status and commentary of research and development on oil sand pyrolysis characteristics with technology and equipment, Chem. Ind. Eng. Prog. 35 (11) (2016) 3484e3490. [37] P.A. Strizhak, R.S. Volkov, G. Castanet, F. Lemoine, O. Rybdylova, S.S. Sazhin, Heating and evaporation of suspended water droplets: experimental studies and modelling, Int. J. Heat Mass Transf. 127 (2018) 92e106. https://doi.org/10.1016/j.ijheatmasstransfer.2018.06.103. [38] D. Antonov, J. Bellettre, D. Tarlet, P. Massoli, O. Vysokomornaya, M. Piskunov, Impact of holder materials on the heating and explosive breakup of two-component droplets, Energies 11 (2018) 3307. https://doi.org/10.3390/en11123307. [39] O. Moussa, D. Tarlet, P. Massoli, J. Bellettre, Parametric study of the micro-explosion occurrence of W/O emulsions, Int. J. Therm. Sci. 133 (2018) 90e97. https://doi.org/10. 1016/j.ijthermalsci.2018.07.016. [40] M.Y. Khan, Z.A. Abdul Karim, A.R.A. Aziz, M.R. Heikal, C. Crua, Puffing and microexplosion behavior of water in pure diesel emulsion droplets during leidenfrost effect, Combust. Sci. Technol. 189 (2017) 1186e1197. https://doi.org/10.1080/00102202.2016.1275593. [41] S.S. Sazhin, O. Rybdylova, C. Crua, M. Heikal, M.A. Ismael, Z. Nissar, A.R.B.A. Aziz, A simple model for puffing/micro-explosions in water-fuel emulsion droplets, Int. J. Heat Mass Transf. 131 (2019) 815e821. https://doi.org/10.1016/j.ijheatmasstransfer.2018.11.065. [42] P.A. Strizhak, R.S. Volkov, M.V. Piskunov, M.V. Zabelin, Transformation of water ball falling in high-temperature gases, Atomization Sprays 27 (10) (2017) 893e911. [43] F.Y. Hagos, O.M. Ali, R. Mamat, A.A. Abdullah, Effect of emulsification and blending on the oxygenation and substitution of diesel fuel for compression ignition engine, Renew. Sustain. Energy Rev. 75 (2017) 1281e1294. https://doi.org/10.1016/j.rser.2016.11.113. [44] P. Geng, E. Cao, Q. Tan, L. Wei, Effects of alternative fuels on the combustion characteristics and emission products from diesel engines: a review, Renew. Sustain. Energy Rev. 71 (2017) 523e534. https://doi.org/10.1016/j.rser.2016.12.080. [45] Z. Yang, Y. Wu, Z. Zhang, H. Li, X. Li, R.I. Egorov, P.A. Strizhak, X. Gao, Recent advances in co-thermochemical conversions of biomass with fossil fuels focusing on the synergistic effects, Renew. Sustain. Energy Rev. 103 (2019) 384e398, https://doi.org/10.1016/j.rser.2018.12.047. [46] H. Li, J. Li, X. Fan, X. Li, X. Gao, Insights into the synergetic effect for co-pyrolysis of oil sands and biomass using microwave irradiation, Fuel (2019) 219e229, https:// doi.org/10.1016/j.fuel.2018.10.139.

Please cite this article as: D.V. Antonov et al., Differences of two-component droplets breakup at the high temperatures, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.005