Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS
March 26-30, 2017, Jeju Island, Korea
ACTS-P00730
DEVELOPMENT OF HIGH HEAT CAPACITY LOOP HEAT PIPE FOR
AUTOMOTIVE APPLICATION
M.P.Shyam1, Chiemi Oka2, Hosei Nagano3, Seiji Yamashita4, Takeshi Bessho4
1
Graduate student, Graduate school of Mechanical Engineering, Nagoya University,
Nagoya, Aichi, 464-8603 Japan
2
Postdoctoral fellow, Department of Mechanical Engineering, Nagoya University, Japan
3
Professor, Graduate School of Mechanical Engineering, Nagoya University, Japan
4
Toyota Motor Corporation, 1 Toyota-cho, Toyota, Aichi, 471-8572
*
M.P.Shyam: [email protected]
ABSTRACT
Loop heat pipes have extensively been used in the field of space engineering. As the need for passive cooling in the
terrestrial field increases, LHP gains prominence due to its zero dependence on power, high heat transfer capability
and lack of moving parts. This paper reports the test results and analysis of a Loop heat pipe designed for automotive
cooling application. A novel block type evaporator was designed and fabricated for the research. The material of the
evaporator wick was stainless steel and water was selected as the working fluid owing to its high merit number.
Grooves were made on both the sides of the block type-evaporator thereby leading to two heat absorbing surfaces.
The total length of the Loop heat pipe was 9.33m. The experimental results showed that the LHP was capable of
starting up at heat loads as small as 50W and as high as 1000W under various conditions. The maximum heat load
that could be transferred by the Loop heat pipe was found to be 2.6kW and the evaporator case temperature at the
maximum heat load was 150ºC. A numerical analysis was also performed to understand the temperature distribution.
The startup and operation were also studied under various condenser cooling conditions. The outcome of this research
confirms that a passive method of cooling can be used in automotive cooling applications.
KEYWORDS: High heat capacity, Loop heat pipe, Passive cooling, Automotive application, Stainless steel wick,
High temperature
1) INTRODUCTION:
Loop heat pipes are gaining popularity in terrestrial applications. Their gravity unaffected behavior and the absence of
moving parts have been the primary driving force for their application on earth [1]. This research focuses on loop heat
pipes for high heat load application. A loop heat pipe (LHP) is a two-phase heat transfer device that uses capillary
action to remove heat from a source and passively move it to a condenser or radiator. This study presents a block type
evaporator with two heat absorbing surfaces which is capable of transporting a maximum heat load of 2.6kW
2) EXPERIMENTAL SETUP AND RESULTS:
A LHP of a total length of 9.33m was fabricated. The dimensions of the LHP can be found in table 1. Two aluminum
heater blocks were attached to the evaporator and 8 cartridge heaters were inserted into the heater blocks to provide
the heat loads. The wick inside the evaporator is made up of stainless steel porous material (SUS304). A variety of
cooling methods were used in the experiments. The primary ones presented are air-forced convection and water-forced
convection. Water is chosen as the working fluid based on its high merit number. During the test, the Vapor line and
Liquid line was covered with thermal insulation material made of ethylene propylene synthetic rubber. The
temperature was measured using T-type thermocouples. An average of 10 thermocouples were used for the Evaporator,
compensation chamber (CC), vapor line, condenser and the liquid line. The schematic representation of the LHP is
shown in Fig. 1.
1
A chiller capable of temperature control from 10ºC to 30ºC is present which is used to control the temperature when
water-forced convection experiments were conducted. The ambient temperature was maintained at 24ºC for all
experiments
2.1) Experimental conditions:
All the experiments were conducted with the evaporator placed in the vertical orientation. The condenser of the LHP
was cooled with the following conditions. The experimental conditions are shown in Table 3.
The experiment was stopped if the following conditions occurred:
1)Dry out of the wick.
2)Formation of vapor pocket in the evaporator.
3)Very high operating temperature.
2.2) Results and Discussion:
The experimental Evaporator case and Compensation chamber temperatures at steady state are provided in the Fig.2.
As you can see the highest heat load of 2.6kW was achieved when the condenser was cooled using forced air
convection.
Table 1 Geometrical characteristics of high capacity loop heat pipe
Heater Block (A5052)
Length[mm]
104
Width[mm]
120
Height[mm]
16
CC (SUS304)
Length[mm]
27
Width[mm]
106
Evaporator(SUS304)
Length[mm]
153
Width[mm]
112
Height[mm]
40
Additional CC (SUS316)
Volume[cc]
2250cc
Vapor Line(SUS304)
Length[mm]
1500
O.D[mm]
19.05
I.D.[mm]
16.57
Liquid Line(SUS304)
Length[mm]
1500
O.D[mm]
9.525
Working Fluid
Height[mm]
36
Wick (SUS304)
Length[mm]
120
Width[mm]
100
Height[mm]
30
Porosity
0.4
Water
Volume
2100cc
Groove
Length[mm]
Height[mm]
Width[mm]
Number
110
2.5
2.5
40
I.D.[mm]
7.745
Condenser (SUS304)
Length[mm]
5000
O.D[mm]
19.05
I.D.[mm]
16.57
Vapor pocket formation occurred in both the cases of water convection-forced as well as natural. Since this experiment
relied on two heater blocks, we were able to conclude the formation of vapor pocket on one side, as the temperature
of the heater block and evaporator on the other side did not rise rapidly. The limit of the water natural convection was
2kW and forced convection was 2.2kW. In the case air convection, when the condenser was cooled using air-natural
convection, the experiment had to be stopped due to high operating temperature at 1.2kW. The same condition
prevailed when the LHP was operating with the condenser cooled by air-forced convection. The operation of the LHP
was not feasible beyond 2.6kW as the operating the temperature of the LHP was very high.
Table 2 Experimental conditions
Type of Convection
Natural convection
Forced convection
Condenser Cooling condition
Air
Water
Air using cooler
Water at 25ºC
2
Fig. 1 Overview figure of LHP
A transient plot for the air forced convection condition is shown in Fig. 3. The time is plotted against the heat load and
various temperatures. As you can see the condenser is at a lesser temperature from the rest of the system. It can be
understood from the plot that the experiment was stopped due to high operating temperature conditions.
Steady State Comparison of Temperatures
160
140
Temperature (Celsius)
120
100
80
60
40
water natural convection Evap avg
water natural convection CC avg
Forced air convection Evap
Forced air convection CC
Forced water convection 25 degrees Evap
Forced water convection 25 degrees CC
Air Natural Convection Evap avg
Air Natural Convection CC avg
20
0
0
500
1000 1500 2000 2500 3000
Heat Load (W)
Fig. 2 Comparison of experimental evaporator case and CC steady state temperatures
3
250
HB average
Heat Load (W)
3000
Evap average
VL average
2500
LL average
Cond average
2000
Heat Load (W)
Temperature (Celsius)
200
CC average
150
1500
100
1000
50
500
0
0
0
2000 4000 6000 8000 10000 12000 14000 16000
Time (sec)
Fig. 3 Transient plot of the temperatures at air-forced convection
3) CONCLUSIONS:
A high heat capacity loop heat pipe was made for the application in Automobiles. The maximum heat transfer
capability of the LHP was found to be 2.6kW.
kW
Q
T
cond
Nomenclature:
Kilowatt
VL
Compensation chamber
LL
Temperature (Celsius)
Evap
Condenser
avg
REFERENCES:
[1] Yu.F. Maydanik-Loop heat pipes, Applied Thermal Engineering 25(2005)635-657
4
Vapor line
Liquid line
Evaporator
Average