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[en] Nanofluids have been introduced as an alternative to conventional fluids to improve energy efficiency in heat transfer systems. However, their stability problems before and after operation cycles can produce inconsistent results in different heat transfer technologies that use them. This review summarizes different experimental results obtained using nanofluids in heat pipes, particularly in two-phase closed thermosyphons, and it focuses on the role of preparation and stability issues of nanofluids before and after their use in these devices. Additionally, the effects of nanofluids on heat pipes’ thermal performance were compiled and compared from available experimental studies in the literature. Nanoparticles’ deposition on the evaporator surface and wick or groove structures were the most common mechanism to explain the reported increase or decrease in the thermal performance of heat pipes. This review also identifies the research problems that need to be solved in order to use nanofluids that outperform conventional fluids in heat pipes.
[en] In a two-step approach the applicability of 10 m long two-phase closed thermosyphons (TPCT) is investigated for a passive heat removal system for spent fuel pools. The basic operational behavior of TPCT is measured for predefined thermal conditions at various pipe diameters (20, 32 and 45 mm) and pipe filling ratios in a laboratory setup. The influence on the thermal operation and the heat flux in dependency on the inner pipe diameter is measured and presented. First, the experiments are performed with direct electric heating and then with indirect water-heating. In the second step, the demonstration facility ATHOS (Atmospheric THermosyphon cooling System) with water tank heating and ambient air cooling is built, in order to investigate in a small-scale model experiment the heat transfer performance of TPCTs towards application-oriented thermal conditions of a spent fuel pool (SFP). First results of the ATHOS experiments are presented, demonstrating the applicability of a TPCT bundle using the ambient air as ultimate heat sink.
[en] Stable nanofluids based on DG-100 grade carbon black and carbon nanotubes have been prepared, and their influence on the maximum heat-transfer capacity and thermal resistance of closed-loop two-phase thermosyphons (TPTs) intended for electronics cooling have been studied. A more than twofold increase in the critical heat flux of these TPTs as compared to those filled with water has been obtained along with a sharp decrease in their thermal resistance. It is suggested that this effect is not only due to the high thermal conductivity of the proposed nanofluids, but is also related to the formation of a specific porous structure hindering the appearance of a vapor film and enhancing the boiling process.
[en] The investigation into a full-scale 27 m high, by 6 m wide, thermosyphon loop. The simulation model is based on a one-dimensional axially-symmetrical control volume approach, where the loop is divided into a series of discreet control volumes. The three conservation equations, namely, mass, momentum and energy, were applied to these control volumes and solved with an explicit numerical method. The flow is assumed to be quasi-static, implying that the mass-flow rate changes over time. However, at any instant in time the mass-flow rate is constant around the loop. The boussinesq approximation was invoked, and a reasonable correlation between the experimental and theoretical results was obtained. Experimental results are presented and the flow regimes of the working fluid inside the loop identified. The results indicate that a series of such thermosyphon loops can be used as a cavity cooling system and that the one-dimensional theoretical model can predict the internal temperature and mass-flow rate of the thermosyphon loop.
[en] This paper presents the derivation of a closed form solution for density driven natural circulation flows (i.e. – thermosiphon) in a thermal system typical of power plants. The assumptions and methodology to properly define the mathematical system are described and an example provided. The solution is shown for a common power plant system. (author)
[en] Kilopower is a power generation system based on nuclear processes for Space, Moon and Mars applications. The system is designed to operate from 1 to 10 kWe and is expected to occupy the gap between fission power systems and thermoelectric systems. The heat generated in the reactor core will be transported by alkali metal heat pipe to the Stirling converters for electric power generation. The Stirling converters need a radiator system to dump the excess of heat. This radiator system is also based on heat pipes. This work aims to analyze the thermo-hydraulic characteristics of the heat extraction system of the micro-reactor for the Stirling converters. This analysis will be made in Ansys Fluent. As boundary conditions, the operation temperature was fixed in 1023K with a thermal power of 1250W in the evaporator, and the external temperature of the condenser in 973K. The analysis results were very close to the experimental results released by the Advanced Cooling Technologies team (ACT). (author)
[en] Highlights: • Atmospheric plate thermosyphon (APT) cooling solar cells was first proposed. • APT can reduce the temperature of PV panels without parasitic energy consumption. • The maximum temperature difference was less than 6 °C. • The heat transfer resistance at the evaporator is between 0.00486 and 0.02368 K/W. - Abstract: Since the heat pipe has no parasitic energy consumption, it is an important method for cooling the photovoltaic. In this paper, a novel type of atmospheric plate thermosyphon (APT) cooling system has been designed, which can be used for the heat dissipation of the single or low concentrated solar cells. In the experiments, the non-condensable gas (NCG) was collected by a gas reservoir. The coolant, ethanol, formed a liquid film on the porous medium and directly cooled the photovoltaic panel. The effect of various parameters such as heat flux density, tilt angle and inlet temperature have been studied. The results demonstrated that APT cooling system could effectively reduce the temperature of PV cells, and the higher heat flux density was, the shorter start-up time. The temperature of evaporator was uniform, and the larger inclined angle was, the greater surface temperature difference which maximumly was 5.6 °C. The thermal resistance at the evaporator was between 0.00486 and 0.02368 K/W.
[en] Two-phase thermosyphons, which use sodium at 960 K as a refrigerant for heat transportation in small nuclear reactors, are promising for manned exploration to Mars. This is because the concentric-tube type thermosyphon may not have a flooding limit, and so the heat transfer performance per unit volume is comparatively large. Moreover, since its external form is a single tube, the reactor core can be made smaller. Experimental investigations of the energy transport properties when using water, R113, ethanol, and nitrogen for the refrigerant, as well as formulation of prediction relations, are progressing for establishing the design of the concentric-tube two-phase thermosyphon. However, the model needs to be improved and fluidization phenomena must be clarified in order to establish a model for predicting the maximum heat transfer rate of the thermosyphon which uses sodium at 960 K as a refrigerant. We have proposed a model based on bubble pump theory in order to take into consideration the rise of liquid level of the heating section. We conducted an experimental study on the flow inside a thermosyphon made of transparent material, and evaluated the maximum heat transfer rate using a low boiling point refrigerant, HFE-7100. As a result, even when the heat transfer rate was close to the maximum, it was shown that the flow in the outer tube of the adiabatic section of the concentric tube is two-phase flow. However, the experimental value of the maximum heat transfer rate was found to be about 30% smaller than the calculated value. We therefore investigated the cause of this difference by making pressure measurements and flow observations, and found that bubble entrainment in the inner tube of the concentric tube of the adiabatic section greatly influences the pressure distribution. It is thought that taking mixing into consideration would help improve the model. (author)