Application of Boiling Heat Transfer to High-Heat-Flux · PDF file127 Suzuki et al.:...

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Suzuki et al.: Application of Boiling Heat Transfer (1/7)

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Application of Boiling Heat Transfer to High-Heat-Flux Cooling

Technology in Power ElectronicsKoichi Suzuki*, Kazuhisa Yuki*, and Masataka Mochizuki**

*Tokyo University of Science, Yamaguchi, 1-1-1, Daigaku-dori, Sanyo-Onoda-shi, Yamaguchi 756-0884, Japan

**Thermal Tech. Div., Fujikura Corp., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan

(Received August 16, 2011; accepted September 12, 2011)

Abstract

Boiling heat transfer is a superior heat transfer technology using the latent heat transport with phase-change. However,

it has been difficult to employ as a cooling technology for electronics because the unstable transition boiling and film

boiling with excessive high temperature are impossible to control. In highly subcooled boiling, the coalescing bubbles

formed on the heating surface collapse to many fine bubbles at the beginning of transition boiling and the heat flux

exceeds the critical heat flux. This boiling regime has been called Microbubble Emission Boiling (MEB). Two models of

cooling device are introduced using subcooled flow boiling with MEB for power electronics, where the maximum heat

flux is 500 W/cm2 (5 MW/m2).

Keywords: MEB (Microbubble Emission Boiling), High Heat Flux, Cooling Technology, Power Electronics

NomenclaturehB ; Heat transfer coefficient in boiling, kJ/m2K

hMEB ; Heat transfer coefficient in MEB, kJ/m2K

q ; Heat flux, W/m2

Tl ; Bulk temperature of liquid, K

Tsat ; Saturation temperature of liquid, K

Tw ; Temperature of heating surface, K

ΔTsub ; Liquid subcooling, K

ΔTsat ; Superheat of heating surface, K

1. IntroductionSince the end of the last century, we have serious prob-

lems with the natural environment, energy resources, and

global warming. In particular, the reduction of CO2 emis-

sions is an important problem to be resolved urgently.

In order to reduce CO2 emission and save fossil fuels,

the development of electric vehicles (EVs) has recently

been accelerated. In the near future, automobiles driven

by petrol or oil will be replaced by EVs or fuel cell vehicles

(FCVs). In the EV power control system, an IC package is

employed for electronic power equipment such as an

inverter. These IC inverters produce large amount of heat

and the maximum heat flux has been predicted at higher

than 300 W/cm2.

Recently, Silicon Carbide, SiC, has attracted interest as

a new power semiconductor material in place of the con-

ventional Silicon. The maximum operating temperature of

SiC is said to be 400~450°C. However, the actual operating

temperature should be lower than 200°C in order to

decrease power loss and to protect circuit parts from high

temperatures. Of course, it is impossible to remove such

high heat flux from the new power IC devices by conven-

tional cooling technologies, namely air cooling or liquid

cooling.

Boiling heat transfer is well known as a superior heat

transfer technology to remove large amounts of heat from

a hot body using the transport of latent heat with a phase-

change. However, there are some difficulties with applying

this cooling method to electronics. The heating surface

becomes covered with coalescing bubbles at the critical

heat flux (CHF), the maximum of nucleate boiling where

the liquid is not fully in contact with the heating surface.

The remaining liquid layer or microlayer below the

coalescing bubbles evaporates rapidly and the surface

begins to dry. The boiling then turns rapidly to film boiling

with the instantaneous temperatures rising through transi-

tion boiling. Finally, the surface is seriously damaged by

the high temperature burnout. Generally, it is impossible

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Transactions of The Japan Institute of Electronics Packaging Vol. 4, No. 1, 2011

to control the transition boiling. So it has been difficult to

apply boiling heat transfer to the cooling technology in

electronics.

In highly subcooled boiling, many microbubbles are

emitted from coalescing bubbles formed on the heating

surface at the beginning of transition boiling and the heat

flux exceeds CHF considerably, as CHF shown in Fig. 1

and 2.[1] This boiling regime has been called Microbubble

Emission Boiling (MEB). The investigation of MEB was

started by Shigeaki Inada, Professor of Gunma National

University, in 1981.[2–5] More recently, MEB research

has been conducted by a group led by Satoshi Kumagai,

Professor of Tohoku National University, for subcooled

flow boiling[6–8] and by Kin-ichi Torikai and Koichi

Suzuki for channel flow boiling.[9, 10] For example, the

maximum heat flux is higher than 1000 W/cm2 in sub-

cooled flow boiling of water with heating surfaces smaller

than 10 mm × 10 mm.[8, 11] According to the investiga-

tions of MEB, the authors proposed two models for cool-

ing devices in power electronics. The maximum cooling

heat flux was 500 W/cm2 in subcooled flow boiling of

water with microbubble emissions under atmospheric con-

ditions. The present paper introduces Microbubble Emis-

sion Boiling and its application to cooling technology in

power electronics.

2. Outline of Microbubble Emission Boiling (MEB)Bubble behaviors in subcooled pool boiling are shown

in Fig. 2 as a typical example of MEB.[1] A copper heating

surface 10 mm in diameter is placed facing upward on the

bottom of the boiling vessel. The surface is polished with

#500 sand-paper. The boiling liquid is a mixture of water,

an aqueous solution of 10 wt% Ethanol and an aqueous

solution of 7.5 wt% Propanol. The boiling is performed in

subcooled quasi-pool boiling under atmospheric condi-

tions. Large coalescing bubbles formed on the heating sur-

face at CHF collapse to many fine bubbles at the beginning

of transition boiling. The liquid is supplied to the heating

surface as the bubbles collapse. The supplied liquid then

evaporates and the boiling bubbles again expand to large

coalescing bubbles at CHF. The process is repeated rap-

idly and the heat flux exceeds the critical heat flux.[9, 10]

Microbubble emission boiling occurs significantly in

highly subcooled flow boiling. Boiling curves with MEB

are shown in Fig. 3 as an example of subcooled flow boil-

ing in a horizontal rectangular channel.[10] As an exam-

ple, a test section for channel flow boiling is shown in Fig.

4. The channel is 17 mm in height, 12 mm in width and 150

mm in length and is set horizontally. The 10 mm × 10 mm

heating surface is the upper surface of a copper heating

block placed at the bottom of the horizontal rectangular

channel. The heating block consists of a straight section

and a base block. A number of cartridge heaters are

Fig. 1 Typical Boiling curve including MEB.

Fig. 2 Video pictures of microbubbles emitted from coalesc-ing bubbles formed on heating surface (10 mm in diameter) in subcooled quasi-pool boiling with MEB at liquid subcooling of 40 K.

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inserted in the base section. Three K-type sheathed ther-

mocouples 0.5 mm in diameter are fitted in the axis of the

straight section with 1 mm, 3 mm and 5 mm from the heat-

ing surface. The surface temperature is estimated by

extrapolating the measured temperature distribution and

the heat flux is calculated by the temperature gradient

using Fourier’s thermal conduction equation in steady

state. The heating surface is polished with #500 sandpaper

prior to the boiling test. The heating block is enveloped

with thermal insulations and the heat loss is estimated at

less than 5% of the axial heat flow in the straight section

according to a number of previous experiments by the

authors.

Distilled water is used for the boiling liquid and the

mean pressure in the channel is maintained near atmo-

spheric pressure. The mass flow rate is 500 kg/m2s (0.5

m/s). Bubble behaviors on the heating surface are

observed using a high-speed video camera through a

transparent glass window assembled in the side housing of

the channel. Pressure fluctuations are measured by an

electronic pressure transducer installed over the heating

surface.

Subcooled boiling was tested for liquid subcooling at 20

K, 30 K, 40 K, 50 K and 60 K.[10] At the beginning of tran-

sition boiling, MEB occurs and the heat fluxes exceed the

CHFs, as shown in Fig. 3. The maximum heat flux reaches

10 MW/m2 at the higher liquid subcooling of 40 K. How-

ever, no MEB occurs at the 20 K liquid subcooling or the

boiling turns instantaneously to transition boiling without

increasing heat flux even if MEB occurs. Therefore, the

liquid subcooling is an important factor for MEB genera-

tion.

The liquid subcooling ΔTsub is defined as the difference

between the saturation temperature of the liquid Tsat and

the bulk liquid temperature, Tl , as indicated in equation

(1). The superheating of the heating surface, ΔTsat , is

defined as the difference between the temperature of the

heating surface, Tw , and the saturation temperature of the

liquid, Tsat , as indicated in equation (2).

ΔTsub = Tsat - Tl (1)

ΔTsat = Tw - Tsat (2)

The heat transfer coefficient is given by the boiling

curves shown in Fig. 3. The heat transfer coefficient in

boiling is based on the superheating of the heating surface

as in equation (3).

hB, hMEB = q/ΔTsat (3)

For example, the heat transfer coefficient hB in nucleate

boiling is about 80,000 W/m2K and hMEB in MEB is

140,000~200,000 W/m2K given by the experimental result

shown in Fig. 3. Those heat transfer coefficients are con-

siderably higher than those in single phase flow heat trans-

fer.

3. Applications of MEB for Cooling TechnologyGenerally, an inverter used for an EV is composed of IC

packages and the cooling surface is considered large, 10

cm × 20 cm for example. The thermal emission rate is pre-

dicted to be higher than 100 W/cm2 (1 MW/m2), because

the thermal emission issued from an inverter is generally

estimated to be at least 20 percent of the maximum power

output of an electric vehicle, which is assumed to be 100

kW. For cooling an inverter with a single channel, it is dif-

ficult to remove the heat from the down-stream section of

cooling surface as shown in Fig. 5. The maximum heat

flux and CHF are indicated for the length of the surface in

Fig. 6 in our experimental results on the subcooled flow

boiling of water in a horizontal rectangular channel. The

heating surfaces are placed at the bottom of a horizontal

Fig. 3 An example of subcooled flow boiling of water with MEB at liquid velocity of 0.5 m/s (500 kg/m2s).

Fig. 4 An example of subcooled flow boiling of water with MEB at liquid velocity of 0.5 m/s (500 kg/m2s).

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rectangular channel 5 mm in height. The temperature of

the distilled water is 40 K of liquid subcooling at the inlet

of the channel and the mean flow velocity is 0.5 m/s (500

kg/m2s). The maximum heat flux and CHF decrease with

the length of the heating surface. The critical heat flux

agrees roughly with the correlation proposed by

Haramura-Katto[12] and Ivey-Morris.[13] For a long heat-

ing surface, the maximum heat flux decreases as shown in

Fig. 6. The experimental data indicate that the limit of cool-

ing length in MEB is less than 50 mm.

According to the experimental results on subcooled flow

boiling of water in a horizontal rectangular channel, the

authors have proposed two models of high-heat-flux cool-

ing device for power electronics.

3.1 Compact High-Heat-Flux Cooling Device (CHCD) [11, 14, 15]

Subcooled flow boiling was conducted for a horizontal

rectangular channel with a flat heating surface placed at

the bottom of the channel.[14] The channel is 5 mm in

height, 24 mm in width, and 300 mm in length for a single

unit. The heating surface is made of copper and is 20 mm

in width and 50 mm in length. The boiling curves at liquid

subcooling of 40 K and liquid velocity of 0.5 m/s (500

kg/m2s) are shown in Fig. 7.[14] Significant microbubble

emission boiling occurs and the heat flux exceeds the

CHF. The maximum heat flux obtained is 500 W/cm2 (5

MW/m2) and the maximum heat removal is 5 kW.

The test section is useful for a high-heat-flux cooling

device where the length of the cooling surface is shorter

than 50 mm. The proposed simple cooling device is named

CHCD (Compact High-Heat-Flux Cooling Device).

A concept of the composite type of CHCD is shown in

Fig. 8.[11] The maximum heat removed from the power

component is 120 kW. However, high pressure fluctuations

are generated in MEB as shown in Fig. 9.[14] MEB is

extremely vigorous reaching a maximum pressure of

700~800 kPa.

In order to reduce the excessive instantaneous pressure

fluctuations, many needle rods 0.5 mm in diameter and 4~4.5

mm in length are assembled in a lattice with 3~5 mm sepa-

ration just above the heating surface from the upper hous-

ing of the rectangular channel, as shown in Fig. 10.[14, 15]

Fig. 6 Maximum heat flux and CHF with length of heating surface in subcooled flow boiling of water in horizontal rectan-gular channel at liquid subcooling of 40 K and liquid velocity of 0.5 m/s (500 kg/m2s).

Fig. 7 Subcooled flow boiling of water at liquid subcooling of 40 K and liquid velocity of 0.5 m/s (500 kg/m2s) under atmo-spheric condition. Heating surface = 50 mmL × 20 mmW.

Fig. 8 A concept of integrated CHCD of 24 CH. Max. cooling heat = 120 kW.

Fig. 5 Surface temperature in subcooled flow boiling with long sized heating surface.

131


In microbubble emission boiling, large coalescing bubbles

divide into small ones. The pressure fluctuations decrease

dramatically and the peak pressures are lower than 200

kPa, as shown in Fig. 11.[14] According to the experiment,

a low noise and high-heat-flux cooling device can be pro-

duced by the pressure reduction needles.

3.2 Passive-Active High-Heat-Flux Cooling Device (PHCD) [11, 16, 17]

For a heating surface longer than 50 mm, it is difficult to

remove the high heat flux from a high-powered electronics

device with a single cooling channel. The liquid subcooling

decreases as it flows along the heating surface and large

coalescing bubbles form on the down-stream section as

shown in Fig. 5. Microbubble emission boiling is gener-

ated on the up-stream section but dry-out can easily occur

in the down-stream section.

An inverter with an IC package is employed for an elec-

tric vehicle. The size of the cooling surface will be 20 cm ×

20 cm for more than 100 kW of poweroutput. In order to

prevent the long heating surface from dry-out, a model of

cooling device with two channels and multi-needle nozzles

is proposed and shown in Fig. 12.[11, 16, 17]

The subcooled liquid is separated into the main channel

and the sub-channel assembled over the head of the main

channel. The subcooled fluid in the sub-channel flows

counter to the main channel flow. The tips of the nozzles

have four small slits and are placed within 1 mm of the

heating surface.

The performance of the cooling device is shown in Fig.

13. In the experiments, the cooling device was tested for a

single unit with a heating surface 100 mm in length × 10

mm in width. The main channel is 5 mm high and the sub-

channel is 10 mm high. Nine nozzles, each 1 mm in diame-

ter and 4.5 mm in length, are aligned on the centerline of

heating surface at 10 mm intervals.

Distilled water is used and the liquid subcooling is 40 K

under atmospheric conditions. With a low thermal load,

the cooling is performed in Passive Mode, where the mean

velocity of the liquid is 0.03~0.05 m/s (30~50 kg/m2s) in

the main channel, as shown in Fig. 13. The maximum cool-

ing heat flux is 500 kW/m2 (50 W/cm2) in the passive

mode. With increasing thermal emission, the flow rate of

Fig. 9 Pressure fluctuations in cooling channel at bubble col-lapse in MEB. Liquid subcooling ΔTsub = 40 K, Liquid veloc-ity = 0.5 m/s (500 kg/m2s), Superheat of heating surface ΔTsat = 43 K.

Fig. 10 Illustration of cooling device with needles assembled overhead of cooling Surface.

Fig. 11 Pressure damping in the channel with the number of needle in MEB. Heating surface = 50 mmL × 20 mmW. Liquid subcooling ΔTsub = 40 K, Liquid velocity = 0.5 m/s.

Fig. 12 Cutting view of Passive-Active High Heat Flux Cool-ing Device (PHCD).

132


the subcooled liquid in the main channel is increased to

0.5 m/s (500 kg/m2s) and the cooling is performed mainly

by the main channel flow until the thermal emission

reaches 2 MW/m2 (200 W/cm2).

In the higher thermal emission region, the subcooled

liquid is supplied into the heating surface directly from the

sub-channel through the multi-needle nozzle. The strong

subcooled jet removes the thin vapor film from the sur-

face. This is Active Cooling Mode. The active mode

removes up to 5 MW/m2 (500 W/cm2). So the dual chan-

nel cooling device was named PHCD (Passive-Active High-

Heat-Flux Cooling Device). The maximum heat flux of 500

W/cm2 is sufficient to cover the cooling limit of future

power electronics for motor vehicles. PHCD is flexible in

terms of the size of cooling surface using multiple-units or

multiple-channels and can adapt to cool variable thermal

loads by continuously changing the cooling mode.

4. ConclusionIn subcooled boiling of water, microbubble emission

boiling (MEB) is generated at liquid subcooling above 20

K under atmospheric conditions and the heat flux exceeds

critical heat flux (CHF). The maximum heat flux is 10

MW/m2 (1 kW/cm2) at liquid subcooling higher than 40 K

and liquid velocity of 0.5 m/s (a mass flow flux of 500

kg/m2s) for a small heating surface of 10 mm × 10 mm

with a cm-sized rectangular channel. The cooling heat flux

decreases with the length of the heating surface. For a

long heating surface with a single flow channel, it is diffi-

cult to remove high heat flux in the down-stream section of

the heating surface.

Two models of compact high-heat-flux cooling devices

using MEB have been proposed for high thermal-emission

electronic devices such as IC packaged inverters used for

electric vehicles or power electronics. They are a CHCD

(Compact High-Heat-Flux Cooling Device) with a single

rectangular channel and a PHCD (Passive-Active High-

Heat-Flux Cooling Device) with two channels and a multi-

needle nozzle.

The maximum length of the heating surface is 50 mm

for CHCD. The maximum cooling heat flux is 5 MW/cm2

(500 W/cm2) at liquid subcooling of 40 K and 0.5 m/s (500

kg/m2s), however, considerable high pressure fluctuation

is induced at the collapse of large coalescing bubbles in

MEB. The strong pressure fluctuations are successfully

reduced by a multi-needle rod placed in lattice-wise above

the heating surface.

PHCD has been proposed for heating surfaces longer

than 50 mm. In the experiments, the test was conducted

for a device with a heating surface 100 mm in length and

10 mm in width. The maximum cooling heat flux of 5

MW/m2 (500 W/cm2) was obtained in active mode with

impinging jets in addition to the main channel flow. The

maximum heat flux is sufficiently higher than the pre-

dicted heat flux of 3 MW/m2 (300 W/cm2) emitted from

the power IC package used for electric vehicles or a com-

pact electric power generator.

Acknowledgement The fundamental study on microbubble emission boil-

ing was supported financially by a Grant-in-Aid for Scien-

tific Research promoted by the JSPS (Japan Society for the

Promotion of Science) in 2002–2005.

The research and development for high-heat-flux cool-

ing technology was carried out as part of the “Project of

Fundamental Technology Development for Energy Con-

servation” promoted by NEDO (New Energy and Indus-

trial Technology Development Organization of Japan) in

2002–2005.

The further experiments with the PHCD cooling device

were conducted in the “Project to develop innovative

seeds” promoted by the JST (Japan Science and Technol-

ogy Agency) in 2006. The authors express our gratitude to

NEDO, JSPS, and JST for their support of the experimen-

tal research and development. Furthermore, the authors

also wish to express their sincere appreciation to Prof.

Hiroshi Kawamura of Tokyo University of Science, Prof.

Haruhiko Ohta of Kyushu University, Dr. Yoshiyuki Abe of

AIST, Dr. Hideo Iwasaki of Toshiba Corp., Prof. Masaru

Ishizuka of Toyoma prefectural University, Dr. Masataka

Mochizuki of Fujikura Corp. and Prof. Johannes Straub of

Fig. 13 An example of cooling performance of PHCD, Liquid subcooling Tsub = 40 K.

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Technical University of Muenchen for their great coopera-

tion in the experimental research and development.

References[1] K. Suzuki, “Microbubble Emission Boiling of Alco-

hol-Water Mixtures,” IASME Transactions, Issue 7,

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[13] H. J. Ivey and D. J. Morris, “On the Relevance of

Vapor Liquid Exchange Mechanism for Subcooled

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