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[en] The critical heat fluxes (CHFs) of two-phase closed thermosyphons with and without fins were studied. The thermosyphons were fabricated using 1.25-mm-thick iron tubes with inner diameters of 16, 21 and 26 mm. The lengths of the evaporator, adiabatic, and condensation sections were 20, 10 and 20 cm, respectively. Pure water, ethanol, and R134a refrigerant were used as the working fluids with 50 % filling rate of the evaporation length. CHF data when using fins of different thicknesses (1.0, 1.5 and 2.0 mm), radii (5, 10 and 15 mm), and spacing (10, 20 and 30 mm) were recorded. The CHF increased with the fin thickness and radius but decreased with the increase in fin spacing. In addition, the CHF increased with the diameter of the thermosyphon tube. Overall, the CHF of thermosyphons with fins was higher than that of thermosyphons without fins regardless of the working fluid
[en] Highlights: • A shaft cooling structure is designed based on loop thermosyphons. • A single loop thermosyphon is studied during heating and cooling of the same tube. • The optimal liquid filling ratio is obtained under the special condition. • Cooling effects of the cooling structure are simulated on the motorized spindle. - Abstract: In this paper, a shaft cooling structure of a grinding motorized spindle was designed based on loop thermosyphons. The evaporation and condensation sections of the loop thermosyphons were located on the same tube due to the thermal conductivity of the shaft. The experimental studies on both heat transfer performance and start-up characteristics of a single loop thermosyphon were performed under the special condition. Then, the cooling effect on the shaft was simulated depending on the obtained experimental data. Results demonstrated that the optimal liquid filling rate of a loop thermosyphon ranged between 50 and 60% under the special condition. Furthermore, a critical value of heating power between 20 W and 40 W was found. When the heating power exceeded this value, the temperature of the evaporation section increased rapidly without any fluctuation. The violent fluctuation of temperature at the upper evaporation section could be utilized as an indicator for the heat transfer limit. Finally, according to the simulation, the maximum temperature of the motorized spindle was reduced by approximately 28% under the effect of the designed cooling structure.
[en] Highlights: • Three parameters are used to evaluate the startup performance. • The startup is faster and overshoot is larger when the distribution is more uneven. • Heating on one evaporator with same Q with the other makes transition time longer. • Heating on one evaporator with same Q with the other makes the overshoot smaller. • Transition time is about twice as much as peak time when peak time exists. - Abstract: Loop thermosyphon with multiple evaporators is a promising device in multi-source heat transfer. The startup performance is very important for its thermal control ability. In this paper, the effect of heating power distribution on the startup of a loop thermosyphon with dual evaporators is investigated experimentally. The startup time and stationarity under different power distributions are analyzed utilizing three parameters: peak time, transition time and temperature (pressure) overshoot. The results show that the startup process is faster and the overshoot of pressure and temperature is larger when the distribution is more uneven; Heating on one evaporator with the same heating power with the other evaporator makes the startup process longer while it makes the overshoot smaller or even disappear; The transition time is approximately twice as much as the peak time when the peak time exists.
[en] There have been many tries to predict heat transfer capability and working limitation of thermosyphon, therefore, there are serval correlations equation show a good agreement with experimental data. However, it is very rare about reports of thermal performance of thermosyphon with relatively long length, which can be applied passive cooling system for nuclear plant. For proper design passive cooling system with thermosyphon for nuclear plant, it is essential that predict thermal performance of relatively long thermosyphon. In this study, we did experiment to figure it out length effect on thermal performance of thermosyphon. In this paper, we showed length effect on thermal performance of thermosyphon with different length. Through present study, we can know following points. 1) There is length effect on count current flooding limitation in thermosyphon, therefore, it should be considered when relatively long thermosyphons would be designed. e.g.) Passive cooling system for nuclear spent fuel pool. 2) Excepting Hashimoto model, which shows 30% error with present experiment results, the other models shows very big difference with current experiment data. So, it should be carefully to predict and design condensation heat transfer when length of thermosyphon is relatively long. 3) The reason, which can effect on big deviance between models and present data for condensation heat transfer, can be consider as different extent of developed film and it can have effect on pressure gradient of consider part, which may influence on condensation heat transfer.
[en] Highlights: • The model considering contact angle for a thermosyphon is developed. • The model with contact angle has lower relative error than without contact angle. • Mechanism of the effect of evaporator wettability on heat performance is discussed. • Mechanism of the effect of filling ratio on heat performance is discussed. • Heat performance is best at filling ratios of 20–30% for hydrophilic evaporator. - Abstract: A thermosyphon is considered an efficient heat dissipation device in engineering fields due to its low thermal resistance. The heat transfer mechanisms for thermosyphons at different evaporator wettability and filling ratios are not well detailed. A model considering evaporator wettability in terms of a contact angle is developed to detail the phase change process to explore the heat transfer mechanism for a thermosyphon in this study. The effects of evaporator wettability and filling ratio on the heat performances of a thermosyphon charged with water are investigated. It is observed that the simulated absolute temperatures with a contact angle are in better agreement with the experimental results with an average relative error of 0.15% than the simulation results without a contact angle (0.28%). The results show that a hydrophilic surface causes bubbles to easily depart the evaporator wall, thereby increasing the heat performance, whereas a hydrophobic surface causes bubbles to adhere to the evaporator wall, decreasing the heat performance. Further study shows that a low filling ratio of 12% will result in drying out, but a high filling ratio of 40% will prevent large bubbles from reaching the liquid surface, thereby decreasing the heat performance. The heat performance is best at filling ratios of 20–30% for an evaporator with a hydrophilic surface.
[en] Highlights: • A performance evaluation index of two-phase thermosyphon loop was proposed. • Real cycles under different conditions were measured and evaluated. • The performance of the “ideal cycle” is the upper limit of all real cycles. • Reasons for lower approaching degree have been identified and illustrated visually. • Ideal cycle can be approached with optimal charge and height difference, no flow drag. - Abstract: For energy conservation, two-phase thermosyphon loops (TPTLs) have been widely used in air conditioning systems. Unfortunately, the evaluation index and design criterion of the TPTL is currently lacking, and causes for poor performance are inconclusive. In this study, an “ideal cycle” (optimal state) of the TPTL was proposed, and an approaching degree ε was defined to evaluate the extent of approaching the “ideal cycle”. Furthermore, a visual experimental platform was developed, and real cycles under different conditions were measured and evaluated. The results indicate that: (1) the performance of the “ideal cycle” is the upper limit of the performances of all real cycles, and the approaching degree provides an effective evaluation index and design criterion; (2) causes for low approaching degree have been identified and illustrated visually, from the perspective of refrigerant distribution, including under-charged refrigerant, over-charged refrigerant, insufficient height difference, and considerable flow resistance; and (3) the “ideal cycle” can be approached, only when the filling ratio (100%) and the height difference (1.2 m) is optimal, and the flow resistance is negligible simultaneously.
[en] Highlights: • A SINDA/FLUINT simulation model of a TPCT charged with R245fa was established. • Good agreement was achieved between simulation model and experimental results. • The effects of relevant factors were investigated on performances of the TPCT. • Performances of the large scale TPCTs were investigated by the developed model. - Abstract: Two-phase closed thermosyphons (TPCTs) are simple and efficient heat exchangers. They have been explored for use in the renewable energy resource utilization marker and low grade thermal energy heat recovery systems. A transient simulation model for a TPCT was established by SINDA/FLUINT with low global warming potential Freon R245fa as working fluid. The TPCT was manufactured from a 40 mm inner diameter (Di), 3 m long smooth copper tube with a wall thickness of 2 mm. It consists of the evaporator, adiabatic and condenser sections with 1 m long for each section. The evaporator section was immersed in a water bath and the condenser section was cooled by recycled water. The effects of water bath temperature Tb and inlet temperature of cooling water Tcw,i were investigated by experimental and simulation methods. The results show the heat transfer rate Q and overall heat transfer coefficient U increase with the increase of Tb, and the decrease of Tcw,i. Good agreement between experimental and simulation results confirms the model is accurate and reliable. The influence of filling ratio (FR) and Di on the performance of TPCT was also investigated. The optimum FR for Di of 30 mm, 40 mm and 50 mm are 15%, 15% and 25% respectively. Moreover performances of 60–150 m long TPCTs were investigated based on the developed model.
[en] The Passive Containment Cooling Systems (PCCSs) were designed to be installed inside the containment and transfer the released heat to the water pool outside the containment. However, the PCCS of AP1000 cannot be applied to the Korean PWRs directly. The penetration adds the risk of radioactive material release by introducing another potential pass way. In order to simplify the PCCS design and eliminate the risk of radioactive material release, the KAIST research team proposed a new PCCS concept, a condenser using a phase change material (PCM) as shown in Fig. 1. The PCM acts as the final heat sink and absorbs the heat. The heat is transferred through the copper thermal conductor. As a simple system, this condenser does not need to penetrate the containment wall for installation. Thus, it has the potential applicability for operating PWRs and can work with other PCCSs to improve the cooling efficiency. The main target nuclear reactor is the APR1400 but it also can be applicable to many other designs. This study aims to see the heat transfer performance of a certain PCM with a given heat transfer structure. PCCS using PCM can be applied to existing nuclear power plants without containment penetration. The Steam to PCM Heat Transfer Experiment setup is planned to test the performance of the PCM in accident conditions. Various PCMs will be tested in different pressure and temperature conditions to secure database and find the optimum material or combination for future application.
[en] Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling is widely used in application of thermal engineering and considerably more efficient than single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circular spots (0.5 - 2 mm in diameter and 1.5 - 3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and low incipience temperature overshoot with water as working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests under heat loads 30 W to 260 W reveal the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation-site density. The surface with hydrophobic spots with diameter 1 mm and pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. A comparison of three evaporator surfaces with identical wettability patterns but with different surface topographies and coating thicknesses is carried out experimentally. The results show superior heat transfer rates and wear resistance on the surface coated with HNTs spots thanks to the large contact angle, great thickness, and durability of the coating layer. (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.