Investigation of Hydrodynamic and Heat Transfer Characteristics of Gas-liquid Taylor flow in Square Microchannel

2019 ◽  
Vol 18 (2) ◽  
Author(s):  
Zunlong Jin ◽  
Qiqi Sun ◽  
Dingbiao Wang ◽  
Yongqing Wang
Keyword(s):  
Heat Transfer ◽  
Void Fraction ◽  
Bubbly Flow ◽  
Bubble Formation ◽  
Flow Characteristics ◽  
Bubble Velocity ◽  
Moving Frame ◽  
Taylor Flow ◽  
Bubble Length ◽  
Square Microchannel

Abstract Heat transfer and flow characteristics under air-water Taylor flow in a square microchannel with T-junction were investigated in this work. Different hydraulic diameters of models were discussed numerically by VOF method. Flow patterns such as bubbly flow, slug flow, annular flow and churn flow were identified by both numerical simulation and experimental methods. Simulation results including bubble formation process, bubble length, bubble velocity, void fraction and heat transfer fit well with literature data. The pressure differential of two sides in gas phase played an important role in bubble development. The gas and liquid superficial velocities were found to have a significant impact on bubble behavior. And the higher liquid viscosity would promote higher bubble velocity, also enhance heat transfer, but weaken the void fraction. The results showed a tiny but not ignorable effect of geometric dimensioning on bubble and liquid slug lengths. An appropriate correlation was proposed to estimate bubble length, and the deviation was −10 ~ + 15 %. By using moving frame of reference technique, the internal circulations inside the moving slugs were displayed more clearly.

Thermal and Flow Characteristics of Water-Nitrogen Taylor Flow Inside Vertical Circular Tubes

2016 ◽  
Author(s):  
Jingzhi Zhang ◽  
Wei Li
Keyword(s):  
Heat Transfer ◽  
Single Phase ◽  
Flow Characteristics ◽  
Phase Flow ◽  
Single Phase Flow ◽  
Liquid Slug ◽  
Pressure Drops ◽  
Taylor Flow ◽  
Separate Model ◽  
Bubble Length

Heat transfer and flow characteristics of Taylor flow in vertical capillaries with tube diameters ranging from 0.5 mm to 2 mm have been investigated numerically with the Volume of Fluid (VOF) method. Streamlines, bubble shapes, pressure drops, and heat transfer characteristics of Taylor flow were investigated in detail. The results indicate that the dimensionless bubble length increases with increasing Re, while the variation of diameters have slight influence on it. A flat tail and sharper nose bubble with longer bubble length and thicker film thickness are obtained at higher Re for the increasing inertia force. Pressure drops in liquid slug region are higher than single phase flow because of the Laplace pressure drop. The flow pattern dependent model and modified separate model in this work can predict the simulation data well with a MAE of 2.416% and 2.289%, respectively. Bo and Re are adopted in the modified separate model to taking surface tension, gravity, inertia, and viscous force into account. The wall temperature Tw increases along X axis in liquid region, and gets its peak at the tail of Taylor bubble region. Nutp, which is about 1.2∼3 times of fully developed single phase flow with constant wall heat flux, is negatively proportional to the dimensionless liquid slug length (Ls*). Taylor flow can enhance the heat transfer efficiently.

Effect of Flow Characteristics on Taylor Flow Heat Transfer

2011 ◽  
Vol 51 (4) ◽  
pp. 2010-2020 ◽  
Author(s):  
Sharon S.Y. Leung ◽  
Raghvendra Gupta ◽  
David F. Fletcher ◽  
Brian S. Haynes
Keyword(s):  
Heat Transfer ◽  
Flow Characteristics ◽  
Taylor Flow ◽  
Flow Heat

Numerical Simulation of Taylor Flow in Micro Circular Tubes

2015 ◽  
Author(s):  
Jingzhi Zhang ◽  
Wei Li
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Single Phase ◽  
Volume Fraction ◽  
Flow Characteristics ◽  
Constant Heat Flux ◽  
Bulk Temperature ◽  
Capillary Tubes ◽  
Taylor Flow ◽  
Gas Volume

Heat transfer and flow characteristics of Taylor flow in micro capillary tubes have been investigated numerically with the Volume of Fluid (VOF) method. A constant heat flux (32kwm−2) is adopted at the tube wall. All seven computational cases have the same Reynolds number (Re=280), Capillary number (Ca=0.006) and homogenous void fraction (β=0.51), while the inlet gas volume fraction varies from 0.2 to 0.8. The results indicate that liquid slug length (Ll), gas slug length (Lg) and cell length (Lc) vary with α, while liquid film thickness δ remains constant. The friction factor f of Taylor flow is higher than single phase flow. The simulation results agree well with the correlation proposed by Kreutzer et al.. The Local Nusselt number (Nux) gets its peak value at the liquid film region, where the temperature difference between wall temperature (Tw) and fluid bulk temperature (Tbx) is smallest. The average Nu (Nuav) is about 2.8 times of single phase. This means that Taylor bubble can enhance the heat transfer coefficient in micro capillary tubes.

Investigation of Characteristics of Gas-Liquid Two-Phase Flows in a Rectangular Microchannel With Return Bends

2011 ◽  
Author(s):  
Akimaro Kawahara ◽  
Michio Sadatomi ◽  
Hideki Matsuo ◽  
Satoshi Shimokawa
Keyword(s):  
Void Fraction ◽  
Scaling Law ◽  
Bubble Velocity ◽  
Liquid Slug ◽  
Two Phase Flows ◽  
Two Phase ◽  
Rectangular Microchannel ◽  
Drift Flux Model ◽  
Bubble Length ◽  
Length Data

Gas-liquid two-phase flows in a horizontal rectangular microchannel with return bends have been investigated. The width and the depth of the microchannel are 240 μm and 230 μm, respectively. T-junction type gas-liquid mixer was used to introduce gas and liquid in the channel. In order to know the effects of liquid properties, distilled water, pure ethanol, 49wt% ethanol aqueous solution and HFE7200 were used as the test liquids, while nitrogen gas as the test gas. The flow pattern, the bubble velocity, the bubble length and the liquid slug length were measured, and the void fraction was determined as the ratio of the gas superficial velocity to the bubble velocity. The bubble velocity at a downstream position from the bend is faster than that at an upstream position, and thus the void fraction is smaller at a downstream position. The bubble velocity data were well correlated with the well-known drift flux model with Kawahara et al.’s distribution parameter correlation. The bubble length data at the upstream and the downstream positions are also correlated with the scaling law proposed by Garstecki et al., irrespective of the test liquids. The liquid slug length data are correlated with an exponential function of the void fraction. The ratio of the bubble length to the bubble pitch is also well correlated with a linear function of the homogeneous void fraction.

scholarly journals Study of air-core vortical flow structure induced by a plughole vortex

2017 ◽  
Vol 823 ◽  
pp. 787-818 ◽  
Author(s):  
Rayhan Ahmed ◽  
Heechang Lim
Keyword(s):  
Bubbly Flow ◽  
Bubble Diameter ◽  
Bubble Formation ◽  
Vortical Flow ◽  
Air Bubbles ◽  
Air Bubble ◽  
Naturally Occurring ◽  
Core Method ◽  
Water Head ◽  
Bubble Length

This paper describes a study of the generation of a plughole vortex and its consequences in a drainpipe during drainage of water from a stationary rectangular tank. The critical and minimum depths of water above the inlet of the drainpipe, where a surface dip starts to develop for drainpipes of various diameters, were examined parametrically. This study explored the following naturally occurring phenomena arising from a plughole vortex. (i) A plughole vortex initially causes a surface dip to develop towards the inlet of the drainpipe and as the surface dip approaches the inlet of the drainpipe it creates a droplet-shaped air bubble. (ii) A unique bubble transformation, i.e. from a droplet-shaped to a donut-shaped bubble ring, occurs just after the separation of the droplet-shaped air bubble from the surface dip. (iii) The donut-shaped bubble ring flows with the drain water and initially causes bubbly flow in the drainpipe. (iv) As the water head above the inlet of the drainpipe decreases, the droplet-shaped bubble size increases, and consequently, the bubble ring size increases and causes slug flow in the drainpipe. (v) As the slugs combine, the flow of the draining water eventually becomes annular flow in the drainpipe. Sounds, such as that of instantaneous fizz and bubble sink draining, were observed to be produced as a result of the bubble formation process. Temporal changes in the shape and size of the air bubbles were studied. Within the range of 0.45–0.6, the ratio of the bubble diameter to the bubble length was found to be linearly proportional to the ratio of the water depth to the diameter of the drainpipe. Several drainage cases were simulated numerically to observe the physics of these naturally occurring phenomena. The shapes and sizes of the vortices induced by plugholes have been visualised and analysed using the vortex core method.

Thermal and Flow Characteristics of Water–Nitrogen Taylor Flow Inside Vertical Circular Tubes

2018 ◽  
Vol 140 (8) ◽  
Author(s):  
Jingzhi Zhang ◽  
Wei Li
Keyword(s):  
Heat Transfer ◽  
Single Phase ◽  
Flow Characteristics ◽  
Numerical Data ◽  
Bond Number ◽  
Phase Flow ◽  
Single Phase Flow ◽  
Pressure Drops ◽  
Taylor Flow ◽  
Bubble Rising Velocity

Heat transfer and flow characteristics of Taylor flows in vertical capillaries with tube diameters ranging from 0.5 mm to 2 mm were studied numerically with the volume of fluid (VOF) method. Streamlines, bubble shapes, pressure drops, and heat transfer characteristics of the fully developed gas–liquid Taylor flow were investigated in detail. The numerical data fitted well with experimental results and with the predicted values of empirical correlations. The results indicate that the dimensionless liquid film thickness and bubble rising velocity increase with increasing capillary number. Pressure drops in liquid slug region are higher than the single-phase flow because of the Laplace pressure drop. The flow pattern dependent model and modified flow separation model which takes Bond number and Reynolds number into account can predict the numerical pressure drops well. Compared with the single-phase flow, less time is needed for the Taylor flow to reach a thermal fully developed status. The Nusselt number of Taylor flow is about 1.16–3.5 times of the fully developed single-phase flow with a constant wall heat flux. The recirculation regions in the liquid and gas slugs can enhance the heat transfer coefficient and accelerate the development of the thermal boundary layer.

Characteristics of Two-Phase Flows in a Rectangular Microchannel With a T-Junction Type Gas-Liquid Mixer

2009 ◽  
Author(s):  
Akimaro Kawahara ◽  
Michio Sadatomi ◽  
Keitaro Nei ◽  
Hideki Matsuo ◽  
Takatoshi Masuda
Keyword(s):  
Pressure Drop ◽  
Void Fraction ◽  
Bubble Velocity ◽  
Velocity Data ◽  
Two Phase Flows ◽  
Two Phase ◽  
Rectangular Microchannel ◽  
Drift Flux Model ◽  
Junction Type ◽  
Bubble Length

In this study, gas-liquid two-phase flows in a horizontal rectangular microchannel have been investigated. The rectangular microchannel has the hydraulic diameter of 0.235 mm, and the width and the depth of 0.24 mm and 0.23 mm, respectively. A T-junction type gas-liquid mixer was used to introduce gas and liquid in the channel. In order to know the effects of liquid properties, distilled water, ethanol and HFE7200 were used as the test liquids, while nitrogen gas as the test gas. The flow pattern, the bubble length, the liquid slug length and the bubble velocity in two-phase flow were measured with a high speed video camera, and the void fraction was determined from the bubble velocity data and the superficial gas velocity data. In addition, the pressure drop was also measured with a calibrated differential pressure transducer. The bubble length data were compared with the calculation by the scaling law proposed by Garstecki et al (2006). The bubble velocity data and/or the void fraction data were well correlated with the well-known drift flux model (Zuber and Findlay, 1965) with a new distribution parameter correlation developed in this study. The frictional pressure drop data were also well correlated with Lockhart-Martinelli method with a correlation of two-phase friction multiplier.

Two-Phase Heat Transfer and Bubble Characteristics in a Microchannel Array

2012 ◽  
Vol 134 (7) ◽  
Author(s):  
T. P. Lagus ◽  
F. A. Kulacki
Keyword(s):  
Heat Transfer ◽  
Bubbly Flow ◽  
Bubble Diameter ◽  
Bubble Formation ◽  
Uniform Heat Flux ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Microchannel Array ◽  
Two Phase Heat Transfer

Heat transfer coefficients and bubble motion characteristics are reported for two-phase water flow in an array of 13 equally spaced microchannels over an area of 1 cm2. Each channel has Dh = 451 ± 38 μm, W/H = 0.8, and L/Dh = 22.2. Uniform heat flux is applied through the base, and wall temperatures are determined from the thermocouple readings corrected for heat conduction effects. The upper surface is insulated and transparent. Single-phase heat transfer coefficients are in a good agreement with comparable trends of existing correlations for developing flow and heat transfer, although a difference is seen due to the insulated upper surface. Two-phase heat transfer coefficients and flow characteristics are determined for 221 < G < 466 kg/m2s and 250 < q < 1780 kW/m2. Heat transfer coefficients normalized with mass flux exhibit a trend comparable to that of available studies that use similar thermal boundary conditions. Flow visualization shows expanding vapor slug flow as the primary flow regime with nucleation and bubbly flow as the precursors. Analysis of bubble dynamics reveals ∼t1/3 dependence for bubble growth. Flow reversal is observed and quantified, and different speeds of the vapor phase fronts are quantified at the leading and trailing edges of vapor slugs once the bubble diameter equals the channel width. Bubble formation, growth, coalescence, and detachment at the outlet of the array are best characterized by the Weber number.

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