Jane Baumann

Steady State and Transient Loop Heat Pipe Modeling

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. However, simple methods cannot meet the challenging needs of LHP modeling such as transient start-up and noncondensible gas (NCG) effects, are often hardware-specific or proprietary, or cannot be used in a vehicle-level analysis. There are many reasons for this hesitancy to use SINDA/ FLUINT as it was intended. First, hardware developers tend to be less versed in analytic methods than the user community they serve. Second, there are political hurdles, such as the fact that ESA contractors are required to use ESA sponsored software. Third, the state-of-the-art in LHPs is not so advanced that the analysts can be ignorant of the complex two-phase thermohydraulic and thermodynamic processes and phenomena involved, and unfortunately most thermal analysts are accustomed only to “dry” thermal control (radiation, conduction, etc.). Fourth, the general-purpose and complete nature of SINDA/FLUINT tends to make it intimidating, especially in light of the third reason listed above. SINDA/FLUINT is not designed strictly for LHPs or even for LHP-like systems; it has been used for everything from nuclear reactor cooling to dynamic models of human hearts and tracheae. The user’s manuals and standard training classes † rarely mention capillary phenomena because only a fraction of SINDA/FLUINT’s users are thus inclined. It is to address this fourth reason that this paper has been written, since the authors can do little to redress the first three problems. This paper summarizes the available modeling capabilities applicable to various LHP design and simulation tasks. Knowledge of LHPs is assumed.

A METHODOLOGY FOR ENVELOPING RELIABLE START-UP OF LHPS

The loop heat pipe (LHP) is known to have a lower limit on input power. Below this limit the system may not start properly creating the potential for critical payload components to overheat. The LHP becomes especially susceptible to these low power start-up failures following diode operation, intentional shut-down of the device, 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. Based on analytical modeling correlated to startup test data, this paper will describe the key parameters driving this low power limit and provide an overview of the methodology for predicting a “safe start” design envelope for a given system and loop design. The amount of incipient superheat was found to be key to the enveloping procedure. Superheat levels have been observed to vary significantly based on evaporator design and even from unit to unit of identical designs. Statistical studies of superheat levels and active measures for limiting superheat should be addressed by both the hardware vendors and the system integrators.

Noncondensible Gas, Mass, and Adverse Tilt Effects on the Start-up of Loop Heat Pipes

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. Based on analytical modeling correlated to start-up test data, this paper will describe how the minimum power required to start the loop is increased due to the presence of mass, noncondensible gas, and adverse tilt. The end-product is a methodology for predicting a “safe start” design envelope for a given system and loop design.

The capillary pumped loop III (CAPL III) flight demonstration description and status

To realize the full benefits of capillary pump loop (CPL) devices, for use in spacecraft thermal control subsystems, a reliable, load sharing, multiple evaporator system must be developed and successfully demonstrated in space. The Capillary Pumped Loop Flight Experiment 3 (CAPL III) will be the second attempt to flight demonstrate a multiple evaporator CPL in space environment. Using the lessons learned from CAPL I, which was flown aboard STS-60 in February 1994, new hardware and concepts are being developed for CAPL III to enable load sharing between evaporators, reliable system start-up/re-start, and reliable continuous operation. Started in May 1996, CAPL III is primarily a joint venture between the Naval Research Laboratory and the NASA-Goddard Space Flight Center, with Swales and Associates, Inc. as an industry partner. The program is scheduled to meet an STS flight opportunity in mid-1998. This paper will present the requirements and the preliminary design description of the CAPL III CPL system.

Experimental investigation of a neon cryogenic capillary pumped loop

This paper describes the first known demonstration of a cryogenic capillary pumped loop (CCPL) using neon as the working fluid. One obvious application for a neon CCPL is in thermally coupling redundant 35 K cryocoolers to a long-wave TR sensor operating at 40 K without the need for additional flexible links and with negligible (off-cooler) parasitic penalties. The test hardware utilized in this investigation is the 3rd-generation CCPL which was developed by Cullimore and Ring Technologies and Swales Aerospace under NASA/GSFC SBIR funding. Since neon melts at 24.5 K, boils at 27 K, and is a gas above 44.5 K, the practical operating range for a neon CCPL is 30-40 K. The specific demonstrations described herein include several start-ups from a supercritical state without the aid of a cooled shroud, power cycling from 0.1-3.5 W, long-term operation, and start-up/operation under 1.0 cm of adverse evaporator elevation. This paper describes the test set-up, analysis, and results associated with this first neon CCPL.

Development testing of the cryogenic capillary pumped loop

This paper describes the continuing development of a promising new technology, a capillary pumped loop (CPL) which operates at cryogenic temperatures. While cryogenic capillary pumped loops have application to passive spacecraft radiators and to long term storage of cryogenic propellants and open-cycle coolants, their application to the integration of spacecraft cryocoolers has generated the most excitement. Without moving parts or complex controls, they are able to thermally connect redundant cryocoolers to a single remote load, eliminating thermal switches and providing mechanical isolation at the same time. The development of the cryogenic CPL presented some unique challenges such as start-up from a super-critical state and management of parasitic heat leaks. This paper describes this new technology and presents the results of continuing development testing.

CAD-based methods for thermal modeling of coolant loops and heat pipes

As air cooling of electronics reaches the limits of its applicability, the next generation of cooling technology is likely to involve heat pipes and single- or two-phase coolant loops. These technologies are not suitable for analysis using 2D/3D computational fluid dynamics (CFD) software, and yet the geometric complexities of the thermal/structural models make network-style schematic modeling methods cumbersome. This paper describes CAD line-drawing methods to quickly generate 1D fluid models of heat pipes and coolant loops within a 3D thermal model. These arcs and lines can be attached intimately or via lineal contact or saddle resistances to plates and other surfaces, whether those surfaces are modeled using thermal finite difference methods (FDM) or finite element methods (FEM) or combinations of both. The fluid lines can also be manifolded and customized as needed to represent complex heat exchangers and plumbing arrangements. To demonstrate these concepts, two distinct examples are developed: a copper-water heat pipe, and an aluminum-ammonia loop heat pipe (LHP) with a serpentined condenser. A summary of the numerical requirements for system-level modeling of these devices is also provided.

Flight Testing of a Cryogenic Capillary Pumped Loop

Future space-based cryogenic systems will require enhanced integration flexibility, lower weight reduced parasitic penalties, better vibration isolation, and a variety of other improvements to meet performance goals. Additionally, there is an increasing need to locate cooling sources remotely from cooled components. In the past flexible conductive links were used and worked well in most cases. However, as the transport lengths increase, conductive couplings become heavier and less effective, and must be replaced by higher performance systems. One available option, which can meet many of these future requirements, is the cryogenic capillary pumped loop (CCPL). The development of the CCPL technology started in 1992, following the success of the room temperature CPLS. The extrapolation of CCPL technology to cryogenic temperatures offers many performance benefits, which are not currently within the reach of traditional heat pipes or conductive links. Specific advantages of the CCPL technology pertaining to cryocooler integration include: (1) greater capillary pumping pressure for improved ground testability; (2) improved mechanical isolation; (3) faster diode shutdown and lower reverse heat leaks; (4) tighter control of detector temperature; (5) variable or fixed conductance operation; and (6) ease of integration due to their flexibility. The applications of CCPL technology are numerous. Military and commercial applications include surveillance satellites, earth observing satellites, deep space observation systems, medical devices, and many other cryogenic systems. Over the past few years, several breadboard and prototype CCPLs have been built and ground tested. A prototype CCPL has demonstrated successful operation between 80K and 110K with heat loads between O.5W and 12W using nitrogen as the working fluid, and 35K and 40K with head loads of 0.25W to 3.5W using neon. In order to verify CCPL performance in a microgravity environment, a flight unit, CCPL-5, was tested onboard 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. The CCPL-5 utilized nitrogen as the working fluid and operated between 75K and 110K. Flight results indicated excellent performance of the CCPL-5 under zero-G environment The CCPL could start from a supercritical condition in all tests, and the loop operating temperature could be tightly controlled regardless of changes in the heat load and/or the sink temperature. In addition, the loop demonstrated successful operation with a heat load of 0.5W as well as with parasitic heat loads alone. There were no noticeable differences between zero-G and one-G operation.

An Excel User Interface for Modeling LHPs in SINDA/FLUINT

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. Unfortunately, the perception that FLUINT is too difficult to learn has precluded many engineers and technologists from learning how to model LHPs. A recently developed Application Programming Interface (API) provides a means of launching and controlling SINDA/FLUINT from a separate stand-alone software program. To support the analysis needs of engineers who are interested in evaluating LHPs and performing system level studies, but who do not have the resources required become fluent with SINDA/FLUINT, a Microsoft Excel-based front end has been developed using this API. The resulting LHP Launch Control Panel is a simple Excel-based user interface requiring no knowledge of SINDA/FLUINT nor any need to purchase licenses if no changes are required to the underlying model. This paper will provide an overview of the LHP Launch Control Panel, discuss typical analysis results, and provide some guidelines on bracketing the LHP performance for stochastic phenomena such as evaporator core state. In addition it will provide information for the advanced user on how to further customize the tool for their needs. The Excel-based LHP model is available in a licensed or unlicensed SINDA/FLUINT mode.