Suppressed Dry-out in Two-Phase Microchannels via Surface Structures

2016 ◽  
Vol 138 (8) ◽  
Author(s):  
Yangying Zhu ◽  
Dion S. Antao ◽  
Tiejun Zhang ◽  
Evelyn N. Wang
Keyword(s):  
Liquid Film ◽  
Annular Flow ◽  
Flow Boiling ◽  
Thin Liquid Film ◽  
Structure Design ◽  
Frame Rate ◽  
Structured Surfaces ◽  
Boiling Regime ◽  
Structured Surface ◽  
Dry Out

We demonstrated suppressed dry-out on structured surfaces during flow boiling in microchannels. We designed and fabricated microchannels with well-defined silicon micropillar arrays (heights of ~25 µm, diameters of 10 µm and pitches of 40 µm) coated with silicon dioxide on the bottom heated channel wall. We visualized the flow fields inside a smooth and structured surface microchannel during the annular flow boiling regime with a high speed camera at a frame rate of 2000 fps. Time-lapse images revealed two distinct dry-out dynamics for the two types of surfaces. For the smooth surface, the thin liquid film broke-up into smaller liquid drops/islands and the surface stayed in a dry state after the drops evaporated. The microstructured surface, on the other hand, preserved the thin liquid film initially due to capillary wicking. Dry patches eventually formed at the center of the microchannel which indicated wicking in the transverse direction (from the sidewalls inward) in addition to wicking in the flow direction. Overall, the structured surface showed less instances of dry-out both spatially and temporally. These visualizations aid in the understanding of the stability of the thin liquid film in the annular flow boiling regime and provide insight into heat transfer enhancement mechanisms by leveraging surface structure design in microchannels.

Modeling of Dry-Out Incipience for Flow Boiling in a Circular Microchannel at a Uniform Heat Flux

2015 ◽  
Vol 137 (2) ◽  
Author(s):  
Amen Younes ◽  
Ibrahim Hassan
Keyword(s):  
Heat Flux ◽  
Liquid Film ◽  
Flow Regime ◽  
Annular Flow ◽  
Flow Boiling ◽  
Operating Conditions ◽  
Inertia Force ◽  
Saturated Flow ◽  
Working Fluids ◽  
Dry Out

Dry-out is an essential phenomenon that has been observed experimentally in both slug and annular flow regimes for flow boiling in mini and microchannels. The dry-out leads to a drastic drop in heat transfer coefficient, reversible flow and may cause a serious damage to the microchannel. Consequently, the study and prediction of this phenomenon is an essential objective for flow boiling in microchannels. The aim of this work is to develop an analytical model to predict the critical heat flux (CHF) based on the prediction of liquid film variation in annular flow regime for flow boiling in a horizontal uniformly heated circular microtube. The model is developed by applying one-dimensional (1D) separated flow model for a control volume in annular flow regime for steady, and sable saturated flow boiling. The influence of interfacial shear and inertia force on the liquid film thickness is taken into account. The effects of operating conditions, channel sizes, and working fluids on the CHF have been investigated. The model was compared with 110 CHF data points for flow boiling of various working fluids, (water, LN2, FC-72, and R134a) in single and multiple micro/minichannels with diameter ranges of (0.38≤Dh≤3.04 mm) and heated-length to diameter ratios in the range of (117.7 (117.7≤Lh/D≤470)470). Additionally, three CHF correlations developed for saturated flow boiling in a single microtube have been employed for the model validation. The model showed a good agreement with the experimental CHF data with mean absolute error (MAE) = 19.81%.

THIN LIQUID FILM DYNAMICS AND BUBBLE BEHAVIOR IN FLOW BOILING

2016 ◽  
Vol 4 (4) ◽  
pp. 279-292 ◽  
Author(s):  
Ke Wang ◽  
Ruinan Lin ◽  
Na Wu ◽  
Weimin Ma ◽  
Bofeng Bai
Keyword(s):  
Liquid Film ◽  
Flow Boiling ◽  
Thin Liquid Film ◽  
Bubble Behavior

scholarly journals Numerical Simulation of Evaporating Two-Phase Flow in a High-Aspect-Ratio Microchannel with Bends

2017 ◽  
Vol 139 (2) ◽  
Author(s):  
Zhenhai Pan ◽  
Justin A. Weibel ◽  
Suresh V. Garimella
Keyword(s):  
Aspect Ratio ◽  
Liquid Film ◽  
High Aspect Ratio ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Fluid Velocity ◽  
Thin Liquid Film ◽  
Heated Wall ◽  
Phase Flow ◽  
Two Phase

Despite the demand for high-performance, two-phase cooling systems, high-fidelity simulations of flow boiling in complex microchannel geometries remains a challenging numerical problem. We conduct a first-principles-based simulation of an evaporating two-phase flow in a high-aspect-ratio microchannel with bends using a volume of fluid-based numerical model. For the case shown, the lower horizontal section of the microchannel has a constant flux of 20 W/cm2 applied to the wetted wall area (heat flux at the base of 133 W/cm2); HFE-7100 vapor and liquid enter the channel at 2 m/s. The three-dimensional channel geometry requires a refined near-wall numerical mesh to resolve thin liquid film flow features. The recently developed saturated-interface-volume phase change model (Int J Heat Mass Trans 93:945-956, 2016) is implemented for prediction of mass and energy exchange across the liquid-vapor interface at a low computational cost (~80 hr; 6-core parallelization on Intel Xeon E3-1245V3). The model reveals transport details including the interface shape and fluid velocity and temperature fields. The interfacial temperature remains fixed at saturation with smooth velocity contours near the interface. The highest evaporation flux is located in the thin liquid film region near the heated wall.

Experimental Comparison of Flow Boiling Heat Transfer in High Aspect Ratio Microchannels With Plain and Grooved Walls

2020 ◽  
Vol 142 (12) ◽  
Author(s):  
Xiao Cheng ◽  
Huiying Wu
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Liquid Film ◽  
High Aspect Ratio ◽  
Heat Sink ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Experimental Comparison ◽  
Dry Out

Abstract The dry-out easily occurs on high-aspect ratio microchannel sidewalls due to the decreasing of liquid film thickness. In this paper, the triangular microgrooves possessing the characteristic of evaporating meniscus were designed on the microchannel sidewalls. The heat sink consisted of 33 parallel microchannels, having a hydraulic diameter of 100 μm and an aspect ratio of 4. A platinum film heater and platinum resistance temperature detectors (RTDs) were integrated on the backside of the heat sink to realize uniform heating and precise temperature measurement, respectively. Flow boiling visualization experiments were carried out by high-speed camera in triangular groove-wall and plain-wall microchannels at mass fluxes of 148–490 kg/m2·s and inlet temperatures of 42 °C and 60 °C. The boiling curve, heat transfer coefficient (HTC), pressure drop, and two-phase flow boiling instability were systematically investigated to assess the flow boiling performances. Thin liquid film was observed in the triangular grooves during the dry-out process, compared to the dry-out in plain-wall microchannels. The oscillations of wall temperature, inlet temperature, and pressure drop were significantly suppressed in triangular groove-wall microchannels. Moreover, the earlier onset of nucleate boiling, improved heat flux, and HTC were realized in triangular groove-wall microchannels compared to plain-wall microchannels. Therefore, triangular groove design on the sidewalls is a promising solution to enhance boiling heat transfer and suppress flow boiling instabilities for high-aspect ratio microchannels.

Temporal Effect and Transient Simulation of Thin Liquid Film in Micro Channels

2013 ◽  
Author(s):  
Yu-Yan Jiang ◽  
Da-Wei Tang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Liquid Film ◽  
Flow Boiling ◽  
Thin Liquid Film ◽  
Liquid Films ◽  
Transient Simulation ◽  
Governing Equations ◽  
Flash Evaporation ◽  
Micro Channels

The evaporation and heat transfer of thin liquid film are crucial factors affecting on the heat transfer performance of boiling bubbles or slugs. For boiling in micro-channels, the flash evaporation of the liquid film may give rise to boiling instability, and the dry-out of the film leads to serious deterioration of the heat transport. The thin liquid film has multi-scale transitions, and hence the phase change and fluid dynamics need to be solved by special governing equations and numerical algorithm. The numerical studies to date have solved the steady state distribution of the film, but the difficulty consists in the transient simulation of time-variant liquid films. In the present study, unsteady form governing equations are developed. With inclusion of the temporal terms, we conducted transient simulations for flat liquid films formed during the flow boiling in micro-channels. The model predicts the developing of drying spot during growth of elongated bubbles. The results show that the film thickness and distribution change quickly in a growth period, which are functions of the heat flux, mass flow rate and the other parameters. The quantitative assessment of these effects helps to clarify the mechanism of boiling instability and the conditions for the occurrence of critical heat flux (CHF). The simulation needs special numerical scheme for time marching and stabilization treatment for the nonlinear terms, where the numerical accuracy and the significance of the temporal effects are also discussed.

A Cost-Effective Modeling Approach for Simulating Phase Change and Flow Boiling in Microchannels

2015 ◽  
Author(s):  
Zhenhai Pan ◽  
Justin A. Weibel ◽  
Suresh V. Garimella
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Phase Change ◽  
Volume Fraction ◽  
Flow Boiling ◽  
Thin Liquid Film ◽  
Cost Effective ◽  
Source Terms ◽  
Time Step ◽  
Two Phase

High-fidelity simulation of flow boiling in microchannels remains a challenging problem, but the increasing interest in applications of microscale two-phase transport highlight its importance. In this paper, a volume of fluid (VOF)-based flow boiling model is proposed with computational expense-saving features that enable cost-effective simulation of two-phase flow and heat transfer in realistic geometries. The vapor and liquid phases are distinguished using a color function which represents the local volume fraction of the tracked phase. Mass conservation is satisfied by solving the transport equations for both phases with a finite-volume approach. In order to predict phase change at the liquid-vapor interface, evaporative heat and mass source terms are calculated using a novel, saturated-interface-volume phase change model that fixes the interface at the saturation temperature at each time step to achieve stability. Numerical oscillation of the evaporation source terms is thus eliminated and a non-iterative time advancement scheme can be adopted to reduce computational cost. The reference frame is set to move with the vapor slug to artificially increase the local velocity magnitude in the thin liquid film region in the relative frame, which reduces the influence of numerical errors resulting from calculation of the surface tension force, and thus suppresses the development of spurious currents. This allows use of non-uniform meshes that can efficiently resolve high-aspect-ratio geometries and flow features and significantly reduces the overall numerical expense. The proposed model is used to simulate the growth of a vapor bubble in a heated 2D axisymmetric microchannel. The bubble motion, bubble growth rate, liquid film thickness, and local heat transfer coefficient along the wall are compared against previous numerical studies.

scholarly journals Liquid film–induced critical heat flux enhancement on structured surfaces

2021 ◽  
Vol 7 (26) ◽  
pp. eabg4537
Author(s):  
Jiaqi Li ◽  
Daniel Kang ◽  
Kazi Fazle Rabbi ◽  
Wuchen Fu ◽  
Xiao Yan ◽  
...  
Keyword(s):  
Heat Flux ◽  
Liquid Film ◽  
Critical Heat Flux ◽  
Thin Liquid Film ◽  
Bubble Formation ◽  
Contact Lines ◽  
Bubble Behavior ◽  
Two Phase ◽  
Structured Surfaces ◽  
Bubble Evolution

Enhancing critical heat flux (CHF) during boiling with structured surfaces has received much attention because of its important implications for two-phase flow. The role of surface structures on bubble evolution and CHF enhancement remains unclear because of the lack of direct visualization of the liquid- and solid-vapor interfaces. Here, we use high-magnification in-liquid endoscopy to directly probe bubble behavior during boiling. We report the previously unidentified coexistence of two distinct three-phase contact lines underneath growing bubbles on structured surfaces, resulting in retention of a thin liquid film within the structures between the two contact lines due to their disparate advancing velocities. This finding sheds light on a previously unidentified mechanism governing bubble evolution on structured surfaces, which has notable implications for a variety of real systems using bubble formation, such as thermal management, microfluidics, and electrochemical reactors.

Numerical Investigation On Bubble Growth and Merger in Microchannel Flow Boiling with Self-rewetting Fluid

2021 ◽  
Author(s):  
Wei Li ◽  
Yuhao Lin ◽  
Yang Luo
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Liquid Film ◽  
Numerical Investigation ◽  
Contact Line ◽  
Bubble Growth ◽  
Flow Boiling ◽  
Thin Liquid Film ◽  
Microchannel Flow ◽  
Thermocapillary Force

Abstract The application of two-phase flow in microchannel needs further research to achieve a more stable and highly-performed heat sink. Utilizing self-rewetting fluid is one of the promising ways to minimize the dryout area, thus increasing the heat transfer coefficient and critical heat flux (CHF). To investigate the heat transfer performance of self-rewetting fluid in microchannel flow boiling, a numerical investigation is carried out in this study utilizing the VOF method, phase-change model and continuum surface force (CSF) model with surface tension versus temperature. Athree-dimensional numerical investigation of bubble growth and merger is carried out with water and 0.2%wt heptanol solution. The single bubble growing cases, two x-direction/y-direction bubbles merging cases and three bubbles merging cases are conducted. Since the bubbles never detach the heated walls, the dryout area and regions nearby the contact line with thin liquid film dominated the heat transfer process during the bubbles' growth and merger. The self-rewetting fluid is able to minimize the local dryout area and achieve the larger thin liquid film area around the contact line due to the Marangoni effect and thermocapillary force, thus result in higher wall heat flux when compared to water. The two x-direction bubbles merging case performed best for heat transfer in the microchannel, in which self-rewetting fluid achieves heat transfer enhancement for over 50 percent compared with water.

Local Heat Transfer of Saturated Flow Boiling in Vertical Narrow Microchannel

2020 ◽  
Vol 142 (7) ◽  
Author(s):  
Junye Li ◽  
Yuhao Lin ◽  
Wei Li ◽  
Kan Zhou
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Liquid Film ◽  
Annular Flow ◽  
Flow Boiling ◽  
High Heat ◽  
Flow Patterns ◽  
High Heat Flux ◽  
Bubble Flow ◽  
Saturated Flow

Abstract An experimental study of saturated flow boiling in a high-aspect-ratio one-side-heating rectangular microchannel was conducted with de-ionized water as the working fluid. ZnO microrods with the average diameter of about 1 μm and length of about 7 μm were synthesized on the Ti wafer surface, which was used to fabricate the heated bottom surface of the microchannel. The ZnO microrod surface appeared to be hydrophobic and the capillary wetting effect on the surface was found after being wet. The heat transfer and pressure drop characteristics of saturated flow boiling in the microchannel were studied and the flow patterns were photographed with a high-speed camera. Almost all the flow patterns observed in this experiment featured the main annular flow and abrupt flush of bubbly flow. Because of the capillary wetting effect on the ZnO microrod surface, the local dryout and rewetting phenomenon did not appear in this study. However, due to the numerous nucleation sites on ZnO microrod surface, the abrupt bubble flow caused much more disruption to the liquid film of annular flow when compared to the regular silicon surface. The abrupt bubble flow flushed through the annular liquid film and caused the fluctuation and nonuniformity of the liquid film and heat transfer deterioration, which was severer in the high heat flux conditions. Otherwise, the capillary effect on the ZnO microrod surface was able to restrict the nonuniformity of the liquid film under high heat flux and low mass flux conditions; thus, the deterioration of heat transfer performances diminished.

Physics of Transition to Annular Flow in Microchannel Flow Boiling Process

2018 ◽  
Author(s):  
Meisam Matin ◽  
Abdy Fazeli ◽  
Saeed Moghaddam
Keyword(s):  
Liquid Film ◽  
Flow Regime ◽  
High Speed ◽  
Annular Flow ◽  
Flow Boiling ◽  
Thermal Characteristics ◽  
Transitional Region ◽  
Boiling Process ◽  
The Difference ◽  
Complex Phenomena

Transition to annular flow regime in microchannels is arguably one of the most complex phenomena in the flow boiling process. The instability of the vapor-liquid interface in this interstitial regime presents an intricate situation in which the interface pattern rapidly changes with the mass flow rate and surface heat flux. Although a few past studies have reported observing this regime, thermohydraulics of the process and flow and boundary conditions under which this transition occurs have remained largely unknown. The main obstacle in deciphering the physics of this process is lack of measurement tools to characterize hydrodynamics and thermal characteristics of this flow regime at microscales. The present study benefits from implementation of a novel test device that enables measuring the liquid film thickness and its rapid variations with micrometer and microseconds spatial and temporal resolutions. It is determined that each flow regime has a unique surface temperature signature that enables its clear distinction without need for high-speed visualization. Based on the dynamics of the flow, we identified that the transitional region is comprised of two regimes coalescing bubbles (CB) and semi-annular flow conditions. The difference between these two flow regimes emanates from motion of liquid film beneath the bubble.

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