Analysis of Flow Instabilities in Two-Phase Natural Circulation

2009 ◽  
Author(s):  
Geping Wu
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Flow Rate ◽  
Natural Circulation ◽  
Flow Instabilities ◽  
High Heat Flux ◽  
Circulation Flow ◽  
Two Phase ◽  
Drift Flux Model ◽  
Circulation Flow Rate

Safety concerns of nuclear reactors have attracted the attention of researchers on flow instabilities in natural circulation boiling loops. In this theoretical study, a drift flux model which solves the conservation equations of mass, momentum and energy applicable to boiling two-phase natural circulation systems is adopted. The influence of two-phase flow parameters such as drift velocity and void distribution parameter on the loop flow rate is weak. The model is used to analysis the effects of heat flux and inlet subcooling on steady circulation flow rate. High circulation flow rate is accompanied by high heat flux and low inlet subcooling. According to the region and number of meeting points which connects the resistance pressure drop curve and the driving pressure drop curve, flow excursion and density-wave instability sometimes may occur. Further, investigations are carried out to study the effect of heat flux and system pressure on the instabilities region in natural circulation.

Two-Phase Natural Circulation Cooling Performance Assessment and Safety Analysis for APR1400 Core Catcher System

2014 ◽  
Author(s):  
Ki Won Song ◽  
Shripad T. Revankar ◽  
Hyun Sun Park ◽  
Bo Rhee ◽  
Kwang Soon Ha ◽  
...  
Keyword(s):  
Void Fraction ◽  
Flow Rate ◽  
Natural Circulation ◽  
Scaling Analysis ◽  
Circulation Flow ◽  
Two Phase ◽  
Cooling Performance ◽  
The Core ◽  
Core Catcher ◽  
Circulation Flow Rate

The two-phase natural circulation cooling performance of the APR1400 core catcher system is studied utilizing a drift flux flow model developed via scaling analysis and with an air-water experimental facility. Scaling analysis was carried out to identify key parameters, so that model facility could simulates two-phase natural circulation. In the experimental apparatus, instead of steam, air is injected into the top wall of the test channel to simulate bubble formation and void distribution due to boiling water in the core catcher channel. Measurement of void fraction critical to the heat transfer between the wall and coolant is carried out at certain key position using double-sensor conductivity probes. Results from the model provide expected natural circulation flow rate in the cooling channel of the core catcher system. The observed flow regimes and the data on void fraction are presented. For a given design of the down comer piping entrance condition bubble entrainment was observed that significantly reduced the natural circulation flow rate.

Dryout Avoidance Control for Multi-Evaporator Vapor Compression Cycles With Transient Heat Flux

2014 ◽  
Author(s):  
Daniel T. Pollock ◽  
Zehao Yang ◽  
John T. Wen
Keyword(s):  
Heat Flux ◽  
Flow Rate ◽  
Coolant Flow ◽  
Coolant Flow Rate ◽  
High Heat Flux ◽  
Vapor Compression ◽  
Transient Heating ◽  
Outer Loop ◽  
Two Phase ◽  
Loop Control

Multiple-evaporator vapor compression cycles may be used for distributed cooling of high heat-flux systems, such as arrays of high-power electronics. Under transient heating conditions, these systems must be carefully controlled to avoid critical heat flux (CHF) due to evaporator dryout. An active control strategy is presented that regulates two-phase flow quality in multiple evaporators in order to avoid critical quality under transient heating conditions. A two-loop control system is used, in which an outer loop uses model-based feedforward combined with evaporator wall temperature feedback to determine the necessary coolant flow rate to avoid CHF, while an inner loop uses system actuators (variable speed compressor, electronic expansion valves) to track to the desired flow rate. An advantage of this approach is that the inner-loop control handles the system complexity arising from pressure coupling and actuator nonlinearity. Additionally, the outer-loop quality control may be applied to other two-phase cooling schemes, for instance pumped systems, by providing coolant flow rate setpoints. Simulations and corresponding experimental controller validation were conducted using a three-evaporator vapor compression testbed with transient imposed heat-flux.

Numerical Prediction of Cooling Capability in Hemispherical Gap Flow Passage for In-Vessel Core Retention

2002 ◽  
Author(s):  
Akihiro Uchibori ◽  
Kenji Fukuda ◽  
Koji Morita ◽  
Tatsuya Matsumoto
Keyword(s):  
Heat Flux ◽  
Void Fraction ◽  
Two Phase Flow ◽  
High Heat Flux ◽  
Phase Flow ◽  
Narrow Gap ◽  
Two Phase ◽  
Flow Passage ◽  
Drift Flux Model ◽  
Gap Flow

A numerical method for thermal hydraulic phenomena in a hemispherical narrow gap flow passage was developed to evaluate a cooling capability with gap formation between the molten core and the reactor pressure vessel.The gap cooling mechanism was modeled as gas-liquid two-phase flow in the narrow gap with two-dimensional spherical coordinate system. The analytical model is based on a modified drift flux model for multi-dimensional two-phase flow analysis. Numerical results showed that liquid phase intrusion into the gap in the counter direction of gas phase upward flow kept down a rise of void fraction as gap cooling phenomena. Under the high heat flux condition,expansion of the high void fraction region due to the counter-current flow limitation was reproduced as a dryout phenomenon. Characteristics of gap cooling limitation predicted by the numerical analyses were verified by comparison with various experimental data and correlations of critical heat flux.

On the Operational Parameters Effects on Two-Phase Pressure Drop Characteristics of Reentrant Copper Microchannels

2016 ◽  
Author(s):  
Daxiang Deng ◽  
Qingsong Huang ◽  
Yanlin Xie ◽  
Wei Zhou ◽  
Xiang Huang ◽  
...  
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Mass Flux ◽  
Heat Sink ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Two Phase ◽  
Cooling Systems ◽  
Phase Pressure

Two-phase boiling in advanced microchannel heat sinks offers an efficient and attractive solution for heat dissipation of high-heat-flux devices. In this study, a type of reentrant copper microchannels was developed for heat sink cooling systems. It consisted of 14 parallel Ω-shaped reentrant copper microchannels with a hydraulic diameter of 781μm. Two-phase pressure drop characteristics were comprehensively accessed via flow boiling tests. Both deionized water and ethanol tests were conducted at inlet subcooling of 10°C and 40°C, mass fluxes of 125–300kg/m2·s, and a wide range of heat fluxes and vapor qualities. The effects of heat flux, mass flux, inlet subcoolings and coolants on the two-phase pressure drop were systematically explored. The results show that the two-phase pressure drop of reentrant copper microchannels generally increased with increasing heat fluxes and vapor qualities. The role of mass flux and inlet temperatures was dependent on the test coolant. The water tests presented smaller pressure drop than the ethanol ones. These results provide critical experimental information for the development of microchannel heat sink cooling systems, and are of considerable practical relevance.

An Experimental Study on Open Natural Circulation Flow Characteristics by Air Injection

2017 ◽  
Author(s):  
Pengjiu Cao ◽  
Xiaxin Cao ◽  
Zhongning Sun ◽  
Ming Ding ◽  
Na Li ◽  
...  
Keyword(s):  
Flow Rate ◽  
Driving Force ◽  
Flow Instability ◽  
Natural Circulation ◽  
Heat Removal ◽  
Air Injection ◽  
Circulation System ◽  
Circulation Flow ◽  
Churn Flow ◽  
Circulation Flow Rate

An open natural circulation system has the characteristics of a simple structure, superior safety performance and strong heat removal capability. However, during long-term operation, the flow instability may occur due to the reduction of the driving force, which will have adverse effects on the heat removal capability and safe operation of the system. Thus, injecting air into the riser is designed in this paper to improve the driving force of the circulation flow, reduce the possibility of flow instability, and increase the heat removal capability. In order to investigate the influence of air injection on the evolution of flow pattern, resistance characteristics and circulation flow rate, the method of visual observation and data analysis is used based on different pore sizes porous media, air injection rate and submergence ratios. The ratio of the driving pressure head to the resistance pressure drop is proposed as the basis for assessing the effect of air injection on the ability of natural circulation. It is found that the driving force of natural circulation increases with the increase of air injection rate, and the circulation flow rate increases obviously when the bubbly flow appears in the riser. However, when the transition from bubbly flow to churn flow appears, the growth of the circulation flow rate slows down because the resistance increases faster than the driving force. Therefore, it can be known that the best performance is obtained when bubbly-churn flow appears in the top of the riser. What’s more, the capacity of lifting water will be reduced and churn flow will appear prematurely when the submergence ratio decreases. This means that in the process of open natural circulation system design, the submergence ratio of the system should be increased as much as possible. Finally, in this paper, it is found that the bubble pump with PS = 0.2 μm has better performance.

Research on Natural Circulation Flow Characteristics of Floating Nuclear Power Plant in Heaving Motion

2017 ◽  
Author(s):  
Li Ren ◽  
Peng Minjun ◽  
Xia Genglei ◽  
Zhao Yanan
Keyword(s):  
Power Plant ◽  
Nuclear Power Plant ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Nuclear Power ◽  
Natural Circulation ◽  
Flow Characteristics ◽  
Circulation Flow ◽  
Drift Flux Model

The FNPP (Floating Nuclear Power Plant) expanded the application field of Integrated Pressurized Water Reactor (IPWR) in the movable marine platform, it is necessary to study the natural circulation flow characteristics in heaving motion on the ocean. From the characteristics of FNPP, by means of THEATRe code which was based on the two-phase drift flux model and was modified by adding module calculating the effect of heaving motion, the simulation model in heaving motion was built. Using the models developed, the natural circulation operating characteristics of natural circulation in heaving motion and the transitions between forced circulation and natural circulation are analyzed. In the case of amplitude limited, the periods of mass flow rate are equal to periods of heaving motion. The oscillation amplitude of mass flow rate increases with the heaving amplitude increase. In the case of period limited, the natural circulation flow rate oscillating amplitude increases with the heaving period increases. The result obtained are not only evaluating FNPP design behavior properly but also pointing out the direction to further optimum design to ensure FNPP operating safety in heaving motion.

Reduced-Order Fluidic Model for Flow Instabilities in Two-Phase Microfluidic Heat Exchangers

2010 ◽  
Author(s):  
Josef L. Miler ◽  
Gamal Refai-Ahmed ◽  
Maxat N. Touzelbaev ◽  
Milnes P. David ◽  
Julie E. Steinbrenner ◽  
...  
Keyword(s):  
Heat Flux ◽  
Phase Change ◽  
Heat Exchangers ◽  
Flow Instabilities ◽  
High Heat Flux ◽  
Time Step ◽  
Two Phase ◽  
Reduced Order ◽  
Flow Response ◽  
Pump Flow Rate

Two-phase microfluidic heat exchangers have the potential to provide high-heat flux cooling with lower thermal resistance and lower pumping power than single-phase heat exchangers. However, the process of phase change in two-phase heat exchangers can cause flow instabilities that lead to microchannel dryout and device failure [1–3]. Modeling these flow instabilities remains challenging because the key physics are highly coupled and occur over disparate time and length scales. This work introduces a new approach to capture transient thermal and fluidic transport with a reduced-order model consisting of fluidic, thermal, and phase-change submodels. The present study presents a reduced-order, transient, multichannel fluidic circuit submodel for integration into this proposed modeling approach. The fluidic submodel is applicable in flow regimes in which a thin liquid film exists around the bubble. Flow response to boiling is modeled considering bubble overpressure. An adaptive time step approach is used to treat the rapid flow response at short time scales after initial bubble vaporization. Using a seeded bubble technique for testing two-phase flow response, the model predicts a stability threshold at 0.015 W of localized superheating for two 100-micron square channels in parallel with a pump flow rate of 0.15 ml/min. Once integrated with the proposed reduced-order thermal and phase change models, this fluidic circuit model will yield criteria for stable two-phase heat exchanger operation considering factors such as pumping pressure, channel geometry, and applied heat flux that can be compared to experimental observations.

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