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[en] In typical pressurized water reactor (PWR), in case that one steam generator (SG) cannot be credited for the primary cooldown, it is necessary to homogenize primary coolant temperature among loops using at least one reactor coolant pump (RCP) for the plant cooldown. If the natural circulation condition is established due to unavailability of all the RCPs, the continuous cooldown using intact SGs causes to disturb the smooth depressurization because it leads to void generation in the top of the non-cooldown SG tube where the high temperature coolant is remained. For this purpose, W.Sakuma, et al. suggested the outline of asymmetric cooldown procedure without any RCPs restart. Since the suggested procedure is based on only one secondary condition (SG dry-out) of non-cooldown SG, and hence the impact of difference of the secondary condition should be investigated. In this paper, the sensitivity analyses were performed to confirm the impact on the asymmetric cooldown procedure , and consequently , it was confirmed that the coolable range used in the procedure was expanded if the water inventory exists in non-cooldown SG. Therefore it was concluded that the coolable range which was defined with the SG dry-out condition in non-cooldown SG can be conservatively applied for the operating procedure. (author)
[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] Heat pipes are passive heat transfer devices, of long lives. Material and testing reactors (MTRs) have residual heat after shutdown. Usually MTRs have also spent fuel storage tanks to compromise heat that need to be removed. Gravity assisted two-phase closed heat-pipe loop (GTPHL) covered by removal of decay heat (or heat after shutdown) with evaporator and condenser lengths each 100 m helical coil shape with outer diameter 15 cm and 3 mm thickness as a passive cooling system for a nuclear spent fuel storage pool. This study proposes a completely passive cooling system using thermosyphon loop for cooling and dissipation of the residual heat of wet spent fuel storage by running as main or alternative cooling system. The design focuses on heat removal from the spent fuel storage tank of a research reactor. The model considers natural convection by air for the condenser part of the heat-pipe loop to confine the residual heat. A numerical simulation, using special design of GTPHLs, was used to investigate the thermal performance of the GTPHL. The effects of heat loads were analyzed. Demineralized water was used as the GTPHL working fluid. The atmospheric air was circulated around the condenser as a cooling system. The thermal performance of the GTPHL is evaluated at heat input ranging from 25 to 15 degree kW with filling ratio of the working fluid of 100%. The results show that a good thermal performance is obtained at high evaporator heat load obtained from nuclear spent fuel storage tank.
[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 control rod drive mechanism (CRDM) in nuclear power plant is usually cooled by outside forced ventilation to make sure it works properly. The forced ventilation outside the CRDM will lead to thermosyphon phenomenon inside the CRDM which results in the temperature rising along the axial direction of the CRDM. It was reported that the location of the drive rod is one of the most important factors which affect the thermosyphon phenomenon. In this study, the effect of the drive rod's location on the axial heat transfer of CRDM is investigated experimentally through measuring the axial temperature distribution of the CRDM under various ventilation speeds when the drive rod locates at both full height and half height of the CRDM. The experimental results show that the thermosyphon phenomenon is intensified for the full height condition comparing with the half height condition and more cooling capacity is achieved under the same ventilation condition. (authors)
[en] Highlights: • Study of HCs and Freon as TPCT working fluids for renewable energy applications. • Experimental performances of TPCTs were studied with eight working fluids. • R245fa/R152a, R600a, and R1234ze were recommended as substitutes for R134a. • Suitability of typical HTC correlations were analysed for the TPCT working fluids. • A simplified Rohsenow correlation was developed to further improve accuracy. - Abstract: Two-phase closed thermosyphons (TPCTs) are simple, efficient, and low cost heat exchangers. They have been explored for use in the renewable energy resource utilization marker and low grade thermal energy heat recovery systems. Freon R134a has been extensively used in refrigeration systems and researched as a working fluid of TPCTs; however; it has high global warming potential and operating pressure. In this paper, an experimental investigation of the performance of TPCTs charged with eight working fluids: R134a, R601, R245fa, R600a, R1234ze, R152a, R245fa/R152a, and R601/R245fa have been carried out. The experimental results showed that R245fa/R152a offered the best performance in maximum heat transfer rate. R134a outperformed the other pure working fluids, while R600a and R1234ze had close performances to that of R134a. R245fa showed marginal improvement at higher operating temperatures. The predictions of six evaporation heat transfer coefficients (HTCs) correlations, including Imura, Shiraishi, Labuntsov, Kutateladze, Cooper, and Rohsenow were compared with the experimental results. In the five constant coefficients and powers correlations, the Shiraishi and Cooper correlations had superior accuracy. The coefficients and powers of the Rohsenow correlations fitted based on the experimental data, while they had the best accuracy. Nusselt and Hashimoto-Kaminaga correlations were chosen to predict the condensation HTCs. Both of them tend to over-predict the condensation HTCs in low heat fluxes while under-predicting in high heat fluxes. The experimental results had greater agreement with Hashimoto and Kaminaga correlations.