Liquid Metal Heat Pipes Research Articles

Experimental Study on Startup Performance of a High-temperature Liquid Metal Heat Pipe with Fins

Experimental Study on Startup Performance of a High-temperature Liquid Metal Heat Pipe with Fins

Key Technologies for Korean Space Heat Pipe Reactor

Sustainable energy is crucial for long-term space missions. Fission surface power is a leading option due to its ability to provide consistent electricity and heat irrespective of the sun’s position or distance. A heat pipe is the primary cooling solution for space fission power systems. It operates without the need for additional electricity or gravity. It offers the advantage of streamlining the reactor design by eliminating components such as pumps and piping. This paper presents the advancements in the Korean space heat pipe reactor development program over the past 4 years. The primary objectives of this program include the development of codes for space heat pipe reactors, the innovative design of a hybrid wick structure, and the evaluation of the thermal performance of liquid-metal heat pipes.

Analysis of geyser boiling in liquid metal heat pipe: Impact on startup and shutdown, fuel temperature, and heat removal performance

Liquid metal heat pipes are efficient heat transfer devices with diverse applications in various industries, including nuclear power plants. The use of liquid metal heat pipes in Heat Pipe-Cooled Nuclear Reactors (HPNRs) has been proposed for high-temperature heat utilization during electricity generation. This study experimentally investigates the impact of geyser boiling on the temperature of the fuel and its effect on heat removal performance using a sodium heat pipe. The investigation includes the influence of both increasing and decreasing power levels. The study reveals significant differences between operating the heat pipe in vertical and horizontal orientations. The thermal resistance and efficiency of the heat pipe are analyzed under different operating conditions. The results show that geyser boiling has a more pronounced impact in unsteady states compared to steady states, leading to non-periodic behavior and fluctuations in temperature and heat removal. The study emphasizes the importance of considering the changing liquid pool in liquid metal heat pipes to prevent or mitigate geyser boiling effects. Modifications to the optimal filling ratios of heat pipes are suggested to enhance stability and efficiency, making liquid metal heat pipes more feasible for HPNR applications.

Operating condition optimization of liquid metal heat pipe using deep learning based genetic algorithm: Heat transfer performance

Liquid metal heat pipes play a critical role in various high-temperature applications, with their optimization being pivotal to achieving optimal thermal performance. In this study, a deep learning based genetic algorithm is suggested to optimize the operating conditions of liquid metal heat pipes. The optimization performance was investigated in both single and multi-variable optimization schemes, considering the operating conditions of heat load, inclination angle, and filling ratio. The single-variable optimization indicated reasonable performance for various conditions, reinforcing the potential applicability of the optimization method across a broad spectrum of high-temperature industries. The multi-variable optimization revealed an almost congruent performance level to single-variable optimization, suggesting that the robustness of optimization method is not compromised with additional variables. Furthermore, the generalization performance of the optimization method was investigated by conducting an experimental investigation, proving a similar performance. This study underlines the potential of optimizing the operating condition of heat pipes, with significant consequences in sectors such as high temperature field, thereby offering a pathway to more efficient, cost-effective thermal solutions.

Experimental investigation on reactivation characteristics of liquid–metal heat pipes after hydrogen inactivation

The use of a hydrogen window provides an effective way for liquid–metal heat pipes (LMHPs) to prevent hydrogen inactivation when operated in a hydrogen-containing atmosphere. This work experimentally investigates the effects of various operational conditions on the reactivation characteristics of LMHPs after hydrogen inactivation. Theoretical analysis and sensitivity analysis are conducted to enhance our understanding and optimize reactivation operations. The results indicate that a slope-shaped distribution of hydrogen buffer enable faster reactivation compared to a near-column shape. A sweep-gas flow rate in 200 mL/min is preferred under test conditions, when both the reactivation time and the pure N2 consumption rate are considered. The working temperature significantly affects the reactivation time, as higher temperatures accelerate the NaH decomposition and increase the hydrogen permeability of hydrogen window. The varying of pressure and preheating temperature of sweep gas under test conditions just have little influence on the reactivation time. In practical conditions, vacuum operations are not recommended due to its extremely low hydrogen-removal rate.

Experimental investigation of thermal behavior of overfilled sodium heat pipe

Operating limitations model of heat pipes are difficult to evaluate the effect of the charging amount of working fluid, which is one of the crucial factors in heat pipe performance of startup and maximum heat transfer. To reduce the knowledge gap on the effect of filling amount, the purpose of this paper is an investigation of the thermal behavior of excess liquid. Overfilled sodium heat pipe having about 260% filling ratio was fabricated and tested with the change of boundary and initial condition. Sodium heat pipe showed good isothermal characteristics that the temperature difference between evaporator and condenser reduced as the development of continuum flow until reaching operating limit condition. The characteristics of an overfilled heat pipe appear at power increased after the continuum flow fully developed to the condenser that sudden temperature rise about 100 °C in the overall heat pipe except the end region of the condenser where a continuous temperature about 50 °C drop occurs. Excess liquid moves toward the condenser end and forms the liquid pool and lessening the active condenser length which reduces the maximum heat transfer of heat pipe and induced the operating limitation. The results showed that this phenomenon only occurs at high heat flux conditions as sufficient vapor flow is required to allow excess liquid to propagate to the condenser and determined the operating limitation of overfilled sodium heat pipe that continuous temperature rises as the liquid propagates to the end of the condenser rather than the abrupt rising such as dry out. However, the operating limitations occur sudden dry-out of the evaporator similar to the conventional results of liquid metal heat pipe when the liquid propagation did not occur. Compared to the entertainment limit model, there was a significant difference in the dry-out point of the experimental results with excessive liquid propagation. It was confirmed that the horizontal operation of overfilled heat pipe was in good agreement with the prediction of capillary limit model within 22.6% of the maximum error.

High-Fidelity Modeling and Experiments to Inform Safety Analysis Codes for Heat Pipe Microreactors

Heat pipe microreactors are reactor designs that primarily use liquid-metal heat pipes to cool the core. The main interest in heat pipes is the fact that they can remove heat passively. This, along with the use of liquid metal, allows the reactor to operate at higher temperatures. Although the use of heat pipes in nuclear reactors is new, liquid-metal heat pipe technology is mature. Nevertheless, experimental data on heat pipes are scarce, and very little is known about their behavior during abnormal operations and close to their thermal limits. Therefore, new experiments and accurate heat pipe simulations are needed to develop reliable closure models. This work describes a joint experimental and numerical investigation into heat pipes that attempts an initial closure of this gap. The numerical and experimental efforts are currently proceeding in parallel, aimed at different aspects of heat pipes. The numerical part is focused on gaps in local closures, and the experiments capture the overall heat pipe behavior.

CFD modeling of liquid-metal heat pipe and hydrogen inactivation simulation

The use of hydrogen window is a potential way for liquid metal heat pipes (LMHPs) to avoid hydrogen inactivation when operated in a hydrogen-containing atmosphere. To reveal the effects of hydrogen permeation on heat transfer in LMHPs in a visual way, we develop a comprehensive CFD model in this work for the first time, and experimental tests are also carried out for model validation. The experimental results show that the top end of an LMHP reduces to below 400 ℃ after hydrogen inactivation, when the heating temperature of the evaporator is maintained at 950 ℃. It takes about 150 min for the LMHP to recover performance by flushing its hydrogen window with nitrogen controlled at 50 mL/min. The CFD model is demonstrated reliable by comparing the predicted heat output and temperature distribution with the experimental results in steady state. The CFD model is capable of reproducing the liquid up-throwing phenomenon, and revealing the impact of hydrogen-buffer formation on heat transfer. In addition, the hydrogen migration along with the returning condensate leads to the formation of a gas blanket covering the inner surface in the condenser, which has a negative effect on the heat transfer as well.

Liquid Metals Heat-Pipe solution for hypersonic air-intake leading edge: Conceptual design, numerical analysis and verification

Embedded propulsion systems will allow future hypersonic aircraft to reach amazing levels of performance. However, their peculiar small-radius air-intake leading edges pose serious challenges from the aerothermodynamic, design, integration, and manufacturing standpoints. This paper discloses the methodology developed in the framework of the H2020 STRATOFLY project and specifically tailored to support the conceptual and preliminary design phases of future high-speed transportation systems. The methodology implements an incremental approach which includes multi-fidelity design, modelling and simulation techniques. The specific application to the MR3, a Mach 8 waverider configuration with an embedded dorsal mounted propulsive subsystem, is reported. Different alternative solutions have been thoroughly analysed, including five liquid metals as fluids (Mercury, Caesium, Potassium, Sodium and Lithium) and relative wick and case materials (Steel, Titanium, Nickel, Inconel® and Tungsten) and three leading-edges materials (CMC, Tungsten with low emissivity painting and Tungsten with high emissivity painting). The analysis of the heat transfer limits (the capillary, entrainment, viscosity, chocking and boiling limits) carried out for all five fluids and relative compatible materials, together with a more accurate FEM analysis, suggest the adoption of a Nickel–Potassium liquid metal heat pipe completely integrated in a platelet air-intake leading edge made of CMC material. Ultimately, the effectiveness of the adopted solution throughout all mission phases has been verified with a detailed numerical model, built upon an electrical analogy.

Parameter sensitivity study on startup characteristics of high temperature potassium heat pipe

Due to high power density, compact structure and easy modular assembly, heat pipe cooled reactor (HPR) is considered as an excellent candidate for removable power supply system. The heat transfer characteristic of liquid metal heat pipe, which is the only device to conduct the heat from reactor core, is extremely important for the safe operation of HPR. In this paper, the heat transfer performances of high temperature potassium heat pipes (Φ28 × 800 mm) are tested under different heating power (300–1000 W), wick mesh (600 and 800), filling charge amount (20 g and 50 g) and inclination angle (0°, 45°, 90°). The results show there is a temperature difference of 170 °C in condensation section at heating power of 336 W. With the increase of heating power, the melting and propagation speed of the working fluid in container increases, and the heat pipe performance is improved. Both wick mesh and filling charge amount increasing can promote the heat transfer performance of heat pipes, the filling charge amount is the main influencing factor to prevent from overheating at evaporation section. However, the heat pipes, having 50 g potassium, have a great geyser boiling phenomenon at 45°and 90° which could threaten the safe operation of HPR. This study provides experimental data support and theoretical basis for design and optimization of liquid metal heat pipes.

Analysis on heat transfer and start-up performance of mercury heat pipe

Mercury heat pipe has the advantages of good thermal stability and low saturated vapor pressure, which is the best choice for the transition from water heat pipe to liquid metal heat pipe. The effects of heating power and heat pipe structure on start-up time and steady-state heat transfer performance of mercury heat pipe were studied by using transient thermal network model. The results showed that: 1) Increasing the length of condenser is beneficial to reducing the start-up time and thermal resistance; 2) Increasing the heating power or wall thickness will reduce the thermal resistance, but increase the start-up time, and increasing the porosity of wick is just the opposite; 3) Increasing the thickness of wick can increase both the start-up time and the thermal resistance.

Capillary character and evaporation heat transfer in the wicks of high temperature liquid metal heat pipe

The study focuses on the capillary character and evaporation in micro-structures of high temperature liquid metal heat pipe, an important component for the cooling of reusable leading edge of hypersonic vehicle. Based on the free surface shape calculated by Surface Evolver, the rise of liquid and evaporation capacity are investigated theoretically and numerically for the sphere and rectangular grooved wick respectively. The results show that rectangular grooved wick presents better capillary character performance mainly due to its smaller flow resistance than the sphere one. And the evaporation capacity is mainly affected by the thermal conducting path and the liquid-vapor interface area, which combined lead to that the rectangular grooved wick also has more superior evaporating performance, much different from the evaporation of the low temperature liquid such as water or octane and so on. It is also found that decreasing the solid-liquid contact angle is helpful to improve both the capillary and evaporation character.

Benefits from heat pipe integration in H2/H2O fed SOFC systems

The Solid oxide fuel cell (SOFC) is an electrochemical energy-conversion device considered as a promising solution because of its high electrical efficiency, fuel flexibility and modularity with relevant environmental benefits. However, high temperatures and thermal gradients cause significant problems for the practical application of SOFC systems. SOFC operating conditions have to be maintained within specific bounds with limitation in the maximum achievable current density in order to avoid excessive degradation. Relevant air flow rate values are also required for a correct thermal management, dealing with the highly exothermicity of the SOFC operation. In this context, the integration of planar liquid metal heat pipes into the stack structure can contribute to reduce thermal gradients within the stack itself while lowering the parasitic consumption of the air blower since less convective cooling is required. In the present work, a model implementing different SOFC cells interposed between two planar heat pipe plates filled with sodium is developed. In particular, an SOFC system fed with a H2/H2O mixture is assessed (the most stringent thermal condition because of the absence of internal reforming). The maximum permissible current density is then evaluated by varying the air utilization factor (AU). The limiting operating conditions are identified by taking into account the following constraints: maximum cell temperature, maximum global and local temperature gradients and heat pipe transport limits. A comparison of the system with and without the presence of heat pipes is then carried out with the aim of highlighting the positive effects derived from their integration. Results from simulations show a remarkable improvement of the system performance with heat pipes, which allow to reach higher values of current densities (and so power densities) with a consequent economic benefit.

A walk-away safe, Very-Small, Long-LIfe, Modular (VSLLIM) reactor for portable and stationary power

This paper describes a Very-Small, Long-LIfe, and Modular (VSLLIM) reactor design, developed at the University of New Mexico’s Institute for Space and Nuclear Power Studies, which offers passive operation and decay heat removal and redundant control. During nominal operation and after shutdown, the VSLLIM reactor is cooled by natural circulation of in-vessel liquid sodium (Na), that is enabled using in-vessel chimney (1–2 m tall) and helically coiled tubes Na-Na heat exchanger, placed at the top of the downcomer. The reactor can generate 1.0–10 MWth, depending on the height of the chimney and the HEX design, at an average fission power density up to 23.47 MWth/m3. The VSLLIM reactor can potentially operate continuously, without refueling for ∼92 and 5.9 full power years (FPY), respectively, and slightly below atmospheric pressure, owing to the sodium low vapor pressure. The core is loaded with hexagonal assemblies of fuel rods with UN enriched to 13.76%, and has independent systems for emergency shutdown and nominal control. It also has two independent systems for safe passive removal of decay heat after shutdown, and following an unlikely malfunction of in-vessel HEX. These are liquid metal heat pipes (LMHPs) embedded in the primary vessel wall, and natural circulation of ambient air along the outer surface of the guard vessel wall. The LMHPs are thermally coupled to thermoelectric elements for generating 10 s of kW of auxiliary DC power, independent of on-site and off-site sources, both during nominal reactor operation and after shutdown. The sodium coolant enters the fast-neutron energy spectrum core at 610 K and exits at ≤755 K, depending on the reactor thermal power. At these temperatures, Na is compatible with the HT-9 Ferritic-Martensitic steel cladding, core structure and reactor vessel. The reactor would be fabricated, assembled and sealed at the factory, and transported by rail, truck or barge to a permanent site and installed underground, to protect against an airplane or a missile impact, and mounted on seismic isolation bearings, to resist earthquakes. At the site, the VSLLIM module could use a superheated steam Rankine cycle or a supercritical CO2 Brayton cycle for electricity generation at high thermal efficiency. Alternatively, a VSLLIM power module, with an open air Brayton cycle, could be deployed on a portable platform or a truck, to provide both electricity and/or process heat at remote sites, hospitals, data centers, natural disaster areas, arid or desert regions, and advanced military bases.

Thermal effects of planar high temperature heat pipes in solid oxide cell stacks operated with internal methane reforming

The presented paper contributes to developing a new thermal control approach for solid oxide cell (SOC) stacks and systems. Integrating planar liquid metal heat pipes to the interconnector structure of the stacks targets a reduction of internal temperature gradients and an enhanced heat extraction from the stack. This work applies 3-D CFD-modelling to discuss the thermal effects of integrated heat pipes in solid oxide cell stacks, in order to evaluate the possible benefits in terms of temperature gradient reduction and heat removal as well as the resulting benefits for stacks and systems. The stack model set-up is described and its functioning is validated with experimental results from thermal short stack measurements with integrated heat pipe interconnectors. The simulation results are discussed with respect to the possible benefits for full-scale stacks of different cell size, in particular regarding internal heat recycling and the beneficial reduction of air ratios.

Low-enrichment and long-life Scalable LIquid Metal cooled small Modular (SLIMM-1.2) reactor

The Scalable LIquid Metal cooled small Modular (SLIMM-1.0) reactor with uranium nitride fuel enrichment of 17.65% had been developed for generating 10–100MWth continuously, without refueling for ∼66 and 5.9 full power years, respectively. Natural circulation of in-vessel liquid sodium (Na) cools the core of this fast energy spectrum reactor during nominal operation and after shutdown, with the aid of a tall chimney and an annular Na/Na heat exchanger (HEX) of concentric helically coiled tubes. The HEX at the top of the downcomer maximizes the static pressure head for natural circulation. In addition to the independent emergency shutdown (RSS) and reactor control (RC), the core negative temperature reactivity feedback safely decreases the reactor thermal power, following modest increases in the temperatures of UN fuel and in-vessel liquid sodium. The decay heat is removed from the core by natural circulation of in-vessel liquid sodium, with aid of the liquid metal heat pipes laid along the reactor vessel wall, and the passive backup cooling system (BCS) using natural circulation of ambient air along the outer surface of the guard vessel wall. This paper investigates modifying the SLIMM-1.0 reactor design to lower the UN fuel enrichment. To arrive at a final reactor design (SLIMM-1.2), the performed neutronics and reactivity depletion analyses examined the effects of various design and material choices on both the cold-clean and the hot-clean excess reactivity, the reactivity shutdown margin, the full power operation life at 100MWth, the fissile production and depletion, the Beginning-Of-Life (BOL) spatial distributions of the fission power in the core at 100MWth, and the negative temperature reactivity feedback effects. Besides decreasing the UN fuel enrichment, other parameters examined are the materials of the followers to the B4C rods for RSS and RC, the rods in the radial blanket assemblies, and the thickness of the scalloped BeO walls of the UN fuel and radial blanket assemblies. Despite ∼13% lower fuel enrichment (15.35%) and ∼22% lower hot-clean excess reactivity ($6.29 versus $8.06 for SLIMM-1.0 at 100MWth), the operation life of the SLIMM-1.2 is ∼6.8% longer (6.3 versus 5.9 FPY for SLIMM-1.0), and the total negative temperature reactivity feedback is slightly smaller. At BOL, the radial blanket assemblies in ring 4, and the six UN fuel assembles in ring 1 of the SLIMM-1.2 core generate 2% and 23% of the total reactor thermal power, respectively. The twelve and eighteen UN fuel assemblies in rings 2 and 3 of the SLIMM-1.2 core generate 38% and 37% of the reactor thermal power, respectively. At EOL, the thermal power generation by the UN assemblies in rings 1 and 2 of the SLIMM-1.2 core decreases to 21.5% and 35.9%, respectively, while that by the assemblies in rings 3 and 4 increases to 37.6% and 5%, respectively.

Frozen start-up performance of a high temperature special shaped heat pipe suitable for solar thermochemical reactors

Renewable but intermittent solar energy, typically marked as a sustainable chemical fuel, has been tremendously used to drive high-temperature thermochemical processes via concentrated solar irradiation. Thus, a novel solar thermochemical reactor which integrated with a high temperature special shaped heat pipe (HTSSHP) using sodium as the working fluid is proposed. The HTSSHP includes a flat disk-shape evaporator and multiple cylindrical condensers. The frozen start-up performance of a second generation of HTSSHP with wall thickness reduction has been tested in an electric heater which showed that the frozen start-up process of the HTSSHP is different from the flat-front model of most traditional liquid metal heat pipes, but the continuum vapor flow regime transition velocity is in the same level. Meanwhile, the transition temperature doesn’t appear with the peak value of temperature variation rate, but at the down peak with about 10–25°C/min for heat flux of 82.5kW/m2. The second generation of HTSSHP can start up successfully under tested heat fluxes and inclination angles without any heat transfer limits occurred, which shows the feasibility of operating HTSSHP under solar dish conditions. Meanwhile, the start-up time is largely reduced compared with the first generation, and it is about 41min under heat flux of 82.5kW/m2.

Preliminary design and thermal analysis of a liquid metal heat pipe radiator for TOPAZ-II power system

An alkali metal heat pipe radiator is proposed for the Russian TOPAZ-II space reactor power system to replace the original pumped loop radiator with three advantages. First, the single point failure problem of the original radiator can be avoided. Second, because only the NaK manifold needs to be armored, the specific mass would be reduced and the reliability and survivability can be improved. Third, the heat pipe has nearly isothermal characteristics and high heat transfer capabilities. In the present paper, the high temperature heat pipes using potassium as working fluid, wire screen mesh as wick layer, made of stainless steel are adopted to remove waste heat by radiation. An integral carbon-carbon fin covering and connecting heat pipes as whole heat transfer radiator is selected for its higher thermal conductivity and lower self-weight. A detailed steady state numerical calculation using the finite element method coupled with finite difference method is performed to predict the heat rejection characteristics of the heat pipe radiator. Heat transfer performances of the new concept heat pipe radiator and the pumped loop radiator are compared with key parameters. The results show that the designed heat pipe radiator satisfies the waste heat rejection requirements of the TOPAZ-II power system under the normal operating conditions and has an ideal redundancy. The isothermal and safety of heat pipe radiator is also better than that of pumped loop radiator.

USES OF LIQUID-METAL AND WATER HEAT PIPES IN SPACE REACTOR POWER SYSTEMS

Water and liquid-metal heat pipes are considered for redundant and passive cooling of nuclear reactors and redundant operation of light-weight heat rejection radiators of space reactor power systems. The systems operate continuously for many years independent of the Sun, are enabling of deep space exploration and could provide 10’s of kilowatts of electrical power for lunar and Martian outposts. Space reactors typically operate at 900 K – 1400 K could be cooled using liquid-metal heat pipes with either sodium (900 –1100 K) or lithium (1100 K–1400 K) working fluids. The heat rejection temperatures dictate the working fluid of the radiator heat pipes; potassium for 600 – 800 K, rubidium for 500 – 700 K, and water for < 500 K. These working fluids would be frozen prior to starting up the power systems in orbit or at destination. The startup of water and liquid-metal heat pipes from a frozen state is complex, involving a number of non-linear mass and heat transport processes that require implementing appropriate procedures, depending on the type of working fluid. This paper reviews operation and design constraints pertinent to the uses of water and liquidmetal heat pipes in space reactor systems, and the modeling capabilities of the startup from a frozen state. In addition to models validation, results of design optimization of liquid-metal and water heat pipes in a number of space reactor power systems are presented and discussed.

Construction of Sodium Heat-Pipe Furnaces and the Isothermal Characteristics of the Furnaces

Due to an excellent temperature flattening ability, annular sodium heat pipes operating from 500 °C to 1200 °C have been widely used as liners for isothermal furnaces to provide uniform and stable temperature zones. In order to develop the capabilities to fabricate liquid-metal heat pipes, an apparatus for fabricating sodium heat pipes was set up at the National Institute of Metrology (NIM), China. In this paper, the newly developed fabrication apparatus, the detailed procedures for manufacturing sodium heat pipes, the sodium heat pipes, the constructed furnaces for realizing the aluminum freezing point, and their isothermal characteristics are described. The experimental results showed that the biggest temperature differences within 150 mm from the bottom of the thermometer well in an aluminum point cell placed in the sodium heat-pipe furnaces were better than 15 mK, when the temperatures of the furnaces were controlled at approximately 657 °C.