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atw 2017-12

atw

atw Vol. 62 (2017) | Issue 12 ı December RESEARCH AND INNOVATION 728 | | Fig. 9. Natural circulation flows under different pressures. pseudo-critical point of water (at the corresponding pressure), hence the coolant is observed to be in the pseudo­ gas state. With increasing fluctuations, the system tends to become unstable. 3.4 Transition of stability When the natural circulation system drops from supercritical pressure to subcritical pressure, the natural circulation flow exhibits a certain trend, as shown in Figure 8. As shown in Figure 8a, the black curve represents heating power of the test section, and the blue curve shows the corresponding mass flow rate of natural circulation. In Figure 8b, the black curve shows the temperature at the outlet of the test section, and the red curve is the temperature at the inlet of the test section. As observed from Figure 8a, the system pressure drops from the supercritical to subcritical value, when the time elapses by about 37,337 seconds. The mass flow rate of the experimental loop tends to oscillate rapidly, and the flow state transitions from the stable to the unstable state. This is due to a rapid increase in the specific volume of supercritical water in the loop, as there is a sudden reduction in the system pressure. The vapor fraction is highly susceptible to changes in the pressure (which cause rapid changes), hence there is a large oscillation in the natural circulation flow and the flow exhibits the instability phenomenon. Figure 8b shows changes in the trends of the inlet and outlet temperatures of the test section. As shown in Figure 8b, instability occurs in the natural circulation when the system pressure drops suddenly, and there are rapid fluctuations in the corresponding outlet temperature ( followed by a rapid rise in the outlet temperature of the test section). During this time, these is a rapid decrease in the inlet temperature of the test section. This is owing to the rapid decrease in the mass flow rate of natural circulation (during instability) with a loss of cooling ability in the test section. This gives rise to the outlet temperature of the test section. Henceforth, the heat transfer deterioration is observed in the experimental setup. An interrupt of normal flow (in the natural circulation loop) causes the cooling water (in the descending tube) to be stopped from flowing through the test section and removing the heat. Hence there is a blockage effect in the inlet of the test section. The accumulation of cold fluid leads to a rapid decrease in the inlet temperature of the test section. 3.5 Parametric analysis of natural circulation flow Figure 9 shows the effect of pressure on natural circulation flows. The effect of heating power has been shown correspondingly in Figure 10. As shown in Figure 9, when the heating power is between 8.1 and 9.8 kW, and the inlet temperature is between 228.1 °C and 254.5 °C, the system pressure changes from the subcritical to supercritical state and there is an increase in the natural circulation mass flow. The cycle period is much more longer in subcritical state. On the other hand, the cycle period is much shorter in a supercritical state. Whether in the supercritical or subcritical state, the amplitude of fluctuation tends to remain constant. This is due to the fact that the fluctuations are transferred much more rapidly in the case of higher pressures, hence reflecting in a shorter cycle period. There are fluctuations in the uniform flow rate, which is closely related to the mutual coupling between the driving force | | Fig. 10. Natural circulation flows with different heating power levels. and resistance of the natural circulation loop. Because the system has a consistent flow resistance, the amplitude of fluctuation remains largely unchanged. As shown in Figure 10, when the system pressure is in the 24.2 to 26.1 MPa range, the inlet temperature is between 244.4 °C and 261.2 °C, and an increase in the heating power tends to change the trend of the natural circulation flow rate. The flow rate of the test segment is much lower when the heat power is 4.0 kW. As the heating power increases to 8.25 kW, there is an increase in the amplitude of the natural circulation mass flow rate. And there is an increased level of fluctuations as the heating power is increased, with longer cycle periods. This is due to the increased density difference between the cold and hot segments of the test section, leading to an increase in the driving pressure head and the corresponding mass flow rate. When the heating power is higher, the amplitude of fluctuation becomes larger, translating to insta bility in the natural circulation flow. 4 Conclusion By conducting an experiment on the natural circulation of supercritical water, an investigation has been made on its thermal characteristics. The fluid exhibits different characteristics in the supercritical region. The following conclusions have been drawn from this study: (1) The flow change law is consistent, as compared with the natural circulation flow rates in pipes of larger diameters. The mass flow rate shows an initial increasing trend with an increase in the heating power, which tends to decrease after reach a peak value. In the smaller tube diameter, the Research and Innovation Supercritical Water Natural Circulation Flow Stability Experiment Research ı Dongliang Ma, Tao Zhou, Bing Li and Yanping Huang

atw Vol. 62 (2017) | Issue 12 ı December peak value of the mass flow rate is much lower. (2) The flow characteristics (of natural circulation) are different in the subcritical and supercritical pressures. Compared to the supercritical natural circulation, the amplitude of fluctuations in the flow rate is relatively small in the subcritical state. (3) The mass flow rate tends to fluctuate across a wider range as the heating power is increased, and the natural circulation flow has a much longer period of fluctuation. At this time, the flow tends to be more unstable. (4) It is easier for the flow instability to occur, when the system pressure (of natural circulation flow) is suddenly reduced from the supercritical to subcritical state. The outlet temperature of the test section may exhibit a sudden increase. The blocking effect is easily generated/observed at the inlet of the test section. Acknowledgement This research is financially supported by the Beijing Natural Science Foundation (3172032), Nuclear Reactor Thermal Hydraulic Technology Key Laboratory (20130901) and Key Laboratory of Nuclear Reactor System Design Technology (2015BJ0151). Reference [1] Dipankar N. Basu, Souvik Bhattacharyya, P.K. Das. Performance comparison of rectangular and toroidal natural circulation loops under steady and transient conditions [J]. International Journal of Thermal Sciences. 2012 (57): 142-151. [2] Misale M., Frogheri M. Stablization of a single-phase natural circulation loop by pressure drops [J]. Experimental Thermal and Fluid Science. 2001(25):277-282. [3] Lin Chen, Bi-Li Deng, Xin-Rong Zhang. Experimental study of trans-critical and supercritical CO 2 natural circulation flow in a closed loop [J]. Applied Thermal Engineering. 2013 (59) :1-13. [4] Attila Kiss, Márton Balaskó, László Horváth, Zoltán Kis, Attila Aszódi. Experimental investigation of the thermal hydraulics of supercritical water under natural circulation in a closed loop [J]. Annals of Nuclear Energy . 2017 (100): 178-203. [5] Jain R., Corradini M.L. A Linear Stability Analysis for Natural-Circulation Loops Under Supercritical Conditions [j]. Nuclear Technology. 2006 (9): 312-323. [6] Ebrahimnia E., Chatoorgoon V., Ormiston S.J.. Numerical stability analyses of upward flow of supercritical water in a vertical pipe [J]. International Journal of Heat and Mass Transfer. 2016 (97): 828-841. [7] Chen Y.Z., Zhao M.F., Yang C.S., et al. An experiment on flow and heat transfer characteristics in natural circulation of supercritical water [C]. The 3 rd China-Canada Joint Workshop on Supercritical-Water-Cooled Reactors, CCSC-2012, Xi’an, China, April 18-20, 2012. [8] Misale M., Garibaldi P., Passos J.C., et al. Experiments in a single-phase natural circulation mini-loop [J]. Experimental Thermal & Fluid Science, 2007, 31(8):1111-1120. [9] Li Jingjing, Zhou Tao, Song Ming qiang, et al. CFD analysis of supercritical water flow instability in parallel channels [J]. International Journal of Heat & Mass Transfer, 2015, (86):923-929. [10] Zhou Tao, Li Jingjing, Ju Zhongyun, et al. The Development and Study on Passive Natural Circulation [J]. Nuclear Safety, 20 Authors Dongliang Ma Tao Zhou Bing Li School of Nuclear Science and Engineering, North China Electric Power University, Beijing, 102026, China Institute of Nuclear Thermalhydraulic Safety and Standardization, North China Electric Power University, Beijing, 102206, China Beijing Key Laboratory of Passive Safety Technology for Nuclear Energy, North China Electric Power University, Beijing, 102206, China Yanping Huang Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu, 610041, China RESEARCH AND INNOVATION 729 Konsequenzen für eine vollständig dekarbonisierte Energieversorgung für Deutschland Friedrich Wagner 1 Einleitung 2016 wurde Deutschland Exportweltmeister mit einem Leistungsbilanzüberschuss von 261 Mrd. €. Über seine wirtschaftlichen Fähigkeiten finanziert der deutsche Staat Sozialleistungen, die trotz der günstigen wirtschaftlichen Entwicklung eine Höhe von 918 Mrd. € erreicht haben. In 2016 belief sich der Primärenergieverbrauch Deutschlands auf 3.718 TWh. Mit einem großen Teil dieser Energie wandelt die deutsche Wirtschaft Rohstoffe in auf dem Weltmarkt gesuchte Produkte um. 80,2 % des Primärenergieverbrauchs Deutschlands basieren auf den fossilen Brennstoffen Kohle, Öl und Gas. Aus diesem Grunde liegt Deutschland bei der CO 2 -Emission weltweit auf dem sechsten Platz. Vor Deutschland liegen nur bevölkerungsreichere Länder. In Europa liegt Deutschland bei der gesamten ausgestoßenen Menge an Treibhaus gasen an der Spitze der Emittenten, und bezüglich des pro- Kopf gerechneten Treibhausgasausstoßes belegt Deutschland nach Luxemburg, Estland, Irland und Tschechien den fünften Rang [1]. Die Minderung dieser Emissionswerte ist ein wichtiges Motiv für die Energiewende und die Dekarbonisierung der Wirtschaft, die Deutschland als nationale Aufgaben mit ambitionierten Zielen ausgerufen hat 1 . Trotz seiner Spitzenstellung auf der Emissionsrangliste trägt Deutschland jedoch „nur“ 2,2 % zu den globalen Treibhausgasemissionen bei. Deutschland kann deshalb das Klimageschick der Erde – ob das 2°-Ziel oder das 1,5°-Ziel oder keines von beiden erreicht wird – nicht durch direkte Aktionen beeinflussen. Diese Ohnmacht steht im Widerspruch zum 1) Siehe zum Beispiel Rede der Bundeskanzlerin Angela Merkel anlässlich des 6. Petersburger Klimadialoges am 19. Mai 2015. Research and Innovation Consequences for a Completely Decarbonised Energy Supply for Germany ı Friedrich Wagner