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.

Numerical Investigation of EHD Conduction Pumping of Liquid Film With Phase-Change

2007 ◽  
Author(s):  
Miad Yazdani ◽  
Jamal Seyed-Yagoobi
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Phase Change ◽  
Flow System ◽  
Thin Liquid Film ◽  
Finite Thickness ◽  
Constant Heat Flux ◽  
Two Dimensional ◽  
Enhanced Heat Transfer ◽  
Constant Heat

Electrohydrodynamic (EHD) conduction pumping is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes, generated by the process of dissociation of the neutral electrolytic species and recombination of the generated ions. This paper numerically investigates the EHD conduction pumping of a thin liquid film in the presence of phase change. The flow system comprises a liquid film flowing over a two-dimensional flat plate while the vapor phase extended far beyond the interface to result in almost motionless vapor. The channel is separated into four different sections: the entrance, electrode, evaporation, and downstream sections. The entrance, electrode and downstream regions are adiabatic while a constant heat flux is applied in the evaporation side. The concept of EHD conduction pumping of liquid film in the presence of phase change is demonstrated in this paper. The enhanced heat transfer due to conduction pumping is evaluated.

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.

Phase-Change Heat Transfer of Ethanol-Water Mixtures: Towards Development of a Distributed Hydrogen Generator

2013 ◽  
Author(s):  
Manoj Kumar Moharana ◽  
Rohan M. Nemade ◽  
Sameer Khandekar
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Phase Change ◽  
Transfer Coefficient ◽  
Single Phase ◽  
Flow Boiling ◽  
Phase Region ◽  
Design Flow ◽  
Two Phase ◽  
Vapor Region

Hydrogen fuel from renewable bio-ethanol is a potentially strong contender as an energy carrier. Its distributed production by steam reforming of ethanol on microscale platforms is an efficient upcoming method. Such systems require (a) a pre-heater for liquid to vapor conversion of ethanol water mixtures (b) a gas-phase catalytic reactor. We focus on the fundamental experimental heat transfer studies (pool and flow boiling of ethanol-water mixtures) required for the primary pre-heater boiler design. Flow boiling results (in a 256 μm square channel) clearly show the influence of mixture composition. Heat transfer coefficient remains almost constant in the single-phase region and rapidly increases as the two-phase region starts. On further increasing the wall superheat, heat transfer starts to decrease. At higher applied heat flux, the channel is subjected to axial back conduction from the single-phase vapor region to the two-phase liquid-vapor region, thus raising local wall temperatures. Simultaneously, to gain understanding of phase-change mechanisms in binary mixtures and to generate data for the modeling of flow boiling process, pool-boiling of ethanol-water mixtures has also been initiated. After benchmarking the setup against pure fluids, variation of heat transfer coefficient, bubble growth, contact angles, are compared at different operating conditions. Results show strong degradation in heat transfer in mixtures, which increases with operating temperature.

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.

Numerical Simulation of Condensation in a Rectangular Minichannel Using VOF Model

2012 ◽  
Author(s):  
Zhenyu Liu ◽  
Bengt Sunden ◽  
Jinliang Yuan
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Phase Change ◽  
Industrial Applications ◽  
Heat Transfer Process ◽  
Numerical Results ◽  
Condensation Heat Transfer ◽  
Source Terms ◽  
Two Phase ◽  
Vof Model

The understanding of two-phase flow and heat transfer with phase change in minichannels is needed for the design and optimization of heat exchangers and other industrial applications. In this study a three-dimensional numerical model has been developed to predict filmwise condensation heat transfer inside a rectangular minichannel. The Volume of Fluid (VOF) method is used to track the vapor-liquid interface. The modified High Resolution Interface Capture (HRIC) scheme is employed to keep the interface sharp. The governing equations and the VOF equation with relevant source terms for condensation are solved. The surface tension is taken into account in the modeling and it is evaluated by the Continuum Surface Force (CSF) approach. The simulation is performed using the CFD software package, ANSYS FLUENT, and an in-house developed code. This in-house code is specifically developed to calculate the source terms associated with phase change. These terms are deduced from Hertz-Knudsen equation based on the kinetic gas theory. The numerical results are validated with data obtained from the open literature. The standard k-ω model is applied to model the turbulence through both the liquid and vapor phase. The numerical results show that surface tension plays an important role in the condensation heat transfer process. Heat transfer enhancement is obtained due to the presence of the corners. The surface tension pulls the liquid towards the corners and reduces the average thermal resistance in the cross section.

scholarly journals Experimental analysis of the evaporation of a thin liquid film deposited on a capillary heated tube: estimation of the local heat transfer coefficient

2021 ◽  
Vol 2116 (1) ◽  
pp. 012110
Author(s):  
L Cattani ◽  
F Bozzoli ◽  
V Ayel ◽  
C Romestant ◽  
Y Bertin
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Film ◽  
Transfer Coefficient ◽  
Heat Conduction Problem ◽  
Thin Liquid Film ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Inverse Heat Conduction Problem ◽  
Two Phase

Abstract The aim of this work is to estimate the local heat flux and heat transfer coefficient for the case of evaporation of thin liquid film deposited on capillary heated channel: it plays a fundamental role in the two-phase heat transfer processes inside mini-channels. In the present analysis it is investigated a semi-infinite slug flow (one liquid slug followed by one single vapour bubble) in a heated capillary copper tube. The estimation procedure here adopted is based on the solution of the inverse heat conduction problem within the wall domain adopting, as input data, the temperature field on the external tube wall acquired by means of infrared thermography.

Simulation of Single Bubble Evaporation in a Microchannel in Zero Gravity With Thermocapillary Effect

2018 ◽  
Vol 140 (11) ◽  
Author(s):  
Wei Li ◽  
Yang Luo ◽  
Jingzhi Zhang ◽  
W. J. Minkowycz
Keyword(s):  
Heat Transfer ◽  
Capillary Tube ◽  
Flow Boiling ◽  
Thin Liquid Film ◽  
Local Heat Transfer ◽  
Heated Wall ◽  
Thermocapillary Effect ◽  
Two Phase ◽  
Thermocapillary Force ◽  
Good Convergence

This paper presents fundamental research on the hydrodynamics and heat transfer surrounding a single elongated bubble during flow boiling in a circular microchannel. A continuum surface force (CSF) model based on the volume of fluid (VOF) method is combined with the thermocapillary force to explore the effects of thermocapillarity for flow boiling in microchannels. To validate the self-defined codes, a two-phase thermocapillary-driven flow and a Taylor bubble growing in a capillary tube are studied. Results of both test cases show good convergence and agreement with data from the earlier literature. The bubble motion and the local heat transfer coefficient (HTC) on the heated wall with respect to time are discussed. It is found that for large Marangoni number (case 3), variation of surface tension has affected the bubble shape and temperature profile. The thermocapillary effect induces convection in a thin liquid film region, which augments the HTCs at specified positions. The numerical investigation also shows that the average HTC increased by 6.7% in case 3 when compared with case 1. Thus, it is very important to study further the effects of themocapillarity and the Marangoni effect on bubble growth in microchannels.

Heat Transfer Mechanisms During Flow Boiling in Microchannels

2003 ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Contact Line ◽  
Single Phase ◽  
Flow Boiling ◽  
Reynolds Numbers ◽  
Nucleate Boiling ◽  
Two Phase ◽  
Mass Fluxes ◽  
Transfer Mechanisms

The forces due to surface tension, inertia, and momentum change during evaporation in microchannel govern the two-phase flow patterns and the heat transfer characteristics during flow boiling. These forces are analyzed in this paper, and two new non-dimensional groups, K1 and K2, relevant to flow boiling phenomenon are derived. These groups are able to represent some of the key flow boiling characteristics, including the CHF. The small hydraulic dimensions of microchannel flow passages present a large frictional pressure drop in single-phase and two-phase flows. In order to keep the pressure drop within limits, the channel lengths are generally shorter and the mass fluxes are generally lower than those with conventional channels (Dh>3 mm). The resulting lower mass fluxes, coupled with small Dh, lead to Reynolds numbers in the range 100–1000. Such low Reynolds numbers are rarely employed for flow boiling in conventional channels. In these low Reynolds number flows, nucleate boiling systematically emerges as the dominant mode of heat transfer. Aided by strong evaporation rates, the bubbles nucleating on the wall grow quickly and fill the entire channel. The contact line between the bubble base and the channel wall surface now becomes the entire perimeter at both ends of the vapor slug. Evaporation occurs at the moving contact line of the expanding vapor slug as well as over the channel wall covered with a thin liquid film surrounding the vapor core. The usual nucleate boiling heat transfer mechanisms, including liquid film evaporation and transient heat conduction in the liquid adjacent to the contact line region, play an important role. The liquid film under the large vapor slug evaporates completely at downstream locations thus presenting a dryout condition periodically with the passage of each large vapor slug. The flow boiling correlation by Kandlikar [1, 2] with (i) the nucleate boiling dominant region equation, and (ii) the laminar flow equation for single-phase all-liquid flow heat transfer coefficient hLO was successful in correlating the available R-134a data for parallel microchannels of 190 μm hydraulic diameter.

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.

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