Natural Circulation Conditions Research Articles

Natural circulation flow resistance correlations in a rod bundle channel under vertical, inclined and rolling motion conditions

Flow resistance characteristics in a 3 × 3 rod bundle channel are investigated under static vertical, inclined and rolling motion conditions. The circulation mode hardly influences the frictional and local resistance and the transition Reynolds number under forced and natural circulation conditions are both around 800. Correlations are proposed to predict the frictional and local resistance coefficients. Under inclined conditions, the friction factors are slightly larger than vertical values due to the radial convection and increase with the inclination angles for the same Reynolds number. Since the flow resistance increases clearly in the laminar regime, a correlation allowing for the inclination angles is proposed to predict the friction factors in the laminar regime under inclined conditions. The spacer grid local resistance is barely influenced by the inclination instead. The natural circulation flowrate and pressure drop fluctuations are quite different from the forced circulation results in previous studies. The amplitude is much larger and no obvious phase difference is observed under natural circulation conditions. The friction factor variation rules are different from the forced circulation results as well. A new correlation is proposed considering the rolling parameters through dimensionless numbers and rolling Reynolds number. It is tested with a mass of experimental data and applicable to friction factors prediction under given conditions.

Numerical study on reverse flow in U-tubes of UTSG under natural circulation

The inverted U-tube steam generator (UTSG) is the key hub between primary side and secondary side in nuclear power plants. However, it is found that reverse flow occurs in UTSG under natural circulation condition, which has a negative effect on the reactor safety. Additionally, in barge-mounted nuclear power plants, the UTSG will incline due to the sea wave. The characteristics of heat and mass transfer in UTSG under the inclined condition will also differ from that on land-based equipment. It is meaningful to do research on reverse flow in U-tubes of UTSG in inclined under natural circulation condition. In this paper, a three-dimensional computational fluid dynamic (CFD) model is carried out to study the reverse flow in UTSG. Three inclined directions at different angles of UTSG are taken into consideration. Through some cases, the temperature profiles and mass flow rates in the primary side of the UTSG are calculated. The influence of secondary side temperature on the reverse flow is studied. The results are obtained that reverse flow most easily occurs in shorter tubes of vertical UTSG under natural circulation condition. As the temperature on the secondary side increases, the pressure drop between inlet and outlet of UTSG is negative, and its absolute value decreases, resulting in a decrease in the total number of reverse flow tubes. Under the inclined conditions, the inclined angle of UTSG has a great effect on the reverse flow. Along the inclined direction, reverse flow occurs more easily in the U-tubes with lower inlet cross-section locations. The results are validated by the estimate code Reactor Excursion and Leak Analysis Program 5 (RELAP5/MOD3.3).

Experimental study on natural circulation flow instability in parallel boiling channels under low pressure

Two-phase flow instabilities in parallel boiling channels under natural circulation condition were experimentally studied at low pressure (0.2–0.6 MPa). The parallel boiling channels composed of two different form channels, namely a 3 × 3 rod bundle channel and a tube channel, were used as the testing section. In the present experiment, the flow oscillation behaviors can be classified into three typical modes based on their frequency spectrum characteristics: (1) small amplitude out-of-phase flow instability induced by the type-1 density wave oscillations in parallel boiling channels, (2) large amplitude out-of-phase flow instability with reversal flow induced by the occurrence of geysering in tube channel and (3) a kind of compound flow instability superposed by geysering and natural circulation instability. The influence of thermal hydraulic parameters on the evolution process of above three flow instabilities were analyzed. The marginal stability boundaries presented by the subcooling number (Nsub) and phase change number (Npch) at different system pressures are obtained and all unstable flow occurs in the negative region of the outlet equilibrium quality in the present experiment. Besides, the effects of asymmetric heating between two parallel channels on NC flow instability were also studied. The asymmetric heating can change the NC flow distribution in parallel boiling channels and reduce the system stability. The occurrence of out-phase oscillations in parallel channels can be suppressed with the increasing of heating power asymmetry.

Analysis of natural circulation frictional resistance characteristics in rod bundle channel

A considerable amount of research has gone into understanding the thermal-hydraulics characteristics of rod bundle channels which is of great importance for the nuclear industry. This paper will show that the development of the correlation of the frictional resistance coefficient in rod bundle channels under natural circulation conditions is well understood. This paper presents experimental studies of the single-phase resistance and obtains an improved correlation of the frictional resistance coefficient under natural circulation conditions based upon thermal-hydraulics characteristics which are well understood. It is also shown that this newly developed correlation closely predicts data.The experimental results show that the frictional resistance coefficient of a rod bundle channel under natural circulation is related to the physical properties of fluid and the size of the rod bundle channel. According to different flow regions, the relationship between the frictional resistance coefficient and Reynolds number is discussed separately, and the expressions of frictional resistance coefficient with Reynolds number in laminar flow region and transition region are obtained. After considering the influence of fluid properties on the frictional resistance coefficient, this paper introduces the kinematic viscosity correction factor to correct the frictional resistance coefficient. Finally, the complete expression of the frictional resistance coefficient was obtained. This newly developed predictive expression is in good agreement with experimental data in both the laminar and transition flow regions, with a relative error within ±5%. Thus we now have a predictor of the frictional resistance coefficient under natural circulation conditions based upon more complete thermal-hydraulic parameters in a static state.

CFD Analysis of the Passive Decay Heat Removal System of an LBE-Cooled Fast Reactor

This paper presents capacity of the passive decay heat removal system (DHRS) operated under the natural circulation conditions to remove decay heat inside the main vessel of the Lead-bismuth eutectic cooled Fast Reactor (LFR). The motivation of this research is to improve the inherent safety of the LFR based on the China Accelerator Driven System (ADS) engineering project. Usually the plant is damaged due to the failure of the main pumps and the main heat exchangers under the Station Blackout (SBO). To prevent this accident, we proposed the DHRS based on the diathermic oil cooling for the LFR. The behavior of the DHRS and the plant was simulated using the CFD code STAR CCM+ using LFR with DHRS. The purpose of this analysis is to evaluate the heat exchange capacity of the DHRS and is to provide the reference for structural improvement and experimental design. The results show that the stable natural circulations are established in both the main vessel and the DHRS. During the decay process, the heat exchange power is above the core decay heat power. In addition, in-core decay heat and heat storage inside the main vessel are efficiently removed. All the thermal-hydraulics parameters are within a safe range. Moreover, the highest temperature occurs at the upper surface of the core. A swirl occurs at the corner of the lateral core surface and some improvements should be considered. And the natural circulation driving force can be further increased by reducing the loop resistance or increasing the natural circulation height based on the present design scenario to enhance the heat exchange effect.

Research of coupled wall conduction effects on forced and natural circulation of LBE in CLEAR-S

The lead based reactor concept is considered to be the most promising reactor option for the Accelerator Driven Subcritical (ADS) system. Lead based reactor engineering validation facility CLEAR-S is a large scale integrated non-nuclear pool type facility which has been built to provide a unique experimental platform for full-scale prototype components test of lead based reactors. In this article, three-dimensional steady state analysis of flow in the hot and cold pools of CLEAR-S was carried out by using computational fluid dynamics (CFD) code FLUENT during forced and natural circulation conditions. The results showed that the simulation had a good agreement with the design parameters of CLEAR-S and the temperature profile was uniform in the whole domain of the hot and cold pools without considering any conduction through the coupled walls. The temperature distribution in the hot and cold pools became non-uniform with the coupled wall conduction and thermal stratification was observed in the cold and hot pools near the bottom wall of the hot pool. This non-uniform distribution of temperature could cause thermal stresses on equipment, which is dangerous for the integrity of the system. During natural circulation conditions, the coupled wall conduction enhanced the natural circulation and kept the hot pool at a relatively low temperature because the cold pool cooled the hot pool through heat transfer between the walls of the hot and cold pools. Therefore, wall conduction has some advantages and disadvantages as it enhances the natural circulation on one hand but disturbs temperature uniformity on the other hand.

Visualized Experiment of Bubble Behaviors in a Vertical Narrow Rectangular Channel Under Natural Circulation Condition

The characteristics of bubble behavior have been in particular interest for decades due to its significant contribution to understanding the mechanism of heat transfer. In the present work, visualized experiment is conducted to study the bubble characteristics in subcooled flow boiling of a narrow rectangular channel under natural circulation. The experiments were performed at pressures of 0.2 MPa, with inlet subcooling ranging from 20 to 60 K and heat flux ranging from 100 kW/m2 to 300 kW/m2. A high-speed digital camera is used to capture the pictures of bubble behaviors. A sequence of image processing algorithms is used deal with the original bubble images to get relevant bubble parameters. We observe the whole process of a single sliding bubble lifetime and found most of bubbles slide along the heating surface after detaching the nucleation sites. Five typical sliding bubble growth paths are observed in the present experimental conditions. According to the analysis of the experimental data, it can be found that the liquid subcooling and wall superheat are the main factors that affect the bubble size during sliding in narrow rectangular channel under natural circulation condition. Due to the difference of driving force, the sliding velocity of bubble in forced circulation is always larger than that in natural circulation. At the same time, the bubble velocity changes significantly at different heat flux and shooting location.

Analysis of operating characteristics of IPWR under natural circulation

Abstract Many integrated pressurized water reactor (IPWR) designs using natural circulation operation mainly to enhance their inherent safety. The operating characteristics of primary coolant are completely different without mechanical pumps. The designs and safety analysis of forced circulation reactors are widely researched, but the natural circulation characteristics of IPWR have not been well studied by literatures. The present work discussed the thermal-hydraulic characteristics of IPWR under natural circulation conditions by using the best estimate codes RELAP5. And the effect of system parameters on natural circulation characteristics of IPWR is also studied. The results show that, the primary coolant average temperature and steam pressure are two key parameters that affect the natural circulation stable operating load. The set value of primary coolant average temperature effects the core outlet temperature and the steam temperature, but the primary coolant flow is basically the same under different primary coolant average temperature but same load conditions. The smaller steam pressure is more conducive to produce superheated steam, but there is risk of two phase flow instability in OTSG secondary side. The rapid load change process under natural circulation indicating that the reactor has a good load tracking characteristics under natural circulation, but the rapid change of primary coolant temperature will cause oscillations in system parameters.

Scaling analysis of core pressure drop in reduced height integral test facility

Abstract Integral test plays essential role to assess the design of the emergency cooling system of nuclear reactors. Different from full height integral test facilities, reduced height integral test facilities have new problems on the pressure drop scaling. This paper mainly focuses on scaling of pressure drop across the core as it is the major pressure drop in primary loop. The analysis of pressure drop across the core has been divided into three terms and each term has been discussed separately based on two conditions: the normal operation condition and natural circulation condition. After that, the total pressure drop ratios under these two conditions have been discussed.

Pre-test analysis of accidental transients for ALFRED SGBT mock-up characterization

In the framework of the Horizon 2020 SESAME project (thermal hydraulics Simulations and Experiments for the Safety Assessment of MEtal cooled reactors), the CIRCE pool facility has been refurbished by ENEA to host HERO test section (Heavy liquid mEtal pRessurized water cOoled tubes) with the aim to test an innovative concept of the steam generator bayonet tubes proposed for ALFRED (Advanced Lead Fast Reactor European Demonstrator) and to provide experimental data for code validation. HERO consists of a bundle of seven bayonet tubes characterized by an active length of six meters (1:1 with the ALFRED steam generator tube length). The main purposes of the research were to investigate the thermal hydraulic behavior of the innovative concept and provide a set of experimental data aiming at validating STH (System Thermal-Hydraulic) code. The calculations were carried out adopting RELAP5-3D©, updating the nodalization scheme validated with the experimental data of CIRCE-ICE campaign. The experiment consists of a transition from forced to natural circulation in a loss of flow accidental scenario; in order to identify the initial conditions of the experiment, several full power conditions were simulated. Starting from the reference steady state conditions, five transient tests were simulated to evaluate the effect of the reduction of the secondary mass flow rate (reduced from the nominal value to simulate the activation of the DHR system) and the effect of the heat losses compensation. According with the calculation, HERO test section offers excellent thermal-hydraulic behavior ensuring a sufficient natural circulation conditions to remove the decay heat in the short term.

Experimental study of direct solar photocatalytic water splitting for hydrogen production under natural circulation conditions

Photocatalytic water splitting for hydrogen production provides a promising route for the future hydrogen economy, being operational in the visible light domain with a potential use of solar radiation. An outdoor pilot demonstration of CPC-based photoreactors has been designed, installed and tested at the State Key Laboratory of Multiphase Flow in Power Engineering to assess its effectiveness in solar photocatalytic hydrogen production. Nine sets of CPC-based photoreactors, each of which is 3.6 m2 in area and 23 L in volume, are connected, controlled and operated in parallel. The high efficiency photocatalyst (Cd1-xZnxS), low concentration sacrifice agents (Na2S and Na2SO3) and deionized water are the raw materials of the pilot system. Two operation models, viz. the natural circulation model and the gas disturbance model, are proposed considering the expense and the efficiency. From our observations, the slurry temperature inside the tubes rises by 20–30 °C from the ambient. The slurry velocity can reach 1.2 m/s in the gas disturbance model, but is as low as 3.5 cm/s in the natural circulation model. The average hydrogen productivity is 184.30 mL/min and accumulated to be 10.321 L/h in the natural circulation model, with the average solar radiation, photocatalyst concentration and sacrifice agents' concentration being 803.8 W/m2, 2.77 g/L and 0.1 mol/L, respectively.

Investigation of Non-cooldown SG Secondary Condition on the Natural Circulation Cooling Procedure

In typical pressurized water reactor (PWR), in case that one steam generator (SG) cannot be credited for the primary cooldown, it is necessary to homogenize primary coolant temperature among loops using at least one reactor coolant pump (RCP) for the plant cooldown. If the natural circulation condition is established due to unavailability of all the RCPs, the continuous cooldown using intact SGs causes to disturb the smooth depressurization because it leads to void generation in the top of the non-cooldown SG tube where the high temperature coolant is remained. For this purpose, W.Sakuma, et al.[1] suggested the outline of asymmetric cooldown procedure without any RCPs restart. Since the suggested procedure is based on only one secondary condition (SG dry-out) of non-cooldown SG, and hence the impact of difference of the secondary condition should be investigated. In this paper, the sensitivity analyses were performed to confirm the impact on the asymmetric cooldown procedure, and consequently, it was confirmed that the coolable range used in the procedure was expanded if the water inventory exists in non-cooldown SG. Therefore it was concluded that the coolable range which was defined with the SG dry-out condition in non-cooldown SG can be conservatively applied for the operating procedure.

Pre-test analysis of protected loss of primary pump transients in CIRCE-HERO facility

In the frame of LEADER project (Lead-cooled European Advanced Demonstration Reactor), a new configuration of the steam generator for ALFRED (Advanced Lead Fast Reactor European Demonstrator) was proposed. The new concept is a super-heated steam generator, double wall bayonet tube type with leakage monitoring [1]. In order to support the new steam generator concept, in the framework of Horizon 2020 SESAME project (thermal hydraulics Simulations and Experiments for the Safety Assessment of MEtal cooled reactors), the ENEA CIRCE pool facility will be refurbished to host the HERO (Heavy liquid mEtal pRessurized water cOoled tubes) test section to investigate a bundle of seven full scale bayonet tubes in ALFRED-like thermal hydraulics conditions. The aim of this work is to verify thermo-fluid dynamic performance of HERO during the transition from nominal to natural circulation condition. The simulations have been performed with RELAP5-3D© by using the validated geometrical model of the previous CIRCE-ICE test section [2], in which the preceding heat exchanger has been replaced by the new bayonet bundle model. Several calculations have been carried out to identify thermal hydraulics performance in different steady state conditions. The previous calculations represent the starting points of transient tests aimed at investigating the operation in natural circulation. The transient tests consist of the protected loss of primary pump, obtained by reducing feed-water mass flow to simulate the activation of DHR (Decay Heat Removal) system, and of the loss of DHR function in hot conditions, where feed-water mass flow rate is absent. According to simulations, in nominal conditions, HERO bayonet bundle offers excellent thermal hydraulic behavior and, moreover, it allows the operation in natural circulation.

Experimental study on mixed convection in an asymmetrically heated, inclined, narrow, rectangular channel

Experimental study on mixed convection in the entrance region of a one-side-heated narrow rectangular channel has been carried out. We have performed a series of experiments under natural circulation conditions with Re ranging from 1000 to 3000 and inclination angle ranging from 0° to 30°. Meanwhile, we conducted flow visualization experiments to identify secondary flow driven by temperature difference between the lower, heated and upper, unheated plate of the channel. It is found that thermal instability can be enhanced by increasing the inclination angle of the channel. The secondary flow induces the onset of thermal instability (OTI) while increasing in Re delays the OTI. A sudden increase of hear transfer coefficient and friction factor has been observed after the OTI. The traditional identification criteria for mixed convection are not suitable for the inclined, narrow rectangular channel with a heated lower side. However, transverse Richardson number can identify mixed convection in the channel. The experiment results, together with image data for the flow condition in the inclined narrow rectangular channel, offer valuable data to improve the engineering design of plate-type fuel assembly and similar heat exchangers.

Experimental and theoretical investigations on effect of U-tube inlet and secondary temperatures on reverse flow inside UTSG

The reverse flow phenomenon in partial tubes of vertically inverted U-tube steam generator (UTSG) during natural circulation conditions is investigated in this paper. By gradually rising the inlet plenum temperature of steam generator simulator at a constant mass flux rate, the phenomenon is observed in a thermal hydraulic test facility. Based on the experiment, the flow and heat transfer model in U-tubes are established to study the effects of U-tube inlet and secondary side temperatures on the reverse flow inside the UTSG. The results show that the mass fluxes will be increased and the inlet temperatures will be decreased in the normal flow U-tubes when the reverse flow occurs, thus the normal flow in remnant tubes tends to be more stable. The space distribution of reverse flow U-tubes depends on the critical pressure drop (CPD) of each U-tube under the local physical conditions, such as the U-tube length, U-tube inlet temperature and secondary side temperature. For one whole steam generator under the natural circulation, the critical mass flow rate (CMFR) of each U-tube will be increased when the U-tube inlet temperature increases or the secondary side temperature decreases, and the larger CMFR indicates the higher possibility that reverse flow occurs in the steam generator. The relationship between the CPD or CMFR and the U-tube length is non-monotonic.

Experimental and numerical study on fluctuations and distributions of fluid temperature under rolling motion conditions

An experimental and numerical study is conducted on a narrow rectangular channel of a loop under natural circulation conditions with rolling motion to investigate temperature fluctuations and mechanisms that influence such fluctuations. Meanwhile, the temperature distribution along the channel under rolling motion conditions is predicted based on a self-developed code. The results show that the cycle-averaged liquid temperature at the outlet of the test section under rolling motion conditions is higher than that under vertical conditions. The waveform of fluctuations in fluid temperature is complex and deviates from the standard sine curve, which is the law of the rolling platform. In the single-phase natural circulation loop, temperature fluctuations at the outlet of the test section are spurred from flow rate fluctuations caused by rolling motion in a dominant manner. Instantaneous temperature fluctuations are successfully predicted from the self-developed code for the single-phase natural circulation loop of a heated narrow rectangular channel under rolling motion conditions. Under rolling motion and constant heat flux conditions, the amplitude of temperature fluctuations in the heated zone increases along the flow direction. Along the flow direction, phases of the temperature crest at different locations of the heated zone exhibit a lag effect. In the adiabatic section between the outlets of the heated zone and test section, the amplitude of temperature fluctuations decreases, and the waveform is smoothed along the flow direction. Moreover, the instantaneous temperature distribution in the heated zone is approximately linear along the flow direction when the flow rate remains positive during rolling motion.

Direct solar photocatalytic hydrogen generation with CPC photoreactors: System development

A new compound parabolic concentrator (CPC) for solar photocatalytic hydrogen production has been designed and estimated in the present study. Parameters of the CPC, i.e., the orientation, the acceptance angle and the absorber tube diameter are designed according to the local meteorological conditions. A north-south orientated, adjustable, single axis tracking truncated CPC with the concentration ratio of 4.22 is designed as one unit. An experimental system including 76 units with the total light area of 103.7m2 is developed and installed. The units have four different angles with inclination being 25°, 35°, 45° and 55°, respectively. The system is operated under the natural circulation condition for most of the time, with high pressure gas perturbing the sedimentary photocatalysts from time to time. NiS-CdxZn1−xS is used as the photocatalyst. Preliminary results show that the mean hydrogen productivity and cumulative hydrogen generation are 7.14NL/h and 42.84NL/day of the horizontal angle 25° row on a typical meteorological day. The average energy conversion efficiency is 0.087%. The daily hydrogen generation of the whole system is accumulated to be 160.34NL.

Liquid Salt Test Loop modeling using TRACE

The objective of this work is to simulate the Liquid Salt Test Loop (LSTL) using the TRACE system code. The LSTL is an experimental facility built at Oak Ridge National Laboratory to test the thermal-hydraulic behavior of a circulating FLiNaK liquid salt. A literature review of the experimental work and the heat transfer and friction correlations for pebble beds was performed to identify the correlations that can be used to simulate the flow of FLiNaK through a pebble bed. The selected correlations were implemented in the TRACE source code and tested using 1-D pipe components. The modified version of TRACE was then used to develop models for each component in the loop. Each component was tested separately to assess its correct physical behavior. The components were integrated into a comprehensive loop model, including the main loop and the air cooling system. The model was used to compute the steady state conditions of the loop, in particular calculation of the pressure distribution and other global operational parameters. The loop-filling transient was analyzed, along with other specific transient conditions such as the pump trip and the loop behavior under natural circulation conditions. The TRACE model was used to help characterize loop thermal hydraulic behavior and to help interpret results from loop shakedown testing.

Experimental investigation of flow and heat transfer for natural circulation flow in an inclined narrow rectangular channel

Narrow rectangular channels are widely used in heat transfer exchangers. We conducted an experimental study under natural circulation conditions to investigate the flow resistance and heat transfer characteristics of water in a one-sided heated narrow rectangular channel. The experiments were performed at pressures of 0.2 MPa and 0.3 MPa, with inlet subcooling temperatures ranging from 30 to 70 K and heat flux ranging from 20 to 90 kW⋅m-2. In a laminar flow regime for a channel with a heated lower plate, both the heat transfer coefficient and friction factor increased with increasing inclination angle. For a channel with a heated upper plate, the inclination angle had an insignificant influence on both the heat transfer coefficient and friction factor. In both transitional and turbulent flow regimes, increase in the Reynolds number suppressed the influence of inclination angle on the heat transfer coefficient and friction factor. The present study suggests that flow resistance and heat transfer in an inclined, narrow rectangular channel with a heated lower plate can be enhanced, as compared to the parameters of a vertical channel. Thermal instability (driven by buoyancy force) is the main factor influencing flow and heat transfer characteristics.

Experimental study of critical heat flux enhancement with hypervapotron structure under natural circulation conditions

The enhancement of critical heat flux with a hypervapotron structure under natural circulation conditions is investigated in this study. Subcooled flow boiling CHF experiments are performed using smooth and hypervapotron surfaces at different inclination angles under natural circulation conditions. The experimental facility, TESEC (Test of External Vessel Surface with Enhanced Cooling), is designed to conduct CHF experiments in a 30mm by 61mm rectangular flow channel with a 200mm long heated surface along the flow direction. The two-phase flow of subcooled flow boiling on both smooth and hypervapotron heating plates is observed and analyzed by the high-speed visualization technology. The results show that both smooth surface and hypervapotron surface CHF data exhibit a similar trend against inclination angles compared with the CHF results under forced flow condition on the same facility in earlier studies. However, the CHF enhancement of the hypervapotron structure is evidently more significant than the one under forced flow conditions. The experiments also indicate that the natural flow rates are higher with hypervapotron structure. The initiation of CHF is analyzed under transient subcooling and flow rate conditions for both smooth and hypervapotron heating surfaces. An explanation is given for the significant enhancement effect caused by the hypervapotron surface under natural circulation conditions. The visualization data are exhibited to demonstrate the behavior of the vapor blanket at various inclination angles and on different surfaces. The geometric data of the vapor blanket are quantified by an image post-processing method. It is found that the thickness of the vapor blanket increases with the increase of the inclination angle. Surface wettability and roughness are measured for both smooth and hypervapotron surfaces before and after the CHF experiments.