arXiv subjects

Zhizhao Che

Publications and source records attributed to Zhizhao Che.

At least 37 records · Page 2Linked to original sources

arXiv2025

Coalescence delay mediated by the gas layer during the impact of hot droplets

Coalescence may not occur immediately when droplets impact a liquid film. Despite the prevalence of the high-temperature condition during the impact process in many applications, the effect of droplet temperature on droplet coalescence is rarely considered. In this study, we experimentally investigate the droplet coalescence during the impact of hot droplets on a liquid film by using color interferometry, high-speed imaging, and infrared imaging. We find that the coalescence of the hot droplet with the liquid film can be delayed which is mediated by the intervening gas layer between the droplet and the film. Compared with droplets at room temperature, the residence time of hot droplets can increase by more than two orders of magnitude. We find that the thickness of the gas layer increases with the droplet temperature, explaining that the thermal delay of coalescence is due to the thicker gas layer. During the hot droplet impact, the temperature gradient at the bottom of the droplet induces Maranogni flow, which can delay the drainage of the intervening gas layer. The results also show that as the Weber number increases, the residence time of the droplet decreases because of the thinner thickness of the gas layer.

physics.flu-dyn↗

arXiv2025

Growth of Elongated Vapor Bubbles During Flow Boiling Heat Transfer in Wavy Microchannels

Flow boiling within microchannels is an efficient heat removal mechanism by utilizing the latent heat of the fluid. Numerical simulations can resolve the small temporal and spatial scales of the vapor bubbles in the flow boiling process, thus accessing many details of the flow which is unattainable experimentally. Most of the previous studies of flow boiling heat transfer used straight microchannels, even though curved microchannels have potential to further enhance the heat transfer performance. In this study, the flow boiling heat transfer in wavy microchannels is studied via numerical simulation by focusing on the growth of the vapor bubbles. Straight and wavy microchannels are considered for comparison and analysis. The numerical simulations are conducted using the finite volume method to solve the flow and the heat transfer, and the volume of fluid (VOF) method to capture the interface dynamics. The effects of key dimensionless numbers on bubble growth rate and wall heat transfer are quantified, including the Weber number, the Capillary number, and the Jakob number. The results indicate that the wavy structure of the microchannel has a strong effect on the bubble growth and the wall heat transfer in microchannels. As the waviness of the microchannel increases, the bubble grows faster because of the bubble deformation by the wavy channel, the bubble moves faster due to the bubble expansion, and the heat transfer is enhanced by the perturbation of the bubble to the flow. With the large velocity perpendicular to the channel flow direction induced by the waviness, the local Nusselt number of the wavy channel could be high up to 2.6 times higher than the straight channel.

physics.flu-dyn↗

arXiv2025

Mist formation during micro-explosion of emulsion droplets

The micro-explosion of emulsion droplets plays an important role in promoting atomization, improving combustion efficiency, and reducing pollutant emissions. In this experimental study of the micro-explosion of emulsion droplets, we find that mist can be generated during the heating of emulsion droplets, and the mist generation is closely related to the micro-explosion process. Combined analysis from high-speed images, gas chromatography, and droplet temperature variation shows that the mist generation is due to the condensation of vapor into small droplets as the temperature decreases. Two micro-explosion modes are observed, intense micro-explosion with a large amount of mist and weak micro-explosion with a small amount of mist. Different emulsified fuels are tested, and mist can be produced for all the emulsified fuels. The mist is quantitatively analyzed via digital image processing. According to the mist concentration curve in the micro-explosion process, the micro-explosion mode can be distinguished. The effects of the water and surfactant contents in the emulsion droplets are studied, and the mists are used to characterize the micro-explosion. Increasing the water content can promote the vaporization of the water phase, increase the strength of micro-explosion, and result in a large amount of mist. Increasing the surfactant content can improve the stability of the emulsion droplet, reduce the probability of intense micro-explosion, and hence reduce the mist concentration.

physics.flu-dyn↗

arXiv2025

Transitions of breakup regimes for viscous droplets in airflow

Droplet breakup is important in many natural and industrial processes, but the current classification of breakup regimes, especially the intermediate breakup regime, is ambiguous. In this study, the transitions of breakup regimes for viscous droplets are investigated experimentally using high-speed imaging taken from a side view and a 45$^{\circ}$ view. Based on the morphology change in the middle of the droplet, the breakup regimes are classified into no-breakup, bag breakup, bag-stamen, low-order multimode, high-order multimode, and shear-stripping breakup. The droplet morphologies in different regimes and the corresponding transitions are discussed in detail. The droplet viscosity dissipates the kinetic energy transferred by the airflow during the initial droplet flattening, and affects the development of the Rayleigh-Taylor instability wave after the flattening. Through the analysis of the droplet deformation and the Rayleigh-Taylor instability with the droplet viscosity taken into account, the transition conditions of different regimes are obtained in a regime map. By further considering the relative velocity loss between the droplet and the airflow, the ranges of the dual-bag breakup in the low-order multimode regime and the droplet retraction in the bag-stamen regime are determined.

physics.flu-dyn↗

arXiv2024

Dynamics of cavitation bubbles inside a small corner

Cavitation is a ubiquitous phenomenon in nature and bubble dynamics in open spaces have been widely studied, but the effects of the wall on the dynamics of cavitation bubbles in confined spaces are still unclear. Here, the dynamics of cavitation bubbles in small corners is studied experimentally, focusing on the interaction of the bubble with the wall. High-speed photography is used to visualize the temporal development of laser-induced cavitation bubbles in a small corner formed by two rigid walls, and the bubble spreading on the corner walls and the bubble migration is analyzed via digital image processing. We identify three distinct modes of bubble collapse, namely, collapse dominated by annular shrinkage, collapse dominated by wall attraction, and collapse governed by a combination of both annular shrinkage and wall attraction. The distribution of different collapse modes at different opening angles of the corner is also analyzed. The displacement and shrinkage of different parts of the bubble surface, as well as the direction and amount of bubble migration, are determined. The results show that the asymmetric structure of the corner leads to asymmetric bubble dynamics, including asymmetric bubble expansion, spreading, contraction, and migration.

physics.flu-dyn↗

arXiv2024

Conjugate Heat Transfer Effects on Bubble Growth During Flow Boiling Heat Transfer in Microchannels

Flow boiling in microchannel heat sinks is an efficient way to dissipate high heat flux by utilizing the large surface-to-volume ratio and high latent heat. Previous studies of boiling heat transfer in microchannels mainly consider the fluid flow in channels only, but often neglect the conjugate effects of the heat conduction in the solid wall, which becomes important for microchannels because of the comparable sizes of the flow channel and the solid wall. In the present study, the effects of conjugate heat transfer on bubble growth during flow boiling in microchannels are examined by numerical simulation. The results indicate that the bubble growth is non-uniform for different bottom wall thicknesses or different solid materials even with the same heat flux at the wall. As the bottom wall thickness increases, the bubble growth rate increases because of the heat conduction in the solid wall along the channel direction. The increased bubble size also increases the perturbation to the flow field, and enhances the thermal convection between the fluid and the wall. For different solid materials, the high-thermal-diffusivity material possesses a higher heat transfer performance because of the quick diffusion of thermal energy from the heat source to the solid-fluid interface.

physics.flu-dyn↗

arXiv2024

Numerical study of cavitation bubble dynamics in a flowing tube

Cavitation in tubes is a common occurrence in nature and engineering applications. Previous studies of cavitation bubble dynamics mainly consider bubbles in stagnant-water tubes, but the dynamics of cavitation bubbles in tubes with flow is not clear. This study investigates the dynamics of cavitation bubbles in tubes with flow by numerical simulations. The results show that, unlike bubbles in stagnant-water tubes, bubbles under the combined effects of water inflow and tube wall confinement exhibit asymmetric behavior along the axis of the tube. The inflow suppresses the development of the bubble interface near the tube inlet, causing that side of the interface to move with the inflow. In contrast, the expansion and contraction of the bubble and the generation of liquid jets occur on the side near the outlet. This feature results in significant asymmetry in the bubble interface, therefore we introduce a skewness parameter to characterize the difference in length between the left and right parts of the bubble during the bubble evolution. The evolution of the bubble significantly affects the mass flow rate at the outlet of the tube, and even leads to backflow during the bubble contraction process.

physics.flu-dyn↗

arXiv2024

Heat transfer enhancement by mist/air two-phase flow in a high-temperature channel

Mist/air two-phase flow is a promising cooling technique for many applications such as internal cooling of gas turbine blades. A significant enhancement of heat transfer can be achieved with a low mass fraction of droplets by utilizing the latent heat of the droplets. Using newly designed atomizers to accurately control the mist droplets, this study experimentally explores the heat transfer performance of mist/air flow in a high-temperature channel with a maximum temperature of 880 K. The effects of the mist/air mass ratio, droplet size, Reynolds number, and wall heat flux are studied. The results show that the cooling performance of the test section can be significantly improved by even adding a small amount of droplets. Considering mist droplets of different sizes, larger droplets can cause more remarkable temperature reduction, while smaller droplets can improve the uniformity of temperature distribution. For large droplets, the cooling effect in the upstream is more obvious than that in the downstream due to the interaction between the wall and the droplets, and with the increase of mist/air mass ratio, the area with obvious cooling extends downstream. The performance of mist/air cooling is tested by increasing the heat flux until the maximum temperature at the outlet reaches a predetermined value. Compared with air-only cooling, the increment in the wall heat flux by the mist/air cooling with a mass ratio of 3% can be up to 18.4%.

physics.flu-dyn↗

arXiv2024

Wall confinement effects on the dynamics of cavitation bubbles in thin tubes

Cavitation is a common phenomenon in nature and has numerous applications. In contrast to a cavitation bubble in a free domain, a cavitation bubble in a thin tube is restricted by the tube wall, which is expected to significantly affect bubble evolution but its mechanism is still unclear. In this study, the dynamics of a cavitation bubble in a thin circular tube is studied by numerical simulation, focusing on the confinement effects of the tube. The results show that besides affecting the size and lifetime of the bubble, the confinement effects of the tube lead to the generation of counter jets and a ring jet during the contraction process of the bubble, and the curvature of the two counter jets determines the ring jet's peak velocity. When the bubble deviates from the midpoint of the tube in the axial direction, the two sides of the bubble along the axial direction show asymmetric behaviors, which results in the bubble migrating toward the midpoint. The tube diameter, tube length, liquid viscosity, and initial bubble position, can significantly influence the degree of confinement effects, which can be characterized by the variations of several key indicators, such as bubble size, lifetime, degree of deformation, counter jet velocity, ring jet velocity, and axial migration of the bubble.

physics.flu-dyn↗

arXiv2024

Ionomer structure and component transport in the cathode catalyst layer of PEM fuel cells: A molecular dynamics study

The transport of water and protons in the cathode catalyst layer (CCL) of proton exchange membrane (PEM) fuel cells is critical for cell performance, but the underlying mechanism is still unclear. Herein, the ionomer structure and the distribution/transport characteristics of water and protons in CCLs are investigated via all-atom molecular dynamics simulations. The results show that at low water contents, isolated water clusters form in ionomer pores, while proton transport is mainly via the charged sites of the ionomer side chains and the Grotthuss mechanism. Moreover, with increasing water content, water clusters are interconnected to form continuous water channels, which provide effective paths for proton transfer via the vehicular and Grotthuss mechanisms. Increasing the ionomer mass content can enhance the dense arrangement of the ionomer, which in turn increases the density of charge sites and improves the proton transport efficiency. When the ionomer mass content is high, the clustering effect reduces the space for water diffusion, increases the proton transport path, and finally decreases the proton transport efficiency. By providing physics insights into the proton transport mechanism, this study is helpful for the structural design and performance improvement of CCLs of PEM fuel cells.

physics.chem-ph↗

arXiv2023

Bridge evolution during the coalescence of immiscible droplets

Hypothesis: Droplet coalescence is a common phenomenon and plays an important role in many applications. When two liquid droplets are brought into contact, a liquid bridge forms and expands quickly. Different from miscible droplets, an extra immiscible interface exists throughout the coalescence of immiscible droplets and is expected to affect the evolution of the liquid bridge, which has not been investigated. We hypothesized that the liquid bridge of immiscible droplets exhibits a different growth dynamics. Experiments: We experimentally study the coalescence dynamics of immiscible droplets. The evolution of the liquid bridge is measured and compared with miscible droplets. We also propose a theoretical model to analyze the effects of immiscibility. Findings: We find that immiscibility plays different roles in the viscous-dominated and inertia-dominated regimes. In the initial viscous-dominated regime, the coalescence of immiscible droplets follows the linear evolution of the bridge radius as that of miscible droplets. However, in the later inertia-dominated regime, the coalescence of immiscible droplets is slower than that of miscible droplets due to the water-oil interface. By developing a theoretical model based on the force balance, we show that this slower motion is due to the immiscible interface and the extra interfacial tension. In addition, a modified Ohnesorge number is proposed to characterize the transition from the viscous-dominated regime to the inertia-dominated regime.

physics.flu-dyn↗

arXiv2023

Deformation and breakup of compound droplets in airflow

Hypothesis: Immiscible liquids are commonly used to achieve unique functions in many applications, where the breakup of compound droplets in airflow is an important process. Due to the existence of the liquid-liquid interface, compound droplets are expected to form different deformation and breakup morphologies compared with single-component droplets. Experiments: We investigate experimentally the deformation and breakup of compound droplets in airflow. The deformation characteristics of compound droplets are quantitatively analyzed and compared with single-component droplets. Theoretical models are proposed to analyze the transition between breakup morphologies. Findings: The breakup modes of compound droplets are classified into shell retraction, shell breakup, and core-shell breakup based on the location where the breakup occurs. The comparison with single-component droplets reveals that the compound droplet is stretched more in the flow direction and expands less in the cross-flow direction, and these differences occur when the core of the compound droplet protrudes into the airflow. The transition conditions between different breakup modes are obtained theoretically. In addition, the eccentricity of the compound droplet can lead to the formation of the thick ligament or the two stamens in the droplet middle.

physics.flu-dyn↗

arXiv2023

Coalescence of immiscible droplets in liquid environments

Hypothesis: Droplet coalescence process is important in many applications and has been studied extensively when two droplets are surrounded by gas. However, the coalescence dynamics would be different when the two droplets are surrounded by an external viscous liquid. The coalescence of immiscible droplets in liquids has not been explored. Experiments: In the present research, the coalescence of two immiscible droplets in low- and high-viscosity liquids is investigated and compared with their miscible counterparts experimentally. The coalescence dynamics is investigated via high-speed imaging, and theoretical models are proposed to analyze the growth of the liquid bridge. Findings: We find that, the liquid bridge $r$ evolves differently due to the constraint from the triple line in the bridge region, which follows $r\propto {{t}^{{2}/{3}}}$ for low-viscosity surroundings. While for high-viscosity surroundings, the liquid bridge grows at a constant velocity ${{u}_{r}}$ which varies with the surrounding viscosity ${{μ}_{s}}$ as ${{u}_{r}}\propto {{μ}_{s}}^{{1}/{2}}$. In the later stage of the bridge growth, the bridge evolution again merges with the well-established power-law regime $r\propto {{t}^{{1}/{2}}}$, being either in low or high-viscosity liquids. Moreover, a new inertia-viscous-capillary timescale is proposed, which unifies the combined influence of inertia, viscous, and capillary forces on the evolution of the liquid bridge in liquid environments, highlighting the joint role of inertia and viscous resistance in the coalescence process.

physics.flu-dyn↗

arXiv2023

Droplet control based on pinning and substrate wettability

Pinning of liquid droplets on solid substrates is ubiquitous and plays an essential role in many applications, especially in various areas, such as microfluidics and biology. Although pinning can often reduce the efficiency of various applications, a deeper understanding of this phenomenon can actually offer possibilities for technological exploitation. Here, by means of molecular dynamics simulation, we identify the conditions that lead to droplet pinning or depinning and discuss the effects of key parameters in detail, such as the height of the physical pinning-barrier and the wettability of the substrates. Moreover, we describe the mechanism of the barrier crossing by the droplet upon depinning, identify the driving force of this process, and, also, elucidate the dynamics of the droplet. Not only does our work provide a detailed description of the pinning and depinning processes, but it also explicitly highlights how both processes can be exploited in nanotechnology applications to control droplet motion. Hence, we anticipate that our study will have significant implications for the nanoscale design of substrates in micro and nano-scale systems and will assist with assessing pinning effects in various applications.

physics.flu-dyn↗

arXiv2023

Molecular dynamics simulation of the coalescence of surfactant-laden droplets

We investigate the coalescence of surfactant-laden water droplets by using several different surfactant types and a wide range of concentrations by means of a coarse-grained model obtained by the statistical associating fluid theory. Our results demonstrate in detail a universal mass transport mechanism of surfactant across many concentrations and several surfactant types during the process. Coalescence initiation is seen to occur via a single pinch due to aggregation of surface surfactant, and its remnants tend to become engulfed in part inside the forming bridge. Across the board we confirm the existence of an initial thermal regime with constant bridge width followed by a later inertial regime with bridge width scaling roughly as the square root of time, but see no evidence of an intermediate viscous regime. Coalescence becomes slower as surfactant concentration grows, and we see evidence of the appearance of a further slowdown of a different nature for several times the critical concentration. We anticipate that our results provide further insights in the mechanisms of coalescence of surfactant-laden droplets.

cond-mat.soft↗

arXiv2023

Breakup of particle-laden droplets in airflow

The atomisation of suspension containing liquid and dispersed particles is prevalent in many applications. Previous studies of droplet breakup mainly focused on homogeneous fluids, and the heterogeneous effect of particles on the breakup progress is unclear. In this study, the breakup of particle-laden droplets in airflow is investigated experimentally. Combining synchronised high-speed images from the side view and the 45$^\circ$ view, we compare the morphology of particle-laden droplets with that of homogeneous fluids in different breakup modes. The results show that the higher effective viscosity of particle-laden droplets affects the initial deformation, and the heterogeneous effect of particles appears in the later breakup stage. To evaluate the heterogeneous effect of particles quantitatively, we eliminate the effect of the higher effective viscosity of particle-laden droplets by comparing cases corresponding to the same inviscid Weber number. The quantitative comparison reveals that the heterogeneous effect of particles accelerates the fragmentation of liquid film and promotes localised rapid piercing. A correlation length that depends on the particle diameter and the volume fraction is proposed to characterise the length scale of the concentration fluctuation under the combined effect of the initial flattening and later stretching during the droplet breakup process. Based on this correlation length, the fragment size distributions are analysed, and the scaling results agree well with the experimental data.

physics.flu-dyn↗

arXiv2023

Melting process of frozen sessile droplets on superhydrophobic surfaces

Superhydrophobic surfaces can exhibit icephobicity in many ways due to their large contact angles and small rolling angles. The melting process of frozen droplets on superhydrophobic surfaces is still unclear, hindering the understanding of surface icephobicity. In this experimental study of the melting process of frozen sessile droplets on superhydrophobic surfaces, we find two types of melting morphologies with opposite vortex directions on a single-scale nano-structured (SN) superhydrophobic substrate and a hierarchical-scale micro-nano-structured (HMN) superhydrophobic substrate. Melting pattern visualizations and flow field measurements showwed Marangoni convection and natural convection occuring in the melting sessile droplets. For the HMN superhydrophobic substrate, the internal flow was found to be dominated by Marangoni convection due to the temperature gradient along the surface of the droplet. For the SN superhydrophobic substrate, Marangoni convection was inhibited by the superhydrophobic particles at the surface of the droplet, which were shed from the fragile superhydrophobic substrate during the freezing--melting process, as confirmed by surface characterizations of the substrate and flow measurements of a water pool. These results will help researchers better understand the melting process of frozen droplets and in designing novel icephobic surfaces for numerous applications.

physics.flu-dyn↗

arXiv2023

Coalescence of immiscible sessile droplets on a partial wetting surface

Droplet coalescence is a common phenomenon and plays an important role in multi-disciplinary applications. Previous studies mainly consider the coalescence of miscible liquid, even though the coalescence of immiscible droplets on a solid surface is a common process. In this study, we explore the coalescence of two immiscible droplets on a partial wetting surface experimentally and theoretically. We find that the coalescence process can be divided into three stages based on the timescales and force interactions involved, namely (I) the growth of the liquid bridge, (II) the oscillation of the coalescing sessile droplet, and (III) the formation of a partially-engulfed compound sessile droplet and the subsequent retraction. In stage I, the immiscible interface is found not to affect the scaling of the temporal evolution of the liquid bridge, which follows the same 2/3 power law as that of miscible droplets. In Stage II, by developing a new capillary timescale considering both surface and interfacial tensions, we show that the interfacial tension between the two immiscible liquids functions as a nonnegligible resistance to the oscillation which decreases the oscillation periods. In Stage III, a modified Ohnesorge number is developed to characterize the visco-capillary and inertia-capillary timescales involved during the displacement of water by oil; a new model based on energy balance is proposed to analyze the maximum retraction velocity, highlighting that the viscous resistance is concentrated in a region close to the contact line.

physics.flu-dyn↗