Begell House Inc.
Multiphase Science and Technology
MST
0276-1459
24
3
2012
EFFECT OF HEAT-TRANSFER SURFACE STRUCTURE ON CRITICAL HEAT FLUX
181-196
10.1615/MultScienTechn.v24.i3.10
Hitoshi
Asano
Department of Mechanical Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
Kei
Kawasaki
Graduate School of Engineering, Kobe University, 1-1, Rokkodai, Nada, Kobe, Japan
Nobuyuki
Takenaka
Department of Mechanical Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
saturated flow boiling
subcooled flow boiling
heat transfer
critical heat flux
thermal spray coating
inclined channel
This study deals with the critical heat flux for saturated and subcooled flow boiling in rectangular narrow channels with boiling heat-transfer enhancement surfaces manufactured by a thermal spray coating. The coatings were fabricated on sandblasted copper plates by vacuum plasma spraying using fine copper particle. HCFC123 and FC72 were used as the working fluid for saturated and subcooled flow boiling, respectively. The one side of the wall at the center of the narrow channel was replaced with the heating surface. The channel height was varied 1, 2, and 4 mm, and the channel width was 20 mm. In the saturated flow boiling experiments, the channel was placed in various inclined angles, and the effect of flow and heating direction to gravity on the critical heat flux had been measured. For subcooled boiling, the flows were horizontal flows with bottom heating, and the effect of inlet subcooling degree on the critical heat flux had been measured. These experimental results were compared with those for a smooth surface. As for the results, for saturated flow boiling, the critical heat flux was very sensitive to the change in the channel inclined angle. The effect of the surface structure on the critical heat flux was minimal. On the other hand, for subcooled flow boiling, the critical heat flux of the coating increased by about 20% in the condition with a large degree of subcooling of 40 K. The reason might be that vapor bubbles generated on the coating were immediately condensed in bulk subcooled liquid due to its smaller diameter. For both surfaces, large pressure fluctuation was observed just before burnout in the flow condition with a large degree of subcooling of 40 K.
BUBBLE TRACKING SIMULATION OF BUBBLE-INDUCED PSEUDOTURBULENCE
197-222
10.1615/MultScienTechn.v24.i3.20
Mohd Hatta bin Mohd
Akbar
Graduate School of Engineering, Kobe University, 1-1, Rokkodai, Nada, Kobe, Japan
Kosuke
Hayashi
Department of Mechanical Engineering, Graduate School Engineering, Kobe
University, 1-1 Rokkodai, Nada, Kobe, Hyogo, 657-8501 Japan
Shigeo
Hosokawa
Faculty of Societal Safety Science, Kansai University, 7-1 Hakubai, Takatsuki,
Osaka 569-1098, Japan
Akio
Tomiyama
Department of Mechanical Engineering, Graduate School Engineering, Kobe
University, 1-1 Rokkodai, Nada, Kobe, Hyogo, 657-8501 Japan
bubble
simulation
large eddy simulation
bubble-induced pseudoturbulence
Bubble tracking simulations of bubbly flows in a rectangular column are carried out to examine the applicability of a bubble tracking method to flows involving the bubble-induced pseudoturbulence. Experimental data of void fraction, mean liquid and bubble velocities, and liquid fluctuation velocity are obtained for validation. As a result, the following conclusions are obtained: (1) The order of liquid fluctuation velocity is similar to that of the bubble relative velocity, and the fluctuation is well scaled in terms of the bubble terminal velocity and the void fraction, as Risso & Ellingsen suggested [J. Fluid Mech., vol. 440, pp. 235-268, 2001]. (2) The Reynolds shear stress is not produced by the bubble motion when both void fraction and liquid velocity are uniform. (3) The Reynolds stress model proposed by Lopez de Bertodano et al. [Int. J. Multiphase Flow, vol. 20, pp. 805-818, 1994] underestimates the normal stress, whereas the bubble tracking simulation with a spatial resolution comparable to the bubble size gives better predictions than the model. (4) The fluctuation in liquid velocity induced by bubbles is partly resolved with a spatial resolution comparable to the bubble size, and subgrid turbulence models do not have much influence on predictions. (5) Eddy viscosity models would be a reasonable choice to capture the shear-induced turbulence in the near-wall region in bubble tracking simulations with an inevitably insufficient spatial resolution for large eddy simulation.
EFFECT OF WALL WETTABILITY ON FLOW CHARACTERISTICS OF ANNULAR TWO-PHASE FLOW
223-238
10.1615/MultScienTechn.v24.i3.30
Tatsuya
Hazuku
Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, 2-1-6 Etchujima, Koto, Tokyo 135-8533, Japan
Akihiko
Kamura
Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, 2-1-6 Etchujima, Koto, Tokyo 135-8533, Japan
Tomoji
Takamasa
Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, 2-1-6 Etchujima, Koto, Tokyo 135-8533, Japan
annular two-phase flow
liquid film thickness
entrainment
wettability
To evaluate the effect of wall surface wettability on the characteristics of upward gas-liquid annular two-phase flow in a vertical pipe, an experimental study was performed using two test pipes: an acrylic pipe and a fluorinated ethylene propylene (FEP) pipe. The working fluids were air and water. The experiments were conducted using a pipe 3 m long with 10 mm inner diameter. Measured contact angles on the acrylic and FEP pipe surfaces were 60° and 80°, respectively. Basic flow characteristics such as liquid film thickness and liquid entrainment rate were respectively measured by a laser focus displacement meter and a suction method. Flow observation using a high-speed video camera was also performed for measurement of the passing frequency of disturbance waves. Data were collected for predetermined superficial gas and liquid velocities under atmospheric conditions ranging from jg0 = 20−80 m s−1 for the gas phase and jf = 0.1−1.0 m s−1 for the liquid phase. Under conditions of relatively high gas flow rate and low liquid flow rate, a reduction in surface wettability by using FEP pipe enhanced the amplitude of the interfacial waves on a liquid film, causing an increase in the entrainment rate and decreases in the liquid film thickness as well as the disturbance wave frequency. Frictional pressure losses in the FEP pipe were 1.1−1.7-fold greater than those in the acrylic pipe. This result could be attributable to an increase in the interfacial shear stress due to the enhancement of the fluctuation of the interfacial waves on the liquid film.
LENGTHS OF BUBBLE AND SLUG AND PRESSURE DROP IN GASLIQUID SLUG FLOW IN MICROCHANNELS
239-256
10.1615/MultScienTechn.v24.i3.40
Akimaro
Kawahara
Advanced Thermal and Fluid Energy System
Division of Industrial Fundamentals
Faculty of Advanced Science and Technology, Graduate School of Science and Technology, Kumamoto University, Chuo-ku,
Kurokami 2-39-1, Kumamoto, Japan
Michio
Sadatomi
Department or Advanced Mechanical System, Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1, Chuo-Ku, Kumamoto City, 860-8555, Japan
Satoshi
Shimokawa
Graduate School of Science and Technology, Kumamoto University, Chuo-ku Kurokami 2-39-1, Kumamoto City, 860-8555, Japan
two-phase flow
slug flow
microchannel
bubble length
liquid slug length
pressure drop
The characteristics of two-phase slug flow, i.e., bubble length and liquid slug length and pressure drop in horizontal circular and rectangular microchannels with T-junction-type gas−liquid mixer have been investigated. The inner diameter of the circular channel was 250 µm, and the hydraulic diameter of the rectangular one was 235 µm. In order to determine the effects of fluid properties, distilled water and aqueous solutions of ethanol having different mass concentrations of 49 and 100 wt% and HFE7200 were used as the test liquid, while nitrogen gas was used as the test gas. The bubble length data for circular and rectangular microchannels were well correlated with Sobieszuk et al.'s [Chem. Eng. Res. Des., vol. 88, pp. 263−269,2010] correlation. The ratio of the bubble length to bubble pitch, which is sum of the bubble length and the liquid slug length, could be correlated with the homogeneous void fraction, regardless of the channel geometries and the liquid properties. The two-phase pressure drop could be reasonably calculated with a unit cell model, except for flows with relatively larger capillary and Weber numbers.
STUDY ON BUBBLE BREAKUP BEHAVIOR IN A VENTURI TUBE
257-277
10.1615/MultScienTechn.v24.i3.50
Shin-Ichiro
Uesawa
University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki, Japan
Akiko
Kaneko
University of Tsukuba, Faculty of Engineering of Information Engineering and
Systems, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8573, Japan
Yasumichi
Nomura
University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki, Japan
Yutaka
Abe
Department of Engineering Mechanics and Energy, Graduate School of System and Information Engineering, University of Tsukuba, 1-1-1, Tennoudai, Tsukuba, Ibaraki, 305-8573, Japan
bubbly flow
micro-bubble
venturi tube
shock
void fraction
We focus on a venturi-tube-type micro-bubble generator that can generate tiny bubbles with a diameter of 10 µm−1 mm in a high gas volume flow ratio. However, the mechanisms of micro-bubble generation are not clarified. The objective of the present study is to reveal the mechanism of the bubble breakup phenomenon. In order to achieve the objectives, we observed bubble behavior in a venturi tube in detail, and measured the pressure and void fraction profiles in the flow direction. By using the measuring results, the gas−liquid mixture velocity, sonic speed, and Mach number were estimated. In the experimental results, the bubbles expanded once into a divergence region of the venturi tube and then contracted rapidly and broke up into a great number of tiny bubbles. The pressure decreased sharply around the throat of the venturi tube and increased at the bubble breakup region. On the other hand, the void fraction increased downstream from the throat and decreased around the bubble breakup point. These results indicate that the flow was supersonic flow between the throat and the bubble breakup point, while it became subsonic flow downstream from the point. Therefore, it was proposed that a shock was present at the bubble breakup point.