Experimental Observation of Nucleate Boiling Entrainment in a Liquid Film

Many studies have been conducted on droplet entrainment in an annular flow regime, but little is known about droplet entrainment caused by nucleate boiling. In this report, visualization results of droplet entrainment caused by nucleate boiling are described. We observed two processes of droplet entrainment. The first one causes bubble bursting at a water surface. The second one causes filament breakup which occurs when the vapor bubble reaches and collapses at the interface between air and liquid. From comparison of the phenomena for the two processes, we found that the diameters of the droplets and vapor bubbles were considerably different. Using the results of this research allows the effect of forced convection to be taken into account. In the future, we plan to expand the amount of data and develop a boiling entrainment model under forced convection conditions.

Anilox Cell Geometries For Printable Electronics and Flexible Packaging

Flexography is a major high-volume printing process used extensively for flexible packaging. The heart of flexographic press is the anilox roll, which meters the flow of ink to the image carrier (plate) by virtue of the engraved cells on the surface. The anilox was original engraved mechanically using a stylus to peck at the surface. This limited the size and profile of the engraved cells. However, laser engraving has enabled much more control with a variety of shapes and aspect ratios. Much has been claimed by the manufacturers for these new designs – improved ink transfer, higher volumes of ink transfer and better half tone reproduction – on the basis of industrial field trials. The objective of the research reported in this thesis has been to quantify the ink release from the anilox to the plate for both traditional cell profiles and the open channel designs.Previously, the ink release was mostly determined by examining the optical density of the print products. The optical density is a qualitative indicator of the ink release from anilox cells. These studies were limited to closed anilox cells with a low ink viscosity, as typically used for graphic prints. This study explores an extended range of anilox cell shapes, including open channel geometries, and the ink viscosities. The ink released from the anilox cells has been be directly measuring and quantified.Experiments were performed printing directly to glass and on flexible packaging at a commercial printers to establish the current industry position. A laboratory scale printability tester was then used to study ink release using three inks: UV Cyan, Carbon and Silver. These represented a link to the graphic experiments in previous published studies, while the Carbon and Silver were highly viscoelastic functional inks used in printed electronics. Four cell geometries were used: laser engraved closed cells, extended hexagonal and wavy channels together with mechanically engraved conventional closed pyramid cells. The laser engraved anilox afforded the opportunity to vary key parameters of cell width, depth, profile and volume. A brief exploration of print speed was also undertaken with exemplar anilox of each cell type. The main study considered ink transfer to a 100% solid plate, as this would allow the ink release to be studied without influence of the plate distorting into the cells thereby extracting more ink. A limited study was then undertaken with a half tone plate to establish the impacted on ink transfer.The amount of ink transferred was highly dependent on the absolute volume of cells, i.e. the amount of ink available on the anilox. The anilox cells with wider, shallower and smaller depth-to-width ratio released a higher proportion of the ink. The ink’s physical characteristics of viscoelasticity and extensional viscosity also determine the proportion of ink transferred.The anilox hexagonal closed cells (typically used in the flexographic printing process) performed best with the low viscosity ink. The information gaining from this study would aid in the design of anilox cell geometries and development of ink characteristics to enhance its capability for functional print applications such as printable electronics. The anilox wavy channels released the greatest proportion of the ink with high viscosity, elastic modulus, and filament breakup time. The anilox wavy channel has the potential to be used for the functional print as it increased the release of paste-like ink. Additionally, it improved the ink lay-down. The anilox engraving technique affected the ink release. The anilox cells, which were engraved by the laser technique, gave greater ink release comparing to the anilox cells, which were engraved by the mechanically engraving technique. The increase of the dot coverage increased the ink release out of the anilox cells because of the increase of the receiving area. However, the increase of the ink release plateaued after the dot coverage of 50% for UV Silver because of its large filament breakup time.The characteristics of ink influenced the ink release out of the anilox cells. Unlike previous work which examined only the ink viscosity, this studied included ink elastic modulus and filament breakup. The ink with high viscosity and elastic modulus, but small filament breakup time gave greatest ink release for all anilox shapes.When the printing speed increased, it decreased the ink release due to two factors; reduction of engagement time between the anilox cells and the plate (reducing time for ink to transfer) and enlargement of the filament extension rate (reducing the amount of ink transfer). The decrease of ink release was affected by the ink characteristics and the anilox cells shapes. The decrease of ink release was significant when UV Cyan ink (small viscosity and elastic modulus) was used with the anilox open cells and wavy channels. Contrarily, the decrease of ink release was insignificant when UV Carbon and Silver inks (large viscosity and elastic modulus) was used with the anilox open cells and wavy channels.The experimental data was analysed and the critical parameters in releasing the ink of the anilox cells were identified. The depth of anilox cell was the most critical parameter; the shallower cell depth released a higher proportion of the ink. The ratio of depth-to-width was the second most important parameter in determining the ink release. The smaller depth-to-width ratio released more ink. The width of anilox cell could not be used as a parameter predicting the ink release because the wider anilox cell did not always release the higher proportion of the ink.

Effects of Pressure on Collision, Coalescence, and Breakup of Raindrops. Part I: Experiments at 50 kPa

Previous breakup experiments have been carried out at laboratory pressures (∼100 kPa). However, raindrop interactions mainly take place higher up in the atmosphere, even in the supercooled part of a cloud where drops can be initiated by shedding from hailstones. Thus, 50 kPa, corresponding to a height of ∼5.5 km in the atmosphere at a temperature of ∼−20°C, was selected to bracket the region of interest for rain. Six drop pairs were studied at 50 kPa and laboratory temperature (∼20°C), one of them with reduced surface tension. The apparatus consists of drop-producing nozzles, acceleration systems, deflectors, a timing and selection control, a pressure regulator, and a photographic unit, mostly set up in a low-pressure chamber. After acceleration to terminal speed, a smaller drop is blown into the path of the larger one while an electronic timing system selects suitable drop pairs that may collide, thereby triggering eight subsequent flashes with a frequency of up to 100 kHz. The results are displayed in terms of a normalized fragment probability per size bin, ready for parameterization in the Part II of this paper. Five drop pairs were studied in 772 individual events. Overall, 51% resulted in filament breakup, 22% in sheet breakup, 7% in disk breakup, and 20% ended in coalescence. No bag breakups were observed. When compared to the 100-kPa results, the fragment numbers increased at large collision kinetic energies (CKEs) by factors of between 2.64 and 4.37 with pressure decreasing from 100 to 50 kPa, and they remained unchanged at low CKE. Detailed diagrams and tables show the results for the different drop pairs and collision categories. Increasing the sensitivity of the optical measurements from 0.05 to 0.01 cm increased the number of recognized fragments by factors up to 4.4, but only for the two higher-CKE cases. The higher resolution did not increase the fragment numbers detected in the lower-CKE range.