Formation mechanism and characteristics of micro layer in micro-gap boiling system

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Experiments were performed using laser extinction method to measure the thickness of the liquid layer formed by growing flattened bubbles between two parallel plates for gap sizes of 0.5, 0.3, and 0.15 mm. Water, toluene , ethanol and HFE7200 were used as the test fluid. A high-speed camera was used to simultaneously measure the bubble growth process and determine the bubble forefront velocity. Vapour bubbles that are generated by boiling in the present mini-gap grows exponentially due to the rapid evaporation of the thin liquid layer, which, in contrast to the steady situations considered previously, makes the phenomena highly transient. Based on the experimental data (together with the previous results for water and toluene) and scaling arguments, a non-dimensional correlation in terms of capillary number and Bond number is proposed.dc201

Characteristics and mechanism on condensate drop movement under bulk surface temperature gradient in Marangoni Dropwise condensation

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.In Marangoni dropwise condensation, condensate drops move spontaneously when a bulk temperature gradient is applied to the condensing surface. It was considered that the velocity of drop movement is strongly affected by the horizontal component of Marangoni force around a condensate drop. And it is easy to infer that the horizontal component of Marangoni force is decided by both strength of Marangoni force and shape (angle) of condensate drop. In previous studies, it was shown that the velocity of condensate drop movement increases with the increase of bulk surface tension gradient and has a strong relation to initial drop distance. Initial drop distance was adopted as a parameter to express the characteristics of drop velocity, since the qualitatively relations among initial drop distance, Marangoni force and shape of condensate drop inferred from the previous experimental results. However, those relations are still not known clearly in detail. Furthermore, the mechanism on the condensate drop movement is also still not well understood. Therefore, in this paper, experimental study was carried out and the quantitative relation between initial drop distance and angle of condensate drop is investigated. In addition, numerical calculations on the condensation process of water-ethanol vapor mixture and the behavior of condensate were performed by using the VOF (Volume Of Fluid) method to qualitatively analyze the mechanisms of condensate drop movement.dc201

Enhancement of critical heat flux using spherical porous bodies in saturated pool boiling of nanofluid

One strategy to address severe nuclear accidents is the in-vessel retention (IVR) of corium debris. IVR consists of the external cooling of the reactor vessel to remove the decay heat from the molten core through the lower head of the vessel. However, heat removal is limited by the occurrence of the critical heat flux (CHF) condition at the outer surface of the reactor vessel. Therefore, we propose a CHF enhancement technique in a saturated pool boiling by the attachment of a honeycomb porous plate (HPP) on the heated surface. However, the reactor vessel on which to install the HPP exhibits curvature, so the key to realizing IVR depends on the placement of the HPP on the curved surface of the reactor vessel. Accordingly, we propose an approach using porous cellulose beads and a nanofluid. Consequently, for the combination of the nanofluid (TiO2, 0.1 vol%) and spherical porous bodies, the CHF is demonstrated to be enhanced by up to a maximum factor of two compared to that of a plain surface of distilled water.journal articl

Precise measurement of effective oxygen diffusivity for microporous media containing moisture by review of galvanic cell oxygen absorber configuration

The performance of polymer electrolyte fuel cells is influenced by moisture control in their gas diffusion layer (GDL). Therefore, to achieve suitable control, it is necessary to clarify the mass transfer characteristics within a GDL by high precision measurement of oxygen diffusivity. We have previously proposed that measurement of the effective oxygen diffusivity in a GDL containing moisture can be achieved using a galvanic cell oxygen absorber and demonstrated this to be an effective technique for the measurement of microporous media. However, the diffusion resistance of a single dry GDL is low, so that the margin of error in the oxygen diffusivity measurement is high. In this study, high precision measurement of the oxygen diffusivity in a GDL was developed further by analysis of the major error factors and modification of the measurement apparatus configuration. The results indicate a reduction in the maximum measurement error from 50% to 20% for a dry GDL with minimal diffusion resistance.journal articl

Improved performance of secondary heat exchanger for latent heat recovery from flue gas using mini-tubes

A new type of heat exchanger, in which flue gas flows inside thin tubes and cool water is on the shell-side, was proposed to develop the performance and compactness of shell and tube type heat exchangers for latent heat recovery from flue gas. The experimental heat transfer characteristics of single tubes were systematically investigated to determine the effects of tube diameter (1.0-5.0 mm) and length (7-100 mm). Furthermore, a correlation between the non-dimensional bulk mean temperature and the ratio of effective tube length to the thermal entrance region was proposed and was correlated well with the measurement data. Prediction of the heat exchanger performance using this correlation was possible. As a result, it was elucidated that the using mini-tubes is remarkably effective to reduce the size of heat exchanger due to enhancement of heat transfer coefficient and enlargement of heat transfer surface. The volume with 1 mm inner diameter of tubes was approximately 5 percent of that with 5 mm in diameter.journal articl

Improvement of Oxygen Diffusion Characteristic in Gas Diffusion Layer with Planar-distributed Wettability for Polymer Electrolyte Fuel Cell

Mass transfer characteristics of gas diffusion layer (GDL) are closely related to performance of polymer electrolyte fuel cells. Therefore, it is necessary to clarify the characteristics of water distribution relating to the microscopic conformation and oxygen diffusivity of GDL. A hybrid type carbon paper GDL with planar-distributed wettability is investigated for control of liquid water movement and distribution due to hydrophobic to hydrophilic areas that provide wettability differences in GDL and to achieve enhancement of both oxygen diffusion and moisture retention. Hybrid GDLs with different PTFE content were fabricated in an attempt to improve the oxygen diffusion characteristics. The effects of different PTFE contents on the oxygen diffusivity and water distribution were simultaneously measured and observed using galvanic cell oxygen absorber and X-ray radiography. The PTFE distribution was observed using scanning electron microscopy. The formation of oxygen diffusion paths was confirmed by X-ray radiography, where voids in the hybrid GDL were first formed in the hydrophobic regions and then spread to the untreated wetting region. Thus, the formation of oxygen diffusion paths enhanced the oxygen diffusion. In addition, the effects of local PTFE content in the hydrophobic region and the optimal amount of PTFE for hybrid GDL were elucidated.journal articl