Simulation of condensation flow in a rectangular microchannel

https://doi.org/10.1016/j.cep.2013.12.004Get rights and content

Highlights

  • The condensation flow in a rectangular microchannel is numerically studied.

  • The simulated flow patterns agree well with the experimental observations.

  • Waves along the interface form necks decreasing the local vapor pressure.

  • The initial bubble size increases as the flow mass flux increases.

  • The initial bubble size increases as the heat flux decreases.

Abstract

The condensation flow of the refrigerant FC-72 in a rectangular microchannel with a 1-mm hydraulic diameter is numerically studied using the volume of fluid (VOF) model. The heat transfer related to the condensation is taken into account by a thermal equilibrium model assuming the interface temperature is at saturation. The numerical method is validated against experiments from the literature and well predicts the flow patterns along the microchannel. The vapor phase in the microchannel forms a continuous column with a decreasing diameter from upstream to downstream. Slugs are periodically generated at the head of the column. Decreasing the wall cooling heat flux or increasing the flow mass flux increases the vapor column length. Waves along the interface cause necks in the column and locally increase the vapor velocity and decrease the pressure, facilitating breakage of the vapor column into slugs. The liquid temperature is close to saturation near the interface and lower downstream and in the thin liquid layer close to the cooling surface. The initial bubble size increases with increasing flow mass flux or decreasing cooling heat flux.

Introduction

Rapid developments in many cutting-edge technologies require efficient heat removal from areas subject to increasingly high heat fluxes. Examples include advanced electronics in computers, automobiles and aircraft. For heat fluxes exceeding 10,000 W m−2, natural and forced convection with air and liquids are incapable of efficiently dissipating the heat; and device temperatures may rise above safe operating levels, resulting in devastating failures. Two-phase boiling flow in microchannels is a prominent candidate for heat transfer at extremely high heat fluxes above 10,000 W m−2. With recent progress in manufacturing, microchannels can be fabricated in small regions exposed to high heat fluxes. Current fabrication methods include wire-sawing and chemical etching.

When heat is removed from the target device via microchannel boiling with two-phase flow in a cooling loop (called a primary loop here), the heat then needs to be efficiently rejected to a secondary loop and finally to the surroundings. Compact condensers, sometimes called micro-condensers, have microchannels arranged to thermally connect the two loops to transfers heat from the primary loop to the secondary loop. Compact condensers are much smaller and less expensive than commercial macrochannel counterparts [1]. Compact condensers, for example with channel hydraulic diameters of 1.11 and 0.80 mm [2], have been successfully used in automobile air conditioners for decades [3]. The compact condenser is also used as one of the key components in micro-refrigeration systems [4], [5], [6].

On the primary loop side of a compact condenser, the refrigerant vapor from the hot target devices, is condensed into liquid in the microchannels with release of the latent heat in the refrigerant to the secondary loop side. Thus, condensation flow and heat transfer in the microchannels are central to the thermal performance of the compact condenser so they have attracted extensive academic and industrial interest. Yang and Web investigated R12 condensation flow in four parallel mini-channels with plain and micro-fin extruded tubes [7]. Yan and Lin [8], Wang et al. [9], and Koyama et al. [2] studied condensation heat transfer in parallel microchannels with hydraulic diameters of 0.81–2.0 mm. The flow patterns which determine the condensation heat transfer rate differ in microchannels from that in conventional macrochannels. Garimella et al. [10] developed a flow pattern map for condensation flow in circular microchannels with special interest on the intermittent flow regime. Hu and Cheng [11] reported on vapor injection flow and related condensation instabilities in microchannels. Hu and Chao [12] found that slug-bubbly flow was the dominant flow pattern and measured the heat transfer and pressure drop for water vapor condensation in a micro-condenser.

While the flow pattern and the associated heat transfer for microchannel condensation flows have been extensively explored using experimental observation and measurements, the details of the flow field, local temperature distribution and interface motion remain to be better understood. This constitutes the primary motivation for the present study. Numerical simulations supplementing experiment studies give additional details on the flow and heat transfer. The volume of fluid (VOF) model proposed by Hirt and Nichols [13] has been extensively used for two-phase flow simulations to track the liquid–vapor interface motion which strongly affects the two-phase flow and heat transfer in microchannels. Tomiyama et al. [14], Gopala and Wachem [15] and Jeon et al. [16] employed the VOF model to study a single rising bubble and found good agreement with experimental observations. The VOF model was also successfully used to study the velocity and local wall shear stress distributions around bubble slugs [17], coalescence of two bubbles [18] and the lift force on a bubble [19].

To take into account the heat and mass transfer at a liquid–vapor interface during phase change, mass and energy sources terms need to be defined in the continuum and energy equations of the VOF model. Yang et al. [20] used the VOF model with self-defined source terms to simulate the boiling flow of R141b in a serpentine tube, with the flow patterns found to agree well with their experiment observations. Huang et al. [21] studied bubble growth in R141b boiling flows in the coiled part of a serpentine tube using the VOF model with the simulations revealing the unusual contraction of bubbles due to the flow re-distribution induced by centrifugal forces in the bends, as was observed in their experiments. Da Riva and Del Col [22] used the VOF model to simulate R134a film condensation in a horizontal circular minichannel and found that the film thickness remained almost constant in the upper half of the channel while the film in the bottom half became thicker due to gravity.

Although a number of studies have investigated condensation flows in microchannels, few have presented detailed flow and heat transfer results. The present study uses a VOF model with phase change to study condensation flow in microchannels. The model is first validated against existing experimental flow patterns from the open literature and then used to study FC-72 condensation flow for different mass fluxes and cooling heat fluxes. Detailed flow and temperature profiles are presented to give for a good understanding of the heat transfer mechanisms.

Section snippets

VOF model

Even though the liquid–vapor interface changes its location and shape with time, the VOF model employs a fixed grid to model two-phase flows. The volume fraction of phase i in each cell is represented by αi, with the sum of the volume fractions for all phases adding up to 1, i.e.:iαi=1

In a liquid–vapor two-phase flow, when a cell is filled with liquid, the liquid volume fraction, αl, is 1 and the corresponding vapor volume fraction, αv, is 0. αl is 0 and αv is 1 when a cell is filled with

Model validation

Before the extensive case studies, the model was first validated against available experimental data. This was done by examining the flow patterns for different regimes of microchannel condensation flow. Kim et al. [1] experimentally studied the condensation flow of FC-72 in parallel, square microchannels with a hydraulic diameter of 1 mm and a length of 29.9 cm with sufficient details on flow patterns for specific flow and thermal conditions. Since the channel structure is exactly the same as in

Results and discussion

The validated model was then used to study the flow and heat transfer details for the FC-72 condensation two-phase flow in the microchannel, including the flow patterns, velocity and temperature profiles and the slug flow. FC-72 is a dielectric fluid which can be safely used for heat transfer in electronic devices without the risk of electrical short-circuits. FC-72 has a saturation temperature of 56 °C at atmospheric pressure which is 10–20 °C above common environmental temperatures so it is

Conclusions

A two-phase VOF model with phase change was used to study condensation flow of FC-72 in a rectangular microchannel with a hydraulic diameter of 1 mm. The simulated flow patterns, including bubbly flow, slug flow, transition flow, wavy annular flow and smooth annular flow regimes, agree well with experimental observations.

In the microchannel, the vapor phase forms a continuous column with a decreasing diameter downstream. Slugs are periodically generated at the head of the column. Decreasing the

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (Nos. U0934006 and 51106083) and Science Fund for Creative Research Groups (No. 51321002).

References (31)

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