Ethanol Droplets Research Articles

New Insight into the Effects of Gaseous CO2 on Spherically Symmetric Droplet Flames

This study investigated the effect of CO2 on the burning behavior and radiative properties of a single ethanol droplet flame in microgravity. Measurements of the droplet burning rate, the flame size and temperature, and the radiative emissions were performed, under microgravity conditions for ethanol droplets burning in N2 and CO2 environments, using the 1.5 s drop tower facilities at the Korea Maritime and Ocean University (KMOU). The non-monotonic sooting behaviors (caused by the elevated O2 concentrations) were found to have a significant influence on radiative heat losses in N2 environments, resulting in non-linear droplet burning behaviors with O2 concentrations. Due to the unique nature of CO2 in microgravity, which absorbs radiative energy from the flame and raises the temperatures of the surrounding gases, the CO2 environments suppressed the radiative heat losses from the flame, regardless of the non-monotonic sooting behavior observed at the higher O2 concentrations. These experimental findings highlight the complicated physics of CO2 gas radiation in microgravity, which has not been quantitatively explored.

Investigation of cellular instabilities and local extinction for two-phase flames under microgravity conditions

The initial mechanism of cellular instabilities on the flame surface was investigated experimentally. Firstly, mono-dispersed ethanol droplets were created via the Wilson cloud principle in a spherical combustion chamber, and aerosol ethanol/air flame propagation was tracked under microgravity conditions. Schlieren images revealed that at the beginning of the cellular structure formation, dark spots formed linking to the signal loss in a density gradient. This phenomenon is assumed to be local extinction and quenching on the flame front due to either liquid droplets’ presence or evaporation. To further investigate the observed phenomenon, stagnation flame experiments were performed for rich propane/air gaseous flames with isolated ethanol droplet injection. The evidence for the local extinction on the flame surface due to droplet passage was found via the Chemiluminescence method. The intrinsic instabilities were found to be triggered by the droplets in cases where the flame cannot recover its initial state.

Measurement of impinging and boiling droplet temperature by two-color/two-dye laser-induced fluorescence technique using continuous-wave laser

The 2-color/2-dye laser-induced fluorescence technique is a non-intrusive method for measuring temperature in millimeter-scale fluids. This study determines optimal conditions for fluorescein disodium (FL) and Sulforhodamine 640 (SRh640) with a low-cost continuous-wave laser to reduce dye saturation and morphology-dependent resonances. The signal images of both dyes, captured by two high-speed cameras, were clearly distinguishable from the background with high contrast under specific concentration conditions. As a result, three concentration combinations were selected based on preliminary experiments, and the fluorescence signal ratio was analyzed to establish a temperature relationship between 32 and 71 °C. The intensities of FL and SRh640 increases and decrease with the temperature, respectively. Errors resulting from laser light attenuation in single-signal images were reduced using the ratiometric method. Solution 1 (CFL = 1.55 × 10−4 / CSRh640 = 3.2 × 10−6 M) exhibited the highest sensitivity at 0.718 (%/ °C) and a strong linear relationship (r2 = 0.997), so it was determined to be suitable concentration for measuring the droplet temperature. The proposed method estimated the temporal temperature variations of ethanol droplets impinging on a heated surface at TW = 80, 150 and 200 °C. At TW = 80 °C, contact boiling initiated without a recoil process, and the heat transfer elevated the droplet's temperature near the surface, leading to bubble formation and growth. As surface temperature increases, the film boiling behavior can be observed. The temporal evolution of the average heat transfer rate was analyzed.

Electric field-influencing combustion behaviors of droplets of ethanol, biodiesel and diesel

The droplet combustion in an electric field is fundamental for the electrospray combustion. In this paper, experimental research was done to investigate the effect of the vertical electric field on the combustion behaviors (including the flame and droplet characteristics) of droplets of ethanol, biodiesel and diesel. The outcomes demonstrated that the flame morphology was controlled by the competition between the electric field force and natural buoyancy. As the electric field strength (E) increased, the upward flame (0 ≤ E < 100 kV/m), quasi-spherical flame (E ≈ 100 kV/m) and downward flame (100 < E ≤ 200 kV/m) successively emerged. This expressed as a decrease in upper flame heights and an increase in bottom flame heights. The fuel order of the flame height variation extent was ethanol < biodiesel < diesel. An increase before 100 kV/m and a decrease after 100 kV/m for the flame width existed, whose value variations were similar in all droplets. These differences in flame size had a direct impact on the magnitude of flame fronts, which in turn determined the area where the fuel vapor and oxygen reacted. Therefore, the combustion duration of the biodiesel and diesel droplets increased before 100 kV/m and decreased after 100 kV/m. Due to the downward movement of the electric arc, the combustion duration of the ethanol droplet increased with the electric field strength. The maximum increases in combustion durations of the ethanol, biodiesel and diesel droplets in the electric field could achieve 14.1%, 18.1% and 21.9%, respectively. This work supplies a further understanding of the droplet combustion in electric fields and the electrospray combustion.

Assessment of Bark Properties from Various Tree Species in Terms of Its Hydrophobicity and Energy Suitability

Wood bark is a by-product of the forestry industry with significant quantitative potential. Due to the higher heating value and hydrophobic character (dictated by the content of hydrophobic extracts in the chemical composition of the bark) facilitating long-term storage and transport, bark is considered a valuable material to be integrated into the energy industry as a substrate for combustion and co-combustion. However, the heterogeneous structure of the biomass causes significant differences in these parameters between different types of bark. For this reason, this study aimed to analyze the energy usefulness and hydrophobicity of 14 species of bark occurring in large amounts across Europe. Bark’s higher heating value has been shown to range from 17.239 MJ·kg−1 ± 0.318 MJ·kg−1 to 21.618 MJ·kg−1 ± 0.375 MJ·kg−1, and most species are extremely hydrophobic in the WDPT (water drop penetration time; 70–10,495 s) and MED (molarity ethanol droplet; 8.5–24%) tests. Nevertheless, the statistically significant differences between the species in the hydrophobicity tests indicate that the storage of individual types of bark should be adapted to the species and preceded by an analysis of its hydrophobicity. It was also shown that the WDPT test seems to be more suitable for the analysis of highly hydrophobic biomasses, due to the possibility of better differentiation of the level of hydrophobicity between species showing the same degree of hydrophobicity. In addition, the best type of bark that could be implemented in the power industry from the point of view of hydrophobicity and combustion characteristics was Silver Birch bark.

Impact and wetting of polysilazane droplets on a metal surface

Spray coating with liquid polymers is a crucial step in producing polymer-derived ceramic (PDC) coatings before curing the precursor. However, the interactions between polymer droplets and target surfaces during the spray coating process have not been adequately characterized in existing literature. This experimental study aims to comprehensively examine the previously unknown droplet impact dynamics for a specific ceramic precursor, Polysilazane, on Inconel surfaces. The diameter of Polysilazane droplets was kept constant, while their impact velocity was varied to achieve different impact conditions. The same parameter range was also explored with water, water-glycerol, and ethanol droplets for comparison purposes. High-speed videos were captured to observe the instantaneous impact process, while low-speed videos were recorded at specific instances to document the gradual wetting process at later stages. The investigation revealed unique spreading dynamics of Polysilazane droplets, including smooth contact angle variation during the transition from impact to lateral spreading, absence of delayed wetting, no formation of wavy structures on the droplet interface during spreading, and no retraction process within the studied conditions (Weber number up to 1200). Conversely, Polysilazane displayed similarities in the normalized spreading curves with other liquids. Additionally, the maximum spread factor of Polysilazane could be well predicted using existing universal scaling models designed for different liquid types. Lastly, Polysilazane exhibited a distinctive post-impact wetting process that persisted for an extended duration. The wetting dynamics of Polysilazane could be effectively described by an empirical exponential law originally developed for partial wetting systems.

Investigation of multi-stage evaporation and wave multiplicity of two-phase rotating detonation waves fueled by ethanol

In this study, a numerical investigation based on the Eulerian-Lagrangian model is conducted to explore a rotating detonation engine (RDE) fueled by liquid ethanol. The focus is on examining the characteristic phenomena of the two-phase rotating detonation wave (RDW) caused by droplet evaporation and varying inlet conditions. To enhance the evaporation of liquid fuel, pre-heated air is used, and both liquid and pre-vaporized ethanol are simultaneously injected. The distribution of ethanol droplets reveals an initial concentration near the injection surface and accumulation in the fuel-refill zone. Here, liquid droplets gradually evaporate after absorbing latent heat from the surrounding gas. The subsequent interactions between the evaporating droplets and the RDW vary with the droplet size. For droplets with diameters of d0 = 5–15 μm, after being swept by the RDW, a secondary evaporation process occurs, leading to an enlargement of the width of the reaction zone. However, the chemical reactions still predominantly take place in close proximity to the detonation front. As d0 further increases, droplet evaporation persists in the post-detonation expansion zone over a long distance until the remaining droplets are fully evaporated and eventually burned in this region. The study also analyzes the extinction of rotating detonations and the emergence of new detonation waves resulting from local explosions and consequent shock collisions. It is demonstrated that variations in the diameter of injected droplets and inlet temperature can lead to different operating modes with varying numbers of RDWs.

Evaporative and non-evaporative droplet impact on a heated phase change material pool: A comparison between ethanol, acetone, and distilled water

The interaction between liquid droplet and phase change material (PCM) underlies the direct contact approach, which is an efficient way for precipitating the melting and solidification processes of PCMs. In this experimental research, to comprehend the direct contact method more profoundly, the impingement of a single droplet on the heated paraffin pool, having a temperature of 70–95 °C, is scrutinized systematically for the first time. Droplet impact solidifies a portion of the pool and may lead to droplet evaporation. Acetone, ethanol, and distilled water are exploited as droplet liquids, whose diameter and impact velocity range from 2.32 to 3.36 mm and from 1.70 to 2.28 m/s, respectively. Through impact visualizations, six distinct regimes are observed, whose regime maps are constructed. Moreover, the effects of fluid properties, pool temperature, and the Weber number on the evolution of crater and jet, crater depth, crater shape, and solidified PCM area (A) generated after the impingement are investigated. It is ascertained that the low PCM temperature instigates the coalescence phenomenon regardless of the Weber number. Additionally, the crater depth is a function of the PCM temperature, Weber number, and Ohnesorge number; thus, a dimensionless correlation is proposed for the maximum crater depth. Furthermore, droplet evaporation augments the extracted heat from the paraffin, provoking a more sizeable solidified area. Interestingly, though absorbing the greatest amount of heat from the PCM, ethanol droplet generates a smaller solidified area than acetone since acetone favorably spreads on the pool surface, enhancing the effective heat exchange area.

Droplet thermometry based on an optimized two dye two-color laser-induced fluorescence concept

In this work two-color laser-induced fluorescence (2c-LIF) is applied to calibrate and measure the temperature in a micrometric monodisperse ethanol droplet chain. A dye mixture of fluorescein disodium (FL) and sulforhodamine 101 (SRh) is used, in order to form a sensitive signal ratio of one temperature dependent and one independent fluorophore. Spectral and planar fluorescence detection via a fiber-coupled spectrometer and a camera system, consisting of two sCMOS cameras, is set up around the droplet chain. Additionally, absorption measurements of the dye mixture in ethanol are conducted using a photo-spectrometer to analyze the temperature sensitivity of the dyes as well as potential re-absorption and fluorescence cross-talk effects. The spectral setup allowed an investigation of the wavelength range in which morphology dependent resonances (MDR) occur at the phase boundary of the droplet. Thus, the optical filters, determining the color channels in the camera system, are chosen to avoid detection of the lasing signal and providing maximal temperature sensitivity at the same time. The calibrated signal ratios are applied in temperature measurements of evaporating heated droplets in the droplet chain, showing the cooling with larger distance from the nozzle.

Effects of solution concentration and surface supercooling on the spreading characteristics of an impact binary droplet

We investigate the effect of liquid properties and surface supercooling on the spreading dynamics of an impact binary ethylene-glycol and ethanol droplet. Surface supercooling and solution concentration, affecting the physical properties of a droplet, jointly determine the maximum spreading factor. At a high concentration of ethylene-glycol, the maximum spreading factor decreases with surface supercooling. At a low concentration of ethylene-glycol, the maximum spreading factor decreases and then increases with surface supercooling. The relation between the maximum spreading factor and surface supercooling is resulting from the opposite surface-supercooling-dependencies of the maximum internal spreading factor and the dimensionless fingering length. The maximum spreading factor decreases with the ethylene-glycol concentration due to increased liquid surface tension and viscosity. A new model of the maximum spreading factor, taking into account the effect of surface cooling in viscous dissipation, is proposed based on energy conservation, which optimizes the prediction of spreading of an impact droplet on a surface with different supercooling degrees.

Multi-objective optimization of multiple droplet impacts on a molten PCM using NSGA-II optimizer and artificial neural network

Embracing an interaction between the phase change material (PCM) and the droplets of a heat transfer fluid, the direct contact (DC) method suggests a cutting-edge solution for expediting the phase change rates of PCMs in thermal energy storage (TES) units. In the direct contact TES configuration, when impacting the molten PCM pool, droplets evaporate, provoking the formation of a solidified PCM area (A). Then, they reduce the created solid temperature, leading to a minimum temperature value (Tmin). As a novelty, this research intends to maximize A and minimize Tmin since augmenting A expedites the discharge rate, and by lowering Tmin, the generated solid is preserved longer, resulting in a higher storage efficacy. To take the influences of interaction between droplets into account, the simultaneous impingement of two ethanol droplets on a molten paraffin wax is surveyed. Impact parameters (Weber number, impact spacing, and the pool temperature) govern the objective functions (A and Tmin). Initially, through high-speed and IR thermal imaging, the experimental values of objective functions are achieved for a wide range of impact parameters. Afterward, exploiting an artificial neural network (ANN), two models are fitted to A and Tmin, respectively. Subsequently, the models are provided for the NSGA-II algorithm to implement multi-objective optimization (MOO). Eventually, utilizing two different final decision-making (FDM) approaches (LINMAP and TOPSIS), optimized impact parameters are attained from the Pareto front. Regarding the results, the optimum amount of Weber number, impact spacing, and pool temperature accomplished by LINMAP and TOPSIS procedures are 309.44, 2.84 mm, 66.89 °C, and 294.98, 2.78 mm, 66.89 °C, respectively. This is the first investigation delving into the optimization of multiple droplet impacts for TES applications.

Modulation of forced isotropic turbulence by an anchored droplet with near-Kolmogorov diameter and varying volatility

Utilizing a fan-stirred chamber and two-dimensional particle image velocimetry, we analyse the modification of homogeneous and isotropic turbulence ( $50 \leq Re_\lambda \leq 140$ , with supplementary data out to $Re_\lambda = 310$ , where $Re_\lambda$ is the longitudinal Taylor Reynolds number) induced by both a non-volatile (water) and a volatile (ethanol) isolated and anchored droplet in the range $(0.3 \leq d/\eta \leq 5.1)$ , where $d/\eta$ is the ratio of droplet diameter to the Kolmogorov length scale. The dissipation rate, $\varepsilon$ , is calculated via the corrected spatial gradient method, and the resultant fields of both turbulent kinetic energy, $k$ , and $\varepsilon$ are presented as spatial heat maps and as shell averages, ${\overline {k_{\Delta r}}}$ and ${\overline {\varepsilon _{\Delta r}}}$ , vs the radial coordinate normalized by the droplet radius, $r/R$ . The dissipation rate near the water droplet surface may exceed the corresponding unladen flow value by a factor of twenty or more. The normalized radius of recovery, $r^*$ , which designates the radial location where ${\overline {k_{\Delta r}}}$ or ${\overline {\varepsilon _{\Delta r}}}$ has returned to within 10 % of the unladen value, is reasonably expressed as $r^* \propto (d/\lambda )^{-C_2}$ in either case, where $\lambda$ is the longitudinal Taylor microscale and $C_2$ is a positive empirical fitting parameter. Recovery of ${\overline {k_{\Delta r}}}$ and ${\overline {\varepsilon _{\Delta r}}}$ may take up to 14 normalized radii when $d/\lambda$ is small. Trend line extrapolation suggests that the attenuation region becomes negligible as $d/\lambda \to 1$ . Ethanol, which evaporates up to five times faster than water, induces a much smaller dissipation spike near the surface. The mass ejection phenomenon appears to reduce the strong near-surface damping of the radial root-mean-square component. However, the radius of recovery trend for fields surrounding a volatile ethanol droplet falls directly in line with the non-volatile water droplet data for both $k$ and $\varepsilon$ , indicating that droplet vaporization has little effect on the far-field return to isotropy.

A Study on the Dynamic Collision Behaviors of a Hydrous Ethanol Droplet on a Heated Surface

This study uses high-speed imaging to investigate the dynamic collision behavior of a single hydrous ethanol droplet in different water/ethanol ratios on a heated horizontal glass surface. The initial droplet diameter varied from 3.3 to 4.1 mm, and the impact velocity was 0.57 m/s. The study covers a range of surface temperatures (373 K to 553 K) and ethanol mass fractions (0% to 100%) to reveal four regimes of droplet-impinging behaviors, including quiescent surface evaporation, puffing or partial boiling, explosive nuclear boiling, and the Leidenfrost effect. The addition of volatile ethanol to less volatile water shifts the droplet collision behavior toward explosive boiling and the Leidenfrost phenomenon. As the ethanol mass fraction increased from 0% to 100%, the superheat limit temperature decreased by approximately 80 K, while the Leidenfrost temperature decreased by at least 100 K. The dimensionless droplet diameter in the regime of droplet spreading with quiescent surface evaporation is influenced by surface temperature, surface tension, and viscosity. Meanwhile, the dimensionless diameter and height of a droplet in the regime of the Leidenfrost phenomenon are mainly influenced by its surface tension. The study concludes that a single parameter, such as the superheat level, Weber number, or Reynolds number, is difficult to describe droplet collision behavior, and multiple factors would be required to best describe droplet collision behavior and establish empirical correlations. However, it is feasible to predict partial collision behaviors by using one of the single parameters under certain conditions.

Evaporation of vertical and pendant ethanol droplets and internal Marangoni convections

Understanding droplet evaporation has broad implications for science and industry. Over the past decades, theoretical models concerning this topic have been extensively studied for better prediction of this process. Although the existing models have shown reliable accuracy both in qualitative and quantitative analysis, most focus on specific physical scenarios and therefore consider only part of the transport mechanisms in the process, e.g., a diffusion-based model. In this study, we numerically investigated the evaporation of vertical and pendant ethanol droplets in an open space and developed a novel comprehensive model for simulation. The model incorporated vapor diffusion, thermal conduction, evaporative cooling, natural convection, and Marangoni stress, showing excellent accuracy in three-dimensional scenes. A sessile droplet evaporating on top of a horizontal substrate was compared. Our analysis started with the temperature distribution surrounding and inside the droplet, from which a complex multi-mechanism coupling behind the concerned issues was implied. We then studied the vapor diffusion and flow features in the gas domain and revealed geometric restriction as an important influencing factor. Next, we focused on the internal flows, demonstrating the dominance of Marangoni convections within the droplet and the characteristics of flow behaviors. The local evaporation flux was identified. Finally, we compared lifetimes in different cases and discussed the effects of droplet volume during evaporation. Overall, a vertical droplet had higher evaporation intensity and internal convection, while pendant and sessile droplets were similar. Our study hopes to provide a reference for ethanol droplet evaporation and possible applications based on such a phenomenon.

Methanol, ethanol and propanol droplets evaporation characteristics with a dilute concentrations of hematite and magnetite nanoparticles

Evaporation characteristics of a single suspended fuel droplet with nanoparticles were experimentally investigated. Three ambient temperatures of 100, 200 and 300 °C were considered. The droplet initial volume of 1.5 µL was used and the whole evaporation process was recorded using a high-speed camera at 500 frame per second. Methanol, ethanol and propanol were selected as the base fuels. Hematite and magnetite nanoparticles were added to the alcohols at concentrations of 100, 200, 300 mg/L to examine their effects on the droplet evaporation. The alcohol/nano additive droplet experienced different evaporation characteristics compared to this of pure alcohol. The variation of normalized square diameter was linear during the evaporation process for the considered blends. Methanol droplet experienced the same evaporation stages with hematite and magnetite along with 10% and 8.3% increase in the evaporation rate respectively at 300 °C and a nanoparticles concentration of 300 mg/L. Ethanol droplet containing hematite and magnetite nanoparticles evaporation rate was retarded by 15.9% and 12.6%, respectively at 300 mg/L and 300 °C. Propanol droplet evaporation was increased by 14.9% with hematite additives. On the other hand, a decrease in the constant by 26% was observed at 300 °C and a concentration of 300 mg/L.

Alterations in aggregate characteristics of thermally heated water-repellent soil aggregates under laboratory conditions

Abstract The heat generated during wildfires modifies soil characteristics, including soil water repellency (SWR) and the water stability of aggregates, which are known to be interrelated. SWR lowers the rate of water entry into aggregates, minimizing aggregate disruption and subsequent erosion. This study aimed to examine these aggregate characteristics (SWR, water stability of aggregates) of thermally heated water-repellent soil aggregates under laboratory conditions. Water-repellent aggregates were collected from Eucalyptus grandis forest soil separately from four soil depths (0–5, 5–10, 10–15, and 15–20 cm) with varying initial repellency levels. Using an automated programmable muffle furnace, aggregates were separately exposed to three heating temperatures, T H (150, 200, 250 °C), three rates of heating (speed of rising temperature to reach relevant T H), R H (200, 400, 800 °C h−1), and three durations of exposure to relevant T H, E D (30, 60, 120 min). The molarity of an ethanol droplet test was used to measure the contact angle (contact angle&gt;90°). The water drop penetration time (WDPT) was also measured. The SWR of aggregates declined with the increasing T H and E D. All aggregates were wettable once exposed to 250 °C. At the lowest T H and E D (150 °C, 30 min), the contact angle was &lt;90° only in the least repellent aggregates collected from 10–15 and 15–20 cm depths. Although R H indicated the least influence on the measured parameters, the slowest R H (200 °C h−1) caused a comparatively greater decline in SWR. Water stability of aggregates increased with heating irrespective of decreasing SWR. Further investigations on heat-induced changes in organic compounds at molecular levels would be necessary to understand the theories for the behavior of aggregates.

Extended model of bouncing boundary for droplet collisions considering numerous different liquids

Sprays are widely utilised in different technical and industrial applications. The produced droplet size spectrum is one of the most important parameters that determine the spray characteristics and performance. Depending on the operational conditions and the spray structure, the droplet size spectrum obtained after atomisation will be of course modified by collisions between droplets. In numerical calculations, the Euler/Lagrangian approach is a favourable method for obtaining relatively accurate predictions of spraying processes. However, for correctly modelling droplet collisions, reliable boundary line correlations are needed for distinguishing the different droplet collision outcomes, which are bouncing, coalescence, and separations. These boundary lines are summarised in so-called collision maps to demark the various outcome scenarios. The result of a binary droplet collision depends on numerous parameters, namely, the kinetic properties (droplet velocities, droplet diameter ratio, impact angle) and the physical properties of gas and droplets (droplet liquid, density and viscosity, surface tension, type of gas phase, gas phase pressure and temperature). The boundary line for bouncing, primarily used in numerical studies so far, was derived based on experiments with ethanol droplets by Estrade et al. (1999). However, this theoretical derivation, based on an energy balance, unfortunately neglects viscous dissipation effects. Therefore, the deviations of the predicted bouncing region are pretty large for liquids with higher viscosity or other physical properties. Hence, a data-driven model is proposed in this paper, with new assumptions and definitions, considering the dissipated energy during the collision and the shape changes (i.e., deformation) of the collision complex. For that purpose, two new parameters are introduced in the model, namely a shape factor and an energy conversion rate, both depending on the impact parameter B, which describes the lateral displacement of droplets at a collision. Therefore, the new bouncing model is related to the degree of droplet deformation. The dependence on B could be described by linear correlations with two parameters for slope and intercept. These parameters are correlated with the Ohnesorge number, and polynomial fitting functions were derived with the help of numerous available experiments for different liquids. The upper limit of Ohnesorge numbers in the present study was Oh<0.35. Due to the dependence on the Ohnesorge number, the proposed model very well captures the effect of liquid properties on the bouncing boundary, however, not necessarily the influences of environmental conditions and interface modifications. The model was validated by comparison with numerous available experimental results.

Variable-Capacitance Sensor to Measure the Evaporation Rate and Lifetime of Microliter Droplets

This article presents a variable-capacitance sensor for sessile droplet evaporation rate and lifetime measurements. The sensor consists of a plane-parallel-electrode capacitor with a commercial glass substrate inserted within. The glass substrate has a concave cavity at its center to deposit a liquid droplet for its measurement. An air gap is left between the substrate and the upper electrode to allow for the droplet’s evaporation. The main advantage of this configuration is that it fixes the position and geometric shape of the droplet. An analytical model based on the local-height approximation is developed to calculate the capacitance of the device, with and without a liquid droplet. Its validity is initially tested comparing with finite element calculations. Then, using the analytical model we simulate the capacitance signal during the evaporation of a sessile droplet and compare it with experimental results finding very good correspondence. The differential capacitance signal follows closely the volumetric changes of the droplet. The evaporation kinetics and lifetime of 3.5 ± 0.1 microliter droplets of ethanol and methanol mixtures are presented. We also show that the evaporation kinetics and lifetime of a droplet can be used to reveal the presence of a surfactant at very low concentrations. The results presented demonstrate the ability of the sensor to monitor the evaporating droplet from start to end.

High spatio-temporal resolution measurement of boiling heat transfer of a falling droplet

In this study, we conducted experiments to establish a technique to simultaneously measure the boiling behavior and corresponding heat transfer fluctuations through a heat transfer surface with high spatio-temporal resolution. In this study, an ethanol droplet was dropped onto a heat transfer surface, and the boiling behavior was photographed by two high-speed cameras, one diagonally above and the other through the heat transfer surface. Simultaneously, heat transfer fluctuations were measured using an infrared camera. Consequently, it was possible to quantitatively measure the fluctuations in the heat flux distribution corresponding to the processes of formation, growth, and collapse of the boiling bubble. In addition, the limit of spatio-temporal resolution that can be measured using this method was examined from the aspect of the thermophysical properties of the heat transfer surface.

Simultaneous double droplet impact on a molten phase change material pool: An experimental investigation

Although momentous in numerous authentic applications, multiple droplet impacts on the liquid pool are overlooked in most studies. Hence, the impingement of simultaneous double ethanol droplets on the molten phase change material (PCM) pool, instigating the evaporation of droplets and the solidification of PCM, is comprehensively scrutinized for the first time. Experiments were carried out through high-speed imaging for various Weber numbers ranging from 179 to 464, pool temperatures from 70 to 95 °C, and several horizontal impact spacing. By altering impact parameters, five distinct regimes were observed in the case of double droplets. Based on observations, regime maps were also established for conducted experiments. Furthermore, the influences of the impact parameters on the central uprising sheet, formed between two impinging droplets, crater width, crater depth, jet height, and solidified PCM area, created after the impact was investigated. Additionally, the time evolution of crater width and depth was compared with the existing theoretical predictions. It was ascertained that boosting either the Weber number or the impact spacing intensifies the droplets' spreading areas, leading to a larger solidified PCM area, whereas temperature inversely affects this parameter. Moreover, increasing the impact spacing diminishes the central uprising sheet height and retards its formation, while raising the Weber number monotonously strengthens the central uprising sheet. The research, whose results have an immediate application in novel energy storage devices containing droplet–PCM interaction, fills the gap between droplet impact and energy storage.