Jun Fukai

Wetting effects on the spreading of a liquid droplet colliding with a flat surface: Experiment and modeling

In this paper an experimental and theoretical study of the deformation of a spherical liquid droplet colliding with a flat surface is presented. The theoretical model accounts for the presence of inertia, viscous, gravitation, surface tension, and wetting effects, including the phenomenon of contact‐angle hysteresis. Experiments with impingement surfaces of different wettability were performed. The study showed that the maximum splat radius decreased as the value of the advancing contact angle increased. The effect of impact velocity on droplet spreading was more pronounced when the wetting was limited. The experimental results were compared to the numerical predictions in terms of droplet deformation, splat radius, and splat height. The theoretical model predicted well the deformation of the impacting droplet, not only in the spreading phase, but also during recoiling and oscillation. The wettability of the substrate upon which the droplet impinges was found to affect significantly all phases of the spre...

Thermal conductivity enhancement of energy storage media using carbon fibers

Carbon fibers with a high thermal conductivity are used to enhance the thermal conductivities of energy storage media. Two types of enhancement techniques were studied. One is the technique using randomly oriented fibers, while the other is that of using a fiber brush. The carbon fibers essentially enhance the effective thermal conductivity of fiber/paraffin composites. For the random type, the fiber length has little effect on the effective thermal conductivity. The fiber brush increases the effective thermal conductivities to the maximum values predicted which is theoretically. The fiber brush is also useful for enhancing the effective thermal conductivities of packed beds of particles.

Modeling of the deformation of a liquid droplet impinging upon a flat surface

This article presents a theoretical study of the deformation of a spherical liquid droplet impinging upon a flat surface. The study accounts for the presence of surface tension during the spreading process. The theoretical model is solved numerically utilizing deforming finite elements and grid generation to simulate accurately the large deformations, as well as the domain nonuniformities characteristic of the spreading process. The results document the effects of impact velocity, droplet diameter, surface tension, and material properties on the fluid dynamics of the deforming droplet. Two liquids with markedly different thermophysical properties, water and liquid tin, are utilized in the numerical simulations because of their relevance in the industrial processes of spray cooling and spray deposition, respectively. The occurrence of droplet recoiling and mass accumulation around the splat periphery are standout features of the numerical simulations and yield a nonmonotonic dependence of the maximum splat radius on time.

Heat transfer and fluid dynamics during the collision of a liquid droplet on a substrate—II. Experiments

Abstract This paper presents a numerical study of the fluid dynamics and heat transfer phenomena during the impingement of a liquid droplet upon a substrate. The theoretical model, based on the Lagrangian formulation, is solved numerically utilizing the finite element method. A deforming mesh is utilized to simulate accurately the large deformations, as well as the domain nonuniformity characteristic of the spreading process. The occurrence of droplet recoiling and mass accumulation around the splat periphery are standout features of the numerical simulations and yield a nonmonotonic dependence of the maximum splat radius on time. The temperature fields developing in both the liquid droplet and the substrate during the impingement process are also determined. To this end, liquid metal and water droplet collisions on different substrates were investigated. Convection effects on the temperature field development were found to be important for the entire history of spreading. These effects resulted sometimes in a practically radial temperature variation at late stages of spreading, particularly so in the cases of high impact velocities.

Effect of carbon-fiber brushes on conductive heat transfer in phase change materials

Brushes made of carbon fibers are used to improve the thermal conductivities of phase change materials packed around heat transfer tubes. The transient thermal responses measured in brush/n-octadecane composites essentially improve as the volume fraction of the fibers and the brush diameter increase. However, there is a critical diameter above which further improvement is not expected due to thermal resistance between the fibers and the tube surface. A two-dimensional heat transfer model describing anisotropic heat flow in the composite is numerically solved. The calculated transient temperatures agree well with the experimental. A simple model is also developed to predict the heat exchange rate between the composite and the heat transfer fluid. The values of the correction factors are identified on the basis of the results for the anisotropic model.

Improvement of thermal characteristics of latent heat thermal energy storage units using carbon-fiber brushes: experiments and modeling

Brushes made of carbon fibers with a high thermal conductivity are inserted on the shell side of a heat exchanger to enhance the conductive heat transfer rates in phase change materials. The experimental results show that the brushes essentially improve the heat exchange rate during the charge and discharge processes even when the volume fractions of the fibers are about one percent. A three-dimensional model describing the heat transfer in the heat exchanger is numerically solved. The model predicts well the experimental outlet fluid temperatures and the local temperatures in the composite.

Internal Flow in Polymer Solution Droplets Deposited on a Lyophobic Surface during a Receding Process

When a polymer solution droplet is deposited on a lyophobic surface, the contact line is moved back to some degree and subsequently pinned. An experimental setup is constructed to investigate not only the receding process but also an internal flow of polystyrene-acetophenone and -anisole solutions. As a result, the time variation of the evaporation rate per unit area during receding does not strongly depend on the initial solute concentration. The average solute concentration at the pinning of the contact line increases as the initial solute concentration increases. A convective circulation flow that is upward at the axis of symmetry is observed. This flow pattern is different from those of pure liquids such as water, acetone, benzene, and so forth, which have been previously reported. Furthermore, the observed flow is enhanced as the initial solute concentration increases, contrary to an increase in the fluid viscosity. To resolve these discrepancies, the mechanism of the flow is numerically investigated using a hemispherical droplet model considering the density and surface tension distributions. The numerical results demonstrate that the circulation flow that is experimentally observed is actually caused. It is also found that the solutal Rayleigh effect initially induces the internal flow, and subsequently the solutal Marangoni effect dominates the flow. Both effects are enhanced as the initial concentration increases because of the evaporative mass balance at the free surface.

Structure of circulation flows in polymer solution droplets receding on flat surfaces

In a previous report where internal flows were experimentally visualized in polymer solution droplets receding on a lyophobic surface [Kaneda et al., Langmuir 2008, 24, 9102-9109], the direction of the circulation flow was found to depend on solvent and solute concentration. To identify the reason for this finding, the internal flow in the droplet is investigated numerically. A mathematical model predicts that double circulation flows initiate after a single flow develops at high Marangoni numbers, while only a single circulation flow develops at low Marangoni numbers. The dependencies of the calculated velocities on the solvent and the initial solute concentration agree qualitatively with experiment. It is concluded that the difference of the flow directions that were investigated experimentally is due to such a change in the flow structures. The effects of the contact angle and dimensions on transport phenomena in a droplet are also discussed.

Optimized combustion of biomass volatiles by varying O2 and CO2 levels: A numerical simulation using a highly detailed soot formation reaction mechanism

To increase syngas production and minimize soot, polycyclic aromatic hydrocarbon (PAH), and CO(2) emissions resulting from biomass combustion, the evolution of biomass volatiles during O(2)/CO(2) gasification was simulated. A highly detailed soot formation reaction mechanism flowing through the reactor, involving 276 species, 2158 conventional gas phase reactions and 1635 surface phase reactions, was modeled as a plug flow reactor (PFR). The reaction temperature and pressure were varied in the range 1073-1873K and 0.1-2MPa. The effect of temperature on product concentration was more emphasized than that of pressure. The effect of O(2)/CO(2) input on product concentration was investigated. O(2) concentration was important in reducing PAHs at low temperature. Below 1473K, an increase in the O(2) concentration decreased PAH and soot production. However, if the target of CO(2) concentration was higher than 0.22 in mass fraction terms, temperatures above 1473K reduced PAHs and increased CO.

Simultaneous estimation of thermophysical properties by periodic hot-wire heating method

Abstract A new unsteady heat flux method is proposed to measure thermal conductivity and diffusivity simultaneously, the principle based on an analytical solution for an infinite hollow cylindrical system with a periodic heat source in the center. Thermal conductivity and diffusivity are determined from the amplitude and phase lag of the temperature response within the cylinder. The method also makes it easy to measure the temperature dependency of thermal properties during a continous heating process. The measurement errors caused by the finite specimen size and the displacement of the thermocouple location are estimated numerically to confirm the accuracy of the measurement method. Effective thermal conductivity and diffusivity for the packed beds of aluminum oxide particles and potassium perchlorate particles are measured. The thermophysical properties measured by this method agree well with those measured by conventional methods such as the hot-wire, periodic heating, and continuous heating methods.