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To solve the thermal control problems of modern spacecraft with complex payloads and configurations, there is an increasing demand for multiple evaporator and/or multiple condenser loop heat pipes (LHPs). As a result, several initiatives, including flight demonstration, are being proposed and research is under way. It is well known that mathematical modeling of a conventional LHP is highly challenging due to the complex two-phase phenomena involved. In the case of multiple evaporator/condenser LHPs, there are additional difficulties since it is necessary to take into account the dynamic interactions between the evaporators and condensers. A mathematical model for multiple evaporator/condenser LHP configurations is developed within EcosimPro. In this paper, the overall formulation and main assumptions of the mathematical model are explained. The simulation results obtained for both steady-state and transient regimes are presented.

Two-Phase Loops with Capillary Pump (Loop Heat Pipes (LHP) and Capillary Pumped Loops (CPL)) are currently among advanced thermal control technologies for aerospace applications. Large numbers of experimental and operational two-phase loops were successfully tested and used in several spacecraft in the past two decades. Novel technologies such as Advanced CPL-LHP, High Performance CPL, miniature LHPs, inversion (reversible, “Push-Pull") LHPs, ramified, multiple evaporator and condenser LHPs and CPLs, for complex thermal control systems are being proposed. This paper presents a state-of-the-art survey and analysis of these technologies. A classification of Two-Phase Loop with Capillary Pump designs is recommended. Basic principles, operational conditions and characteristics, temperature control and start-up initiation are discussed. The use of thermal control systems based on Two-Phase Loops with Capillary Pump for space applications is reviewed and summarized.

Loop Heat Pipes (LHPs) are two phase heat transport devices where the fluid circulation is achieved by capillary forces. Because of their high heat transport capability, robustness, reliability and compactness, they are becoming standard thermal control devices in several applications in space, aeronautics and electronics industry. Several mathematical models have been developed to predict the behavior of these devices. However, due to the complexity of the two-phase phenomena involved in LHPs, current models cannot simulate several performance characteristics. This paper presents an LHP mathematical model developed using the software simulation tool EcosimPro. The results of the mathematical model have been compared with the hardware test data for code validation. Results in both, steady and transient conditions, are presented and discussed.

The main objective of this study is to develop a mathematical model for the simulation of the thermal characteristics of two-phase capillary pumped devices. The mathematical model presented in this paper is an extension of the earlier mathematical model developed for a conventional heat pipe. The three-dimensional incompressible energy, momentum and mass conservation equations are solved by using the finite element method. Except in the wick region, the viscous terms in the governing equations are neglected. However, the pressure drops due to frictional losses are introduced. The interface between vapor and liquid phases is assumed static and only converged steady-state solutions are retained. The reservoir dynamic is not modeled. The energy, momentum and mass jump conditions are written across the interface. The resulting set of equations is solved iteratively until the overall mass conservation is satisfied between the evaporator and condenser.

This study focuses on the mathematical modeling of the evaporator section of the two-phase heat transfer devices: heat pipes, loop heat pipes and capillary pumped loops. Although the heat pipe technology made its first public appearance in the early forties, some operational aspects of two-phase systems are still not well understood, and research in this area continues. The evaporation and condensation process, taking place in these systems is among the most complex phenomena encountered in engineering applications. In this study, full three-dimensional incompressible energy, momentum and mass conservation equations are solved by using the finite element method to predict thermal operational characteristics of the two-phase heat transfer devices. The main focus of the study is the modeling of the phase transition region in the evaporator section.

In this paper, experimental work performed on a breadboard Loop Heat Pipe (LHP) is presented. The test article was built by DCI for the Geoscience Laser Altimeter System (GLAS) instrument on the ICESat spacecraft. The thermal system requirements of GLAS have shown that ammonia cannot be used as the working fluid in this LHP because GLAS radiators could cool to well below the freezing point of ammonia. As a result, propylene was proposed as an alternative LHP working fluid since it has a lower freezing point than ammonia. Both working fluids were tested in the same LHP following a similar test plan in ambient conditions. The thermal performance characteristics of ammonia and propylene LHP's were then compared. In general, the propylene LHP required slightly less startup superheat and less control heater power than the ammonia LHP. The thermal conductance values for the propylene LHP were also lower than the ammonia LHP. Later, the propylene LHP was tested in a thermal vacuum chamber.

Loop Heat Pipes (LHPs) are being considered for cooling of military combat vehicles and spinning spacecraft. In these applications, it is important to understand the effect of an accelerating force on the performance of LHPs. In order to investigate such an effect, a miniature LHP was installed on a spin table and subjected to variable accelerating forces by spinning the table at different angular speeds. Several patterns of accelerating forces were applied, i.e. continuous spin at different speeds and periodic spin at different speeds and frequencies. The resulting centrifugal accelerations ranged from 1.2 g's to 4.8 g's. This paper presents the second part of the experimental study, i.e. the effect of an accelerating force on the LHP operating temperature. It has been known that the LHP operating temperature under a stationary condition is a function of the evaporator power and the condenser sink temperature when the compensation temperature is not actively controlled.

Loop Heat Pipes (LHPs) are being considered for cooling of military combat vehicles and spinning spacecraft. In these applications, it is important to understand the effect of an accelerating force on the performance of LHPs. In order to investigate such an effect, a miniature LHP was installed on a spin table and subjected to variable accelerating forces by spinning the table at different angular speeds. Several patterns of accelerating forces were applied, i.e. continuous spin at different speeds and periodic spin at different speeds and frequencies. The resulting centrifugal accelerations ranged from 1.2 g's to 4.8 g's. This paper presents the first part of the experimental study, i.e. the effects of an accelerating force on the LHP start-up. Tests were conducted by varying the heat load to the evaporator, condenser sink temperature, and LHP orientation relative to the direction of the accelerating force.

A parametric study of performance characteristics of a Loop Heat Pipe (LHP) is presented. A mathematical model, based on the steady-state energy conservation equations, is used. The calculations are performed by varying the operation conditions (heat load, sink and ambient temperatures, and elevation) and the LHP design parameters (working fluid, transport length size, external thermal conductance of the condenser and wick properties). The results are illustrated on LHP performance curves (saturation temperature as a function of applied power). All the results are compared with a baseline configuration to analyze the effects of different parameters. Operating limits due to various constraints such as heat transport limit, capillary pressure limit and the vapor pressure limit are discussed.

In this paper, results of the NRL LHP experimental studies, conducted by Naval Research Laboratory (NRL) and NASA Goddard Space Flight Center, will be presented. Emphasis in this test program is to examine the “turnkey” startup of the NRL LHP and its operational characteristics. Series of tests were performed, including startup tests, power cycling tests, low power tests, and high power tests. The NRL LHP has demonstrated very robust operations throughout the tests. In addition, hysteresis was found at low power operations. Importance of the two-phase dynamics in the evaporator core is realized, which has shown significant effects on loop operations.