Hiroyasu Ohtake

Experimental Study on Fundamental Phenomena of Boiling Using Heat Transfer Surfaces With Well-Defined Cavities Created by MEMS (Effect of Spacing Between Cavities)

Pool nucleate boiling heat transfer experiments were performed for water using heat transfer surfaces having unified cavities. Cylindrical holes of 10μm in diameter and 40μm in depth were formed on a mirror-finished silicon wafer of 0.525mm in thickness using Microelectromechanical systems technology. The test heat transfer surface was heated by a semiconductor laser beam. Experiments were conducted in the range of up to 4.54×104W∕m2. The temperature of the back side of the heat transfer surface was measured by a radiation thermometer. When the spacing between cavities was S∕Lc<0.8, the horizontal and declining coalescence of bubbles on the neighboring cavities were dominant. Strong thermal and bubble coalescence interactions between nucleation sites were observed in this situation. This vigorous bubble coalescence created strong convection. The heat carried by this convection accounted for a large part of the heat transfer. As the cavity interval became wide, S∕Lc≥1.2, the horizontal and the declining coalescence of the bubbles ceased. The coalescence was limited to the vertical or no coalescence. The thermal and bubble coalescence interactions between the nucleation sites became quite low, to the extent of being negligible. The bubbles themselves were key in carrying heat away from the heat transfer surface.

Correlations of Wave Characteristics for a Liquid Film Falling Down Along a Vertical Wall

The behavior of a liquid film that flows down countercurrently along the inner surface of a circular pipe was examined. In the experiments of the present study, silicone oils of 500 cS, 1000 cS, and 3000 cS, as well as water, were used as the liquid phase. The gas phase was air. The vertically oriented test section was a circular pipe of 30 mm in inner diameter and 5.4 m in length. The substrate thickness of the silicone films, the film Reynolds numbers of which were quite low, was close to the mean film thickness, while the water film substrate was much thinner than the mean film thickness. Waves were observed on the substrate. Waves of a certain amplitude were confirmed to exist, even on the silicone films near the flooding occurrence, where the film Reynolds number was quite low. The mean film thicknesses of the silicone films, as well as that of the water film, were well expressed by applying the universal velocity profile to the film flow. When the film Reynolds number was lower than 600, the wave velocity was well predicted as the velocity of small perturbation waves on a laminar film. As the film Reynolds number became large, the wave velocity became slower than the small perturbation wave velocity. The correlation for the wavelength was developed based on the present experimental results. Combining this correlation with the Nosoko correlations and modifying the constants and exponents of the parameters in the equations, new correlations for the wave velocity and maximum film thickness were proposed. These new correlations were used to predict the wave velocity and the maximum film thickness to an accuracy of within 15%.

Study on Mechanism of Condensation Heat Transfer of Water Jet in Steam Injector

It has been proposed to introduce a steam injector into boiling water reactors as a feed water heat exchanger and a safety injection pump. In the present paper, the heat transfer characteristics in the steam injector were examined. The nozzle size of the water jet in the steam injector that was used in the present experiments was 5 mm. The length of the mixing section of the water jet and the steam flow in the injector was 53 mm. Subcooled water was supplied to the water nozzle. Saturated steam at approximately 0.1 MPa was also supplied to the mixing section of the steam injector. Water jet velocities tested in the present experiments were in the range of 9.7 ∼ 21 m/s. The velocities corresponded to the Reynolds number of 5.9×104 ∼ 1.5×105 . Radial and axial temperature distributions of the water jet in the steam injector were measured. Velocity distributions of the water jet were also measured. From the measured temperature and the velocity distributions, heat exchange rates from the steam flow around the water jet to the water jet were derived. The obtained results indicated that the heat exchange rates were greatly larger than those of usual turbulent flow in a pipe. A flow state of the water jet was also visually examined. The results of the visual observation revealed that the interface between the water jet and the steam flow was very wavy. It was supposed that the wavy motion on the water jet surface created the effective-large-internal circulation flow in the water jet, which resulted in the tremendously effective heat transport into the center portion of the water jet. From the pictures of the water jet surface recorded by a high speed video camera, characteristics of waves on the surface; the wave height, the wave velocity, the wave length and the wave frequency, were obtained. The heat transfer of the water jet in the steam injector was correlated with the wave characteristic properties. The heat transfer of the water jet was also analyzed by using the commercial CFD code of STAR-CD. When the wavy interface was introduced into the STAR-CD code analysis, the radial heat transport was drastically improved. This analytical result also supported that the tremendously more effective radial heat transport than that of the usual turbulent flow was caused by the wavy motion of the water jet surface.© 2008 ASME

Experimental Study on Nucleation Site Interaction During Pool Nucleate Boiling by Using Three Artificial Cavities

Pool nucleate boiling heat transfer experiments were performed for water using heat transfer surfaces having unified cavities. Cylindrical holes of 10 μm in diameter and 40 μm in depth were formed on a mirror-finished silicon wafer of 0.2 mm in thickness using Micro-Electro Mechanical Systems (MEMS) technology. This silicon plate was used as the heat transfer surface. The test heat transfer surface was heated by a semiconductor laser beam. Experiments were conducted in the range of up to 1.35 × 105 W/m2 . When the cavity spacing was narrow such as S = 1 or 2 mm, the convection created by the departure of coalesced bubbles played a main role in heat transfer when the heat flux was low. As the heat flux was increased, the coalesced bubbles absorbed enough heat to become large while the frequent bubble departure was maintained. As a result of it, the latent heat term in heat transfer became large to approximately 40%. When the cavity spacing was wide such as S = 4 mm, a bubble absorbed heat enough to become large before departure and the coalescence of bubbles were not prominent. Thus, the latent heat term took approximately 50% in heat transfer. With an increase in the heat flux, the vertical coalescence became to happen quite frequently. This coalescence made convection vigorous to increase the heat transfer. As a result of it, the convection term increased to 60% and the latent term decreased to 40%.Copyright

Study on Micropump Using Boiling Bubbles

A micropump was developed using boiling and condensation in a microchannel. The length and hydraulic diameter of the semi-half-circle cross-section microchannel having two open tanks at both ends were 26 mm and 0.465 mm, respectively. A 0.5 X 0.5 mm 2 electrically heated patch was located at the offset location from the center between both ends of the microchannel, at a distance of 8.5 mm from one end and at a distance of 17 mm from the other end. The microchannel and the two open tanks were filled with distilled water The heating patch was heated periodically to cause cyclic formation of a boiling bubble and its condensation. By this procedure, flow from the short side (8.5 mm side) to the long side was created. The flow rate increased as the heating rate was increased. The obtained maximum average flow velocity and flow rate were 10.4 mm/s and 2.16 mm 3 /s, respectively. The velocity of an interface between the bubble and the liquid plug during the condensing period was much faster than that during the boiling period. During the condensing period, the velocity of the interface at the short channel side (8.5 mm side) was faster than that at the long channel side (17 mm side). The equation of motion of liquid in the flow channel was solved in order to calculate the travel of liquid in the flow channel. The predicted velocities agreed well with the experimental results. The velocity differences between the short side and the long side, as well as those between the boiling period and the condensing period, were expressed well by the calculation. Liquid began to move from the stationary condition during both the boiling and the condensing periods. The liquid in the inlet side (short side) moved faster than that in the outlet side (long side) during the condensing period because the inertia in the short side was lower than that in the long side. Since the condensation was much faster than boiling, this effect was more prominent during the condensing period. By iterating these procedures, the net flow from the short side to the long side was created.

Frictional Pressure Drops of Single-Phase and Gas-Liquid Two-Phase Flows in Circular and Rectangular Microchannels

The frictional pressure drops of single-phase and two-phase flows in mini-pipes and mini-rectangular channels were investigated experimentally. The friction factors and the critical Reynolds number were measured by using water single-phase and gas-water two-phase flows through circular and rectangular channels respectively. Diameter of the circular pipes was 0.5, 0.25 and 0.17 mm, respectively; dimension of the rectangular channels was 0.2 × 20 mm, 0.2 × 10 mm, 0.2 × 5 mm and 0.2 × 2 mm, respectively. The experimental results for water single-phase flow in the circular tubes show that the measured friction factor agreed well with the conventional Poiseuille’s equation (λ = 64/Re) in laminar flow regime; the laminar-turbulent transition Reynolds number was approximately 2300 in a range of the present experimental conditions for each diameter. On the other hand, the experimental results for water flow in the rectangular channels slightly differed from the conventional equation (λ = 96/Re). For the two-phase flow experiments, pressure drops and flow patterns were collected over 0.01 < jG < 15 m/s for the superficial gas velocity and 0.01 < jL < 2 m/s for the superficial liquid velocity. Test gas was pressurized argon; test liquid was water. The argon gas was mixed with water through a coaxial annular nozzle to make two-phase flow. The observed flow patterns were slug, churn and annular flows; bubbly flow pattern was not observed in a range of the present experimental conditions. Time-averaged void fraction and two-phase friction pressure drops were also obtained. The two-phase friction multipliers were shown to be in good agreement with a correlation presented by Mishima-Hibiki in the experimental range considered in the present report.Copyright

Correlations of Characteristics of Waves on a Film Falling Down on a Vertical Wall

Behavior of a liquid film flows down counter-currently on the inner surface of a circular pipe was examined. In experiments, silicon oils of 500, 1000 and 3000 cSt as well as water were used as the liquid phase. The gas phase was air. A test section vertically oriented was a circular pipe of 30 mm in inner diameter and 5.4 m in length. The substrate thickness of the silicone film, where the film Reynolds number was quite low, was close to the mean film thickness while the water film substrate was much thinner than the mean film thickness. Waves were on the substrate. It was confirmed that there were waves with some certain amplitude even on the silicone films near the flooding occurrence where the film Reynolds number was quite low. The mean film thicknesses of silicon films as well as the water film were well expressed by applying the universal velocity profile to the film flow. When the film Reynolds number was lower than 600, the wave velocity was well predicted as the velocity of small perturbation waves on the laminar film. As the film Reynolds number becomes large, the wave velocity becomes slower than the small perturbation wave velocity. The correlation for the wave length was developed based upon the present experimental results. Combining it with the Nosoko correlations and modifying constants and exponents, new correlations for the wave velocity and the maximum film thickness were proposed. The correlations could predict these within 15% accuracy.Copyright

Study on Flow Boiling Heat Transfer and Two-Phase Flow Pressure Drop in Flat Mini-Channel

Flow and the heat transfer characteristics of boiling two-phase flow of water in flat mini-rectangular-channels were examined. The cross-sections tested were 1.0×10 to 0.2×10 mm and the flow channel length was 250 mm. Single phase flow pressure drop was well expressed by the method for the usual size in the present experimental range. Boiling heat transfer of 0.5 mm high and 10 mm wide cross section was similar to that of the usual size. However, that 0.2 mm high and 10 mm wide cross section was a little different from that of the usual size. An increase in the heat flux after the onset of nucleate boiling on the boiling curve is milder than that of the usual size. Thus, the critical heat flux was lower than that of the usual size. Flow patterns observed in the present experiments were a little different from the Baker flow pattern chart. Consistent agreement was not obtained between the present results of the two-phase flow pressure drop and predictions by the methods for the usual size and also for a mini tube. Subcooled boiling was observed widely in the test section. This made it difficult to determine the local conditions such as quality that was necessary to calculate the Lockhart-Martinelli parameter for the two-phase flow pressure drop prediction.Copyright

Study on Condensation Heat Transfer of Micro Structured Surfaces

Condensation heat transfer experiments for steam were performed by using mirror-finished copper surfaces, mirror-finished silicon surfaces and silicon surfaces with micro grooves or micro pins on it. The micro-grooves and the micro-pins were created by the MEMS technology. The film- and also the drop-wise condensation were observed on the copper surface. The film-wise condensation heat flux was in good agreement with the values of the Nusselt equation. It was approximately one-tenth of the drop-wise condensation heat flux. The condensation on the mirror-finished silicon surface was the drop-wise condensation. The heat flux was approximately one-tenth of the drop-wise condensation heat flux on the copper surface. The condensation on the micro-grooved and the micro-pin silicon surfaces was film-wise. The condensation heat fluxes were approximately one-tenth of the copper surface film-wise condensation heat flux. When the contact angle was smaller than 70 degree, the condensation was film-wise and when larger than the value, drop-wise. It seemed that the hollow parts of the micro-grooved or the micro-pin surface were filled with condensate first after the condensation was initiated. It made the surface hydrophilic and the condensation film-wise.Copyright

Experimental Study on Fundamental-Microscopic Mechanism of Boiling by Using MEMS Technique: Examination From Aspect of Time Series Chaos Analysis

Pool nucleate boiling heat transfer experiments were performed for water by using well-controlled and -defined heat transfer surfaces. The cavities were formed on a mirror-finished silicon plate by utilizing Micro-Electro Mechanical Systems (MEMS) technology. Those had the exactly same cylindrical shape; 10μm in diameter and 40 μm in depth, respectively. The back side of the silicon heat transfer surface was heated by applying a Laser beam. The back side surface temperature was measured with a radiation thermometer. Bubble behavior was recorded with a high speed video camera. In the single cavity case, the reconstructed return maps from the time series data of the bubble diameter exhibited strong correlation even if the delay time was increased until 0.166 ms. It was suggested that the bubble diameter from the single cavity can be predicted deterministically over long time period. In the triple cavity case, when the cavity spacing was narrow, the bubbles frequently coalesce to the bubbles generating from neighboring cavities. When the cavity spacing became wide, the bubbles coalescence was suppressed. The reconstructed return maps of the bubble diameter indicated that when the cavity spacings ≤ 3 mm, the bubble diameter after a few m seconds could not be predicted. This loss of the predictability for the bubble diameter was caused by the interaction/coalescence from the neighboring cavities. The three-dimensional reconstructed attractors of the surface temperature of the cavity position were examined. The attractors of the single cavity case and the triple cavities with S = 4 mm case were quite similar and like a ball. This suggested that when S ≥ 4 mm, the interaction between cavities disappeared and the chaotic complexity might not appear in the surface temperature variation. When S = 1 ∼ 3 mm, the attractors were conical and the chaotic complexity might exist.Copyright