Single-phase Natural Circulation Loop Research Articles

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.

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.

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.

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.

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.

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.

Numerical investigations on dynamic characteristics of series coupled single-phase natural circulation loops

On the backdrop of the Fukushima accident, the importance of passive heat removal systems based on coupled natural circulation loops (NCLs) has gained more attention towards nuclear safety. However, the transient behavior of these coupled NCLs is quite complex due to their regenerative feedback effects. In this connection, a numerical model based on the finite volume method is developed to investigate the transient performance of series-coupled NCLs. In this study, a double pipe heat exchanger (HX) is employed, while most of the published works in literature considered only point contact and plate type HXs. The model developed is validated extensively against the benchmarked single and coupled NCL experimental facilities. The model has additional features such as wall conduction, equation of state (EOS) for properties, wall-fluid heat transfer coefficient correlations, free convection type heat exchanger, variable sections, different heater and cooler orientations, etc. The grid and time-step independence studies are carried out for the actual application of the model. The model is then employed to investigate the horizontally and vertically coupled NCLs. The effects of essential parameters such as power, pressure, aspect ratio, wall thickness, wall material, initial flow and temperatures, loop diameter, ambient heat loss and heater-cooler orientations etc., are investigated on the flow and thermal transients of coupled NCLs. Subsequently, a comparison of single and coupled NCLs is carried out. Finally, a sensitivity analysis is carried out by defining the Sensitivity Factor (SF), and the dominant parameters which influence the transients of coupled NCLs are highlighted. The findings of this study can be helpful for the design of coupled NCLs for conventional and nuclear applications for the purpose of safety.

Experimental Study on the Dynamic Behavior of Start-Up from Rest in a Single-Phase Natural Circulation Loop

Experimental Study on the Dynamic Behavior of Start-Up from Rest in a Single-Phase Natural Circulation Loop

Adequacy of single phase natural circulation loop in parabolic trough collector

A larger diameter single phase natural circulation loop is analysed, with a focus on parabolic trough application. Since the geometry chosen is highly unstable, loop tilt is invoked, which resulted in steady state. Although, inclination of 2° initiated the stability process, for better response 4°–6° is suggested. To generate good number of data, 1-D model is validated and extended to different tilts and solar irradiance. Although loop fluid flow rate enhances with irradiance level, higher tilt negates the performance level notably. The gist of the present experimental cum theoretical study is, the application of a natural circulation loop in parabolic trough is conceivable with desired modifications in the present system.

Experimental and computational study of a low-pressure natural circulation loop

The experimental and analytical study of single-phase flow and heat transfer in natural circulation loop has been carried out. Experiments were performed on water and ethanol that are the liquids with significantly different thermophysical properties. Experimental apparatus was a rectangular shaped loop with vertical flow up leg. The flow up and flow down legs of the loop are joined to the separator-condenser at the top of the loop. The upper limit of heat flux densities in the experiments was set with the consideration for flow regime to remain in single phase state along the whole heated length. Wall temperature time records being registered at different distances from the inlet to the heated zone indicate the occurrence of temperature fluctuations near the exit from heated zone even at relatively low heat flux densities. This fact displaces a complex changing of velocity profiles along the tube with vortex formation and occurrence of flow instability. Experimental data on longitudinal wall temperature distributions of heated section have been used to test a modified method of hydraulic calculation of the loop. It was pointed out that in spite of long year (since early 1950s) experimental, analytical and numerical investigations of natural circulation loops no suitable predicting recommendations for heat transfer and friction have been proposed till today for engineering hydraulic calculations of single-phase natural circulation loops.

Stability enhancement in large diameter rectangular natural circulation loops using flow restrictors

Single-phase natural circulation loops (SPNCL) finds application in district heating reactors and low-power pressurized water reactors. It is not only adopted as a backup operation mode during accidental scenarios but also as the normal operating mode in several small and medium-sized advanced nuclear power plants. Stable operation is desired in such plants for smooth power generation and hence, the stability analysis of single-phase natural circulation systems assumes significance. The horizontal heater and horizontal cooler combination produce a maximum flow rate for a specific heater power but also is the most unstable compared to other configurations. Out of many instability restraining techniques, the orifice plate has been targeted by very few. The present experimental study focuses on the effect of using orifice plates (β = 0.90, 0.77, 0.64, and 0.51) on the stability of the 38.89 mm diameter rectangular loop with a horizontal heater and cooler configuration. The various operating procedures (start-up from rest, power raise from a stable steady-state, and power step down from an unstable state) have shown different stability/instability thresholds. Further, a hysteresis phenomenon was observed between the operating procedure of power raise from a stable state and the power step down from an unstable state, both exhibiting different stability thresholds. The increase in the stability threshold for β = 0.9, 0.77, and 0.64 for the operating procedure of start-up from rest was 75%, 110%, and 225% respectively compared to without orifice. Since the heater power was limited to 1100 W, the orifice of β = 0.51 was seen to be completely stable for the entire range of all three operating procedures for the same cooler conditions. From the loss coefficient (K), the effective length of the loops for β = 0.9, 0.77, 0.64, and 0.51 increased by 0.36, 1.32, 4.72, and 21.8 m respectively. Accordingly, the steady-state analytical solution with and without orifice found a good agreement with the experimental results. Thus, the present experimental investigations show that with the judicious choice of the flow restrictor, an unstable natural circulation system can be stabilized although the steady-state flow decreases.

Experimental Study of Thermal Behavior of a Single-phase Natural Circulation Loop with Vertical Heater and Cooler

The natural circulation systems have been widely applied to various technologies, for example, safety systems in molten salt reactors. Hence the instability of the natural circulation system became an exciting topic to study. This study aimed to find out the effect of horizontal width variation on the thermal behavior of a single-phase natural circulation loop (NCL) with vertical heater and vertical cooler. Three loops with horizontal widths of 50 cm, 100 cm, and 150 cm, respectively, were designed for experimental studies. Meanwhile, the vertical height of all three loops was 100 cm. The cooler was installed vertically on the upper arm of the loop to cool the fluid. The heater was installed vertically on the forearm on the other side of the loop to heat the fluid. K-type thermocouples integrated with the data acquisition system were used to measure the temperature distribution in several zones of the loop directly. The conclusion obtained in this study was a single-phase natural circulation loop with vertical heater and vertical cooler did not indicate a reversal of the direction of heat transfer at each operating power and geometry configuration. This study is expected to be one of the references to the design of a single-phase natural circulation loop as an advanced nuclear reactor passive safety system.

Thermo-hydraulic performance of connected single-phase natural circulation loops characterized by two different inner diameters

In this paper, an experimental study regarding the natural circulation in connected vertical rectangular loops is presented. Typically, the geometry of the natural circulation loops (NCL) is rectangular or toroidal and the analysis is focused on the thermo-hydraulic behaviour of a single working loop. In this study, a new approach referred to several NCLs simultaneously operated was investigated. In the experimental setup, the lower heated sections of the NCLs are connected. The tests were carried out by varying the internal diameter of the loops to investigate the influence of the mutual interaction between two adjacent NCLs. The data were analyzed by the modified Vijayan's correlation showing a good agreement. A simple relationship is proposed to normalize the data both in terms of inner tube diameter and heat sink temperature. The results obtained with two NCLs characterized by different internal diameters could be considered as the first attempt to try optimizing several connected NCLs.

Effects of various modeling assumptions on steady-state and transient performances of single-phase natural circulation loop

It is necessary to investigate the relative effect of various phenomena before neglecting the same for simulating the single-phase natural circulation loop to obtain an acceptable prediction. Hence, the present study focuses on the validation and effect of different assumptions related to Boussinesq approximation, property variation, bend effect, heat loss, axial fluid and wall conduction on the transient and steady-state characteristics. Effect of assumptions for different working fluids (water, brines and hybrid nanofluids), and assess the effect of different heat flux distributions, like uniform, linear, non-linear, sinusoidal and Gaussian, applied to heater are also explored. Mass flow rate (predicts heat transport capability), effectiveness (predicts heat transfer ability) and entropy generation rate (predicts irreversibility) are evaluated. The results disclose that the Boussinesq approximation with constant other properties under-predicts the steady-state mass flow rate and effectiveness and over-predicts the entropy generation rate. This assumption cannot be applied for all working fluids. The fluid and solid conduction is the crucial parameter for simulating transient characteristics. Gaussian heat flux distribution shows the highest mass flow rate and non-linear increasing heat flux shows the highest effectiveness and entropy generation rate. This study will help to consider reasonable assumptions for the numerical simulation.

Single-Phase Natural Circulation Loop Using Oils and Ternary Hybrid Nanofluids: Steady-State and Transient Thermo-Hydraulics

Abstract Steady-state and transient behaviors of single-phase natural circulation loop (SPNCL) are investigated using four thermal oils (Therminol VP1, Paratherm CR, Dowtherm A, and Dowtherm Q) and water-based ternary hybrid (various combinations of different nature and shaped nanoparticles: Al2O3, Cu, carbon nanotube (CNT) and graphene) nanofluids as loop fluid. The influences of nanoparticle volume concentration and loop height-to-width ratio on the mass flow rate and total entropy generation rate of SPNCL are investigated. Results disclose that ternary hybrid nanofluids enhance flow initiation, reduce fluctuation and are expected to attain a steady-state faster than water. Steady-state mass flow rate increases/decreases for ternary hybrid nanofluid depending on the shape of the nanoparticle, and the total entropy generation rate decreases as compared to water. Thermal oil shows a higher mass flow rate and total entropy generation rate as compared to water. Al2O3–Cu–CNT–water and Paratherm CR show the best result among all ternary hybrid nanofluids and thermal oils, respectively. The nanoparticle shape decides the optimum nanoparticle volume fraction. Increasing the height-to-width ratio decreases the total entropy generation and upsurges the mass flow rate at specified input power. The optimum height-to-width ratio depends on the loop fluid.

Effect of geometrical parameters on the performance of nanofluid-based single phase natural circulation mini loops

Single phase natural circulation loops are widely used passive systems in heat transfer applications. Since achievement of stable flow and highly efficient heat transfer is the main concern of recent studies, effects of geometry and the working fluid on the single phase natural circulation loops’ performance are getting attention. In this study, aspect ratio and pipe diameter effect on thermo-hydraulic performance of mini SPNCLs (SPNCmLs) has been investigated numerically by developing a 3D steady model. Water based Al2O3 nanofluid (1, 2, 3 vol. %) was used as a working fluid. Performance and characteristics of the loop for different working fluids is evaluated by using temperature distributions, mass flow rates and derived non-dimensional parameters. It is shown that pipe diameter effect on the heat transfer performance is more significant compared to aspect ratio. Moreover, nanofluids have higher temperatures and thus effectiveness values compared to water. In order to generalize the SPNCmL performance Num and Ress correlations were developed within the accuracy of ±10%.