Koichi Suzuki

Subcooled Boiling in the Ultrasonic Field—On the Cause of Microbubble Emission Boiling

Subcooled quasi-pool boiling for water and for ethanol aqueous solutions of 10% by weight (10wt%) and 50wt% and ethanol in an ultrasonic field was experimentally performed for the upward flat heating surface of a copper block with 10 mm diameter under atmospheric conditions. Tested liquid subcooling was 15 K, 20 K, and 25 K for water and aqueous solutions of ethanol and 20 K, 30 K, and 40 K for 100wt% ethanol. At 20 K of liquid subcooling for water and ethanol aqueous solutions, no microbubble emission boiling (MEB) has been observed in quasi-pool boiling. Even if MEB occurs, the heat flux levels off and it turns easily to film boiling. In an ultrasonic field, MEB occurs remarkably. Then the heat flux increases to higher than the ordinary critical heat flux as observed in highly subcooled boiling. The experimental results show that the ultrasonic vibration introduces instability of the interface of liquid and vapor and accelerates MEB at 20 K of liquid subcooling for water and aqueous solutions of ethanol. At 15 K of liquid subcooling for water and aqueous solutions, no effect of ultrasonic vibration is observed. However, at 25K of liquid subcooling, the ultrasonic vibration extends MEB region to higher superheating of the heating surface for aqueous solutions of ethanol. The maximum heat flux in MEB decreases with increasing of ethanol concentration and becomes critical heat flux for 100wt% ethanol. No effect of ultrasonic vibration on boiling is observed for the 100wt% ethanol in these experiments.

Subcooled Flow Boiling in a Minichannel

It has been considered that dry-out occurs easily in boiling heat transfer for a small channel, a mini- or microchannel, because the channel was easily filled with coalescing vapor bubbles. In the present study, the experiments of subcooled flow boiling of water were performed under atmospheric conditions for a horizontal rectangular channel for which the size is 1 mm height and 1 mm width, with a flat heating surface of 10 mm length and 1 mm width placed on the bottom of the channel. The heating surface has a top of copper heating block and is heated by ceramic heaters. In the high heat flux region of nucleate boiling, about 70–80% of the heating surface was covered with a large coalescing bubble and the boiling reached critical heat flux as observed by high-speed video. In the beginning of transition boiling, coalescing bubbles were collapsed to many fine bubbles and microbubble emission boiling was observed at liquid subcooling higher than 30 K. The maximum heat flux obtained was 8 MW/m2 (800 W/cm2) at liquid subcooling of higher than 40 K and a liquid velocity of 0.5 m/s. However, the surface temperature was very much higher than that of a centimeter-scale channel. The high-speed video photographs indicated that microbubble emission boiling occurs in the deep transition boiling region.

Proposal of experimental setup on boiling two-phase flow on-orbit experiments onboard Japanese experiment module "kIBO"

Boiling is one of the efficient modes of heat transfer due to phase change, and is regarded as promising means to be applied for the thermal management systems handling a large amount of waste heat under high heat flux. However, gravity effects on the two-phase flow phenomena and corresponding heat transfer characteristics have not been clarified in detail. The experiments onboard Japanese Experiment Module KIBO in International Space Station on boiling two-phase flow under microgravity conditions are proposed to clarify both of heat transfer and flow characteristics under microgravity conditions. To verify the feasibility of ISS experiments on boiling two-phase flow, the Bread Board Model is assembled and its performance and the function of components installed in a test loop are examined.

Microgravity experiments on boiling and applications: research activity of advanced high heat flux cooling technology for electronic devices in Japan.

Abstract: Research and development on advanced high heat flux cooling technology for electronic devices has been carried out as the Project of Fundamental Technology Development for Energy Conservation, promoted by the New Energy and Industrial Technology Development Organization of Japan (NEDO). Based on the microgravity experiments on boiling heat transfer, the following useful results have obtained for the cooling of electronic devices. In subcooled flow boiling in a small channel, heat flux increases considerably more than the ordinary critical heat flux with microbubble emission in transition boiling, and dry out of the heating surface is disturbed. Successful enhancement of heat transfer is achieved by a capillary effect from grooved surface dual subchannels on the liquid supply. The critical heat flux increases 30‐40 percent more than for ordinary subchannels. A self‐wetting mechanism has been proposed, following investigation of bubble behavior in pool boiling of binary mixtures under microgravity. Ideas and a new concept have been proposed for the design of future cooling system in power electronics.

Friction Factor Correlations for Compressible Gaseous Flow in a Concentric Micro Annular Tube

Compressible momentum and energy equations were solved numerically for concentric micro annular tubes with slip velocity and temperature jump wall boundary conditions. The results were expressed in the form of the product of friction factor and Reynolds number (f · Re) for a quasi-fully developed condition and for the ranges of Re < 1000 and Ma < 1. The numerical methodology was based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The outer tube radius ranged from 5 to 40 µm with radius ratios of 0.2, 0.5, and 0.8. The length to hydraulic diameter ratio was more than 100. The stagnation pressure was chosen in such a way that the exit Mach number ranged from 0.1 to 1.0, and the outlet pressure was fixed at the atmospheric pressure. For the case of incompressible slip flow, f · Re is calculated as a function of radius ratio and Knudsen number. For high speed flows, the values of f · Re for compressible slip flow were higher than those for incompressible slip flow due to compressibility effects. Also, f · Re correlation for compressible slip flow was obtained from compressible no-slip flow and incompressible slip flow as a function of Mach and Knudsen numbers and radius ratio. In addition, a correlation for microchannel, microtube, and micro annular tube was obtained from the authors previous work.

Bubble Behavior in Subcooled Pool Boiling of Water under Reduced Gravity

Subcooled pool boiling of water was conducted in reduced gravity performed by a parabolic flight of aircraft and a drop‐shaft facility. A small stainless steel plate was physically burned out in the subcooled water by AC electric power during the parabolic flight. Boiling bubbles grew with increasing heating power but did not detached from the heating surface. The burnout heat fluxes obtained were 200 ∼ 400 percent higher than the existing theories. In the ground experiment, boiling bubbles were attached to the heating surface with a flat plate placed over the heating surface, and the experiment was performed by the same heating procedure as practiced under the reduced gravity. Same burnout heat fluxes as under the reduced gravity were obtained by adjusting the plate clearance to the heating surface. As the heating time extended longer than the reduced gravity duration, the burnout heat fluxes decreased gradually and became constant. Contact area of bubbles with heating surface was observed using a transp...

Proposal of a Micro/Mini Cooling Device Using Fins-Installed Porous Media for High Heat Flux Removal Exceeding 1000W/cm2

Heat transfer characteristics of micro-sized bronze particle-sintered porous heat sinks and copper minichannel-fins heat sinks are experimentally investigated in order to clarify the feasibility of a newly proposed micro/mini cooling device using fins-installed porous media. Regarding the porous heat sinks, fin effect toward more inside of the porous medium is promoted by sintering the porous heat sink on the heat transfer surface, which results in increasing the heat transfer performance up to 0.8MW/m2 K at heat flux of 8.2MW/m2 though there still remains a large pressure loss issue. In addition, the results clarify that the heat exchanging area exists only in the vicinity of the heat transfer surface. As to the minichannel-fins heat sinks, the influence of the channel width and the fin thickness are evaluated in detail. As a result, the minichannel-fins heat sink having the narrower channel width (i.e. scale effect) and lower porosity (i.e. thicker fin thickness with larger heat capacity) achieves higher heat transfer performance up to 0.10MW/m2 K at 8.3MW/m2 . However, rapid increase of pressure loss, which is occasionally observed in a microchannel due to vapor bubbles choking the narrow channel, still remains as an issue under flow boiling conditions in the minichannel. Finally, heat transfer performance of the fin-installed porous heat sink is numerically predicted by the control volume method. The simulation confirms that the heat transfer coefficient at each wall superheat of 0 and 30 degrees has performance 2.5 times and 2.0 times higher than that of the normal fins, which indicates that this heat sink coupling the micro and mini channels has high potential as efficient cooling method under high heat flux conditions exceeding 10MW/m2 .Copyright

Supersonic Flow at Micro-Tube Outlet

The boundary layer is formed on micro-channel walls and its thickness becomes 0 at the exit of the channel. And, it plays a role of a wall of a converging and diverging nozzle and the flow becomes supersonic at the micro-channel outlet. Then outlet Mach number is beyond unity. This fact is not widely known. Therefore, experimental investigations on behavior of super sonic flow at the outlet of straight micro-tubes whose diameter ranges from 150 to 500 μm are conducted. The stagnation pressure ranges 379 from to 812 kPa. The successive expansion and recompression waves of under-expanded state were visualized by Schlieren method and a high-speed camera. The numerical investigations are also performed for straight micro-tubes with diameter ranging from 50 to 400 μm. Numerical methodology is based on the aribitary-Langrangian-Eulerian (ALE) method. The stagnation pressure was chosen in such a way that the Mach number at the tube outlet ranges from 1.0 to 1.6. The ambient back pressure is fixed at the atmospheric pressure. The flow at the tube outlet change from the over-expanded to the under-expanded state. It is observed that the recompression and expansion waves are alternately formed in downstream of the micro-tube outlet in both experiments and numerical computations. The experimental correlation for the distance from the micro-tube outlet to the Mach disk as a function of pressure at the outlet was proposed for the prediction of outlet pressure of micro-tube in under-expanded.Copyright

High heat flux transport by microbubble emission boiling

In highly subcooled flow boiling, coalescing bubbles on the heating surface collapse to many microbubbles in the beginning of transition boiling and the heat flux increases higher than the ordinary critical heat flux. This phenomenon is called Microbubble Emission Boiling, MEB. It is generated in subcooled flow boiling and the maximum heat flux reaches about 1 kW/cm2(10 MW/m2) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s for a small heating surface of 10 mm×10 mm which is placed at the bottom surface of horizontal rectangular channel. The high pressure in the channel is observed at collapse of the coalescing bubbles and it is closely related the size of coalescing bubbles. Periodic pressure waves are observed in MEB and the heat flux increases linearly in proportion to the pressure frequency. The frequency is considered the frequency of liquid-solid exchange on the heating surface.For the large sized heating surface of 50 mm length×20 mm width, the maximum heat flux obtained is 500 W/cm2 (5 MW/m2) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s. This is considerably higher heat flux than the conventional cooling limit in power electronics. It is difficult to remove the high heat flux by MEB for a longer heating surface than 50 mm by single channel type. A model of advanced cooling device is introduced for power electronics by subcooled flow boiling with impinging jets. Themaxumum cooling heat flux is 500 W/cm2 (5 MW/m2).Microbubble emission boiling is useful for a high heat flux transport technology in future power electronics used in a fuel-cell power plant and a space facility.

Subcooled boiling with nano-coating heating surface

In this study, the effect of the nano-coating heating surface was examined to improve boiling heat transfer. A silane coupling agent, which produces self-assembled monolayer (SAM), was used to change the wettability on the heating surface. Since the coated thickness of SAM is very thin, it does not fill the cavity on the heating surface. By means of the nano-coating heating surface, boiling curve of pool boiling was investigated at the liquid subcooling of 50 K. As a result, a hydrophobic surface obtained by SAM improved the heat transfer coefficient in the nuclear boiling regime. Furthermore, it became clear that a heating surface coated with fluorinated silane copling agent did not prevent the occurrence of microbubble emission boiling (MEB).