Splashing correlation for single droplets impacting liquid films under non-isothermal conditions

The phenomena of droplet impact can be encountered in modern applications, including heat exchangers, electronic cooling devices, and internal combustion engines. The underlying mechanisms of two-phase flows, coupled with heat and mass transfer processes, such as evaporation, condensation, and boiling, increase the complexity of physical systems. This requires a thorough investigation of interfacial dynamics associated with temperature gradients and mass transfer phenomena. However, existing studies in the literature mainly focus on droplets impacting liquid films under ambient conditions, neglecting thermal effects. Therefore, the main objective of this work is to evaluate the influence of the liquid film temperature on the droplet impact outcomes, namely splashing and crown dynamics. An experimental facility was designed and adapted to account for both ambient and non-isothermal conditions. The droplet is released from a hypodermic needle until impacting onto the liquid film. The impact surface is positioned above an aluminum block, which acts as a heat source, heating the liquid film by conduction. Water, n-decane, and n-heptane are the fluids considered for the experiments. Study cases include varying the droplet impact velocity and liquid film temperature, encompassing both spreading and splashing regimes for isothermal and non-isothermal conditions. Experimental results show that an increase in the liquid film temperature induces the transition from spreading to splashing regimes across all fluids. Qualitatively, this translates from smooth to irregular crowns due to the formation of cusps in the crown rim. These structures are the main source of secondary atomization, which is promoted by increasing the liquid film temperature. Transitional regimes may display several irregularities, in which splashing is visualized at non-isothermal conditions, followed by the reduction/suppression of the splashing occurrence for higher liquid film temperatures. These require further attention in order to fully comprehend fluid and heat flow, such as Marangoni stresses and local evaporation.

The agitated thin film evaporator (ATFE), which is known for its high efficiency, force the material to form a film through the scraping process of a scraper, followed by evaporation and purification. The complex shape of the liquid film inside the evaporator can significantly affect its evaporation capability. This work explores how change in shape of the liquid films affect the evaporation of the materials with non-Newtonian characteristics, achieved by changing the structure of the scraper. Examining the distribution of circumferential temperature, viscosity, and mass transfer of the flat liquid film shows that the film evaporates rapidly in shear-thinning region. Various wavy liquid films are developed by using shear-thinning theory, emphasizing the flow condition in the thinning area and the factors contributing to the exceptional evaporation capability. Further exploration is conducted on the spread patterns of the wavy liquid film and flat liquid film on the evaporation wall throughout the process. It is noted that breaking the wavy liquid film on the evaporating wall during evaporation is challenging due to its film-forming condition. For which the fundamental causes are demonstrated by acquiring the data regarding the flow rate and temperature of the liquid film. The definitive findings of the analysis reveal a significant improvement in the evaporation capability of the wavy liquid film. This enhancement is attributed to increasing the shear-thinning areas and maintaining the overall shape of the film throughout the entire evaporation process.

The wall combustion is one of the soot and unburned hydrocarbon formation sources in the engine cylinders, which is affected by both spray and wall parameters. The impingement dynamics of the droplets on liquid films have been widely studied. However, there is less focus on the droplet impingements on hot liquid film, which is more representative as a pre-wall combustion condition. This work investigates the impingement dynamics of ethanol on heated glycerol liquid film. The Weber number (10 ≤ We ≤ 275) and liquid film temperature (70°C ≤ T ≤ 175°C) are two main parameters that lead to a comprehensive understanding of impinging phenomena. In the experiment, a high-speed camera was used to visualize the droplets impingement behaviors, which could be classified to six categories: deposition-spreading, rebounding-sputtering, rebounding-floating, stable crown-spreading, stable crown-sputtering, and splash crown-sputtering. The critical temperature for sputtering is about 125°C, independent of the We. The dynamic phenomena were quantified by the diameter and height of the crown, sputtering time and droplet size distribution. The dimensionless diameter, dimensionless height, and maintenance time of the crown all increase with the increasing We or temperature. When the We is greater than 172, the dimensionless diameter increases less. The relationship between the maximum dimensionless height of the crown and the We is H*max = 0.0026 We. The change of the crown diameter with time is independent from the temperature. Additionally, the sputtering time decreases with the increasing We and temperature. For the diameter distribution of sputtering droplets, the fractions of the larger sputtering droplets increased at low We, while the smaller droplets increased their contributions at high We. With the increase of temperature, the proportion of small diameter sputtering droplets increases.

Study of the coalescence/splash threshold of droplet impact on liquid films and its relevance in assessing airborne particle release

Development of a two-line DLAS sensor for liquid film measurement.

Simultaneous measurement of liquid film thickness and temperature on metal surface

The droplet impact phenomena onto liquid films are a field extensively researched for over a century, which are driven by many practical applications such as heat exchangers, internal combustion engines and spray cooling. Despite the extensive work on wetted surfaces, the influence of temperature on droplet outcome, local evaporation/boiling effects, and liquid film stability has been overlooked in the literature. Therefore, the main objective of this work is to evaluate the influence of the liquid film temperature on the crater and jet dynamics. The experimental setup was designed for this purpose, in which a borosilicate glass surface that contains the liquid film is placed over an aluminium block with embedded cartridge heaters, heating it by conduction. Water, n-decane and n-heptane are the fluids adopted for the experiments due to their differences in thermophysical properties and saturation temperature. Different conditions are considered, which include two dimensionless thicknesses, h∗=1.0 and h∗=1.5, and a range of dimensionless temperatures, θ=0, θ=0.2, θ=0.4 and θ=0.6. Qualitative and quantitative analyses are performed regarding crater evolution, and central jet height and breakup measurements, respectively. Evaporation rate measurements are required due to the influence on the liquid film thickness variation. Qualitative results show that temperature differences promote the formation of recirculation zones near the impact surface and the crater boundaries, as well as the influence on the crater shape and curvature. In terms of the quantitative analysis, the central jet height measurements for the n-heptane and n-decane reveal that higher values of the dimensionless temperature lead to an increase in the jet height, as well as promoting and increasing the occurrence and number of secondary droplets, respectively. Water follows a similar trend with the exception of θ=0.2, which can be explained by a time scale analysis.

The impact of droplets on liquid films is ubiquitous in natural and industrial processes, and surfactants can significantly alter the impact process by changing the local surface tension. Here we study the impact of droplets on liquid films in the presence of surfactant using high-speed photography, and reveal the flow pattern by dye-tracing. The effects of the droplet size and speed, and the initial film thickness on the impact process are elucidated. The results show that the flow is significantly affected by adding surfactant to the droplet, the liquid film, or to both phases. In particular, the film dye patterns form concentric circles and flower-shaped structures at low and high droplet Weber numbers, respectively. We also show how surfactant-induced Marangoni stresses modify these flow patterns, and alter the characteristics of the phenomena associated with the impact process, such as the propagation of capillary waves, the evolution of the crown, and the formation of secondary droplets. During the impact of surfactant droplets on thin water films, the Marangoni stresses can be sufficiently strong so as to drive film dewetting.

Experimental and numerical study on the falling film flow process on the outer wall of dome cylinder

Evaporation and boiling are processes that occur in many industrial applications involving multiphase flows. For liquid films, however, studies are scarce regarding heat and mass transfer mechanisms and require further research. The main objective of this work is to evaluate bubble formation and detachment, followed by the impact phenomena. Therefore, an experimental setup was built and adapted for this purpose. A borosilicate glass impact surface is placed over a heat source, which consists of an aluminum block with four embedded cartridge heaters that heat the liquid film by conduction. Water and n-heptane are the fluids adopted for the experimental study, as the differences in thermophysical properties allow for a wider range of experiments. Study cases include dimensionless temperatures of θ > 0.6 for similar impact conditions. In terms of bubble formation, n-heptane displays smaller bubble diameters and higher release rates, whereas water exhibits larger bubbles and lower rates. Qualitatively, liquid film temperatures close to the saturation temperature do not reveal a direct influence on the crown development and posterior secondary atomization. For later stages of the impact, the central jet height and breakup are influenced by the film temperature, which is associated with the variation of thermophysical properties.

The process of the droplet impact onto the liquid film on the rough solid surface, as one of the basic multiphase problems, is very important in many fields of science and engineering. On the other hand, the problem is also very complicated since there are many parameters that may influence the process of the droplet impact on the rough solid surface with a liquid film. Up to now, there are still little research on this problem, and to gain a better understanding on the physical mechanics of the droplet impact onto the film on the rough solid surface, it is desirable to conduct a detailed study. To clearly understand the physical phenomena appearing in the process of droplet impact on the liquid film, a parametric study on this problem is also carried out based on a recently developed lattice Boltzmann method in which a MRT lattice Boltzmann model is used to solve the Navier-Stokes equations, and the other is adopted to solve the Cahn-Hilliard equation that is used to depict the interface between different phases. In this paper, the effects of the relative thickness of film ( h ), the relative width of cavity ( d *) and the relative depth of cavity ( L *) on the dynamic behavior of interface are investigated in detail, and the velocity and pressure fields are also presented. In order to reduce the influence of lattice, we fix the lattice to be 600×120 for gas, which is fine enough to give accurate results. In addition, in our simulations, We =500, Re =480, viscosity ratio and density ratio are set to be 2:1. The numerical results first show that, the phenomena of crown and entrainment can be observed obviously during the process of droplet impact onto the liquid film on the rough interface when We and Re are large. The radius of spray ( r ), which is formed by the droplet impact onto liquid film, is related to time through the relation r / 2 R ≈ α U t / 2 R when h is small, which is coincident with the result of droplet impact onto the liquid film on smooth surface, and additionally the coefficient α would decrease with the increase of h . However, this relation seems not accurate for the case with a large h , and simultaneously, the splashing phenomenon has not been observed. Secondly, the relative width of cavity d * plays an important role on the phenomena of splashing. When d *=1, there will be two small droplets through the splashing phenomenon (left half part), then with this parameter increase, the number of small droplet and the point where the splashing occur will also change, and there also are much difference in relation of spray radius and time. Actually, if d * is small, the coefficient α would first decrease and then increase with the increase of d *, while if d *>8, the cavity width would only have a little influence on the behavior of spray. Finally, it is also found that the pressure change near the cavity bottom is small at different L *, that is to say, the relative depth of cavity L * seems to has no apparent effect on the formation of spray, but it brings a great influence on the splashing of spray and the movement of the droplet which is produced in the process of splashing.

: In this paper, a fixed three-dimensional model was applied to simulate liquid film spreading process and heat transfer process. The so-called volume of fluid (VOF) method was used to deal with the interface between the liquid and gas phase by using Fluent 14.5. It was found that two adjacent columns both spread along the axial direction and collide with each other at the midpoint between two columns and a crest is formed. The composition and heat is mixed in the liquid film fully at the crest, which broke the velocity and temperature boundary layers, promoting the heat transfer between liquid film and tube wall. Besides, the temperature of liquid film is increased with the circumferential direction and reaching the maximum temperature at the bottom of the tube.

Droplet impact on the solid surfaces is widespread in nature, daily life, and industrial applications. The spreading characteristics and temperature evolution in the inertial spreading regime are critical for the heat and mass transfer process on the solid-liquid interface. This work investigated the spreading characteristics and temperature distribution of the thin liquid film in the inertial rapid spreading regime of droplet impact on the heated superhydrophilic surfaces. Driven by the inertial and capillary force, the droplet rapidly spreads on the superhydrophilic surface, resulting in a high temperature center in the impact center surrounded by a the low-temperature ring. The formation of the unique the low-temperature ring on the heated superhydrophilic surface is due to the much smaller time scale of rapid spreading than that of heat transfer from the hot solid surface to the liquid film surface. CFD numerical simulation shows that the impacting droplet spreads and congests in the front of liquid film, leading to the formation of vortex velocity distribution in the liquid film. Increasing We number and wall temperature can accelerate the heat transfer rate of liquid film and shorten the existence time of the low-temperature ring. The findings of the the low-temperature ring on the superhydrophilic surface provide the guidelines to optimization of surface structures and functional coatings for enhancing heat transfer in various energy systems.

Surface roughness can have a critical effect upon the splashing threshold and dynamics of a drop impacting on either a dry or rough solid surface or one coated by a thin fluid film. As most coating applications and spray systems quickly evolve to a state where the droplets impinge upon fluid deposited by preceding droplets, the combined contributions of surface roughness and a pre-deposited thin liquid film of comparable thickness upon droplet impingement dynamics are examined. For comparison, we include results for droplets impacting on a smooth, dry surface and a smooth surface wetted by a thin fluid film. The inclusion of surface roughness considerably lowers the splashing threshold and alters the splashing dynamics such that differences in fluid surface tensions between 20.1 and 72.8 dynes/cm or viscosities between 0.4 and 3.3 cP have little effect.

The process of the droplet impact onto the liquid film, as one of the basic multiphase problems, is very important in many fields of science and engineering. On the other hand, the problem is also very complicated since there are many parameters that may influence the process of the droplet impact on the liquid film. To clearly understand the physical phenomena appearing in the process droplet impact on the liquid film, a parametric study on this problem is conduced based on a recently developed lattice Boltzmann method in which a lattice Boltzmann model is used to solve the Navier-Stokes equations, and the other is adopted to solve the Cahn-Hilliard equation that is used to depict the interface between different phases. In this paper, we mainly focus on the effects of the Reynolds number (Re), the Weber number (We), the relative thickness of film (h) and the surface tension () on the dynamic behavior of interface between different phases, and the velocity and pressure fields are also presented. It is found that with the increase of Re and We, the phenomena of crown and entrainment can be observed obviously during the process of droplet impact onto the liquid film, and the radius of the crown seems not dependent on the We and Re where the relative thickness of film and surface tension are fixed to be 0.5 and 0.003. However, when Re becomes much larger, the splashing phenomenon is produced, and the small droplets caused by the splashing can fall and then impact onto the liquid film again. We also find that if the relative thickness of film is small, the surface tension, Re and We are set to be 0.003, 480 and 500, the film can break up during the process of the droplet impact onto the liquid film, while with the increase of relative thickness, although more liquid are induced in the splashing process, the film cant break up. In addition, with the increase of surface tension, the resistance which prevents the change of interface becomes large, and thus the change of interface is not large when the droplet impacts onto liquid film, as expected. And finally, a quantitative study on the relation between the radius of crown (formed by droplet impact onto liquid film) and the time is also performed, and the expression r/(2R) Ut/(2R) where the parameter is about 1.0 and is also independent of We and Re, can be used to describe the relation.