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NASA NTRS2002

Miniature Heat Transport System for Spacecraft Thermal Control

Loop heat pipes (LHP) are efficient devices for heat transfer and use the basic principle of a closed evaporation-condensation cycle. The advantage of using a loop heat pipe over other conventional methods is that large quantities of heat can be transported through a small cross-sectional area over a considerable distance with no additional power input to the system. By using LHPs, it seems possible to meet the growing demand for high-power cooling devices. Although they are somewhat similar to conventional heat pipes, LHPs have a whole set of unique properties, such as low pressure drops and flexible lines between condenser and evaporator, that make them rather promising. LHPs are capable of providing a means of transporting heat over long distances with no input power other than the heat being transported because of the specially designed evaporator and the separation of liquid and vapor lines. For LHP design and fabrication, preliminary analysis on the basis of dimensionless criteria is necessary because of certain complicated phenomena that take place in the heat pipe. Modeling the performance of the LHP and miniaturizing its size are tasks and objectives of current research. In the course of h s work, the LHP and its components, including the evaporator (the most critical and complex part of the LHP), were modeled with the corresponding dimensionless groups also being investigated. Next, analysis of heat and mass transfer processes in the LHP, selection of the most weighted criteria from known dimensionless groups (thermal-fluid sciences), heat transfer rate limits, (heat pipe theory), and experimental ratios which are unique to a given heat pipe class are discussed. In the third part of the report, two-phase flow heat and mass transfer performances inside the LHP condenser are analyzed and calculated for Earth-normal gravity and microgravity conditions. On the basis of recent models and experimental databanks, an analysis for condensing two-phase flow regimes, pressure gradients, and local heat transfer coefficients using ammonia, propylene, and R134, are carried out.

Ochterbeck, Jay M.↗

NASA NTRS2012

Phase Change Material for Temperature Control of Loop Heat Pipe Compensation Chamber

The compensation chamber (CC) of a loop heat pipe (LHP) requires a stable temperature. In the current technology, the CC is commonly cold biased by exposing it to deep space, and active heater control is used to maintain its temperature stable. Heater power required for a typical LHP with a 300 W heat transport capacity is as much as 25 W. If multiple LHPs are used, heater power requirement could be a bigger issue. Also heater controller or temperature sensor failure could lead to a LHP or mission failure. This paper presents an innovative concept of using phase change material (PCM) for temperature control of LHP CC. Some of the electronics and/or detector assembly waste heat is stored in the PCM and transferred to the LHP CC by constant conductance heat pipes (CCHPs). Phase change of the PCM occurs at a constant temperature and maintains the LHP CC temperature stable. It eliminates heater power and active heater control for the CC. It also increases the reliability of LHP.

Choi, Michael K.↗

NASA NTRS2001

Evaluation of GLAS Demonstration Model Loop Heat Pipe Thermal Vacuum Performance with Various Fluid Charges

Two loop heat pipes (LHPs) are to be used for tight thermal control of the Geoscience Laser Altimeter System (GLAS) instrument, planned for flight in late 2001. The LHPs are charged with Propylene as a working fluid. One LHP will be used to transport 110 W from a laser a radiator, the other will transport 190 W from electronic boxes to a separate radiator. The application includes a large amount of thermal mass in each LHP system and low initial startup powers. This along with some non-ideal flight design compromises, such as a less than ideal charge level for this design concept with a symmetrical secondary wick, lead to inadequate performance of the flight LHPs during the flight thermal vacuum test in October of 2000. This presentation focuses on identifying; the sources of the flight test difficulties by modifying the charge and test setup of the successfully tested Development Model Loop Heat Pipe (DM LHP). While very similar to the flight design, the DM L14P did have several significant difference in design and method of testing. These differences were evaluated for affect on performance by conforming the DM LHP to look more like the flight units. The major difference that was evaluated was the relative fill level of the working fluid within the concentrically design LHP compensation chamber. Other differences were also assessed through performance testing including starter heater size and "hot biasing" of major interior components. Performance was assessed with respect to startup, low power operation, conductance, and control heater power. The results of the testing showed that performance improves as initial charge increases, and when the starter heater is made smaller. The "hot biasing" of the major components did not appear to have a detrimental effect. As a result of test results of the DM LHP, modifications are being made to the flight units to increase the fluid charge and increase the watt-density of the starter heater.

Baker, Charles↗

NASA NTRS2004

Thermal Performance of a Multi-Evaporator Loop Heat Pipe with Thermal Masses and Thermoelectric Coolers

This paper describes thermal performance of a loop heat pipe (LHP) with two evaporators and two condensers in ambient testing. Each evaporator has an outer diameter of 15mm and a length of 76mm, and has an integral compensation chamber (CC). An aluminum mass of 500 grams is attached to each evaporator to simulate the instrument mass. A thermoelectric cooler (TEC) is installed on each CC to provide heating as well as cooling for CC temperature control. A flow regulator is installed in the condenser section to prevent vapor from going back to the evaporators in the event that one of the condensers is fully utilized. Ammonia was used as the working fluid. Tests conducted included start-up, power cycle, heat load sharing, sink temperature cycle, operating temperature control with TECs, and capillary limit tests. Experimental data showed that the loop could start with a heat load of less than 10W even with added thermal masses. The loop operated stably with even and uneven evaporator heat loads, and even and uneven condenser sink temperatures. The operating temperature could be controlled within +/- 0.5K of the set point temperature using either or both TECs, and the required TEC control heater power was less than 2W under most test conditions. Heat load sharing between the two evaporators was also successfully demonstrated. The loop had a heat transport capability of 120W to 140W, and could recover from a dry-out when the heat load was reduced. The 500-gram aluminum mass on each evaporator had a negligible effect on the loop operation. Existing LHPs servicing orbiting spacecraft have a single evaporator with an outer diameter of about 25mm. Important performance characteristics demonstrated by this LHP included: 1) Operation of an LHP with 15mm diameter evaporators; 2) Robustness and reliability of an LHP with multiple evaporators and multiple condensers under various test conditions; 3) Heat load sharing among LHP evaporators; 4) Effectiveness of TECs in controlling the LHP operating temperature; and 5 ) Effectiveness of the flow regulator in preventing vapor from going back the evaporators.

Ku, Jen-Tung↗

NASA NTRS2004

Mathematical Modeling of Loop Heat Pipes with Multiple Capillary Pumps and Multiple Condensers: Stead State Stimulations - Part 1

Loop Heat Pipes (LHPs) have proven themselves as reliable and robust heat transport devices for spacecraft thermal control systems. So far, the LHPs in earth-orbit satellites perform very well as expected. Conventional LHPs usually consist of a single capillary pump for heat acquisition and a single condenser for heat rejection. Multiple pump/multiple condenser LHPs have shown to function very well in ground testing. Nevertheless, the test results of a dual pump/condenser LHP also revealed that the dual LHP behaved in a complicated manner due to the interaction between the pumps and condensers. Thus it is redundant to say that more research is needed before they are ready for 0-g deployment. One research area that perhaps compels immediate attention is the analytical modeling of LHPs, particularly the transient phenomena. Modeling a single pump/single condenser LHP is difficult enough. Only a handful of computer codes are available for both steady state and transient simulations of conventional LHPs. No previous effort was made to develop an analytical model (or even a complete theory) to predict the operational behavior of the multiple pump/multiple condenser LHP systems. The current research project offered a basic theory of the multiple pump/multiple condenser LHP operation. From it, a computer code was developed to predict the LHP saturation temperature in accordance with the system operating and environmental conditions.

Hoang, Triem T.↗

NASA NTRS2004

Thermal performance of a multi-evaporator loop heat pipe with thermal masses and thermal electrical coolers

This paper describes thermal performance of a loop heat pipe (LHP) with two evaporators and two condensers in ambient testing. Each evaporator has an outer diameter of 15mm and a length of 76mm, and has an integral compensation chamber (CC). An aluminum mass of 500 grams is attached to each evaporator to simulate the instrument mass. A thermal electric cooler (TEC) is installed on each CC to provide heating as well as cooling for CC temperature control. A flow regulator is installed in the condenser section to prevent vapor from going back to the evaporators in the event that one of condenser is fully utilized. Ammonia was used ad the working fluid. Tests conducted included start-up, power cycle, heat load sharing, sink temperature cycle, operating temperature control with TECs, and capillary limit tests. Experimental data showed that the loop could start with a heat load of less than 1OW even with added thermal masses. The loop operated stably with even and uneven evaporator heat loads, and even and uneven condenser sink temperatures. The operating temperature could be controlled within +/-0.5K of the set point temperature using either or both TECs, and the required TEC control heater power was less than 2W under most test conditions. Heat load sharing between the two evaporators was also successfully demonstrated. The loop had a heat transport capability of 120W to 140W, and could recover from a dry-out when the heat load was reduced. The 500-gram aluminum mass on each evaporator had a negligible effect on the loop operation. Existing LHPs servicing the orbiting spacecraft have a single evaporator with an outer diameter of about 25mm. Important performance characteristics demonstrated by this LHP included: 1) Operation of an LHP with 15mm diameter evaporators; 2) Robustness and reliability of an LHP with multiple evaporators and multiple condensers under various test conditions; 3) Heat load sharing among LHP evaporators; 4) Effectiveness of TECs in controlling the LHP operating temperature; and 5) Effectiveness of the flow regulator in preventing vapor from going back the evaporators.

loop heat pipes↗

NASA NTRS2010

Thermoelectric Converter for Loop Heat Pipe Temperature Control: Experience and Lessons Learned

This paper describes the theoretical background and implementation methodology of using a thermoelectric converter (TEC) for operating temperature control of a loop heat pipe (LHP). In particular, experimental results from ambient and thermal vacuum tests of an LHP are presented for illustrations. The most commonly used state-of-the-art method to control the LHP operating temperature is to cold bias its compensation chamber (CC) and use an electrical heater to maintain the CC at the desired set point temperature. Although effective, this approach has its shortcomings in that the electrical heater can only provide heating to the CC, and the required power can be large under certain conditions. An alternative method is to use a TEC, which is capable of providing both heating and cooling to the CC. In this method, one side of the TEC is attached to the CC, and the other side is connected to the evaporator via a thermal strap. Using a bipolar power supply and a control algorithm, a TEC can function as a heater or a cooler, depending on the direction of the current flow. Extensive ground tests of several LHPs have demonstrated that a TEC can provide very tight temperature control for the CC. It also offers several additional advantages: (1) The LHP can operate at temperatures below its natural operating temperature at low heat loads; (2) The required heater power for a TEC is much less than that for an electrical heater; and (3) It enhances the LHP start-up success. Although the concept of using a TEC for LHP temperature control is simple, there are many factors to be considered in its implementation for space applications because the TEC is susceptible to the shear stress and yet has to sustain the dynamic load under the spacecraft launch environment. The added features that help the TEC to withstand the dynamic load will inevitably affect the TEC thermal performance. Some experiences and lessons learned are addressed in this paper.

Ku, Jentung↗

NASA NTRS2010

Investigation of Loop Heat Pipe Survival and Restart After Extreme Cold Environment Exposure

NASA plans human exploration near the South Pole of the Moon, and other locations where the environment is extremely cold. This paper reports on the heat transfer performance of a loop heat pipe (LHP) exposed to extreme cold under the simulated reduced gravitational environment of the Moon. A common method of spacecraft thermal control is to use a LHP with ammonia working fluid. Typically, a small amount of heat is provided either by electrical heaters or by environmental design, such that the LHP condenser temperature never drops below the freezing point of ammonia. The concern is that a liquid-filled, frozen condenser would not restart, or that a thawing condenser would damage the tubing due to the expansion of ammonia upon thawing. This paper reports the results of an experimental investigation of a novel approach to avoid these problems. The LHP compensation chamber (CC) is conditioned such that all the ammonia liquid is removed from the condenser and the LHP is nonoperating. The condenser temperature is then reduced to below that of the ammonia freezing point. The LHP is then successfully restarted.

Golliher, Eric↗

NASA NTRS2013

Thermal Performance of ATLAS Laser Thermal Control System Demonstration Unit

The second Ice, Cloud, and Land Elevation Satellite mission currently planned by National Aeronautics and Space Administration will measure global ice topography and canopy height using the Advanced Topographic Laser Altimeter System {ATLAS). The ATLAS comprises two lasers; but only one will be used at a time. Each laser will generate between 125 watts and 250 watts of heat, and each laser has its own optimal operating temperature that must be maintained within plus or minus 1 degree Centigrade accuracy by the Laser Thermal Control System (LTCS) consisting of a constant conductance heat pipe (CCHP), a loop heat pipe (LHP) and a radiator. The heat generated by the laser is acquired by the CCHP and transferred to the LHP, which delivers the heat to the radiator for ultimate rejection. The radiator can be exposed to temperatures between minus 71 degrees Centigrade and minus 93 degrees Centigrade. The two lasers can have different operating temperatures varying between plus 15 degrees Centigrade and plus 30 degrees Centigrade, and their operating temperatures are not known while the LTCS is being designed and built. Major challenges of the LTCS include: 1) A single thermal control system must maintain the ATLAS at 15 degrees Centigrade with 250 watts heat load and minus 71 degrees Centigrade radiator sink temperature, and maintain the ATLAS at plus 30 degrees Centigrade with 125 watts heat load and minus 93 degrees Centigrade radiator sink temperature. Furthermore, the LTCS must be qualification tested to maintain the ATLAS between plus 10 degrees Centigrade and plus 35 degrees Centigrade. 2) The LTCS must be shut down to ensure that the ATLAS can be maintained above its lowest desirable temperature of minus 2 degrees Centigrade during the survival mode. No software control algorithm for LTCS can be activated during survival and only thermostats can be used. 3) The radiator must be kept above minus 65 degrees Centigrade to prevent ammonia from freezing using no more than 135 watts of heater power. 4) The LHP reservoir control heater power is limited to 15 watts with a 70 percent duty cycle. 5) The voltage of the power supply can vary between 26 volts direct current and 34 volts direct current during the spacecraft lifetime. A design analysis shows that a single LTCS can satisfy these requirements. However, shutdown of· the LHP is particularly challenging and the shutdown heater must be wired in series with two reservoir thermostats and two CCHP thermostats at different set points. An LTCS demonstration unit has been tested to verify these performance characteristics experimentally prior to proceeding to the final LTCS design and fabrication. Test results showed that the LHP shutdown scheme would be able to shut down the LHP as designed and the reservoir control heater can maintain the ATLAS mass simulator within the plus or minus 1 degrees Centigrade accuracy under various combinations of the heat load, sink temperature, and power supply voltage.

Ku, Jentung↗

NASA NTRSDate not supplied

Simulated Lunar Gravity Testing of VIPER Loop Heat Pipe

NASA’s Volatiles Investigating Polar Exploration Rover (VIPER) mission consists of a rover designed to explore the lunar south pole. One of the main challenges faced by the rover during the lunar polar exploration is the adverse thermal environment. Temperatures can fluctuate more than a 100°C between day and night; potentially dropping to −246°C in permanent shadow regions. To maintain the rover components within temperature limits, VIPER’s Thermal Management System (TMS) relies heavily on Loop Heat Pipes (LHPs). To assist the design of the thermal management system, an Engineering Design Unit (EDU) LHP has been tested in several opportunities under thermal vacuum (TVAC) environment. Of particular interest was the LHP performance in lunar gravity. To that end, the EDU LHP was tested at the nominal orientation within the rover assembly, vertical, and inclined such that the gravitational component acting on the LHP evaporator and condenser was 1/6g (lunar gravity is 1/6 of Earth gravity) when compared to the vertical configuration. This paper will examine the LHP conductance as one the key parameters to assess the effect of the gravity on the LHP performance.

Loop Heat Pipe↗

NASA NTRS1998

Mathematical Modeling of Loop Heat Pipes

The primary focus of this study is to model steady-state performance of a Loop Heat Pipe (LHP). The mathematical model is based on the steady-state energy balance equations at each component of the LHP. The heat exchange between each LHP component and the surrounding is taken into account. Both convection and radiation environments are modeled. The loop operating temperature is calculated as a function of the applied power at a given loop condition. Experimental validation of the model is attempted by using two different LHP designs. The mathematical model is tested at different sink temperatures and at different elevations of the loop. Tbc comparison of the calculations and experimental results showed very good agreement (within 3%). This method proved to be a useful tool in studying steady-state LHP performance characteristics.

Kaya, Tarik↗

NASA NTRS2000

Testing of a Loop Heat Pipe Subjected to Variable Accelerating Forces

This paper presents viewgraphs of the functionality of a loop heat pipe that was subjected to variable accelerating forces. The topics include: 1) Summary of LHP (Loop Heat Pipe) Design Parameters; 2) Picture of the LHP; 3) Schematic of Test Setup; 4) Test Configurations; 5) Test Profiles; 6) Overview of Test Results; 7) Start-up; 8) Typical Start-up without Temperature Overshoot; 9) Start-up with a Large Temperature Overshoot; 10) LHP Operation Under Stationary Condition; 11) LHP Operation Under Continuous Acceleration; 12) LHP Operation Under Periodic Acceleration; 13) Effects of Acceleration on Temperature Oscillation and Hysteresis; 14) Temperature Oscillation/Hysteresis vs Spin Rate; and 15) Summary.

Ku, Jentung↗

NASA NTRS2005

Using Thermoelectric Coolers to Enhance Loop Heat Pipe Performance

Contents include the following: Loop Heat Pipe (LHP) operating temperature. LHP start-up issues. How Thermoelectric Cooler (TECs) can enhance LHP performance: start-up; operating temperature control. Experimental studies: LHP with one evaporator and one condenser; LHP with two evaporators and two condensers. Conclusion.

Ku, Jentung↗

NASA NTRS2011

GOES Type III Loop Heat Pipe Life Test Results

The GOES Type III Loop Heat Pipe (LHP) was built as a life test unit for the loop heat pipes on the GOES N-Q series satellites. This propylene LHP was built by Dynatherm Corporation in 2000 and tested continuously for approximately 14 months. It was then put into storage for 3 years. Following the storage period, the LHP was tested at Swales Aerospace to verify that the loop performance hadn t changed. Most test results were consistent with earlier results. At the conclusion of testing at Swales, the LHP was transferred to NASA/GSFC for continued periodic testing. The LHP has been set up for testing in the Thermal Lab at GSFC since 2006. A group of tests consisting of start-ups, power cycles, and a heat transport limit test have been performed every six to nine months since March 2006. Tests results have shown no change in the loop performance over the five years of testing. This presentation will discuss the test hardware, test set-up, and tests performed. Test results to be presented include sample plots from individual tests, along with conductance measurements for all tests performed.

Ottenstein, Laura↗

NASA NTRS2011

Loop Heat Pipe Operation Using Heat Source Temperature for Set Point Control

Loop heat pipes (LHPs) have been used for thermal control of several NASA and commercial orbiting spacecraft. The LHP operating temperature is governed by the saturation temperature of its compensation chamber (CC). Most LHPs use the CC temperature for feedback control of its operating temperature. There exists a thermal resistance between the heat source to be cooled by the LHP and the LHP's CC. Even if the CC set point temperature is controlled precisely, the heat source temperature will still vary with its heat output. For most applications, controlling the heat source temperature is of most interest. A logical question to ask is: "Can the heat source temperature be used for feedback control of the LHP operation?" A test program has been implemented to answer the above question. Objective is to investigate the LHP performance using the CC temperature and the heat source temperature for feedback control

Ku, Jentung↗

NASA NTRS2011

DeCoM Validation

Deepak Condenser Model (DeCoM) was validated in a two-step process. The first step validated DeCoM/TTH/FloCAD against a simplified 1D (1D fluid flow and 1D heat flow) scenario. Through this simplification the fluid calculations were simple enough that the results were comparable to hand calculations. Once a method was compared against 1D case, the next step was the verification against a 2D test case. 2D is only for heat flow, while the fluid flow is still 1D. The test data was obtained from the project Geoscience Laser Altimeter System (GLAS) which performed a Looped Heat Pipe (LHP) test and contained necessary condenser information; such as a test point and condenser configuration. From the GLAS LHP test report, the construction of the LHP condenser and correlation was roughly possible. The assumptions and limitations are discussed, within this paper, of the 2D correlation. The success of DeCoM is illustrated by the results after correlation to the test data. Results show the differences between the model and the test setup, as well as explaining the probable cause for the differences; another set of results were produced after taking into account these differences, such as parasitic heat leak. Substantial effort has been made in listing the possibilities to further develop DECOM, thus to make it more reliable, user-friendly, and better integrated with generic SINDA models. The comparison of DeCoM to FloCAD and TTH LHP should not be taken as a definite answer to choosing DeCoM as a condenser modeler, rather as an alternate to obtaining preliminary results without the complexity of TTH LHP and license of FloCAD.

Patel, Deepak↗

NASA NTRSDate not supplied

Hybrid Thermal Control System for Extreme Thermal Environments

NASA’s return to the moon brings about many challenges, including issues with survival on the Lunar surface. Marshall Space Flight Center (MSFC) has been developing a hybrid thermal control system that can be utilized for various surface assets that must survive in extreme lunar environments. This scalable system is targeted for human-rated systems and utilizes a combination of a- pumped fluid loop (PFL) and a loop heat pipe (LHP) with thermal control valve (TCV). Pumped fluid loops have a long history of use in human rated systems. They can collect large amounts of waste heat and transport it over long distances in rovers, habitats, and other systems. By utilizing a non-toxic working fluid in the habitable volume, the PFL is easily serviceable by the crew and does not pose a risk during any unexpected leaks or failures. The addition of a LHP for the exterior heat transport and rejection adds several benefits to the system for extreme environment survival. The quantity of LHP scan be tailored to optimize heat rejection for different systems and allows for larger adiative surfaces. By utilizing a TCV in combination with the LHP, there can be high heat transfer during the daytime with minimal heat transfer during the night. The TCV passively controls the amount of heat transfer through the LHP based on the environmental temperature. A prototype system has been tested in bench top and thermal vacuum conditions. Results are presented showing the feasibility of the system concept.

Thermal Control↗

NASA NTRS2023

Hybrid Thermal Control System for Extreme Thermal Environments

Thermal Control↗