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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] Reprocessing of nuclear fuel requires concentration of uranium, plutonium and other active effluent streams at various stages for purification, storage or solidification. This is usually achieved by evaporation and in U.K. plant such processes are often carried out under reduced pressure. For high throughput streams, there are considerable advantages in using vertical thermosiphon systems for evaporation and for recovery of nitric acid. However, data for such systems at reduced pressure is limited and the development by John Brown E and C Ltd of a computer program for reliable prediction of thermosiphon performance was carried out on behalf of British Nuclear Fuels Plc using data from operating plant. (author)
[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] This paper deals with experimental and theoretical investigations on a thermosyphon with throughflow in a figure-of-eight loop. The one-dimensional energy and momentum conservation equations are solved with suitable assumptions and analytical solutions are obtained for the steady-state case for different throughflow inlet and outlet points. The steady-state flow rates obtained with hot and cold leg injections are compared. Comparison of predicted steady-state flow rates with experimental data shows agreement within ±15%. The transient behaviour has been obtained numerically using the finite difference method and compared with experimental data. (author)
[en] Experimental investigations were carried out to study heat transfer characteristics of titanium (commercially pure titanium, TA2)/water two-phase closed thermosyphon (Ti/H2O TPCT). Experiments of copper/water (Cu/H2O) TPCT with same dimension and manufacturing process had also been performed for contrast. Experimental results show that there are no remarkable differences of heat transfer coefficients in evaporator (he) between the two kinds of TPCTs, whereas surprisingly the experimental results of heat transfer coefficient in condenser (hc) of Ti/H2O TPCTs are about 2-3 times more than that of Cu/H2O TPCTs, moreover the Nusselt's theoretical correlation based on laminar filmwise condensation is not suitable for simulating the hc of Ti/H2O TPCTs. Experimental results and theoretical analysis of surface free energy difference between condensate and solid surface indicate that the mixed condensation mode with dropwise and filmwise condensation coexisting on titanium surface result in the higher hc for Ti/H2O TPCTs. Experiments on condensation mechanism of titanium surface are ongoing to further validate the point.
[en] A water target was designed to enhance cooling efficiency using a thermosyphon, which is a system that uses natural convection to induce heat exchange. Two water targets were fabricated: a square target without any flow channel and a target with a flow channel design to induce a thermosyphon mechanism. These two targets had the same internal volume of 8 ml. First, visualization experiments were performed to observe the internal flow by natural convection. Subsequently, an experiment was conducted to compare the cooling performance of both water targets by measuring the temperature and pressure. A 30-MeV proton beam with a beam current of 20 μA was used to irradiate both targets. Consequently, the target with an internal flow channel had a lower mean temperature and a 50% pressure drop compared to the target without a flow channel during proton beam irradiation. - Highlights: • The flow mechanisms inside a proton beam–irradiated water target are elucidated. • A flow channel was designed inside the water target to introduce a thermosyphon. • The pressure of a water target with a flow channel is 50% lower than a target without it.
[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°