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Energy Procedia 34 (2013) 298 306

1876-6102 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of COE of Sustainalble Energy System, Rajamangala University of TechnologyThanyaburi (RMUTT)doi:10.1016/j.egypro.2013.06.758

10th Eco-Energy and Materials Science and Engineering

(EMSES2012)

Effect of Filling Ratios and Adiabatic Length on Thermal

Efficiency of Long Heat Pipe Filled with R-134a

Thanaphol Sukchana*, Chaiyun Jaiboonma

Heat Transfer Technology Laboratory

Mechanical Department, Faculty of Engineering, Pathumthani University

140 Moo. 4 T.Banklang A.Muang Pathumthani 12000, Thailand

Abstract

The effect of R-134a filling ratio and adiabatic length on thermal efficiency of the long heat pipe are investigated. An

experimental heat pipe is fabricated from the straight copper tube having an inner diameter of 11.3 mm. The

evaporator and condenser length are 100 mm. The R-134a used as the working fluids filled in the heat pipe was

tested. Filling ratios of the working fluid to heat pipe volume at 10, 15 and 20% and input heat flux rates of 1.97 to

9.87 kW/m2were carried out in order to determine the thermal efficiency and optimum condition. The inlet cooling

water temperature was kept constant at 20oC. The adiabatic lengths of heat pipe for these experiments were 300, 500

and 700 mm (adiabatic length ratios 0.60, 0.71 and 0.77). The results show that the filling ratios have more

significant effect on the thermal efficiency than adiabatic length. The optimum filling ratio of 15% and 5.92 kW/m2

of heat flux are found to be favorable at shorter adiabatic length.

2013 The Authors. Published by Elsevier B.V.

Selection and/or peer-review under responsibility of COE of Sustainalble Energy System, Rajamangala

University of Technology Thanyaburi (RMUTT)Keywords: Thermal effieiency; Optimum filled; Adiabatic; Long heatpipe; R-134a

* Corresponding author. Tel.: +66-2975-6999; fax: +66-2979-6728.

E-mail address:[email protected].

Available online at www.sciencedirect.com

2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of COE of Sustainalble Energy System, Rajamangala University of TechnologyThanyaburi (RMUTT)

Open access under CC BY-NC-NDlicense.

Open access under CC BY-NC-NDlicense.
http://creativecommons.org/licenses/by-nc-nd/3.0/http://creativecommons.org/licenses/by-nc-nd/3.0/

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300 Thanaphol Sukchana and Chaiyun Jaiboonma / Energy Procedia 34 (2013) 298 306

distilled water proposed by Noie [5] with filled ratio are 30, 60 and 90% of evaporator volume the

adiabatic length ratio are 0.22, 0.29 and 0.37, filling ratio 60% is the optimum charge filled. Nitrogen is

the working fluid experimentally by Jiao et al. [6] the filling ratio is 4.5-20.2% of total volume and

adiabatic length ratio 0.4 and 0.75, it found that the optimum charge filled must be more than 10%. The

filling ratio is 0.21-1.41 times of evaporator volume proposed by Long and Zhang [7] with adiabatic

length ratio is 0.18, the filling ratio 1.02 is the optimum charge filled. Refrigerant is the working fluid

filled with FC-84, FC77 and FC-3283 studied by Jouhara and Robinson [8] as filling ratio 1.38 times ofevaporator volume and adiabatic length ratio is 0.5. R-134a filled 25-100% of evaporator volume

proposed by Grooten and Geld [9] the adiabatic length ratio is 0.12, the results show that the 62% filled

ratio is optimum charge filled. The R-134a filling ratio is 0.35-0.8 of evaporator volume and adiabatic

length ratio is 0.23 proposed by Ong and Alahi [10], the optimum filling ratio is 0.8. R22, R123, R134a,

ethanol and water as the working fluids. Payakaruk et al. [11] proposed that the filling ratio are 50, 80 and

100% of evaporator volume while the aspect ratios are 5, 10, 20, 30 and 40; it found that the optimum

condition is 50% of filling ratio, Le/d of 5 and tilt angle is 40o. Long heat pipe with adiabatic length

between 0.7 to 1.8 m proposed by Arab et al. [12] the distilled water as working fluid with filling ratio 30,

50 and 70%, and filling ratio 70% results to high efficiency of heat pipe. Air heat exchangers with longheat pipe to studied by Hagens et al.[13] the filling ratio of R-134a 19 and 59%, heat pipe is long 1.5 m

and adiabatic length ratio is 0.15, the optimum filling ratio is 19%. Experimentally thermosyphon solar

collector by Nada et al. [14] with a copper tube 0.92 m of length, adiabatic length ratio is 0.15. A solar

collector with wickless heat pipe proposed by Hussein et al. [15] was charge filled with distilled water 10,

20 and 35%, heat pipe is long 1.5 m and adiabatic length ratio is 0.03, the optimum filling ratio is 10% for

the elliptical cross section tube and 20% for the circular cross section tube. R-134a refrigerant as theworking fluid experimental by Ibrahim et al. [16] to study the heat transfer characteristics of a rotating

triangular thermosyphon with total length of pipe of 1500mm, adiabatic length ratio is 0.2 and filling

ratios are of 10, 30 and 50 of the evaporator section volume. The thermosyphon are obtained at the filling

ratio of 30 %.

The results of former researchers were presented the effect of filling ratio on thermal efficiency of heat

pipe with different design to study. No measurements with heat pipe that long adiabatic (adiabatic length

ratio is higher than 0.5) have been found in literature except those [2] and [6]. The objective of this paper

is to study the effect of filling ratio and adiabatic length on the thermal efficiency of long heat pipe withR-134a as the working fluid. And the optimum of filling ratio of long heat pipe that it has adiabatic length

ratio higher than 0.5.

2. Experimental Apparatus and Test Procedure

The schematic diagram of an experimental apparatus is shown in Fig. 1. The experimental apparatus

loop consists of the R-22 refrigeration loop, test section, cold water loop and record data system. Thewater is chilled by refrigeration R-22 system and controlled water temperature by temperature controller.

The close-loop of cold water consists of a 0.15 m3 storage tank and a cooling coil immersed inside a

storage tank. When the water temperature is reached to the desired level, the cold water is, then, pumped

out of the storage tank passed through a flow controlled valve to test section (Condenser section), and

return to the storage tank. The flow rates of the cold water are controlled by adjusting valve and measuredby digital weight scale with the accuracy of 0.01% of full scale. The test section is fabricated from

straight copper tube with 12.7 mm outer diameter, 11.3 mm inner diameter. The evaporator section of the

heat pipe is inserted into the cylindrical electrical heater which is supplied by 220V AC power supply,

while the condenser section is inserted into cooling chamber. Six point type K thermocouples are installed

in order to measure the temperature of evaporator section, adiabatic section and condenser section by

mounting on the heat pipe outside surface wall and fixed with special glue as shown in Fig. 2.

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Thanaphol Sukchana and Chaiyun Jaiboonma / Energy Procedia 34 (2013) 298 306 301

Cold water tankCondensing unit

Water pump

By pass valve

T

T

Testsectio

n

Cond

enser

sectio

n

Evaporator

sectio

n

Adiabaticse

ctio

n

60

Beaker

Digital weight

0.00 g

Fig. 1. Schematic diagram of experimental apparatus

100 mm 100 mmVarious length 300-700 mm

T1 T2 T4 T5 T6

20 60 80 20

Evaporator

section

Condenser

section

Adiabatic section

T3

Fig. 2. Schematic diagram of test section

3. Operating Condition

The heat pipe is assigned to the same length of evaporator section and condenser section at 100 mm

each. The adiabatic length of 300, 500 and 700 mm (adiabatic length ratio 0.60, 0.71 and 0.77 (Eq.1)) are

tested, while the condenser section are used to condense the vapor in the pipe by supplying 20oC cold

water from refrigeration R-22 system. Experiments are conducted with constant heat fluxes of 1.97, 3.95,

5.92, 7.90 and 9.87 kW/m2at tilt angle of tested heat pipe of ranged of 15, 30, 45, 60, 75 and 90

o. R-134a

filling ratio of 10, 15 and 20% of heat pipe volume were performed, while the constant heat flux was

supplied to the evaporator section by using 220V AC electric heater.

Table 1. Accuracy and uncertainty of measurements

Instruments Accuracy Uncertainty

Thermocouple type K,Data logger (oC)

0.1% 0.1

Digital weight scales(1000g)

0.01% 0.01 1.0

Digital power meter 0.1% 0.1 1.0

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The supplied power is calculated by multiplying the supply voltage and current measured from digital

power meter, (Eq.2). The temperatures at each position are recorded three times by a data logger. The

mass flow rates of cooling water are measured by the digital weight scale. The uncertainty and accuracy

of measurement are shown in Table 1.

4. Results and Discussion

The adiabatic length ratio of the thermosyphon is calculated as follows,

total

adia

L

LAR (1)

The input power at evaporator section is calculated using the supply voltage and current measurements

from digital power meter,

VIQin (2)

The energy removes the condenser section which is determined by performing an energy balance

across the condenser section,

)( ,, inwoutwpout TTcmQ (3)

The thermosyphon efficiency can be calculated from the ratio of cooling capacity rate of coolant and

electric supplied power,

100xQ

Qe

in

ou t (4)

The effective overall thermal resistance of the thermosyphon is by applying the electrical analogue in

the form,

in

ce

Q

TTRexp (5)

Fig. 3 show the variation of the heat pipe efficiency with heat pipe tilt angle at 50% charge amount of

R-134a of adiabatic section volume. It can be seen that the heat pipe efficiency increases with increasing

tilt angle and become decreases when the tilt angle more than 60o. The tilt angle is 60

oas the tests tilt

angle of this experimental. The effect of the different R-134a filling ratios (10, 15 and 20% of heat pipevolume ) and heat fluxes (from 1.97- 9.87 kW/m2) and adiabatic length ratio 0.60, 0.71 and 0.77 on heat

pipe thermal efficiency are shown in Fig. 4. The input power at evaporator section, the heat transfer rate

calculates from energy removal at the condenser section and the heat pipe efficiency is calculated from

equation (2), (3) and (4) respectively. From Fig.4, the heat pipe thermal efficiency tends to increase with

heat flux until 5.92 kW/m2and after that the considerable drop in thermal efficiency is observed at these

filling ratios. This is because the increasing of heat flux leads to an increase in evaporator temperature

which promotes the higher evaporation heat transfer rate to working fluid. It is also found that maximum

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heat pipe thermal efficiency is obtained at filling ratio 15% of heat pipe volume. Considering to an

increase adiabatic length, the experimental results show the similar trend for thermal efficiency obtained

from the increasing heat fluxes and filling ratios. Therefore, the heat pipe thermal efficiency tends to

increase as the optimum heat flux increases. For adiabatic length of heat pipe, the thermal efficiency

depends on the filling ratio, adiabatic length of heat pipe and space for the vapor of working fluid.

Filling ratio = 50% of adiabatic

Adiabatic length ratio = 0.33

Heat flux = 3.95 kW/m2

Tilt angle of heat pipe, degree

20 30 40 50 60 70 80 90 100

Heatpipethermalefficiency,

%

0

20

40

60

80

100

Fig. 3. Variation of heat pipe thermal efficiency with tilt angle of heat pipe for R-134a

Heat pipe length = 900 mm


Heat flux, kW/m2

2 4 6 8 10

Heatpipethermal

efficiency,

%

20

40

60

80

100

Filling ratio 20%

Filling ratio 15%

Filling ratio 10%

Fig. 4. Variation of heat pipe thermal efficiency with heat flux for different filling ratio at adiabatic length ratio 0.77

Fig. 5 shows the influence of heat flux on heat pipe thermal efficiency for different adiabatic length atfilling ratio 15%. Reduction of heat pipe thermal efficiency for long adiabatic length is due to the longer

time to flow by gravity and more liquid film of working fluid from condenser section to evaporator

section owing to the higher vapor space of heat pipe. Therefore, the heat pipe thermal efficiency also

depends on the adiabatic length of heat pipe. The influence of heat flux on evaporator surface temperature

for different adiabatic length at filling ratio15% is shown in Fig. 6. The results indicate that an increase of

heat flux leads to increase the evaporator surface temperature. This is due to high heat flux causes an

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increase of the evaporation of working fluid leading to increase pressure in the heat pipe affecting higher

saturation temperature of working fluid in heat pipe. However, the filling ratios of 10 and 20% are not

found to be favorable (The data not shown). Moreover, it is found that adiabatic length has slightly effect

to the evaporator surface temperature.

Filling ratio = 15%

Heat flux, kW/m2

0 2 4 6 8 10 12

Heatpipethermalefficiency,

%

20

40

60

80

100




Fig. 5. Variation of heat pipe thermal efficiency with heat flux for different adiabatic length at filling ratio 15% of total volume

Filling ratio = 15%

Heat flux, kW/m2

0 2 4 6 8 10 12

Evaporatorsurfacetemperature,

oC

35

40

45

50

55

60




Fig. 6. Variation of evaporator surface temperature with heat flux for different adiabatic length at filling ratio15% of total volume

Fig. 7 shows variation of overall thermal resistance with heat flux for different adiabatic length atfilled ratio 15% of heat pipe volume. The overall thermal resistance can be calculated from the ratio of

different temperature between the evaporator section and condenser section and supplied power at

evaporator section with equation (5). It can be seen that the thermal resistance tends to decrease with

increasing heat flux. This is because the heat flux increases which results in lower of the temperature

difference between evaporator section and condenser section. Therefore, the overall thermal resistance

decreases since heat flux has increased. However, the filling ratio must be optimum with the heat pipe

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volume. This is because the filled ratio which results to evaporate space and saturated temperature of

working fluid.

Filling ratio = 15%

Heat flux, kW/m

2

0 2 4 6 8 10 12

Thermalresistance,

K/W

.2

.3

.4

.5

.6

.7

Adiabatic length ratio = 0.60Adiabatic length ratio = 0.71


Fig. 7. Variation of overall thermal resistance with heat flux for different adiabatic length at filling ratio 15%

5. Conclusions

Heat pipe technology has been used in wide variety of application in the various heat transfer devices

especially in the electronic component. Some application of heat pipe has long distance of adiabatic

section. The importance of heat transfer capability is limited by the filling ratio of working fluid and

length of heat pipe, especially adiabatic section. Experimental data of the thermal efficiency of heat pipe

at 0.60, 0.71 and 0.77 of adiabatic length ratios with R-134a working fluid and 60 degree of tilt angle at

different heat fluxes are presented. The optimum filling ratio for each adiabatic length is 15% of heat pipe

volume and maximum performance for 11.3 mm inner diameter and evaporator length 100 mm at 5.92

kW/m2of heat flux.

Acknowledgements

This work is supported by Mechanical Department, Faculty of Engineering, Pathumthani University,

Thailand for financial and laboratory for this study.

References

[1] Zhen-Hua Liu,; Yuan-Yang Li,; and Ran Bao. 2011. Compositive effect of nanoparticle parameter on thermal performanceof cylindrical micro-grooved heat pipe using nanofluids.International Journal of Thermal Sciences50 : 558 - 568.

[2] Yu-Hsing Lin,; Shung-Wen Kang,; and Hui-Lun Chen. 2008. Effect of silver nano-fluid on pulsating heat pipe thermalperformance.Applied Thermal Engineering28 : 1312 1317.

[3] Paisarn Naphon,; Pichai Assadamongkol,; and Teerapong Borirak. 2008. Experimental investigation of titanium nanofluidson the heat pipe thermal efficiency.International Communications in Heat and Mass Transfer35: 1316 1319.

[4] Paisarn Naphon,; Dithapong Thongkum,; and Pichai Assadamongkol. 2009. Heat pipe efficiency enhancement with

refrigerant nanoparticles mixtures.Energy Conversion and Management50 : 772 776.

8/10/2019 1-s2.0-S1876610213010011-main

9/9


[5] S.H. Noie. 2005. Heat transfer characteristics of a two-phase closed thermosyphon. Applied Thermal Engineering 25 :495 506.

[6] B. Jiao,; L.M. Qiu,; X.B. Zhang,; and Y. Zhang. 2008. Investigation on the effect of filling ratio on the steady-state heattransfer performance of a vertical two-phase closed thermosyphon.Applied Thermal Engineering28 : 1417 1426.

[7] Z.Q. Long,; and P. Zhang. 2012. Impact of cooling condition and filling ratio on heat transfer limit of cryogenic

thermosyphon. Cryogenics52: 66 76.[8] A. K. Mozumder1,: A. F. Akon1,; M. S. H. Chowdhury,; and S. C. Banik. 2010. Performance of Heat Pipe for Different

Working Fluids and Fill Ratios. Transaction of the Mech. Eng. Div., The Institution of Engineers, Bangladesh Journal ofMechanical Engineering, Vol. ME 41, No. 2, December.

[9] M. H. M. Grooten.; and C. W. M. van der Geld. PREDICTING HEAT TRANSFER IN LONG, R-134a FILLEDTHERMOSYPHONS. Department of Mechanical Engineering, Technische Universiteit Eindhoven, Postbus 513, Eindhoven,

Netherlands.[10] K.S. Ong.; and Md. Haider-E-Alahi. 2003. Performance of a R-134a-filled thermosyphon. Applied Thermal Engineering

23 : 2373 2381.[11] T. Payakaruk,; P. Terdtoon,; and S. Ritthidech. 2000. Correlations to predict heat transfer characteristics of an inclined

closed two-phase thermosyphon at normal operating conditions.Applied Thermal Engineering20 : 781-790.

[12] M. Arab,; M. Soltanieh,; and M.B. Shafii. 2012. Experimental Investigation of Extra-Long Pulsating Heat PipeApplication in Solar Water Heaters.Experimental Thermal and Fluid Science42 : 6-15

[13] H. Hagens,; F.L.A. Ganzevles,; C.W.M. van der Geld,; and M.H.M. Grooten. 2007. Air heat exchangers with long heat

pipes Experiments and predictions.Applied Thermal Engineering27 : 2426 2434.[14] S.A. Nada a,; H.H. El-Ghetany,; and H.M.S. Huss. 2004. Performance of a two-phase closed thermosyphon solar collector

with a shell and tube heat exchanger.Applied Thermal Engineering24 : 1959 1968.

[15] H.M.S. Hussein,; H.H. El-Ghetany,; and S.A. Nada. 2006. Performance of wickless heat pipe flat plate solar collectorshaving different pipes cross sections geometries and filling ratios.Energy Conversion and Management47 : 1539 1549.

[16] E. Ibrahim,; M. Moawed,; and N. S. Berbish. 2012. Heat transfer characteristics of rotating triangular thermosyphon.HeatMass Transfer48 : 1539 1548.

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