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Startup performance of a liquid-metal heat pipe in near-vacuum and gas-loaded modes
Abstract
Understanding the startup behavior of a heat pipe from frozen state has
received a lot of attention in recent years. In liquid-metal heat pipes,
a calculated amount of inert gas is filled as one of the safest means to
start them from a frozen state. In the present study, an arterial-type
sodium heat pipe with a long transport section has been tested for
startup performance in both vacuum and gas-filled modes. A comparison
was necessary to contrast the relative merits of gas-filled mode startup
from the conventional vacuum mode. The 2-m-long heat pipe with 2.0-Torr
argon started very smoothly from frozen state for suddenly applied
evaporator loads up to 1.1 kW. Only less than 5% of the total length
remained as inactive condenser due to the gas loading. The same heat
pipe in the vacuum mode had large temperature spikes at the evaporator
and heater during the frozen startup indicating a rough startup
behavior.
... Beyond their mere presence, NCGs can strongly influence the thermal and hydrodynamic characteristics of the heat pipe during startup. Several studies have shown that these gases interact with both the liquid metal pool and the vapor phase, creating local pressure gradients and disturbing flow patterns (Shaubach & Gernet, 1992;Ponnappan & Chang, 1994). As the vapor flows from the evaporator to the condenser, NCGs may accumulate at the condenser end of the heat pipe, forming a barrier that impedes the vapor condensation and reduces overall heat transfer area (Zhang et al., 2023). ...
... Numerical analysis of high-temperature heat pipe startup from frozen state with one-dimensional compressible vapor flow coupled the twodimensional heat conduction. (Ponnappan & Chang, 1994) Na Experiment Startup performance of liquid-metal heat pipe in near-vacuum and gas-loaded modes. (Yamamoto et al., 1994) Hg Experiment A preheating operation of 30 hours is required to reach a steady state. ...
This study investigates the startup behavior of sodium heat pipes, focusing on how different startup methods, the presence of non-condensable gases (NCGs), and flow instabilities near the mixing layer affect thermal performance and operational stability. Four startup methods were evaluated, ranging from slow, incremental power increases to rapid, one-step power applications. Thermal instabilities were observed to emerge at a critical power of 50.43 W, corresponding to an operating temperature of 340 °C. Slow startups initiated below this threshold enabled a gradual displacement of NCGs toward the condenser, resulting in a uniform temperature distribution and extended effective heat transfer lengths. At 200 W, the effective length exceeded 800 mm in slow startups, whereas rapid startups showed shorter lengths due to mixing at the vapor-NCG interface. At 1000 W, rapid startups exhibit significant portions of the heat pipe remaining below the sodium melting point of 97.8 °C, particularly near the condenser. This occurs due to incomplete displacement of NCGs during the initial phase of startup, leading to uneven temperature distributions and inactive regions. The abrupt vaporization of sodium causes unstable flow patterns that prevent the vapor from fully engaging the condenser region. Slow startups, by contrast, gradually transition the entire pipe into operation, minimizing inactive regions and maintaining a more uniform temperature profile. These results underscore the need to manage startup rates carefully, especially at higher power levels, to ensure complete activation of the heat pipe. The results validate a theoretical model treating the flow near the mixing layer as compressible in time and incompressible in space. This approach successfully modeled the interface dynamics and instability mechanisms caused by rapid interactions between sodium vapor and NCGs. The findings demonstrate that gradual power increases are shown to maximize thermal performance and operational stability. Future research should refine startup methodologies, develop strategies to mitigate instabilities, and improve the interaction between sodium vapor and NCGs for high-temperature applications. This study provides critical insights into optimizing sodium heat pipe performance in high-temperature applications, particularly for advanced nuclear reactors and other demanding thermal management systems.
... The steady-state parameters such as the heat pipe temperature rise. Another experiment (Ponnappan and Chang, 1994) tested the starting performance of sodium heat pipes in both vacuum and gas-loaded modes, found that the start-up time of gasloaded mode is longer than that of vacuum mode. However, heat pipe in the vacuum mode had large temperature spikes at the evaporator and heater during the frozen start-up process indicating a rough start-up behavior. ...
... The frozen start-up process is also validated using the experimental data of Ponnappan and Chang (1994). The results are shown in Fig. 14 and Table 5, the HPTAC-NCG program accurately describes the ''heat front" interface where the temperature moves steeply from the evaporator to the condenser during the start-up. ...
The high-temperature lithium heat pipe utilized in the reactor will be irradiated by neutrons, causing non-condensable gas (NCG) to generate within and deteriorate thermal performance. This study develops a high-temperature heat pipe transient analysis code that takes into account the NCG effect. Heat pipe frozen start-up and key parameters sensitivity study are numerically evaluated. The NCG influence can be well described by the code. When compared to heat pipes without NCG, the start-up time of heat pipes with 25% NCG volume fraction increases by 54.2%. However, the existence of NCG can make the start-up process go more smoothly. The temperature drop in the NCG area changes from 40 K to 395 K as the NCG volume fraction increases from 5% to 25%. Increased input power can lower the volume fraction of NCG and hence diminish the negative thermal effects. This work provides foundation for transient analysis of high temperature heat pipe containing NCG.
... Ochterbeck (Ochterbeck, 1997) studied NCG's effect on the startup of the high-temperature heat pipe. They found that a certain number of NCG could improve the safety of the frozen startup of the heat pipe; Ponnappan (Ponnappan et al., 1990;Ponnappan and Chang, 1994) added the Ar into the sodium heat pipe to test the startup characteristic. Their results indicated that the Ar could make the startup more stable while extending the startup time; Anand (Anand, 2019) studied the NCG's effect on the Loop heat pipes. ...
... According to the aforementioned in the introduction, the NCG could improve the startup of the sodium heat pipe though it generally impedes the phase change in many conditions. The above microscopic phenomenons coincide with these conclusions of the macroscopic experiments (Ochterbeck, 1997;Ponnappan et al., 1990;Ponnappan and Chang, 1994;Anand, 2019;FUKUZAWA and FUJII-E, 1978). And the special trend of the NCG effects is also an extension for these experiments. ...
Understanding the evaporation of the thin liquid sodium film is important for deeply studying the heat transfer inside sodium heat pipe. In the present work, molecular dynamics is employed to investigate the evaporation of the thin liquid sodium film. The simulation system is a cuboid and consists of an upper gold wall and a bottom one. The sodium fluid exists between the walls. For studying the Non-Condensable Gas’s (NCG) effect on the evaporation, the Ar is added into the evaporation system. Based on the startup and normal operation of the sodium heat pipe, eight different cases are studied. The equilibrium simulation is achieved by setting the same temperature of the solid walls. The Mass Accommodation Coefficients (MACs) of eight cases are acquired. By setting the walls at different temperatures, the non-equilibrium is built. The evolution of the evaporating liquid film is observed. The net evaporation flux (Jnet), the heat transfer coefficient (h) at the liquid–gas interface are obtained. It is found that the NCG’s effects on the Jnet are achieved through its influence on the MAC. When the variation of the MAC is above 27%, NCG’s effects on the Jnet are more obvious. In 600–700 K, 840–1020 K, the NCG suppresses the evaporation heat transfer and the maximum decrement is 150–200 kW∙m⁻²∙K⁻¹. Whereas in 690–810 K, the NCG has an enhancement effect and the growing magnitude is 15–40 kW∙m⁻²∙K⁻¹. This study could provide the mechanism supplement and extension for the macroscopic investigation of the sodium heat pipe.
... Ponnappan et al. (1990) found that the sodium HTHP could start easily from the frozen state under the NCG loading. Another experiment (Ponnappan and Chang, 1994) investigated the startup performance in vacuum and gas-loaded modes, which the latter took significantly longer. The sodium HTHP in vacuum mode, on the other hand, exhibited rough start-up behavior as seen by high temperature spikes at the evaporator and heater during the frozen start-up process. ...
Heat pipe cooled reactors (HPCRs) have broad application prospects due to their advantages, such as high power density, compact structure, lower cost, and easy modular assembly. Numerous countries have engaged in extensive research and development of HPCR conceptual designs. The heat from the reactor is removed by high temperature heat pipes (HTHPs), which generally employ alkali metals as the working fluid, such as potassium, sodium, and lithium. Understanding the thermal-hydraulic performance of HTHPs is essential for the safe and efficient operation of a reactor. Therefore, the objective of this paper is to provide a comprehensive review of HPCR conceptual designs developed by various countries in recent years. The research progress of HTHPs on flow and heat transfer performance is reviewed, with an emphasis on both transient and steady-state characteristics. Research progress, as well as the issues that need to be focused on in future research, are discussed in detail.
... Cotter firstly proposed general heat pipe theory in 1965 and divided heat pipe startup into three basic transient modes (Cotter, 1965). After this theory, many experiments of the startup or transient operation of LMHPs have been conducted by researchers to validate with Cotter's theory (Deverall et al., 1972;Tolubinskii et al., 1978;Tilton et al., 1988;Ponnappan and Chang, 1994;Dickinson et al., 1998;Glass et al., 1999). Besides these experiments, numerical models and codes about heat pipes simulation have been developed (Levy, 1971;Brovalsky et al., 1976;Costello et al., 1986;Jang, 1988Jang, , 1995Seo and El-Genk, 1988;Faghri, 1992, 1993;Tournier and El-Genk, 1996). ...
Heat pipe radiator, featured with remarkable advantages in heat transfer efficiency and inherent safety with small specific mass and little weight, are widely adopted to the heat-rejection system for space nuclear power reactors. In this paper, physical and numerical models are developed to obtain the startup and transient behaviors of radiator unit with a potassium (K) heat pipe covered by fin under space environment. The heat transfer limit theory is adopted as criteria for heat pipe operation success. Numerical results indicated that according to the internal vapor flow regimes, the K heat pipe startup could be divided into three distinct stages. The K heat pipe started up from frozen state successful and rapidly until the expected operation state is reached. Among the different heat transfer limits, only the sonic limit due to choked flow restricts the K heat pipe during the second stage. Overall, for the heat-rejection system of space nuclear power reactor, the heat pipe radiator unit can effectively radiated waste heat to the space environment in 5 min and responses fast under transient conditions.
... The liquid metal heat pipes (LMHP) for hightemperature range can be used in many industrial applications including isothermal high temperature heater, thermometric calibration tools, die casting and mold system, glass forming process , and solar power system (Brost and Crroll, 1995; Rosenfeld and Ernst, 1997). In previous studies on LMHP's, heat transfer limit, operation characteristics, and compatibility were investigated and most of the research works were on cylindrical geometry (Faghri et al., 1991a: 199!b ; Ponnappan and Chang, 1994 ; Jang, 1995 ; Moraga and Jacobson, 1987 ). In recent works, however, various geometries were studied depending on special demand and applications. ...
The isothermal characteristics of a rectangular parallelepiped sodium heat pipe were investigated for high-temperature applications.
The heat pipes was made of stainless steel of which the dimension was 140 m(L) ×95m(W) ×46m(H) and the thickness of the container
was 5 mm. Both inner surfaces of evaporator and condenser were covered with screen meshes to help spread the liquid state
working fluid. To provide additional path for the working fluid, a lattice structure covered with screen mesh wick was inserted
in the heat pipe. The bottom surface of the heat pipe was heated by an electric heater and the top surface was cooled by circulating
coolant. The concern in this study was to enhance the temperature uniformity at the bottom surface of the heat pipe while
an uneven heat source up to 900 W was in contact. The temperature distribution over the bottom surface was monitored at more
than twenty six locations. It was found that the operating performance of the sodium heat pipe was critically affected by
the inner wall temperature of the condenser region where the working fluid may be changed to a solid phase unless the temperature
was higher than its melting point. The maximum temperature difference across the bottom surface was observed to be 114°C for 850 W thermal load and 100°C coolant inlet temperature. The effects of fill charge ratio, coolant inlet temperature and operating temperature on thermal
performance of heat pipe were analyzed and discussed.
To enhance isothermal characteristics of glass-farming surface, a rectangular parallelepiped heat pipes was fabricated, tested, and analyzed. The working fluid was sodium and the wall material was stainless steel 304. The dimension of the heat pipe was 210 (L) 140(W) 92(H)mm. A lattice structure covered with screen mesh was inserted to promote return of working fluid. The bottom side of heat pipe was heated electrically and the top side was cooled by liquid circulation. The temperature distribution at the bottom surface was of major concern and was monitored to determine isothermal characteristics. A frozen start-up of rectangular parallelepiped liquid metal heat pipe was tested. The operating mode of the sodium heat pipe was affected by the temperature of cooling zone, input heat flux, and the operating temperature of heat pipe. The heat pipe operated in a normal fashion as long as the heat flux was over 5.78W/cm, and the inside wall temperature of condenser part was above 95 The maximum temperature difference at the bottom surface was observed to be 32 when the operating temperature of the heat pipe was operating normally around 50. The result showed that a sodium heat pipe was very effective in reducing significantly the temperature difference in the glass-forming surface.
Experiments were performed to evaluate the effect of non-condensable gases and axial conduction on the transient performance of copper-water wicked heat pipes. An existing transient network model for wicked heat pipes was extended to incorporate the effects of axial conduction and non-condensable gas. The different components were modeled by a larger number of smaller elements in both axial and radial directions. The model predictions of the steady and transient response of the vapour and wall temperature of the heat pipes were in good agreement with the experimental results. The non-condensable gases and axial conduction did not significantly affect the transient response during the heat-up phase; however, it significantly slows down the cool-down phase.
Fundamental heat transfer experiments were carried out for three kinds of heat pipes which may be applied to turbine cooling in future aero-engines. In the turbine cooling system with a heat pipe, heat transfer rate and start-up time of the heat pipe are the most important performance criteria to evaluate and compare with conventional cooling methods. Three heat pipes are considered, called heat pipe A, B and C, respectively. All heat pipes have a stainless steel shell and nickel sintered powder metal wick. Sodium(Na) was the working fluid for heat pipes A and B; heat pipe C used eutectic sodium-potassium(NaK). Heat pipes B and C included non-condensible gas for rapid start-up. There were fins on the cooling section of heat pipes. In the experiments, an infrared image furnace supplied heat to the heat pipe simulating turbine blade surface conditions.
In the results, heat pipe B demonstrated the highest heat flux of 17 to 20 W/cm2. The start-up time was about 6 minutes for heat pipe B and about 16 minutes for heat pipe A. Thus adding non-condensible gas effectively reduced start-up time. Although NaK is a liquid phase at room temperature, the start-up time of heat pipe C (about 7 to 8 minutes) was not shorter than the heat pipe B. The effect of a gravitational force on heat pipe performance was also estimated by inclining the heat pipe at an angle of 90 degrees. There was no significant gravitational dependence on heat transport for heat pipes including non-condensible gas.
Development activity in heat pipe and thermosyphon technology in the Americas from January 1990 through December 1995 is surveyed. This period was selected because of the lack of a regional report for the Americas at the 8th International Heat Pipe Conference and the high level of activity since 1990. Heat pipe related journal articles and conference papers as well as books by American authors are cited. This report is by no means comprehensive. Only representative citations are given, as their vast number from various sources make selection in every report section mandatory. Because coverage of a six year period is attempted, a bibliographic form was selected rather than the more customary summary form. Mention is also made of some products developed by commercial heat pipe manufacturers in the US and Canada who do not normally publish their work. Introduction During this period a number of conferences were devoted in whole or in part to various aspects of heat pipe research and deve...
Liquid-metal heat pipes have exhibited difficulties starting up from a frozen-state. Inert gas loading is a possible solution to the frozen-state startup problem. A few research papers give results of this technique. The applicability of the method to heat pipes with arterial grooves and long adiabatic lengths is unknown. The present study deals with the diffusion-controlled startup analysis and testing of an argon-loaded, 2-m-long, stainless steel-sodium heat pipe of the double-walled type with artery channel and long adiabatic section. A two-dimensional, quasisteady state, binary vapor-gas diffusion model determined the energy transport rate of vapor at the diffusion front. The analytical solution to the diffusion problem provided the vapor flux, which in turn was used in the one-dimensional transient thermal model of the heat pipe to predict the time rate of change of temperature and position of the hot front. The experimental test results successfully demonstrated the startup of a gas-loaded sodium heal pipe and validated the diffusion model of the startup.
An area of major concern for high-temperature liquid metal heat pipes used in waste heat rejection systems of spaceborne power plants is the behavior of these devices under transient load conditions during the startup, the shutdown, and the operational variation of thermal loads. This paper considers the transient characteristics of a gas-loaded liquid metal heat pipe pertaining only to the startup from the frozen state. The approach used was to test a specially designed and fabricated 2-m-long sodium heat pipe under simulated heat input and rejection conditions. Both vacuum and gas-filled mode performances were obtained for comparison purposes. It was demonstrated that gas-loading did not adversely affect the priming of the long adiabatic artery groove. The startup from the liquid state was smoother than from the frozen state. Test results are presented on transient temperature profiles at specified axial locations and under different conditions.
High temperature liquid metal heat pipes have exhibited difficulties starting up from frozen state due to inherent low near-room temperature vapor pressures associated with working fluids. Inert gas loading is a possible solution to the frozen state startup problem. A few research papers give results of this technique. The applicability of the method to heat pipes with arterial grooves and long adiabatic lengths is unknown. the present study deals with the design, fabrication and startup testing of a gas loaded sodium heat pipe of the double walled type with grooved artery channel and long adiabatic section. Artery, Startup, Noncondensible gas, Frozen start, Diffusion model, Reservoir wick, Heat front, High temperature, Liquid metal, Sodium, Transient analysis.
Liquid metal heat pipes have exhibited difficulties starting up from
frozen state. Inert gas loading is a possible solution to the frozen
state startup problem. A few research papers give results of this
technique. The applicability of the method to heat pipes with arterial
grooves and long adiabatic lengths is unknown. The present study deals
with the diffusion controlled startup analysis and testing of an argon
loaded 2 m long stainless steel sodium heat pipe of the double walled
type with artery channel and long adiabatic section. A two-dimensional,
quasi-steady state, binary vapor-gas diffusion model determined the
energy transport rate of vapor at the diffusion front. The analytical
solution to the diffusion problem provided the vapor flux which in turn
was used in the one-dimensional transient thermal model of the heat pipe
to predict the time rate of change of temperature and position of the
hot front. The experimental test results successfully demonstrated the
startup of a gas loaded sodium heat pipe and validated the diffusion
model of the startup.
A comprehensive review and analysis of all aspects of heat pipe technology pertinent to the design of self-controlled, variable conductance devices for spacecraft thermal control is presented. Subjects considered include hydrostatics, hydrodynamics, heat transfer into and out of the pipe, fluid selection, materials compatibility and variable conductance control techniques. The report includes a selected bibliography of pertinent literature, analytical formulations of various models and theories describing variable conductance heat pipe behavior, and the results of numerous experiments on the steady state and transient performance of gas controlled variable conductance heat pipes. Also included is a discussion of VCHP design techniques.
Development of a High Temperature Furnace with Variable Conductance Heat Pipe
- References Brost
- O Groll
- M Mack
References 'Brost, O., Groll, M., and Mack, H., "Development of a High Temperature Furnace with Variable Conductance Heat Pipe," Pro-ceedings of the 6th International Heat Pipe Conference, Grenoble, France, May 25-29, 1987, pp. 697-702.
Study of Vapor-Gas Front Temperature Characteristics in Sodium Coaxial Heat Pipes
- V L Tolubinsky
- E N Shevchek
- L V Kudritskaya
Tolubinsky, V. L, Shevchek, E. N., and Kudritskaya, L. V., "Study of Vapor-Gas Front Temperature Characteristics in Sodium Coaxial Heat Pipes," Proceedings of the 6th International Heat Pipe Conference, Grenoble, France, May 25-29, 1987, pp. 326-329.
Space Reactors Los Alamos National Lab
- W A Ranken
Ranken, W. A., "Space Reactors," Los Alamos National Lab., Progress Rept. LA-9146-PR, Los Alamos, NM, April-June 1981, pp. 17-22.
A Liquid Metal Variable Conductance Heat Pipe for Space Radiators
- R Ponnappan
- J E Beam
- E T Mahefkey
Ponnappan, R., Beam, J. E., and Mahefkey, E. T., "A Liquid Metal Variable Conductance Heat Pipe for Space Radiators," 7th Symposium on Space Nuclear Power Systems, Albuquerque, NM, Jan. 7-11, 1990.
Development of a Dou-ble-Wall Artery High-Capacity Heat Pipe Spacecraft Thermal Con-trol, Design and Operation
- R Ponnappan
- E T Mahefkey
Ponnappan, R., and Mahefkey, E. T., "Development of a Dou-ble-Wall Artery High-Capacity Heat Pipe," Spacecraft Thermal Con-trol, Design and Operation, edited by P. E. Bauer and H. E. Collicott, Vol. 86, Progress in Astronautics and Aeronautics, AIAA, New York, 1983, pp. 202-221.