Near-critical Condition Research Articles

Preliminary conceptual design with neutronics and thermal-hydraulics assessments of FuSTAR

The fluoride-salt-cooled high-temperature advanced reactor (FuSTAR) is an advanced small modular reactor, which aims to the electric supplement and clean heat production to western China. In order to have good economy and safety, FuSTAR needs to have the characteristics of small size, light weight, deep burnup, and inherent safety. A conceptual design of FuSTAR is proposed in this study with neutronics and thermal-hydraulics assessments. In this study, its concept is established with two-region fuel to flatten core power, and the neutron spectrum of the core is in the thermal spectrum to improve the utilization of thermal neutrons and reduce fuel inventory. Neutronics and thermal-hydraulics analyses are also carried out, and established the core layout and material ratio under the near-critical condition. Burnup depth of the fuel, power distribution, neutron flux distribution, flow and heat transfer in rod-bundles channel, and 1/6 core porous media analysis are also investigated. The parameters of the designed core are evaluated quantitatively from various aspects. The results show that the core design in this study has sufficient shutdown depth, the equilibrium cycle is 3.24 years, and the discharge burnup depth is 106 GWd/MtU. The helical cruciform fuel (HCF) adopted by this reactor has a better flow and heat transfer capacity than cylindrical-type fuel at the inner channel. The analysis of 1/6 core porous media shows that the peak fuel temperature under normal operating condition is 1009.3 K, which is far below the limit of 1573 K. Relevant design and analysis results can provide the reference for the subsequent small modular FHR design and construction.

High-fidelity neutronics and thermal-hydraulics analysis of helical cruciform fuel based on small modular FHR

Fluorine High-temperature Reactor (FHR) is a novel reactor type without water-cooling, but with a high outlet temperature. Small Modular FHR is convenient for modular assembly and transportation. Therefore, it is suitable for remote and water shortage areas to provide efficient and clean nuclear energy. The neutronics and heat transfer of the fuel are the key issues when the small modular FHR has high thermal power output. This research proposes the fuel – TRISO (TRi-ISOtropic) particles dispersed in Helical Cruciform Fuel (HCF). HCF can be self-supported in radial direction, no additional grid-spacers are required, which can decrease the pumping power and improve the efficiency of the reactor system. For small modular FHR, it can be disassembled, transported, and assembled better without grid-spacers and simplifying the core structure. The heat transfer capacity of the coolant also could be further improved because of the special structure of HCF. Based on the Monte-Carlo particle transport code MCNP, the neutronic characteristic from the assembly to the whole core of FHR is studied. By obtaining the fission power distribution in near-critical condition, the thermal-hydraulics of the single most heated HCF and 7 bundle HCF in the heated assembly are calculated. In order to preliminary analyze the thermal-hydraulics of the whole core scale, the thermal-hydraulics analysis of porous media is carried out for 1/6 core. The isotropic resistance tensor and thermal equilibrium porous model have been introduced with reasonable approximation, and porous resistance relation has been carried out. The radial power peak factor is 1.185 and the axial power peak factor is 1.435 in near-critical condition, which proves the arrangement of the HCF and the control rods can flatten the power distribution effectively. The average temperature of the coolant outlet in the active region of the core is 974.44 K, and the average temperature of the upper plenum outlet is 972.85 K. High coolant outlet temperature can ensure effective supply of high-temperature process heat. And the highest temperature of the fuel is 1271.49 K, which is lower than the limitation 1573 K. The parameters of this research can supply the neutronics and thermal-hydraulics references for further FHR design.

Mechanistic Understanding of Delayed Oil Breakthrough with Nanopore Confinement in Near-Critical Point Shale Oil Reservoirs

Recently, significant breakthroughs have been made in exploring northeast China’s shale part of the Q formation. On-site observation reveals that the appearance of oil at the wellhead is only seen a long time after fracturing in many wells. Strong coupling between phase behavior and relative permeability curves in the reservoir with the near-critical point initial condition restricts the efficient development of this kind of shale oil. A series of compositional models are constructed to address the issues to reveal the cause of the late oil breakthrough. Nanopore confinement is checked by including this phenomenon in the numerical model. Before the simulations, the work gives detailed descriptions of the geology and petrophysics background of the target formation. Simulation results show that the delayed oil breakthrough is highly related to the coexistence of three phases at the beginning of production, which is not seen in common reservoirs. The extended period of purely water production complicates subsurface flow behavior and hinders the increase of medium- and long-term oil production. Early-time production behavior in such reservoirs is associated with the gas–liquid relative permeability curves and initial water saturation. Oil–water relative permeability curves affect the water-cut behavior depending on wetting properties. The potential oil-wet property slows down oil breakthroughs. Conceivably, purely gas and water phases exist due to the nanopore confinement of crude oil phase behavior; thus, the late oil production is barely related to the gas–liquid relative permeability curves.

임계점 부근의 이산화탄소-메탄 혼합물의 관내 응축 열전달 특성 연구

The condensation heat transfer coefficient and pressure drop for pure CO₂ and CO₂+CH₄ mixture were investigated under the near-critical condition, which simulates the CO₂ transporting conditions under Carbon Capture and Storage (CCS) process. The experimental facility consisted of a test section, heat exchangers, mass flow meters, temperature sensors, a magnetic gear pump, and a differential pressure transducer. The test section made with a copper tube was assembled with Polyvinyl Chloride (PVC) pipe to form a double tube. The condensation temperature and mole fraction of CH₄ in the CO₂ mixture ranged from 20℃ to 30℃, and 1% to 5%, respectively. The mass flux was changed from 500, 600, and 700 kgm-2s-1. The two-phase flow pattern under the set conditions was estimated to be the annular flow. When the mole fraction of methane in the mixture increased from 1% to 5% at 20℃ of condensation temperature, the average condensation heat transfer coefficients and the pressure drop for CO₂+CH₄ decreased from 4.97% to 19.9%, and 30.0% to 54.5%, respectively, compared to those for pure CO₂

Double-averaged kinetic energy budgets in flows over mobile granular beds: insights from DNS data analysis

The equations for double-mean, form-induced and spatially averaged turbulent energy budgets are employed to analyse data from direct numerical simulations of turbulent open-channel flows over transitionally rough mobile beds with intermediate flow submergence. Two scenarios were considered related to (i) near-critical bed condition, and (ii) fully mobile bed condition. The bed was composed of a layer of mobile spherical particles moving on the top of one layer of fixed particles of the same size. Data analysis showed the leading energy exchanges between double-mean, form-induced, turbulent flow field contributions as well as particle motions. Above the fixed particles tops, the turbulent flow receives kinetic energy directly from the mean flow as well as from moving bed particles, which in turn also receive energy from the mean flow. For near-critical bed condition, particle aggregations enhanced mean-flow heterogeneity, strengthened turbulent stresses and their effects on the flow, while at increased bed-mobility, energy transport mechanisms became weaker and conversions induced by viscous stresses and pressure became stronger.

In-tube condensation heat transfer characteristics of CO2 with N2 at near critical pressure

The condensation heat-transfer coefficient and pressure drop for pure CO2 and CO2 + N2 mixtures were investigated under the near-critical condition, which simulates the CO2 transporting conditions under Carbon Capture, Transportation, and Storage (CCS). The experimental apparatus consists of a test section, heat exchangers, mass flow meters, temperature sensors, a magnetic gear pump, and a differential pressure transducer. The test section made with a copper tube was assembled with Polyvinyl Chloride (PVC) pipe to form a double tube. The condensation temperature and mole fraction of N2 of the CO2 mixtures ranged from 20 °C to 30 °C, and 1 to 5%, respectively. The mass flux was changed from 500, 600 and 700 kg·m−2s−1. The average heat transfer coefficient of the CO2 + N2 mixtures decreased by 2.23 to 15.9% based on the average heat transfer coefficient of pure CO2, and the pressure drop decreased by 53.4 to 77.3% at the condensation temperature of 25 °C with increase of the mole fraction of N2.

Production and characterization of ultrafine aspirin particles by rapid expansion of supercritical solution with solid co-solvent (RESS-SC): expansion parameters effects

Micronization of polar drugs by the rapid expansion of supercritical solution (RESS) process is not successful in near critical pressure due to the extremely low solubility of the drugs in near-critical CO2. In this study, for the first time, micronization of aspirin was successfully performed by the rapid expansion of supercritical solution with solid co-solvent (RESS-SC) process in near-critical pressure by using menthol as solid co-solvent. To achieve this aim, some experiments were conducted to study the influences of expansion pressure ranging from 1 to 8 bar, pre-expansion temperature ranging from 30 to 70 °C, spray distance ranging from 2 to 6 cm, nozzle diameter ranging from 300 to 700 µm, and nozzle types (orifice - capillary) on the morphology and size of aspirin particles. The obtained particles were characterized by x-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. Utilizing RESS-SC process in near-critical condition, aspirin particles in the range of 0.17–6.61 µm were obtained.

Characteristics of a Sensitive Well Showing Pre-Earthquake Water-Level Changes

Water-level data recorded at a sensitive well next to a fault in central Japan between 1989 and 1998 showed many coseismic water-level drops and a large (60 cm) and long (6-month) pre-earthquake drop before a rare local earthquake of magnitude 5.8 on 17 March 1997, as well as 5 smaller pre-earthquake drops during a 7-year period prior to this earthquake. The pre-earthquake changes were previously attributed to leakage through the fault-gouge zone caused by small but broad-scaled crustal-stress increments. These increments now seem to be induced by some large slow-slip events. The coseismic changes are attributed to seismic shaking-induced fissures in the adjacent aquitards, in addition to leakage through the fault. The well’s high-sensitivity is attributed to its tapping a highly permeable aquifer, which is connected to the fractured side of the fault, and its near-critical condition for leakage, especially during the 7 years before the magnitude 5.8 earthquake.

Development of safety analysis code for SCWR and its LOCA analysis of CSR1000

In this study, Transient Analysis Code Of SCWR (TACOS), with the ability of simulating transients or accidents under both supercritical water conditions and subcritical water conditions, has been developed. A 3-equation model for supercritical water flow and single phase flow, and a 4-equation drift-flux model for two-phase mixture are adopted. Coupling of the 3-equation model and 4-equation model is utilized to perform simulation of transition between supercritical and subcritical conditions. Furthermore, an illustration of heat transfer models suitable for potential boiling crisis under near-critical pressure condition is introduced and presented. The code TACOS is then applied to simulate SWAMUP trans-critical test to validate its feasibility of simulating trans-critical transients. Furthermore, validations with the Edward-O’Brian blow-down experiment data and other codes’ results are illustrated to evaluate its capacity in simulating the fast blow-down progress under both sub-critical condition and supercritical condition.At last, the Loss of Coolant Accidents (LOCAs) with a small break size and a large break size occurring at the main coolant line of Chinese Super-Critical Water-cooled Reactor (CSR1000) are simulated. The results show that CSR1000 with its official safety system design can be cooled down during the considered accidents and code TACOS has good feasibility for safety analysis of supercritical water systems.

Hydraulic performance and fouling characteristics of a membrane sequencing batch reactor (MSBR) for landfill leachate treatment under various operating conditions.

This study investigates the hydraulic performance and the fouling characteristics of a bench-scale membrane sequencing batch reactor (MSBR), treating mature landfill leachate under various time-based operating conditions. The MSBR system operated initially under a high-flux condition (Period 1) which resulted in a rapid trans-membrane pressure (TMP) rise due to intense fouling. Following the characterization of Period 1 as super-critical, the system was subsequently operated under a near-critical condition (Period 2). The overall filtration resistance analysis showed that cake layer formation was the dominant fouling mechanism during Period 1, contributing to 85.5% of the total resistance. However, regarding the MSBR operation during Period 2, adsorption was found to also be a dominant fouling mechanism (Days 1 to 47), contributing to 29.1% of the total resistance. Additionally, the irregular total resistance variation, which was observed during the subsequent operation (Days 48 to 75), and the respective filtration resistance analysis suggested also the formation of an initial sludge cake layer on the membrane surface, contributing to the 47.7% of the total resistance.

Near-critical GLUT1 and Neurodegeneration.

Recent articles have drawn renewed attention to the housekeeping glucose transporter GLUT1 and its possible involvement in neurodegenerative diseases. Here we provide an updated analysis of brain glucose transport and the cellular mechanisms involved in its acute modulation during synaptic activity. We discuss how the architecture of the blood-brain barrier and the low concentration of glucose within neurons combine to make endothelial/glial GLUT1 the master controller of neuronal glucose utilization, while the regulatory role of the neuronal glucose transporter GLUT3 emerges as secondary. The near-critical condition of glucose dynamics in the brain suggests that subtle deficits in GLUT1 function or its activity-dependent control by neurons may contribute to neurodegeneration. © 2017 Wiley Periodicals, Inc.

Numerical simulation of transonic nozzle flows in a near-critical condition

Numerical simulation of transonic nozzle flows in a near-critical condition

Effect of Evaporation Temperature on the Performance of Organic Rankine Cycle in Near-Critical Condition

Considering the large variations of working fluid's properties in near-critical region, this paper presents a thermodynamic analysis of the performance of organic Rankine cycle in near-critical condition (NORC) subjected to the influence of evaporation temperature. Three typical organic fluids are selected as working fluids. They are dry R236fa, isentropic R142b, and wet R152a, which are suited for heat source temperature from 395 to 445 K. An iteration calculation method is proposed to calculate the performance parameters of organic Rankine cycle (ORC). The variations of superheat degree, specific absorbed heat, expander inlet pressure, thermal efficiency, and specific net power of these fluids with evaporation temperature are analyzed. It is found that the working fluids in NORC should be superheated because of the large slope variation of the saturated vapor curve in near-critical region. However, the use of dry R236fa or isentropic R142b in NORC can be accepted because of the small superheat degree. The results also indicate that a small variation of evaporation temperature requires a large variation of expander inlet pressure, which may make the system more stable. In addition, due to the large decrease of latent heat in near-critical region, the variation of specific absorbed heat with evaporation temperature is small for NORC. Both specific net power and thermal efficiency for the fluids in NORC increase slightly with the rise of the evaporation temperature, especially for R236fa and R142b. Among the three types of fluids, dry R236fa and isentropic R142b are better suited for NORC. The results are useful for the design and optimization of ORC system in near-critical condition.

An investigation into the formation and conversion of metal complexes to metal oxide nanoparticles in supercritical water

Due to unavoidable formation of metal complexes derived from nitrate solutions in high temperature water, the current research aimed at study on such intermediates in order to find the best condition for the complex-free synthesis of metal oxides in supercritical water (SCW). Here, aqueous solutions of several metal nitrate salts including zinc(II), copper(II), manganese(II), cerium(III), magnesium(II), nickel(II), silver(I), and zirconium(IV) have been treated as the precursors for the metal oxide nanoparticle synthesis in a batchwise SCW reactor. The experiments were conducted at two temperatures with different reaction properties, near-critical condition (400°C) in which water preserves its ionic properties relative to the second point with higher temperature (500°C) that the mechanism of reaction inclines to the free radical mechanism due to the falling of dielectric constant (ɛ). Based on the results of X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectra, and thermogravimetric analysis (TGA), it was revealed that some nitrate, hydroxide, and especially hydoxynitrates complexes with ionic origins through their synthesis path, as the byproducts of metal oxide formation reaction, were formed at low temperatures. However, these complexes decomposed into the metal oxide nanoparticles at higher temperature. Complementary experiments on copper nitrate solution showed that temperature is a key parameter in the formation and decomposition of metal complexes in the SCW treatment of metal nitrate solutions used in this research work. Finally, a general mechanism describing the formation and conversion of metal complexes during the treatment of aforementioned metal nitrate solutions and, consequently, synthesis of metal oxide nanoparticles was suggested.

Conversion of sugarcane bagasse to gaseous and liquid fuels in near-critical water media using K2O promoted Cu/γ-Al2O3–MgO nanocatalysts

Bagasse conversion to H2, CO and light gaseous hydrocarbons as gaseous fuels, and higher alcohols and ethers as liquid fuels and fuel additives were performed in a basic water medium with near-critical condition in presence of potassium promoted Cu/γ-Al2O3–MgO catalysts. The catalysts were extensively characterized using ICP, XRD, TPR, BET, CO chemisorption and TEM techniques. In order to investigate support stability at reaction condition, XRD test also was carried out for used catalysts. Maximum dispersion of 48% and minimum average particles sizes of 8.4 nm were obtained for Cu20–K7.5/γ-Al2O3–MgO catalyst. Copper and potassium effects on quality and quantity of gaseous and liquid products were investigated. The maximum amounts of H2 (10 mmol g−1 of bagasse) and total produced gases (41 mmol g−1 of bagasse) were obtained with unpromoted Cu20/γ-Al2O3–MgO catalyst. Addition of K increased the bagasse conversion to liquid fuels. Potassium made the process more selective for alcohols and ethers production. Maximum amount of alcohols and ethers (83.3 mmol g−1 of bagasse) was obtained for Cu20–K7.5/γ-Al2O3–MgO catalyst.

Structural overloading evaluation based on the identification of subcritical crack increments

A complex approach using fractographic and FE analysis was used to assess the load level of a lower root spar assembly under conditions, which were close to the critical load of the structure.A Damage Tolerance scheme of determining the conditions for subcritical crack increments was used. The stress state for the measured subcritical increments in an airplane structure was determined using a proposed iterative solution to match the material R curve to the J-integral solution. A structural stress analysis of the root of the wing structure assigned a load level to the crack state identified above. The numerical FE solutions of the J-integral along the identified crack fronts were generated using ABAQUS SW at the Strength of Structures Dpt., Aerospace Research and Test Establishment.The analysis provides information regarding the near-critical loading condition of the wing structure.

PRELIMINARY INVESTIGATION ON THE SPREADING ANGLE OF A NEAR-CRITICAL RP-3 KEROSENE JET

The spreading angle of a near-critical RP-3 kerosene jet into a quiescent subcritical nitrogen chamber through a simple injector was experimentally investigated through a schlieren imaging system. The jet spreading angles affected by ambient pressure, injection pressure, and injection temperature are separately analyzed. It is found that the jet spreading angle increases with the increased injection pressure, but decreases with increased injection temperature. However, the jet spreading angle shows an unusual drastic changing trend when the ambient pressure is just from 2.5 to 3.0 MPa. These phenomena are analyzed based on the injection dynamics and injectant physical proprieties. It is believed that the sensitivity of the physical properties of the injectant in near-critical condition is responsible for the drastic changing trends of the jet spreading angle. The momentum of the injectant is a dominating factor instead of the density ratio that affects the jet spreading angle under this experimental condition.

Heat transfer modeling for supercritical methane flowing in rocket engine cooling channels

To investigate the methane behavior inside rocket engine cooling channels, a test article has been specifically designed by the Italian Aerospace Research Center. In this study, the expected wall and coolant behavior of this test article is analyzed by a three-dimensional conjugate heat transfer model. Different coolant pressure and surface roughness levels are considered in order to understand their influence on the heat transfer capability of the cooling system. Results show that heat transfer deterioration may occur in principle when methane is operated in near-critical condition. However, the resultant wall temperature peak reduces for increasing coolant pressure and for increasing surface roughness.

The Numerical Calculation of the Temperature Field of Water-Wall Tube of the Supercritical Boiler under Near-Critical Condition

The spirally water wall are distributed on the plane and divided into seven sections. The mean heat flux of each section is determined using furnace thermal calculation in sections. The value of the pressure and temperature concerning the working medium at inlet and outlet of each tube are established by calculating enthalpy increasing. The vaporizing processes of the working medium in the spiral tube under the three working conditions: 70%BMCR, 65%BMCR and 60%BMCR are analyzed. The state of the working medium and the possibility of the heat transfer deterioration are estimated. More reliable references are provided for the appearance of the heat transfer deterioration.

Numerical Analysis of Deterioration in Heat Transfer to Near-Critical Rocket Propellants

Liquid propellants, which are typically used for regenerative cooling of rocket thrust chambers, can flow in channels at supercritical pressures and in the neighborhood of pseudocritical temperature (near-critical fluid). This could be for instance the case for the envisioned liquid-oxygen/liquid-methane engines with chamber pressures larger than about 50 bar. When the fluid is in such a near-critical condition, deterioration in heat transfer can occur if the heat transfer level is higher than a threshold value. Aiming to improve flow prediction capabilities for the design of such systems, the present study is devoted to numerical simulations of near-critical fluids flowing in uniformly heated straight tubes. After code validation against experimental data of near-critical-hydrogen flow, numerical simulations of near-critical-methane flow in heated tubes are carried out, each characterized by a different wall heat flux. Results are discussed in detail and the near-critical-methane flow condition that exhibits the heat transfer deterioration is identified and emphasized.