Use of Two-Phase CFD Simulations to Develop a Boiling Heat Transfer Prediction Method for Slug Flow Within Microchannels

2015 ◽  
Author(s):  
Mirco Magnini ◽  
John R. Thome
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Slug Flow ◽  
Boiling Heat Transfer ◽  
Prediction Method ◽  
Liquid Slug ◽  
Modeling Framework ◽  
Cfd Simulations ◽  
Two Phase ◽  
Elongated Bubble

This work presents a new boiling heat transfer prediction method for slug flow within microchannels, which is developed and benchmarked against the results of two-phase CFD simulations. The proposed method adopts a two-zone decomposition of the flow for the sequential passage of a liquid slug and an evaporating elongated bubble. The heat transfer is modeled by assuming transient heat conduction across the liquid film surrounding an elongated bubble and sequential conduction/convection within the liquid slug. Embedded submodels for estimating important flow parameters, e.g. bubble velocity and liquid film thickness, are implemented as “building blocks”, thus making the entire modeling framework totally stand-alone. The CFD simulations are performed by utilizing ANSYS Fluent v. 14.5 and the interface between the vapor and liquid phases is captured by the built-in Volume Of Fluid algorithm. Improved schemes to compute the surface tension force and the phase change due to evaporation are implemented by means of self-developed functions. The comparison with the CFD results shows that the proposed method emulates well the bubble dynamics during evaporation, and predicts accurately the time-averaged heat transfer coefficients during the initial transient regime and the terminal steady-periodic stages of the flow.

Two-Phase Flow Phenomena for Boiling in Two Parallel Microchannels

2004 ◽  
Author(s):  
H. Y. Li ◽  
P. C. Lee ◽  
F. G. Tseng ◽  
Chin Pan
Keyword(s):  
Heat Transfer ◽  
Slug Flow ◽  
Boiling Heat Transfer ◽  
Two Phase Flow ◽  
Nucleate Boiling ◽  
Vapor Flow ◽  
Phase Flow ◽  
Two Phase ◽  
Silicon Bulk ◽  
Flow Phenomena

Boiling heat transfer and corresponding two-phase flow phenomena are of significant interest for the design of a compact evaporator. The present work investigates experimentally, using a high-speed digital CCD camera, the two-phase flow phenomena for boiling in a silicon-based, two parallel trapezoid microchannels, which were prepared by the combination of silicon bulk micro machining and Pyrex-silicon wafer bonding. Onset of nucleate boiling, bubbly flow, slug flow, and partial dry out slug flow are typically observed along the flow direction. The appearance of the partial dryout slug flow may degrade the nucleate boiling heat transfer in the microchannel. At a low flow rate, reversed vapor flow is observed. In such a flow pattern, liquid droplets are formed intermittently on the inner wall of top Pyrex glass due to vapor condensation. Moreover, the reversed vapor flow usually accompanies with large magnitude two-phase flow oscillations.

Heat Transfer Characteristics in a Slug Unit

2010 ◽  
Author(s):  
Valery Babin ◽  
Dvora Barnea ◽  
Lev Shemer
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Slug Flow ◽  
Video Camera ◽  
Transfer Mechanism ◽  
Heat Transfer Mechanism ◽  
Vertical Pipe ◽  
Two Phase ◽  
Taylor Bubble ◽  
Hydrodynamic Parameters

Heat transfer mechanism in two-phase flows and particularly in vertical slug flow is of high interest both for basic hydrodynamic research and for industrial applications. Two-phase slug flow is highly complicated and only a limited number of heat transfer studies have been carried out. The flow field around a single Taylor bubble propagating in a vertical pipe can be subdivided into three distinct hydrodynamic regions: the gas bubble surrounded by a thin liquid film, a highly turbulent liquid wake in the vicinity of the bubble bottom, and the far wake region. Experimental and theoretical works have been presented during the last decades investigating the hydrodynamic parameters in each region. Due to the complexity and intermittent nature of slug flow the existing data on the heat transfer in slug flow is limited to a narrow range of operational conditions. To improve the understanding of the heat transfer mechanism in slug flow a new experimental setup was constructed. A part of the vertical pipe wall was replaced by a thin metal foil heated by electrical current. An IR video camera was used to determine the temporal variation of the instantaneous temperature field along the foil. The video camera was synchronized with a sensor that determined the instantaneous location of the Taylor bubble. The results of the instantaneous heat transfer measurements along the liquid film and in the wake of the Taylor bubble can be correlated with the detailed velocity measurements carried out in the same facility (Shemer et al. 2007). The effect of the local hydrodynamic parameters on the heat transfer coefficient in each region is examined.

Computational Study of Saturated Flow Boiling Within a Microchannel in the Slug Flow Regime

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
Mirco Magnini ◽  
John R. Thome
Keyword(s):  
Heat Transfer ◽  
Slug Flow ◽  
Flow Boiling ◽  
Periodic Regime ◽  
Liquid Slug ◽  
Modeling Framework ◽  
Saturated Flow ◽  
Bubble Liquid ◽  
Continuous Stream ◽  
Multiple Bubbles

This paper presents a fundamental study of the flow dynamics and heat transfer induced by a slug flow under saturated flow boiling in a circular microchannel. Numerical simulations are carried out by utilizing the commercial CFD solver ansys fluent v. 14.5, with its built-in volume of fluid (VOF) method to advect the interface, which was improved here by implementing self-developed functions to model the phase change and the surface tension force. A continuous stream of bubbles is generated (by additional user-defined functions) by patching vapor bubbles at the channel upstream with a constant generation frequency. This modeling framework can capture the essential features of heat transfer in slug flows for a continuous stream of bubbles which are here investigated in detail, e.g., the mutual influence among the growing bubbles, the fluid mechanics in the liquid slug trapped between two consecutive bubbles, the effect of bubble acceleration on the thickness of the thin liquid film trapped against the channel wall and on other bubbles, and the transient growth of the heat transfer coefficient and then its periodic variation at the terminal steady-periodic regime, which is reached after the transit of a few bubble–liquid slug pairs. Furthermore, the results for a continuous stream of bubbles are found to be quite different than that of a single bubble, emphasizing the importance of modeling multiple bubbles to study this process. Finally, the outcomes of this analysis are utilized to advance a theoretical model for heat transfer in microchannel slug flow that best reproduces the present simulation data.

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.

Two-Phase Concurrent Separated Flow Model for Boiling Heat Transfer in Narrow Vertical Rectangular Space

2005 ◽  
Author(s):  
Liang-Ming Pan ◽  
Xiangfei Liang ◽  
Ming-Dao Xin ◽  
Tien-Chien Jen ◽  
Qinghua Chen
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Heat Transfer Enhancement ◽  
Boiling Heat Transfer ◽  
Separated Flow ◽  
Thermal Engineering ◽  
Transfer Enhancement ◽  
Two Phase ◽  
Micro Channels ◽  
Vapor Liquid

Compared with conventional channels, narrow and micro channels have significant characteristic of heat transfer enhancement. With smooth internal surface, such channels can efficiently avoid encrustation at the washing effect of the high-speed liquid. Moreover, heat transfer elements can be easily assembled. This type of channels have been adopted extensively in many engineering applications, e.g. microelectronic cooling, Advanced Nuclear Reactor, cryogenic, aviation and space technology and thermal engineering. In recent years, much work was focused upon flow patterns, heat transfer and pressure drop. Almost everyone thought the heat transfer enhancement mechanism of narrow and micro channels to be bubbles’ deformation and disturbance, which is insufficient to explain the heat transfer enhancement. In present work, an innovative model of quasi-one-dimensional vapor liquid two-phase concurrent separated flow was proposed for boiling heat transfer in vertical narrow rectangular space. Numerical results such as boiling heat transfer coefficient and liquid film thickness were obtained. Comparison of model results with reported experimental correlation indicates that the proposed model can predict heat transfer in narrow channels correctly, with the relative deviation less than 14%. Numerical simulating result confirms that heat conduction through liquid film is the predominant mechanism of boiling heat transfer in vapor liquid separated flow region in a vertical narrow rectangular space.

Boiling Heat Transfer in a Horizontal Small-Diameter Tube

1993 ◽  
Vol 115 (4) ◽  
pp. 963-972 ◽  
Author(s):  
M. W. Wambsganss ◽  
D. M. France ◽  
J. A. Jendrzejczyk ◽  
T. N. Tran
Keyword(s):  
Heat Transfer ◽  
Flow Pattern ◽  
Slug Flow ◽  
Boiling Heat Transfer ◽  
Small Diameter ◽  
Heat Fluxes ◽  
Mean Deviation ◽  
Physical Parameters ◽  
Transfer Coefficients ◽  
Two Phase

Results of a study on boiling heat transfer of refrigerant R-113 in a small-diameter (2.92 mm) tube are reported. Local heat transfer coefficients are measured for a range of heat flux (8.8–90.75 kW/m2), mass flux (50–300 kg/m2s), and equilibrium mass quality (0–0.9). The measured coefficients are used to evaluate 10 different heat transfer correlations, some of which have been developed specifically for refrigerants. High heat fluxes and low mass fluxes are inherent in small channels, and this combination results in high boiling numbers. In addition, based on a flow pattern map developed from adiabatic experiments with air-water mixtures, it has been shown that small-diameter channels produce a slug flow pattern over a large range of parameters when compared with larger-diameter channels. The effects of high boiling number and slug flow pattern lead to domination by a nucleation mechanism. As a result, the two-phase correlations that predicted this dominance also predicted the data the best when they properly modeled the physical parameters. The correlation of Lazarek and Black (1982) predicted the data very well. It is also shown that a simple form, suggested by Stephan and Abdelsalam (1980) for nucleate pool boiling, correlates the data equally well; both correlations are within a mean deviation of less than 13 percent. Results are applicable to boiling in compact heat exchangers.

Measurement of Liquid Film Thickness in Micro Tube Annular Flow

2010 ◽  
Author(s):  
Hiroshi Kanno ◽  
Youngbae Han ◽  
Yusuke Saito ◽  
Naoki Shikazono
Keyword(s):  
Heat Transfer ◽  
Film Thickness ◽  
Liquid Film ◽  
Annular Flow ◽  
Liquid Film Thickness ◽  
Phase Flow ◽  
Two Phase ◽  
Micro Scale ◽  
Film Model ◽  
Annular Film

Heat transfer in micro scale two-phase flow attracts large attention since it can achieve large heat transfer area per density. At high quality, annular flow becomes one of the major flow regimes in micro two-phase flow. Heat is transferred by evaporation or condensation of the liquid film, which are the dominant mechanisms of micro scale heat transfer. Therefore, liquid film thickness is one of the most important parameters in modeling the phenomena. In macro tubes, large numbers of researches have been conducted to investigate the liquid film thickness. However, in micro tubes, quantitative information for the annular liquid film thickness is still limited. In the present study, annular liquid film thickness is measured using a confocal method, which is used in the previous study [1, 2]. Glass tubes with inner diameters of 0.3, 0.5 and 1.0 mm are used. Degassed water and FC40 are used as working fluids, and the total mass flux is varied from G = 100 to 500 kg/m2s. Liquid film thickness is measured by laser confocal displacement meter (LCDM), and the liquid-gas interface profile is observed by a high-speed camera. Mean liquid film thickness is then plotted against quality for different flow rates and tube diameters. Mean thickness data is compared with the smooth annular film model of Revellin et al. [3]. Annular film model predictions overestimated the experimental values especially at low quality. It is considered that this overestimation is attributed to the disturbances caused by the interface ripples.

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