Loop Heat Pipe

Space Technology & Application International Forum – STAIF 2004 Conference of Thermophysics on Microgravity American Institute of Physics (AIP) Conference Proceedings, Albuquerque, NM, USA Vol. 699, Issue 1, ISBN 0-7354-0171-3

Loop Heat Pipe: Design and Performance During Operation Thiago Dutra1, Roger R. Riehl2 1

Federal University of Santa Catarina – Mechanical Engineering Department – Laboratory of Combustion and Thermal Systems Engineering – Florianópolis, SC Brazil, 88040-900 2

National Institute for Space Research – Space Mechanics and Control Division – DMC/Satélite Av. Dos Astronautas 1758, São Jose dos Campos, SP, Brazil, 12227-010 Phone: 55 12 3945-6178; Fax: 55 12 3945-6226; E-mail: [email protected]

Abstract. Loop heat pipes (LHPs) have been extensively investigated and considered for the thermal control of satellites and other space equipments, but some geometric limitations, as well as the use of hazardous working fluids must be considered. Focusing on such concerns, a LHP was designed and built to accomplish certain requirements towards its future application in space missions. The designing procedure had to consider some limitations, such as a reduced scale capillary evaporator and the use of an alternative working fluid. Thus, an experimental LHP was built and tested for acetone as the working fluid to manage up to 70 W of heat transfer rate. The experimental results showed a good thermal management performance of the proposed LHP for the imposed limitations to its design. The proposed LHP presented to be a reliable thermal management device for applying in future space missions, especially when considering the use of a less hazardous working fluid.

INTRODUCTION Loop heat pipe (LHP) is a two-phase thermal management device, usually applied in the thermal control of satellites, spacecrafts, electronics and structures, and operates by acquiring heat from a source and dissipating it in a sink. It is considered a reliable thermal control device as it can dissipate large amounts of heat while keeping a tight control of the heat source temperature. The components of a LHP are a capillary evaporator, condenser, liquid and vapor lines and a compensation chamber (or two-phase reservoir). Heat is acquired by the system through the capillary evaporator, which is responsible by evaporating the working fluid and generating the capillary forces that will drive the fluid. Then, vapor flows in the vapor line towards the condenser, where it is condensed and flows back to the evaporator by the liquid line. The compensation chamber, which is the two-phase reservoir, is responsible for establishing the loop’s operation pressure and temperature, as well as the right amount of working fluid in the system. LHPs operate passively by means of capillary forces generated by the capillary evaporator and present several advantages such as no moving parts, the working fluid operates at its pure state and very little power consumption. It is considered to be more effective than capillary pumped loops (CPL), as the control of the temperature in the two-phase reservoir is not required and pre-conditioning procedures are usually not necessary. Depending on the power applied to the capillary evaporator, the working fluid flow rate may increase or decrease, which will determine the amount to working fluid that must be supplied or removed by the compensation chamber. In the same way, the percentage of the condenser used to condense the working fluid will be determined by the applied power, which will directly affect the temperature in the compensation chamber. As the system will passively seek the equilibrium, the entire loop will reach the steady state operation and the compensation chamber will establish the loop’s operation temperature. For low flow rates, the LHP will operate under variable conductance, as the compensation chamber will hold both liquid and vapor. When it is working at its maximum designed capacity, the LHP operates at constant conductance as the compensation chamber holds only liquid. Thus, the LHP operation temperature is very dependent whether the system operates at variable or constant conductance.

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Even though capillary pumped loops (CPLs) are still under development by researchers to be used in space and ground applications (Bazzo and Riehl, 2003), the reliability that LHPs have proven in many microgravity tests have made CPLs fall into a second option as thermal control devices due to the necessity of controlling the two-phase reservoir temperature. Several investigations have already been performed with LHPs in the past ten years, as this innovating technology was first tested at the former Soviet Union in the mid- 80’s (Maidanik et al., 1985). North et al. (1997) reported one of the first informations about LHPs using ammonia as the working fluid. This new technology used a bi-disperse wick material. This investigation showed that the LHP could reach heat fluxes around 0.07 MW/m2. Ku (1999) described the operation characteristics of LHPs regarding their capabilities on handling high heat fluxes and the constraints that are important to consider while designing such a system. Studies have reported the capability of LHPs in efficiently transporting and dissipating heat fluxes, while keeping a tight controlled temperature of the heat source in both space and ground environments. Kurwitz and Best (1997) investigated LHPs startups during parabolic flight and concluded that the system presents an operation very similar to CPLs. Reliable startups and steady state operation were also verified by Baker et al. (1998) while testing a LHP flight model aboard the Columbia Space Shuttle. Even though several experimental tests involving LHPs have been performed, some uncertainties related to their design are still unknown and rely only on the designer experience. Some mathematical models have been presented in the past, which are mentioned to predict the LHPs operation, such as those reported by Kaya and Hoang (1999), Mulholland et al. (1999) and Watson et al. (2000). The guidance given by these models can perfectly be used for new LHP designs with good degree of correlation. The issues related to the use of ammonia in LHPs as thermal control systems in space, due to the ammonia freezing point, have been reported by Goncharov and Kolesnikov (2002). As ammonia freezes at – 78 ºC, the condenser design must prevent such a condition. In order to avoid such a problem, other working fluids might be considered, such as propylene. Upon considering ammonia and propylene as working fluids, several issues related to safety must be taken, as their use in an environment where people are present is dangerous. Thus, other working fluids must be considered for such applications. Considering the issues regarding designing and testing LHPs for future space missions, the objective of this paper is presenting a LHP experiment designed using Kaya and Hoang (1999) model and considering the particular characteristics related to its design, as well as the need of using an alternative working fluid.

DESIGNING AND CONSIDERATIONS During the design of the proposed LHP, many factors had to be taken into account. Considering the current need for heat dissipation for this specific device, the LHP had to be designed to accomplish the heat management of up to 70 W and had to present a reduced scale capillary evaporator. In this case, the design of the set capillary evaporator/compensation chamber had to be carefully considered because it should present a compact configuration, while being able to promote a good thermal performance and the heat source temperature should not exceed 85 ºC. Despite the need to construct compact components able to fit restricted areas, with a special attention to the set capillary evaporator/compensation chamber, the most important variable was the working fluid to be used. In the present design, two working fluids were selected, being anhydrous ammonia and acetone. A current issue is regarding the use of hazardous working fluid such as ammonia. Even though such a working fluid present an outstanding Figure of Merit when compared to other fluids, it is extremely dangerous and several safety steps have to be taken in order to use it in capillary pumping systems. In the current Brazilian Space Program, there is an orientation to substitute ammonia by another working fluid that should present a reduced risk and be easier to handle. Considering that the range of heat to be dissipated is considerably low, acetone has become an interesting option together with other alcohols such as methanol and ethanol. Despite the lower Figure of Merit for these working fluids when compared to ammonia, they present several advantage such as: sub-atmospheric working pressure for the operation temperature range (within –60 and 80 ºC), reduced handling risks, less expensive distillation process and filling devices and less probability of freezing in space conditions.

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Studies have already being performed comparing the thermal performance of capillary pumping systems that use ammonia and acetone, such as the one presented by Riehl and Bazzo (2003). In this investigation, it could be verified that both working fluids presented the same thermal performance, despite a higher superheating obtained when acetone was used as the working fluid, using a reduced scale capillary evaporator. For heat loads applied to the capillary evaporator of up to 70 W, the levels of superheating are not expected to be a concern, as the heat dissipation area does not present a major issue in the present design. Thus, in the present investigation, acetone has been considered as the working fluid for testing the proposed LHP for the maximum heat load to be managed. A design procedure for CPL/LHP systems was developed based on the model proposed by Kaya and Hoang (1999) and applied in the present investigation. Mathematical simulations using this procedure showed that the proposed LHP was able to manage up to 70 W of heat applied to the evaporator, while keeping the heat source temperature below 85 ºC with a maximum superheating level (difference between the capillary evaporator and the compensation chamber temperatures) below 15 ºC. Such a level of superheating was expected due to the reduced size of the capillary evaporator and its configuration (effective length, number of axial grooves). Prior to initiating all the experimental procedures, extensive tests were performed to check whether the set capillary evaporator/compensation chamber would perform as expected, such as bubble tests and testing it using a static pressure procedure.

EXPERIMENTAL APPARATUS An experimental apparatus to test a LHP was constructed according to the calculation results given by the design procedure above mentioned, in order to perform the thermal management of up to 70 W using a LHP with acetone as the working fluid. Certain constraints had to be carefully considered as it was intended to have a reduced scale capillary evaporator, with an active length of less than 70 mm. Such a characteristic was imposed to the LHP design due to an interest in applying it in future microsatellite programs and also to check the degree of freedom that the design would present, in regard to miniaturization related to the capillary evaporator. Thus, a LHP was designed and built with the geometric characteristics as shown by Table 1. Such a LHP was built and tested under ground conditions, but it has also been scheduled to be tested under microgravity conditions in the near future and thus, this system was also built to accomplish the standard qualification process for two-phase thermal control system. The set capillary evaporator/compensation chamber was built with ASTM 304L stainless steel and the rest of the loop was built with ASTM 316L stainless steel. Figure 1 presents the LHP configuration tested in laboratory. For the capillary evaporator design, the effective thermal conductivity was 0.38 W/m-ºC. Figure 2 presents the set capillary evaporator/ compensation chamber assembly and end view. TABLE 1. Geometric characteristics of the experimental LHP. Capillary Evaporator Total Length (mm) Active Length (mm) Outer/Inner Diameter (mm) Material UHMW Polyethylene Wick Mean Pore Radius (µm) Permeability (m2) Porosity (%) Diameter (OD/ID) mm Grooves height, width, angle Number of Axial Grooves

Value 100.0 67.0 19.0 / 16.0 304L SS Value 7.0 10-13 50 16.0 / 7.0 2.12/3.22/28º 7

Liquid Line Outer Diameter (mm) Inner Diameter (mm) Length (mm) Material Compensation Chamber Outer Diameter (mm) Length Screen mesh Volume (cm3) Material

Value 4.85 2.85 1100 316L SS Value 45.0 25.0 200 65.0 316L SS

Vapor Line Outer Diameter (mm) Inner Diameter (mm) Length (mm) Material Condenser Outer Diameter (mm) Inner Diameter (mm) Length (mm) Material

Value 4.85 2.85 800 316L SS Value 6.35 4.35 1200 316L SS

The capillary evaporator had a primary wick made of UHMW polyethylene with 7 axial grooves machined at its outer diameter. The secondary wick was ASTM 304L stainless steel screen mesh number 200, which covered the entire length of the capillary evaporator, compensation chamber and the connection between each other. The capillary evaporator was internally machined with microgrooves, in order to enhance the heat exchange rate between the primary wick, the evaporator housing and the working fluid. The returning liquid from the condenser was connected to this set, which presented a bayonet that delivered sub-cooled liquid to the evaporator liquid core. The entire set capillary evaporator/compensation chamber was welded, which did not cause any damage to the internal plastic parts. The compensation chamber presented an internal volume enough to hold the volume of both condenser and vapor line while the LHP was operating at its highest designed power. 53

Twenty type-T Omega thermocouples with a deviation of 0.3 ºC at 100 ºC were used to monitor the LHP operation. The thermocouples were connected to a HP 3292A data acquisition system, which was used to monitor the temperatures throughout the loop. A silicon skin heater (15 mm X 65 mm, with a resistance of 11.7 Ω) was placed at the top of the capillary evaporator using an aluminum plate that was in thermal contact with the evaporator, with an angle of applied heat of 120º. The condenser was in thermal contact with an aluminum plate with 300 mm X 300 mm and 12.4 mm of thickness. This plate was also in thermal contact with a heat exchanger with internal channels, which was connected to a constant temperature cooling bath, set at –5 ºC for all tests, circulating a mixture of 50% water and 50% ethylene-glycol. The acetone liquid inventory for the tests was 47 grams, for a fraction of the compensation chamber occupied by the working fluid of 50% in the cold mode.

FIGURE 1. LHP Experimental Test Bed.

FIGURE 2. Capillary Evaporator/Compensation Chamber Assembly and End View.

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The tests were performed for a range of heat applied to the capillary evaporator from 2 to 70 W, where both startup and heat load profile procedures were analyzed. For the startup tests, the objective was to investigate the LHP capability on initiating its operation, where the required superheating and time to start and reach the steady state regime were verified. For the heat load profile tests, the objective was investigating the LHP capability in handling sudden changes in the capillary evaporator heat load. It was also intended to investigate how the LHP operation conditions would interact in order to reach the operation temperature for a given heat load. In this case, it was also verified the time and level of superheating required to reach the steady state.

RESULTS AND DISCUSSION Experimental tests were carried out without temperature control of the compensation chamber and pre-conditioning procedures for both startups and heat load profile tests. The startup tests for the designed LHP presented to be very reliable, but some interesting characteristics were observed at both high and low heat applied to the evaporator. For low heat load, little superheating (difference between the evaporator and compensation chamber temperatures) was verified and a considerable time to reach steady state was necessary. For high heat load, higher superheating was verified (as high as 14 oC) and the time for reaching the steady state was basically the same as that verified for low heat loads. The short active length of the capillary evaporator and the reduced number of grooves could explain the high superheating levels. Figure 3 presents the startup results for 10 W and 50 W applied to the LHP. The liquid side temperatures stabilized faster than the vapor side temperatures due the interaction between the ambient and operation conditions that guide the LHP to reach the steady state condition. The dynamics in reaching the loop operation temperature was very similar when comparing low and high heat loads and no sudden changes or temperature overshooting of the capillary evaporator were verified in any test. The presented experimental results were in accordance to what it was predicted by the designing procedure used. 50

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FIGURE 3. Experimental Tests for the LHP.

One specific test presented very interesting results, which was for a heat load of 5 W. In this case, the capillary evaporator presented a high superheating prior to initiating its operation. After its initiation, the entire loop reached the steady state in a very short time, as it can be verified in Fig. 4a. This behavior was verified in all three re-tests performed for this specific heat load, which could be explained by the geometric particularities of the LHP design, along with the use of acetone as the working fluid. An important experimental test had to be carried out for the LHP, which as related to its operation at a reduced heat load applied to the capillary evaporator. Tests with a heat applied of 2 W are interesting to be investigated,

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considering that the LHP might operate in a so-called “sleeping mode”. In such a mode, the LHP is not shut down when the thermal control of a certain equipment/component is not required and its operation can be continued. In this case, very little power is required to keep its operation, avoiding the startup procedure every time the LHP is required to start its operation. Such a test is required to be performed in case of applying the LHP in the thermal control of satellites and components, in order to verify its capability in operating at reduced heat loads. Figure 4b presents the LHP startup test operating at 2 W. The tests with the LHP operating in the “sleeping mode” show the robust design of such a system, as it could start operating at very low power applied to the capillary evaporator. Considering the overall thermal performance of the LHP, it has presented good operation for the range of thermal management that has been designed. 50

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FIGURE 4. LHP Experimental Results.

The LHP was also tested on power cycles where the objective was analyzing its capability on handling changes on the heat load applied to the capillary evaporator, as well as to check its continuous operation along time. Tests were performed for certain levels of heat loads applied to the evaporator and then reducing it to 2 W, as presented by Fig. 5. As verified during the startup tests, the LHP presented a reduced superheating to start its operation. After reaching the steady state, the heat load was then decreased to 2 W, where it was verified that the system took less time to reach the steady state when compared to the startup test for 2 W and did not show any depriming tendency. After reaching the steady state, the heat load was then increased again, where it shows that the LHP was able to handle the changes in the heat applied without presenting any depriming or temperature overshooting tendency. The results show that the proposed LHP design is robust, with great potential to be used in future space missions. Due to the reduced active length of the capillary evaporator, number of axial grooves and the use of acetone as the working fluid, the heat source temperature was considerably high (around 80 ºC) for high heat loads (70 W), but it was in agreement to the design specifications. Upon considering the degree of correlation between the experimental and calculated results, given by the design procedure, Fig. 6 presents the overall test results for the LHP in steady state conditions according to the applied heat load and operation temperature. The design procedure presented a good capability in predicting the LHP experimental behavior, as the relative error between the calculated and experimental results was less than 5%. From the experimental tests, it was possible to verify that the LHP design presented an acceptable performance considering a less effective working fluid such as acetone, as well as a reduced active length of the evaporator. The performance of the LHP system tested presented very reliable and according to what it was expected, following the predictions made by the design procedure given by Kaya and Hoang (1999). The tests performed for selected power cycles also presented reliable results, as the LHP showed to handle the changes on the heat applied very well. 56

The development of the capillary evaporator is still undergoing, where another primary wick with more axial grooves will be applied in an attempt to reduce the level of superheating. Despite the superheating levels verified during the experimental tests, the proposed LHP has presented a reliable behavior for future space missions. TC01 TC02 TC06 TC09 TC12 TC13 TC20 Power

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CONCLUSIONS This paper presented a proposed LHP experiment, which was designed to accomplish the thermal management of up to 70 W, using acetone as the working fluid. The selection of acetone was due to a tendency in substituting hazardous working fluids in two-phase capillary pumping systems, which present a high risk. Even though acetone presents lower thermal efficiency when compared to anhydrous ammonia, it can perfectly be used as working fluid 57

when the thermal management of low power levels is required and also avoiding freezing concerns when applying the LHP in space conditions. The proposed LHP was built in accordance to a design procedure, based on Kaya and Hoang (1999) model, following some restrictions applied to the design related to the need of a reduced scale capillary evaporator. Tests were performed for a heat load range between 2 and 70 W applied to the capillary evaporator, without preconditioning procedures and temperature control of the compensation chamber. The tests presented the good performance of the proposed LHP, where reliable startups were observed even when it had to operate in the sleeping mode, i.e., at 2 W. An interesting behavior was verified when the LHP was operating at 5 W, as the system reached the steady state in a very short time, which could be explained by the geometric particularities of the LHP design and the use of acetone as the working fluid. The LHP also presented reliable operation for selected power cycles, as the system was able to handle the changes on the power applied to the capillary evaporator without presenting a tendency of depriming. The experimental tests were in accordance to the calculated results given by the design procedure, which showed that the proposed LHP could be used in future thermal management applications in both ground and space applications. Further experimental tests are scheduled to be performed in order to better develop the proposed LHP for qualifying it to be used in future space missions.

ACKNOWLEDGMENTS This work was supported by the National Council of Research (CNPq/Brasília/Brazil), grant number 300778/01-5 and the Brazilian Space Agency (AEB/Brasília/Brazil).

REFERENCES Baker, C. L., Bienert, W. B. and Ducao, A. S., “Loop Heat Pipe Flight Experiment,” 28th International Conference on Environmental Systems, Danvers, MA, July 13-16, 1998, paper # 981580. Bazzo, E., Riehl, R. R., “Operation Characteristics of a Small Scale Capillary Pumped Loop,” Applied Thermal Engineering, Elsevier Science Press, Vol. 23, No 6, pp. 687-705, (2003). Goncharov, K., Kolesnikov, V., “Development of propylene LHP for Spacecraft Thermal Control,” Proceedings of the 12th International Heat Pipe Conference, Moscow-Kostroma-Moscow, Russia, 19-24 May, 2002, pp. 171-176. Kaya, T. and Hoang, T., “Mathematical Modeling of Loop Heat Pipes and Experimental Validation,” AIAA Journal of Thermophysics and Heat Transfer, Vol. 13, No. 3, 1999, pp. 214-220. Ku, J., “Operating Characteristics of Loop Heat Pipes,” SAE – Society of Automotive Engineers, 1999, paper # 1999-01-2007. Kurwitz, C. and Best, F. R., “Experimental Results of Loop Heat Pipe Startup in Microgravity,” American Institute of Physics Conference Proceedings, Vol. 387, No. 1, 1997, pp. 647-652. Maidanik, Y. F., Vershinin, S., Kholodov, V., and Dolgirev, J., “Heat Transfer Apparatus,” 1985, U.S. patent No. 4515209. Mulholland, G., Gerhart, C., Gluck, D. and Stanley, S., “Comparison Between Analytical Predictions and Experimental Data for a Loop Heat Pipe,” American Institute of Physics Conference Proceedings, Vol. 458, No. 1, 1999, pp. 805-810. North, M. T., Sarraf, D. B., Rosenfeld, J. H., Maidanik, Y. F. and Vershinin, S., “High Heat Flux Loop Heat Pipe,” American Institute of Physics Conference Proceedings, Vol. 387, No. 1, 1997, pp. 561-566. Riehl, R. R., Bazzo, E. “Comparison Between Acetone and Ammonia on the Thermal Performance of a Small-Scale Capillary Pumped Two-Phase Loop,” Space Technology and Applications International Forum (STAIF2003), AIP Conference Proceedings 654, Albuquerque, NM, Vol. 654, Issue 1, 2003, pp. 80-87. Watson, H., Gerhart, C., Mulholland, G. and Gluck, D., “Steady-State Operation of a Loop Heat Pipe with Analytical Prediction,” Proceedings of the ASME Heat Transfer Division, Vol. 366-4, 2000, pp. 457-462.

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