INVESTIGATION OF BUBBLE FREQUENCY IN SLUG FLOW REGIME FOR FLOW BOILING IN A SINGLE ROUND UNIFORMLY HEATED MICRO-CHANNEL

2016 ◽  
Author(s):  
Amen Younes ◽  
Ibrahim Hassan ◽  
Lyes Kadem
Keyword(s):  
Flow Regime ◽  
Slug Flow ◽  
Flow Boiling ◽  
Micro Channel ◽  
Bubble Frequency

Investigation of Bubble Frequency for Slug Flow Regime in a Uniformly Heated Horizontal Microchannel

2017 ◽  
Vol 139 (6) ◽  
Author(s):  
Amen Younes ◽  
Ibrahim Hassan ◽  
Lyes Kadem
Keyword(s):  
Flow Pattern ◽  
Flow Regime ◽  
Slug Flow ◽  
Flow Boiling ◽  
Operating Conditions ◽  
Bubble Frequency ◽  
Drift Flux Model ◽  
Wide Range ◽  
Bubble Length ◽  
Bubble Zone

Slug flow is an essential flow pattern observed in microchannels where its transition boundaries in microchannels are characterized by two complex hydrodynamic phenomena, the bubble confinement and the bubble coalescence. Slug flow may be classified in terms of bubble size into two major zones: isolated bubble zone and coalescence bubble zone. In this paper, a semi-analytical model is developed for predicting the main characteristics of isolated bubble zone for flow boiling in a horizontal microchannel. The influences of surface tension, shear, and inertial forces have been taken into account. The model is developed on the basis of drift flux model, and a fully developed slug unit is chosen as a control volume for deriving the equations of motion. The effects of main operating conditions, mass and heat fluxes, on bubble length and bubble frequency have been investigated. The boundaries of slug flow regime have been identified based on the most proper diabatic flow pattern maps available in the literature for the chosen database. The model has been validated using the database available in the literature for flow boiling of R134a and R245fa in 0.509 mm and 3.0 mm inner diameter horizontal mini-tubes, respectively, and over wide range of mass fluxes (300≤G≤1000 kg/m2 s). This study has shown that the mass flux has a significant effect on the slug length and the bubble frequency. The model gave a good agreement with the experimental data of bubble length and bubble frequency with a mean absolute error (MAE) of 18.0% and 27.34%, respectively.

Size Effect on Adiabatic Gas-Liquid Two-Phase Flow Map and Void Fraction in Micro-Channels

2006 ◽  
Author(s):  
Renqiang Xiong ◽  
J. N. Chung
Keyword(s):  
Linear Relationship ◽  
Flow Regime ◽  
Slug Flow ◽  
Phase Flow ◽  
Micro Channel ◽  
Two Phase ◽  
Void Fractions ◽  
Micro Channels ◽  
Mini Channels ◽  
Churn Flow

Adiabatic gas-liquid two-phase flow patterns and void fractions in micro-channels were experimentally investigated. Using nitrogen gas and water, experiments were conducted in near-square micro-channels with hydraulic diameters of 0.209mm, 0.412mm and 0.622 mm, respectively. The main objective was focused on the effects of micro-scale channel sizes. Gas and liquid superficial velocities were varied from 0.06-72.3 m/s and 0.02-7.13 m/s, respectively. Four defined flow patterns including bubbly-slug flow, slug-ring flow, dispersed-churn flow and annular flow were observed in both micro-channels of 0.412 mm and, 0.622 mm in hydraulic diameter. In the micro-channel of 0.209 mm, the bubbly-slug flow turned into the slug-flow and the dispersed-churn flow was not observed. The current flow regime maps showed that the transition lines shifted towards higher gas superficial velocity due to the stronger surface tension effect as the channel size was reduced. The micro-channel flow regime maps were found to be quite different from those of mini-channels. Measured time-averaged void fractions in our micro-channels held a non-linear relationship with the homogeneous void fraction as oppose to the relatively linear trend for the mini-channels. A new correlation was proposed to predict the non-linear relationship that fits most of the experimental data of the current three micro-channels and those of the 0.1 mm diameter tube reported by Kawahara et al. within ± 15%.

Adiabatic Gas-Liquid Two-Phase Flow Patterns in Microchannels

2006 ◽  
Author(s):  
Renqiang Xiong ◽  
J. N. Chung
Keyword(s):  
Surface Tension ◽  
Flow Regime ◽  
Slug Flow ◽  
Flow Patterns ◽  
Phase Flow ◽  
Micro Channel ◽  
Surface Tension Effect ◽  
Two Phase ◽  
Micro Channels ◽  
Churn Flow

Flow patterns of adiabatic gas-liquid two-phase flow in micro-channels were experimentally investigated. Using nitrogen and water, experiments were conducted in square micro-channels with hydraulic diameters of 0.209mm, 0.412mm and 0.622 mm, respectively. Gas and liquid superficial velocities were varied from 0.06–72.3 m/s and 0.02–7.13 m/s, respectively. Four defined flow patterns, bubbly-slug flow, slug-ring flow, dispersed-churn flow and annular flow, were observed in micro-channels of 0.412 mm and, 0.622 mm. In the micro-channel of 0.209 mm, the bubbly-slug flow became the slug-flow and the dispersed-churn flow disappeared due to the surface tension effect and the smooth gas-liquid interface. The flow regime maps for the current three micro-channels were constructed and showed the transition lines shifted to higher gas superficial velocity due to the stronger surface tension effect with a smaller channel size in micro-channels. They were also compared with some other micro-channel flow regime maps and the mini-channel flow regime map based on the Weber number model, which showed the flow map for the micro-channel is significantly sensitive to the working fluid, channel geometry and channel size and the flow regime criteria developed for mini-channels should not be applied for micro-channels without further verification.

An Analytical Model of Flow Boiling Heat Transfer for Slug Flow in a Single Circular Horizontal Micro-Channel

2012 ◽  
Author(s):  
Amen M. Younes ◽  
Ibrahim Hassan
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Film ◽  
Transfer Coefficient ◽  
Slug Flow ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Micro Channel ◽  
Vapor Quality ◽  
Flow Boiling Heat Transfer

Slug flow is one of the most common flow patterns that occur during flow boiling in horizontal micro-channels. In the present work, an analytical model of flow boiling heat transfer is developed for slug flow in a single circular horizontal micro-channel under a uniform heat flux. The heat transfer is affected mainly by the liquid film thickness confined between the vapor slug and the channel wall. For more physical and reliable flow boiling heat transfer model, the liquid film thickness variation and pressure gradient effects on the flow boiling heat transfer coefficient are considered. The influence of vapor quality on heat transfer coefficient, vapor velocity and liquid film velocity is studied. The model is constructed based on the conservation equations of the separated two phase flow. The interphase surface is assumed to be smooth and the flow is a laminar flow. The obtained model applied for flow boiling of R-134a refrigerant in the slug flow at a narrow vapor quality range (0.0 < x < 0.1). The heat transfer coefficient showed a high increase close to the low vapor quality while decreases gradually after the peak. Furthermore, the vapor velocity increases linearly by increasing the vapor quality while, the liquid film velocity decreases.

Flow Regime Classification and Transition of Flow Boiling Through Porous Channel

2009 ◽  
Author(s):  
Bofeng Bai ◽  
Xiaojie Zhang ◽  
Maolong Liu ◽  
Wang Su
Keyword(s):  
Heat Flux ◽  
Flow Regime ◽  
Mass Flux ◽  
Slug Flow ◽  
Flow Boiling ◽  
Particle Diameter ◽  
Experimental Parameter ◽  
Flow Regimes ◽  
Two Phase ◽  
Porous Channels

In the present research, a visual experiment was carried out on the flow regimes of the porous channels in the spherical fuel element nuclear reactor. Boiling two-phase flow in different porous channels composed of particles with diameters of 4mm, 6mm and 8mm were studied respectively, and four different flow regimes occur within the experimental parameter range: bubbly flow, bubbly-slug flow, slug flow and slug-annular flow. The effects of heat flux, mass flux and particle diameter on the flow regimes were obtained. Bubbles and slugs deform, coalesce and break up more frequently, and increase in both number and size with the increase of the heat flux; bubbles and slugs tend to decrease in number and size with higher mass flux and particles of smaller diameters. At higher mass flux, a higher heat flux is needed to get the same flow regime that occurs at lower mass flux; with particles of smaller diameter, a higher heat flux is needed to get the same flow regime as that of particles of larger diameter. The flow regime map and flow regime transition have been proposed by modifying the void fractions of Tung/Dhir model.

Quantitative Visualization of Vapor Bubble Growth in Diabatic Vapor-Liquid Microchannel Slug Flow

2015 ◽  
Author(s):  
Todd A. Kingston ◽  
Justin A. Weibel ◽  
Suresh V. Garimella
Keyword(s):  
Flow Regime ◽  
Vapor Bubble ◽  
Slug Flow ◽  
Flow Boiling ◽  
Flow Behavior ◽  
Circular Cross Section ◽  
Vapor Flow ◽  
Liquid Slug ◽  
Two Phase ◽  
Vapor Liquid

Slug flow is a commonly encountered flow regime in microchannels due to the influence of surface tension and vapor confinement at small length scales. Few experimental studies have considered diabatic vapor-liquid slug flow, owing to difficulties in generating a well-controlled and repeatable slug flow regime; generation of vapor by wall heating typically leads to large, stochastic variations in the vapor bubble characteristics. To facilitate the study of flow behavior and vapor-liquid interfaces under precisely controlled conditions, a diabatic, one-component, two-phase microchannel flow was generated by separately injecting HFE-7100 vapor and liquid into a T-junction. Injection at independently controllable liquid and vapor flow rates allows the creation of vapor-liquid slug flow patterns in a downstream borosilicate microchannel of circular cross-section with a 500 μm inside diameter. The outside surface of the microchannel was coated with a 100 nm-thick layer of indium tin oxide (ITO) to generate a uniform wall heat flux via Joule heating while allowing full optical access for flow visualization. The growth of individual vapor bubbles was quantitatively visualized at different imposed heat fluxes, in terms of the percentage change in vapor bubble length along the heated microchannel. The results demonstrate the ability of the T-junction to generate diabatic, one-component, two-phase microchannel slug flow that is suitable for generating results for the validation of flow boiling models.

Bubble Characteristics in a Developing Vertical Gas–Liquid Upflow Using a Conductivity Probe

1999 ◽  
Vol 122 (1) ◽  
pp. 138-145 ◽  
Author(s):  
Junping Zhang ◽  
Norman Epstein ◽  
John R. Grace ◽  
Kokseng Lim
Keyword(s):  
Flow Regime ◽  
Slug Flow ◽  
Flow Region ◽  
Flow Regimes ◽  
Chord Length ◽  
Bubble Velocity ◽  
Gas Velocity ◽  
Bubble Frequency ◽  
Local Bubble ◽  
Bubble Characteristics

Experiments were carried out in an 82.6-mm-dia column with a perforated distributor plate. Conductivity probes on the axis of the column were used to measure local bubble properties in the developing flow region for superficial air velocities from 0.0018 to 6.8 m/s and superficial water velocities from 0 to 0.4 m/s, corresponding to the discrete bubble, dispersed bubble, coalesced bubble, slug, churn, bridging, and annular flow regimes. Bubble frequency increased linearly with gas velocity in the discrete and dispersed bubble regimes. Bubble frequency also increased with gas velocity in the slug flow regime, but decreased in the churn and bridging regimes. Bubble chord length and its distribution were smaller and narrower in the dispersed than in the discrete bubble regime. Both the average and standard deviation of the bubble chord length increased with gas velocity in the discrete, dispersed, and churn flow regimes. However, the average bubble chord length did not change significantly in the slug flow regime due to the high population of small bubbles in the liquid plugs separating Taylor bubbles. The bubble travel length, defined as the product of local gas holdup and local bubble velocity divided by local bubble/void frequency, is used to correlate bubble characteristics and to characterize the flow regimes. [S0098-2202(00)00101-2]

On Instabilities in Horizontal Two-Phase Flow

1994 ◽  
Vol 59 (12) ◽  
pp. 2595-2603
Author(s):  
Lothar Ebner ◽  
Marie Fialová
Keyword(s):  
Differential Equation ◽  
Flow Regime ◽  
Slug Flow ◽  
Annular Flow ◽  
Two Phase Flow ◽  
Phase Flow ◽  
Two Phase ◽  
Annular Flows ◽  
Flow Regime Maps

Two regions of instabilities in horizontal two-phase flow were detected. The first was found in the transition from slug to annular flow, the second between stratified and slug flow. The existence of oscillations between the slug and annular flows can explain the differences in the limitation of the slug flow in flow regime maps proposed by different authors. Coexistence of these two regimes is similar to bistable behaviour of some differential equation solutions.

Visualization of R134a flow boiling in micro-channels to establish a novel bubbly-slug flow transition criterion

2018 ◽  
Vol 91 ◽  
pp. 230-244 ◽  
Author(s):  
Xuejiao Li ◽  
Li Jia ◽  
Chao Dang ◽  
Zhoujian An ◽  
Qian Huang
Keyword(s):  
Slug Flow ◽  
Flow Boiling ◽  
Flow Transition ◽  
Micro Channels ◽  
Transition Criterion

scholarly journals Flow Pattern Map of Flow Boiling in a Rectangular Channel Filled with Porous Media

Energies ◽  
2021 ◽  
Vol 14 (9) ◽  
pp. 2440
Author(s):  
Youngwoo Kim ◽  
Dae Yeon Kim ◽  
Kyung Chun Kim
Keyword(s):  
Porous Media ◽  
Flow Pattern ◽  
Flow Regime ◽  
Annular Flow ◽  
Flow Boiling ◽  
Rectangular Channel ◽  
Flow Patterns ◽  
Flow Pattern Map ◽  
Mist Flow ◽  
Churn Flow

A flow visualization study was carried out for flow boiling in a rectangular channel filled with and without metallic random porous media. Four main flow patterns are observed as intermittent slug-churn flow, churn-annular flow, annular-mist flow, and mist flow regimes. These flow patterns are clearly classified based on the high-speed images of the channel flow. The results of the flow pattern map according to the mass flow rate were presented using saturation temperatures and the materials of porous media as variables. As the saturation temperatures increased, the annular-mist flow regime occupied a larger area than the lower saturation temperatures condition. Therefore, the churn flow regime is narrower, and the slug flow more quickly turns to annular flow with the increasing vapor quality. The pattern map is not significantly affected by the materials of porous media.

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