Nanofluids are metallic or nonmetallic, nanometer-sized particles dispersed in liquid. They can be used in various fields to increase heat transfer rates, as the thermal conductivity of nanofluids can be increased significantly. Nanofluids may be used as a good alternative coolant in spray cooling applications. This study conducted experiments to compare spray characteristics, such as droplet diameters and velocities, between water and alumina nanofluid sprays. The mass ratio of alumina nanoparticles was varied from 0.2 to 0.5 weight percentages (wt.%) and the spray injection pressure was varied between 0.2 and 0.3 MPa. The local distributions of droplet sizes and velocities along the spray axial and radial directions were measured by a laser doppler instrument. Generally, the spray characteristics of nanofluid sprays is significantly different from that of water sprays. The average droplet diameters of the fluids tested increased in an approximately linear manner with the increase in the mass ratio of nanoparticles up to 0.4 wt.%, whereas the average droplet velocities decreased. In the case of the nanofluid spray of 0.5 wt.%, the increase in droplet diameters and the decrease in droplet velocities were much more marked, departing from the linear relationship. This unusual behavior could also be observed in the local distributions of droplet diameters and velocities along the axial and radial directions. Further research studies are required to reveal how the addition of nanoparticles affects the atomization mechanism of nanofluids. The difference in the spray characteristics of nanofluid sprays from that of water sprays should be taken into consideration when the cooling effectiveness of nanofluids and water in spray cooling is compared.

The drop impact onto porous surfaces has important applications in many fields, such as painting, paper coating,drug delivery and cosmetic sprays. In most of these applications, the optimisation of the deposition process is carried out empirically, without a proper understanding of the physics and a theoretical modelling of the spreading and the imbibition phenomena. The purpose of this study is to analyse droplet impacts on metallic meshes to define a general modelling strategy of the impact regimen on particular 2D regular porous surfaces. The application of this structure is relevant in process like filtration but also in the medical field, considering for example reconstructive surgery. By analysing the impact of droplets of water, acetone and a mixture of glycerol and water, having a diameter and an impact velocity in a range of 1.5-3mm and 2-4m/s, respectively, on meshes with a pore size ranging between 25 and 400 µm, a regime map was built considering 6 different impact outcomes. The outcomes were characterised by a deposition of the droplet on the substrate, or a partial imbibition, or a total imbibition. By increasing the impact velocity, a splash region was defined, which is still characterised by a final deposition, a partial imbibition and a total imbibition. It is found that the most influencing parameters are closely linked to the liquid properties and the impact velocity, more specifically liquid surface tension plays a major role in defining the impact outcome. In the case of Acetone, the lower surface tension brings to an almost instantaneous total imbibition whereas the experiments conducted using water and glycerol solution, showed a major distribution of the deposition regimes with respect to the other outcomes, due to the effect of a higher viscosity. It was found that the geometrical characteristics of the mesh such as pore size and wire diameter, play an important role as well in defining the total imbibition outcome. Finally, the defined transition maps, shows that for a certain combination of physical properties and initial condition,the outcome of the droplet impact is predictable.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4973

In this study, the evaporation characteristics of water or nanofluid droplets on a heated surface was investigated by visualization of the evaporation process and evaluation of the heat transfer coefficient using the droplet temperature measured. The evaporation characteristics was compared between water and nanofluid droplets and the effects of the mass ratio of nanofluid and the inclination of heated surface were analyzed. The heat transfer rate of nanofluid droplet was higher than that of water droplet. The heat transfer coefficient was increased with the increase of the mass ratio of nanofluid. The effect of the inclination of heated surface was much higher than that of fluid type used, which indicates that the inclination of heated surface should be considered as one of influential parameters in the spray cooling process.

In this study digital holographic microscopy system for measurements of 3-D velocities of particles in MR fluid is developed. Holograms are recorded using either a CCD camera with a double pulse laser or a high-speed camera with a continuous laser. To process recorded holograms, the correlation coefficient method is used for focal plane determination of particles. To remove noise and improve the quality of holograms and reconstructed images, a Wiener filter is adopted. The two-threshold and image segmentation methods are used for binary image transformation. For particle pairing, the match probability method is adopted. The developed system will be applied to measurements of the characteristics of unsteady 3D particle velocities in MR fluids through the next stage of this study.

This paper experimentally studied the dynamic behavior of a droplet impacting upon a liquid film, by investigating the effects of the droplet velocity and thickness of the liquid film on the impact behavior of the droplet. The formation of the crown, central jet, and disintegrating droplet from the central jet were visualized by time-delay photography. The time evolutions of the diameter and height of the crown and the height of the central jet were obtained, and the size of the disintegrating droplet from the central jet was measured. The crown diameter and the central jet height were mostly affected by the droplet velocity and the thickness of the liquid film, respectively, while the crown height was influenced by both the droplet velocity and the thickness of the liquid film. The diameter and height of the crown were higher for the case of the faster impacting droplet and thinner liquid film. On the other hand, the height of the central jet was higher for the case of the faster impacting droplet and thicker liquid film. The size of the disintegrating droplet from the central jet heavily depends on the velocity of the impacting droplet. Namely, a larger droplet is produced by a faster impacting droplet.

In this paper, the effects of a step edge and a stationary droplet on the dynamic behavior of a droplet impacting upon a wall are experimentally studied. The main parameters were the distance from the step edge to the center of the impacting droplet and the center-to-center distance between the stationary and impacting droplets. Photographic images showed the coalescence dynamics, shape evolution and contact line movement of the impacting droplet. The spread length is presented for the step edge and two coalescing droplets. The droplets exhibited much different dynamic behavior depending on the location of the step edge. The momentum of the impacting droplet was better transferred to the stationary droplet as the center-to-center distance between the two droplets was reduced, resulting in more spreading of the coalescing droplet.

In this study, the behavior of water or nanofluid droplets impacting upon a hot surface was investigated by visualization of impacting phenomena with time-delayed photographic technique. Changing the mass ratio of nanofluid and the temperature of the heated surface, the characteristics of the spreading behavior and the diameter of spreading liquid film was compared between water and nanofluid droplets. The impacting droplet spreaded as a liquid film after impact and nanofluid droplets spreaded more widely than water droplets. After reaching the maximum diameter, water droplets shrinked more than nanofluid droplets. Based on this, the heat transfer area from a hot surface to impacting nanofluid droplets would be wider than that of impacting water droplets. Considering individual impacting droplet only, spray cooling using nanofluid would be better than using water.

Nanofluids are that metallic or nonmetallic nanometer-sized particles are dispersed in liquid. They can be used in various fields to increase the heat transfer rate because the thermal conductivity of nanofluids can be increased significantly. Nanofluids may be used as a good alternative of coolants in spray cooling. This study conducted experiments to compare the characteristics of sprays between water and nanofluid sprays. The radial distributions of droplet velocities and diameters of water, 0.2% wt.(weight), and 0.5% wt. $Al_2O_3$ nanofluids at the pressure of 0.2 and 0.3 MPa were measured by laser doppler instruments. The radial distributions of droplet diameters and velocities at two axial positions with water and 0.2% wt. nanofluid sprays didn't show much difference. A big difference, however, was observed between 0.5% wt. nanofluid and water sprays. With the increase of the mass of nano-particles, the average droplet diameters were increased and the average droplet velocities were decreased.

Digital holographic particle velocity measurement system can be a promising optical tool for the measurements of three dimensional particle velocities. In this research, validation experiments for the digital holographic particle velocity measurement system were conducted with measuring the velocities of glass beads on a rotating disk. Uncertainty analysis was performed to identify the sources of all relevant errors and to evaluate their magnitudes. The measurement results of particle velocities obtained with digital holographic method are compared reasonably well with the known values within acceptable range of errors. Moreover, digital holographic method showed better performance compared with that of optical holographic system.

In this paper an experimental study is presented to investigate the dynamic behavior of impacting droplet onto a liquid film. The main parameters are the droplet velocity and the thickness of the liquid film.Photographic images are presented to show the formation of crown, central jet and disintegrating droplet from the central jet. The emphasis is on presenting the time evolution of crown diameter, crown height, central jet height and the size of disintegrating droplet from the central jet. The diameter and height of crown are higher for faster droplet and thinner liquid film. On the other hand, the height of central jet are higher for faster droplet and thicker liquid film. The size of disintegrating droplet from the central jet heavily depends on the droplet velocity; Larger droplet is produced with faster falling droplets.

Lensless in-line digital particle holography (DPH) can be thought of as a special case of photography, and its depth of focus (DoF) characteristics combine those of both photography and holography. The effect of important parameters such as the pixel size and the number of pixels in a charge-coupled device (CCD), the object's distance, the wavelength of the laser, and the particle diameter on the DoF of a reconstruction image in lensless in-line DPH was investigated using simulation holograms. The DoF is directly proportional to the object distance and the particle size. As the wavelength of the laser increases, the DoF decreases. The DoF is inversely proportional to the pixel size and to the number of pixels in a CCD. Instead of expressing nonspecific proportional relationships for DoF, the DoF prediction equations for typical CCD cameras were obtained using DoF data from many simulation holograms and efficient data-fitting software. Finally, the DoF prediction equations were verified using real holograms from a calibration target for several cases. Good agreement between measured and predicted values was confirmed.

To enhance the capability of digital particle holography as a tool for flow field measurements, several effective methods are developed. The correlation coefficient method was used to accurately locate the focal plane of particles and the optimal factors of this method were discussed. To remove noises and improve the quality of holograms and reconstructed images, the Wiener filter was adopted. The two-threshold and image segmentation methods were used to obtain high quality binary images from which we can get good results of particle extraction. Based on the above methods, an in-line digital particle holographic system was applied to channel flow field and the axial velocities of channel flow were measured. The feasibility of these methods is verified by quantitative measurement results which are in good agreement with the theoretical predictions.

In this study, a digital particle holographic system and its application to channel-flow measurements were investigated. A double-exposure hologram recording system that is capable of recording digital holograms in a short time interval was developed. A correlation coefficient method was used to determine the focal plane of particles. The Wiener filter was used to remove noises and improve image quality. Two-threshold and image segmentation methods were used for binary image transformation. The cross-correlation method was used for particle pairing. The developed system was employed to study channel flow fields, and the axial velocities of channel flow were measured. The measurement errors are acceptable, and this proves the feasibility of using the digital particle holographic system as a good tool for flow-field measurements.

In-line digital particle holography is applied to measure the characteristics of spray droplets. Common reconstruction methods were considered and the best one was selected. Several important parameters at the time of hologram recording, such as the object distance and the region of laser beam used, are discussed. The feasibility of the correlation coefficient (CC) method for focal plane determination of 3-D droplets was verified. A double exposure hologram recording system with synchronization system for time control was established, and two digital spray holograms were obtained in a short time interval. For post-processing of reconstruction images, the two-threshold and the image segmentation methods were used in binary image transformation. Using the CC method and some image processing techniques applied to droplets in each double exposure image, the spatial positions of droplets used to evaluate the three dimensional droplet velocities were easily located, which proved the feasibility of in-line digital particle holographic technology as a good measurement tool for spray droplets.

A modified 3-omega method applied to a suspended platinum microwire was employed to measure the thermal conductivity and convective heat transfer coefficient of a water-based single-walled carbon nanotube (CNT) solution (metallic single-wall nanotubes with 1.33 nm diameter and 1.14 wt% concentration), and an expression for calculating the convective heat transfer coefficient in such a free convective fluid was introduced. The measurement technique was validated for three model systems including vacuum, air and deionized water. It is found that there is excellent agreement between these three model systems with theoretical predictions. In addition, the frequency dependence on the third harmonic response measured in deionized water reveals the existence of a very low working frequency below 60 mHz. The thermal conductivity and convective heat transfer coefficient of the nanofluid (water-based single-wall CNT solution) were determined to be 0.73 +/- 0.013 W m(-1) K-1 and 14 900 +/- 260 W m(-2) K-1, respectively, which correspond to an enhancement of 19.4% in thermal conductivity and 18.9% in convective heat transfer as compared to water.

Periodic cellular metals with open, periodic cell topologies have received much attention owing to their potential for multi-functionality such as load bearing, thermal dissipation, and actuation. Recently, a new technique, known as wire-woven bulk Kagome, has been introduced, which is used for fabricating multi-layered Kagome with truss periodic cellular metal. The fabrication of the wire-woven bulk Kagome is based on a concept where continuous helical wires are systematically assembled in six directions. Besides its excellent load-bearing capability with light weight, the wire-woven bulk Kagome has potential for a heat dissipation media because of the high ratio of surface area to volume and low flow resistance. This article presents the experimental results of the fluid flow and heat transfer characteristics of the multi-layered wire-woven bulk Kagome composed of aluminum 1100 helix wires. Under forced-air convection conditions, the friction factor and heat transfer rate of the wire-woven bulk Kagome specimen were investigated for two specimen orientations. The results showed that the friction factor of the wire-woven bulk Kagome was mainly affected by the flow blockage area, and the heat transfer characteristics depended on the open-area ratio. In addition, the results were compared with other heat dissipation media (e.g., open-celled foams, woven screens, lattice-frame materials, and cast Kagome structures). The results showed that the heat transfer performance of the multi-layered aluminum wire-woven bulk Kagome competed favorably with the best heat dissipation media currently available.