Single-phase Natural Circulation Research Articles

Assessment of MARS-KS prediction capability for natural circulation flow in passive heat removal system

Considering that system analysis codes are used for the evaluation of the performance of Passive Safety Systems (PSSs), it is important to investigate the capability of the system analysis code to reliably predict the heat transfer and natural circulation flow, which are the main phenomena governing the performance of a PSS. Since MARS-KS has been widely validated for heat transfer models, this study focuses on evaluating its capability to predict the single and two-phase pressure drops and natural circulation flow. The straight pipe simulation results indicate that the pressure drop predictions are reliable within ±5 % error margin for the single-phase flow and the errors of pressure drop up to −30 % for the two-phase flow. Through single-phase natural circulation flow analysis, it is concluded that the use of the appropriate K-factor modeling based on the flow regimes is important since the natural circulation flow rate in MARS-KS is mainly affected by the form loss factor modeling. With two-phase natural circulation flow analysis, this study emphasizes the behavior of the system could change significantly depending on the two-phase wall friction and pressure loss modeling. With the analysis results, modeling considerations for the PSS performance evaluation with the system analysis codes are proposed.

A novel magnetohydrodynamic power generation system using low-temperature liquid metal coupled with a rectangular single-phase natural circulation loop

A novel magnetohydrodynamic power generation system using low-temperature liquid metal coupled with a rectangular single-phase natural circulation loop

Computational and analytical study on the stability and utility aspects of single-phase natural circulation loop in parabolic trough collector

The use of passive heat transport devices in solar thermal energy devices is critical to make the systems more reliable and sustainable. Hence, linking single phase natural circulation loop to parabolic trough collector is the apt solution. In this regard, a 3-D computational model is built and validated for steady state and transient results. To acquire realistic results, solar irradiance pattern over the receiver is explored from SolTrace® and applied to the loop. The upper threshold of the horizontal heater horizontal cooler loop is found and validated using linear stability analysis. Since the loop is unstable at lower loads, instability restraining techniques viz. loop inclination (θ = 4°) and orifice insertion (β = 0.7) are employed. Although loop inclination gave desired results, the orifice couldn't improve the behavior due to loop geometry. The type of heat flux pattern does affect the initial transients of the loop, but it does not affect the steady state results in any manner. The loop was also tested with Therminol VP 1 and compared the stability aspects with water. In the end, intending NCL application in PTC, transient analysis was undertaken considering whole day solar flux distribution, which gave convincing results.

CFD investigation of fluid and heat transfer in a single-phase natural circulation toroidal loop

Natural circulation occurs due to a temperature-induced density gradient, typically observed within closed loops where the fluid is heated in the upper section and cooled in the lower section. This phenomenon finds numerous engineering applications, particularly as a passive heat removal mechanism in nuclear reactors, significantly enhancing their safety. However, the computational modeling of natural circulation poses challenges due to its complex, multidimensional, and nonlinear characteristics. Furthermore, in such systems, fluid dynamics and thermal aspects are intricately intertwined. Hence, the objective of this study is to evaluate the fluid and thermal behavior of a toroidal loop under natural circulation using the Computational Fluid Dynamics (CFD) ANSYS CFX package. A 2D model is employed, along with the Boussinesq approximations, as they are well-suited for toroidal systems. Turbulence is calculated using the k−ϵ. Various modified Grashof Numbers (Gr) normalized by the geometric parameter (NG) are utilized, ranging from 5.4×103 to 4.7×105, for two different Prandtl numbers. The lowest Grm/NG value in each simulation represents the convection lower limit (flow initiation), while the highest value indicates the convergence limit, beyond which a steady state cannot be achieved. Temperature and velocity profiles are presented, and their variations with the power level are discussed. The resulting Reynolds of Steady State (Ress) are plotted against Grm/NG values, demonstrating good agreement with experimental data. As the Prandtl Number increases, the system is observed to enter an unstable zone at an earlier stage. Finally, it is observed that multiple steady states exist for Pr=0.7, and their implications are discussed.

PWR natural circulation at a small break LOCA

In this paper, a one-dimensional quasi-steady model has been developed to analyse the potential impacts of reduced primary coolant inventory on natural circulation. This model focuses on the development of expressions for the coolant inventory, temperature distributions, and mass flow rate. The model encompasses a range of fluid flow scenarios, including single-phase, combined single-and-two-phase, and two-phase natural circulation, as well as the phenomenon of reflux condensation. The model is also utilized to examine the impact of reactor core power on natural circulation parameters. Comparisons with experimental findings and earlier research are made, highlighting a satisfactory level of agreement with the analytical findings. Based on the findings of this study, it has been observed that within the range of primary coolant inventory of 100–60% and core decay power of 1.5–5% of full power, natural circulation in a PWR type system has the potential to serve as an effective mechanism for transferring core decay heat to the secondary loop. The flow rates of natural circulation in pressurized water reactors (PWRs) are observed to have a significant dependence onthe amount ofliquid inventory, whereas their relationship with power level is comparatively less pronounced. The flow rate of a pressurized water reactor (PWR) system tends to exhibit its highest value within a specific range of inventory, typically falling between 60% and 80%. The transition between the different modes of natural circulation is observed to exhibit a smooth transition, with varying system inventories.

Analysis of a parallel-channel natural circulation loop in the passive residual heat removal system of the NHR200-II reactor

The passive residual heat removal (PRHR) system of the NHR200-II nuclear heating reactor is a parallel-channel single-phase natural circulation loop. The reverse-flow phenomenon in parallel-channel natural circulation loops will cause insufficient heat removal capacity in the PRHR system. In this study, a simplified mathematical model was established for the PRHR system of NHR200-II. The mechanism of the reverse flow phenomenon in the PRHR system was analyzed using a pressure drop-flow rate diagram, and the effects of cooler elevation and loop resistance on the number of reverse flow branches were discussed. The analysis reveals that the reverse flow phenomenon of the PRHR system can be suppressed by increasing the elevation of the air cooler. Furthermore, there exists a critical elevation of the air cooler. It is not feasible to suppress the reverse flow by changing the friction coefficients of either the PHE branches or the air cooler branch when the air cooler elevation is less than the critical value. Oppositely, the number of reverse flow branches can be reduced or even eliminated by increasing the resistance of the PHE branches or reducing the resistance of the air cooler branch when the air cooler elevation is greater than the critical value.

Numerical simulation of a toroidal single-phase natural circulation loop with a k-kL-ω transitional turbulence model

The wall friction correlations of oscillatory natural circulation loops are highly loop-specific, making it difficult to perform 1-D system simulations before obtaining specific experimental data. To better predict the friction characteristics, the nonlinear dynamics of a toroidal single-phase natural circulation loop were numerically investigated, and the transition effect was considered. The k-kL-ω transitional turbulence and k-ω SST turbulence models were used to compute the flow characteristics of the loop under different heating powers varying from 0.48 to 1.0 W/cm2, and the results of both models were compared with previous experiments. The mass flow rates and friction factors predicted by the k-kL-ω model showed a better agreement with the experimental data than the results of the k-ω SST model. The oscillation frequencies calculated using both models agreed well with the experimental data. The k-kL-ω transitional turbulence model provided better friction-factor predictions in oscillatory natural circulation loops because it can reproduce the temporal and spatial variation of the wall shear stress more accurately by capturing the movement of laminar, transition turbulent zones inside unstable natural circulation loops. This study shows that transition effects are a possible explanation for the highly loop-specific friction correlations observed in various oscillatory natural circulation loops.

The Scaling Analysis and Numerical Simulation of Single-phase Natural Circulation

In this work, scaling analysis and numerical simulation of the single-phase natural circulation loop with water are presented. First, grid-dependency testing and model validation are performed. Then, according to the assumption of the boundary conditions, the effect of the heat input and structural parameters on the heat transfer characteristic is studied, including the boundary condition and temperature of the steam fluid. The results show that the scaling model can accurately express the flow heat transfer characteristics of the original model, and can reach a steady state under different working conditions. The formation of stable natural cycles is the result of coupling the boundary conditions of the heating and cooling segments under the fixed structure reference. The mass flow rate on the steady state decrease with the increase of the external fluid temperature in the cooling section, and as the heat transfer coefficient of the external convection heat transfer coefficient increases.

Dynamic scaling characteristics of single-phase natural circulation based on different strain transformations

Dynamic scaling characteristics of single-phase natural circulation based on different strain transformations

Study of a Square Single-Phase Natural Circulation Loop Using the Lattice Boltzmann Method

Natural circulation loops are thermohydraulic circuits used to transport heat from a source to a sink in the absence of a pump, using the forces induced by the thermal expansion of a working fluid to circulate it. Natural circulation loops have a wide range of engineering applications such as in nuclear power plants, solar systems, and geothermic and electronic cooling. The Lattice Boltzmann Method was applied to the simulation of this thermohydraulic system. This numerical method has several interesting features for engineering applications, such as parallelization capabilities or direct temporal convergence. A 2D model of a single-phase natural circulation mini-loop with a small inner diameter was implemented and tested under different operation conditions following a double distribution function approach (coupling a lattice for the fluid and a secondary lattice for the thermal field). An analytical relationship between the Reynolds number and the modified Grashof number was used to validate the numerical model. Two regimes were found for the circulation, a laminar regime for low Reynolds numbers and a non-laminar regime characterized by a traveling vortex near the heater and cooler’s walls. Both regimes did not present flux inversion and are considered stable. The recirculation of the fluid can explain some of the heat transfer characteristics in each regime. Changing the Prandtl number to a higher value affects the transient response, increasing the temperature and velocity oscillations before reaching the steady state.

Scale analysis using FSA methodology for single-phase natural circulation in a simplified RCCS

Improving the safety and reliability of innovative Generation IV nuclear reactors can be achieved by adopting passive safety systems for accident prevention. These systems rely only on physical phenomena such as natural circulation and eliminate the need for an external response in the event of an accident. While passive natural circulation systems are widely used in modern nuclear reactors, their complexity and high thermal power output make them difficult to simulate at full scale. One solution is to study reduced-scale systems that are more feasible for experimental investigation. The correct scale procedure is essential to ensure that the results of these systems represent those expected in a real facility. This article describes a small-scale, single-phase water natural circulation circuit representing a simplified reactor cavity cooling system. The standard scale method is combined with an innovative fractional scaling analysis to determine the conditions and similarity parameters for a test facility. This analysis outlines the necessary simplifications required to achieve a certain similarity between natural circulation systems in the transient regime. The methodology is applied to two studies from the literature, demonstrating the feasibility of comparing different passive heat removal systems.

Experimental study on energy-exergy performance of single-phase natural circulation loop using mono/hybrid nano-oils

Thermal/vegetable oil-based mono/hybrid nanofluids (Nano-oils) may be potential working fluids in single-phase natural circulation loops for medium temperature applications due to their favorable thermophysical properties (high thermal expansion coefficient and low viscosity) and high Prandtl number; however, not studied yet. Hence, the present study investigates the different nano-oils in a natural circulation loop. Therminol-VP1 and soybean-based mono/hybrid nano-oils with different nanoparticles (Al2O3, CuO, SiC, and MWCNT) are used at a volume fraction of 0.1%. The influence of power input and loop inclination on the energy-exergy performance parameters (Effectiveness, Entropy generation rate and Induced mass flow rate) are presented. The use of nano-oil shows enhanced heat exchanger effectiveness and lower entropy generation rate, whereas the induced mass flow rate may increase or decrease depending on nanoparticle shape and power input. Al2O3+MWCNT nano-oil exhibits superior performance, with a maximum mass flow rate reduction of 8%, effectiveness enhancement of 36%, and entropy generation reduction of 20% as compared to base oil. Therminol-VP1 shows quick flow establishment than soyabean oil. The usage of nano-oil is more beneficial at the higher power input. Loop inclination reduces the mass flow rate while increasing the effectiveness and entropy generation; however, mass flow reduction is much lower for counter-clockwise inclination.

Passive mitigation of condensation-induced water hammer by converging-diverging structures for offshore nuclear power plants

The equal-height-difference passive heat removal system (EHDPHRS) utilizes the ocean environment as an infinite heat sink that can better suit ocean nuclear power plants (ONPPs). However, a structure with an equal-height-difference can easily cause reverse flow of subcooled water, leading to serious condensation-induced water hammer (CIWH). In this paper, a fully visual experimental facility with same geometric characteristics as the EHDPHRS was established with the aim of studying the CIWH phenomenon. First, two kinds of CIWH modes were observed by the results of the visual experiment, namely the intermittent CIWH and the continuous CIWH. The typical feature of the intermittent CIWH is that the single-phase natural circulation occurs intermittently. Then, the converging-diverging structure (CDS) was designed to avoid the CIWH events in the EHDPHRS. The effectiveness of CIWH prevention was evaluated employing the velocity field, which was obtained by the PIV experimental system. It was found that the CDS can effectively inhibit the further reverse flow of subcooled water and thus eliminate the triggering mechanism of CIWH events. Finally, a dimensionless number was used to balance CIWH prevention and natural circulation capacity. It was shown that when the dimensionless number is in the range of 0.67–0.80, the CDS can eliminate the CIWH events while ensuring the natural circulation capacity.

Investigation of external magnetic field effect on the performance of ferrofluid-based single-phase natural circulation loops

The goal of this study is to investigate the performance of a Single-Phase Natural Circulation mini Loop (SPNCmL) operating under the influence of an external magnetic field (EMF).For this purpose, a numerical SPNCmL model working with Fe3O4 ferrofluids (1–3 vol%.) under the influence of an EMF is developed to reflect the effect of a NdFeB permanent magnet with a remanence of 1.22 T located at the outlet of the cooler-end for the magnetic field generation. System characteristics such as temperature difference at heater-end (ΔTheater) and maximum temperature (Tmax) and performance in terms of effectiveness (ε) are investigated. In addition, the effect of EMF on boundary layer energy transport along the cooler-end is evaluated in terms of the change in the local Nusselt number. Applying an EMF dramatically affects the system performance in terms of an increase in ΔTheater and ε, respectively up to 34% and 25% compared to those with water. Tmax values are obtained by up to 9% higher for Fe3O4 ferrofluids compared to water, while applying EMF results in an increment in Tmax by up to 5%. Improved heat transfer performance by employing EMF at the cooler-end outlet of the SPNCmLs emphasizes their potential in cooling applications.

Effect of hot leg length on the steady-state performance of single-phase natural circulation based tube-in-tube thermosyphon heat transport device with heat sink directly above the heat source

Thermosyphon heat transport device (THTD) is a compact natural circulation loop to enhance the heat transport distance and the power rating compared to existing devices like heat pipe. Hence it finds application in indoor solar cooktops and diverse passive fuel cooling systems to eliminate Fukushima type accidents in advanced nuclear power plants. THTDs for the nuclear application have the heat sink directly above the heat source in the same vertical leg. Practically no study is reported, even for simple rectangular loops with heat sink directly above the heat source in the same vertical leg. All reported studies for rectangular loops with vertical heater vertical cooler configuration are for the heat source in the vertical up leg and heat sink in the other vertical down leg. The main issue with a heat sink located directly above the heat source in the same vertical leg is that it leads to instability with recirculation loops (bidirectional flow in the cooler with the heavy coolant coming down along the cooled wall and light fluid rising along the adiabatic wall) in the cooler. Earlier reported studies indicated that at higher powers, the instability is suppressed. In this paper, another method of eliminating this instability was experimentally studied, which involved increasing the hot leg length and, thereby the elevation difference. The main finding is that the instability can be avoided if the hot leg length (equivalent to increasing the height) is increased. The same finding may hold good even for the rectangular loops with heat sink directly above the heat source in the same vertical leg. Besides, the study also revealed that the same one dimensional steady-state dimensionless flow equation applicable to simple rectangular loops is found to be valid for the tube-in-tube type THTD considered in the present work.

Preliminary code development for single-phase liquid natural circulation flow simulation based on two-phase compressible model

It is of great significance to study the thermal-hydraulic characteristics of Lead-based Fast Reactor. In these years, the computational fluid dynamics method has become an attractive way in thermal-hydraulic and safety simulations of liquid metal cooled reactors. In order to investigate thermal-hydraulic characteristics of single-phase natural circulation flow in liquid metal cooled reactors, a self-developed CFD code was developed in this paper. The new code was proposed based on the two-phase compressible model, including liquid and vapor governing equations and a series of auxiliary models. The commercial CFD software ANSYS FLUENT was used to verify and validate the new code by comparing their simulation results under a typical heat driven cavity flow case. The comparison showed that the results simulated by the new code had good agreement with the ones calculated by ANSYS FLUENT. Furthermore, by coupling the above CFD model with the fuel pin heat transfer model, the application was conducted based on a full-scale reactor steady-state simulation of a natural circulation driven LBE-cooled fast reactor. From the results, it can be concluded that the new code can simulate and evaluate thermal-hydraulic behaviors of the LBE-cooled reactor well.

Experimental and analytical study on the stability of a low aspect ratio single-phase natural circulation loop coupled to a parabolic trough collector

Use of passive heat transport systems in renewable energy eliminates operator and external power requirements, thereby reducing the cost. Hence, a horizontal heater and horizontal cooler rectangular natural circulation loop (NCL) of low L/D (110) and aspect ratio (0.54) is coupled to a parabolic trough in the present work. Experimental investigations of the loop's steady state and stability characteristics were carried out at various powers corresponding to the solar irradiances using water as the heat transfer fluid. SolTrace was used to determine the heat loads at different direct normal irradiance (DNI) for the selected loop and trough dimensions. The upper threshold for stability was experimentally observed for the first time and reported here. Stability analysis carried out for different loop diameters revealed that the power at which the upper threshold of instability occurs reduces significantly with increase in loop diameter, enabling its observation in the low-pressure low temperature loop employed in the present study. Stabilizing techniques like orifice insertion and loop inclinations were also studied in detail. The results showed stability at higher heat loads (> 350 DNI), while the instability at lower heat loads can be curbed using an orifice (β = 0.5) or a loop inclination (> 4°). The small inclination can eliminate multiple steady state solutions and hence stabilizes the loop. Besides, the reduction in natural circulation flow rate with inclination is much smaller compared to the orifice making its heat transport capability higher. Additionally, the steady state results were validated with 1-D analytical model giving a good match.

Numerical study on the influence of input power and tube shape on a single-phase natural circulation for passive residual heat removal heat exchanger

The passive residual heat removal heat exchanger (PRHR HX) plays an important role in cooling the reactor core under accident conditions. However, few studies have considered the exact conditions of the natural circulation loop (NCL) for the PRHR HX, such as different initial temperatures of fluid in the NCL and the cooler, and configuration of the heat exchanger tubes. In this study, analyses with these considerations are conducted. Influences of different heating power inputs and the two configurations (spiral-shaped and C-shaped heat exchanger tubes) on heat transfer performance are discussed. The CFD computational setting was qualitatively verified and transient computations of 600 s were conducted. The computational results suggest that the oscillation of the first 40 s is associated with difference of the initial temperatures of the fluid in the NCL and the cooler. It is found that the frequency increases first and then the amplitude with increase of heating power. The system reaches a neutrally-stable condition with heating power between 500 W and 750 W, and then it reaches a neutrally-stable condition earlier with an increasing heating power, which also results in a higher frequency and a larger amplitude. In addition, the loop flow rate is positively correlated with the average convective heat transfer coefficient. On the other hand, replacing a C-shaped tube with a spiral-shaped tube insignificantly affects the flow oscillation in the loop, and the more complex heat exchanger tube structure could enhance its heat transfer performance. The results of the study reveal the characteristics of the NCL for the PRHR HX.

Experimental thermal-hydraulic characteristics of single-phase natural circulation loop using water-based hybrid nanofluids

Nanoparticle shape strongly affects the thermal-hydraulic behavior of nanofluid-enhanced single-phase natural circulation loop (SPNCL) but is yet to be explored experimentally. Hence, various mono/hybrid nanofluids having dissimilar shaped nanoparticles (Al2O3+Water, Al2O3+CuO + Water, Al2O3+SiC + Water, Al2O3+MWCNT + Water) with a 0.1% volume concentration, are used in SPNCL to investigate mass flow rate (MFR), heat exchanger effectiveness (HEE) and total entropy generation rate (TEGR). The effect of power input, coolant inlet temperature and loop inclination (both counter-clockwise and clockwise) on transient and steady-state performances are presented. The result reveals that mono/hybrid nanofluid shows enhanced HEE and lower TEGR as compared to base fluids, whereas the MFR may increase or decrease since the performance of SPNCL is significantly impacted by the nanoparticle's shape. Spherical-shaped nanoparticle-dispersed nanofluids show early flow initiation and higher MFR than cylindrical-shaped nanoparticle-dispersed nanofluids. MFR and TEGR rise with higher power input, whereas HEE declines for all working fluids. MFR and TEGR increase as the coolant inlet temperature rises, whereas HEE decreases for all working fluids. The loop inclination decreases the MFR, while it increases the HEE and TEGR. The Counter-clockwise inclination of the loop shows a higher MFR than the clockwise inclination at the same angle of inclination.

Thermal-Hydraulic Investigation of Steady-State Single-Phase Natural Circulation of an Integral PWR-Type SMR Test Rig (iPSTR)

The main aim of the natural circulation of the primary coolant in a nuclear reactor is to reject heat from the reactor core to the steam generator without using a circulation pump. In this work, a vertical heater–vertical cooler, high-temperature, high-pressure, nonuniform-diameter, single-phase natural circulation loop is proposed. The rig contains a spacer grid assembly of electrical heaters in a core with a conical section in its upper plenum and a double helical coil steam generator. The proposed loop is analyzed using RELAP5 and various analytical models. First, these models are benchmarked with experimental data from the Facility to Investigate Natural Circulation in SMART or FINCLS. The model results are found to be in good agreement with the experimental data. The same models are then employed to investigate the proposed natural circulation facility, named the Integral PWR-type SMR Test Rig (iPSTR), to investigate the mass flow rate as a function of geometric and process parameters. Core power input was varied from 5 to 82.5 kW at a maximum system pressure of 10 bar and a maximum elevation difference of thermal centers of 3400 mm. Elevation differences of the thermal centers and diameter of core are found to be important parameters that affect thermal-hydraulic performance significantly. However, cone angle, spacer grid, and system pressure are found to have no significant effect on the performance of the iPSTR. Moreover, the proposed iPSTR is found to possess higher Reynolds number compared with the existing facilities.