Blended Droplets Research Articles

Experimental investigation on evaporation, puffing and vapor jetting of multi-component fuel droplets with high-volatility difference

The current study presents an experimental investigation into the evaporation, puffing and vapor jetting characteristics of multi-component fuel droplet with high-volatility difference at 773 K. The multi-component fuel blends consist of biodiesel as a lower-volatile component, while 2, 5-dimethylfuran (DMF) is used as the higher volatile constituents. The DMF content is systematically varied from 0 to 100 %vol, employing both the fiber-support method and a magnified high-speed backlight illumination technique. The results show that the biodiesel-DMF blended droplets only exhibited weak rupture phenomena: puffing and vapor jetting. In puffing, the thinning of the liquid film primarily arises from the squeezing effect of bubbles, while in vapor jetting, the reduction of the liquid film is predominantly a consequence of the evaporation of the liquid film. Anomalous behavior occurred during the evaporation process of biodiesel-DMF droplets. The lower-volatile component within the droplets did not contribute to an accelerated evaporation of the blended droplets; instead, they resulted in the suppression of their evaporation. In comparison to the theoretical value, the evaporation duration of blended fuel droplets increased by 56.6 %, 50.6 %, and 78.9 %, respectively. This outcome is attributed to the combined effects of five factors: an increase in the overall thermal resistance, a reduction in heat and mass transfer within the gas phase, the occurrence of condensation and incomplete micro-explosion. Notably, the primary factor plays a significant role as the main contributor to this phenomenon, with its contribution to the reduction in droplet evaporation rate ranges from 28 % to 59 %.

A study on puffing and micro-explosion characteristics of diesel/ethanol/tetrahydrofuran blend fuel droplets

ABSTRACT Tetrahydrofuran (THF) is a biofuel that can be added to improve the solubility of ethanol in diesel. To explore the effect of surfactants, the puffing and micro-explosion characteristics of ethanol diesel/THF (DTE) blend droplets are investigated experimentally in comparison to a counterpart blend of ethanol diesel/biodiesel (DBE). Results show that with a rise in temperature, the puffing delay time decreases rapidly opposite to the fluctuation ratio; however, the puffing delay time rises with the ethanol content, whereas the fluctuation ratio remains about constant. Moreover, the tetrahydrofuran mainly participates in the early evaporation stage resulting in more prone to the edge mode for DTE droplets, which is in contrast to the DBE droplets, which prefer the inner mode. The micro-explosion strength of DTE droplets is lower than that of DBE due to the more likely occurrence of the edge mode, but the corresponding growth rate of DTE (15.2%, 25.4%, and 56.8%) with temperatures of 573 K, 623 K, and 723 K is higher than that of DBE droplets (8.8%, 20.4%, and 48.7%). A numerical model is also established for the present blend droplets, and the simulations reveal that the THF may additionally influence the heat absorption by the liquid and hence the superheat point of ethanol, which is responsible for the bubble formation and the micro-explosion.

Investigation of single drop evaporation characteristics of two-component fatty acid methyl esters at high temperature

To investigate the evaporation and micro-explosion characteristics of blended droplets at 773 K and 973 K ambient temperatures, methyl palmitate (MP) and methyl oleate (MO) are selected as typical fuels in this study. The MP concentration ranges from 10 % to 50 % to investigate the micro-explosion characteristics of the blended droplets. No micro-explosion that occurs during its whole lifetime when MP is pure, but the micro-explosion can be observed in all blended droplets. Interestingly, the micro-explosion intensity, average evaporation rate, micro-explosion delay time, and evaporation duration of droplet all reach the optimum value when the MP concentration is 20 % at 773 K and 973 K. The micro-explosion can be divided into three categories by intensity, namely strong micro-explosion, weak micro-explosion, and puffing. The strong micro-explosions occur at 973 K when the concentration is 20 %. It can be inferred that micro-explosion is the critical factor in shortening the evaporation duration. Cleavage reaction that occurs in the MO produced bubbles to increase the micro-exploration intensity. Meanwhile, the polymerization that carried out in the MO inhibits the evaporation with producing solid. It is observed experimentally that the residual volume caused by the polymerization decreases with increasing MP concentration. Furthermore, a new relationship is established in terms of the evaporation rate, micro-explosion intensity and ambient temperature by numerical fitting.

Experimental study on the evaporation characteristics of Jatropha curcas oil methyl ester (JME)-ethanol blended droplets

The evaporation and micro-explosion characteristics of Jatropha curcas oil methyl ester (JME) at 10%, 20%, and 30% ethanol by volume at two ambient temperatures were investigated using droplet evaporation experiments. The effects of different temperatures and ethanol content on parameters such as the normalized square diameter of droplets, micro-explosion intensity, number of micro-explosions, droplet evaporation duration and evaporation rate were analyzed. The experimental results show that adding ethanol component to the JME droplets increases the droplet expansion micro-explosion, and increasing ambient temperature promotes the droplet micro-explosion intensity and the number of micro-explosions. The intensity and number of micro-explosions of blended droplets are related to the concentration of ethanol. In addition, the micro-explosion intensity is numerically divided into strong micro-explosion and weak micro-explosion. Different concentrations of ethanol improve the evaporation characteristics of droplets differently. In general, the higher the concentration of ethanol, the shorter the droplet evaporation duration and the greater the evaporation rate. It was found that increasing the ethanol concentration from 20% to 30% had the most significant effect on droplet duration. Notably, the phenomenon of vapor clouds due to non-isothermal condensation caused by Stefan outflow was discovered.

Experimental study on the micro-explosion characteristics of biodiesel/1-pentanol and biodiesel/ methanol blended droplets

Alcohols and biodiesel are promising alternative biofuels for their excellent physicochemical properties and renewable property. Compared with low-carbon alcohols, high-carbon 1-Pentanol has better fuel characteristics. The objective of the present study was to reveal the effects of different 1-pentanol addition ratios on the micro-explosion characteristics of blended droplets. Single droplet suspension experiment under atmospheric pressure and 700 °C was carried out, and results were compared with that of methanol. The droplet had three initial diameters (1.281, 1.451, and 1.537 mm). The droplet evaporation images were recorded at 500 fps with a resolution of 1024 × 1024 pixels by a high-speed video camera, and the corresponding droplet diameter in the evaporation process was obtained by a MATLAB program. A new dimensionless micro-explosion intensity was proposed. The results showed that when the 1-pentanol volume content was 50% (B50P50), the micro-explosion number and total micro-explosion intensity reached the maximum. With the increase of initial droplet diameter, the micro-explosion number, total micro-explosion intensity and droplet lifetime increased. Compared with biodiesel/1-pentanol blended droplets, biodiesel/methanol ones had better micro-explosion characteristics and shorter droplet lifetime. White fog formed in the evaporation process of blended droplets and pure biodiesel ones. In short, the addition of methanol and 1-pentanol caused micro-explosions, changed the evaporation behavior of biodiesel, and reduced droplet lifetime, which could be considered as an alternative fuel blended with biodiesel.

Evaporation Patterns of Dextran-Poly(Ethylene Glycol) Droplets with Changes in Wettability and Compatibility.

The dextran–PEG system is one of the most famous systems exhibiting phase separation. Various phase behaviors, including the evaporation process of the dextran–PEG system, have been studied in order to understand the physicochemical mechanism of intracellular phase separation and the effect of condensation on the origin of life. However, there have been few studies in dilute regime. In this study, we focused on such regimes and analyzed the pattern formation by evaporation. The specificity of this regime is the slow onset of phase separation due to low initial concentration, and the separated phases can have contrasting wettability to the substrate as evaporation progresses. When the polymer concentration is rather low (<5 wt%), the dextran–PEG droplets form a phase-separated pattern, consisting of PEG at the center and dextran ring of multiple strings pulling from the ring. This pattern formation is explained from the difference in wettability and compatibility between dextran and PEG upon condensation. At the initial dilute stage, the dextran-rich phase with higher wettability accumulates at the contact line of the droplet to form a ring pattern, and then forms multiple domains due to density fluctuation. The less wettable PEG phase recedes and pulls the dextran domains, causing them to deform into strings. Further condensation leads to phase separation, and the condensed PEG with improved wettability stops receding and prevents a formed circular pattern. These findings suggest that evaporation patterns of polymer blend droplets can be manipulated through changes in wettability and compatibility between polymers due to condensation, thus providing the basis to explore origins of life that are unique to the process of condensate formation from dilute systems.

Experimental study on the evaporation characteristics of biodiesel-ABE blended droplets

At an atmospheric pressure, the effects of different temperatures and acetone-butanol-ethanol (ABE) content on the evaporation characteristics of biodiesel-ABE blended droplets suspended on a thermocouple fiber were investigated. The experimental results showed that micro-explosion did not occur for B80A20 (BX1AX2 represented that the volume fractions of biodiesel and ABE were X1% and X2%, respectively) blended droplets at 530 and 730 °C. The micro-explosion intensity increased first and then decreased，and reached the maximum at B40A60. Besides, it increased with temperature at certain ABE content. The fluctuation delay decreased first and then increased with ABE content, and the minimum was obtained at B60A40. In addition, it decreased with the temperature. The variation trend of micro-explosion delay was similar to that of fluctuation delay. The equivalent evaporation rate in the transient heating stage (k1) increased with the increase of ABE content, but decreased with the increase of temperature. Among all the blended droplets, the B40A60 droplets had the shortest lifetime. It can be speculated that there is an optimal ABE content between 40% and 60%, which makes the biodiesel-ABE blended droplets have the best evaporation performance. That is, the blended droplets have stronger micro-explosion and ejection, shorter fluctuation delay time and lifetime.

Effect of 2, 5-dimethylfuran concentration on micro-explosive combustion characteristics of biodiesel droplet

Micro-explosive combustion is considered as an effective way to improve combustion efficiency in combustor. In this work, the micro-explosive combustion characteristics of biodiesel and DMF blended droplet are investigated using high-speed backlit image technique. The droplet is suspended at the intersection of two quartz fiber and ignited by a pair of electrodes. Flame temperature and soot concentration are measured using two-color pyrometry method. Moreover, two new parameters are proposed to analysis droplet micro-explosion, namely droplet instability and breakup index. The results shown that the instability of DMF25BD75, DMF50BD50 and DMF75BD25 droplets is mainly caused by bubbles coalescence, internal circulation of bubbles, micro-explosion and vapor jetting. The low-temperature flame region is mainly located near the droplet surface and flame edge. However, the soot concentration is highest in these two regions. The combustion of blended droplet includes a micro-explosive combustion stage and a stable combustion stage. Four modes of secondary atomization is determined according to droplet breakup index. When the DMF concentration is 50% and 75%, the burning rate is 9% and 37.5% higher than diesel, which is expected to realize rapid and high-efficient combustion.

Analytical and experimental study on boiling vaporization and multi-mode breakup of binary fuel droplet

In this paper, the boiling vaporization and breakup of single droplets of n-butanol, n-hexadecane and their binary mixtures with 10-50 w.t.% n-butanol have been studied. The experiments of droplet evaporation with an elevated ambient temperature range of 537-609 K were conducted through the pendant drop method and high-speed camera technology. The mathematical method was also proposed to predict the heterogeneous superheat limit and nucleation rate within the droplet. The experimental results indicate that mono-component droplets only go through the transient heating and stable evaporation phases. While for bi-component droplets, the characteristics show strong two-phase flow instability. With increasing n-butanol concentration or ambient temperature, the heterogeneous nucleation will occur inside the blended droplet, which experiences fluctuation evaporation between the transient heating and stable evaporation phases. The droplet vaporization characteristics gradually change from normal evaporation to partial rupture or passive breakup and then to micro explosion which reduces the droplet lifetime considerably. The novelty of this work is that various ambient temperature conditions and compositions are adopted to study the droplet evaporation to identify the conditions that favor flash boiling, and a new analytical model of heterogeneous nucleation is proposed, which can be used to estimate the temperature at the instant of bubble nucleation.

Transcritical evaporation and micro-explosion of ethanol-diesel droplets under diesel engine-like conditions

A transcritical multi-component evaporation and micro-explosion sub-model is suggested to investigate the micro-explosion of blended ethanol-diesel droplets under diesel engine-like conditions. A homogeneous theory is used to qualitatively predict the bubble generation, and the subsequent bubble growth leads to the final explosion. The evaporation rate under subcritical state is calculated by a multi-component diffusion sub-model along with an isothermal-isobaric flash. The finite thermal conduction and mass diffusion equations are solved inside the multi-component droplets. The transcritical transition is defined by the time instant that the droplet surface attains the mixture critical temperature, then the finite regression rate is controlled by the thermal diffusion, and the critical mixing surface moves continuously inward. The suggested model is validated against the experimental data. Based on the numerical results, the heat transfer rate in ambient and the superheat limit are found to be the predominated factors on the micro-explosion under low pressures. However, with accelerating pressure, the net result of competition among the evaporation rate, heating rate, transcritical transition, as well as the bubble growth rate determines the micro-explosion for small micron droplets (<24 μm). When the ambient temperature is higher than 1000 K, the enhanced regression rate caused by the transcritical transition makes the droplet lifetime too short to explode. Finally, the suggested sub-model is implemented into the OpenFOAM code. The spray of blended ethanol-diesel under diesel engine-like conditions is simulated, and the occurrence of micro-explosion is confirmed and found to show large effects on the spray characteristics.

Experimental investigation on ignition, combustion and micro-explosion of RP-3, biodiesel and ethanol blended droplets

Based on an infusion set, measuring cylinder and different types of micro needles, a method for measuring the volume of micro-droplet was established. The same volume of aviation fuel (RP-3), biodiesel and ethanol mixed droplets were sent to a heatable experimental device to study the combustion characteristics. Through a movable slide rail with adjustable speed, the mixed droplets suspended on the high-temperature resistant wire were transported to a distance of about 2 mm from the preheating plug for heating. The ingle-lens-reflex (SLR) camera and a high-speed camera were used to capture the ignition characteristics and instantaneous combustion characteristics of mixed droplets during heating, respectively. The burning mixed droplets video captured by the SLR camera was processed into color pictures by Adobe Premiere Pro CC software. Through the processing and analysis of these images, the ignition, combustion and micro-explosion characteristics of mixed droplets during combustion were studied. The equation for calculating the micro-explosion intensity was established, the micro-explosion intensity of binary mixed droplets and ternary mixed droplets was calculated and analyzed.

Experimental study on evaporation and micro-explosion characteristics of biodiesel/n-propanol blended droplet

The evaporation and micro-explosion characteristics of biodiesel/n-propanol blended droplets at 573, 673 and 773 K ambient temperatures are studied using high-speed backlight imaging technique. The results show that the droplet evaporation is relatively stable at 573 K. However, micro-explosions occur at 673 and 773 K, the micro-explosion intensity increases with ambient temperature. The calculated superheat limit of n-propanol is 490 K. The blended droplet micro-explosion occurrence time is associated with the n-propanol concentration. The lower content of n-propanol, the earlier blended droplet micro-explosion occurs, and vice versa. The micro-explosion intensity and evaporation rate of the blended droplet first increase and then decrease as the n-propanol concentration increases. The micro-explosion delay time of the blended droplet first decreases and then increases with the increase of n-propanol concentration. Interestingly, the micro-explosion intensity, evaporation rate and micro-explosion delay time of droplet all reach the optimum value when the n-propanol concentration is 50%. Moreover, the oil membrane formation mechanisms of the soluble blended droplet with two different blended structures (n-propanol in biodiesel and biodiesel in n-propanol) are proposed.

Experimental investigation of evaporation characteristics of biodiesel-diesel blend droplets with carbon nanotubes and nanoceria as nanoadditives

The evaporation characteristics of biodiesel–diesel blends mixed with different concentrations of nanoadditives, namely, carbon nanotubes (CNTs) and ceria nanoparticles (NPs), at ambient temperatures of 673 K and 873 K under normal gravity were researched. The nanoadditives were dispersed in dosages of 50 ppm CNTs + 50 ppm NPs, 100 ppm CNTs + 100 ppm NPs, 250 ppm CNTs + 250 ppm NPs, and 500 ppm CNTs + 500 ppm NPs with 80% diesel blended with 20% jatropha biodiesel (B20D80). The results show that the nanoadditive has dual influence during evaporation processes at 673 K; the nanoadditives accelerate the evaporation rate at concentrations of 50 ppm CNTs + 50 ppm NPs and 100 ppm CNTs + 100 ppm NPs; conversely, they retard the evaporation rate at concentrations of 250 ppm CNTs + 250 ppm NPs and 500 ppm CNTs + 500 ppm NPs. However, the four concentrations of nanoadditives accelerate the evaporation rate at 873 K, whereas the level of evaporation acceleration is different. The CNTs has higher heat transfer efficiency than ceria NPs, whereas CNTs aggregate more easily and prolong the evaporation owing to their long length. Finally, the 100 ppm CNTs and 100 ppm NPs are suitable concentrations for nanoadditive in that it can effectively accelerate droplet evaporation at 873 K and 673 K.

An experimental study of the puffing and evaporation characteristics of acetone–butanol–ethanol (ABE) and diesel blend droplets

Puffing and evaporation characteristics of acetone–butanol–ethanol (ABE) and diesel blend droplet under different ABE contents and ambient temperatures were investigated experimentally by using droplet suspension method. Both thermocouple wire and quartz fiber wire were adopted in this experiment to realize the measurement of droplet temperature, and minimize the effect of suspension wire. The experimental results show that, adding ABE component in diesel results in the droplet puffing, and increasing of ambient temperature promotes droplet puffing. With the increasing of ABE content, the onset point of puffing decreases first and then increases, and the average expansion speed and the frequency of puffing increase first and then decrease. Thus, an optimal volume blend ratio exists for puffing, which is found to be around 40%–50% ABE content. The occurrence of puffing decreases the duration of transient heating phase, and promotes the evaporation in the fluctuation evaporation phase. Thus, the overall evaporation duration decreases first and then increases with the ABE content increasing due to the effect of puffing at higher ambient temperature.

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

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.

Pressure Effect in Droplet Combustion of Blended Fuel on Ethanol and Kemiri Sunan (Reutealis Trisperma (Blanco) Airy Shaw) Biodiesel

Bio oil from seeds of Kemiri Sunan (reutealis trisperma (Blanco) airy shaw) plant is particularly attractive to be studied since its high potential as alternative of biodiesel. This paper describes the results of a fundamental study of the combustion characteristics of a single droplet of blended fuel of Kemiri Sunan (reutealis trisperma (Blanco) airy shaw) biodiesel and ethanol in various levels of ambient pressure. The ambient pressures of the burning chamber were varied at 1, 3, and 5 bars. The fuels were prepared by mixing the biodiesel with the ethanol at concentrations of 0%, 10%, 20%, and 30% vol/vol. The single droplets of blended fuels were suspended using micro-syringe on a tip of the thermocouple. It was ignited and combusted using an electrical heater. The ignition and combustion processes of the single droplets were recorded using a high-speed camera. It was found that the ignition delay of the droplet decrease with the increase of ambient pressure and the concentration of ethanol in blended fuels. Also, the burning rate of blended droplets increased with increasing biodiesel concentration and pressure. The maximum droplet temperature slightly increased during the combustion with the increasing ethanol concentration, but not with increasing ambient pressure.

Experimental study on evaporation characteristics of lubricating oil/gasoline blended droplet

The auto-ignition of lubricating oil/gasoline blended droplet is one of super-knock origins in gasoline direct injection engine. This paper investigates the effect of ambient temperature and blended ratio on evaporation characteristics of lubricating oil/gasoline blended droplet. The experimental results indicate that the evaporation process of blended droplets can be divided into three periods. The second evaporation period may be rapid evaporation or fluctuation evaporation, which is depended on ambient temperature and gasoline content. Gasoline is the main reason of bubble generation inside blended droplets. The bubble generation, puffing and micro-explosion becomes more violent with the increase of gasoline content. The blended droplet lifetime decreases with the increase of ambient temperature. The evaporation residue is a colloidal substance at low ambient temperature and it is a hard solid material at high ambient temperature.

An experimental study of the burning characteristics of acetone–butanol–ethanol and diesel blend droplets

Acetone–Butanol–Ethanol (ABE), the intermediate product to produce bio-butanol is used as an alternative fuel directly to eliminate needless production costs. In this study, the droplet burning characteristics of neat ABE, diesel and ABE-diesel blends (10%, 20%, 30%, 50% of ABE (vol%)) fuels are investigated by the droplet free falling technique under atmospheric pressure. The initial droplet temperature and diameter are about 300 K and 235 μm respectively. The ambient temperature around the flat-flame burner is about 1123 K, and the residual oxygen concentration is 21 vol%. The results show that the addition of ABE not only increases the average burning rate and the ignition delay of droplets, but also reduces soot emissions. Meanwhile, ABE-diesel blends droplets occur micro-explosion at the end of flame because of the large difference in volatility between ABE components and diesel, which distinctly shortens the burning duration of ABE-diesel blends. In addition, with the increase of ABE content, the micro-explosion performance and overall burning rates increase first and then decrease, which indicates the existence of an optimal volume blend ratio around 30% ABE content for ABE-diesel blends that makes explosion performance and overall burning rates reach the top.

Evaporation of pure and blended droplets of diesel and alcohols (C2–C9) under diesel engine conditions

ABSTRACTAn improved multicomponent evaporation model was developed to study the evaporation characteristics of the pure diesel and alcohol (C2–C9) as well as their blended droplets. In this model, the Predictive Soave–Redlich–Kwong (PSRK) equation of state (EOS) was employed to evaluate the vapor–liquid equilibrium for pure and blended droplets. The results indicated that the model predictions agree very well with the experimental measurements. The evaporation characteristics of different pure alcohol droplets were analyzed, and the influence of the addition of alcohols in diesel on the evaporation behavior of droplets under various conditions was discussed.

Numerical modeling of acetone–butanol–ethanol and diesel blends droplet evaporation process

To investigate the evaporation characteristics of acetone–butanol–ethanol (ABE) and diesel blends, a multi-component evaporation model for ABE–diesel blend droplet coupled with six components to represent diesel and Universal Functional Activity Coefficient (UNIFAC) method has been built and validated with droplet fiber-suspension evaporation experimental results. The evaporation characteristics of ABE–diesel blend as well as the effects of ABE mass fraction and ambient temperature are analyzed. The results show that ABE–diesel blend droplets evaporate faster than diesel, the addition of ABE affects the early stage of evaporation process, especially at high temperature, which lead to the occurrence of internal gasification observed in experiments. With increasing the ABE mass fraction, the evaporation rate increases, and the internal gasification is more prone to occur later but preform more intensively. The droplet lifetime decreases non-linearly with the increasing ambient temperature, and the evaporation of low volatile components are enhanced significantly. Meanwhile, the increasing ambient temperature leads to the performance of internal gasification faster and more intense.