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Study on flow modes of open natural circulation with low height difference under low power conditions
International Journal of Advanced Nuclear Reactor Design and Technology xxx (xxxx) xxx
Contents lists available at ScienceDirect
International Journal of Advanced Nuclear Reactor Design and Technology journal homepage: http://www.keaipublishing.com/en/journals/ international-journal-of-advanced-nuclear-reactor-design-and-technology/
Study on flow modes of open natural circulation with low height difference under low power conditions Li Yi a, *, Sun Jianchuang b, Peng Hang a, Xiang Cheng a, Biao Quan a, Xiaxin Cao b, Zhou Jian b, Ding Ming b a b
Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu, 610041, China Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, Harbin Engineering University, Harbin, 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 March 2019 Received in revised form 27 November 2019 Accepted 2 December 2019 Available online xxx
Open natural circulation systems are widely applied in the field of energy and chemical industries. Under some special conditions, open natural circulation systems with low height difference need to be applied, but research on this type of systems is not sufficient. In this paper, the flow modes of sucha natural circulation system were experimentally studied at low power level by adopting steam heating. In addition, the flow modes and physical phenomena of the natural circulation system were analyzed in detail under different heating power conditions. It is found that there are four flow modes at different power levels in the natural circulation system due to the influence of non-condensable gas and subcooled boiling. The inlet and outlet temperatures of heating section are analyzed in different flow modes. The result shows that the outlet temperature of heating section can be used as a judgment criterion for different flow modes. © 2019 Published by Elsevier B.V. on behalf of Xi'an Jiaotong University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Low height difference natural circulation Open natural circulation Non-condensable gas Subcooled boiling Flow mode
1. Introduction As an important part of the passive safety system, the natural circulation system, especially the open natural circulation system, has been widely used in energy and chemical industries due to its simple structure and inherent safety [1,2]. Many scholars have studied this system in recent years. For example, Kyung et al. [3,4] conducted a detailed study on the two-phase flow characteristics of the open natural circulation system and analyzed the mechanism of periodic flow drift. Guo et al. [5] studied the flow phenomena and mechanism of the open natural circulation system during the startup process. Zhu et al. [6] and Hou et al. [7,8] conducted experimental and mechanistic analysis on the instability of the geyser flow in the natural circulation system. The above-mentioned open natural circulation system has great difference in position and height difference between the cold and heat sources, and the system has large natural circulation capability and strong anti-
* Corresponding author. E-mail address: [email protected] (L. Yi).
Production and Hosting by Elsevier on behalf of KeAi
interference ability. For floating nuclear power plants, due to space geometry limitations, there may be no height difference between the coldsource and heat source of the open natural circulation system, and the height difference between the cold source center and the heat source center is small, resulting in poor natural circulation capability [9]. Even if the outlet fluid of the heating section is not saturated, the flow of the system will be greatly affected by the generation and collapse of the bubble caused by the subcooled boiling.And with the integration and miniaturization of nuclear power equipment, this natural circulation system has broad application prospects.At present, there is little research on this kind of lowheight difference natural circulation system [10,11]. In this paper, the experimental study on the lowheight difference open natural circulation system is carried out at low heating power level. 2. Experimental device and method The experimental device of the low height difference natural circulation loop is shown in Fig. 1. The natural circulation loop mainly includes a heating section, a cooling water circuit, a water tank and an air-cooling tower. The heating section is a verticaltubular heat exchanger, and the heating method is steam heating.The cold pipe section of the cooling water circuit comprises lower horizontal section 1 and 2, which are vertically connected on
https://doi.org/10.1016/j.jandt.2019.12.001 2468-6050/© 2019 Published by Elsevier B.V. on behalf of Xi'an Jiaotong University. This is an open access article under the CC BY-NC-ND license (http://creativecommons. org/licenses/by-nc-nd/4.0/).
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
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Fig. 1. Sketch of experimental loop.
the same horizontal plane.The heat pipe section comprises an upper horizontal section and an inclined section, which are also vertically connected,and the angle between the inclined section and the horizontal plane is 4 . After the cooling water is heated in the heating section, it flows into thewater tank (the intermediate heat sink) through the heat pipe section, and then is circulated and cooled by the air-cooling tower to transfer the heat in the water tank to the atmosphere (the final heat sink). In order to deeply study the flow characteristics of this natural circulation system, a plurality of temperature and pressure measurement points are arranged in the cold and hot pipe sections. The cooling water temperature is measured by K-type armored thermocouple of 1 class precision. The cooling water pressure is measured by pressure sensorof 0.5 class precision. The flow rate is measured by electromagnetic flowmeter of 0.5 class precision. The gas space pressure of the pressure tank is measured by hightemperature pressure sensorof 0.2 class precision. All measurement signals are acquired in real time through the NI data acquisition system. During the experiment, the electric heating system is used to control the pressure of the pressure tank to adjust the heating power of the heating section. The cooling water inlet temperature is regulated by controlling the flow of the fan and the temperature control pipeline in the air-cooling tower. The measurement signals for temperature, pressure and flow in the natural circulation loop are collected in real time. The heat rejection power of the natural circulation loop is calculated based on the heating section enthalpy rise and the natural circulation flow rate. When the subcooled boiling occurs in the heating section, the bubble generated by it is gradually condensed and annihilated in the heat pipe section, and the latent heat is released, causing an increase in the temperature of the heat pipe section fluid. Therefore, when calculating the heat rejection power of the system, the enthalpy corresponding to the measuring point with the highest temperature along the inclined pipe is taken as the outlet enthalpy.
3. Experimental results and analysis 3.1. Flow mode analysis The natural circulation flow is divided into four flow modes according to its oscillation frequency and amplitude. The experimental results are shown in Fig. 2. It can be seen from Fig. 2 that during the pressure tank boosting process, the heating section
Fig. 2. Flow modes of the natural circulation system.
power is gradually increased, and the natural circulation flow of the natural circulation system experiences four different flow modes, respectively, steady flow (mode 1), periodiclow frequency oscillation (mode 2), continuous high frequency oscillation (mode 3) and intermittent high frequency oscillation (mode 4). The stability of the system is described by the stability threshold [12], that is, the ratio of the flow fluctuation valueDG to the average valueG, which is 5% in this paper. When DG =G < 5%, system flow is defined as stable; when DG =G 5%, system flow is defined as oscillation. 3.1.1. Steady flow When the inlet temperature is 21 C and the pressure tank pressure is stable at 0.46 MPa, the system is in a steady flow state.In this flow mode, the variation of parameters with time is shown in Fig. 3. As can be seen from Fig. 3(a), the fluctuation amplitude of the natural circulation flow is less than 5% (the stability threshold). The outlet temperature of the heating section is 67 C. The temperature fluctuation of T2 is up to 4 C, and that of T3, T4 and T5 is within 1 C. In the steady flow mode, sincethe natural circulation flow rate issmall, the fluid at the outlet of the heating section is not uniformly mixed, resulting in a large amplitude of the T2 temperature fluctuation. After passing through the 90 elbow, the fluid is sufficiently
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
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Fig. 4. Parameter changes with time in periodic low frequency oscillation mode.
the incubation period, and then enter the heat pipe section, causing a large flow oscillation.
Fig. 3. Parameter changes with time in steady flow mode.
mixed, so the fluctuationamplitudes of T3, T4 and T5 are reduced and the temperature valuesare substantially the same. 3.1.2. Periodic low frequency oscillation When the inlet temperature is 21 C and the pressure tank pressure is stable at 0.61 MPa, the flow rate of natural circulation oscillates periodically at low frequencies.In this flow mode, the variation of parameters with time is shown in Fig. 4. As can be seen from Fig. 4, the maximum fluctuation amplitude of the natural circulation flow is equal to 5% (the system stability threshold), and the flow rate periodically oscillates witha period between 100 and 180s. The outlet temperature of the heating section is about 75 C, and it fluctuates periodically.The amplitude of the fluctuation of T3T5 is within 3 C. The fluctuation frequency of the heating section outlet temperature is the same as the oscillation frequency of the flow rate, but there is a certain delay. In addition, in this flow mode, the maximum fluctuation amplitude of the inlet and outlet pressure of the heating section is increased to 10 kPa, and the pressure fluctuation occurs in synchronization with the flow fluctuation. The natural circulation system emits heat from the circulating cooling water to the atmosphere by circulating cooling in an aircooling tower. Therefore, the non-condensable gas in the cooling water at the inlet of the heating section cannot be eliminated and continues to exist. At the same time, because the height difference between the cold source center and the heat source center is small, the driving forces of the flow is small, and the natural circulation flow is small. Part of the non-condensable gas will accumulate at the top of the heating section header to form larger bubbles during
3.1.3. Continuous high frequency oscillation When the inlet temperature is 21 C and the pressure tank pressure is stable at 0.71 MPa, the natural circulation flow is in continuous high frequency oscillation mode, and the variation of parameters with time is shown in Fig. 5. As can be seen from Fig. 5, the amplitude of the natural circulation fluctuation is equal to 5% (the system stability threshold), and the oscillation period is between 5 and 10s. The outlet temperature of heating section is 82 C, and there is fluctuation. The amplitude of the fluctuation of T3-T5 is within 2 C. The pressure at the inlet and outlet of the heating section is continuously oscillated at a high frequency with an amplitude of 10 kPa. Compared with the periodic lowfrequency oscillation, the oscillation frequency of this flow mode increases, but the oscillation amplitude is substantially unchanged. As the pressure of the pressure tank increases, the heat flux of the heating section increases, and subcooled boiling occurs in the heating section. Due to the high degree of subcooling of the main fluid in the heating section(DTsub > 19 C),bubbles produced by subcooled boiling are completely condensed and collapsed. Therefore, the frequency of the natural circulation flow oscillation
Fig. 5. Parameter changes with time in continuous high frequency oscillation mode.
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
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increases, but the amplitude does not change significantly, which is consistent with the experimental conclusion of Yang Ruichang(2005).
3.1.4. Intermittent high frequency oscillation When the inlet temperature is 21 C and the pressure tank pressure is stable at 0.8 MPa, the natural circulation flow exhibitsintermittent high frequency oscillation.In this flow mode,the variation of parameters with time is shown in Fig. 6.As can be seen from Fig. 6, the amplitude of the natural circulation flow fluctuation is greatly increased, and the maximum oscillation amplitude can reach 35% of the average value.The flow oscillation is intermittent, and in the oscillation region,the oscillation periodis about 2.5s. In addition, thefluctuation amplitude of the heating section outlet temperature T3 is increased to 5 C, and T3 is up to 92 C.The pressure fluctuation frequency of the inlet and outlet of the heating section is the same as the flow rate, and the oscillation amplitude is increased to 20 kPa.The minimum value of theheating section outlet pressure P1 is 2.28 kPa, indicating that there is a certain vapor contentat the heating sectionoutlet and it condenses at the position of P1 to form an instantaneous negative pressure. According to the above analysis, in this flow mode, both the heating sectionoutlet temperature and the natural circulation flow rate increase as the power level increases, thereby increasing the amount of precipitation of the non-condensable gas.At the same time, as the degree of supercooled boiling increases, so does the production of bubbles that cannot be completely condensed.The non-condensable gas and vapor accumulate at the top of the header simultaneously.And after anincubation period of accumulation, the bubbles enter the heat pipe section with the fluid.In this flow mode, the volume of the bubble is large, and the instantaneous introduction of the bubble increases the flow resistance of the fluid, resulting in a decrease in the flow rate.The decrease in flow rate leads to an increase in the heating sectionoutlet temperature, and then the vapor content increases.However, since the mainstream fluid is still subcooled (DTsub > 6 C), most of the bubbles produced by the subcooled boiling are condensed and annihilated, so the natural circulation flow still exhibits high frequency oscillation in the oscillation region.In this flow mode, as the amount of bubble generation and annihilation increases, so does the amplitude of the flow oscillation.
Fig. 6. Parameter changes with time in intermittent high frequency oscillation mode.
3.2. Flow mode transition criterion The low height differenceopen natural circulation system has four flow modes when the heating sectionoutlet temperature is less than 92 C.This paper gives the temperature distribution of the inlet and outlet of the heating section at different flow mode transition points, as shown in Fig. 7. It can be seen from Fig. 7 that for different inlet temperatures,the outlet temperature T3 at flow mode transition point are substantially the same, regardless of the inlet temperature.The outlet temperature corresponding to each flow mode transition point is 75 C, 80 C and 85 C.That is, when T3 is less than 75 C, the system is in steady flow mode; when T3 is between 75 C and 80 C, the system is in periodic low frequency oscillation mode; when T3 is between 80 C and 85 C, the system is in continuous high frequency oscillation mode; when T3 is between 85 C and 90 C, the system is in intermittent high frequency oscillation mode.According to the above analysis, the mechanism of different flow modes is mainly related to non-condensable gas and subcooled boiling, and they are all greatly affected by the heating sectionoutlet temperature T3.Therefore, at low power levels, the flow mode of the lowheightdifference natural circulation system can be determined by the heating section outlet temperature T3. 4. Conclusion In this paper, the experimental study on the flow mode of lowheightdifference open natural circulation system is carried out at low power level.The flow behavior and mechanism of the system are analyzed in detailunder different flow modes.The following three conclusions are obtained. (1) According to the characteristics of natural circulation flow variation, the system is divided into four flow modes: steady flow, periodic low frequency oscillation, continuous high frequency oscillation and intermittent high frequency oscillation.The periodic lowfrequency oscillation is mainly caused by the precipitation and accumulation of non-condensable gas; the continuous highfrequency oscillation is mainly caused by subcooled boiling; the intermittent highfrequency oscillation is formed by the combination of subcooled boiling and non-condensable gas. (2) The heating sectionoutlet temperature corresponding to different flow mode transition points is different:the
Fig. 7. Transition temperatures of different flow modes.
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
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temperature of the transition point for the steady flow and the periodic lowfrequency oscillation is about 75 C; the temperature of the transition point for the periodic lowfrequency oscillation and the continuous highfrequency oscillation is about 80 C; the temperature of the transition point for the continuous high frequency oscillation and the intermittent high frequency oscillation is about 85 C. (3) The transition of different flow modes is related to the heating sectionoutlet temperature, and is independent of the inlet temperature. Therefore, the outlet temperature can be used as acriterion for determining the flow mode. References [1] T. Zhou, J.J. Li, Z.Y. Ju, et al., The development and study on passive natural circulation, Nucl. Saf. (2013). [2] Y.B. Li, H. ZHANG, J.J. Xiao, T. Jordan, Study of natural convection driven by containment condensation in late phase of accident [J], Ann. Nucl. Energy 114 (2018) 1e10. [3] I.S. Kyung, S.Y. Lee, Experimental observations on flow characteristics in an
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open two-phase natural circulation loop, Nucl. Eng. Des. 150 (1) (1994) 163e176. I.S. Kyung, S.Y. Lee, Periodic flow excursion in an open two-phase natural circulation loop, Nucl. Eng. Des. 162 (2e3) (1996) 233e244. X.Q. Guo, Z.N. Sun, J.J. Wang, Experimental study on start-up characteristics of open natural circulation system, Nucl. Tech. 37 (4) (2014) 69e74. X.T. Zhu, X.X. Cao, M. Ding, et al., Validation of REALP5 code for simulation of geyser flow instability under low pressure natural circulation condition, At. Energy Sci. Tech. 48 (8) (2014) 1406e1410. X.F. Hou, Z.N. Sun, Q.N. Sun, et al., Experimental study on geysering in an open natural circulation system, Harbin Eng. University. 37 (4) (2016) 556e561. X.F. Hou, Z.N. Sun, W.J. Lei, Capability of relap5 code to simulate the thermalhydraulic characteristics of open natural circulation, Ann. Nucl. Energy 109 (2017) 612e625. J.Y. Yu, B.S. Jia, Study of one design of passive residual heat removal system of nuclear reactor, Chi J Nucl Sci Eng 23 (1) (2003) 32e38. J. Peng, L. Yu, Research on the passive residual heat removal system operating characteristic of marine nuclear power plant, Ship Sci Tech 64e67 (2012) 95. S.C. Fan, J.C. Lu, S.N. Peng, et al., Research on characteristics of passive residual heat removal system for modular reactor, Nucl. Power Eng. 35 (1) (2014) 152e155. R.C. Yang, Y.W. Wang, F. Wang, et al., Experimental study of flow instability in a natural circulation system with subcooled boiling, Nucl. Power Eng. 26 (4) (2005) 317e322.
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
Contents lists available at ScienceDirect
International Journal of Advanced Nuclear Reactor Design and Technology journal homepage: http://www.keaipublishing.com/en/journals/ international-journal-of-advanced-nuclear-reactor-design-and-technology/
Study on flow modes of open natural circulation with low height difference under low power conditions Li Yi a, *, Sun Jianchuang b, Peng Hang a, Xiang Cheng a, Biao Quan a, Xiaxin Cao b, Zhou Jian b, Ding Ming b a b
Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu, 610041, China Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, Harbin Engineering University, Harbin, 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 March 2019 Received in revised form 27 November 2019 Accepted 2 December 2019 Available online xxx
Open natural circulation systems are widely applied in the field of energy and chemical industries. Under some special conditions, open natural circulation systems with low height difference need to be applied, but research on this type of systems is not sufficient. In this paper, the flow modes of sucha natural circulation system were experimentally studied at low power level by adopting steam heating. In addition, the flow modes and physical phenomena of the natural circulation system were analyzed in detail under different heating power conditions. It is found that there are four flow modes at different power levels in the natural circulation system due to the influence of non-condensable gas and subcooled boiling. The inlet and outlet temperatures of heating section are analyzed in different flow modes. The result shows that the outlet temperature of heating section can be used as a judgment criterion for different flow modes. © 2019 Published by Elsevier B.V. on behalf of Xi'an Jiaotong University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Low height difference natural circulation Open natural circulation Non-condensable gas Subcooled boiling Flow mode
1. Introduction As an important part of the passive safety system, the natural circulation system, especially the open natural circulation system, has been widely used in energy and chemical industries due to its simple structure and inherent safety [1,2]. Many scholars have studied this system in recent years. For example, Kyung et al. [3,4] conducted a detailed study on the two-phase flow characteristics of the open natural circulation system and analyzed the mechanism of periodic flow drift. Guo et al. [5] studied the flow phenomena and mechanism of the open natural circulation system during the startup process. Zhu et al. [6] and Hou et al. [7,8] conducted experimental and mechanistic analysis on the instability of the geyser flow in the natural circulation system. The above-mentioned open natural circulation system has great difference in position and height difference between the cold and heat sources, and the system has large natural circulation capability and strong anti-
* Corresponding author. E-mail address: [email protected] (L. Yi).
Production and Hosting by Elsevier on behalf of KeAi
interference ability. For floating nuclear power plants, due to space geometry limitations, there may be no height difference between the coldsource and heat source of the open natural circulation system, and the height difference between the cold source center and the heat source center is small, resulting in poor natural circulation capability [9]. Even if the outlet fluid of the heating section is not saturated, the flow of the system will be greatly affected by the generation and collapse of the bubble caused by the subcooled boiling.And with the integration and miniaturization of nuclear power equipment, this natural circulation system has broad application prospects.At present, there is little research on this kind of lowheight difference natural circulation system [10,11]. In this paper, the experimental study on the lowheight difference open natural circulation system is carried out at low heating power level. 2. Experimental device and method The experimental device of the low height difference natural circulation loop is shown in Fig. 1. The natural circulation loop mainly includes a heating section, a cooling water circuit, a water tank and an air-cooling tower. The heating section is a verticaltubular heat exchanger, and the heating method is steam heating.The cold pipe section of the cooling water circuit comprises lower horizontal section 1 and 2, which are vertically connected on
https://doi.org/10.1016/j.jandt.2019.12.001 2468-6050/© 2019 Published by Elsevier B.V. on behalf of Xi'an Jiaotong University. This is an open access article under the CC BY-NC-ND license (http://creativecommons. org/licenses/by-nc-nd/4.0/).
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
2
L. Yi et al. / International Journal of Advanced Nuclear Reactor Design and Technology xxx (xxxx) xxx
Fig. 1. Sketch of experimental loop.
the same horizontal plane.The heat pipe section comprises an upper horizontal section and an inclined section, which are also vertically connected,and the angle between the inclined section and the horizontal plane is 4 . After the cooling water is heated in the heating section, it flows into thewater tank (the intermediate heat sink) through the heat pipe section, and then is circulated and cooled by the air-cooling tower to transfer the heat in the water tank to the atmosphere (the final heat sink). In order to deeply study the flow characteristics of this natural circulation system, a plurality of temperature and pressure measurement points are arranged in the cold and hot pipe sections. The cooling water temperature is measured by K-type armored thermocouple of 1 class precision. The cooling water pressure is measured by pressure sensorof 0.5 class precision. The flow rate is measured by electromagnetic flowmeter of 0.5 class precision. The gas space pressure of the pressure tank is measured by hightemperature pressure sensorof 0.2 class precision. All measurement signals are acquired in real time through the NI data acquisition system. During the experiment, the electric heating system is used to control the pressure of the pressure tank to adjust the heating power of the heating section. The cooling water inlet temperature is regulated by controlling the flow of the fan and the temperature control pipeline in the air-cooling tower. The measurement signals for temperature, pressure and flow in the natural circulation loop are collected in real time. The heat rejection power of the natural circulation loop is calculated based on the heating section enthalpy rise and the natural circulation flow rate. When the subcooled boiling occurs in the heating section, the bubble generated by it is gradually condensed and annihilated in the heat pipe section, and the latent heat is released, causing an increase in the temperature of the heat pipe section fluid. Therefore, when calculating the heat rejection power of the system, the enthalpy corresponding to the measuring point with the highest temperature along the inclined pipe is taken as the outlet enthalpy.
3. Experimental results and analysis 3.1. Flow mode analysis The natural circulation flow is divided into four flow modes according to its oscillation frequency and amplitude. The experimental results are shown in Fig. 2. It can be seen from Fig. 2 that during the pressure tank boosting process, the heating section
Fig. 2. Flow modes of the natural circulation system.
power is gradually increased, and the natural circulation flow of the natural circulation system experiences four different flow modes, respectively, steady flow (mode 1), periodiclow frequency oscillation (mode 2), continuous high frequency oscillation (mode 3) and intermittent high frequency oscillation (mode 4). The stability of the system is described by the stability threshold [12], that is, the ratio of the flow fluctuation valueDG to the average valueG, which is 5% in this paper. When DG =G < 5%, system flow is defined as stable; when DG =G 5%, system flow is defined as oscillation. 3.1.1. Steady flow When the inlet temperature is 21 C and the pressure tank pressure is stable at 0.46 MPa, the system is in a steady flow state.In this flow mode, the variation of parameters with time is shown in Fig. 3. As can be seen from Fig. 3(a), the fluctuation amplitude of the natural circulation flow is less than 5% (the stability threshold). The outlet temperature of the heating section is 67 C. The temperature fluctuation of T2 is up to 4 C, and that of T3, T4 and T5 is within 1 C. In the steady flow mode, sincethe natural circulation flow rate issmall, the fluid at the outlet of the heating section is not uniformly mixed, resulting in a large amplitude of the T2 temperature fluctuation. After passing through the 90 elbow, the fluid is sufficiently
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
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Fig. 4. Parameter changes with time in periodic low frequency oscillation mode.
the incubation period, and then enter the heat pipe section, causing a large flow oscillation.
Fig. 3. Parameter changes with time in steady flow mode.
mixed, so the fluctuationamplitudes of T3, T4 and T5 are reduced and the temperature valuesare substantially the same. 3.1.2. Periodic low frequency oscillation When the inlet temperature is 21 C and the pressure tank pressure is stable at 0.61 MPa, the flow rate of natural circulation oscillates periodically at low frequencies.In this flow mode, the variation of parameters with time is shown in Fig. 4. As can be seen from Fig. 4, the maximum fluctuation amplitude of the natural circulation flow is equal to 5% (the system stability threshold), and the flow rate periodically oscillates witha period between 100 and 180s. The outlet temperature of the heating section is about 75 C, and it fluctuates periodically.The amplitude of the fluctuation of T3T5 is within 3 C. The fluctuation frequency of the heating section outlet temperature is the same as the oscillation frequency of the flow rate, but there is a certain delay. In addition, in this flow mode, the maximum fluctuation amplitude of the inlet and outlet pressure of the heating section is increased to 10 kPa, and the pressure fluctuation occurs in synchronization with the flow fluctuation. The natural circulation system emits heat from the circulating cooling water to the atmosphere by circulating cooling in an aircooling tower. Therefore, the non-condensable gas in the cooling water at the inlet of the heating section cannot be eliminated and continues to exist. At the same time, because the height difference between the cold source center and the heat source center is small, the driving forces of the flow is small, and the natural circulation flow is small. Part of the non-condensable gas will accumulate at the top of the heating section header to form larger bubbles during
3.1.3. Continuous high frequency oscillation When the inlet temperature is 21 C and the pressure tank pressure is stable at 0.71 MPa, the natural circulation flow is in continuous high frequency oscillation mode, and the variation of parameters with time is shown in Fig. 5. As can be seen from Fig. 5, the amplitude of the natural circulation fluctuation is equal to 5% (the system stability threshold), and the oscillation period is between 5 and 10s. The outlet temperature of heating section is 82 C, and there is fluctuation. The amplitude of the fluctuation of T3-T5 is within 2 C. The pressure at the inlet and outlet of the heating section is continuously oscillated at a high frequency with an amplitude of 10 kPa. Compared with the periodic lowfrequency oscillation, the oscillation frequency of this flow mode increases, but the oscillation amplitude is substantially unchanged. As the pressure of the pressure tank increases, the heat flux of the heating section increases, and subcooled boiling occurs in the heating section. Due to the high degree of subcooling of the main fluid in the heating section(DTsub > 19 C),bubbles produced by subcooled boiling are completely condensed and collapsed. Therefore, the frequency of the natural circulation flow oscillation
Fig. 5. Parameter changes with time in continuous high frequency oscillation mode.
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
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increases, but the amplitude does not change significantly, which is consistent with the experimental conclusion of Yang Ruichang(2005).
3.1.4. Intermittent high frequency oscillation When the inlet temperature is 21 C and the pressure tank pressure is stable at 0.8 MPa, the natural circulation flow exhibitsintermittent high frequency oscillation.In this flow mode,the variation of parameters with time is shown in Fig. 6.As can be seen from Fig. 6, the amplitude of the natural circulation flow fluctuation is greatly increased, and the maximum oscillation amplitude can reach 35% of the average value.The flow oscillation is intermittent, and in the oscillation region,the oscillation periodis about 2.5s. In addition, thefluctuation amplitude of the heating section outlet temperature T3 is increased to 5 C, and T3 is up to 92 C.The pressure fluctuation frequency of the inlet and outlet of the heating section is the same as the flow rate, and the oscillation amplitude is increased to 20 kPa.The minimum value of theheating section outlet pressure P1 is 2.28 kPa, indicating that there is a certain vapor contentat the heating sectionoutlet and it condenses at the position of P1 to form an instantaneous negative pressure. According to the above analysis, in this flow mode, both the heating sectionoutlet temperature and the natural circulation flow rate increase as the power level increases, thereby increasing the amount of precipitation of the non-condensable gas.At the same time, as the degree of supercooled boiling increases, so does the production of bubbles that cannot be completely condensed.The non-condensable gas and vapor accumulate at the top of the header simultaneously.And after anincubation period of accumulation, the bubbles enter the heat pipe section with the fluid.In this flow mode, the volume of the bubble is large, and the instantaneous introduction of the bubble increases the flow resistance of the fluid, resulting in a decrease in the flow rate.The decrease in flow rate leads to an increase in the heating sectionoutlet temperature, and then the vapor content increases.However, since the mainstream fluid is still subcooled (DTsub > 6 C), most of the bubbles produced by the subcooled boiling are condensed and annihilated, so the natural circulation flow still exhibits high frequency oscillation in the oscillation region.In this flow mode, as the amount of bubble generation and annihilation increases, so does the amplitude of the flow oscillation.
Fig. 6. Parameter changes with time in intermittent high frequency oscillation mode.
3.2. Flow mode transition criterion The low height differenceopen natural circulation system has four flow modes when the heating sectionoutlet temperature is less than 92 C.This paper gives the temperature distribution of the inlet and outlet of the heating section at different flow mode transition points, as shown in Fig. 7. It can be seen from Fig. 7 that for different inlet temperatures,the outlet temperature T3 at flow mode transition point are substantially the same, regardless of the inlet temperature.The outlet temperature corresponding to each flow mode transition point is 75 C, 80 C and 85 C.That is, when T3 is less than 75 C, the system is in steady flow mode; when T3 is between 75 C and 80 C, the system is in periodic low frequency oscillation mode; when T3 is between 80 C and 85 C, the system is in continuous high frequency oscillation mode; when T3 is between 85 C and 90 C, the system is in intermittent high frequency oscillation mode.According to the above analysis, the mechanism of different flow modes is mainly related to non-condensable gas and subcooled boiling, and they are all greatly affected by the heating sectionoutlet temperature T3.Therefore, at low power levels, the flow mode of the lowheightdifference natural circulation system can be determined by the heating section outlet temperature T3. 4. Conclusion In this paper, the experimental study on the flow mode of lowheightdifference open natural circulation system is carried out at low power level.The flow behavior and mechanism of the system are analyzed in detailunder different flow modes.The following three conclusions are obtained. (1) According to the characteristics of natural circulation flow variation, the system is divided into four flow modes: steady flow, periodic low frequency oscillation, continuous high frequency oscillation and intermittent high frequency oscillation.The periodic lowfrequency oscillation is mainly caused by the precipitation and accumulation of non-condensable gas; the continuous highfrequency oscillation is mainly caused by subcooled boiling; the intermittent highfrequency oscillation is formed by the combination of subcooled boiling and non-condensable gas. (2) The heating sectionoutlet temperature corresponding to different flow mode transition points is different:the
Fig. 7. Transition temperatures of different flow modes.
Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001
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temperature of the transition point for the steady flow and the periodic lowfrequency oscillation is about 75 C; the temperature of the transition point for the periodic lowfrequency oscillation and the continuous highfrequency oscillation is about 80 C; the temperature of the transition point for the continuous high frequency oscillation and the intermittent high frequency oscillation is about 85 C. (3) The transition of different flow modes is related to the heating sectionoutlet temperature, and is independent of the inlet temperature. Therefore, the outlet temperature can be used as acriterion for determining the flow mode. References [1] T. Zhou, J.J. Li, Z.Y. Ju, et al., The development and study on passive natural circulation, Nucl. Saf. (2013). [2] Y.B. Li, H. ZHANG, J.J. Xiao, T. Jordan, Study of natural convection driven by containment condensation in late phase of accident [J], Ann. Nucl. Energy 114 (2018) 1e10. [3] I.S. Kyung, S.Y. Lee, Experimental observations on flow characteristics in an
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Please cite this article as: L. Yi et al., Study on flow modes of open natural circulation with low height difference under low power conditions, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.12.001