Heat transport capability and compensation chamber influence in loop heat pipes performance

Applied Thermal Engineering 26 (26) 1158 1168 www.elsevier.com/locate/apthermeng Heat transport capability and compensation chamber influence in loop heat pipes performance Roger R. Riehl a, *, Tulio C.P.A. Siqueira b a National Institute for Space Research Space Mechanics and Control Division DMC/Satélite Av. dos Astronautas 1758, São Jose dos Campos, SP, 12227-, Brazil b Universidade Federal de Ouro Preto Departamento de Engenharia de Controle e Automação Ouro Preto, MG, 354-, Brazil Received 15 May 25; accepted 25 October 25 Available online 15 December 25 Abstract The development of the loop heat pipe technology for application in future space missions requires that certain aspects related to the operation of this device in regard to the heat transport, geometry and selected working fluid must be carefully considered. As efforts have been focused in the construction of loop heat pipes able to manage up to 8 W of applied heat using an alternative working fluid, designing and testing these devices have shown important results. Two loop heat pipes have been built and tested, where they differ from each other on their compensation chamber geometry and use high grade acetone as working fluid, in substitution of the so-used ammonia. Life tests have shown reliable operation for both loop heat pipes with successful startups and continuous operation without temperature overshoot or evaporator dryout. The life tests results investigation have generated important data that has been applied on the design and construction of loop heat pipes toward their use in future space applications. Ó 25 Elsevier Ltd. All rights reserved. Keywords: Loop heat pipe; Thermal control; Evaporator and compensation chamber 1. Introduction The application of loop heat pipes (LHPs) in several space missions have gained interest as this device presents passive thermal control. As a two-phase thermal control device that operates by means of capillary forces generated in the evaporator, it has shown reliability and remarkable development during the last years. Even though LHPs have been presented as a promising technology with several successful applications, certain particularities concerning their miniaturization and design still need to be carefully considered. * Corresponding author. Tel.: +55 12 3945 6178; fax: +55 12 3945 6226. E-mail address: rriehl@dem.inpe.br (R.R. Riehl). Several space applications for LHPs and capillary pumped loops (CPLs) have presented interesting applications as well as reliability during their operation. Some issues related to the use of passive two-phase thermal control devices have been already presented [1 3]. Issues related to the design and applications have been investigated but LHPs still require particular investigations depending on the application for this specific device, selected working fluid, maximum heat load management and temperatures as each design is unique. The miniaturization of LHPs has gained efforts and interesting results have been obtained [4 6]. As a reliable two-phase thermal management device, LHP has been applied in the thermal control of satellites, spacecrafts, electronics and structures. It operates by acquiring heat from a source and dissipating it in a sink and it can manage large amounts of heat while keeping a tight control of the heat source temperature. 1359-4311/$ - see front matter Ó 25 Elsevier Ltd. All rights reserved. doi:.16/j.applthermaleng.25..37

R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 1159 Nomenclature T temperature ( C) _Q heat load (W) TR thermal resistance ( C/W) UHMW ultra high molecular weight Subscripts compensation chamber Evap evaporator Several space applications using LHPs have been developed, such as those applied to the NASAÕs GLAS laser instrument [1] and the Mars Rovers [2]. As a constant effort to improve the LHP technology, new options of working fluids have been suggested and used [7,8], but they still require further investigations in order to accept them for space applications. Currently, there is a wish to use alternative working fluids such as acetone instead of ammonia due to its less hazard effects and simple manipulation, as well as reduced costs for distillation, purification and charging. Certain aspects related to the compensation chamber temperature influence on the LHP design has been investigated [9] and the miniaturization of this device still require proper considerations []. The advances on the development of the LHP technology and the variations of its application have been presented by Maidanik [11]. The components of a LHP are: capillary evaporator with an internal wick structure, condenser, liquid and vapor lines and 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 operation pressure and temperature, as well as the working fluid inventory in the system. Loop heat pipes operate passively by means of capillary forces generated in the capillary evaporator as it presents a fine porous wick structure. Loop heat pipes also present several advantages compared to regular systems such as no moving parts, the working fluid operates at its pure state and no power consumption is required. The issues related to the use of anhydrous ammonia in LHPs as thermal control systems in space, due to the ammonia freezing point, have been already discussed [8]. In the case of using ammonia, the condenser design must prevent freezing conditions. In order to avoid this problem, other working fluids should be considered, such as propylene [3]. 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 depending on the application. Upon verifying the current need for thermal dissipation in satellites, a research program focusing on the development of the loop heat pipe technology has been undergoing. As limited areas for integration of this device are a concern in the current development, some issues related to the geometric characteristics of the LHPs have to be properly addressed. Life tests have been performed using high grade acetone in order to use the data for qualification of this working fluid for use in future space applications. Thus, this paper presents the development of two loop heat pipes designed to promote the same thermal management, only differing from each other on their compensation chamber geometric characteristics. The results gathered in this investigation have been used to better develop the LHPs in order to apply them as thermal control devices in future space applications, with the objective of using the informations to improve the LHPs design. 2. Loop heat pipe design and important parameters The LHP technology development has been focused in the current thermal management requirements of this institute, as well as the application of an alternative working fluid. The informations presented in this paper are part of a long-term chronogram to be performed towards to use of LHPs in future space missions. This project has defined objectives that must be accomplished, such as: development of a stainless steel loop heat pipe, with fine pore UHMW polyethylene as primary wick; use of alternative working fluids that result in reduced hazard during manipulation, along with reduced costs for purification and charging; thermal management of up to 8 W, for a maximum heat source (evaporator) temperature of 85 C. Particularities of this program are related to the use of less hazard working fluids instead of ammonia in systems operating within the temperature range of 2 and 85 C. Thus, other fluids have been considered during the development of this program, with special attention to acetone due to its reduced cost and compatibility with the other components of the loop. This fluid presents acceptable application in LHPs but life tests related to

116 R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 continuous operation still require extensive investigation. In this case, the continuous development of a capillary evaporator with UHMW polyethylene as primary porous wick has been carried out, due to its reduced cost and chemical compatibility with many working fluids. This material has been already used in capillary pumped loops applied in space missions, which has presented reliable operation over the time [12]. Parallel investigations focusing on capillary evaporators that use other materials such as sintered metal wicks and ceramics have been also carried out and the results of this investigation will be presented in future reports. As the geometric parameters of LHPs are extremely important for their proper operation, an issue that has been considered is related to the compensation chamber geometry. This component is important when considering the proper operation temperature control (specially when considering the heat leak from the capillary evaporator to the compensation chamber) and the LHP integration. As the capillary evaporator and the compensation chamber forms only one component in a LHP, their design must be carefully carried out to promote the desirable device operation. However, some issues related to the compensation chamber geometric characteristics can potentially make the LHP integration in the satellite a concern, specially where there is reduced available area, as well as issues related to its construction and the operation in microgravity conditions. This last concern is specially related to the liquid/vapor distribution in the compensation chamber prior to the LHP startup and during its operation. Investigating the development of LHPs in the past, different compensation chamber geometric characteristics were used, but indications on which one could lead to better results as well as any potential operation/integration problems are rare. Several results have been presented for LHPs with different compensation chamber configurations, showing their potential as thermal management devices, such as: dual compensation chambers with their diameters greater than the capillary evaporator [13]; compensation chamber detached from the capillary evaporator body [14 16]; the capillary evaporator and the compensation chamber are an integral part with the same outer diameters [17,18]. When designing a LHP, a well defined compensation chamber geometry must already be presented and proper considerations related to this important variable (i.e. thermal resistance, connection with the capillary evaporator, secondary porous wick structure configuration, etc.) have to be addressed. Therefore, it becomes interesting to investigate the LHP thermal behavior whether the compensation chamber used is detached or it is an integral part of the capillary evaporator, in order to better establish the most indicated configuration towards the accomplishment of the development of such a technology. 3. Loop heat pipes setups and test procedures Considering the important above-mentioned parameters and issues, two LHPs were designed and built to accomplish the same thermal management using acetone as the working fluid. Acetone has been considered as a potential working fluid as it presents to be less hazard than ammonia, less expensive distillation/charging procedures and freezes at a lowest temperature ( 93.15 C for acetone while ammonia freezes at 78 C [19]). Acetone also presents a reduced operation pressure for the above-mentioned operation temperature range, which potentially reduces risks. This last information is important when evaluating the application of this working fluid in space missions. Previous investigations toward the design and tests of a LHP for future space applications have already addressed the potential application of acetone as working fluid and showed the reliable operation of this device [8]. The LHPs, named as Thermal Control Devices (TCD), differ from each other on the compensation chamber geometric characteristics, which are defined as TCD-LHP (Fig. 1a) and TCD-LHP2 (Fig. 1b). Both LHPs were designed according to a computer code [2] developed to determine the best geometric configuration and thermal performance for such devices and selected working fluid. The computer code is based on a twophase pressure drop model applied for the capillary evaporator and condenser and the thermal model proposed by Kaya and Hoang [21] that was applied for the entire system. The geometric characteristics of both LHPs are presented in Table 1. The LHPs were designed to operate with acetone as the working fluid, managing a maximum applied heat load on the evaporator of 8 W. One issue related to the use of polyethylene is the maximum evaporator temperature, which should not exceed 12 C, according to the wick manufacturerõs instruction. However, during the simulations with the computer code the maximum evaporator temperature was calculated around 78 C for the TCD-LHP geometry and 85 C for the TCD-LHP2 and the experimental tests could be used to verify these calculations. For the capillary evaporator design, the effective thermal conductivity was.38 W/m C. Following the project requirements, the entire experimental setup was built in stainless steel grade 316L (ASTM) tubing with the described geometric characteristics. The grooves were axially machine on the polyethylene wick outer diameter, in a total number of 15. The evaporator housing was machined with micro threads

R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 1161 Fig. 1. Set capillary evaporator/compensation chamber characteristics. (a) TCD-LHP and (b) TCD-LHP2. on its inner diameter (15 threads/cm, 1 mm in depth to the right and left) to improve the heat transfer process between the evaporator housing, the primary wick and the working fluid. Both sets of capillary evaporator/ compensation chamber were welded using mechanical orbital welding system to be in agreement with space requirements as both LHPs had to be built to accomplish the standard qualification process for space application of two-phase thermal control systems. A data acquisition system was used to monitor the temperature readings throughout the LHPs, as each of them had 2 type-t Omega thermocouples distributed as presented by Fig. 2. The thermocouples have a deviation of ±.3 C at C and were connected to a Agilent 3292A data acquisition system, which was performing the readings at a sample frequency of.5 Hz and saving the data on a spreadsheet file for further analysis. Heat was applied to each capillary evaporator through an aluminum saddle (screwed to the evaporator) that was in thermal contact by using high thermal conductivity heat sink compound. Each saddle had a kapton skin heater (7 mm 15 mm, 14 X) used to simulate the heat loads, connected to a DC power supply with accuracy of ±1.%. Each condensation area was an aluminum heat exchanger with embedded channels,

1162 R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 Table 1 LHPs geometric characteristics Capillary evaporator Liquid line Total length (mm) Outer diameter (mm) 4.85 Active length (mm) 67 Inner diameter (mm) 2.85 Outer/inner diameter (mm) 19./16.5 Length (mm) 85 Material Stainless steel grade 316L (ASTM) Material Stainless steel grade 316L (ASTM) UHMW polyethylene wick Condenser Pore radius (lm) 6 Outer diameter (mm) 4.85 Permeability (m 2 ) 13 Inner diameter (mm) 2.85 Porosity (%) 5 Length (mm) Diameter (OD/ID) mm 16.5/7. Material Stainless steel grade 316L (ASTM) Grooves height, width, angle 2. mm/2.5 mm/26 Number of axial grooves 15 Compensation chamber Vapor line Volume (cm 3 ) 2 Outer diameter (mm) 4.85 Screen mesh # 2 Stainless steel grade 4L (ASTM) Inner diameter (mm) 2.85 TCD-LHP OD/ID/length (mm) 45/43/25 Length (mm) 55 TCD-LHP2 OD/ID/length (mm) 19/17/95 Material Stainless steel grade 316L (ASTM) Material Stainless steel grade 316L (ASTM) Fig. 2. Schematics of the LHPs experimental test bed and instrumentation. (a) TCD-LHP and (b) TCD-LHP 2. circulating a coolant (mixture of 5% water and 5% ethylene-glycol by volume) at a rate of 9 L/min at 5 C. Each LHP condenser was in thermal contact to its own heat exchanger through an aluminum plate

R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 1163 ( mm mm 4 mm thick). High grade acetone with minimum purity of 99.98% was used as working fluid. Prior to charging the LHPs for the life tests, the acetone was twice distilled and out-gassed in order to remove the excess of water and any other impurities that could contribute to the non-condensable gases (NCGs) generation. All tests for both LHPs were performed without preconditioning procedures, for a controlled room temperature ranging from 18 to 2 C. The liquid inventory for testing both LHPs was 25 g, keeping the compensation chamber with a void fraction of 5% in the cold mode. The compensation chamber geometric characteristics represent important differences between each other, which can directly affect the startups of either LHP specially when considering the void fraction in the cold mode. Before either LHP is started, the liquid in the compensation chamber will occupy 5% of its internal volume. In the case of the geometric characteristics of TCD-LHP, the liquid present in the compensation chamber will contribute to flood the entire capillary evaporator. This is an ideal condition for the startup, as the liquid in the compensation chamber will apply a static pressure that will be benefic for the capillary evaporator (Fig. 3a). In the case of the geometric characteristics of TCD-LHP2, the volume occupied by the liquid in the compensation chamber will be the same in the capillary evaporator. Then, part of the capillary evaporator will present both vapor and liquid phases, which represent a critical condition for the startup (Fig. 3b). The results presented in this paper are related to the operation of both LHPs in horizontal orientation, which have been undergoing extensive life tests in laboratory conditions for the past 2 years. This kind of information Table 2 Heat loads applied on the evaporators Profile Power levels (W) Startup power (W) 1 2-2--2-4 2 2 4--6-5-2-8 4 3 2--2--5-2 2 4 6-5-8-2-4-6 5 2-5-1-2-1-5 2 is important to be considered, as the behavior of the LHPs along time must be carefully analyzed in order to investigate the NCGs generation and influence for this specific working fluid. Upon using an alternative working fluid such as acetone, the information related to life tests are considerably important to qualify this fluid and the LHPs for space applications. The LHPs were tested on a heat load profile basis, following the power management for this device during operation, according to the mission application requirements. A total of five heat load profiles applied to the evaporator were tested as presented by Table 2. Important attention was particularly given to profile 5, where reduced heat loads were administrated to the capillary evaporator. The level of 1 W is required for the LHP operating at the so-called sleeping mode, while the thermal control of components is not required and capillary evaporator startup procedures should be avoided [8]. During all tests, the LHPs capability in handling the heat load changes and their performance until reaching the steady-state condition were analyzed. Such analyses were important to be made due to the LHP requirement to promote the thermal management properly without presenting temperature overshooting or capillary evaporator dryout. 4. Experimental results and discussion Fig. 3. Liquid/vapor distribution in the capillary evaporator/compensation chamber. (a) TCD-LHP and (b) TCD-LHP 2. During the tests with the TCD-LHP, the startups were instantaneous at all times showing a robust design and operation. The transients were considerably long (up to 45 min) for such a small scale LHP and reduced active length of the capillary evaporator but the steady-states were achieved without temperature overshooting or oscillations. The TCD-LHP presented fast responses to the changes on the heat loads applied to the evaporator, as presented by the experimental results in Figs. 4 and 5. In the graphs, is the average temperature of thermocouples TC-2 to TC-4 and is the average temperature of thermocouples TC-5 to TC-8. The heat source temperature was below the limit of 85 C at the maximum applied heat load (8 W), which was within the levels predicted during the LHP design and simulation using the computer code [2] and further verified by a mathematical analysis

1164 R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 5 2 W 2 W W 2 W 4 W 2 W 5 W 1 W 2 W 1 W 5 W 4 2 TC1 TC9 TC Cold Shock 25 2 15 5 TC1 TC9 TC 12 24 36 (a) 1 22 3 (a) 8 4 W W 6 W 5 W 2 W 8 W 8 7 6 5 4 2 TC1 TC9 TC Cold Shock Cold Shock Cold Shock 7 6 5 4 2 Profile 1 Profile 2 Profile 3 Profile 4 Profile 5 12 24 36 (b) Fig. 4. TCD-LHP profile tests. (a) TCD-LHP on profile 1 and (b) TCD-LHP on profile 2. 2 4 5 6 7 8 9 Q (W) Fig. 5. TCD-LHP profile tests and overall results. (a) TCD-LHP on profile 5 and (b) TCD-LHP overall results: evaporator temperature. (b) [22]. Even using an alternative working fluid, the system presented continuous operation along time. Temperature oscillations were not observed during all tests even at lower heat loads and no tendency of evaporator potential failures were verified. The TCD-LHP has presented satisfactory experimental results during its life tests in laboratory conditions, without any indication of non-condensable gases influence during its operation. In Fig. 4b for a test performing profile 2, it is clear that when the heat load applied to the evaporator was changed from a low to a higher power, the LHP presented an indication of cold shock. This behavior was observed previously by Nikitkin and Bienert [23] for a system operating with ammonia, which is common in high power LHPs and it is defined as a rapid flow of sub-cooled liquid in the evaporator core, causing its temperature drop. However, such a behavior became evident in this particular LHP even when operating with an alternative working fluid. This can be explained by the fact that acetone presents a higher mass flow rate for the same heat load and operation temperatures when compared to ammonia. Thus, as the heat load is changed, a higher mass flow rate will cause a faster transfer of sub-cooled liquid towards the capillary evaporator and its temperature will drop, directly reflecting in the compensation chamber, which would characterize a potential cold shock. However, further investigations of potential cold shocks must be conducted to properly address this issue. An important test performed is related to the TCD- LHP operating in the sleeping mode (profile 5), as presented by Fig. 5a. This test is particularly important to evaluate the LHP capability in operating with reduced heat loads as this mode can be applied when the device is not promoting the thermal control but its continuous operation is necessary to avoid the startups transients. Even with such reduced heat loads, the capillary evaporator presents continuous operation and no tendency of failure. However, due to the reduced mass flow rate of the working fluid, the vapor was condensed before reaching the condenser due to the back heat conduction

R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 1165 in the vapor line. On the same way, greater temperatures at the compensation chamber inlet were observed as the sub-cooling was practically lost due to the heat leaks to the environment, which was not considered an issue to the LHP operation. Sudden changes on the heat loads were easily handled by the system, which presented acceptable performance during all tests. During the tests for power level of 1 W, the LHP presented stable operation and the capillary evaporator was continuously performing the pumping activity even at such a reduced power. The TCD-LHP presented a minimum evaporator temperature of 21.9 C at 2 W and a maximum of 75.5 C for a power level of 8 W, as presented on the overall test results by Fig. 5b, where the characteristic U-shape curve of LHP operation can be observed, as already discussed in Ref. [24]. The minimum and maximum compensation chamber temperatures for the TCD-LHP were 18.1 C (at 2 W) and 38.3 C (at 8 W), respectively. The entire test program was accomplished for this specific design as it shows to be in accordance to what it was expected for such a device during space missions. The NCG generation during the life tests with TCD- LHP has not shown an issue to be considered, as the operation temperatures were always varying within the thermocouples deviation range. Over the time the TCD-LHP has been tested, there has never been an indication of NCG influence on the deviceõs operation. Under the same conditions, the TCD-LHP2 was tested and the heat load profile results are presented by Figs. 6 and 7. The startups at all power levels were successful and the transients were a little faster than with the previous compensation chamber configuration. Some instabilities during the transient were verified when testing the LHP with this configuration, which did not represent an issue for its operation along time. The maximum evaporator temperature was reached for 8 W (85 C), which was according to the temperature limit imposed by the project parameters and predicted by the computer code. The minimum and maximum compensation chamber temperatures for the TCD-LHP2 were 22 C (at 2 W) and 56.5 C (at 8 W), respectively, which shows the reduced thermal conductivity between the evaporator and compensation chamber for this LHP. These results show that both LHP configuration can accomplish the requirements using the alternative working fluid, proving that acetone can be properly used in the substitution of other fluids. Just as observed before, the TCD-LHP2 presented an indication that the cold shock was taking place when the heat loads were changed from low to high levels, as verified in Fig. 6b for tests with profile 2. This behavior was also related to the sudden increase on the mass flow rate, delivering faster sub-cooled liquid to the evaporator core. However, this phenomena should be better investigated in order to avoid any operational concerns in 5 4 2 9 8 7 6 5 4 2 2 W 2 W W 2 W 4 W TC1 TC9 TC 5 15 2 25 (a) 4 W W 6 W 5 W 2 W 8 W TC1 TC9 TC Cold Shock Cold Shock 1 22 3 Fig. 6. TCD-LHP2 profile tests. (a) TCD-LHP2 on profile 1 and (b) TCD-LHP2 on profile 2. future applications, specially with LHPs operating below W. The tests with the TCD-LHP2 operating in the sleeping mode (profile 5), as presented by Fig. 7a, also show the reliable operation of this device in very reduced mass flow rates and heat loads. This shows the reliable operation of TCD-LHP2 when it must operate within the range that was designed for. The overall evaporator temperatures tests results are presented by Fig. 7b, where the U-shape curve for the TCD-LHP2 can be observed. Just as observed before, the NCGs have not indicated any influence on the TCD-LHP2 operation during the life tests period, as the evaporator temperatures variation were within the thermocouples deviation. It could be noticed that the evaporator temperatures were higher for the TCD-LHP2 than during the tests with TCD-LHP, as well as the compensation chamber temperatures. This factor directly influenced the superheating (difference between the evaporator and compensation chamber temperatures), which were higher for the (b)

1166 R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 35 2 W 5 W 1 W 2 W 1 W 5 W 1.4 25 2 15 TC1 TC9 TC Thermal Resistance ( C/W) 1.2 1..8.6.4 TCD-HLP TCD-LHP2 5.2 9 1 22 3 (a). 2 4 5 6 7 8 9 Q (W) Fig. 8. Thermal resistances for both LHPs tested. 8 7 6 5 4 2 2 4 5 6 7 8 9 Q (W) Profile 1 Profile 2 Profile 3 Profile 4 Profile 5 Fig. 7. TCD-LHP2 profile tests and overall results. (a) TCD-LHP2 on profile 5 and (b) TCD-LHP2 overall results: evaporator temperatures. TCD-LHP. Consequently, this behavior was reflected on the temperatures throughout the loop. This characteristics also influences the liquid/vapor interfaces in the capillary evaporator, compensation chamber and condenser, which are directly related to each LHP operation temperatures. The interaction between the evaporator and the compensation chamber must be proper evaluated as described by previous investigations, which will allow the design optimization and acceptable LHP operation. This behavior is mainly due reduced thermal resistances verified for the TCD-LHP2 when compared to TCD-LHP, as presented by Fig. 8, where the thermal resistances for each LHP were calculated as (with uncertainty of ±5%) TR ¼ T evap T = Q. _ ð1þ For the TCD-LHP, the thermal resistances varied from.17 C/W (for W) and 1.33 C/W (for 2 W). Higher values of thermal resistances were observed for the LHP at lower heat loads (1 5 W) due to the heat losses on the liquid line, as very reduced flow rates were verified. The (b) TCD-LHP2 presented thermal resistances varying from.9 C/W (for 2 W) to 1.3 C/W (for 1 W). These results represent the particularities of LHPs characteristics and their potential for application as thermal control devices. The design of the LHPs predicted these levels of thermal resistances, which do not represent a concern for this particular application. The different cross section areas existent in the evaporator and compensation chamber of the TCD-LHP directly influence the thermal resistances and thus contributing for a higher superheating. On the same way, due to the same cross section area of the capillary evaporator and compensation chamber of TCD-LHP2, the thermal resistances are lower than those found on the TCD-LHP and directly influence the heat source and compensation chamber temperatures due to a greater heat leak, as well as the liquid/vapor interfaces throughout the device. In fact, the influence of the heat leak in this particular configuration can be reduced by inserting an additional thermal resistance between the capillary evaporator and the compensation chamber. This extra thermal resistance can be a geometric transition similar to that used on the TCD-LHP, keeping the compensation chamber with the same diameter as the capillary evaporator. Such a geometric transition would work to reduce the heat leak which certainly would minimize its influence in the operation temperature and thus reducing the heat source temperature. However, this factor has not been a major issue for the current project development, as the heat source temperatures have been within the acceptable range for the LHPs operation. In fact, the TCD-LHP2 presents a more indicated geometric configuration for this specific application, as its integration becomes easier. However, proper evaluation of the geometry influence on the heat source and compensation chamber temperatures must be carefully considered in order to keep them within the required levels.

R.R. Riehl, T.C.P.A. Siqueira / Applied Thermal Engineering 26 (26) 1158 1168 1167 With the continuous development of the proposed LHPs it has been possible to improve the devices performance for future space applications. Continuous improvement on their geometry are necessary in order to refine the project and present the most optimized configuration for the set capillary evaporator/compensation chamber. Life tests in laboratory conditions with both LHPs are still undergoing for better analyses of the issues that must be considered for the proper operation in space conditions. Performing life tests with both LHPs using acetone as working fluid has shown the potential of using this substance for space applications in two-phase thermal control devices due to its characteristics related to low working pressure and hazard conditions. The results and informations gathered during the life tests are important to evaluate the NCGs generation and influence as well as future qualification of this working fluid. 5. Conclusions The conclusions that could be taken from the tests with both TCD-LHP and TCD-LHP2 are summarized as follows: both LHPs presented reliable startups and operation during all tests, with acceptable temperature control of the heat source at a maximum of 85 C at the highest power applied of 8 W as pointed by the project requirements; acetone has presented to be a good choice as working fluid during the life tests over the time, which has been important for the accomplishment of the project goals. The NCGs issue has not shown to be a concern when using this fluid as both LHPs have presented reliable operation over the time without any indication of potential influence in their operation. The variation on the operation temperatures during the life tests were always within the thermocouples deviation and thus show no indication of NCGs influence. The informations gathered from the experimental tests with both LHPs are important to qualify this working fluid for future space applications; the TCD-LHP presented thermal resistances within 3.7% and 54.7% higher than the TCD-LHP2, which are mainly due to the geometric differences of the compensation chambers and also due to the distribution of the liquid/vapor interfaces in the capillary evaporator, compensation chamber and condenser; the TCD-LHP presented higher superheating levels due to the higher thermal resistance between the evaporator and the compensation chamber; higher evaporator and compensation chamber temperatures were achieved for the TCD-LHP2, which were mainly due to the reduced thermal resistance of this configuration that is also affected by the heat leak; both LHPs have shown good capability in managing the heat load profiles without any tendency of temperature overshooting or evaporator failure. Qualification tests are scheduled to be performed in the near future, which will follow the requirements for two-phase thermal control devices. The differences found in the results related to the geometric characteristics of each LHP tests are important to be evaluated for future designs improvements and applications. Acknowledgement This work has been supported by Fundação de Amparo a Pesquisa no Estado de São Paulo (FAPESP/ Brazil), grants 3/8365-6 and 3/11477-. References [1] T.D. Swanson, G.C. Birur, NASA thermal control technologies for robotic spacecraft, in: Proceedings of the 12th International Heat Pipe Conference, Moscow Kostroma Moscow, Russia, 19 24 May, 22, pp. 26 34. [2] G.C. Birur, M.T. Pauken, K.S. Novak, Thermal control of mars rovers and landers using mini loop heat pipes in: Proceedings of the 12th International Heat Pipe Conference, Moscow Kostroma Moscow, Russia, 19 24 May, 22, pp. 189 194. [3] K. Goncharov, V. 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