Spray cooling is a promising technique which is used to remove large amounts of heat from surfaces. It is characterized by uniform heat removal, low droplet impact velocity and better cooling efficiency when compared to other cooling schemes. It can be used in electronic cooling, and other applications. However, due to the multiple impacts of droplets, the film fluid dynamics and morphology are quite complicated. Moreover, the effect of heat transfer under spray cooling is not well understood due to the large number of interdependent variables such as impact spacing, impact angle, droplet diameter, droplet velocity and droplet frequency to name a few. An experimental approach is proposed and used to minimize and control key independent variables to determine their effects on surface temperature and heat transfer cooling mode. The effects of droplet impact angle and spacing on different heat flux conditions are studied. The film thickness is also obtained to further investigate the relationship between the independent variable and the observed heat transfer mechanism. The study of coherent droplet impingement on an open surface is experimentally characterized using high speed imaging and infrared thermography. Single stream droplet impingent cooling with different impact angle is also studied. Temperature distribution and impact crater morphology are obtained under different heat flux conditions. Film thickness inside droplet impact craters is measured to understand the relationship between minimum surface temperature and film thickness. Next, double streams droplet impingement cooling with different spacings and impact angles are investigated. The optimum spacing is found to reduce the droplet-to-droplet collision and to minimize splashing, resulting in enhanced heat transfer and better use of the cooling fluid. The film thickness is also measured to understand the relationship between the heat transfer results and the controllable independent variables. The results and conclusions of this study are useful in understanding the physics of spray cooling and can be applied to design better spray cooling systems.

As a logical extension of previous work conducted into viscoelastic atomization, initially motivated by the need to improve spray coating transfer efficiencies, an experimental investigation into the spray-surface interaction for a number of Newtonian and non-Newtonian substitute test liquids is presented. Three model elastic liquids of varying polymer molecular weight and three inelastic liquids of varying shear viscosity were sprayed upon a moving surface to isolate the effect of elasticity and shear viscosity, respectively, on spray impaction behavior. In addition, two liquids exhibiting shear thinning behavior and an industrial top of rail liquid friction modifier, KELTRACK, for use in the railroad industry, were included in the spray tests. High-speed photography was used to examine the impingement of these liquids on the surface. Ligaments, formed as a consequence of a liquid’s viscoelasticity, were observed impacting the surface for 300K PEO, 1000K PEO, and KELTRACK. These ligaments were broadly classified into four groups, based on their structure. Splashing of elastic liquid ligaments and droplets led to filamentary structures being expelled from the droplet periphery, which were then carried away by the atomizing air jet, leading to reductions in transfer efficiency. The effect of increasing elasticity amongst the three varying molecular weight elastic solutions was shown to increase the splash threshold; a similar effect was noted with increasing shear viscosity. Attempts were made at quantifying a critical splash-deposition limit for all test liquids however due to imaging system limitations, no quantitative conclusions could be made. For KELTRACK, both droplets and ligaments spread and deposited on the rail surface upon impact, with no observed splash or rebound. Splash was only noted when droplets impinged directly on a previously deposited liquid film and even then, splashing was well contained. Thus, KELTRACK’s current rheological formulation proved to be very effective in ensuring high coating transfer efficiencies.

As a logical extension of previous work conducted into viscoelastic atomization, initially motivated by the need to improve spray coating transfer efficiencies, an experimental investigation into the spray-surface interaction for a number of Newtonian and non-Newtonian substitute test liquids is presented. Three model elastic liquids of varying polymer molecular weight and three inelastic liquids of varying shear viscosity were sprayed upon a moving surface to isolate the effect of elasticity and shear viscosity, respectively, on spray impaction behavior. In addition, two liquids exhibiting shear thinning behavior and an industrial top of rail liquid friction modifier, KELTRACK, for use in the railroad industry, were included in the spray tests. High-speed photography was used to examine the impingement of these liquids on the surface. Ligaments, formed as a consequence of a liquids viscoelasticity, were observed impacting the surface for 300K PEO, 1000K PEO, and KELTRACK. These ligaments were broadly classified into four groups, based on their structure. Splashing of elastic liquid ligaments and droplets led to filamentary structures being expelled from the droplet periphery, which were then carried away by the atomizing air jet, leading to reductions in transfer efficiency. The effect of increasing elasticity amongst the three varying molecular weight elastic solutions was shown to increase the splash threshold; a similar effect was noted with increasing shear viscosity. Attempts were made at quantifying a critical splash-deposition limit for all test liquids however due to imaging system limitations, no quantitative conclusions could be made. For KELTRACK, both droplets and ligaments spread and deposited on the rail surface upon impact, with no observed splash or rebound. Splash was only noted when droplets impinged directly on a previously deposited liquid film and even then, splashing was well contained. Thus, KELTRACKs current rheological formulation proved to be very effective in ensuring high coating transfer efficiencies.

Spray cooling is one of the most promising technologies in applications which require large heat removal capacity in very small areas. Previous experimental studies have suggested that one of the main mechanisms of heat removal in spray cooling is forced convection with strong mixing due to droplet impingement. These mechanisms have not been completely understood mainly due to the large number of physical variables, and the inability to modulate and control variables such as droplet frequency and droplet size. Our approach consists of minimizing the number of experimental variables by controlling variables such as droplet direction, velocity and diameter. A study of heat transfer for single and multiple droplet impingements using HFE- 7100 as the cooling fluid under constant heat flux conditions is presented. Monosized single and multiple droplet trains were produced using a piezoelectric droplet generator with the ability to adjust droplet frequency, diameter, velocity, and spacing between adjacent droplets. In this study, heaters consisting of a layer of Indium Tin Oxide (ITO) as heating element, and ZnSe substrates were used. Surface temperature at the liquid-solid interface was measured using Infrared Thermography. Heat transfer behavior was characterized and critical heat flux was measured. Film thickness was measured using a non-invasive optical technique inside the crown formation produced by the impinging droplets. Hydrodynamic phenomena at the droplet impact zone was studied using high speed imaging. Impact regimes of the impinging droplets were identified, and their effect on heat transfer performance were discussed. The results and effects of droplet frequency, droplet diameter, droplet velocity, and fluid flow rate on heat flux behavior, critical heat flux, and film morphology were elucidated. The study showed that forced heat convection is the main heat transfer mechanism inside the crown formation formed by droplet impingement and impact regimes play an important role on heat transfer behavior. In addition, this study found that spacing among adjacent droplets is the most important factor for multiple droplet stream heat transfer behavior. The knowledge generated through the study provides tools and know-how necessary for the design and development of enhanced spray cooling systems.

An analytical model is developed to quantify the heat transfer to droplets impinging on heated superhydrophobic (SH) surfaces. Integral analysis is used to incorporate an apparent temperature jump at the superhydrophobic surface as a boundary condition. This Thesis considers the scenario of both isotropic and anisotropic slip, as would be realized on post-cavity style and rib-cavity style SH surfaces. This thermal model is combined with a hydrodynamic model which incorporates velocity slip at the surface. Use of the two models allows determination of the overall cooling effectiveness, a metric outlined in contemporary work. The effect of varying velocity slip and temperature jump is determined for impact Weber numbers ranging from 20 to 150 and surface temperatures ranging from 60 to 100°C. The model results are compared to experiments and good agreement is shown. Heat transfer to a drop impacting superhydrophobic surfaces is decreased when compared to conventional surfaces. A correlation function for the total heat transfer (cooling effectiveness) as a function of relevant parameters is found for isotropic surfaces with a good fit. Anisotropic rib-cavity surfaces are compared to isotropic surfaces to explore the impact of anisotropic slip on the cooling effectiveness, with similar trends seen to that for isotropic surfaces. It's determined that anisotropic surfaces can be modeled with minimal error as an isotropic surface with a temperature jump length equal to the anisotropic surface's average temperature jump length.

The effect of superhydrophobicity on liquid water impingement on a flat horizontal surface was explored. The surfaces combined a hydrophobic surface chemistry with a patterned microstucture in order to produce high contact angles with water. Three sets of experiments were performed, one for jet impingement and two for droplet impingement, which advance previous work in characterizing the interaction of water and superhydrophobic surfaces.Jet impingement experiments were performed to characterize a transitional regime between an unsubmerged and a completely submerged superhydrophobic surface by varying an imposed downstream depth. For low downstream depths, the surface remained unsubmerged and displayed only break up of the thin film, while at high downstream depths, the surface was completely submerged and only a hydraulic jump occurred. Within the transition, the surface was partially submerged and both thin film breakup and a hydraulic jump were observed. Experiments were performed for three Reynolds numbers, Re, ranging from 1.9 x 104 to 2.2 x 104 (based on the volume flow rate). For all Re, the transition was characterized by a reduction in the hydraulic jump radius as downstream depth increased. Also, as Re increased, the downstream depths over which the transition occurred was greater. When a droplet impinges on a surface covered with a liquid film, a thin liquid wall, or crown, forms and propagates outward. Here a comparison of this crown dynamic was made for smooth hydrophilic surfaces and superhydrophobic (SH) surfaces patterned with post or rib microfeatures. Due to the high contact angle of the SH surfaces, a relatively thick film (h ≈ 5 mm) of water was required to maintain a film. This resulted in negligible differences between the surfaces utilized. Droplet train impingement on the same post and rib SH surfaces was also investigated. When each individual droplet impinged on the surface, a crown formed which spread out radially until reaching a semi-stable or regularly oscillating breakup diameter. At this point, the water would either build up or breakup into droplets or filaments and then continue radially outward. In some cases the crown would break up, causing splashing. A comparison to previous experiments on hydrophilic surfaces shows a distinct difference in splashing at low frequency. The breakup diameter was measured over a Weber number range of 72-2800. The data was collapsed as a function of a combination of the Reynolds number (Re), Capillary number (Ca), and Strouhal number (St), resulting in Re0.7CaSt. The rib SH surface displayed an elongated breakup due to the anisotropic surface features. The breakup diameter for the droplet train was compared to the breakup diameter which has been shown to occur with a jet impinging on a SH surface.

This dissertation explores the physics of droplet impingement on superhydrophobic surfaces. The research is divided in three categories. First, the effect of a slip boundary condition on droplet spreading/retracting is considered. A model is developed based on energy conservation to evaluate spreading rates on surfaces exhibiting isotropic and anisotropic slip. The results show that larger slip causes the droplet to spread out farther owing to reduced friction at the interface for both slip scenarios. Furthermore, effects of slip become magnified for large Weber numbers due to the larger solid-liquid contact area during the process. On surfaces with anisotropic slip, droplets adopt an elliptical shape following the azimuthal contour of the slip on the surface. It is common for liquid to penetrate into the cavities at the superhydrophobic interface following droplet impact. Once penetrated, the flow is said to be in the Wenzel state and many superhydrophobic advantages, such as self-cleaning and drag-reduction, become negated. Transition from the Wenzel to the Cassie state (liquid resides above the texture) is referred to as dewetting and is the focus of the second piece of this dissertation. Micro-pillar pitch, height and temperature play a role on dewetting dynamics. The results show that dewetting rates increase with increasing pillar height and increasing surface temperature. A scaling model is constructed to obtain an explanation for the experimental observations and suggests that increasing pillar height increasing the driving dewetting force, while increasing surface temperature decreases dissipation. The last piece of work of this dissertation entails droplet impingement on superheated surfaces (100°C - 400°C). We find that the Leidenfrost point (LFP) occurs at a lower temperature on a hydrophobic surface than a hydrophilic one, where the LFP refers to the lowest temperature at which secondary atomization ceases to occur. This behavior is attributed to the manner in which vapor bubbles grow at the solid-liquid interface. Also in this work, high-speed photographs reveal that secondary atomization can be significantly suppressed on a superhydrophobic surface owing to the micro-pillar forest which allows vapor to escape hence minimizing bubble formation within the droplet. However, a more in-depth study into different superhydrophobic texture patterns later reveals that atomization intensity can significantly increase for small pitch values given the obstruction to vapor flow presented by the increased frequency of the pillars.

Spray cooling has proven to be efficient in managing thermal load in high power applications. Reliability of electronic products relies on the thermal management and understanding of heat transfer mechanisms including those related to spray cooling. However, to date, several of the key heat transfer mechanisms are still not well understood. An alternative approach for improving the heat transfer performance is to change the film dynamics through surface modification. The main goal of this study is to understand the effects of nano-scale features on flat heater surfaces subjected to spray cooling and to determine the major factors in droplet impingement cooling to estimate their effects in the spray cooling system. Single droplet stream and simultaneous triple droplet stream with two different stream spacings (500 μm and 2000 μm), experiments have been performed to understand the droplet-surface interactions relevant to spray cooling systems. Experiments have been conducted on nano-structured surfaces as well as on flat (smooth) surfaces. It is observed that nano-structured surfaces result in lower minimum wall temperatures, better heat transfer performance, and more uniform temperature distribution. A new variable, effective thermal diameter (de), was defined based on the radial temperature profiles inside the impact zone to quantify the effects of the nano-structured surface in droplet cooling. Results indicate that larger effective cooling area can be achieved using nano-structured surface in the single droplet stream experiments. In triple stream experiments, nano-structured surface also showed an enhanced heat transfer. In single stream experiments, larger outer ring structures (i.e. larger outer diameters) in the impact crater were observed on the nano-structured surfaces which can be used to explain enhanced heat transfer performance. Smaller stream spacing in triple stream experiments reveal that the outer ring structure is disrupted resulting in lower heat transfer. Lower static contact angle on the nano-structured surface has been observed, which implies that changes in surface properties result in enhanced film dynamics and better heat transfer behavior. The results and conclusions of this study should be useful for understanding the physics of spray cooling and in the design of better spray cooling systems.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23503号 / 工博第4915号 / 新制||工||1768(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 黒瀬 良一, 教授 花崎 秀史, 教授 岩井 裕 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

In this work, a two-phase lattice Boltzmann method (LBM) approach is implemented to investigate the hydrodynamic behavior of a single droplet impingement on a dry surface. LBM is a recently developed powerful technique to compute a wide range of fluid flow problems, especially in applications involving interfacial dynamics and complex geometries. Instead of solving the non-linear Navier-Stokes equations, which are complicated partial differential equations, LBM solves a set of discretized linear equations, which are easy to implement and parallelize. The fundamental idea of LBM is to recover the macroscopic properties of the fluid which obeys Navier-Stokes equations, by using simplified kinetic equations that incorporate the essential physics at the microscopic level. Considering the numerical instability induced by large density difference between two phases during the LBM simulations, the particular LBM scheme used in this study has its benefits when dealing with high density ratios. All the simulations are conducted for density ratio up to 50 in a three-dimensional Cartesian coordinate system, and three important dimensionless numbers, namely Weber number, Reynolds number and Ohnesorge number, are used for this study. To validate this multiphase LBM approach, several benchmark tests are conducted. First, the angular frequency of an oscillating droplet is calculated and compared with the corresponding theoretical value. Errors are found to be within 6.1% for all the cases. Secondly, simulations of binary droplet collisions are conducted in the range of 20