Splashing of molten tin droplets on a rough steel surface

We photographed the impact of molten metal droplets on a flat plate. From these images we measured droplet dimensions during spreading and counted the number of fingers around a splashing drop. Experiments were done using stainless steel substrates with average roughness of 0.06, 0.07, 0.56, and 3.45 μm respectively. The temperature of the substrate was kept at either 25 or 240 °C. Droplet diameter (2.2 mm) and impact velocity (4 m/s) were kept constant, giving a Reynolds number (Re) of 31 135 and Weber number (We) of 463.Raising substrate roughness from 0.06 to 0.56 μm enhanced the tendency of droplet to splash, whereas increasing roughness even further to 3.45 μm suppressed splashing. This behaviour was attributed to changes in droplet solidification rate with surface roughness. A simple model of droplet spreading was used to estimate thermal contact resistance between the droplet and surface. Increasing surface roughness was found to raise thermal contact resistance and reduce heat transfer from the droplet to the substrate, delaying the onset of solidification and reducing splashing. The number of fingers formed around a droplet splashing on a smooth surface could be predicted reasonably well by a model based on Rayleigh-Taylor instability theory. Increasing surface roughness reduced the number of fingers while enlarging their size.     

Roughness effects of gas diffusion layers on droplet dynamics in PEMFC flow channels

Water management remains one of the major challenges in optimising the performance of PEMFCs, in which liquid accumulation and removal in gas diffusion layers (GDLs) and flow channels should be addressed. Here, effects of GDL surface roughness on the water droplet removal inside a PEMFC flow channel have been investigated using the Volume of Fluid method. Rough surfaces are generated according to realistic GDL properties by incorporating RMS roughness and roughness wavelength as the main characteristic parameters. Droplet dynamics including emergence, growth, detachment, and removal in flow channels with various airflow rates are simulated on rough substrates. The influences of airflow rate on droplet dynamics are also discussed by comparing the detachment time and droplet morphology. The liquid removal efficiency subject to different surface roughness parameters is evaluated by droplet detachment time and elongation, and regimes of detachment modes are identified based on the droplet breakup location and detachment ratio. The results suggest that rough surfaces with higher RMS roughness can facilitate the removal of liquid inside flow channel. Whilst surface roughness wavelength is found less significant to the liquid removal efficiency. The results here provide qualitative assessments on identifying the key surface characteristics controlling droplet motion in PEMFC channels.     

Metal droplet deposition on non-flat surfaces: effect of substrate morphology

In droplet-based deposition processes, substrate morphology plays an important role in the bonding quality between splats and substrate. This paper presents a numerical investigation of metal droplet impact and solidification on two types of non-flat surfaces, namely a pre-molten pool and a wavy surface. We solve the axi-symmetric incompressible Navier-Stokes equations and an enthalpy formulation of the energy equation to simulate both the flow field and the heat transfer during droplet spreading and solidification. The results show that for efficient droplet deposition on molten pools, the pool size and droplet impact velocity should be kept relatively small. Droplet impingement on non-flat surfaces is almost always accompanied by splashing. The degree of splashing decreases with the increase in surface roughness height. Increasing surface roughness wavelength improves droplet spreading and hence solidification.     

Dynamic characteristics of molten droplets and hot particles falling in liquid pool

The dynamic characteristics of molten droplets and hot particles at the very beginning of their fall into coolant pools are presented. The falling course of a single droplet or a single hot particle was recorded by a high-speed camera and a curve of velocity vs. time was obtained. Emphasis was placed on the effects of the droplet’s size and temperature, the coolant’s temperature and properties, and the droplet’s physical properties on the moving behavior. The results for the all cases showed that the velocity of a falling droplet/particle decreased rapidly but rebounded shortly, at the beginning of droplet/particle falling in the coolant. Following such a V-shaped evolution in velocity, the droplet/particle slows down gradually to a comparatively steady velocity. An increase in either coolant temperature or droplet temperature results in a larger velocity variation in the “J-region”, but a smaller deceleration when it moves out of the “J-region”. The elevated volatility of a coolant leads to a steeper deceleration in the “J-region” and beyond. The bigger size of a particle leads to a greater velocity variation in the “J-region” and terminal velocity. A high melting point and thermal conductivity as well as lower heat capacity contribute to dramatic variation in the “J-region” and low terminal velocity.     

Dynamic characteristics of molten droplets and hot particles falling in liquid pool

The dynamic characteristics of molten droplets and hot particles at the very beginning of their fall into coolant pools are presented. The falling course of a single droplet or a single hot particle was recorded by a high-speed camera and a curve of velocity vs. time was obtained. Emphasis was placed on the effects of the droplet’s size and temperature, the coolant’s temperature and properties, and the droplet’s physical properties on the moving behavior. The results for the all cases showed that the velocity of a falling droplet/particle decreased rapidly but rebounded shortly, at the beginning of droplet/particle falling in the coolant. Following such a V-shaped evolution in velocity, the droplet/particle slows down gradually to a comparatively steady velocity. An increase in either coolant temperature or droplet temperature results in a larger velocity variation in the “J-region”, but a smaller deceleration when it moves out of the “J-region”. The elevated volatility of a coolant leads to a steeper deceleration in the “J-region” and beyond. The bigger size of a particle leads to a greater velocity variation in the “J-region” and terminal velocity. A high melting point and thermal conductivity as well as lower heat capacity contribute to dramatic variation in the “J-region” and low terminal velocity.     

Behaviour of a heavy fuel oil droplet on a hot surface

When heavy fuel oil sprayed in droplets burns in water heating boilers, there are cases when the zones of incomplete combustion are present. The volatile compounds and tar contained in the droplets burn out there and the carbon starts to accumulate on the pipes of the screen. Combustion of a fuel droplet on a solid surface is less investigated than that of the droplet falling down in hot air. In this work, the burnout of a droplet of a heavy fuel oil has been measured on a hot surface whose temperature varies in the interval from 400 to 700 °C. Times of evaporation of volatile compounds and burnout of the resulting carbon residue were measured. Changes of the form of the carbon residue depending on the surface temperature were recorded. Ceramic, quartz and stainless steel surfaces were used. The effect of surface roughness was additionally examined. In the case of a droplet of the heavy fuel oil dropped on a hot surface, heat transfer into the droplet is very sudden. The surface wetting condition is important, as it determines evaporation and boiling. Another difference from a freely falling droplet is oxidation of the pure coke because the oxygen diffusion is possible only from one side of the space.     

Characterization of the heat transfer accompanying electrowetting or gravity-induced droplet motion

Electrowetting (EW) involves the actuation of liquid droplets using electric fields and has been demonstrated as a powerful tool for initiating and controlling droplet-based microfluidic operations such as droplet transport, generation, splitting, merging and mixing. The heat transfer resulting from EW-induced droplet actuation has, however, remained largely unexplored owing to several challenges underlying even simple thermal analyses and experiments. In the present work, the heat dissipation capacity of actuated droplets is quantified through detailed modeling and experimental efforts. The modeling involves three-dimensional transient numerical simulations of a droplet moving under the action of gravity or EW on a single heated plate and between two parallel plates. Temperature profiles and heat transfer coefficients associated with the droplet motion are determined. The influence of droplet velocity and geometry on the heat transfer coefficients is parametrically analyzed. Convection patterns in the fluid are found to strongly influence thermal transport and the heat dissipation capacity of droplet-based systems. The numerical model is validated against experimental measurements of the heat dissipation capacity of a droplet sliding on an inclined hot surface. Infrared thermography is employed to measure the transient temperature distribution on the surface during droplet motion. The results provide the first in-depth analysis of the heat dissipation capacity of electrowetting-based cooling systems and form the basis for the design of novel microelectronics cooling and other heat transfer applications.     

3D multiphase lattice Boltzmann simulations for morphological effects on self-propelled jumping of droplets on textured superhydrophobic surfaces

Coalescence induced droplets self-propelled jumping on textured superhydrophobic surfaces (SHS) is numerically simulated using multiple–relaxation–time (MRT), and three dimensional (3D) multiphase isothermal lattice Boltzmann method. Symmetric boundary conditions and parallel computation with OpenMP algorithm are used to accelerate computational speeds. Simulation results for velocity field show that the downward velocity of the droplet is reverted to upward direction due to the counter action of the wall to the contact base of the droplet during the period of droplet deformation on the texture. For a fixed droplet diameter, the spacing of the microstructure is found to play a key role on jumping velocity of the coalescence droplet, and an optimal spacing of the microstructure exists for a maximum jumping velocity. For a texture with small spacings, the adhesion force due to surface tension is large because of the large contact area which results in a decrease of its jumping velocity. On the other hand, for a texture with large roughness spacings, the lower contour of the droplet will fall into the texture, which will also decrease droplet jumping velocity. Simulation results for jumping velocities are used to explain large differences in measured jumping velocities of small droplets (with radius less than 20 μm) on hierarchical textured and nanostructured surfaces in existing experiments.     

Characterisation of the spray cooling heat transfer involved in a high pressure die casting process

A systematic experimental study was conducted to examine the heat transfer characteristics from the hot die surface to the water spray involved in high pressure die casting processes. Temperature and heat flux measurements were made locally in the spray field using a heater made from die material H-13 steel and with a surface diameter of 10 mm. The spray cooling curve was determined in the nucleate boiling, critical heat flux, as well as the transition boiling regimes. The hydrodynamic parameters of the spray such as droplet diameters, droplet velocities, and volumetric spray flux were also measured at the position in the spray field identical to that of the test piece. Droplet size and velocity distribution were measured using a PDA system. A new empirical correlation was developed to relate the spray cooling heat flux to the spray hydrodynamic parameters such as liquid volumetric flux, droplet size, and droplet velocity in all heat transfer regimes. The agreement between experimental data and predicted results is satisfactorily good.     

Oscillation characteristics of droplets on solid surfaces with air flow

Visual experiments were conducted to observe droplet oscillation on horizontal surfaces with air flow. The different Cu test surfaces were polished with different grit sandpapers. Two liquid drop oscillation modes, forward‐backward and upward‐downward, were visually observed in the experiments. Additionally these two modes were observed to transition from one to another under some conditions. The results indicate that the oscillating characteristics were closely dependent on surface roundness, droplet size, and air flow velocity. A larger radius and higher air speed would decrease the oscillation frequency, while the frequency would initially decrease and then increase as the roughness changed from the smooth to rough plates. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 13–19, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20098     

Development of an Improved Multi-Component Vaporization Model for Application in Oxygen-Enriched and EGR Conditions

An improved multi-component vaporization model was developed and applied to predict the vaporization characteristics of a fuel droplet under oxygen-enriched and exhaust gas recirculation (EGR) conditions in this study. The model was constructed including an improved enthalpy diffusion sub-model, a corrected Stefan flow velocity, and temporal variation of thermal physical properties. The computational results were validated with the experimental data and satisfactory agreements between the predictions and measurements can be achieved. The effects of O₂, CO₂ concentration, and EGR on the vaporization rate of fuel droplet in various ambient temperature and pressure conditions were investigated.     

Leidenfrost Droplets on Microstructured Surfaces

The lifetime of a droplet deposited on a hot plate decreases when the temperature of the plate increases, but above the critical Leidenfrost temperature, the lifetime suddenly increases. This is due to the formation of a thin layer of vapor between the droplet and the substrate, which plays a double role: First, it thermally insulates the droplet from the plate, and second, it allows the droplet to “levitate.” The Leidenfrost point is affected by the roughness or microstructure of the surface. In this work, a silicon surface with different microstructured regions of square pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing. The Leidenfrost point was identified in experiments using water droplets ranging in size from 8 to 24 μl and the behavior of the droplets was recorded using high-speed digital photography. The Leidenfrost point was found to vary by up to 120°C for pillar spacings from 10 to 100 μm. If the droplet is placed on the boundary between structured sections, the droplet becomes asymmetric and may move or spin. An axisymmetric computational fluid dynamics (CFD) model is also presented that shows qualitative agreement with experimental observations.     

液滴撞击不同固体表面的数值模拟研究

采用CLSVOF(coupled level set and volume of fluid)方法,对低We情况下液滴撞击不同固体表面的过程进行了数值模拟研究。分析了液滴撞击平板表面的动态过程,构建了液滴衰减振荡的数学模型,探究了不同的表面润湿性、撞击速度及表面微尺度结构对液滴动态特性的影响。结果表明:液滴撞击固体表面的过程包含铺展、回缩、振荡等多个现象,其最大铺展因子及振荡周期随着表面接触角的增大而减小,随着撞击速度的增大而增大;撞击表面的微尺度结构会对液滴的动态特性产生影响,微尺度结构会对液滴的铺展及回缩运动产生阻碍作用,导致液滴的振荡特性减弱;液滴在矩形沟槽表面达到最大铺展因子的用时最短,在三角形沟槽表面的最大铺展因子最小。     

Theoretical investigation on the mechanism of surface temperature non-uniformity formation in spray cooling

In this study, the effects of droplet velocity non-uniformity, SMD (Sauter mean diameter) distribution non-uniformity, droplet number non-uniformity, and heating power on the fluid film thickness, fluid film velocity, and surface temperature distribution were investigated, and then the surface temperature non-uniformity correlations in non-boiling regime and nucleate boiling regime were correlated. The results show that: with the decreasing of the spray parameters non-uniformity, the fluid film thickness on the heating surface becomes more uniform, and the fluid film velocity increases, thus the surface temperature non-uniformity decreases. The highest surface temperature appears in the centre of the heating surface, and the lowest is nearby the position where the fluid film appears. The droplet number non-uniformity contributes the largest portion of impact on the surface temperature non-uniformity, followed by the droplet velocity non-uniformity. The effect of droplet SMD non-uniformity is the minimal. Finally, the surface temperature non-uniformity correlations in non-boiling regime and nucleate boiling regime were correlated with a mean absolute error of 20%.     

Motion of droplets induced by the Marangoni force on a wall with a temperature gradient

Motion of silicone oil and water droplets induced by the Marangoni force was numerically simulated by using two‐ and three‐dimensional second‐order finite difference methods with the CIP and the level set methods. The surface tension was introduced by the continuum surface force (CSF) method. The results clearly showed the flow induced by the Marangoni force and the dependence of droplet velocity on droplet size, contact angle, temperature gradient, and fluid properties. The Marangoni force balanced with the viscous force in the small contact angle case; on the other hand, in the large contact angle case, it balanced with the normal component of surface tension. As for the effect of fluid properties on droplet motion, the temperature coefficient of surface tension had a much larger effect than did viscosity, thermal diffusivity, or surface tension. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 81–93, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20004     

Fringe probing of an evaporating microdroplet on a hot surface

The phenomenon of droplets impacting and evaporating on a hot surface is of interest in many areas of engineering. Quantitative measurement of these processes provides great help to reveal the physics behind. A novel technique was developed to quantitatively measure the volume evolution and contact diameter of an evaporating microdroplet on a hot surface utilizing interference fringe scattering method. In this method, fine fringes produced by the interference of two coherent laser beams was scattered by the droplet and projected onto a screen. The profile and volume of the droplet can be derived from the spatial fringe spacing on the screen. The number of total fringes measurable on the screen was used to determine the instantaneous contact diameter of the microdroplet. Validation experiments demonstrated that the measurement errors are less than ±5% and ±1% for microdroplet volume and contact diameter, respectively. By using this method, the dynamic of droplet impingement, evaporation and boiling using ethanol, pure water and water solution of a surfactant (sodium dodecyl sulfate) with impact velocity of 7.5 m/s and diameters ranged from 0.19 to 0.46 mm were investigated.     

Experimental investigation of anodized/ spray pyrolysed nanoporous structure on heat transfer augmentation

This paper analyzes the effects of nanoporous surface on heat transfer temperaments of assorted thermal conducting materials. A phenomenal proposal of wielding the surface roughness to ameliorate the heat transfer rate has been discovered. The maximum increase of heat transfer rate procured by nanoporous layers is 133.3% higher than the polished bare metals of surface roughness 0.2 μm. This plays an imperative role in designing compact refrigeration systems, chemical and thermal power plants. Experimental results picture a formidable upswing of 58.3% heat transfer in chemically etched metals of surface roughness 3 μm, 133.3% in nanoporous surface of porosity 75–95 nm formed by electrochemical anodization, and porosity of 40–50 nm formed by spray pyrolysis increases the heat transfer by 130%. Effects of porosity, flow velocity and scaling on the energy transfer are also scrutinized. This paper also analyzes the multifarious modes of nanoporous fabrication, to contrive both prodigious and provident system.     

液滴撞击超疏水管状壁面的动态特性研究

利用相界面追踪的CLSVOF(coupled level set and volume of fluid)方法,对液滴在不同速度下撞击超疏水管状壁面的演化过程展开研究。研究结果表明,不同初速度会对液滴在管壁上的动态特性产生较大影响。当撞击速度较小时,液膜会以整体的形态反弹;继续增大撞击速度后,由于受到外缘液膜的牵扯,液膜内部开始出现断裂;撞击速度足够大时,液膜发生解体破碎。同时给出了对于初速度在2.00m/s以内、直径为2.58mm的水滴撞击管径为8.00mm的超疏水壁面时运动状态发生转变的临界条件。     

Heat Transfer and Flow Characteristics Inside Droplet Formed on Water Surface

Abstract

Dynamics and heat transfer of a silicone oil droplet formed on a water surface are investigated. The silicone oil droplet is heated by the water in a constant temperature container. Temperature and velocity fields inside the droplet are simulated in line with the experimental conditions. The influence of the droplet volume on the flow and heat transfer characteristics is also incorporated in the analysis. The oil droplet pinning and its geometric features for different droplet volumes are examined. Temperature predictions are validated with a thermal camera data. It is found that temperature predictions agree well with the thermal camera data. The constant temperature heating of the water container wall gives rise to two counter rotating circulation cells inside the water, which in turn modifies temperature and flow fields in the water. The flow direction occurs from the droplet top region towards the droplet–water interface. The heated fluid in the region close to the droplet–water interface is carried by the flow current to the droplet sides giving rise to temperature increase in these regions. The values of the Bond number attains greater than unity. The Nusselt and the Bond numbers increase with the droplet volume.     

Transient burning of a convective fuel droplet

The transient burning of an n-octane fuel droplet in a hot gas stream at 20 atmosphere pressure is numerically studied, with considerations of droplet regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension. An initial envelope flame is found to remain envelope in time, and an initial wake flame is always transitioned into an envelope flame at a later time, with the normalized transition delay controlled by the initial Reynolds number and the initial Damkohler number. The initial flame shape is primarily determined by the initial Damkohler number, which has a critical value of Da₀=1.02. The burning rates are modified by the transition, and are influenced by the intensity of forced convection which is determined by initial Reynolds number. The influence of surface tension is also studied as the surface temperature is non-uniform. Surface tension affects the liquid motion at the droplet surface significantly and affects the change of surface temperature and burning rate modestly. The influence of surface tension generally increases with increasing initial Reynolds number within the range without droplet breakup. We also studied cases with constant relative velocity between the air stream and the droplet. The results show that in these cases the initial envelope flame still remains envelope, but the evolution from an initial wake flame to an envelope flame is inhibited. Validation of our analysis is made by comparing with a published porous-sphere experiment Raghavan et al. (2005) [6] which used methanol fuel.