Ichiro Tanasawa

Advances in condensation heat transfer

Publisher Summary Condensation represents the change of phase from the vapor state to the liquid state because of cooling. It is considered one of the most important heat-transfer processes in many energy-conversion systems, such as electric power generation plants. This chapter emphasizes on the areas of condensation heat transfer that have made progress in the past 15 years. It introduces various types of condensation and examines the wettability of the surface. The transport process at the vapor-liquid interface and the arguments on whether the condensation coefficient takes the value of unity are discussed. Furthermore, the chapter reviews dropwise condensation and film condensation in detail. It also describes the techniques of enhancement of condensation heat transfer. The usage of surface tension force is one of the most sophisticated ways for augmentation of condensation because it does not require extra energy. The chapter concludes with discussion of future trends in research on condensation heat transfer.

Onset of oscillatory flow in a Czochralski growth melt and its suppression by magnetic field

Abstract Parameter sensitivity on the onset of oscillatory flow and the effect of magnetic field on such an oscillatory flow in the Czochralski melt are investigated. A defect in the crystal known as growth striations seems to be caused by a macroscopic oscillation of the flow, heat and mass transfer in the melt. An oscillatory flow is observed in a model Czochralski melt under a certain condition. It is due to interaction between natural and forced convection in the rotating container. Similar oscillation of flow and temperature fields is also obtained by numerical simulation. The result reveals that the oscillatory flow in the melt is inherent to the system. The critical Reynolds number for the onset of oscillation and the period of oscillation are decreased with the increasing Prandtl number and Rayleigh number. Further, it is confirmed that the oscillatory flow is possible to occur in the real system and can be suppressed by applying the external magnetic field.

Study on silicon melt convection during the RF-FZ crystal growth process: II. numerical investigation

Silicon melt convection during the floating zone (FZ) crystal growth process under radio-frequency (RF) heating and the effect of the externally applied magnetic field on the RF-FZ silicon melt convection have been investigated numerically. The main purpose of the study is to clarify the characteristics of the silicon melt convection under the RF heating and the effect of externally applied magnetic field on such melt convection. The numerically obtained flow characteristics are almost the same as previously reported experimental results. Further, the fluctuation of melt convection disappears when an external vertical magnetic field is applied. The required minimum magnetic flux density to suppress the convection fluctuation is around 0.2 T.

Study on silicon melt convection during the RF-FZ crystal growth process: I. experimental flow visualization

Abstract Silicon melt convection during the floating zone (FZ) crystal growth process under radio-frequency (RF) heating has been observed directly by using an X-ray radiography system with particle tracking velocimetry (PTV) in order to investigate the effect of the RF-induced electromagnetic field on the silicon melt convection. The result reveals that the silicon melt convection is fully affected by the RF induced electromagnetic field and the direction of convection becomes opposite to the natural convection. The obtained maximum velocity was 0.1xa0m/s which was 4–5 times higher than CZ system and the velocity field is fluctuated randomly. This will probably cause strong striation in the crystal.

Active enhancement of evaporation of a liquid drop on a hot solid surface using a static electric field

Abstract This study aims at further improvement of evaporation or boiling heat transfer using an electric field. Active enhancement using an electric field would be extremely effective in film boiling, but favorable effects would be expected on nucleate and transition boiling, and on the maximum and minimum heat fluxes. In this paper the results of our preceding studies are first summarized. We have found that the application of an electric field enhances, to a great extent, the evaporation of a liquid drop on a hot solid surface. The time needed for a single drop to evaporate completely reduces to one-twentieth of that for evaporation without an electric field, when 300 V is applied to a drop of ethanol. The results of an additional experiment are presented in this paper. The heat transfer coefficient during evaporation of a liquid drop on a platinum surface is measured while the volume of the drop suspended at the tip of a thin needle nozzle is kept constant. The voltage applied between the drop and the heat transfer surface is varied from 0 V to 2000 V (for R113) or to 250 V (for ethanol). The maximum enhancement ratios of the evaporative heat transfer coefficients at the highest voltages are 7.6 for ethanol and 2.8 for R113.

Experimental techniques in natural convection

Experimental techniques in natural convection heat transfer employed in the authors laboratory are introduced. The techniques are mostly related to visualization of flow, temperature field, and heat flux distribution in fluids. Three topics are presented, the first being natural convection in a horizontal rectangular liquid layer driven by surface tension and buoyancy. The patterns of flow were visualized by suspending fine aluminum flakes in the liquid. At the same time, the distribution of the temperature gradient in the liquid was visualized by an optical method making use of the refraction of light. The second topic is the onset of oscillatory convection in the Czochralski growth melt. In this case a forced flow due to rotation of the crystal and the vessel is superimposed on the buoyancy convection, resulting in an oscillatory flow under certain circumstances. The flow pattern and the temperature distribution in the liquid were visualized simultaneously by suspending in the liquid a microencapsulated temperature-sensitive liquid crystal. Periodical oscillation of the flow and the temperature was clearly recognized. The third topic is the rollover of double liquid layers that were stratified stably due to a density difference. A small-scale experiment was carried out to clarify the basic mechanism of rollover. The tracer method was used to visualize boundary layer flow along the vertical side wall and the shadowgraph technique to visualize the density distribution in the liquid layers. The article emphasizes the importance of visual observation in the investigation of natural convection phenomena.

A study on the effect of non-condensible gas in the vapor film on vapor explosion

Abstract The effect of the non-condensible gas contained in the vapor film during vapor explosion is investigated. Vapor explosion, on a small scale, is initiated by dropping a globule of molten tin into water. Quite different phenomena are observed when the space above the pool of water is filled with steam instead of an air-steam mixture, indicating that the non-condensible gas mixed in the vapor film affects vapor explosion greatly and makes the process stochastic.

Facilitated diffusion of carbon dioxide in whole blood and hemoglobin solutions

The values of effective permeability (Kroghs diffusion coefficient) for carbon dioxide have been measured in horizontal stationary layers of whole blood and hemoglobin solutions in quasi-steady state, with the goal of understanding the specific nature of facilitated diffusion of carbon dioxide occurring in these media. The average partial pressure of carbon dioxide within the layer ranged from 0.74 kPa (5.6 mm Hg) to 15.7 kPa (118 mm Hg). Facilitation effects were significant in hemolysed blood and in hemoglobin solutions at low pCO2; the facilitation factor was up to 2.3. Facilitation effects were considerably less for intact blood; the facilitation factor of intact blood with hematocrit 45% was 0.3. The presence of the red cell membrane appears to have a negative effect on facilitation of carbon dioxide transport.

Two-dimensional Marangoni and buoyancy convection related to crystal growth techniques in space

Abstract The natural convection in the horizontal liquid layer driven both by surface tension gradient and buoyancy was investigated experimentally and theoretically. The fluid motion was visualized by mixing fine flakes of aluminum into the liquid. At the same time, the temperature field was visualized by the Mach-Zehnder interferometry and the local temperature gradient distributions were visualized by making use of the refraction of parallel incident light. The numerical analysis using the Galerkin method was also carried out which agreed well with the experimental results. It was found that the velocity and temperature fields of Marangoni convection were varied depending on the aspect ratio of the liquid layer and on the coupled buoyancy convection.

Study on EHD convection

EHD convection in a dielectric liquid which is subjected to a nonuniform dc electric field is analysed both experimentally and theoretically. Velocity and temperature fields are visualized using liquid crystal capsules and making use of laser light refraction. An EHD model, which agrees with experimental results, is proposed and a computer simulation of EHD convection is carried out. Finally, the generation of space charge is discussed.