Simultaneous droplet impingement dynamics and heat transfer on nano-structured surfaces

This study examines the hydrodynamics and temperature characteristics of distilled deionized water droplets impinging on smooth and nano-structured surfaces using high speed (HS) and infrared (IR) imaging at We = 23.6 and Re = 1593, both based on initial drop impingement parameters. Results for a smooth and nano-structured surface for a range of surface temperatures are compared. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. The near surface average droplet fluid temperatures are evaluated for conditions of evaporative cooling and boiling. Also included are surface temperature results using a gold layered IR opaque surface on silicon. Four stages of the impingement process are identified: impact, boiling, near constant surface diameter evaporation, and final dry-out. For the boiling conditions there is initial nucleation followed by severe boiling, then near constant diameter evaporation resulting in shrinking of the droplet height. When a critical contact angle is reached during evaporation the droplet rapidly retracts to a smaller diameter reducing the contact area with the surface. This continues as a sequence of retractions until final dry out. The basic trends are the same for all surfaces, but the nano-structured surface has a lower dissipated energy during impact and enhances the heat transfer for evaporative cooling with a 20% shorter time to achieve final dry out.     

Contact angle calculations from the contact/maximum diameter of sessile drops

This paper presents two algorithms for computing the contact angle of sessile liquid drops given data about the drops. The first yields the contact angle given the volume, surface tension and maximum diameter (or contact diameter) of a single drop. This algorithm is an extension of existing algorithms based on knowledge of the maximum diameter or of the contact diameter of a drop. A sensitivity analysis is included for this algorithm, allowing estimates to be made of the error in computed contact angle caused by errors in the measurement of the volume and/or diameter. The second algorithm requires only the volume and maximum or contact diameter of two different drops as input, and it produces both the contact angle and surface tension as output. Both algorithms are based on Newton's method applied to a function whose value is computed by solving a system of ordinary differential equations obtained from the Laplace equation of capillarity. The techniques are applicable to both hydrophobic and hydrophilic surfaces. Copyright © 2000 John Wiley & Sons, Ltd.     

Sessile drop deformations under an impinging jet

The problem of steady axisymmetric deformations of a liquid sessile drop on a flat solid surface under an impinging gas jet is of interest for understanding the fundamental behavior of free surface flows as well as for establishing the theoretical basis in process design for the Aerosol \({{\rm Jet}^{\circledR}}\) direct-write technology. It is studied here numerically using a Galerkin finite-element method, by computing solutions of Navier–Stokes equations. For effective material deposition in Aerosol \({{\rm Jet}^{\circledR}}\) printing, the desired value of Reynolds number for the laminar gas jet is found to be greater than ~500. The sessile drop can be severely deformed by an impinging gas jet when the capillary number is approaching a critical value beyond which no steady axisymmetric free surface deformation can exist. Solution branches in a parameter space show turning points at the critical values of capillary number, which typically indicate the onset of free surface shape instability. By tracking solution branches around turning points with an arc-length continuation algorithm, critical values of capillary number can be accurately determined. Near turning points, all the free surface profiles in various parameter settings take a common shape with a dimple at the center and bulge near the contact line. An empirical formula for the critical capillary number for sessile drops with \({45^{\circ}}\) contact angle is derived for typical ranges of jet Reynolds number and relative drop sizes especially pertinent to Aerosol \({{\rm Jet}^{\circledR}}\) printing.     

不同壁面条件下液滴撞击铺展特性的模拟研究

液滴撞击不同润湿性壁面的传热流动问题在自然界和工业生产中广泛存在。研究采用CLSVOF方法,引入描述壁面润湿特性的动态接触角,并考虑液滴物性参数随温度的变化,建立液滴撞壁模型,模拟研究液滴撞击流动行为,通过与实验对比验证,确定模型有效性。在此基础上,对传热作用下考虑壁面润湿性的液滴撞击问题展开研究,探讨壁面传热作用对液滴撞击铺展特性的影响。研究表明,在撞击过程中,液滴先铺展后逐渐收缩,与静态接触角模型相比,采用动态接触角模型所得的液滴流动特性与实验结果更加吻合;随着接触角增大,液滴在撞壁初期不易铺展,随后则易于收缩;虽然固液传热作用会影响液滴铺展直径,但不改变液滴的运动趋势。     

Heat transfer augmentation between impinging circular air jet and flat plate using finned surfaces and vortex generators

An experimental investigation is performed to study the effect of the finned surfaces and surfaces with vortex generators on the local heat transfer coefficient between impinging circular air jet and flat plate. Reynolds number is varied between 7000 and 30,000 based on the nozzle exit condition and jet to plate spacing between 0.5 and 6 nozzle diameters. Thermal infrared imaging technique is used for the measurement of local temperature distribution on the flat plate. Fins used are in the form of cubes of 2 mm size spaced at a pitch of 5 mm on the target plate and hexagonal prism of side 2.04 mm and height of 2 mm spaced at a pitch of 7.5 mm. Vortex generators in the form of a equilateral triangle of side 4 mm are used. Effect of number of rows of vortex generators, radius of a row, number of vortex generators in a row and inclination angle (i.e., the angle between the plane of the target plate and the plane of the vortex generators) on Nusselt number is studied. It is observed that the heat transfer coefficient between the impinging jet and the target plate is sensitive to the shape of the fin. The increase in the heat transfer coefficient up to 77% depending on the shape of the fin, nozzle plate spacing and the Reynolds number is observed. The augmentation in the heat transfer for the surfaces vortex generators are higher than that of the finned surfaces. The heat transfer augmentation in case of vortex generator is as high as 110% for a single row of six vortex generators at a radius of 1 nozzle diameter as compared to the smooth surface at a given nozzle plate spacing of 1 nozzle diameter and a Reynolds number of 25,000 at extreme radial location.     

Compositive effects of orientation and contact angle on critical heat flux in pool boiling of water

An experimental study was carried out to investigate the effects of heat transfer surface orientation and the solid–liquid contact angle on the boiling heat transfer and critical heat flux (CHF) in water pool boiling using a smooth heat-transfer surface under atmospheric pressure. The orientation angle was ranged from 0° (up-facing horizontal position) to 180° (down-facing horizontal position) with a pace of 45°. The three kinds of heat transfer surfaces having different solid–liquid contact angles were the normal surface with a contact angle of 55°, the hydrophilic surface with a contact angle of 30° and the superhydrophilic surface with a contact angle of 0°. The experimental results indicate that orientation and contact angle have complex, coupling effects on heat transfer and CHF. A predicting correlation for the CHF which takes the effects of both orientation and contact angle into account is established. The predicting correlation agrees reasonably well with the experimental data.     

Impinging sweeping jet and convective heat transfer on curved surfaces

The application of an impinging sweeping jet, which oscillates periodically with a large angle, to convective heat transfer has received attention owing to its capability to provide a more spatially uniform and enhanced heat removal rate when compared to a steady jet. Herein, we study how the surface curvature affects the heat transfer performance of a sweeping jet and couple it with the representative flow characteristics. Heat transfer measurement and quantitative flow visualization are conducted experimentally for concave and convex surfaces as well as a flat surface. Whereas concave surfaces have a better heat transfer rate than a flat surface, the enhancement of the heat transfer is relatively small for a convex surface. For both concave and convex surfaces, the Nusselt number does not increase monotonically with the curvature magnitude but has a peak for a moderate curvature. The variation in heat transfer performance with the surface curvature is correlated with the phase-averaged velocity profile of the wall jet deflected after an impingement and the turbulence kinetic energy inside the jet. For both concave and convex surfaces, the wall jet becomes thinner than a flat surface in general, which contributes to improved heat transfer. However, whereas the turbulence kinetic energy is significantly larger for a concave surface of a moderate curvature than that of a flat surface, the turbulence kinetic energy for a convex surface is reduced from that of a flat surface, resulting in degradation of the heat transfer performance.     

Experimental investigation of parameters effect on heat transfer of spray cooling

The effects of spray height, nozzle spray angle, inlet pressure and spray incident angle on heat transfer of spray cooling were studied by an experimental method. Multi-points thermocouples and infrared imaging device were used to measure temperature distribution on heating surface. A Doppler anemometry and a camera were applied to study the spray flow field. The mechanism of heat transfer of spray cooling was concluded on the basis of experimental data and spray characteristics. It is showed that parameters affect heat transfer by way of changing the flow field on the heating surface. Heat transfer performance can be optimized by a smaller spray angle nozzle, which sprays at a smaller spray height and a higher inlet pressure. The effect of incident angle on heat transfer depends on nozzle spray angle and the definition of distance of nozzle to surface.     

Heat transfer from an open-wedge cavity to a symmetrically impinging slot air jet

Heat transfer from an open-wedge cavity to a symmetrically impinging slot air jet is investigated at the present study. The effect of the cavity angle was mainly examined on the Nusselt number distribution. Based on the results, heat transfer was generally poor at the vicinity of the apex, rising to form a maximum at the impingement and then followed by a moderate decline at further distances. The region of maximum heat transfer on the surfaces shifted outward the cavity as the cavity angle was decreased. Also, average Nusselt number over an effective length of the surface remained almost constant and independent of the cavity angle for a specified jet Reynolds number and nozzle-to-apex spacing.     

Effect of salt spray corrosion on air-side hydrophilicity and thermal-hydraulic performance of copper-fin heat exchangers

The effect of salt spray corrosion on the air-side hydrophilicity and the thermal-hydraulic performance of copper-fin heat exchangers were experimentally investigated. Artificial accelerated method of salt spray corrosion on the copper-fin heat exchangers was used for simulating the actual corroded heat exchangers. The experimental results show that, the contact angles increase with the increase of salt spray corrosion hours, which results in the degradation of the hydrophilicity of copper fin. The air-side heat transfer coefficients decrease and pressure drops increase with the increase of corrosion hours. The effect of salt spray corrosion on the heat transfer coefficients and pressure drops become more obvious with the increase of inlet air velocity. The heat transfer coefficients of the corroded copper-fin heat exchangers decrease by 4.4–34.0% and the pressure drop increase by 5.2–26.1% comparing with those of the uncorroded copper-fin heat exchanger at the inlet air velocity ranging from 0.5 to 2.0 m/s.     

Experimental study on condensation heat transfer of steam on vertical titanium plates with different surface energies

Visual experiments were employed to investigate heat transfer characteristics of steam on vertical titanium plates with/without surface modifications for different surface energies. Stable dropwise condensation and filmwise condensation were achieved on two surface modification titanium plates, respectively. Dropwise and rivulet filmwise co-existing condensation form of steam was observed on unmodified titanium surfaces. With increase in the surface subcooling, the ratio of area (η) covered by drops decreased and departure diameter of droplets increased, resulting in a decrease in condensation heat transfer coefficient. Condensation heat transfer coefficient decreased sharply with the values of η decreasing when the fraction of the surface area covered by drops was greater than that covered by rivulets. Otherwise, the value of η had little effect on the heat transfer performance. Based on the experimental phenomena observed, the heat flux through the surface was proposed to express as the sum of the heat flux through the dropwise region and rivulet filmwise region. The heat flux through the whole surface was the weighted mean value of the two regions mentioned above. The model presented explains the gradual change of heat transfer coefficient for transition condensation with the ratio of area covered by drops. The simulation results agreed well with the present experimental data when the subcooling temperature is lower than 10 °C.     

Experimental study of evaporation of sessile water droplet on PDMS surfaces

Evaporation of sessile water droplet on polydimethylsiloxane （PDMS） surfaces with three different curing ratios （5：1, 10：1, and 20：1） was experimentally investigated in this paper. We show that the constant contact radius （CCR） evaporation on surface with high curing ratio lasts longer than that with low curing ratio. We also measured Young＇s moduli of PDMS films by using atomic force microscopy （AFM） and simulated surface deformation of PDMS films induced by sessile water droplet. With increasing curing ratio of PDMS film, Young＇s modulus of PDMS film is getting lower, and then there will be larger surface deformation and more elastic stored energy. Since such energy acts as a barrier to keep the three-phase contact line pinned, thus it will result in longer CCR evaporation on PDMS surface with higher curing ratio.     

Speckle technique for dynamic drop profile measurement on rough surfaces

A technique has been developed for measuring three-dimensional instantaneous drop profiles on rough surfaces. The surface is illuminated using a laser and images are captured of the resulting speckle pattern with and without the drop in place. The analysis consists of finding the contact line, measuring the deformation of the speckle field caused by refraction of light at the drop surface, then reconstructing the drop using simulated annealing optimization to find the drop shape whose shift vector field best matches the one measured. An error analysis of the technique was performed using a Monte Carlo technique and comparisons to sideview drop images for a large sample of drops. Mean contact angle measurement error was found to be −1.6° with a 1 − σ error bound of −6.9°, +2.0°.     

Infrared visualization of thermal motion inside a sessile drop deposited onto a heated surface

Drop evaporation is a basic phenomenon but the mechanisms of evaporation are still not entirely clear. A common agreement of the scientific community based on experimental and numerical work is that most of the evaporation occurs at the triple line. However, the rate of evaporation is still predicted empirically due to the lack of knowledge of the governing parameters on the heat transfer mechanisms which develop inside the drop under evaporation. The evaporation of a sessile drop on a heated substrate is a complicated problem due to the coupling by conduction with the heating substrate, the convection/conduction inside the drop and the convection/diffusion in the vapor phase. The coupling of heat transfer in the three phases induces complicated cases to solve even for numerical simulations. We present recent experimental results obtained using an infrared camera coupled with a microscopic lens giving a spatial resolution of 10 μm to observe the evaporation of sessile drops in infrared wavelengths. Three different fluids fully characterized, in the infrared wavelengths of the camera, were investigated: ethanol, methanol and FC-72. These liquids were chosen for their property of semi-transparency in infrared, notably in the range of the camera from 3 to 5 μm. Thus, it is possible to observe the thermal motion inside the drop. This visualization method allows us to underline the general existence of three steps during the evaporating process: first a warm-up phase, second (principal period) evaporation with thermal-convective instabilities, and finally evaporation without thermal patterns. The kind of instabilities observed can be different depending on the fluid. Finally, we focus on the evolution of these instabilities and the link with the temperature difference between the heating substrate and the room temperature.     

Thermal diagnostics of highly heated surfaces using water-spray cooling

Spray cooling of heated surfaces is common in many industrial applications, notably steelmaking, because of its high heat dissipating ability. Control of the surface heat flux rate contributes to better products; it is shown that the heat transfer rate depends on the mass fraction of liquid. Quantitative information regarding the parameters affecting spray cooling is relatively scarce. The rate of heat transfer from a plate due to impinging of an array of water jets was investigated numerically through the solution of heat conduction equation. The simulation is carried out for eight different sprayers, in the range of ejected fluid pressure between 1 bar and 3 bars. Experimental data are used to study the influence of the function g(x, y), of dispersed water, on heat transfer variations across the surface at the temperature of 600°C. Curves were generated showing time histories of the steel temperature for the removal of high heat fluxes of the order of 300 W/cm². It was also observed that for lower temperatures, the predicted local heat transfer coefficient increased significantly. In memory of H. Mzad’s father Mebrouk.     

Effects of oxidation and surface roughness on contact angle

Contact angle is known to be a parameter that effects boiling. This study was undertaken to measure contact angle of high and low surface tension fluids on copper and aluminum surfaces.Data were taken for polished, oxidized, and rough surfaces. A simple, yet fairly accurate method of measuring the static equilibrium contact angle of a solid/liquid interface is presented. The principles of a line light source and tilting plate were modified and then combined in the design of this apparatus. The angles obtained and their variation with the solid surface properties were in good agreement with previously published data. The contact angle of distilled water o of the organic fluids and refrigerants tested were in the range of 2–5°. Roughness and oxidation reduce the contact angle. If the depth of the roughness is less than 0.5 μm contact angle. The apparatus is fairly simple in construction, is inexpensive, and has good reproductibity. The measured angles were then compared to those measured with the sessile drop method.     

Hydrodynamics of contact of a falling drop with a liquid free surface

A fine structure of the flows developing during primary contact of freely falling drops with a deep quiescent fluid is studied using the macrophotography and high-speed video filming methods. Water drops falling in water, alcohol, and oil, as well as drops of oil, petroleum, and aqueous solutions of salt or alcohol falling in water are investigated. The work is focused on the visualization of the finespray scattering from the primary contact area. The collisions of small droplets with the surface of the submerging drop are first recorded. The direction of the spray and streamer scattering is determined by the surface tension coefficients of the coalescing liquids. The conditions under which the spray droplets collide with the drop surface are determined.     

Influence of surface topography in the boiling mechanisms

The present paper addresses the qualitative and quantitative analysis of the pool boiling heat transfer over micro-structured surfaces. The surfaces are made from silicon chips, in the context of pool boiling heat transfer enhancement of immersion liquid cooling schemes for electronic components. The first part of the analysis deals with the effect of the liquid properties. Then the effect of surface micro-structuring is discussed, covering different configurations, from cavities to pillars being the latter used to infer on the potential profit of a fin-like configuration. The use of rough surfaces to enhance pool boiling mainly stands on the arguments that the surface roughness will increase the liquid–solid contact area, thus enhancing the convection heat transfer coefficient and will promote the generation of nucleation sites. However, one should not disregard bubble dynamics. Indeed, the results show a strong effect of bubble dynamics and particularly of the interaction mechanisms in the overall cooling performance of the pair liquid–surface. The inaccurate control of these mechanisms leads to the formation of large bubbles and strong vertical and horizontal coalescence effects promote the very fast formation of a vapor blanket, which causes a steep decrease of the heat transfer coefficient. This effect can be strong enough to prevail over the benefit of increasing the contact area by roughening the surface. For the micro-patterns used in the present work, the results evidence that one can reasonably determine guiding pattern characteristics to evaluate the intensity of the interaction mechanisms and take out the most of the patterning to enhance pool boiling heat transfer, when using micro-cavities. Instead, it is far more difficult to control the appearance of active nucleation sites and the optimization of the patterns allowing a reasonable control of the interaction mechanisms and in particular of horizontal coalescence, when dealing with the patterns based on micro-pillars. Hence, providing an increase of the liquid contact area by an effective increase of the roughness ratio is not enough to assure a good performance of the micro-structured surface. Despite it was not possible to clearly evidence a pin–fin effect or of an additional cooling effect due to liquid circulation between the pillars, the results show a significant increase of the heat transfer coefficient of about 10 times for water and 8 times for the dielectric fluid, in comparison to the smooth surface, when the micro-patterning based on pillars is used.     

A numerical study of droplet motion/departure on condensation of mixture vapor using lattice Boltzmann method

Droplet motion/departure, which is governed by external force acceleration coefficient, droplet radius and surface wettability on solid surfaces under external forces such as gravitational force, play a significant role in characterizing condensation heat transfer, especially when high fractional non-condensable gases (NCG) present. However, due to the challenge in visualizing the vapor/steam velocity field imposed by droplet motion/departure, the detailed mechanism of droplet motion/departure on condensing surfaces has not been completely investigated experimentally. In this study, droplet motion/departures on solid surfaces under external forces and their interactions with steam flow are simulated using two dimensional (2D) multiphase lattice Boltzmann method (LBM). Large external force acceleration coefficient, droplet radius and contact angle, lead to large droplet deformation and high motion/departure velocity, which significantly shortens the droplet residual time on the solid surface. Our simulation shows that steam vortices (lateral velocity) induced by droplet motion/departure can greatly disturb the vapor flow and would be intensified by increasing external force acceleration coefficient, droplet radius, and contact angle. In addition, the location of vortex center shifts in the ascending direction with increase of these factors. The average lateral velocities induced by droplet motion/departure at various conditions are obtained. The mass transfer resistance is substantially reduced owing to the droplet motion/departure, leading to an enhanced heat flux. The experimental results are compared to validate the influence of droplet motion/departure on condensation heat transfer performance, especially for steam–air mixture with the presence of high fractional NCG.     

Heat transfer and evaporation/condensation problems based on the linearized Boltzmann equation

A polynomial expansion procedure and an analytical discrete-ordinates method are used to solve four basic problems, all based on the linearized Boltzmann equation for rigid-sphere interactions, that describe heat transfer and/or evaporation–condensation between two parallel surfaces or for the case of a semi-infinite half space. Relevant to the case of two surfaces, the basic problem of heat transfer driven by a temperature difference at two confining walls described by a general Maxwell gas–surface interaction law (a mixture of specular and diffuse reflection) is solved for the case where different accommodation coefficients can be used for each of the two bounding surfaces. In addition, the classical problem of “reverse temperature gradient” in the theory of evaporation and condensation is also solved for the case of two parallel liquid–vapor interfaces kept at different temperatures. In regard to half-space applications, an evaporation/condensation problem based on a presumed known interface condition and a heat-conduction problem (with no net flow) driven by energy flow from a bounding surface with know properties are each solved with what is considered a high degree of accuracy.