Formation Mechanism and Characteristics of a Liquid Microlayer in Microchannel Boiling System

2010 ◽  
Vol 132 (12) ◽  
Author(s):  
Yaohua Zhang ◽  
Yoshio Utaka ◽  
Yuki Kashiwabara
Keyword(s):  
Bubble Growth ◽  
Weber Number ◽  
Kinematic Viscosity ◽  
High Speed ◽  
Growth Process ◽  
Gap Size ◽  
High Speed Camera ◽  
Region I ◽  
Laser Extinction ◽  
Region Ii

Experiments were performed using the laser extinction method to measure the thickness of the liquid film formed by growing flattened bubbles in a microchannel for gap sizes of 0.5 mm, 0.3 mm, and 0.15 mm. Water, ethanol, and toluene were used as test fluids. A high-speed camera was also used to simultaneously measure the bubble growth process. It was confirmed that the gap size and bubble forefront velocity determined the initial microlayer thickness. The variation trend of the microlayer thickness relative to the velocity of the interface was divided into two regions: region I, where the velocity is small and the thickness increases linearly with increasing velocity, and region II, where the thickness is almost constant or decreased slightly with increasing velocity. Furthermore, a nondimensional correlation for investigating the effects of test materials and gap sizes on microlayer thickness is presented. An analysis of the results showed that the boundaries of the two regions correspond to a Weber number of approximately 110, and in the region where the Weber number was smaller than 110, the thickness of the microlayer was thinner for the liquid whose value of ρ0.62ν0.42σ−0.62 was relatively small. However, for the region where Weber number was larger than 110, the smaller the kinematic viscosity of the liquid, the thinner the microlayer became.

Characteristics of Microlayer Thickness Formed During Boiling in Microgaps

2009 ◽  
Author(s):  
Yaohua Zhang ◽  
Yoshio Utaka ◽  
Yuki Kashiwabara ◽  
Takumi Kamiaka
Keyword(s):  
Bubble Growth ◽  
Weber Number ◽  
Kinematic Viscosity ◽  
High Speed ◽  
Growth Process ◽  
Region I ◽  
Result Analysis ◽  
Laser Extinction ◽  
Region Ii ◽  
Velocity Region

Experiments were performed to measure the thickness of the liquid film formed by growing flattened bubbles in microgaps with laser extinction method for gap sizes of 0.5, 0.3 and 0.15mm. Water, ethanol and toluene were used as working fluids. High-speed camera was also taken to measure the bubble growth process simultaneously. It was confirmed that the gap size and bubble forefront velocity determined the initial microlayer thickness. The variation trend of the microlayer thickness relative to the velocity of interface was divided into two regions: region I where the velocity is small and the thickness increases linearly with increasing velocity; region II where the thickness almost constant or decreased slightly. Furthermore, the non-dimensional correlation for investigating the effects of test materials and gap sizes on micro-layer thickness was presented. With result analysis, the boundaries of the two regions were at Weber number of 80 approximately. And at the region where Weber number was smaller than 80 the thickness of mirolayer was thinner for the liquid whose surface tension coefficient is relatively large. But for the region where Weber number was bigger than 80, the smaller kinematic viscosity of liquid is, the thinner thickness of microlayer became.

Formation Mechanism and Characteristics of Liquid Microlayer in Mini-Gap Boiling System

2010 ◽  
Author(s):  
Yaohua Zhang ◽  
Yoshio Utaka
Keyword(s):  
Experimental Data ◽  
Surface Tension ◽  
Liquid Film ◽  
Quartz Glass ◽  
Weber Number ◽  
High Speed ◽  
Dielectric Fluid ◽  
Gap Size ◽  
Small Surface ◽  
Laser Extinction

Experiments were performed using the laser extinction method to directly measure the thickness of the liquid film between the bubbles and heating plates in a mini-gap formed by two parallel vertical quartz glass plates for the dielectric fluid HFE-7200 at gap sizes of 0.15, 0.3 and 0.5mm. High-speed movies were also taken to measure the velocity of bubble forefront simultaneously. It was confirmed that the microlayer thickness was determined by the gap size and velocity of bubble forefront. At the region of small Weber number, microlayer thickness of HFE-7200 was obviously thicker than that of water, toluene and ethanol at the same velocity and gap size for its small surface tension. Furthermore, by nondimensional analyzing of experimental data, the empirical correlation proposed in previous study which was based on water, toluene and ethanol is still reliable for HFE-7200.

Experiments on the Injection of Elliptical Liquid Jets Into a Low Speed Airflow

2018 ◽  
Author(s):  
Yosef Rezaei ◽  
Mehran Tadjfar
Keyword(s):  
Experimental Investigation ◽  
Weber Number ◽  
Momentum Flux ◽  
High Speed ◽  
Flux Ratio ◽  
Major Axis ◽  
Liquid Jets ◽  
Liquid Jet ◽  
High Speed Camera ◽  
Subsonic Crossflow

An experimental investigation was performed to study the physics of liquid jets injected into a low subsonic crossflow. The jets are issued from elliptical and circular injectors with equivalent exit area. The liquid jet was visualized using shadowgraph technique and a high speed camera was used to record the instantaneous status of the jet. The liquid / air momentum flux ratio and air Weber number were varied to examine their effects on different parameters of the flow like liquid jet column trajectory, breakup point and breakup regimes. The major axis of the elliptical nozzle was aligned parallel and perpendicular to the air crossflow direction. Two different breakup modes were observed, column breakup and bag breakup. Based on the obtained results some characteristics of injected liquid jets into the air crossflow such as penetration depth and the trajectory of liquid jet were affected by changing the nozzle exit shape.

Bubble Growth in Surfactant Solutions

2010 ◽  
Author(s):  
G. Hetsroni ◽  
A. Mosyak
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Bubble Growth ◽  
High Speed ◽  
Surfactant Solution ◽  
Life Time ◽  
Channel Length ◽  
Gap Size ◽  
High Speed Video ◽  
Surfactant Solutions

The presence of surfactant additives in water was found to enhance significantly the boiling heat transfer. The objective of the present investigation was to compare the bubble growth in water to that of a surfactant solution with negligible environmental impact. The study was conducted to clarify the effect of the heat flux on the dynamics of bubble nucleation. The bubble growth under condition of pool boiling in water and surfactant solutions was studied using high speed video technique. The bubble generation was studied on a horizontal flat surface; where the natural roughness of the surface was used to produce the bubbles. At heat flux of q= 10 kW/m2 the life-time and the volume of bubble growth in surfactant solution did not differ significantly from those of water. The time behavior of the contact angle of bubble growing in surfactant solution is qualitatively similar to that of water. At a heat flux of q= 50 kW/m2, boiling in surfactant solution, when compared with that of pure water, was observed to be more vigorous. Surfactant promotes activation of nucleation sites; the bubbles appeared in a cluster mode; the life-time of each bubble in the cluster is shorter than that of a single water bubble. The detachment diameter of water bubble increases with increasing heat flux, whereas analysis of bubble growth in surfactant solution reveals the opposite effect: the detachment diameter of the bubble decreases with increasing heat flux. Natural convection boiling of water and surfactants at atmospheric pressure in narrow horizontal annular channels was studied experimentally in the range of Bond numbers Bo = 0.185–1.52. The flow pattern was visualized by high-speed video recording to identify the different regimes of boiling of water and surfactants. The channel length was 24mm and 36mm, the gap size was 0.45, 1.2, 2.2, and 3.7mm. The heat flux was in the range of 20–500 kW/m2, the concentration of surfactant solutions was varied from 10 to 600 ppm. For water boiling at Bond numbers Bo<1 the CHF in restricted space is lower than that in unconfined space. This effect increases with increasing the channel length. For water at Bond number Bo = 1.52, boiling can almost be considered as unconfined. Additive of surfactant led to enhancement of heat transfer compared to water boiling in the same gap size, however, this effect decreased with decreasing gap size. For the same gap size, CHF in surfactant solutions was significantly lower than that in water. Hysteresis was observed for boiling in degraded surfactant solutions.

Crystallization of Spherical Single Crystal of Silicon from Undercooled Droplet under Containerless Processing

2005 ◽  
Vol 475-479 ◽  
pp. 2603-2606
Author(s):  
Zeng Yun Jian ◽  
Kazuhiko Kuribayashi ◽  
Wan Qi Jie
Keyword(s):  
Single Crystal ◽  
Thin Plate ◽  
High Speed ◽  
Growth Process ◽  
Present Model ◽  
High Speed Camera ◽  
Undercooled Liquid ◽  
Containerless Processing ◽  
Critical Undercooling

The images of the growing crystal in the growth process of the undercooled droplets of silicon were lively recorded by using a high-speed camera. The number of crystal that nucleated spontaneously from the undercooled liquid was found to decrease to 1 when the undercooling was higher than 5K. The morphology of the growing single crystal of silicon was a thin plate. A model for predicting the critical undercooling of growing single crystal of silicon from undercooled liquid has been developed. The theoretically predicted value of the undercooling from present model for silicon is in agreement with the experimentally measured result.

scholarly journals Microlayer dynamics during the growth process of a single vapour bubble under subcooled flow boiling conditions

2021 ◽  
Vol 931 ◽  
Author(s):  
Gulshan Kumar Sinha ◽  
Surya Narayan ◽  
Atul Srivastava
Keyword(s):  
Bubble Growth ◽  
High Speed ◽  
Growth Process ◽  
Flow Boiling ◽  
Rectangular Channel ◽  
Working Fluid ◽  
Vapour Bubble ◽  
Subcooled Flow Boiling ◽  
High Speed Cinematography ◽  
Fringe Patterns

The phenomena of microlayer formation and its dynamic characteristics during the nucleate pool boiling regime have been widely investigated in the past. However, experimental works on real-time microlayer dynamics during nucleate flow boiling conditions are highly scarce. The present work is an attempt to address this lacuna and is concerned with developing a fundamental understanding of microlayer dynamics during the growth process of a single vapour bubble under nucleate flow boiling conditions. Boiling experiments have been conducted under subcooled conditions in a vertical rectangular channel with water as the working fluid. Thin-film interferometry combined with high-speed cinematography have been adopted to simultaneously capture the dynamic behaviour of the microlayer along with the bubble growth process. Transients associated with the microlayer have been recorded in the form of interferometric fringe patterns, which clearly reveal the evolution of the microlayer beneath the growing vapour bubble, the movement of the triple contact line and the growth of the dryspot region during the bubble growth process. While symmetric growth of the microlayer was confirmed in the early growth phase, the bulk flow-induced bubble deformation rendered asymmetry to its profile during the later stages of the bubble growth process. The recorded fringe patterns have been quantitatively analysed to obtain microlayer thickness profiles at different stages of the bubble growth process. For Re = 3600, the maximum thickness of the almost wedge-shaped microlayer was obtained as δ ~ 3.5 μm for a vapour bubble of diameter 1.6 mm. Similarly, for Re = 6000, a maximum microlayer thickness of δ ~ 2.5 μm was obtained for a bubble of diameter 1.1 mm.

Effect of Dual Frequency Ultrasound on the Bubble Formation in a Capillary Tube

2016 ◽  
Author(s):  
Benwei Fu ◽  
Nannan Zhao ◽  
Guoyou Wang ◽  
Hongbin Ma
Keyword(s):  
Bubble Growth ◽  
High Speed ◽  
Capillary Tube ◽  
Bubble Formation ◽  
High Speed Camera ◽  
Dual Frequency ◽  
Heating Section ◽  
Nucleation Sites ◽  
Ultrasonic Sound ◽  
Frequency Ultrasound

A visual experimental was conducted to determine the effect of dual frequency ultrasound on the bubble formation and growth in a capillary quartz tube. The ultrasonic sound was applied to the heating section of a capillary tube by using electrically-controlled piezoelectric ceramics made of Pb-based lanthanum-doped zirconate titanates (PLZTs). The bubble formation and growth were recorded by a high speed camera. Experimental results show that the bubble formation and growth depend on PLZT frequency. When a dual frequency ultrasound (154 kHz and 474 kHz) was used, the nucleation sites for bubble formation were significantly increased and the bubble growth rate enhanced.

On the Fragmentation of Drops

1986 ◽  
Vol 108 (1) ◽  
pp. 109-114 ◽  
Author(s):  
M. J. Tan ◽  
S. G. Bankoff
Keyword(s):  
Shock Wave ◽  
Shock Tube ◽  
Weber Number ◽  
High Speed ◽  
Strong Shock ◽  
High Speed Camera ◽  
Pressure Shock ◽  
Critical Weber Number ◽  
Strong Shock Waves

Fragmentation of mercury drops falling through a bubbly aqueous liquid by a pressure shock wave was investigated by means of a shock tube capable of operating at driver pressures up to 3 MPa. The responses to moderately strong shock waves (up to 1.7 MPa) were photographed by a high-speed camera at rates of up to 4400 frames per second. The results show the existence of a critical Weber number, (We)cr = 17, for drop fragmentation. Qualitative characterization of the shock-drop interactions for single mercury drops is provided.

Sign in / Sign up

Export Citation Format

Share Document