Single-phase Natural Circulation Research Articles

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.

Validation of SAS4A/SASSYS-1 for predicting steady-state single-phase natural circulation

The validation of system analysis codes for nuclear reactor systems is required for the development and application of these computational tools. Designed as a comprehensive system analysis code for advanced nuclear reactors, SAS4A/SASSYS-1 requires validation of its physics model for capturing single-phase natural circulation behavior. To support the validation of SAS4A/SASSYS-1, high-precision experiments are performed capturing steady-state single-phase natural circulation on a scaled facility with comprehensive instrumentation. Dedicated tests are performed quantifying the critical modeling parameters, and a single-phase natural circulation benchmark dataset is obtained with well-documented uncertainty and comprehensive facility description. The validation is then performed against the dataset examining the capability of SAS4A/SASSYS-1 in simulating steady-state single-phase natural circulation. The experimental facility is modeled in the candidate code. Solution verification is performed using Richardson-extrapolation-based estimators which quantify and restrict numerical errors from discretization. Input uncertainty provided by the benchmark dataset is forward propagated through the candidate code, quantifying the output uncertainty in a Monte Carlo approach. The composition of the output uncertainty is also quantified through a variance-based sensitivity analysis. With the uncertainty quantified for each individual condition, a detailed comparison between the simulation results and experimental data is performed covering the whole dataset. The results show consistent agreement for all primary parameters. The current validation activity provides a valuable benchmark dataset for the validation of system analysis codes in capturing single-phase natural circulation and demonstrates satisfactory prediction capability of SAS4A/SASSYS-1 for steady-state single-phase natural circulation.

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.

Fundamental evaluation of the effect of pipe diameter, loop length and local losses on steady-state single-phase natural circulation in square loops using the 1D network code Flownex

In this study, steady-state single-phase natural circulation of water in square loops is simulated using a 1D network code. Fundamental explanations are given for the behaviour of the buoyancy force and mass flow rate. The methodology and model are validated by the very good agreement with the generalized correlation of the predicted steady-state Reynolds numbers, as a function of the modified Grashof numbers and loop geometry numbers. For a given pipe diameter and loop length, it is shown that the mass flow rate and buoyancy force increases when the power increases. However, due to the effect of the increase in the mass flow rate, the actual increase in the temperature rise over the heater, which is responsible for the buoyancy force, is only 66% of that associated with the increase in the power. Increasing the pipe diameter increases the mass flow rate significantly and decreases the buoyancy force decreases markedly, due to the significant decrease in the frictional resistance. For a given power and pipe diameter, when no local losses are assumed, the mass flow rate is independent of the loop length. When local losses are accounted for, the mass flow rate, however, increases with increasing loop length until it reaches an asymptotic value equal to the mass flow rate for no local losses. For a given power, pipe diameter and loop length, the mass flow rate increases when the inlet temperature increases, due the non-linear nature of the density of the water as a function of temperature.

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%.

Experimental analysis of phase change material integrated single phase natural circulation loop

Natural circulation loops (NCLs) are mainly consists of three sections i.e., Heat source, heat sink, and adiabatic section. The working fluid in the NCL flows due to density difference caused by the temperature difference between the heat source and heat sink. Single-phase NCLs are used in many types of energy systems, such as solar heaters, nuclear reactors, geothermal power production, engine and computer cooling. Most of the engineering devices are operated with transient heat flux which creates the problems like thermal instabilities and shortage of energy supply. Phase change materials were used for thermal energy storage applications. Thermal energy storage is required to diminish the gap between demand and supply by storing the extra thermal energy during the charging time and utilized in peak time. The current work presents a novel method of integrating phase change materials (PCMs) within the evacuated solar tube collectors of a Single-Phase natural circulation loop (SPNCL) for reducing the instabilities. In this method, the Heat Transfer Equipment (HTE), the pipe is integrated with phase change material, where heat is effectively accumulated and stored for an extended period of time due to thermal insulation of evacuated tubes. The benefit of this method includes improved functionality by delaying release of heat, thus providing heat during the hours of high demand or when the solar intensity is insufficient. The work compares the experimental data with and without Phase Change Material. The experiment is performed for various flow rates and various heat fluxes. SPNCL with PCM evaporator shows the best thermal performance compare to the without PCM.

Numerical investigation on the heat transfer coefficient jump in tilted single-phase natural circulation loop and coupled natural circulation loop

The present study investigates the effect of inclination on the buoyancy driven systems. There are extensive studies on the cavity systems which exhibit a heat transfer coefficient jump upon the change in the inclination as the system moves from one steady state to another. The present numerical study makes use of 2-D and 3-D CFD simulations to investigate if a similar behaviour occurs in natural circulation systems such as a Natural Circulation Loop and a Coupled Natural Circulation Loop. The study identifies a combination of flow pattern rearrangement and flow direction reversal as the phenomena responsible for the heat transfer coefficient jump. The study also examines carefully as to why the same was not reported previously. The CFD study is validated thoroughly with the numerical and experimental data available in the literature. A parametric study is also conducted to examine the effects of the aspect ratio, the Rayleigh number, the Prandtl number and the inclination with the plane of the Natural Circulation Loop.

Closed-loop experimental investigation of single-phase natural circulation flow phenomena based on temperature and heating power variations

Experimental investigation of single-phase, low pressure natural circulation flow phenomena has been carried out. The study aims to critically analyze the thermal hydraulic parameters (temperature and heating power) variations as it affects transfer of heat in a closed-loop rectangular natura l circulation facility having a single heated channel at predetermined inlet sub cooled temperature varied from 40 to 80 °C. Heating power varies between 7 kW−13 kW to portray several conditions of the reactor power in response to temperature changes while sustaining single-phase flow circulation through the hydraulic loop. Real-time signals of thermal hydraulic responses are observed via a graphical interface for data acquisition when stable flow or self-sustained flow oscillations are achieved. Results obtained were analyzed at several sections of the hydraulic circuit which are basically pressure drops and flow rate due to temperature differences and heating power increments. These results and observations made through them thus contribute to the database which is important for characterizing natural circulation phenomena and acquisition of dominants parameters for the scaling analysis of future designs.

CFD analysis on flow and heat transfer characteristic in natural circulation system with subchannels under rolling conditions

The combination of advancement in nuclear technology and ship engineering is a new technology of nuclear application which can not only marine capability and provide power for remote islands, but also help solve the practical issues of high cost, high pollution and high pressure in cutting down emissions in the petroleum exploitation field. The CFD analysis in this paper focuses on the exploration of single-phase natural circulation flow and heat transfer characteristics for rod bundle under rolling conditions. The rolling version CFD tool is developed and compared with the calculated results by system code. The natural circulation system which consists of a 2 by 2 subchannel is established. This paper mainly focuses on the effect of different rolling parameters on natural circulation flow and heat transfer characteristics for rod bundle loop under rolling motion. The thermal–hydraulic performance varies periodically under rolling motion. The variation period equals to rolling period. The stronger the rolling intensity is, the greater the fluctuations of different thermal–hydraulic parameters are. The average values of mass flux and heating section temperature difference under rolling conditions are lower than those under non-rolling conditions, proving that rolling motion may enhance heat transfer. Meanwhile, the inlet temperature of heating section under rolling conditions is lower compared with that under non-rolling conditions. With increasing rolling intensity, the effect of heat transfer enhancement is greater.

Heat transfer characteristics of redan structure in large-scale test facility STELLA-2

The construction of STELLA-2 facility is on-going to demonstrate the safety system of PGSFR and to provide comprehensive understanding of transient behavior under DBEs. Considering that most events are single-phase natural circulation flow with slow transient, STELLA-2 was designed with reduced-height of 1/5 length scale. The ratio of volume to surface area in the vessel can relatively increase resulting in excessive heat transfer. Therefore, a steady-state thermal-hydraulic analysis was performed and the effect of design change to reduce the heat transfer through redan was investigated. The heat transfer through single wall redan in STELLA-2 was 3% of the core power, comparable to 1% of the core power in PGSFR. By applying the insulated redan, about 70% of decrease effect was observed. The effect on transient behavior was also evaluated. The conclusion of this study was directly applied to the STELLA-2 design and the modified version is under construction.

Investigation of a nanofluid-based natural circulation loop

The Natural Circulation Loop (NCL) has many applications in nuclear and renewable energy systems including solar heaters and geothermal power derivation. Applying nanofluids in NCL has distinctive advantageous, including higher stability and effectiveness of NCL. The present study is a numerical investigation of a rectangular single-phase natural circulation loop utilizing Water, water-based Al2O3, and TiO2 nanofluids as working fluids. Both steady-state and transient simulations are in good agreement with experimental data. The effects of heating power and nanoparticle concentration on the stability and flow transitions were investigated in this study. Flow rate and heat transfer capacity enhancement were significantly observed by applying nanofluids in NCL compared to Water. The comparison of effectiveness factor, temperature difference over heater, and mass flow rate of NCL utilize different working fluids, revealed thermal performance improvement by particle concentration increment, and this is more considerable for the nanofluids containing bigger particle size.

Single-phase natural circulation in a rectangular loop with a finite capacity heat sink positioned below the heater section

One of the passive safety systems employed in nuclear reactors is a large water pool positioned at an elevation, which acts as a heat sink to remove decay heat from the reactor core in the event of an accident using the natural circulation phenomena. A large mass of water at an elevated position has concerns regarding the safety of support structure, and therefore an alternative methodology where the heat sink is positioned just below the heater section is investigated in this study. Results of an experimental and numerical investigation for the thermal-hydraulic behavior of single-phase natural circulation in a rectangular loop of height 4300 mm and width 1900 mm with the heat sink located at an elevation just below the heater section is reported here. The heat sink was a finite one, a constant volume insulated tank filled with water, whereas the heat source was a constant heat flux type. The loop was constructed with circular pipe elements with a nominal inner diameter equal to 40 mm. The heater section was 1m in height and three input power values equal to 1.15 kW, 1.55 kW, and 2 kW were used in the study. The flow behavior in a single and twin parallel heater system with a single riser was studied. The loop fluid is initially in a single-phase and becomes two-phase as the heat sink temperature rises, but only the results for the loop fluid in the single-phase regime are presented in the current study. The measured mass flow rate, after an initial transient, settles down to values which slowly decreases with time. The average temperature of the heat sink is shown to rise continuously with increasing time, demonstrating the effectiveness of the heat sink in removing the energy input to the working fluid in the heater section. However, the heat sink removes only 60–70% of the input heat while the rest is either lost to the ambient or utilized to raise the enthalpy of the working fluid.The Nusselt numbers in the heater section are also presented which steadily rise after an initial transient. In addition, the values are much higher than those for the corresponding Reynolds number values in forced convection due to the influence of buoyancy-induced convection. The Nusselt number is well correlated by the expression Nu‾=1.35(Ra*‾)0.3 for 2×105≤Ra*‾≤6.5×105and 600≤Re≤1000. Numerical predictions for loop flow rate, temperature variation, and sink temperature using the RELAP5/MOD3.2 code for the experimental configuration of the current study are presented which compare well with the measurements the maximum deviation being less than 10%. Numerical results for pressurizing the working fluid and permitting boiling in the heat sink to drive the system towards steady-state operation are also presented.

Investigation of performance improvement of a household freezer by using natural circulation loop

Growing number of household refrigerators and freezers also increase the overall energy consumption and number of household refrigerators have grown nearly 3% per year. On the other hand, investigations showed that household refrigerators have a significant technical energy saving potential of up to 30–40%. In this study, a single-phase natural circulation loop is considered as a suction line heat exchanger between the high and low pressure sides of conventional refrigeration cycle of a household chest freezer. The potential heat transfer by the single-phase natural circulation loop is studied by a CFD analysis, considering the possible temperature range for refrigeration cycle and geometrical parameters that affect the loop performance. The performance map of this refrigeration cycle is presented for different compressor suction and discharge pressures (LP/HP), discharge temperatures (T2) and different compressor speeds. The results indicate that the highest amount of heat transfer is obtained for the single-phase natural circulation loop with aspect ratio of 0.8 and pipe diameter of 6.53 mm. Moreover, the single-phase natural circulation loop provides 11.5% efficiency improvement for the household freezer for maximum compressor speed at lowest T2 and LP/HP. Considering reduction in required compressor work, SPNCmL assisted refrigeration system has a wider operating range.

Investigations on the dependence of the stability threshold on different operating procedures in a single-phase rectangular natural circulation loop

During the past years, several studies were focused on the instability of single-phase rectangular natural circulation systems. However, experimental identification of instability threshold and categorization of instability regimes is of prime interest in many industrial applications which have been addressed by very few researchers that too limited to lower diameter loops only. Hence, in the present study, a larger diameter rectangular loop with horizontal heater horizontal cooler (HHHC) configuration has been designed to generate single phase natural circulation data on the steady state and stability behavior. Effect of different operating procedures (i.e., start-up from rest, power raising from stable steady state, and power reduction from an unstable state) on stability behavior are studied. Most of the oscillatory regimes are identified and noticed that the operating procedure can influence the instability threshold. The instability threshold was found to increase with the lowering of the coolant temperature and increasing its flow rate. The operating procedures i.e., power raising from stable steady state and start-up from rest gave the maximum and minimum values of the stability threshold respectively. As opposed to this, the minimum instability threshold in smaller diameter loops was reported for the operating procedure with power reduction from an unstable state. The experimental results confirm the occurrence of the hysteresis phenomenon. Due to the larger diameter and lower Lt/D ratio of the loop, all the oscillatory behaviors were aperiodic in nature in contrast to the observations in smaller diameter loops.

Experimental analysis of four parallel single-phase natural circulation loops with small inner diameter

In this paper, results from a new experimental setup consisting of four rectangular natural circulation loops are presented. The loops are vertically oriented, parallel and connected at the bottom; each one has an independent heat source of variable power and a common heat sink, and are studied under different configurations with varying parameters. The loops are symmetrical with 7 mm inner diameter, using distilled water as the working fluid transporting heat from the source to the heat sink by means of natural circulation. The experimental data are compared with the Vijayan’s correlation showing a good agreement.

Leakage estimation of the high-pressure and high-temperature natural circulation helium loop

Natural circulation loop systems are promising a reasonable option to provide the emergency decay heat removal from a nuclear reactor during an electrical power outage. The present paper deals with an experimental study on the influence of leakages on the behaviour of a single-phase natural circulation loop. The research shows the thermodynamic and hydraulic processes in a large scale natural circulation loop during several measurements. The experimental device was designed to provide heating and cooling power of 250 kW with the maximum operating temperature and pressure 520 °C and 7 MPa, respectively. The continuous loss of coolant is very dangerous because it can result in coolant overheating.

Experimental Investigation of Natural Circulation in a Single-Phase Loop with Different Widths

The natural circulation loop is one of the design concepts of a cooling system in new advanced reactors that has attracted many researchers to develop it. This study aimed to perceive the effect of horizontal width variation on the thermal behavior of a single-phase Natural Circulation Loop (NCL). NCL apparatus with a vertical heater and a vertical cooler was designed for experimental study. The height of the loop was 100 cm while the width of the loop was varied at 50 cm and 100 cm. The heater was designed using Nichrome wire on the outside of the stainless pipe while the cooler was designed using pipe-in-pipe with water flowing through the annulus. Arduino microcontroller and K-type thermocouple sensors were used in temperature data acquisition. XAMPP software was used in data recording. The results of this study indicated that the loop of a 100 cm width has a difference in the temperature of the fluid coming out of the heater and entering the heater that reaches 156,0% higher than the loop of a 50 cm width at the same input voltage. This study is supposed to be one of the references for a single-phase natural circulation loop.

Effect of Nanofluid Thermophysical Properties on the Performance Prediction of Single-Phase Natural Circulation Loops

Specifying nanofluids’ thermophysical properties correctly is crucial for correct interpretation of a system’s thermo-hydraulic performance and faster market-uptake of nanofluids. Although, experimental and theoretical studies have been conducted on nanofluids’ thermophysical properties; their order-of-magnitude change is still a matter of debate. This numerical study aims to reveal the sensitivity of single phase natural circulation loops (SPNCL), which are the passive systems widely used in solar thermal and nuclear applications, to thermophysical property inputs by evaluating the effects of measured and predicted nanofluid thermophysical properties on the SPNCL characteristics and performance for the first time. Performance and characteristics of an SPNCL working with water-based-Al2O3 nanofluid (1–3 vol.%) for heating applications is evaluated for different pipe diameters (3–6 mm). The thermal conductivity effect on SPNCL characteristics is found to be limited. However, viscosity affects the SPNCL characteristics significantly for the investigated cases. In this study, Grm ranges are 1.93 × 107–9.45 × 108 for measured thermophysical properties and 1.93 × 107–9.45 × 108 for predicted thermophysical properties. Thermo-hydraulic performance is evaluated by dimensionless heat transfer coefficients which is predicted within an error band of ±7% for both the predicted and measured thermophysical properties of the data. A Nu correlation is introduced for the investigated SPNCL model, which is useful for implementing the SPNCL into a thermal system.

Heat transfer enhancement based on single phase natural circulation loops

Heat transfer between two streams of hot and cold fluids separated by a plate is an unavoidable exergy destruction process as a whole, i.e. the exergy gain from the cold fluid cannot fully cover the exergy loss of the hot. However, it is noticed that the amount of exergy destruction can be minimized by introducing the work done by the external gravity force that the natural convection of the third fluid in a closed loop driven by the hot and cold fluids themselves can enhance their heat transfer. Based on this idea, a heat transfer enhancement unit of a closed rectangular loop was constructed, the two parallel legs of which were vertically penetrated through a horizontal plate with one half on the hot side and the remained half on the cold side. In order to clarify the mechanism of heat transfer enhancement for the natural circulation loop (NCL), a numerical simulation was performed for a comparison between the heat transfer rates of the NCL and the copper fin with the same shape and size. The effects of side length, velocities of heating and cooling air, and temperature difference on the heat transfer rate were investigated in detail. The results show that the heat transfer rate of the NCL can be larger than that of the copper fin while the temperature difference is over a critical value.

Experimental study on a single-phase natural circulation loop and its steady-state solution

A practical case of a single-phase natural circulation (SPNC) loop having a considerable circulation length and non-uniform structures is investigated via experimental and analytical approaches. The target facility (Facility to Investigate Natural Circulation in SMART, FINCLS) corresponds to the type of vertical heater and vertical cooler (VHVC), and the maximum height and total circulation length are 12.2 and 34.5 m, respectively. A series of SPNC tests comprising 29 steady-state conditions are performed under a wide range of working conditions: 0.5 MPa (60 °C) to 15.0 MPa (300 °C). Unexpected characteristics of the natural circulation (NC) flow rate are observed for two test groups under high-temperature conditions. An NC loop (NCL) model for a steady-state solution that reflects both heat-loss and heat-sink effects is developed by employing the lumped-component method and the moving-boundary algorithm. The NC flow rates for 29 steady-state conditions predicted by the NCL model agree well with measurements (within 5% error), and the unexpected characteristics of the NC flow rate are clearly demonstrated via analysis of the NCL model.