Characteristics of an Open Loop Pulsating Heat Pipe - CiteSeerX

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PHP performance [5] and thus higher heat fluxes could ... experimental apparatus was built to test a PHP .... When testing the OLPHP with acetone, it could be.
2004-01-2509

Characteristics of an Open Loop Pulsating Heat Pipe Roger R. Riehl National Institute for Space Research – INPE/DMC/Satelite

Copyright © 2004 SAE International

ABSTRACT This paper presents an experimental investigation of an open loop pulsating heat pipe (OLPHP), where several issues related to its performance were evaluated. Tests were conducted with different working fluids for the OLPHP operating at both vertical and horizontal orientations. The experimental results show that the system presented better performance when operating at horizontal orientation, as lower evaporation section temperatures were achieved. Regarding the working fluids used, the system showed better performance when acetone was used on vertical orientation and methanol on horizontal orientation.

INTRODUCTION Pulsating heat pipes (PHPs) are passive two-phase thermal control devices first introduced by Akachi et al. [1]. Mainly, PHPs consist of a capillary tube bent in several curves to form parallel passages. In this application, reduced diameter channels are used, which are directly influenced by the selected working fluid. The vapor plugs generated by the evaporation of the working fluid push the liquid slugs toward the condensation section and this motion causes flow oscillations that guide the device operation [2]. There are several applications for PHPs, from electronics and structural thermal control as well as microgravity thermal control. Due to the simple construction, light weight and low cost, PHPs have gained attention to be used in space radiators, in order to give a more isothermal characteristic for this component. Integrating such a device does not represent a major issue and it could be easily used in several other applications. There are two possible configurations for PHPs, being as an open loop and as a closed loop. In the open loop configuration, one end of the tube is pinched off and welded, while the other end may present a service valve for evacuation and charging. The closed loop configuration has both ends connected and the fluid is allowed to circulate. Issues related to the design and operation of PHPs are still under investigation and this subject still represents an open field to be explored.

Considering the sections of a PHP, it presents an evaporation and a condensation section and may also present an adiabatic section. The tube does not present a wick structure and its construction is very simple. As any other two-phase passive thermal control device, heat is acquired from the source through the evaporation section transferring it to the working fluid where the slug/plug pumping action will be generated. The fluid then flows by the adiabatic section towards the condensation section. On a closed loop configuration, the fluid is allowed to circulate and after being condensed, the fluid returns to the evaporation section to complete the loop. On the open loop configuration, the liquid circulation is not possible. In this case, it is believed that a counter-current liquid/vapor flow occurs in order to promote the proper device operation [3]. Therefore, better understanding on this behavior is still a motivation for further investigations. Previous investigations have already addressed the operation and thermal behavior of PHPs. Delil [4] presented a survey on pulsating/oscillating devices suitable to be used in microgravity and supergravity environments. Important contributions related to the PHPs on closed loop configuration were given by Charoensawan et al. [5], Khandekar et al. [6,7], Rittidech et al. [8] and Tong et al. [9]. The critical issues and an approximate operation behavior of PHPs have been addressed by Groll and Khandekar [10]. The factors that directly affect the PHPs operation, such as effects of latent heat, capillary resistance, etc have been already determined [11]. As a relatively new field, most of the theory involved on PHPs design and operation were derived from the classic two-phase flow theory, which could be used as a first approach in analyzing such a device. Evaluation of the slug/plug flow transitions are required, with special attention to the flow pattern transition and dynamics involved in such an application as reduced diameters are used. The PHP operation presents some unique characteristics and a very interesting thermal behavior. One particularity of PHP operation is that it presents thermodynamics instabilities associated with the plug/slug dynamics, even though such a dynamics is in mechanical equilibrium. The vapor plugs formation and collapse presents a chaotic behavior that is difficult to

model. During its operation, metastable conditions of the working fluid (at both liquid and vapor phases) are present, which are directly related to the thermohydrodynamics particularities of this device [5-7]. Quasistationary modeling have showed great potential in predicting PHPs operation, which was in accordance to experimental results [12]. The plug/slug flow dynamics is directly dependent on the applied power to the evaporation section, tilt angle and amount of noncondensable gases [13]. A mathematical model has addressed the operation of PHPs, where the characteristic chaotic behavior can be reflected which were in accordance to experimental results [14]. Other mathematical models have been formulated to describe the PHP operation, considering the geometric parameters and the effect of working fluid [15] as well as the heat transfer effects on its operation [2]. The pulsating action (plug/slug) is the motion force for the PHP, which is directly influenced by the inner tube diameter. The factors that influence the plug/slug formation in reduced diameters must be observed for this application, such as the correct working fluid selection, surface tension and shear stress effects, etc. Without this pumping action, the device will operate as a solid bar conducting heat from one end to another. As an important parameter for the proper PHP operation, the critical bubble diameter is directly related to the selected working fluid and can be estimated from the Bond number as

[

Bo = d c g ( ρ l − ρ v ) / σ

]

0. 5

(1)

where Bo≤ 2.0.

• Latent heat of vaporization ( ilv ): high values of ilv are desirable and important regarding the ClausiusClayperon relation, which can reflect little temperature drop driving force. On the other hand, this parameter should present a reduced value in order to result in faster bubble generation and collapse; the PHP process is more likely to be due to sensible heat [2,7]; • Surface tension: it is desirable that the working fluid presents a low surface tension, although this parameter in conjunction with dynamic contact angle hysteresis may generate additional pressure drop [7]. Looking at the Bond number definition, low surface tension fluids are only possible for PHPs with small diameters to have the slug/plug distribution, in which this parameter should be used as a guide when selecting the proper working fluid. Following the above-mentioned parameters, an experimental apparatus was built to test a PHP configured as an open loop. The device was tested with five different working fluids, which were used to compare the PHP performance. The results gathered in this investigation have the objective to compare the thermal performance of the PHP with different working fluids and orientations.

EXPERIMENTAL SETUP A representation of the device used during the experimental tests is shown by Fig. 1. A pulsating heat pipe configured as an open loop (OLPHP) was built using a capillary copper tubing with 3.0 mm OD, 1.5 mm ID, 4 m long to form 13 parallel channels with 12 curves.

This variable should be used as an upper limit for the maximum tube inner diameter, as an important parameter for the vapor plug formation. If the conditions for the vapor plug formation are satisfied, the PHP would present a satisfactory operation. Another factor that directly influences the PHP performance is the number of turns, as the increase of this parameter will increase the PHP performance [5] and thus higher heat fluxes could be dissipated. For the proper working fluid selection, the ClausiusClayperon relation

(dP / dT )T

sat

= ilv / (Tsat vlv )

(2)

could be applied, where high values for the magnitude of the derivative (dP/dT)Tsat (slope) must be achieved. A comparison of this parameter related to several working fluids was presented by Khandekar et al. [7]. This represents that a small change in the saturation temperature will result in a large influence in the saturation pressure, which will directly affect the pumping forces of the PHP during its operation. Other important parameters should also be evaluated, such as:

Figure 1. PHP test apparatus.

The critical diameters for all working fluids used in this investigation are presented in Table 1. Upon using the

Table 1. Critical diameters for the working fluids (Tsat = 20°C). Fluid

Water

Methanol

Acetone

Isopropyl Alcohol

Ethanol

dc (mm)

5.5

3.4

3.5

3.4

3.6

A 6.35 mm OD tubing was welded to one end of the capillary tube in order to allow the connection to a charging valve. The apparatus was placed on a support that allowed its adjustment on vertical and horizontal orientations, as well as any inclination angle (Ω). The OLPHP was built with three regions: evaporation, adiabatic and condensation, each one being 100 mm long and 300 mm wide. For the tests under vertical orientation, the evaporation section was always below the condensation section. For the tests under horizontal orientation, all sections were at the same plane. Both evaporation and adiabatic sections were thermally insulated, while the condensation section was open to the surrounding air. The condensation was promoted by forced air, using a fan able to deliver air at a velocity of 2.5 m/s. The tests were conducted in a controlled room environment, with temperatures ranging between 18 to 20 °C. The evaporation section was in thermal contact with two aluminum plates, where a kapton skin heater (280 mm X 25 mm, 11.5 Ohms) was attached to them to deliver the desired heat load. The heat loads were administrated by a DC power supply with deviation of ± 1%. Twenty type-T Omega thermocouples (deviation of ±0.3 °C at 100 °C) were used to monitor the device performance during the tests. A total of 6 thermocouples were located at each section (evaporation, adiabatic and condensation) and the remaining two were at the condensation section inlet and outlet. The thermocouples were connected to an Agilent 34970A data acquisition system used to monitor all temperatures, where the temperatures were read and saved in a spreadsheet. The temperatures were scanned every 3 seconds. The OLPHP was tested using five different working fluids: isopropyl alcohol (2-propanol), ethanol, methanol, acetone and water. These fluids were selected according to their Clausius-Clayperon relation and also for the maximum saturation pressure at the evaporation section temperatures. The PHP was under vacuum before charging it with any working fluid, which was able to hold -5 a vacuum level of 10 mbar. The minimum purity of all working fluids used was 99.8%, which were out-gassed prior to charging the PHP and the filling ratio for all tests was 50%. The maximum evaporation section temperature was limited to a range between 110 and 120 °C, which resulted in tests with heat loads up to 50 W. All experimental tests were performed on a heat load profile basis, where the intention was evaluating the OLPHP behavior related to the working fluid and applied power.

EXPERIMENTAL RESULTS AND DISCUSSION For the startup of the OLPHP, the expected behavior was similar to other two-phase thermal control devices such as capillary pumped loops and loop heat pipes. After initiating the administration of the heat load, an increase on the evaporation section temperature would be expected with consequent drop, indicating that the slug/plug pumping action had been initiated, which would result in temperature oscillations. With an increase of the heat load, the oscillations become more intensive and it can be detected by slight temperature decreasing. This behavior was detected by Khandekar et al. [7] during flow observation through transparent tubes in their experiment. For the tests under vertical orientation, the OLPHP presented startup at different power levels for the different working fluids used. Using 2-propanol, the device presented a more intensive temperature oscillation for 35 W, as shown by Fig. 2. Until reaching this power level, the OLPHP was operating with minor flow oscillations captured on the adiabatic section. From this point, stronger pulsations could be captured by the thermocouple readings, which presented amplitudes of up to 5 °C. Such a characteristics indicates the dynamic operation of the OLPHP and its particularities, showing an interesting thermal behavior that could be considered unique among other passive two-phase thermal control devices. 120

10 W

15 W

25 W

35 W

45 W

50 W

100

Average Temperature (°C)

above mentioned tube inner diameter, the OLPHP should operate with all the selected working fluids.

Evaporator

80

Start of intensive temperature oscillations

60

Adiabatic

40

20 Condenser

0 0

4000

8000

12000

16000

20000

24000

28000

32000

Time (sec)

Figure 2 – OLPHP vertical test with 2-propanol.

For all other working fluids, the OLPHP behavior was very similar, only being different on the highest power applied to the evaporation section. This factor was directly influenced by the selected working fluid, which would result in a lower or higher maximum heat load. The more intensive temperature oscillations were occurring at different power levels as well, such as presented by Fig. 3 for tests with ethanol. Even though few pulsations were captured by the thermocouple readings since 5 W of applied power, the stronger

oscillations were initiated at 25 W. At this point, great temperature oscillations could be captured. For this specific working fluid, the temperature oscillations could be up to 8 °C. 120

10 W

5W

15 W

35 W

25 W

45 W

Evaporator

Start of intensive temperature oscillations

120 10 W

5W

80

15 W

35 W

25 W

45 W

100

60

Average Temperature (°C)

Average Temperature (°C)

100

temperature analysis. For this specific working fluid, it was expected to obtain a poor performance with the OLPHP, with higher operation temperatures for the maximum level of heat load. However, tests with this working fluid did not show this tendency, as presented by Fig. 5, but variations on the adiabatic and condensation sections temperatures could be observed which were due to the slug/plug flow dynamics.

Adiabatic

40

20 Condenser

0 0

4000

8000

12000

16000

20000

24000

28000

32000

Evaporator

80 Start of intensive temperature oscillations

60

40

Adiabatic

20 Condenser

Time (sec) 0 0

Figure 3 – OLPHP vertical test with ethanol.

120 10 W

15 W

25 W

35 W

12000

16000

20000

24000

28000

32000

45 W

50 W

Figure 5 – OLPHP vertical test with distilled water.

Tests with methanol, on the contrary as the one with water, were expected to give better thermal performance. However, they presented to be very close to the behavior for distilled water, regarding to the evaporation section temperature at the highest heat load. Figure 6 presents the test results. Temperature oscillations were observed since the beginning of the test, which were specially noticed on the adiabatic section. However, the more intensive temperature oscillations initiated at 35 W, where it was observed that the oscillations could be as much as 8 °C. The adiabatic section temperature presented strong fluctuations, showing that the slug/plug flow dynamics play an important role on the device thermal behavior.

100

140 10 W

Evaporator

15 W

25 W

35 W

45 W

50 W

120

80

60

Start of intensive temperature oscillations

Average Temperature (°C)

Average Temperature (°C)

8000

Time (sec)

When testing the OLPHP with acetone, it could be observed that a more intensive slug/plug pumping action was initiated at 10 W (Fig.4). For this working fluid, the OLPHP presented more even temperature distribution along the evaporation, adiabatic and condensation sections. Another important aspect observed during the operation of the OLPHP with acetone was the lower evaporation section temperature at high heat load levels. This could be explained by the lower latent heat of vaporization that this specific working fluid presents. This important aspect observed during the experimental tests could lead to the use of acetone at higher heat loads for the same device geometric characteristics.

5W

4000

Adiabatic

40

20

Condenser

100

Evaporator

80

Start of intensive temperature oscillations

60

Adiabatic

40

0 0

5000

10000

15000

20000

25000

30000

35000

20

Condenser

Time (sec)

Figure 4 – OLPHP vertical test with acetone.

0 0

Tests with distilled water presented smooth operation of the OLPHP, even though an evident device startup could not be observed from the evaporation section

5000

10000

15000

20000

25000

Time (Sec)

Figure 6 – OLPHP vertical test with methanol.

30000

120

5W

10 W

15 W

25 W

35 W

50 W

45 W

100 Evaporator

80

First start of intensive temperature oscillations

60

Intensive temperature oscillations re-started at this point

Adiabatic

40

20

Condenser

0 0

5000

10000

15000

20000

25000

30000

35000

Time (sec)

Figure 8 – OLPHP horizontal test with 2-propanol.

For ethanol, the intensive pulsations started at 5 W, as presented by Fig. 9. Comparing the experimental results for ethanol between horizontal and vertical orientation, it was observed that lower temperatures were verified and the OLPHP could operate at higher power levels. For 45 W, great perturbations were verified which indicated instabilities during the OLPHP operation without any tendency of dryout in the evaporation section.

120

100

80

60

80 40

Ethanol Water 2-Propanol Methanol Acetone

20

70

Evaporator

Average Temperature (°C)

Evaporation Section Temperature (°C)

propanol presents an uneven continuous operation when varying the heat load. The more intensive pulsations were only observed for certain levels of heat loads, and it has presented to be a particularity of this working fluid.

Average Temperature (°C)

Upon comparing the evaporation section temperature for all working fluids tested, as presented by Fig. 7, it could be noticed that acetone and 2-propanol have better results for the lowest observed evaporation section temperature along with ethanol. On the same way, methanol showed close thermal performance as water with higher evaporation section temperatures. From this comparison, it is possible to conclude that for higher heat loads, acetone could be stated as the best working fluid to be used as it presents the lowest value for ilv and one of the highest Clausius-Clayperon relation. It is interesting to point that the working fluid selection is an important parameter to select the tube inner diameter in order to have the slug/plug pumping action. This parameter is also important when evaluating the best thermal behavior for an specific OLPHP related to the evaporation section (heat source) temperature. Thus, the working fluid selection is a very important variable that should be carefully considered as a design parameter, which could lead to a worse or better thermal performance of the OLPHP. In this case, proper selection of the working fluid must be performed when evaluating the Clausius-Clayperon relation as well as the Bond number as a guide for estimating the maximum tube inner diameter.

60 50

0 0

10

20

30

40

50

60

Q (W)

Figure 7 – Comparison between all working fluids - vertical.

Upon testing the OLPHP under horizontal orientation, it could be noticed that lower evaporation section temperatures were achieved for the same heat load levels. The OLPHP showed an equivalent behavior when compared to the vertical orientation tests, where pulsations were captured by the thermocouples. The pulsations could be observed from the temperature readings taken during the OLPHP operation and they were move evident, for the tests under horizontal orientation, on the adiabatic and condensation sections. For tests with 2-propanol, as presented by Fig. 8, the more intensive pulsations were initiated for 10 W. The thermocouple readings could capture the pulsations, which stopped when 15 W was applied. The intensive pulsations were resumed when 35 W was applied. This behavior indicates that at horizontal orientation, 2-

Adiabatic

Start of intensive 40 temperatureoscillations

30

Condenser

20 5W

10 W

15 W

25 W

10000

15000

35 W

45 W

50W

10 0 0

5000

20000

25000

30000

35000

Time(sec)

Figure 9 – OLPHP horizontal test with ethanol.

Just as observed during the tests at vertical orientation, acetone presented more even temperature distribution along all sections, as presented by Fig. 10. Lower temperatures were verified when compared to the tests at vertical orientation even though at the evaporation section, the temperatures were close to those for water. During the tests with this specific working fluid, the intensive temperature oscillations started at 5 W.

100 5W

10 W

15 W

35 W

25 W

45 W

50 W

Average Temperature (°C)

90 80

Evaporator

70 60 50 Start of intensive temperature

Adiabatic

40 oscillations 30 20

Condenser

10 0 0

4000

8000

12000

16000

20000

24000

28000

32000

36000

Tests with the OLPHP using methanol presented pulsations since the beginning of the operation at 5 W until its termination, as presented by Fig. 12. The OLPHP performance was considerably better when comparing the evaporation section temperatures, as at vertical orientation and 50 W, the device presented temperature around 112 °C while at horizontal orientation and 50 W, the temperature was around 68 °C. Similarly to what it was observed during the tests with acetone, the adiabatic and condensation sections temperatures were very close for high power levels, indicating the particular dynamics of the OLPHP operation. For power levels above 15 W, greater oscillations were taking place during the OLPHP operation and they started to reflect on the evaporation section temperatures where oscillations were also observed.

Time (sec)

80 5W

Figure 10 – OLPHP horizontal test with acetone.

10 W

15 W

25 W

35 W

45 W

50 W

For the tests with the other working fluids (water and methanol), the OLPHP presented the same behavior, showing lower temperatures when compared to vertical orientation tests. For the specific case of water, the OLPHP did not present an indication of pulsations during its operation, even though some oscillations were continuously captured and became more evident above 25 W, as presented by Fig. 11. Even when the power was increased, the pulsations continued to be not expressive as verified when the OLPHP was operating with the other fluids. Probably, this effect was caused by the elevated surface tension that water presents related to the other working fluids, along with the worse Clausius-Clayperon relation. For tests under horizontal orientation and considering that the channels are very reduced, the surface tension and shear stress effects were very expressive, resulting in a poor operationability of the OLPHP. This behavior still requires further investigations in order to better evaluate the surface tension effects, as this is an isotropic parameter.

Average Temperature (°C)

70 Evaporator

50

Adiabatic

Start of intensive

40 temperature oscillations

30 Condenser

20 10 0 0

5000

10000

15000

20000

25000

30000

35000

Time (sec)

Figure 12 – OLPHP horizontal test with methanol.

Differently to what it was observed for the tests at vertical orientation, methanol has presented better results, followed by ethanol and acetone. The test results are presented by Fig. 13.

100

120

Evaporation Section Temperature (°C)

90 Evaporator

80

Average Temperature (°C)

60

70 60

Adiabatic

50 40 Condenser

30 20 5W

10 W

15 W

25 W

35 W

100

80

60

40

Ethanol Water 2-Propanol Methanol Acetone

20

45 W

10

0 0

0 0

5000

10000

15000

20000

25000

30000

Time (sec)

Figure 11 – OLPHP horizontal test with water.

35000

10

20

30

40

50

60

Q (W)

Figure 13 – Comparison between all working fluids - horizontal.

The OLPHP operating at both horizontal and vertical orientations showed an average interval of oscillations as

Table 2. Average interval of oscillations at 50W. Fluid

Water

Vertical 22 to 28 (sec) Horizontal 25 to 32 (sec)

Methanol Acetone

Isopropyl Alcohol

Ethanol

24 to 40

20 to 22

20 to 32

16 to 24

20 to 25

18 to 28

11 to 18

22 to 40

Figures 14 and 15 presents the results for the thermal conductance based on the evaporation and condensation sections, for the OLPHP operating at vertical and horizontal orientations respectively, calculated by the following relation (with deviation of ±5% represented by the error bars)

Gcalc = Q / (Tevap − Tcond )

(3)

Thermal Conductance (W/°C)

0.8

0.7

0.6

1.4 1.2 1.0 0.8 0.6

Ethanol Water 2-Propanol Methanol Acetone

0.4

0.4

Ethanol Water 2-Propanol Methanol Acetone

0.3

0.2 20

30

40

50

0.0 0

10

20

30

40

50

60

Q (W)

Figure 15 –Thermal conductance – horizontal orientation.

Upon evaluating the overall efficiency of the presented OLPHP, the ratio of conductance is defined as

η = Gcalc / Gref

(4)

Gref = kCu A / Leff

(5)

where

The range obtained for the tests under vertical orientation were between 3.3 and 5.0 and for the tests under horizontal orientation were between 3.6 and 11.7. These results are comparable with the best results obtained by Khandekar et al. [11], which was 6.56. In average, the variation of the overall efficiencies for vertical orientation was ± 19% and for horizontal orientation was ± 53%. The results gathered during this investigation presented the potential in using OLPHP as passive thermal control devices for many applications. Therefore, for microgravity applications, proper qualification and life tests are required in order to better establish the limitations of the OLPHP.

0.5

10

1.6

0.2

The experimental results presented the potential of using the OLPHP as a thermal control device. The use of different working fluids could be taken as a comparative guidance for future applications of PHPs and also for the better understanding of the phenomenon involved on their operation.

0

1.8

Thermal Conductance (W/°C)

presented by Table 2, which was defined on the base of intervals between chaotic pulses. These results show that the oscillatory slug/plug motion in the PHP is dependent on the working fluid and the applied power.

60

CONCLUSION

Q(W)

Figure 14 –Thermal conductance – vertical orientation.

It could be verified that, for tests at vertical orientation, acetone, ethanol and 2-propanol presented higher values when compared with the other fluids. On the same way, for tests at horizontal orientation, methanol, ethanol and acetone presented higher thermal conductance when compared to the other fluids. The analysis made on these results point the better performance achieved with acetone, methanol and ethanol during all tests, which show the great potential in using these working fluids in OLPHPs and also in other passive two-phase thermal control devices for a certain operation temperature range.

This paper presented an experimental investigation of an open loop pulsating heat pipe. Tests were performed for five different working fluids at both vertical and horizontal orientations. The conclusions that could be taken from this investigation are as follows: • From the experimental results, for filling ratio of 50 % and vertical orientation, it could be observed that acetone presented the best results. On the other way, water has presented the worse performance. At horizontal orientation, methanol presented better performance and water presented the worse performance once again; • At vertical orientation tests, the OLPHP presented to be not too sensitive regarding the working fluid. At horizontal orientation, the device presented to be

more sensible. The effective thermal conductance varied ± 19% at vertical orientation and ± 53% at horizontal orientation for different working fluids; • The overall efficiency of the OLPHP, defined as the ratio between calculated thermal conductance and conductance of empty copper tubes, was within 3.3 and 11.7; • The average interval of oscillations between pulses was within 11 and 40 seconds.

ACKNOWLEDGMENTS The author wishes to acknowledge Dr. Valeri V. Vlassov (INPE-DMC) for his valuable comments and discussions.

REFERENCES 1. Akachi, H., Polášek F., Štulc P., “Pulsating Heat th Pipes”, Proceedings of the 5 International Heat Pipe Symposium, 1996, pp.208-217, Melbourne, Australia. 2. Zhang, Y., Faghri, A., “Heat Transfer in a Pulsating Heat pipe with an Open End”, International Journal of Heat and Mass Transfer, Vol. 45, 2002, pp. 755764. 3. Riehl, R. R., “Evaluation of the Thermal-HydroDynamics Behavior of an Open Loop Pulsating Heat Pipe”, National Institute for Space Research (INPE) Report, 2003, 35p. 4. Delil, A. A. M., “Pulsating and Oscillating Heat Transfer Devices in Acceleration Environments from Microgravity to Supergravity”, SAE paper # 2001-012240. 5. Charoensawan, P., Khandekar, S., Groll, M., Terdtoon, P., “Closed Loop Pulsating Heat Pipes Part A: Parametric Experimental Investigations”, Applied Thermal Engineering, 2003, Vol. 23, pp. 2009-2020. 6. Khandekar, S., Charoensawan, P., Groll, M., Terdtoon, P., “Closed Loop Pulsating Heat Pipes Part B: Visualization and Semi-Empirical Modeling”, Applied Thermal Engineering, 2003, Vol. 23, pp. 2021-2033. 7. Khandekar, S., Dollinger, N., Groll, M., “Understanding Operational Regimes of Closed Loop Pulsating Heat Pipes: An Experimental Study” , 2003, Applied Thermal Engineering, Vol. 23, pp. 707-719. 8. Rittidech, S., Terdtoon, P., Murakami, M., Kamonpet, P., Jompakdee, W., “Correlation to Predict Heat Transfer Characteristics of a Closed-End Oscillating Heat Pipe at Normal Operating Condition” , 2003, Applied Thermal Engineering, Vol. 23, pp. 497-510. 9. Tong, B. Y., Wong, T. N., Ooi, K. T., “Closed-Loop Pulsating Heat Pipe” , 2001, Applied Thermal Engineering, Vol. 21, pp. 1845-1862. 10. Groll, M., Khandekar, S., “Pulsating Heat Pipes: Progress and Prospects”, Proceedings of the

International Conference on Energy and the Environment, 2003, Shanguai, China, May 22-24. 11. Khandekar, S., Schneider, M., Schäfer, P., Kulenivic, R., Groll, M., “Thermofluiddynamic Study of Flat Plate Closed Loop Pulsating Heat Pipes”, Microscale Thermophysical Engineering, Taylor and Francis, 2002, ISSN 1089-3954, pp. 303-318, Vol. 6/4.

12. Borisov, V., Buz, V., Coba, A., Kuznetzov, I., Zacharchenko, A., Smyrnov, G., “Modeling and Experimentation of Pulsating Heat Pipes”, th Proceedings of the 12 International Heat Pipe Conference, 2003, Moscow-Kostroma-Moscow, 1924 May, pp. 220-225. 13. Qu, W., Ma, T., “Experimental Investigation on Flow and Heat Transfer of Pulsating Heat Pipes”, th Proceedings of the 12 International Heat Pipe Conference, 2003, Moscow-Kostroma-Moscow, 1924 May, pp. 226-231. 14. Dobson, R. T., “Theoretical and Experimental Modeling of an Open Oscillatory Heat Pipe Including Gravity”, International Journal of Thermal Sciences, Vol. 43, 2004, pp. 113-119. 15. Sakulchangsatjatai, P., Terdtoon, P., Wongratanaphisan, T., Kamonpet, P., Murakami, M., “Operation Modeling of Closed-End and ClosedLoop Oscillating Heat Pipes at Normal Operation Condition”, Applied Thermal Engineering, Vol. 24, 2004, pp. 995-1008.

DEFINITIONS, ACRONYMS, ABBREVIATIONS A dc g ilv Gcalc Gref kCu Leff Q Tcond Tevap Tsat vlv

ρl ρv σ η

2

Area (m ) Critical bubble diameter (m) 2 Gravity acceleration (9.81 m/s ) Latent heat of vaporization (J/kg) Calculated thermal conductance (W/°C) Reference thermal conductance (W/°C) Copper thermal conductivity (W/m°C) Effective length (m) Heat load (W) Average condensation section temperature (°C) Average evaporation section temperature (°C) Saturation temperature (°C) 3 Specific volume (m /kg) 3 Liquid density (kg/m ) 3 Vapor density (kg/m ) Surface tension (N/m) Ratio of conductance

CONTACT Roger R. Riehl National Institute for Space Research Space Mechanics and Control Division INPE/DMC/Satelite Av. dos Astronautas 1758, 12227-010 Sao Jose dos Campos, SP – Brazil Phone: 55 12 3945-6178 / Fax: 55 12 3945-6226 E-Mail: [email protected]