Special Issue

Automobile Electronics

The Application of the Thermal Simulation Technology
to the Products in the Energy and in the Automotive Fields
Mamoru Shimada*, Masami Ikeda*

Furukawa Electric is providing the heat sink products which are applicable fromthe cooling of the several watts (W) elements in the electronics field to the several kW of heat treatment in the energy field. At first, the thermal simulation technology started
as the technology for the product design in the electronics field. Then, it came to play an important role in the time reduction of the product design and the optimization of the designing. At
present, it is used in the designing of almost all the fields from the energy to the automotive
fields. In these fields, the thermal simulations were difficult because the heat values of the elements (which were the cooling goals) were larger than those in the electronics field, andbecause the sizes of the heat sinks (which were the analysis goals) were large. In this paper, we
will explain the improvements in the analysis methods and in the analysis accuracy when applying the simulation technology to the designing focusing on the heat sinks in the energy field,
and we will introduce the development of the simulation technology to the automotive field.

ABSTRACTINTRODUCTION

Furukawa Electric has commercialized heat sink products
which are applicable in various fields, focusing on heatpipe-based products. By running a simulation using thermo-fluid software, the number and the costs of the test
production are reduced and the designs are optimized
simultaneously. In the first place, the simulation started tobe applied for the products in the electronics field such as
in notebook PCs. At present, it is used for the designingof various heat sink products and heat control products.
In recent years, along with the wide spread understandings of the importance of the thermal designing,
advanced heat sink designing, which uses the limited
space efficiently, became essential1).
Figure 1 shows the heat values and the heat densities
of various semiconductor elements. The main coolinggoals among electronics devices are the arithmetic elements for the signal processing such as the central processing units (CPUs) and the graphics processing units(GPUs). They have properties of having a small element
size and a large heat density.
On the other hand, as the Figure 1 shows, the coolinggoals in the energy and in the automotive fields are thepower devices which have large heat values such as thyristors, insulated gate bipolar transistors (IGBTs) anddiodes. A heat sink for the energy field, which is commercialized as a POWER KICKER, is used for their cooling2).
* Automotive System & Device Laboratories,
Research & Development Division

Thick heat pipes of 12.7 mm or more in diameter are
mainly used for the cooling of these elements, and they
are making a significant contribution because they cantransport a large heat value at a small heat resistance. In
particular, the adoption of the heat- pipe-based coolingdevices is increasing and replacing the conventional
ebullient cooling device for the railway power sources.
That is because they have the advantages that they areenvironmentally friendly and can save the weight of theheat sink systems.

300
Heat density (W/cm2 )1.

Termination

LD, LED
100

CPU, MPU

30

Thyristor
10

Transistor
IC, Chip

3

1

10

IGBT, IPM
100
Heat value (W)

1k

10 k

Figure 1 Heat values and heat densities of typical
semiconductor devices.

The thermal simulation in the energy field, especiallythe heat sinks for railway power sources, had the difficulties which were different from the ones in the electronics
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The Application of the Thermal Simulation Technology to the Products in the Energy and in the Automotive Fields

products. However, the accuracy which is equivalent tothat in the thermal simulation in the electronics field
became possible. This paper will explain the application
methods of the thermal simulation technology to thedesigning of the heat sink products in the energy field,
and will introduce the analysis examples of the heat sinkdevices for automobiles.

2.

THE HEAT PIPES AND THE VARIOUS
HEAT SINK COMPONENTS IN THE
THERMAL SIMULATION

Figure 2 shows the schematic of the heat sink. Heat sinkproducts are generally consisting of a heat receiving
block, which is connected to heating elements, heat
pipes, radiation fins and cooling fans. Among them, theheat receiving block and the radiation fins are solid thermal conductivity substances. They can be used as thermal conductivity components without modification in thegeneral thermal analysis software. With respect to thecooling fans, they are applicable as a standard model in a
number of software. In many cases, the P-Q (static pressure-air volume) curve gives not only the function of regulating the air volume depending on the pressure loss ofthe system but also gives the rotational components of
the airflow generated by the rotation of the fan blades.
Compared to these components, the application of theheat pipe model to the thermal analysis software was limited. However, in recent years, some examples show its
application in a form of thermal network components.

Heat
releaseRadiation fin

Heat pipe
Heat
receiving
plate

Heat
transport

Cooling fan

Heat input
Figure 2 Schematic of the heat sink.

Figure 3 shows the inner structure of the heat pipe. The
heat pipe has a simple structure where a small amount ofworking fluid is included under the decompression state
in a metal container. While the working fluid circulates by
repeating the phase transition of evaporating in the heating section and condensing in the cooling section, theheat is transported with the flow of the vapor3). In addition,a mechanism called a wick is set in a heat pipe to reflux
the working fluid, which is in the liquid phase of being
generated in the condensing section, to the evaporating

section. In recent years, from the environmental consciousness, water is used as working fluid and copper isused as a heat pipe container in most cases. Table 1
shows the three major and superior properties of the heatpipe as a heat transport device.

Adiabatic
sectionCondensing(cooling)
sectionWick
Vapor flow

Evaporating(heating)
sectionHeat-inContainer

Heat-out
Evacuated

Return of condensed
working fluid
Liquid flow

Figure 3 Schematic of the heat pipe.

Table 1

Properties of the heat pipe as a heat transport device.

Long distanceheattransportation

The temperature gap does not become large
when the heat transport distance becomes
longer because the heat resistance of the heatpipe is small dependent on the length.

Isothermal
property

The temperature in the heat pipe is always
stable because the working fluid transports theheat from the high temperature part to the lowtemperature part autonomously.

Heat density
conversion

The size of the heating section and that of thecooling section can be different. Heat can bereleased and retrieved from larger or smallerarea.

Although the structure of the heat pipe is simple, tohandle the gas-liquid phase change in the working fluid
directly by a simulation becomes a large burden to acomputer. In particular, in the scale which targets thewhole heat sink, the analysis in a practical period of time
is difficult. Therefore, in many cases, it is handled by asimple model in analysis such as in Figure 4. First, in theheat sink analysis, the heat pipes are modeled as a stick
which has uniformed solid-state properties. Then, theconditions which are equivalent to the heat resistances
between the heat input source and the working fluid andbetween the working fluid and the heat release component are given by setting coefficients of heat transfer inthe space between the heat receiving block and between
the fins. In the case where heat pipe components themselves are treated as heat transport components, a sufficiently large thermal conductivity is set to avoid that theheat transport property in the heat pipe element itself has
a large impact on the heat resistance between the heatreceiving block and the fins. In certain thermal analysis
software, the heat pipe components which are built basedon the same idea are available. The reproduction of theheat transport property in the heat pipe by a simulation
becomes possible by using the software.

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The Application of the Thermal Simulation Technology to the Products in the Energy and in the Automotive Fields

αh(coefficient of heat transfer
in evaporation section)αc(coefficient of heat transfer
in condensation section)

λ(thermal conductivity)
Element Heat receiving
block

Heat pipe

Radiation
fin

Figure 4 Example of the analysis model of the heat pipe.

Figure 5 shows an example of the simulated temperature distribution map of the heat sink for electric devices
in which the heat pipes were not used. In this example,
the temperature distribution in the base plate was large
because the sizes of the heating elements were small andtheir heat densities were large. In such a simple heat sink,its thermal property can be estimated to some extent
without using a simulation but using a simple heat calculation tool 4).

Figure 6 Example of the temperature distribution of a heat sinkobtained by a simulation (with heat pipes).

3.

PROBLEMS IN THE ANALYSIS OF
LARGE HEAT SINKS AND THEIR SOLUTIONS

The application of the thermal simulation technology to
the designing had been difficult in large heat sink products in the energy and in the automotive fields. Table 2
shows its three major causes.

Table 2

Figure 5 Example of the temperature distribution of a heat sinkobtained by a simulation (without heat pipes).

Figure 6 shows an analysis example of the heat sinkusing heat pipes. Six heat pipes were used in this example. The design was that heat was released from the finswith the cooling air from the centrifugal fans located in thecenter. Assuming the thermal performance of the heatsink with multiple heat transport routes and with complicated flow of the cooling air as in this example is difficult.
However, an accurate heat resistance can be obtained by
calculation including the airflow using a simulation. At
present, the heat designing with simulations is crucial forthe heat sink devices using heat pipes.

Differences between the large heat sinks in the energy
field and the heat sinks in the electronics field in theperformance of simulations.

Productsize

The heat sinks in the electronics field arecomparatively small and the typical size is about 100
mm. The heat sinks for the energy field are large andtheir maximum size reaches 1000 mm.

Airflow

The flow of cooling air is laminar in the small heatsinks. In some cases, it is turbulent in large heatsinks.

Cooling
condition

The cooling conditions are regulated by the air
volume in the small heat sinks. They are regulated by
the wind speed in a particular point in the large heatsinks.

The first cause is the size difference between the analysis models of the heat sink products in the electronics
field and of the ones in the energy and in the automotive
fields. The typical size of the heat sink products for theelectronics field is about 100 mm, which is small.
Therefore, the time required for an analysis is 1 – 2 hours,
which is short. And the optimal designs can be achievedby repeating the analysis using multiple structuraldesigns and being assigned with designing parameters in
a comparatively short period of time. On the other hand,in a POWER KIKER for railway vehicles, which is an
example of the large heat sinks, the maximum size of aside reaches about 1000 mm. Because of this, the number of elements reaches several tens of millions when amesh division is applied with an accuracy of the samelevel of the heat sinks for PCs. In such a case, the analyFurukawa Review, No. 44

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The Application of the Thermal Simulation Technology to the Products in the Energy and in the Automotive Fields

sis may last for several days, thereby increasing the cost.
When the calculating time is long and the calculating costis large, the advantages can hardly be obtained by the
optimization of the designing using a simulation.
The second cause is the difference of the states of the
airflow between the fins. In the heat sinks in the electronics field, the airflow is laminar because the intervals
between the fins are narrow. On the other hand, in thelarge heat sinks, the intervals between the fins are generally wide, and the turbulence flow occurs in some cases
when the Reynolds number reaches about several thousand due to the large wind speed between the fins.
However, in the simulation of the heat sink, there was a
problem that the element breakdown for the applicationof the standard turbulent flow model was difficult.
The third cause is the difference in the thermal design
specifications. In general, in the heat sinks in the electronics field, their thermal properties are defined as the heat
resistance against the specified air volume in many cases
because they are capable of measuring the heat resistance accurately with air channels. Therefore, quantitativeevaluations with the result of the simulation are easy. On
the other hand, in the large heat sinks, the measurement
evaluation with accurately-controlled air channels is difficult because their sizes are large. For this reason, inmany cases, their thermal properties are regulated by the
relationship between the wind speed and the thermal
resistance in a particular point on the front side of the finswhen the cooling air is blown with the heat release part
covered with the ducts in a thermal evaluation. Thatcaused the difficulty in the quantitative comparison withthe analysis result.
To address the problem of having a large number ofmeshes due to the large size, the shape of the analysis
model was simplified. Although a simulation using adesign made by a three-dimensional CAD is common atpresent, the analysis without changing the product shape
was difficult and the simplification of the shape of theanalysis model was common at the time the capability ofcomputers was incomparably low. For example, in largesized heat-pipe-based heat sinks, the fixing of the fins bypressing the heat pipes into the fins with burring work is astandard method (Figure 7). However, a fine element
breakdown is required to reproduce the shape of the finburring by a simulation, and it causes the increase in thenumber of the elements in the whole analysis model. To
overcome this, the analysis was conducted with the finmodel without the burring. The number of the meshes
was reduced by the methods such as correcting with thecoefficient of heat transfer to counter the influence of the
reduction in the contact areas between the heat pipes
and the fins.

Figure 7 Schematic of the heat pipe and the fin burring.

To address the problem of having a larger Reynolds
number for the airflow between the fins, a turbulent modelof a low Reynolds number type, which is provided for thecase in which a standard turbulent model is difficult to be
applied, was used. To overcome the problem of “thedestabilization of the thermal calculation” which is likely
to happen in the model, we mitigated by discussing withthe software vendor. We finally prevented the problem by
establishing conditions of the element breakdown to
avoid the destabilization of the calculation and by standardizing it.
To address the problem where the quantitative comparison between the measured result and the analysis result
was difficult, we conducted the analysis with the positionof adhering to the measured evaluation state with simulation. A gap was generated between the wind speed which
was given as a boundary condition in a simulation andthe wind speed at a specific point in the analysis. To overcome this, the condition of the boundary wind speed wascontrolled by a user subroutine per cycle in an analysis,
then, the wing speed at the specific point was converged
to the designated value. Thereby, the repetition of theanalysis for matching the wind speed with the specificvalue was avoided.
Figure 8 shows an example of the comparison between
the analysis result and the measured result of the heatsink in railway vehicles with the methods introduced
above. In this product, when the wind speed is low, theairflow between the fins is between turbulent flow andlaminar flow. In the analysis, whether to treat the airflow
as turbulent flow or laminar flow needed to be decided,thus the gap between the analysis result and the measured result became large. However, in the region of thelarge wind speed which was estimated to be in a full turbulent state with a large Reynolds number, the analysis
gap of the heat resistances was within 5%.

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Heat resistance (˚C/W)

The Application of the Thermal Simulation Technology to the Products in the Energy and in the Automotive Fields

Wind direction

Measured value
Analysis value(turbulent flow)
Analysis value(laminar flow)
Wind speed (m/s)

Figure 8 An example of the comparison between the measured
value and the analysis value of the POWER KICKER for
railway vehicles.

4.

EXAMPLE OF THE DESIGN OPTIMIZATION IN THE HEAT SINK IN RAILWAY
VEHICLES

We will explain the actual example of the optimization of
the heat sink designing by a simulation. Figure 9 shows
the first trial design of the POWER KICKER. We investigated the design problems of this first trial design by a simulation. As the analysis result in Figure 10 shows, although
the fin pitches on the leeward side were set narrow andthe heat release areas were set wide, the temperature of
the heating elements became higher as they were closer
to the leeward side. The reason is that the fins on the leeward side were difficult to release heat and the temperatures in the elements gradually rose because the temperature of the air which was flowing between the fins gradually rose with the heat released by the fins as it moved to
the leeward side. Also, it was confirmed that the heat loadtended to concentrate into a part of the heat sinks causedby the positional relationship between the heating elements and the heat pipes which were used in the heatsinks.

Figure 10 Temperature distribution of the POWER KICKER for
railway vehicles (first trial design).

Given this factor, we studied the optimization of thedividing methods of the fins in the windward and in theleeward directions, the ratio of each fin pitch and the two
designing elements of the heat pipe arrangement by simulations. The result was that the ΔT (value of the temperature rise) of the heat sink became smaller when the division number of the fins was increased from two to three.
Also, when the ratio of the neighboring fins was 2:1, theairflow became smooth and the ΔT of the elements on
the leeward side became smaller. Although we studied
the arrangement of the heat pipes, the improvement in
the arrangement did not lead to the enhancement of thethermal performance because it brought the reduction in
the number of the heat pipes which could be arranged.
Therefore, the arrangement in the first design was adopted.
Figure 11 shows the composition of the final design. It
divided the fins into three parts along with the airflow.
Restraining the rise in the air temperature by widening thespace between the fins on the windward side, the fins on
the leeward side became capable of releasing heat effectively. Figure 12 shows the analysis result of the final
design. As it shows, the ΔT of the heat sink was loweredby 6˚C and the temperature gap between the heating elements was reduced by 70%.

Figure 9 First trial design of the POWER KICKER for railway
vehicles.

Figure 11 Final design of the POWER KICKER for railway
vehicles.

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The Application of the Thermal Simulation Technology to the Products in the Energy and in the Automotive Fields

Wind direction

problems because these vehicles are mounted with thebatteries of a large capacity.

Upper caseZero layer
insulating plate
CoppersleeveFigure 12 Temperature distribution of the POWER KICKER for
railway vehicles (final design).

Micro heat pipe
Absorberplate

In the case of the large heat sinks for railways, a sufficient design optimization was difficult in many cases timewise and cost wise because each trial evaluation took
one to two months. However, this product could achieve
the finally targeted heat release performance only with atrial between the first trial design and the final design byrunning 30 ways of simulations in the process of designing. At present, the simulation method of the large heatsinks for railway vehicles is standardized in the company
and is used in almost all designing of the products.

5.

EXAMPLES OF THE SIMULATION OF
THE HEAT SINK DEVICES FOR AUTOMOBILESIn automobiles, a number of semiconductor elements
such as integrated circuits (ICs) are becoming mounted
in line with the rapid advancement of the electrificationand the computerization of automobiles for an accurate
control of the drive trains targeting the improvement of
the fuel efficiency and the enhancement of the comfort in
the living space. In hybrid electric vehicles (HEVs), inelectric vehicles (EVs) and in fuel cell vehicles (FCVs), thedrive-trains are electrified. When the cooling of the powersource or of the electronic control unit (ECUs) in engine
vehicles is required, the water-cooling system which isoriginally mounted on them is commonly used. However,in recent years, the cooling technology of air-coolingwhich is lightweight and has a lot of flexibility in the layout
is becoming the focus in the automotive field.
Figure 13 shows the cooling system of a junction box inwhich heat pipes were applied. The design of this productis that the heat in the junction is accumulated by the heat
pipes and then, it is transported to the chassis. The heatis finally released to the outside from the vehicle body.
This design enabled the heat release without using awater-cooling system or cooling fans.
In HEVs and EVs which are increasing recently, the size
reduction in the battery packs and a cooling method
which prevents the deterioration of the batteries are theHeat sink
Bus bar/Laminated productof insulating plates
Metal collar
Lower case

Figure 13 Heat-pipe-based heat sink for a junction box.

Figure 14 shows an example of the simulation of a heatsink for the cooling of batteries. In this example, four battery cells are cooled simultaneously in a heat sink. The
method is the heat is transported to the heat release partin a distant place by the heat transport function of theheat pipes, and the heat is released by air-cooling. In thisdesign, the temperature fluctuation between the cells
could be smaller with the isothermalization effect of theheat pipes and the fluctuation of the time degradation
speed could be smaller.

Battery cell

Figure 14 Analysis example of the heat sink for the cooling ofbatteries.

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In addition, as the electrification of the automobiles
advances, the cooling method of the semiconductors
which have large heat values such as inverters and converters became one of the problems, as well as in electrictrains. Figure 15 shows an example of the simulation of
the heat sink for inverters. The heat values in the inverters
change constantly along with the change in the running
condition of the vehicle. In the indicated heat-pipe-basedheat sink, a rapid change in the temperature along withthe change in the heat values in the elements are
restrained, and rapid heat release by transporting theheat to the fins with the heat pipes which had a small
thermal capacity and had a quick response became possible simultaneously by optimizing the thermal capacity of
the receiving block.

6.

CONCLUSION

This paper introduced the expansion of the simulation
technology of the designing of heat sink products in theelectronics, in the energy and in the automotive fields.
Heat sink products in the energy and in the automotive
fields have large sizes, and the large sizes caused someproblems when simulations were conducted. However,
the analysis accuracy of within 5%, which was equivalent
to that of the small heat sinks in the electronics field, wasachieved by devising an analysis method. As speed isalways required in the designing and the development ofheat-related products, we will progress in the reduction of
the trial times and the designing efficiency to meet theneeds by simulations.
In addition, thermal simulation software has been evolving. They can be used as a tool by the product designers
with ease because they have been installed with convenient physical models and their user interfaces have been
upgraded. However, as the case that various turbulent
models are used depending on the purpose shows, thethermo-fluid analysis is comparatively a difficult field.
Therefore, it should be used with a confirmation of theaccuracy by continuously comparing to the measured
data. Given this condition, we will continue the study ofenhancing the analysis accuracy and we will make efforts
to advance the simulation technology to ultimately
achieve the analysis accuracy of within 5% in all products.

REFERENCES

Figure 15 Analysis example of the heat sink for inverters.

1) Kunimine (supervision): The newest thermal design technique andelectronics cooling technology, CMC Publishing Co., Ltd., (2011)(in Japanese)
2) Takahashi and others: Environment-Friendly Vertical Air-Cooling
Heatsink: Heatpipe-Based Heatsink “POWER KICKER”, Furukawa
Review, No.27 (2005), 30.
3) Edited by Japan Association for Heat Pipes: Heat pipes in actual
use, Nikkan Kogyo Shimbun Ltd., (2001). (in Japanese)
4) Kimura and others: Thermal Design Simulation Software“HIEROH”, Furukawa Review, No.27 (2005), 52.

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