Evaporation Characteristics Of Droplets Research Articles

Experimental Study on the Influence of Microwave Energy Pulse Width and Duty Cycle on Evaporation and Ignition Characteristics of ADN-Based Liquid Propellant Droplets

This study investigates the evaporation and ignition characteristics of a single droplet of ammonium dinitramide (ADN)-based liquid propellant utilizing a waveguide resonant cavity device, in conjunction with a high-speed photographic imaging system and testing system. Experimental methods are employed to analyze the impact of microwave pulse width and duty cycle on the puffing and meicro-explosion phenomena of the droplet, as well as the delay time and duration of ignition. The experimental findings reveal that increasing the duty cycle enhances the ignition success rate and diminishes flame development time. Specifically, elevating the microwave duty cycle from 60% to 80% reduces the ignition delay time of the droplet from 132.8 ms to 88.1 ms, and the ignition duration from 23.1 ms to 19.9 ms. Furthermore, an increase in microwave energy pulse width expedites the combustion process of the flame and influences plasma generation. Increasing the pulse width of microwave energy from 20 µs to 40 µs prolongs the ignition delay time from 140.3 ms to 200.5 ms and extends the ignition duration from 56.7 ms to 77.8 ms. Additionally, it is observed that a higher duty cycle leads to a more pronounced puffing phenomenon that initiates earlier. In contrast, a higher pulse width results in a more pronounced puffing phenomenon that commences later. This study provides a thorough investigation into the microwave ignition mechanism of ADN-based liquid propellants, offering theoretical insights into the ignition and combustion stability of such propellants in microwave-assisted ignition systems.

Physics Of Two-Stage Evaporation Of Volatile Droplets Suspended In An Ultrasonic Levitator

The evaporation process of mono-component droplets in an ultrasonic levitator is experimentally monitored, focusing on three types of volatile liquids: methanol, ethanol, and 2-propanol. This investigation reveals a ‘two-stage’ evaporation process characterized by an initial rapid evaporation stage followed by a significantly slower evaporation stage; a pattern consistently observed across all three volatile liquids. However, for droplets of comparable size, methanol droplets evaporate faster in the rapid stage than ethanol and iso-propanol droplets and enter the slow evaporation stage earlier. On the other hand, during the slow evaporation stage, 2-propanol droplets exhibit the lowest evaporation rate compared to methanol and ethanol droplets. To explain the two-stage evaporation and the varying evaporation characteristics of mono-component droplets with different volatile components, the streaming patterns within the levitator, with and without a suspended droplet, are visualized and quantified using Particle Imaging Velocimetry (PIV). Two acoustic-induced jet flows, originating from the transducer and reflector of the levitator and considered as Eckart streaming, are observed within the empty levitator. These jet flows move toward the levitated droplet, influencing its evaporation process. Additionally, Stefan flow, carrying vapor away from the surface of the volatile droplet, is clearly observed due to rapid evaporation. The competition between Stefan flow and Eckart streaming ultimately determines the external streaming patterns around the investigated droplets. A theory combining the external streaming patterns and vapor enrichment is proposed to interpret the two-stage evaporation and the corresponding characteristics of the evaporating droplets suspended in the levitator. Furthermore, this study captures boundary-driven acoustic streaming near the droplet surface, known as Rayleigh–Schlichting streaming, whose dimensions and motion vary with the different liquid components and the properties of the corresponding vapor. In summary, the study highlights the presence of Eckart streaming in the ultrasonic levitator and Stefan flow outside an evaporating volatile droplet. Considering these flow patterns is essential for explaining the ‘two-stage’ evaporation and the relevant characteristics within the ultrasonic levitator.

Physics Of Two-Stage Evaporation Of Volatile Droplets Suspended In An Ultrasonic Levitator

The evaporation process of mono-component droplets in an ultrasonic levitator is experimentally monitored, focusing on three types of volatile liquids: methanol, ethanol, and 2-propanol. This investigation reveals a ‘two-stage’ evaporation process characterized by an initial rapid evaporation stage followed by a significantly slower evaporation stage; a pattern consistently observed across all three volatile liquids. However, for droplets of comparable size, methanol droplets evaporate faster in the rapid stage than ethanol and iso-propanol droplets and enter the slow evaporation stage earlier. On the other hand, during the slow evaporation stage, 2-propanol droplets exhibit the lowest evaporation rate compared to methanol and ethanol droplets. To explain the two-stage evaporation and the varying evaporation characteristics of mono-component droplets with different volatile components, the streaming patterns within the levitator, with and without a suspended droplet, are visualized and quantified using Particle Imaging Velocimetry (PIV). Two acoustic-induced jet flows, originating from the transducer and reflector of the levitator and considered as Eckart streaming, are observed within the empty levitator. These jet flows move toward the levitated droplet, influencing its evaporation process. Additionally, Stefan flow, carrying vapor away from the surface of the volatile droplet, is clearly observed due to rapid evaporation. The competition between Stefan flow and Eckart streaming ultimately determines the external streaming patterns around the investigated droplets. A theory combining the external streaming patterns and vapor enrichment is proposed to interpret the two-stage evaporation and the corresponding characteristics of the evaporating droplets suspended in the levitator. Furthermore, this study captures boundary-driven acoustic streaming near the droplet surface, known as Rayleigh–Schlichting streaming, whose dimensions and motion vary with the different liquid components and the properties of the corresponding vapor. In summary, the study highlights the presence of Eckart streaming in the ultrasonic levitator and Stefan flow outside an evaporating volatile droplet. Considering these flow patterns is essential for explaining the ‘two-stage’ evaporation and the relevant characteristics within the ultrasonic levitator.

Experimental Study of the Effect of the Initial Droplet Diameter on the Evaporation Characteristics of Unsymmetrical Dimethylhydrazine Droplets in a Subcritical Environment

The evaporation characteristics of unsymmetrical dimethylhydrazine droplets with different initial diameters in a subcritical environment were experimentally investigated with the temperature–pressure separation technique. The evaporation processes of unsymmetrical dimethylhydrazine droplets with different initial diameters in this environment have the same general pattern. All the studied droplets exhibit a short transient heating phase and a steady-state evaporation phase obeying d2. Notably, the expansion of the transient heating phase gradually increases with increasing ambient pressure. The change in diameter squared ∆d2max increases from 1.03% at 1 MPa to 12.48% at 5 MPa. Under subcritical conditions, the evaporation rate decreases linearly with decreasing droplet diameter, and the droplet evaporation lifetime increases linearly. Changes in the initial droplet diameter may still have a large effect on droplets smaller than those studied here. When the ambient pressure is not greater than 3 MPa, the change in the steady-state evaporation time for both medium- and large-diameter droplets accounts for more than 70% of the variation in the droplet evaporation lifetime. As the ambient pressure increases to 4 MPa and 5 MPa, the percentage of the change in the transient heating time contributing to the variation in the droplet evaporation lifetime gradually increases to more than 45%.

Evaluation of the spatial distribution of aerosols produced by various respiratory activities

This study investigated the dispersion and evaporation characteristics of droplets and droplet nuclei emitted during human respiratory activities. A specially designed wind tunnel was filled with purified air, wherein selected subjects performed various respiratory activities with their heads positioned inside. An optical particle sizer was used to collect particles with sizes of 0.3–10 μm at 63 points in front of the mouth. The dilution factors were analyzed to investigate the impact of combining the exhaled airflow with ambient air on droplet evaporation. At a distance of 0.01 m from the mouth opening, the volume concentration of the particles was the highest during breathing, followed by coughing and speaking. The volumetric concentration of particles decreased with an increase in the distance from the inlet for all activities. The spatial volume concentration distribution of particles showed that coughing tended to disperse the particles in the forward direction, whereas speaking tended to disperse them laterally. Utilizing these findings in CFD analysis can provide in-depth insights into dispersion and evaporation dynamics. This can contribute significantly to the development of preventive measures through the implementation of proactive HVAC systems to effectively remove infectious particles and control the spread of infectious diseases. Future studies should explore a wider range of particle sizes and advanced sampling techniques for a clear understanding of respiratory particle dynamics and infection control strategies.

Evaluation of droplet evaporation characteristics and its influencing factors in high-temperature gaseous environment

Droplet evaporation is widely present in nature and industrial production, involving heat and mass transfer between the droplet and the surrounding gas environment. This study investigates the evaporation of individual spherical droplets in a high-temperature gas environment under both stationary and forced convection conditions, using theoretical and numerical approaches. The results show that the established evaporation model can effectively reflect the temperature change during droplet evaporation. When the droplet evaporates in a gaseous environment, the equilibrium temperature of the droplet is influenced by environmental parameters such as temperature and humidity. Compared to environmental temperature, humidity has a more significant impact on the equilibrium temperature of the droplet, making it the dominant factor affecting the evaporation temperature. The evaporation rate of the droplet is influenced by environmental humidity. When the air humidity is below 0.5, the variable-rate evaporation aligns closely with the numerical simulation results (droplet lifetime). When the air humidity is above 0.5, the simulation results are slightly higher but still have a small relative error. As the environmental humidity increases, the evaporation rate of the droplet is suppressed, leading to an increase in its lifetime. Establishing an appropriate droplet evaporation model not only deepens our understanding of the internal mechanisms of droplet evaporation but also provides practical guidance for enhancing heat and mass transfer in droplets.

The Effect of Blended Palm Oil Biodiesel Droplet Properties on Evaporation Characteristics

The evaporation characteristics of a single fuel droplet of diesel fuel (DF) and the Malaysian palm oil biodiesel-diesel blend (B10-B50) were studied experimentally. The fuel droplet properties consisted of different ratios of palm oil biodiesel mixtures which could influence the droplet behaviour when impinged on the heated surface. By employing the hot surface deposition test (HSDT) method, the droplets impinging on the heated surface of an aluminum alloy plate were observed to evaluate the droplet's evaporation characteristics including the evaporation lifetime and maximum evaporation point (MEP) of the tested fuels. Furthermore, the deposit’s surface temperature for dry condition (timp=7 seconds) and wet condition (timp=3 seconds) test were also measured during the deposition test. Finally, the deposit development on the heated surface was evaluated through a logarithmic expression of MR/mD = αNDβ. The results showed that the evaporation lifetime of each test fuel decreased with increasing hot plate surface temperature. Furthermore, the MEP was the highest for B30 (380°C), followed by B20 and B40 (375°C), B10 and B50 (365°C), and DF (360°C). In the dry condition test, the recorded average minimum and maximum deposit surface temperatures were observed to be within the range of Td=295°C to Td=325°C for DF and Td=290°C to Td=350°C for B10-B50 fuels. On the other hand, for the wet condition test, the recorded average minimum deposit surface temperatures for both DF and B10-B50 fuels were consistently lower than the hot plate temperature. DF exhibited the most variation, ranging from Td=200°C to Td=300°C, whereas, for B10-B50, the recorded deposit surface temperature was only ranging from Td=150°C to Td=200°C.

Evaporation rates of water droplets with soluble and insoluble additives

The relevance. Caused by a wide practical use of water with typical soluble and insoluble additives in the following technologies. They are: thermal and fire purification of liquids, polydisperse fire extinguishing, combustion of waste-derived slurry fuels, cleaning of heat-loaded surfaces, creation of heat agents based on combustion products. Any change in liquid component composition leads to a change in its thermophysical properties, heating rates and phase transformations. So far, there is not enough experimental data on evaporation rates of water droplets with typical soluble and insoluble additives. The aim. Experimental determination of evaporation rate of water droplets with typical additives under various heating methods. The methods. To register sizes of moving droplets, optical registration methods based on a high-speed video camera were used. To register the temperature of combustion products, a measuring complex, consisting of an NI 9219 board and four chromel-alumel fast-response thermocouples, was used. The authors used four methods of droplet heating: in the flow of combustion products with the dominance of convective heat transfer, in a tubular muffle furnace with the dominance of radiative heat transfer, on a solid surface with the dominance of conductive heat transfer, and in a flame with the dominance of mixed (convective and radiative) heat transfer. The results. The authors have determined the ranges of change in evaporation rates of water droplets with typical (soluble and insoluble) additives for various heating methods with the dominance of convective, radiative, and conductive heat transfer. The effect of an additive type and concentration, heating method and heating medium temperature on of droplet evaporation characteristics was established.

Boiling regimes of a single droplet impinging on a superheated surface: Effect of the surrounding medium

Impinging droplets on a hot surface is a significant phenomenon in many industrial applications, such as spray cooling. Previous studies reported various morphological behaviours affecting the droplet's wetting, heat transfer, and evaporation characteristics. However, there is an ambiguity in understanding the influence of the surrounding medium on the resulting boiling regimes. The present work aims to study this aspect through an experimental investigation using high-speed photography and infrared thermography. During the experiments, the degassed FC-72 liquid droplets have impinged onto a heated chromium-plated calcium fluoride (CaF2) surface with a constant impact velocity. Three ambient conditions, such as saturated vapour (heated vapour), air at liquid saturation temperature (heated air), and air at room temperature (ambient air), are used. The surface temperature is varied in each ambience until the complete droplet dewetting, i.e., the Leidenfrost phenomenon has appeared. In this study, the stages of boiling are perceived as film evaporation, excessive and nominal bubbly boiling, transition, and Leidenfrost. With different surrounding media, the onset of a regime and its boiling morphology are distinct. During the film evaporation regime, the droplet heat transfer is high for the impact in ambient air, and the contact line evaporation is dominant in the vapour medium. In contrast, heat transfer is improved in a vapour medium during the bubbly boiling process, reaching up to 30% enhancement compared to the droplets in air surroundings. It is attributed to increased nucleation sites and excessive bubbling compared to air media. Correlations from the literature are also revisited to understand the onset of bubbly boiling and the droplet breakup at the Leidenfrost point. An overall comparison of droplet heat transfer during the boiling regimes in surrounding media is discussed in this paper.

Experimental Investigation of the Water Droplet Evaporation on Inclined Surfaces by Taguchi and ANOVA Optimization Analysis

Abstract We experimentally investigated the evaporation characteristics of a sessile water droplet on a glass substrate with different surface roughness levels. The influence of five parameters is evaluated for the evaporation process: substrate temperature (30 °C, 45 °C, and 60 °C), surface roughness (P0, P600, and P60), droplet volume (3, 5, and 8 µL), water droplets initial temperature (30 °C, 40 °C, and 60 °C), and inclination angle (0 deg, 45 deg, and 75 deg) of the glass substrate. The Taguchi orthogonal array design of L27 is utilized to establish minimal candidate trial points for experimental works, and more trials have been conducted to quantify the effects accurately. Then, analysis of variance (ANOVA) has been used to evaluate the evaporation times for the sessile droplets. The results indicate that evaporation times decrease with rising substrate temperatures, increasing substrate inclination angle, and increasing initial water droplet temperatures. In contrast, evaporation times rise with increasing surface roughness and droplet volumes. After evaluation of the ANOVA analysis, surface roughness levels and droplet volumes are considered the most influential parameters after substrate temperatures, which is the most effective parameter on the evaporation times. On the other hand, initial water droplet temperatures and substrate inclination angle are less effective considering droplet evaporation times. A linear regression fit was derived via ANOVA analysis for the evaporation time, and the best mean deviation was found to be 10% from the experiments. The experimental outcomes were compared to previous research, and correlations were derived. The proposed correlation has given good results considering experimental and literature data.

Parametric Study on the Evaporation Characteristics of Biodiesel-Diesel-Methanol Microemulsion Droplets

ABSTRACT In order to investigate the evaporation characteristics of biodiesel-diesel-methanol microemulsions, an experimental method of droplet evaporation with post processing techniques is established. The droplet normalized square diameter (NSD), droplet temperature and images during evaporation process are used to analyze the evaporation characteristics at different ambient temperatures. The results indicate that the methanol volume percentage, initial volume of droplet and ambient temperature have evident influence on the evaporation characteristics of droplets. When the initial volume of droplet is 0.5 μL, BD95M5 (biodiesel-diesel blend (95%)+methanol (5%)) and BD85M15 (biodiesel-diesel blend (85%)+methanol (15%)) droplets do not show the puffing and micro-explosion phenomena during evaporation process, but BD75M25 (biodiesel-diesel blend (75%)+methanol (25%)) droplet exhibits micro-explosion evidently at ambient temperature of 573K. Puffing is observed for BD85M15 droplet, and micro-explosion is for BD75M25 droplet at ambient temperature of 773K. The droplet lifetime is decreased and the mean evaporation rate (MER) is increased with the increase of ambient temperature and methanol volume percentage in microemulsion. The evaporation process ends at 5.938 s/mm2, 5.417 s/mm2 and 4.792 s/mm2 for BD95M5, BD85M15 and BD75M25, respectively, and MERs of them are 0.087 mm3/s, 0.095 mm3/s and 0.108 mm3/s at ambient temperature of 573K. The MERs of BD95M5, BD85M15 and BD75M25 are 0.225 mm3/s, 0.244 mm3/s and 0.309 mm3/s, respectively, at ambient temperature of 773K. With the increase of initial volume of droplet, the occurrence of puffing and micro-explosion is strengthened. The MER of droplets with initial volume of 0.6 μL is the largest, which increases by 13.1% and 12.8% compared with 0.4 μL droplets and 0.5 μL droplets, respectively.

Investigation on sub-to-supercritical transition of diesel: Gas–liquid interface properties

ABSTRACT By establishing a high-temperature droplet evaporation test device, the influence of ambient temperature on droplet morphology and droplet diameter is studied using diesel oil (D100). The variation of droplet evaporation characteristics is studied. The phase transition model of n-heptane in a supercritical environment is established and verified by the molecular dynamics simulation method. The diffusion coefficient, radial distribution function (RDF), surface tension, interfacial thickness, and liquid film temperature of n-heptane liquid film are analyzed. The results show that when the environmental conditions change from subcritical to supercritical, the diffusion coefficient decreases first and then increases from 8.55 × 10−4 cm/s−1 to 71.75 × 10−4 cm/s−1. The peak value of RDF decreases from 1627 to 438, and the smaller peaks after the main peaks tend to be smooth, indicating that n-heptane appears as a gas under supercritical conditions, and the phase transition changes from evaporation to diffusion. Cases 5(T = 973 K P = 5 MPa), 6(T = 573 K P = 7 MPa), 8(T = 773 K P = 7 MPa), and 9(T = 973 K P = 7 MPa) are high-supercritical calculation cases, with MSD of 1.71 × 105 Å2, 4.03 × 105 Å2, 2.49 × 105 Å2, 4.07 × 105 Å2, compare with case 1(T = 573 K P = 3 MPa), MSD increased by 11 times. When T/Tc ≥1.5 and P/Pc ≥1.5, n-heptane undergo three stages of transition from subcritical state to supercritical state. The first stage is the subcritical evaporation stage. With the evaporation of n-heptane, the fuel surface tension gradually disappears, the subcritical evaporation stage ends, and the transition stage begins. When the liquid film temperature exceeds the critical value, the transition phase ends, and the supercritical diffusion phase begins. In addition, when the ambient pressure is 5 MPa and 7MPa, the ambient temperature rises from 773 K to 973 K. The proportion of the transition process in the evaporation process increases by 19% and decreases by 3%, respectively.

Experimental investigation on evaporation characteristics of RP-3 aviation kerosene droplet above the critical temperature under various pressure conditions

The evaporation of aviation kerosene droplets in the aeroengine combustion chamber is one of the basic processes during liquid fuel combustion, which directly affects fuel atomization, air-fuel mixture formation and the combustion quality. However, the data are rarely reported due to the difficulty of conducting experiments in high temperature and pressure conditions. In this study, a series of experiments were carried out to investigate the evaporation characteristics of RP-3 aviation kerosene droplet. The environmental pressure and temperature of droplet evaporation ranged from 1 and 20 bar and 400 and 600°C respectively. An individual RP-3 kerosene droplet with initial diameter between 0.9 and 1.1 mm was suspended at a thermocouple bead with 50 μm diameter to measure droplet temperature. Droplet diameter during the whole evaporation process was recorded simultaneously by a high-speed camera. Experimental results indicated that at lower evaporation rate, RP-3 kerosene droplet evaporation experienced two processes: transient heating and equilibrium evaporation periods. While under high evaporation rates, the droplet only underwent equilibrium evaporation period without droplet expansion. At all the studied temperatures, droplet temperature kept increasing during the entire evaporation process without reaching a stable temperature. In most cases, the transient evaporation rate (K) of RP-3 kerosene droplet first increased rapidly until reaching the peak, and then followed by a monotonous decrease. However, at 20 bar above 500°C conditions, the temporal K curve exhibited a double-peak feature. Above the critical temperature of the fuel, increasing ambient pressure and temperature both can promote the heating and evaporation. However, the promotion of increasing ambient temperature to droplet evaporation was weakened at high ambient pressure. The quantitative correlations of droplet lifetime and evaporation rate constant with pressure were proposed under different ambient temperatures.

Effects of pulse power on evaporation characteristics of an n-heptane droplet in a microtube

When combustion instability arises in liquid rocket engines, heat release fluctuations cause local temperature oscillations and affect the evaporation of fuel droplets. In this study, to investigate the responses of droplet evaporation characteristics to temperature oscillations, an n-heptane droplet is formed by continuous fuel injection, and a pulse power is applied to generate temperature oscillations. The pulse power outputs a square wave with a frequency of 0.1–10 Hz and a power of 16.6–30.0 W. A transient heat transfer and droplet evaporation model is established, and measured droplet diameters serve to validate the model. The effects of heating frequency, heating power, and waveforms on droplet vaporization are studied. The results show that pulse power induces oscillations in air temperature, droplet diameter, and evaporation rate, which correspond to the applied heating frequency. Due to heat transfer within the tube, a phase lag is observed in the air temperature relative to the applied power. The dynamic equilibrium between the evaporation rate and fuel flow rate causes the evaporation rate phase to lag behind the droplet diameter phase by a constant of 0.5π, and to lead slightly ahead of the air temperature phase. Increases in heating frequency reduce the amplitudes of air temperature, droplet diameter, and evaporation rate while maintaining their averages constant. Decreases in heating power lower the air temperature, consequently increasing the evaporation rate amplitude and average droplet diameter. At 16.6 W, the droplet descends due to exceeding the critical diameter of 2.02 mm. Replacing the square wave of pulse power with a sinusoidal or triangular wave only results in a decline in air temperature amplitude, consequently reducing the evaporation rate amplitude by an equivalent degree. The amplitude percentage ratio between them remains unchanged at 4.77 but is subject to interference from their phase difference. As frequency increases from 0.1 to 1 Hz, the phase difference rises from −0.08π and converges to 0, while the amplitude percentage ratio increases from 4.77, converging to 4.87.

Numerical Simulation of Palm Biodiesel Droplet Evaporation at Various Temperatures and Pressures

This study uses numerical simulations to determine the evaporation characteristics of palm biodiesel droplets under normal gravity at temperatures ranging from 473 to 873 K and pressures ranging from 0.2 to 5 bar. A transient, two-phase, axisymmetric volume of fluid (VOF) model is employed to model transport processes across phases. The palm methyl ester is modelled as a single-component fuel with temperature- and pressure-dependent thermophysical properties. The study compares the obtained evaporation rates with those available in the literature and presents the effects of external parameters in experimental setups. Additionally, the study investigates the effects of changing the oxygen/nitrogen composition of the environment at elevated temperatures. The results show that elevated temperatures enhance the evaporation rate at all pressures and oxygen contents due to significantly enhanced thermal conductivity and droplet surface temperatures, while elevated pressures decrease the evaporation rates. Across the pressure range, evaporation rates decrease by 219%, 213%, and 196% for temperatures of 473, 673, and 873 K, respectively. Furthermore, increasing oxygen concentration in the environment can also increase the evaporation rate; however, the effect is less noticeable with a 6.4% increase at 473 K and more significant with a 27.8% increase at 873 K across the selected oxygen composition range.

Experimental investigation on droplet evaporation characteristics during combustion of future and current aviation fuels with range of properties

Abstract Currently there is lack of knowledge on how new types of alternative fuels and their storage conditions change the droplet evaporation characteristics. Liquid fuel is commonly stored in wide varieties of containers, where fuel characteristics may change because of the exposure to the material of the container. This study evaluates the impact of different storage containers on droplet evaporation characteristics of different fuels. It was found that there is a distinct phase transition between high volatility to low volatility phase in each fuel stored in steel drums verses fuel that is stored in plastic drums. The type of fuel contaminated by polymer additive has a high impact on fuel droplet burn rates. Polymer additives also have an impact on nucleate boiling, sub-droplets and soot particles. The burning rate constant, K, of selected pure aromatics, various fuel mixtures and Jet A with different cetane numbers have been evaluated. Research has shown that in the low volatility combustion phase a trend reduction of lowest boiling point pure aromatic burning rate to the highest boiling point pure aromatic burning rate is obvious. Irregular change in droplet diameter between the high volatility phase and low volatility phase during the combustion of aromatics blend was observed. This work has also evaluated the relationship between burning rates and cetane numbers.

Vapor-shielding effect and evaporation characteristics of multiple droplets

This study investigates the vapor shielding effect and evaporation characteristics of multiple droplets, considering the droplet-to-droplet distance, and suggests an evaporation rate model that considers vapor accumulation. Particle image velocimetry (PIV) is applied to visualize the internal flow of the evaporating droplets. The vapor distribution and local evaporation flux are analyzed through numerical simulations under quasi-steady-state assumptions. The results show that droplet adjacency delays the lifetime of the multiple droplets, compared to the single-droplet evaporation time, due to the vapor-shielding effect. Further, the water vapor of a single droplet reveals axisymmetric distributions, while the multiple droplets show vapor accumulation at the adjacent region. The evaporation flux is also dramatically suppressed in the adjacent region in the case of multiple droplets. Based on these findings, a new evaporation rate model is suggested in this research; the model considers the non-uniform distribution of vapor that rises due to the evaporation of adjacent droplets. This model can accurately estimate the evaporation rate for both center and side droplets.

Evaporation and Autoignition Characteristics of JP-10 Droplets with Hyperbranched Polyester as Additive

It was found in our previous work that hyperbranched polyester (HPE) can generate radicals and accelerate the chemical reactions of hydrocarbon fuels used as initiators. In this work, the evaporation and autoignition characteristics of JP-10 droplets with or without HPE were investigated using the high-speed backlight imaging technique in detail. The results indicate that the puffing and micro-explosion phenomena of HPE-blended JP-10 droplets can accelerate fuel evaporation and autoignition. When a 0.1% mass concentration of HPE was used, the droplet lifetime was reduced by 16.5% in evaporation at 850 K and 18.0% in autoignition at 900 K. A mechanism of HPE that promotes puffing and micro-explosions was proposed by analyzing droplet images of combustion and SEM images of combustion residues. Overall, this study provides a method for improving the evaporation and autoignition performance of JP-10.

A study of blending carbon nanoparticles made of coconut shell (fullerene C60) in vegetable oils on the droplet evaporation characteristics

The droplet evaporation characteristics of crude vegetable oils, including crude coconut oil (CCO), crude palm oil (CPO), crude jathropa oil (CJO), and crude sunflower oil (CSFO) mixed with and without nanocarbon (fullerene C60) were investigated. The nanocarbon was produced from the waste biomass-based coconut shell. The droplet evaporation experiments were performed in an atmospheric chamber using a K type thermocouple and equipped with a Nikon D3300 camera at 60 frames per second. Droplet temperatures were measured in a real-time basis at a sampling frequency of 1 kHz. The crude vegetable oils were mixed with fullerene at 5 mg/L. In this study, the droplet evaporation behaviors of fullerene addition were studied and compared to pure vegetable oils. The results reported that the bubble formation and the flame propagation were formed faster in the cases of crude vegetable oils blended with fullerene by as much as 44% and 22%, respectively. The trend of early bubble formation was CCO5 > CPO5 > CJO5 > CSFO5. However, the initial flame propagation profile was CCO5 > CJO5 > CSFO5 > CPO5. A lot of local hot spots were identified for the blended vegetable oils with fullerene. The micro-explosion mechanism of blended vegetable oils with fullerene was obtained based on the droplet visualization. At a normal temperature, fullerene molecule was well attached to the carbonyl chain. The electronegativity between fullerene and triglyceride influenced the attachment form of fullerene to carbonyl chain with respect to droplet temperature. Finally, the addition of fullerene increased in the fuel reactivity, the droplet temperature, and shortened the ignition delay.

Mixture formation characteristics and feasibility of methanol as an alternative fuel for gasoline in port fuel injection engines: Droplet evaporation and spray visualization

Under the background of energy saving and emission reduction, methanol is considered an ideal alternative fuel for gasoline because of its renewability, high combustion efficiency and low emissions, while an issue is the cold start of methanol port fuel injection engines. To comprehensively understand the underlying mechanisms and the feasibility of methanol as a substitute of gasoline, a suspension method was first used to investigate the evaporation characteristics of methanol and gasoline droplets at low temperatures. Then an optical intake port was constructed based on the outer contour modification strategy and an optical distortion correction algorithm was proposed. The effects of inlet temperature, inlet flow rate and injection pressure on spray characteristics of methanol and gasoline were comparatively investigated. In addition, the influence of valve temperature on the characteristics of spray impingement on a valve was studied. The results showed that the difference between the evaporation rates of methanol and gasoline was small at 60–90 °C. The difference became obvious when the temperature was below 60 °C or above 90 °C. Reducing the ambient humidity may reduce this difference and improve the cold start performance of methanol engines. A higher inlet temperature slightly increased the spray area. A higher inlet flow rate and injection pressure significantly increased the spray area, but also caused serious wet wall problem. From the point of view of spray in the port, methanol was a feasible substitute for gasoline in the presence of inlet flow and at low injection pressure of 6 bar. The valve temperature should exceed 200 and 230 °C for methanol and gasoline, respectively to form vapor films that prevent fuel adhesion onto the valve. However, methanol may not be a suitable alternative fuel for gasoline when spray directly impacted on the valve in port fuel injection engines due to its much smaller spray area.