Natural Circulation Loop Research Articles

The steady behavior of the supercritical carbon dioxide natural circulation loop

The steady state behavior of thermodynamically supercritical natural circulation loops (NCLs) is investigated in this work. Experimental steady state results with carbon dioxide are presented for supercritical pressures in the range of 80–120 bar, and temperatures in the range of 20–65 °C. Distinct thermodynamic states are reached by traversing a set of isochors. An equation for the prediction of the steady state of NCLs at supercritical pressures is presented, and its performance is assessed using empirical data. Changes of mass flow rate as a result of independent changes of thermodynamic state, heating rate, driving height and viscous losses are shown to be accurately captured by the proposed equation. Furthermore, close agreement between the predicted and measured mass flow rate is found when the measured equipment losses are taken into account for the comparison. Subsequently, the findings are put forward in aid of the development of safe, novel supercritical natural circulation facilities.

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

Numerical Analysis of Flow Stability in a Natural Circulation Loop

Flow modeling of a natural circulation loop is of high importance in ensuring the stability of the system. As a system relying on buoyancy force, several parameters play a crucial role in determining stability. In this study, a one-dimensional numerical model is employed to predict the mass flow rate and stability due to the variation of heated riser and cooled faller locations. The result shows that the highest flow rate of 0.344 kg/s is obtained as the heater is located at the lowest position, while the cooler remains at the top. Furthermore, a stability map is plotted to show the condition for the stability, whereby the result demonstrates that the location with a relatively low mass flow rate exhibits a higher chance of oscillating flow.

Overview of separate effect and integral system tests on the passive containment cooling system of SMART100

SMART100 has a containment pressure and radioactivity suppression system (CPRSS) for passive containment cooling system (PCCS). This prevents overheating and over-pressurization of a containment through direct contact condensation in an in-containment refueling water storage tank (IRWST) and wall condensation in a CPRSS heat exchanger (CHX) in an emergency cool-down tank (ECT). The Korea Atomic Energy Research Institute (KAERI) constructed scaled-down test facilities, SISTA1 and SISTA2, for the thermal-hydraulic validation of the SMART100 CPRSS. Three separate effect tests were performed using SISTA1 to confirm the heat removal characteristics of SMART100 CPRSS. When the low mass flux steam with or without non-condensable gas is released into an IRWST, the conditions for mitigation of the chugging phenomenon were identified, and the physical variables were quantified by the 3D reconstruction method. The local behavior of the non-condensable gas was measured after condensation inside heat exchanger using a traverse system. Stratification of non-condensable gas occurred in large tank of the natural circulation loop. SISTA2 was used to simulate a small break loss-of-coolant accident (SBLCOA) transient. Since the test apparatus was a metal tank, compensations of initial heat transfer to the material and effect of heat loss during long-term operation were important for simulating cooling performance of SMART100 CPRSS. The pressure of SMART100 CPRSS was maintained below the design limit for 3 days even under sufficiently conservative conditions of an SBLOCA transient.

Parallel connected natural circulation loops using different working fluids: experimental results

Natural Circulation Loops (NCLs) are closed-loop systems that transport heat from a source to a heat sink without a pump, relying on free convection of the working fluid. Previous research has focused on the stability and influence of different operative parameters on NCLs. This experimental study investigates the thermo-hydraulic performance of three parallel-connected NCLs with small inner diameters using three different working fluids: deionized water, glycol aqueous solution (50+50% wt), and FC-43 dielectric fluid. This study examines the steady-state behaviour of the NCLs at various heat sink temperatures and heat powers. The results indicate that the common one-dimensional model for a single loop’s steady-state can be applied to this configuration and for all the tested fluids, and a proposed figure of merit can describe the working fluids and predict their steady-state behaviour.

Dynamic characterization of supercritical natural circulation loop under periodic excitation

Characterization of supercritical natural circulation loop under the influence of periodic power profile has not been explored in open literature, despite the promise of producing intriguing dynamics. Present study employs a one-dimensional numerical model to explore the transients on imposing a sinusoidal excitation on the heater power. The resultant dynamics are found to be strongly correlated with the heat transfer regime. The time period of excitation plays a more dominating role compared to the amplitude in deciding the characteristics. System attains dynamic steady-state and persists with a periodic profile at both low and high power levels. But introduction of natural frequency emanates into flow reversal at power levels within the transition regime. Chaotic oscillations can be identified for an intermediate band of imposed frequency, transforming to bidirectional pulsing at higher time periods. Frequency locking within the chaotic zone and frequency pushing for double-periodic nature is also identified, and the appearance of chaos is substantiated using the Poincaré map.

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.

Research on characteristic of flow instability in a two-phase natural circulation loop with parallel once-through steam generators

When there are two sets of parallel once through-steam generators(OTSG) in the natural circulation loop, the thermal-hydraulic characteristics are complicated. Flow instability often occurs in such loops. In order to alleviate the harm caused by flow instability and enhance the natural circulation capacity, it is necessary to study the characteristics of flow instability in depth. In this paper, a thermal-hydraulic computational model is developed for a two-phase natural circulation loop with two parallel OTSGs based on the RELAP5 program. And the model is validated. The thermal-hydraulic parameters of the loop were calculated for different decay heat conditions. The flow instability of the loop is found to be characterized by a clear division into three zones in the decay heat interval. The concept of “load-flow ratio” is proposed to analyze the critical point of flow instability under different operating conditions. The reason for the flow instability zones of the loop is explained. The conclusions of the relevant research can provide a basis for an in-depth study of the characteristics of flow instability and the development of operational schemes to mitigate flow instability.

Experimental study on flow and stability characteristics of horizontal heater and cooler natural circulation loop equipped with air injection

Experimental study on flow and stability characteristics of horizontal heater and cooler natural circulation loop equipped with air injection

Analytical Solution for the Equation of the Simplest Natural Circulation Loop of Working Fluid in a Steam Boiler

Analytical Solution for the Equation of the Simplest Natural Circulation Loop of Working Fluid in a Steam Boiler

Experimental study of pressure drop oscillation in a supercritical carbon dioxide natural circulation loop

Pressure drop oscillation (PDO) is a common type of instability in a two-phase system. The PDO may occur in a supercritical fluid system, with limited prior research on the matter. In this study, an experimental study on PDO of supercritical carbon dioxide (S-CO2) in a natural circulation loop has been carried out. The influence of thermal parameters and compressible volume has been investigated. The experimental result shows that PDO occurs in a natural circulation loop of supercritical carbon dioxide. The location and compressibility capacity of the pressurizer greatly impacts the instability characteristics of the natural circulation loop. The large volume pressurizer installed at the outlet of hot section helps to stabilize the system. The amplitude and the period are influenced by the heat flux and the location of the compressible volume. Increasing the heat flux will slightly increase the amplitude and decrease the period. This work helps to clarify the mechanism of PDO of S-CO2 in the natural circulation loop and provides a reference for the safe design and operation of S-CO2 energy conversion systems.

Irregular pentagon loop for nuclear reactor natural circulation system test apparatus

The natural circulation loop has become a crucial topic for investigating a heat transfer system without a recirculation pump. This study aimed to analyze the steady-state condition of an irregular pentagon natural circulation loop design. The loop was constructed using stainless pipes and equipped with a real-time data acquisition and control system capable of handling high-temperature fluids without human contact. The data acquisition and control system was based on Python programming and operated on a Raspberry Pi mini computer. The heater in the loop was designed from a pipe wrapped around a nichrome wire, and the voltage source could be adjusted using Variac. The cooling system utilized the wind from the blower flowing in the annulus between the pipe inside the larger diameter pipe. The experimental results were used as parameters and boundary conditions in modeling using ANSYS Fluent software. The results showed that the deviation of the modeled temperature from the experimental results was below 1.5%. In addition, the results provided that the higher the power applied to the heater, the greater the Reynolds number value at steady-state, with the difference in value between the highest and lowest power reaching 38.7%. This research contributes to the understanding of the natural circulation system as an efficient heat transfer method and can potentially be used in the design of passive safety systems for generation IV nuclear reactors.

Natural circulation oscillation in a seawater loop during startup

Aiming to improve nuclear safety by using seawater as an alternative emergency coolant during an accident, the knowledge of two-phase flow instabilities and heat transfer performances of seawater is needed. Startup experiments from the quiescent state at room temperature of 20–23 °C are carried out by raising heating power with 3.5 wt% artificial seawater in a natural circulation loop. De-ionized water is adopted as well for the comparative study. In this paper, a type of natural circulation oscillation at the modified heat fluxes of 383.70–530.62 kW/m2 and 404.05–594.97 kW/m2, respectively, for seawater and de-ionized water, with intermittent inlet mass flow rates is exclusively investigated, as a part of the experimental studies of two-phase flow instabilities during the startup of a natural circulation loop. Visualizations of two-phase flow behaviors at the power of 2206 W in seawater reveal the unique bubble foam regimes at the outlet of the heated section, tiny bubbles at the inlet of the riser, and the possible elongated slug bubbles at the central region of the riser.The natural circulation oscillations for two fluids are characterized by four distinct stages in sequence: calm state (stage 1), accumulations of foam/slug bubbles in seawater/de-ionized water (stage 2), propagations of foam/slug bubbles in seawater/de-ionized water (stage 3), and restorations (stage 4). The same period for stages 1, 3, and 4 for both fluids are 25 s, 4 s, and 5 s, respectively. For stage 2, the period measured as 12.5 s in seawater is significantly shorter than that recorded as 45 s in de-ionized water, leading to earlier systematic natural circulation rates. It is possibly caused by the much stronger accumulation effect of tiny bubbles in the riser, resulting in vast buoyant forces by draining more liquid out of the riser. The intermittent peak inlet mass flow rates of 0.017 kg/s and 0.023 kg/s are recorded, respectively, for seawater and de-ionized water. It would not induce a pressure drop increase over the riser while cause a very high pressure drop oscillation of 4.67 kPa in de-ionized water. Slower circulation rates induce smaller wall temperature drops of 3.6 °C in seawater than that of 5.2 °C in de-ionized water. Under zero flow rate conditions, the wall temperatures are mainly kept at 117–119 °C, accompanied by bubble foam without ruptures in seawater, while that are about 115 °C with slug bubbles due to bubble coalescences in de-ionized water. It demonstrates that the immobile foam may deteriorate the heat transfer under pool-like conditions.

Evaluation of possible leakage causes in a natural circulation helium loop

With the development of IVth. Generation nuclear reactors, the safety systems have to be adjusted to comply with the specific requirements of these new reactor designs and handle any nuclear accident in the best possible way. The Fukushima accident showed that the decay heat present after the reactor shutdown can still be a threat, and robust and reliable safety systems must be introduced. The natural circulation loop represents such a system since the main driving force is partially supplied by the decay heat itself. To investigate the possibilities of using a natural circulation system, we built an experimental helium loop, where the thermodynamic and hydraulic properties of the system were studied. During the measurements, coolant leakages were observed. The scale of the experimental device is close to the scale of a real application; therefore, any unexpected behavior should be properly investigated and reported, especially if the phenomena involve concerns about possible coolant leakage. This paper reports and analyzes measured leakage rates during several measurements on a large-scale, high-temperature, high-pressure natural circulation loop.

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.

Efficacy of natural circulation loop based parabolic trough collector- An analytical approach

ABSTRACT Realizing the importance of the natural circulation loop (NCL), which is a passive heat transport device, an analytical study is undertaken to demonstrate the functioning of an NCL-based parabolic trough collector (PTC). Multiple validated analytical models were incorporated to evaluate the system performance. The parametric study involved variations in loop height, loop length, and solar radiation. The loop flow rate enhances by increasing its height or the receiver length. At maximum radiation, 1.3 kg/s flow rate was observed for loop geometry of 5 m height and 7.8 m length against 0.79 kg/s for 1 m height. The flow rate predicted for a 10 m length and 1 m height loop was 0.87 kg/s. Since instability rises with height and heat loss and receiver temperature increase with receiver length, a trade-off between the two is necessary to function NCL-based PTC properly. Otherside, though the higher heat output with an increase in the receiver length is evident, a noticeable drop in thermal efficiency (≈7%) during the peak hours of solar radiation is a matter of concern. Further, the outlet temperature was not affected by the loop height for the defined flow condition of the process fluid. Like in conventional PTC systems, the process fluid’s exit temperature or flow rate can also be maintained constant during a day operation by varying the flow condition. The proposed model can deliver ≈ 1500 liters of hot water in six hours with an average 68% thermal efficiency without external power, in contrast to the necessity of notable pumping power and associated overhead charges in conventional PTC. Further, the system’s stability was predicted using linear stability analysis, and stability maps were generated for different loop heights and found that as the loop height decreases, the unstable zone diminishes. Though the steady-state data points are in the unstable flow region, stable operation is expected by incorporating pertinent instability restraining technique, that is, loop tilt.

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.

Linear and Nonlinear Stability Analysis of Molten Salt Natural Circulation Loop

Abstract A molten salt natural circulation loop (MSNCL) has been setup to study the steady-state, transient, and stability characteristics of molten nitrate salt, mixture of sodium nitrate, and potassium nitrate in 60:40 ratios by weight. Natural circulation experiments in a temperature range of 290 °C–600 °C have been performed. A semi-analytical linear model was derived for stability analysis of rectangular natural circulation loops with conventional localized surface heating and cooling. The developed model includes the effect of wall thermal inertia, variable internal heat transfer coefficient, finite secondary side heat transfer coefficient at cooler, and heat losses for predicting the stability map. These effects are incorporated considering their significant role in modeling high-temperature molten salt-based natural circulation systems. The developed model has been first validated with the experimental data of water loop available in literature. The developed model is then validated with the experimental data generated in MSNCL. Validation of in-house developed nonlinear model has also been performed against the same experimental data. The comparison of both linear and nonlinear stability analysis with the experimental data shows good agreement and articulates the importance of various parameters which have been included in the developed model.

Heat transfer study of natural circulation serpentine-shaped loops

Wide range of industrial applications have been utilized natural circulation loops due to heat transport achieved without the aid of any fluid-moving machinery. Moreover, it has been commonly used either two-phase-rectangular- or square-shaped loops owing these geometries promote satisfactory heat transfer and buoyancy forces performance, however, these two geometries require structural elements of considerable size for certain applications. Therefore, this study thoroughly examined a novel design of 10- and 14-elbow-single-phase-serpentine-shaped loops set in vertical configuration for improving cold water dispensers’ systems in a so called side-by-side refrigerator with the size restriction of its cabinet dimensions. Furthermore, when the application was not constricted to the use of water, numerical tests of heat transfer analysis for three working fluids commonly employed in industrial processes, namely, ethylene glycol, ethanol, and air, also was done. Both loops had the same surface area exposed to the heat source and heat sink, along with the total volume within the loops and the pipeline diameter were equal in the two cases. From the procured results, on one hand, when water was used and Reynolds number was increasing, employing the 10-elbow loop, an increase in internal Nusselt number of 24.5% was obtained. On the other hand, when ethanol was used an increase of Nusselt number of 27.28% was achieved. Finally, a great influence of the height of the loop on the increase of buoyancy forces was observed, hence, higher values for Rayleigh number were obtained when 14-elbow loop was utilized instead of 10-elbow loop; nonetheless, higher values of Rayleigh number did not result in greater values of Nusselt number, concluding that a smaller number of elbows promoted a better heat transfer performance under the operation conditions of the present work.

Numerical investigation of natural circulation loop with supercritical CO2 on the thermal control system of micro spacecrafts

Aiming at the heat dissipation problem under the strict constraints of mass, volume, and power consumption in micro spacecrafts, the feasibility of applying a natural circulation loop (NCL) to the thermal control system is performed for the CubeSats (Cube satellites) of orbiting operations or the micro-rovers of extraterrestrial explorations. A 3-dimensional NCL model under variable gravity conditions corresponding the environment of aerospace is established, and the supercritical CO2 as the coolant. The heat transfer and thermal performance of NCL under gravity range of 10−6 g to 10 g are analyzed numerically, including the Earth orbit, the surface of the Moon and the Mars etc. The calculation results present that the gravity has a great influence on the flow circulation characteristics and thermal performance of the NCL. The applicability of employing NCL on the thermal control system of micro spacecrafts is validated with the gravity no less than 0.05 g. It is worth noting that, by designing the range of temperature from inlet to outlet of the heated tube covering the large specific heat region, the superior thermal performance can be obtained on account of the strong heat-carrying capacity of supercritical fluid, even under the low gravity with a small mass flow rate. Finally, a flow correlation is proposed considering the effect of gravity, flow regime, buoyancy force, and geometric factors, which is suitable on the NCL in the thermal control system of micro spacecrafts, and the R2 is 99.77%.