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atw Vol. 62 (2017) | Issue 7 ı July 484 AMNT 2017 ı COMPETENCE PRIZE | | Fig. 3. Thermal non-steady-state operating thermosiphon. and transports the heat by sensitive instead of latent heat transfer. Superheated vapor bubbles grow to the dimension of the pipe diameter and suddenly push the liquid above in the condenser section (geyser boiling). At this moment the evaporator temperature drops to its initial value and the temperature in the adiabatic section and condenser rises abruptly. Similar fluctuations depending on the overheated pool can also be observed in the pressure measurements. Besides these periodical temperature and pressure fluctuations of geyser boiling, this boiling regime is also recognizable by an audible cracking sound within the pipe. The cooling circuit expeditiously removes the heat from the condenser and the initial temperatures are reached again. The procedure of overheating and flashing starts all over again. This pulsating operation of the thermosiphon could be the result of multiple parameters and influences, which limit the performance and prevent the pipe from stable operation. For instance, the surface of the inner pipe wall of the thermosiphon is possibly not rough enough and the resulting lack of evaporation nuclei effects the overheating of the liquid pool. Furthermore, the high driving temperature difference seems to demand high heat transfer rates for a steady-state operation. Each driving temperature difference is related to a certain heat transport performance. This oscillating geyser operation mode is known to occur especially for high filling ratios and high density differences [7, 10]. difference between the heat source i.e. an average evaporator wall temperature and the heat sink by means of the inlet temperature of the double-pipe chiller is shown. A vertical pipe with 32 mm inner diameter and a filling ratio of 100 % is investigated for varying heating power from 1.3 kW to 2 kW and different coolant inlet temperatures from 10 °C to 30 °C. For each coolant temperature, a linear trend between the transferred heat and the driving temperature is observed. An increased heat transfer rate leads to an increase in the overall temperature difference. By comparing the overheated evaporator wall, the temperature difference between evaporator and condenser in the pipe and the temperature difference in the chiller it becomes apparent that the increase of the driving temperature difference is mainly due to an increase in the wall overheating of the evaporator. The temperatures in evaporator and condenser section in the pipe even converge with increasing heat transfer rate. Another apparent trend shown in Figure 4, is that an increase in the coolant inlet temperature in connected to a decrease in the overall temperature difference at same heat transfer rate. At a constant driving temperature difference, the measurements at higher coolant temperatures and therefore higher heat transfer rates showed an increase in the stability of temperatures and pressures. It is assumed, that low operation temperature in combination with low heat transfer rates promote fluctuations in the temperatures and geyser boiling. The spreading of measurements shown in Figure 4 is presumably provoked by variations in the volume flow of the coolant. The volume flow rate in the secondary cooling circuit is manually adjusted and for all shown results approximately 4.8 l/min. Conclusion and future work The thermal operating characteristic of long thermosiphons (10 m) were investigated varying their filling ratio (100 %, 70 %) and their thermal boundary conditions considering the operating conditions for passive heat removal in spent fuel pools. So far the experimental results imply a tendency that the filling ratio of 70 % operates at higher performances than the 100 % FR for same diameters. With larger pipe diameter, the transferred heat increases by trend, according to the increase of the transfer area. Overall, the heat transfer performance increases with higher driving temperatures. Especially at low operating temperatures, the thermosiphon operates in a periodic geyser boiling. In this context, the minimum driving temperature difference has to be Influence of the heat sink In Figure 4 the influence of the heat sink on the heat transfer rate and the corresponding driving temperature | | Fig. 4. Transferred heat vs. temperature difference between heat source and heat sink for varying inlet temperatures in the double-pipe cooler. AMNT 2017 Experimental Investigation on Passive Heat Transfer by Long Closed Two-Phase Thermosiphons ı Claudia Graß, Rudi Kulenovic and Jörg Starflinger
atw Vol. 62 (2017) | Issue 7 ı July investigated. For abnormal and accidental operation modes of a spent fuel pool steady-state operation of the thermosiphon was reached. Experiments for normal operation pool temperature (45 °C) triggered geyser boiling without exception. The investigation of the operational behavior of long thermosiphons (≥10 m) is still in progress and the influencing parameters, like filling ratio and geometry will be further examined. An explicit relation between the heat transfer performances of the thermosiphons at defined temperature boundary conditions cannot be derived yet. It is expected that the performance limits of the thermosiphon are significantly higher than the reached steady-state operations. Acknowledgement The presented work is funded by the German Federal Ministry of Economic Affairs and Energy (BMW, project no. 1501515) on basis of a decision by the German Bundestag. References [1] D.A. Reay, P.A. Kew, Heat pipes: theory, design and applications, 2006. [2] A. Faghri, Heat Pipes Science and Technology, Taylor&Francis, London, 1995. [3] M. Groll, S. Rösler, Operation Principles and Performance of Heat Pipes and Closed Two-Phase Thermosyphons, J. Non-Equilib. Thermosyn. (17), 91-151, 1992. [4] Zhenqin Xiong, Cheng Ye, Minglu Wang, Hanyang Gu, Experimental study on the subatmospheric loop heat pipe passive cooling system for spent fuel pool, Progress in Nuclear Energy (79) 40-47, 2015. [5] Zhenqin Xiong, Hanyang Gu, Minglu wang, Ye Cheng, The thermal performance of a loop-type heat pipe for passively removing residual heat from spent fuel pool, Nuclear Engineering and Design (280) 262-268, 2014. [6] B. Jiao, L.M. Qiu, Z.H. Gan, X.B. Zhang, Determination of the operation range of a vertical two-phase closed thermosiphon, Heat Mass Transfer (48), 1043-1055, 2012. [7] Kate Smith, Samuel Siedel, Anthony J. Robinson, Roger Kempers, The effects of bend angle and fill ratio on the performance of a naturally aspired thermosiphon, Applied Thermal Engineering, 2016. [8] M. Kannan and E. Natarajan, Thermal Performance of a two-phase closed thermosiphon for waste recovery system, Journal of Applied Sciences (10), issue 5, 413-418, 2010. [9] J. Seo, J.-Y. Lee, Length effect on entrainment limitation of vertical wickless heat pipe, Int. J. Heat Mass Transfer (101), 373-378, 2016. [10] I. Khazaee, R. Hosseini, S.H. Noie, Experimental investigation of effective parameters and correlation of geyser boiling in a two-phase closed thermosiphon, Appl. Therm. Eng. (30), 406-412, 2010. Authors Claudia Graß Rudi Kulenovic Prof. Dr.-Ing. Jörg Starflinger Universität Stuttgart Institut für Kernenergetik und Energiesysteme (IKE) Pfaffenwaldring 31 70569 Stuttgart, Germany 485 KTG INSIDE Inside KTG-Nord Veranstaltungsbericht: Leistungsbetrieb, Nachbetrieb, Rückbau und mehr – Herausforderungen und Perspektiven für PreussenElektra Am Abend des 31. Mai 2017 hielt Dr. Erwin Fischer, Mitglied der Geschäftsführung der PreussenElektra GmbH, Hannover, einen Vortrag im Rahmen einer Veranstaltung der Sektion Nord der KTG. Gut 30 KTG-Mitglieder und Gäste konnte der Sprecher der Sektion Nord, Dr. Hans-Georg Willschütz, zum Vortrag begrüßen. Dr. Erwin Fischer gliederte den Vortrag in folgende fünf Teile: i) Die aktuelle Situation der PreussenElektra (PEL) und die Entwicklung in der jüngeren Vergangenheit ii) Das Steuerungsmodell der PEL. iii) Die Situation im Bereich Rückbau. iv) Das neue Geschäftsfeld „Nukleare Dienstleitungen“. v) Externe begleitende Herausforderungen. Zunächst wurde die derzeitige Situation der PEL bezüglich laufender, stillgelegter und im Rückbau befindlicher Kernkraftwerke dargelegt. Wesentliche Kennzahlen der PEL sind z. B. etwa 2.100 Mitarbeiter an den Standorten und in der Zentrale Hannover, mehr als 300 Reaktorbetriebsjahre Erfahrung seit 1971 mit einer kumulierten Erzeugung von über 2.200 TWh (was etwa dem vierfachen Jahresstromverbrauch Deutschlands entspricht), viele Rekorde in den Bereichen Verfügbarkeit und Stromerzeugung von Anlagen wie Brokdorf, Unterweser, Grohnde, Grafenrheinfeld und Isar. Aktuell hat sich PEL mit drei Kompetenzfeldern positioniert: „Leistungsbetrieb“, „Stilllegung und Rückbau“, sowie „Nukleare Dienstleistungen“. Die drei noch in Betrieb befindlichen Anlagen sollen sicher und nachhaltig bis zum gesetzlich festgelegten Abschalttermin betrieben werden. Im zweiten Teil des Vortrags wurde das Steuerungsmodell der PEL dargelegt, welches insbesondere der Sicherstellung von Exzellenz bei der Einhaltung der nuklearen Sicherheit beim Betrieb der Anlagen verpflichtet ist. Hierzu werden vier Steuerungselemente verfolgt: i) Steuerungsziele zum Erreichen von Operational Excellence, ii) Steuerungsebenen: Mensch-Technik- Organisation, iii) Steuerungsprinzip: Kontinuierlicher Verbesserungsprozess und iv) konkrete Verbesserungsmaßnahmen für jedes Jahr. Zum Thema Rückbau ging Dr. Fischer einerseits auf die generischen zeitlichen, organisatorischen und rechtlichen Schritte eines Rückbauprozesses ein. Des Weiteren wurden die spezifischen Zeithorizonte für die einzelnen PEL- Anlagen dargelegt sowie einige größere Detailprojekte – wie z. B. die Zerlegung der Reaktordruckbehälter- Einbauten – beleuchtet. Im vierten Teil des Vortrags wurde das relativ junge Geschäftsfeld „Nukleare Dienstleistungen“ erläutert. Ziel ist es, die umfassende Expertise der PEL in den Bereichen erfolgreicher Betrieb und Rückbau international weiter zu KTG Inside
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