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[en] Highlights: • Annular thermosyphon shows higher heat transfer compared to concentric thermosyphon. • Annular thermosyphon requires new prediction model for flooding limit. • Fill ratio of working fluid affects the flooding limit of the annular thermosyphon. - Abstract: A passive in-core cooling system (PINCs) based on hybrid heat pipe can be adopted to enhance the passive safety of advanced nuclear power plants. A hybrid heat pipe is a heat transfer device that takes the dual roles of neutron absorption and heat removal by combining the functions of a heat pipe and a control rod. To observe the effect of neutron absorber material inside the heat pipe and fill ratio of the working fluid on the thermal performances of heat pipe including operation limit, an annular thermosyphon heat pipe (ATHP) that contains a neutron absorber inside a concentric thermosyphon heat pipe (CTHP) was experimentally studied in the condition of various fill ratios. The ATHP showed lower thermal resistances in the evaporator region with a maximum reduction of 20% compared to those of a CTHP. In terms of the operational limits, the ATHP showed a lower entrainment limit than the CTHP due to a smaller cross-section for vapor path in the evaporator region, which resulted in high shear at the vapor–liquid interface. In addition, increasing the fill ratio enhanced the entrainment limit by 18%.
[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] This paper presents an analytical model for a thermosyphon loop developed for cooling air inside a telecommunication cabinet. The proposed model is based on the combination of thermal and hydraulic management of two-phase flow in the loop. Experimental tests on a closed thermosyphon loop are conducted with different working fluids that could be used for electronic cooling. Correlations for condensation and evaporation heat transfer in the thermosyphon loop are proposed. They are used in the model to calculate condenser and evaporator thermal resistances in order to predict the cabinet operating temperature, the loop's mass flow rate and pressure drops. Furthermore, various figures of merit proposed in the previous works are evaluated in order to be used for selection of the best loop's working fluid. The comparative studies show that the present model well predicts the experimental data. The mean deviation between the predictions of the theoretical model with the measurements for operating temperature is about 6%. Besides, the model is used to define an optimal liquid and vapor lines diameters and the effect of the ambient temperature on the fluid's mass flow rate and pressure drop. - Highlights: • Modeling of thermosyphon loop for cooling telecommunication cabinet. • The cooling system operates with zero electrical consumption. • The new correlations are proposed for condensation and evaporation heat transfer. • FOM equation is defined for selecting the best working fluid. • The proposed model well predicts the experimental data and operating temperature
[en] Several tests have been carried out with two miniature periodic two-phase thermosyphons (PTPTs), which have been developed for thermal control applications. A PTPT is a wickless device which can operates even against gravity. The two PTPTs have the same condenser and accumulator and different evaporators: the evaporator Type A, which can contain a large amount of liquid (20 x 10-6 m3) and which can be tilted up to 90o, and the evaporator Type B, which has an internal volume of 5 x 10-6 m3 and can operate just in horizontal orientation. Their unsteady and their periodic steady state performances have been studied and compared with those of several miniature loop heat pipes quoted in the references. The paper shows that the PTPT thermal resistances are similar to those of miniature LHPs, even those of PTPTs are less influenced by the arrangement of the condenser and the evaporator with respect to the gravity. The thermal resistance, as the PTPT steadily operates, is about 0.55 K/W with a heat load of 110 W. The main experimental observation on the PTPT unsteady behaviour is that their start-ups are rather smooth in the most cases. However the start-up performances does not depend on any configuration and orientation of the loop element.
[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] A new generation of in-flight entertainment systems (IFEs) used on board commercial aircrafts is required to provide more and more services (audio, video, internet, multimedia, phone, etc.). But, unlike other avionics systems most of the IFE equipment and boxes are installed inside the cabin and they are not connected to the aircraft cooling system. The most critical equipment of the IFE system is a seat electronic box (SEB) installed under each passenger seat. Fans are necessary to face the increasing power dissipation. But this traditional approach has some drawbacks: extra cost multiplied by the seat number, reliability and maintenance. The objective of this work is to develop and evaluate an alternative completely passive cooling system (PCS) based on a two-phase technology including heat pipes and loop thermosyphons (LTSs) adequately integrated inside the seat structure and using the benefit of the seat frame as a heat sink. Previous works have been performed to evaluate these passive cooling systems which were based on loop heat pipe. This paper presents results of thermal tests of a passive cooling system of the SEB consisting of two LTSs and R141b as a working fluid. These tests have been carried out at different tilt angles and heat loads from 10 to 100 W. It has been shown that the cooled object temperature does not exceed the maximum given value in the range of tilt angles ±20° which is more wider than the range which is typical for ordinary evolution of passenger aircrafts. -- Highlights: ► A passive cooling system has been developed for avionics application. ► The system consists of loop thermosyphons and a passenger seat as a heat sink. ► Successful system tests have been run at heat loads to 100 W and angle tilts to 20°
[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] Highlights: • Experimental investigations on an internally finned vertical thermosyphon. • Two fluids – water and acetone considered. • Optimum fill ratio determined to be 50%. • Addition of internal fins at the condenser leads to improved thermal performance. - Abstract: This paper reports the results of an experimental investigation of heat transfer from an internally finned thermosyphon charged with either water or acetone. Six constant area fins with a rectangular cross section are placed internally along the length at the condenser section. The ratio of initial liquid pool volume to the evaporator volume, known as the filling ratio in a thermosyphon system, has been varied in this study. Experiments are carried out for filling ratios of 20, 50, and 80% for two working fluids (i) water and (ii) acetone. Results show that a fill ratio of 50% gives better heat transfer performance. Providing internal fins at the condenser produces additional condensation which improves the thermal performance of the thermosyphon by 17% in terms of the temperature reduction at the source and sink and 35.48% in terms of reduction in thermal resistance at lower heat inputs. The thermosyphon is tested between power levels of 50 and 275 W.