Shigefumi Nishio

Observation of boiling structures in high heat-flux boiling

Abstract This study is an attempt to attain a good understanding of boiling structures such as liquid-solid contact patterns and bubble structures near the boiling surface. This was accomplished by observing both the dynamic behavior of liquid-solid contact from below the surface and the sectional views of bubbles in a quasi-two-dimensional boiling space. Results of the observation indicate that the boiling structures at heat fluxes near the CHF point are different from the physical image given by the so-called macrolayer model. Based on this fact, we propose a new concept of the contactline-length density describing the contribution of evaporating thin liquid layers to high-heat-flux boiling heat transfer.

Oscillation-induced heat transport: Heat transport characteristics along liquid-columns of oscillation-controlled heat transport tubes

It is known that heat transfer between two liquid-reservoirs maintained at different temperatures and connected to each other by a capillary bundle is markedly enhanced by oscillatory flow in the bundle. In this paper, first, the effects of physical properties of the working liquid on the enhanced heat flow rate are investigated. Second, the optimum operating condition maximizing the ratio of the enhanced heat flow rate to power input is examined. Finally, to increase further the ratio, a novel type with a phase shift between oscillatory flows in adjacent tubes is proposed.

Vapor-film-unit model and heat transfer correlation for natural-convection film boiling with wave motion under subcooled conditions

Abstract This report presents a heat transfer model and correlation of natural-convection film-boiling heat transfer with interfacial wave motion under subcooled conditions. First, the vapor-film-unit model developed for saturated film boiling in our previous report was extended to subcooled film boiling along inclined and vertical flat-plates and also around horizontal Cylinders of large diameter. Next, based on the vapor-film-unit model, a general form of heat transfer correlation of film boiling with wave motion was developed. Comparison with experimental data showed that the present heat transfer model and correlation can predict the effects of the fluid properties, liquid subcooling, wall superheat, the geometry and the size of the heat transfer surface on film-boiling heat transfer with wave motion.

Heat transfer of dilute spray impinging on hot surface (simple model focusing on rebound motion and sensible heat of droplets)

Abstract In the present paper, focusing on the effects of the rebound motion and sensible heat of droplets on spray-cooling heat transfer in the high temperature region, a simple model was developed to predict the heat flux distribution of a dilute spray impinging on a hot surface. In the model, the local heat flux was regarded as the sum of the heat flux components by droplets, induced air flow, and radiation. To estimate the heat flux component by droplets, it was assumed that the heat flux upon droplet impact is proportional to the sensible heat which heats up the droplet to the saturation temperature and the proportional factor C is constant. In addition, to take account of the contribution of the heat flux upon impact of rebounded droplets, it was assumed that the flight distance of droplets during rebound motion is distributed uniformly from 0 to L max (maximum flight distance) . The values of C and L max determined by experimental data of local heat flux indicate that the assumptions employed in the present model is valid at least as the first order approximation.

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.

Pool boiling heat transfer to saturated liquid helium from coated surface

Abstract Boiling experiments were performed at atmospheric pressure to study the influence of surface coatings on the pool boiling heat transfer to saturated liquid helium from flat surfaces. The parameters of interest were the coating material and the thickness, and the boiling surface inclination. The heat transfer characteristics were related to the surface temperature of the coating and the influences of the coating on boiling heat transfer were discussed. It was established, based on the concept of cold-end recovery of a normal zone, that an insulative coating has an optimum thickness when the coating is utilized to raise the maximum operating current of a superconductor.

Heat Transport Characteristics in Closed Loop Oscillating Heat Pipes

Heat transport rates of micro scale SEMOS (S elf-E xciting M ode O scillating) heat pipe with inner diameter of 1.5mm, 1.2mm and 0.9mm, were investigated by using R141b, ethanol and water as working fluids. The effects of inner diameter, liquid volume faction, and material properties of the working fluids are examined. It shows that the smaller the inner diameter, the higher the thermal transport density is. For removing high heat flux, the water is the most promising working fluid as it has the largest critical heat transfer rate and the widest operating range among the three kinds of working fluids. A one-dimensional numerical simulation is carried out to describe the heat transport characteristics and the two-phase flow behavior in the closed loop SEMOS heat pipe. The numerical prediction agrees with the experimental results fairly well, when the input heat through was not very high and the flow pattern was slug flow. This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.Copyright

Single-phase laminar-flow heat transfer and two-phase oscillating-flow heat transport in microchannels

This article focuses on the research field of heat transfer of single-phase laminar flow and two-phase self-exciting oscillating flow in microchannels. First, to make prominent the special features of microthermal systems, the definition of the term “nanothermal systems” is discussed from the viewpoint of local equilibrium. Next, to show the special features of flow and heat transfer in microchannels, some thermal functions appearing in microchannels are introduced. Further, focusing on flow and heat transfer characteristics of single-phase laminar liquid-flow in microchannels, research is introduced that shows that the results obtained for tubes larger than 0.1 mm in inner diameter are in good agreement with the conventional analyses. Finally, Japanese research and recent results at IIS on Micro SEMOS heat pipes (M-SEMOSs) are introduced, and it is shown that a M-SEMOS of 0.5 mm in inner diameter can transport a significant amount of heat.

OSCILLATION-CONTROLLED HEAT TRANSPORT TUBE ( NUMERICAL SIMULATION OF OVERALL THERMAL RESISTANCE )

In this report, overall thermal resistance of oscillation-controlled heat transport tubes under the optimum operating condition is investigated by numerical simulation. Results obtained from the simulation can be summarized as follows. This kind of heat transport tube consists of heating/cooling regions, a fully developed column region, and undeveloped column regions between the former two regions. Heat transfer coefficients at inner walls of the heating/cooling region are near those in the fully developed laminar flow for small amplitudes, but increase for larger amplitudes. Within the conditions of the present numerical simulation, major resistance exists in the fully developed column region for amplitudes smaller than 50 mm, and it is possible to realize values of overall thermal conductance 10 times larger than the thermal conductance of a copper rod. Finally, based on the results of numerical simulation, a simplified model for the overall thermal resistance is presented.

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.