P01
電場を付与したマイクロチャネル上薄液膜蒸発を利用した高熱流束廃熱デバイスに関する研究
Study on High Heat Flux Thermal Management Devices
Utilizing Thin Liquid Film Evaporation onMicrochannels with Applied Electric Fields
ISMA NURIN BINTI RAZMAN1, 今井良二 1,
ISMA NURIN BINTI RAZMAN1, Ryoji IMAI1,1

1.

室蘭工業大学，Muroran Institute of Technology#1,

Introduction

Recently, in accordance with the rapid growth of the aviation industry, it is assumed that spacecraft will
become more larger and more advanced, and aircraft are transitioning towards electrification, wheretraditional mechanical and hydraulic systems are replaced with electric and electronic components. This isdue to the growing emphasis on reducing the environmental impact of aviation on a global scale, thus
improving cost efficiency, reducing weight, and enhance system reliability. However, there are still several
technological challenges associated with the growing trend in electrification of current engines. One of theconcerns are regarding effective thermal management of the cooling system, where new solutions to dissipate
higher heat loads released by the high-power electric components is required. Moreover, cooling extremely
high heat densities with minimal refrigerant flow can be a challenge, especially in downsized engines wherespace is limited. Efficient thermal management becomes crucial to ensure that the engine components areadequately cooled without compromising performance, durability, or fuel efficiency.
Miniaturizing cooling systems while effectively managing high heat densities is indeed a complex
challenge. As an example, microelectromechanical systems (MEMS) devices often generate heat due to theirminiaturized size and high functionality, making effective thermal management crucial. Japan Society ofMechanical Engineers (JSME) have formulated a technology roadmap for high heat flux removal that outlined
a framework for addressing the challenges posed by escalating heat densities in electronic devices and thenecessity for efficient cooling solutions [1]. Based on the roadmap, this study is setting the goal of heatreducible heat fluxes of 300 [W⁄cm2 ] as a strategy to overcome the current cooling by means of MEMS of
100 [W⁄cm2 ]. In this study, we are exploring thin liquid film evaporative cooling system in microchannel as apotential solution for managing heat, which involves creating thin films of coolant that evaporates quickly to
1 of 8dissipate heat within a confined space. However, the evaporation rate of the liquid is extremely fast, and it isdifficult to form and maintain a stable liquid film. This study suggests that thin liquid film is maintained stably
by replenishing the liquid with the combination of capillary force acting on the microchannel and electrostaticforce generated by the application of an electric field. Therefore, the final purpose of this study is to develop acompact cooling device by optimizing the efficiency of thin liquid film evaporative cooling systems in
microchannel in an approach to address the challenges of cooling high heat densities within compact spaces
and aiming the heat removal of 300 [W⁄cm2 ] . The establishment of this technology can contribute toadvancements in thermal management technologies for which demand will increase in the future.

2.

Overview of High Heat Flux Removal Device

2.1 Heat Transfer Characteristics in Thin Liquid Film Evaporation

In the context of high heat flux removal utilizing thin liquid film evaporation, the cooling systemleverages the principle of evaporation to dissipate heat efficiently, making it a potential solution for managing
heat in miniaturized systems. Thin liquid film evaporation cooling involves creating a thin layer of liquid
coolant on a surface in contact with a heat source. Figure 1 shows the thinning of the liquid film on the rightside. It is known that a three-phase interface is formed in this area, which involves the interaction of threephases: the liquid film, the solid substrate, and the vapor phase. Interfacial evaporation occurs at this interface,where heat is conducted from the high heat flux source through the liquid film to the liquid-vapor interface.
The phase change associated with interfacial evaporation is key to reducing thermal resistance. As the liquid
molecules transition to the vapor phase, they absorb latent heat, where considerable amount of energy isrequired to change the phase without a temperature change. This phase change absorbs a significant amountof heat, hence, effectively enhances the heat removal capacity of the system.

High heat flux is obtained
Water droplet

Figure 1. Thin liquid film evaporation.

2.2 MicrochannelFigure 2 shows a diagram of the microchannel, where it can be seen that it is constructed with microsized grooves that have uniform width and depth in the order of microns. This geometric consistency andappropriate wettability of the microchannel allows the liquid to spread evenly along the grooves, hence,ensuring that the liquid is confined within the microchannel, facilitating the formation of a thin andcontrolled liquid film. As shown in Figure 3, a three-phase interface can be formed on the walls of themicrochannel when liquid is supplied. The controlled flow of liquid into the microchannel, coupled withthe efficient evaporation process, results in a stable and continuous thin liquid film evaporation under
2 of 8

minimum refrigerant flow rate within the microchannel.
Evaporation

Triple-phaseboundary

Refrigerant

Microchannel

Thermal load

Figure 2. Microchannel.

Figure 3. Structure of a microchannel and formation of
three-phase interface.

2.3 Refrigerant Supply

Figure 4 provided illustrates the mechanism of the cooling system device used in this study, bywhich refrigerant is supplied from both ends of the microchannel through a pressure difference. As
shown in Figure 5, the atmospheric pressure and the liquid pressure are equally balanced in the manifold.
However, as the refrigerant moves from the source manifold into the microchannel, a pressure differenceis generated between the atmospheric pressure and the liquid pressure within the grooves, which acts asa driving force that propels the refrigerant through the microchannel. As evaporation causes the liquidfilm to thin, capillary forces help replenish the liquid from the surrounding areas, ensuring a continuous
supply to the evaporating region. Capillary action, driven by surface tension and the geometry of themicrochannels, prevents the liquid from pooling and encourages it to spread uniformly.

Liquid isdriven bycapillaryforce

Manifold
Steam
Thermal load

Figure 4. Schematic diagram of refrigerant supplied

Figure 5. Gas-liquid pressure difference between

from both ends of a microchannel.

manifold and microchannel.

2.4 Method of Applying Electric Field

In cases of extremely high heat fluxes, the three-phase interface can transition into boiling conditions,where rapid vapor generation occurs within the liquid film, hence, the capillary driving pressure can nolonger counteract the losses incurred by liquid and vapor pressure. Boiling enhances the heat removal
capacity but can also introduce risk like dry-out when the capillary limit is reached. This can lead tooverheating and reduced cooling effectiveness. Therefore, in order to address these constraints, theElectrohydrodynamic (EHD) technique stands out for its ability to combine with the capillary structures
3 of 8like microgrooves [2].
The introduction of an electric field at narrow electrode spacing can enhance the drive of themovement of the liquid within the grooves of a microchannel cooling system as shown in Figure 6. Thismechanism of induction in a microchannel with Electrohydrodynamic (EHD) effects involves thegeneration of electric fields within the microchannel when a voltage difference is applied. Then, theelectric field exerts a force on charged particles present in the fluid, which results to Coulomb force,describing the interaction between charged particles. As the Coulomb force acts on the charged particle,
it induces movement in the liquid [3].
To produce a Coulomb force within a liquid, it is necessary to establish a volumetric charge density
or space charge. This objective can be accomplished through three primary approaches: conduction,injection, and induction. This study focuses on the induction method for achieving this phenomenon.
Within the induction process, the space charge arises due to variations or breaks in electric conductivity.
When an electric field is present, a volumetric charge density emerges within the zone of conductivity
gradient. This induced charge density experiences attraction or repulsion, consequently initiating liquidmovement [4]. Figure 7 shows the schematic diagram for the induction between two electrodes.
This combination of capillary and electrostatic forces will enhance the cooling process and mitigate
the challenges associated with the risk of dry-out, thus contribute to achieving the heat removal of
300 [W⁄𝑐𝑚2 ] aimed. By altering the electric field’s properties dynamically, we can control the liquid’sbehavior, allowing for responsive adjustments in the cooling process. The positioning of electrodes thatgenerate the electric field needs to be optimized for effective liquid manipulation. In this study, thecooling system device is constructed with the approach of electrodes configuration parallel to themicrochannel. Due to the increased electric field strength resulting from the narrow electrode spacing,
lower voltage levels are sufficient to induce significant wetting behavior changes. Using lower voltages
translates to lower energy consumption while achieving the desired cooling efficiency of themicrochannel.

SIDE VIEWTOP VIEWElectrode

Plastic LidHigh voltage
power supply

Microchannel

Glasselectrode

Refrigerant

ElectrodeLiquid is driven byattractive force of static
electricity with anarrow electrodespacing

Lid
FRONT VIEW

Steam
Thermal load

Figure 6. Overall view of the cooling system

Triple-phase boundary
Liquid film

Figure 7. Induction between two electrodes.

device.

4 of 8

3.

Experiment on Visualization of Capillary & Electric Force

3.1 Experimental theory

The primary objective of this experiment is to visually and quantitatively study the interplay between
capillary and electric forces with liquids within microchannels. In previous study, experiments wereconducted using microchannels fabricated from silicon wafers with good wettability. It was confirmedthat water was driven into the microchannels both at room temperature and when the microchannels wereheated, hence demonstrating the ability of capillary forces to draw liquid into the microchannels. The
penetration of water into the microchannels was observed to occur faster at higher temperatures. Thisaligns with expectations, as increased temperature generally enhances capillary action due to reduced
surface tension. But due to risk of dry-out as mentioned, electric field is introduced into the system. Sinceextremely high voltages are involved, it is crucial first to visualize and comprehend how liquids are driven
within microchannels under the influence of voltage. Developing techniques to visualize this behavior
and analyzing the flow patterns at different voltages will provide essential insights into the interactions
between voltage, electrostatic forces, and capillary action that could contribute to the optimization designof microchannels cooling systems.

3.2 Visualization of Capillary and Electric Force Experimental Setup

The configuration of electrodes in a microchannel cooling system can significantly influence thebehavior of liquids within the grooves and subsequently impact heat transfer and cooling efficiency.
Parallel electrode placement which involves positioning the electrodes alongside the microchannel wallswith the electric field lines running parallel to the liquid flow direction, can indeed introduce certain
challenges and complexities. Factors such as electrode surface irregularities, variations in spacing, and thenature of the liquid affecting the wetting behavior requires advanced fabrication techniques in achieving
consistent results. Contrarily, perpendicular electrode placement which involves positioning theelectrodes perpendicular to the microchannel walls, intersecting the flow direction allows simplifying
electrode fabrication and placement. In this study, prior to parallel electrode placement experiment,
perpendicular electrode placement experiment is conducted. This sequence provides a baseline forreference and comprehension of the potential challenges and complexities associated with parallel
electrodes configuration.
The visualization of capillary and electric force for perpendicular electrode configuration
experimental setup is shown schematically in Figure 8. The experimental setup consists of tested
microchannel, copper electrodes, DC voltage supply, PTFE board, petri dish, and clamps. Distilled water
was used in the experiments as tested liquid due to its well-defined properties and minimal impurities. It
has low conductivity, making it sensitive to electric fields and providing a clear response to voltage
variations. The microchannel is made out of borosilicate glass, with the dimensions shown in Figure 9.
The experiment involves gradually supplying voltage to the electrodes with electrode spacing of 40mm.
These electrodes generate electric fields across the microchannel. As the voltage is applied, the electric
5 of 8field interacts with the liquid, influencing the movement of the liquid from the petri dish into the grooves.
The rise of the liquid in the grooves in response to the electric field is documented using images and videos.

PTFE board
L-shaped copper
Electrodeclamp

microchannel
CopperElectrodeDistilledwater

Figure 8. Diagram of the visualization of capillary and electric force experiment.

Figure 9. Dimension of the microchannel.

6 of 8

Result

The recorded visual data is analyzed to quantify the rise of the liquid in the grooves under different
voltage conditions. Figure 10 shows the experimental data of the capillary and electric force experimentrecorded. During the experiment, it is observed that the rise of the liquid across multiple grooves in the
microchannel is non-uniform. The recorded experimental values focus on the grooves with the mostsignificant differences from the initial condition (voltage 0 [V]), which prioritizes areas where the electricand capillary forces had a more substantial impact. This allows for the extraction of valuable data evenwhen uniformity is challenging to achieve.
The graph shows the non-uniformity in the rise of liquid across multiple grooves in the microchannel
during the experiment. The data of the 2nd row and 3rd row from the left side of the microchannel is bothtaken to compare the difference in the behavior of the liquid.

24.0

20.0

Height of liquid [mm]

4.

16.0

Rise of liquid after 3 minutes
12.0

8.0

4.0

0.00

400

800

1200

1600

2000

2400

2800

3200

Voltage [V]
3rd row of microchannel

2nd row of microchannel

Figure 10. Experiment values of the capillary and electric force experiment.

7 of 8

5.

Conclusion

In conclusion, the study findings shed light on the crucial role of voltage in influencing the behaviorof the liquid within the microchannel. The observed rise in liquid height as voltage increases underscores
the substantial impact of the electric field. With the combination of capillary force, it is evident that theapplication of voltage is a key factor in inducing and controlling fluid movement throughElectrohydrodynamic (EHD) effects in microgrooves.
However, it is important to acknowledge the presence of non-uniformity in the absolute height of
the liquid across the multiple grooves in the microchannel. While the voltage-induced response is clear,
the variations in liquid height might stem from factors beyond direct control. Subtle microscopic
variations in grooves and minor inconsistencies in electrode effects could contribute to this non-uniform
behavior. As such, future research efforts could focus on minimizing these sources of non-uniformity toenhance the precision and predictability of the induced fluid movement.
In essence, this study advances our understanding of fluid manipulation within microchannels
through EHD effects, emphasizing the significance of voltage while acknowledging the complexities
associated with achieving absolute uniformity in liquid behavior. These insights pave way for further
investigations aimed at refining techniques for precise fluid control in microfluidic applications andcooling system.

6.

References[1] A. Yabe, "JSME 技術ロードマップ," 日本機械学会誌, vol. 119, no. No. 1170, p. 304, 2016.

[2] I. Saad, S. Maalej and M. Zaghdoudi, "Combined effects of heat input power and filling fluid charge on
the thermal performance of an electrohydrodynamic axially grooved flat miniature heat pipe," Applied
Thermal Engineering, vol. 134, pp. 469-483, April 2018.
[3] M. Lackowski, A.Krupa and D. Butrymowicz, "Dielectrophoresis flow control in microchannels," Journalof Electrostatics, vol. 71, pp. 921-925, October 2013.
[4] M. Daaboul, P. Traoré, P. Vázquez and C. Louste, "Study of the transition from conduction to injection in
an electrohydrodynamic flow in blade-plane geometry," Journal of Electrostatics, vol. 88, pp. 71-75, August2017.

© 2023 by the authors. Submitted for possible open access publication under theterms and conditions of the Creative Commons Attribution (CC BY) license(http://creativecommons.org/li censes/by/4.0/).

8 of 8