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Loop Heat Pipe (LHP) Technology has become an accepted technology for thermal management of spacecraft. Despite the acceptance of the technology, there remains a lack of understanding about how to analyze these devices, creating the myth that inadequate tools exist for modeling LHPs and CPLs (capillary pumped loops). LHPs are two-phase devices with a very tight thermal coupling to their environment. Consequently these systems must be modeled in a manner capable of capturing both thermal and fluid behavior. Over the past decade various programs have been written to model LHPs and CPLs, however the majority of them have been either design-specific or focused on capturing a specific behavior within the system. SINDA/FLUINT is a thermal/fluid modeling code fully capable of capturing both heat transfer and fluid dynamics of an LHP. In addition it provides the flexibility to model any portion of the system in as much or as little detail as needed.

The NASA-standard thermohydraulic analyzer, SINDA/FLUINT (Ref 1), has been used to model various aspects of loop heat pipe* (LHP) operation for more than 12 years. Indeed, this code has many features that were specifically designed for just such specialized tasks, and is unique in this respect. Furthermore, SINDA is commonly used at the vehicle (integration) level; has a large user base both inside and outside the aerospace industry; has several graphical user interfaces, preprocessors, postprocessors; has strong links to CAD and structural tools; and has built-in optimization, data correlation, parametric analysis, reliability estimation, and robust design tools. Nonetheless, the LHP community tends to ignore these capabilities, yearning instead for “simpler” methods.

This paper describes the flight test results of the fifth generation cryogenic capillary pumped loop (CCPL-5) which flew on the Space Shuttle STS-95 in October of 1998 as part of the CRYOTSU Flight Experiment. This flight was the first in-space demonstration of the CCPL, a lightweight heat transport and thermal switching device for future integrated cryogenic bus systems. The CCPL-5 utilized nitrogen as the working fluid and operated between 75K and 110K. Flight results indicated excellent performance of the CCPL-5 in a micro-gravity environment. The CCPL could start from a supercritical condition in all tests, and the reservoir set point temperature controlled the loop operating temperature regardless of changes in the heat load and/or the sink temperature. In addition, the loop demonstrated successful operation with heat loads ranging from 0.5W to 3W, as well as with parasitic heat loads alone.

In recent years, loop heat pipe (LHP) technology has transitioned from a developmental technology to one that is flight ready. The LHP is considered to be more robust than capillary pumped loops (CPL) because the LHP does not require any preconditioning of the system prior to application of the heat load, nor does its performance become unstable in the presence of two-phase fluid in the core of the evaporator. However, both devices have a lower limit on input power: below a certain power, the system may not start properly. The LHP becomes especially susceptible to these low power start-ups following diode operation, intentional shut-down, or very cold conditions. These limits are affected by the presence of adverse tilt, mass on the evaporator, and noncondensible gas in the working fluid.

Despite the wide use of the capillary pumped loop (CPL) and loop heat pipe (LHP) technologies, fundamental confusion persists about CPL and LHP operation, their uses and limitations, and even the similarities and distinctions between the western-heritage CPLs and Russian-heritage LHPs. It is therefore the purpose of this paper, written by engineers who have participated in developments on both sides of the Atlantic Ocean, to explain these issues to potential users of these emerging thermal control technologies.