Supercritical Water Flow Research Articles

Numerical study on the hydraulic and thermal performance of internally ribbed tubes in supercritical pressure and sub-critical two-phase flows

This paper numerically investigates the hydraulic and thermal performances of internally ribbed tubes under both sub-critical and supercritical water flows. A real case has been adopted to compare and validate the results with the experimental data. Heat transfer behavior and pressure drop characteristics have been analyzed under different fluid enthalpies. 15 and 25 MPa pressures are used to simulate the water flow inside the ribbed tube. Results indicate that computational method can provide a reliable framework which is capable of predicting the ribbed tubes performance. In sub-critical two-phase flow, vapor volume fraction increases continuously along the tube. It has been observed that the tube maintains the liquid near the wall region and delay the critical heat flux to the post dry-out condition. In supercritical flow, the spiral flow has been captured inside the tube. In addition, the heat transfer enhancement due to the pseudocritical thermophysical properties of the water is resolved.

Flow and heat transfer of supercritical water in the vertical helically-coiled tube under half-side heating condition

Flow and heat transfer of supercritical water in the vertical helically-coiled tube were investigated numerically using RNG k-ε model. The inner side of the coil was heated by constant heat flux and the outer side was adiabatic. Heat transfer performance under half-side heating is obtained and compared with that under uniform heating at the same heat transfer rate. Effect of specific heat, secondary flow velocity and buoyancy are discussed. The decrease of specific heat in the near-wall region, and the attenuated turbulence caused by the strong buoyancy force lead to a decrease in heat transfer for half-side heating. The deviations of heat transfer correlations on predicting half-side heating condition are evaluated, and a new heat transfer correlation for supercritical water in the vertical helically-coiled tube under inner half-side heating is proposed. The investigation into different statistical parameters shows that it is superior to the existing correlations.

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.

Peculiarities of Composition of Hydrocarbon and Heteroatomic Substances Obtained during Conversion of Kashpir Oil Shale in Supercritical Water

The composition of oily fractions (OFs) formed by the Kashpir oil shale conversion in a flow of water vapor and supercritical water under uniform heating from 300 to 550°C is studied by means of gas-chromatography/ mass-spectrometry, IR, and 1H NMR spectroscopy. The maximum yield of the oil fraction is observed at 360–390°C. Alkanes, alkenes, and alkyl-derivatives of naphthalene and benzothiophene are the major components of the OFs formed below 440°C. A significant predominance of even alkanes and alkenes over their odd homologies is detected above 440°C. The peculiarities in the varying of the content of specific groups and individual polyaromatic substances are discussed.

Study on the effects of system parameters on entropy generation behavior of supercritical water in a hexagon rod bundle

In this study, a numerical simulation was performed to investigate the entropy generation behavior of the supercritical water flow in a hexagon rod bundle by using various system parameters, i.e., system pressure, heat flux, and mass flow rate. The Speziale–Sarkar–Gatski Reynolds stress model was verified using experimental data and then used to predict the flow and heat transfer of the supercritical water. The influence mechanism of the system parameters on the entropy generation behavior was discussed in detail. It was found that the system pressure has a minor effect on the entropy generation within the heat transfer enhancement region, while the effects of the mass flow rate and heat flux are significant. The entropy generation of the supercritical water in an inner sub-channel of the hexagon rod bundle decreases with the increasing mass flow rate and increases with the increasing heat flux. A non-dimensional entropy generation is defined to fully evaluate the comprehensive effects of the mass flow rate and heat flux. The results show that the increase in the mass flow rate has a greater effect on the reduction in non-dimensional entropy generation in the low–mass-flow-rate region rather than in the high–mass-flow-rate region. Based on the simulation results, the criteria of non-dimensional entropy generation of 0.4 is set up and the relationship between the mass flow rate and heat flux is determined to balance the minimum irreversibility loss and offer parameters value for practical application in the design strategy of a supercritical water-cooled reactor.

The conditions of similarity and generalized dependences for calculating convective heat transfer in supercritical pressure coolants

In this report the assessment of the results of recent experimental investigations of heat transfer in turbulent flow of supercritical water and modeling fluids (carbon dioxide, Freon) in vertical channels of different geometry (tubes, annular gaps and rod bundles) is presented. The conditions of similarity and the system of criteria, which determine the intensity of heat exchange in the fluids near the critical point, are considered. Due to the small hydraulic diameter of the heat exchange channels in the core of nuclear reactors it is possible to neglect the gravitational forces compared to the acceleration caused by the thermal inertia effects and the forces of viscosity. Based on these ideas two comprehensive criteria were proposed. Their application in the basic equation of heat transfer suggested by the authors earlier for the normal regimes satisfactorily (with an error of 20–25%) describes the features of change of heat transfer coefficient in the deteriorated and mixed regimes of heat transfer. The system of equations suitable for engineering calculation of heat transfer in channels of nuclear reactors cooled with supercritical pressure water was developed.

Numerical study of oscillatory flow instability in upward flow of supercritical water in two heated parallel channels

A 3-D numerical study of turbulent flow of supercritical water flowing upward in two heated parallel channels with constant applied wall heat flux was developed using a RANS model in the Computational Fluid Dynamics (CFD) code ANSYS CFX. Oscillatory flow instabilities were investigated using the standard k-ε turbulence model with scalable wall functions. The effects of changes to the grid sizes and the time step size on flow instabilities were studied first. Then the instability thresholds of experimental cases were obtained with the CFX code. For comparison purposes, Chatoorgoon’s 1-D non-linear SPORTS code was also used to determine the instability boundary. These two new numerical results were compared with the experimental data and previous numerical results by other investigators. Additionally, the effects on the instability thresholds of changing the outlet plenum volume, the turbulent Prandtl number, the turbulence inlet conditions, the channel outlet K factor, the maximum iterations per time step in the transient analysis, and the order of the transient scheme were examined.

Analysis of entropy generation behavior of supercritical water flow in a hexagon rod bundle

The numerical simulation was carried out to investigate the local entropy generation in turbulent mixed convection heat transfer of supercritical water within the inner sub-channel of SCWR. The SSG Reynolds stress model with enhanced wall treatment was used as a turbulence model. The circumferential heterogeneity of heat transfer was introduced into the entropy generation analysis. The mechanisms of entropy generation in the near wall region within the boundary layer and the region away from the wall were discussed in detail. The results show that the buffer layer has great influence on heat transfer enhancement, and both the viscous sub-layer and buffer layer act significantly in the entropy generation with respect to circumferential heterogeneity. Usually, the entropy generation in the narrow gap is relatively larger than that in the sub-channel center when comparing the contribution of entropy production caused by heat transfer to that caused by energy dissipation. The simulation findings may provide a theoretical direction for the conceptual design of SCWR.

Numerical investigation on the secondary flow of supercritical water in a four-head internally ribbed tube

In this paper, the secondary flow characteristics of supercritical water in a vertical, four-head internally ribbed tube are examined with the computational fluid dynamics (CFD) technique using CFX 14.0. The performance of the forced convection heat transfer in the circumferential and radial directions is determined and the corresponding heat transfer mechanism is analyzed. The simulation begins with sensitivity analyses of the turbulence model, near-wall treatment, and mesh dependency. The SSG Reynolds stress model with an enhanced wall treatment gives the best predictions with the experimental data. Based on the results of further simulations, the local forced convection heat transfer in the circumferential and radial directions of the tube are not uniform. The local heat transfer near the rib top surface is much better than that near the substrate surface. Variation in the bulk enthalpy has little effect on the basic structure of the secondary flow. The individual vortex that forms near the rib top surface creates a region of relative low pressure and augments the heat transfer near this region. The heat transfer heterogeneity is more pronounced in the great specific heat region due to the drastically changed physical properties. Because the substrate is the weak point of heat transfer, the internal structure of the tube in this region must be optimized.

Numerical analysis on nonuniform heat transfer of supercritical pressure water in horizontal circular tube

A numerical simulation is conducted in this paper aiming to further reveal the nonuniform heat transfer mechanism of supercritical pressure water in horizontal circular tube. The governing equations are solved by finite volume method, and the renormalization group k-ε turbulence model (RNG k-ε model) with enhanced wall treatment is utilized to predict the coupled wall-to-fluid heat transfer. A good agreement between the numerical results and experimental data indicates high accuracy and reliability of the numerical method. On this basis, the nonuniform heat transfer characteristics of supercritical water and corresponding mechanism are analyzed, and the lateral secondary flow and the criteria for onset of buoyancy effect on heat transfer are also discussed. The numerical results exhibit that: (1) the RNG k-ε turbulence model with enhanced wall treatment is recommended; (2) the strong circumferential nonuniformity in heat transfer exists in horizontal tube due to the flow asymmetry, and the variation of heat flux and pressure can bring on a remarkable change in heat transfer; (3) the lateral secondary flow results from the buoyancy and thermal acceleration, and the interactive influence between secondary flow and thermophysical properties of supercritical water exerts significant effects on heat transfer; and (4) the criterion of Jackson–Hall Gr/Re2.7 accurately evaluates the onset of buoyancy effect and predicts heat transfer deterioration for horizontal flow of supercritical water.

Numerical investigation of supercritical water turbulent flow and heat transfer characteristics in vertical helical tubes

Heat transfer of supercritical water turbulent flow in helical and straight tubes is simulated using the SST k−ω model to investigate the effects of property variations, buoyancy forces and centrifugal forces. Compared with constant property water flow, centrifugal forces near the wall are decreased due to lower densities while centrifugal forces in the center are increased due to higher densities, which result in stronger secondary flow intensity for supercritical water flow. Due to large buoyancy forces, the secondary flow is stretched. The maximum secondary flow velocity near the bottom wall region is 17.6% higher than that near the top wall region. For heating conditions, the axial velocities are accelerated in the flow direction and result in increased secondary flow intensities. However, when the bulk temperature reaching the pseudo-critical temperature, the temperature distributions are flattened and the non-uniformities of wall temperature distributions are decreased due to the higher effective thermal conductivities.

Effect of heat and mass flux on heat transfer characteristics of water forced convection inside vertical and inclined rifled tubes

Abstract In this paper the subcritical and supercritical water flow inside a rifled tube has been investigated experimentally. A comparison between vertical upward flow and a 20° inclined upward flow inside a specifically designed rifled tube for various heat and mass flux values has been conducted. The operating pressures for the tests were 15, 21.5, 22.5, 25 and 28 MPa, heat fluxes were 300, 400 and 500 kW/m2 and mass fluxes were 600, 800 and 1000 kg/m2 s. The low mass flux condition has been considered due to its important applications in boiler industries for once-through vertical and spirally wound tubes boilers. The results were shown as inner wall temperature versus fluid bulk enthalpies and convection heat transfer coefficient distribution for various fluid bulk enthalpies for various flow conditions inside the both vertical and inclined tubes. The results showed that the effect of inclination on heat transfer enhancement is incentive for subcritical pressure flows. Also the results showed the higher effect of mass flux on heat transfer in near-critical and supercritical pressures in comparison with the subcritical pressure conditions.

Effect of inclination on frictional pressure drop of supercritical water flows in internally ribbed tubes: An experimental study

Internally ribbed tubes are highly advantageous for heat transfer enhancement in high pressure water flows. But these tubes increase the frictional pressure drop in comparison with smooth tubes. In this paper the frictional pressure drop characteristics of subcritical, near critical and supercritical water upward flows inside the inclined rifled tube with inclinations angles of 5, 20, 30, 45 and 90° with respect to the horizontal plane are studied experimentally. The test pressures were 15, 21.5, 22.5, 25 and 28MPa, the heat and mass fluxes were kept at 400kW/m2kW/m2 and 800kg/m2s. The effect of heat and mass fluxes on frictional pressure drop in inclined tubes was also investigated. The results showed the least frictional pressure drop for both subcritical and supercritical flows occurs at 5°. The effect of mass flux on frictional pressure drop in all water flow conditions was more considerable than the heat flux.

Analysis and generalization of experimental data on heat transfer to supercritical pressure water flow in annular channels and rod bundles

Experimental data on heat transfer to supercritical pressure water presented at ISSCWR-5, 6, and 7 international symposiums—which took place in 2011–2015 in Canada, China, and Finland—and data printed in recent periodical scientific publications were analyzed. Results of experiments with annular channels and three- and four-rod bundles of heating elements positioned in square or triangular grids were examined. Methodology used for round pipes was applied at generalization of experimental data and establishing of correlations suitable for engineering analysis of heat exchange coefficient in conditions of strongly changing water properties in the near-critical pressure region. Empiric formulas describing normal heat transfer to supercritical pressure water mowing in annular channels and rod bundles were obtained. As compared to existing recommendations, suggested correlations are distinguished by specified dependency of heat exchange coefficient on density of heat flux and mass flow velocity of water near pseudo-critical temperature. Differences between computed values of heat exchange coefficient and experimental data usually do not exceed ±25%. Detailed statistical analysis of deviations between computed and experimental results at different states of supercritical pressure water flow was carried out. Peculiarities of deteriorated heat exchange were considered and their existence boundaries were defined. Experimental results obtained for these regimes were generalized using criteria by J.D. Jackson that take the influence of thermal acceleration and Archimedes forces on heat exchange processes into account. Satisfactory agreement between experimental data on heat exchange at flowing of water in annular channels and rod bundles and data for round pipes was shown.

Numerical investigation of supercritical water flow in a vertical pipe under axially non-uniform heat flux

The authors conducted a numerical investigation of water flow in a vertical pipe during the transition from sub-to supercritical conditions when non-uniform power is applied axially at the wall. The Computational Fluid Dynamics calculations were first assessed with experimental results found in literature, where uniform heat flux was applied. Next, the model was used to perform calculations of four various cases with non-uniform heat flux distribution along the axis, however, the average value is equal to the value of the reference axially uniform-heated case. Results showed high dependency of the wall temperature distributions on the shape of the power distribution curve applied at the wall. It was found that if the peak value of heat flux is close or behind occurrence of heat transfer deterioration (HTD) phenomenon, the wall temperature increases rapidly to higher than in the reference case. However, if the peak is moved to the front part of the pipe, it is possible to achieve lower temperatures than when uniform heat flux was applied. Calculations of convective heat transfer i.e. Nusselt number show that currently, only two correlations were able to follow the trend of the results i.e. the Bishop's and Ornatsky's formulas. From the sensitivity calculations, it was found that the wall temperature is much more dependent on the mass flux than the operating pressure since the temperature variation is much higher and it is followed by the change of the position where HTD occurs.

Investigation on the drag coefficient of supercritical water flow past sphere-particle at low reynolds numbers

Supercritical water fluidized bed is novel reactor for the efficient gasification of coal to produce hydrogen. The Euler-Euler and Euler-Lagrange methods can be used to simulate the flow behaviors supercritical water fluidized bed. The accuracy of the simulated results with the two methods has a great dependence on the drag coefficient model, and there is little work focused on the study on particle?s drag force in supercritical water. In this work, the drag coefficients of supercritical water flow past a single particle and particle cluster. The simulated results show that the flow field and drag coefficient of single particle at supercritical condition have no difference to that at ambient conditions when the Reynolds number is same. For the two-particles model, a simplification of particle cluster, the drag coefficients of the two particles are identical at different conditions for the same Reynolds number. The variation characteristics with the Reynolds number and particles? positions are also same.

УЧЕТ СИЛ ПЛАВУЧЕСТИ ПРИ МОДЕЛИРОВАНИИ ТЕЧЕНИЯ СВЕРХКРИТИЧЕСКОЙ ВОДЫ В ВЕРТИКАЛЬНЫХ ТРУБАХ

The results of computer simulation of supercritical water flow in tubes are presented. The regularities of the influence of buoyancy forces on the flow characteristics are investigated.

Synthesis of ferroelectric K1-xNaxNb1-yTayO3 nanoparticles using a supercritical water flow system

Ta doped alkali niobate, K1-xNaxNb1-yTayO3 (KNNT) nanoparticles (ca. 60–80nm) were continuously synthesized from Nb/Ta-sol with polyacrylic acid as a dispersant and NaOH/KOH solutions using a supercritical water flow system. The syntheses were carried out at temperatures from 400 to 480°C, at pressures from 25 to 30MPa, and at reaction times from 0.5 to 8s. Single phase KNNT was obtained at 480°C and 25MPa for which the Na/(Na+K) and Ta/(Ta+Nb) molar ratios of the products increased with an increase in the precursor molar ratios with slopes of 2.7 and 1.0 for Na/(Na+K) and Ta/(Ta+Nb), respectively. Single phase K0.5Na0.5Nb0.7Ta0.3O3 was obtained from the precursor composition with initial Na/(Na+K) and Ta/(Ta+Nb) molar ratios of 0.19 and 0.3, respectively. Crystallinity increased with an increase in the reaction time and the particles formed at 0.5s were spherical-shaped and they were cube-shaped at the longer reaction times. Single KNNT phase was predominantly produced at high temperatures (ca. 420–480°C) and moderate pressures (ca. 25–30MPa), and was formed at 420°C, 25MPa where the density of water was less than 150kgm−3. The ferroelectric property of KNNT particles sintered at 1085°C was determined to be 96.0pC/N.

Heat Transfer to Sub- and Supercritical Water Flowing Upward in a Vertical Tube at Low Mass Fluxes: Numerical Analysis and Experimental Validation

Heat transfer to supercritical water (SCW) flowing upward in a vertical heated tube at low mass fluxes (G ≤ 20 kg/m2 s) has been numerically investigated in COMSOL Multiphysics and validated with experimental data. The turbulence models, essential to describing local turbulence, in COMSOL have been checked under conditions in which empirical heat-transfer correlations are available, and it is concluded that the shear-stress transport (SST) turbulence model gives the most accurate results. The numerical results obtained show a buoyancy induced circulation of the fluid resulting from gravitational force acting on density gradients as well as a thin thermal boundary layer with a steep temperature gradient at the inner wall and a flat temperature profile in the bulk fluid. The heat-transfer coefficient of SCW is enhanced near the pseudocritical temperature (Tpc) and is deteriorated at temperatures above Tpc. A new heat-transfer correlation has been developed and validated with experimental data.

Conversion of tar in a counter flow of supercritical water at a temperature varying along the reactor axis

The article reports the conversion of tar (empirical formula CH1.47N0.01S0.007) continuously introduced into a counter flow of supercritical water (SCW) at 30 MPa in a tubular reactor with a temperature gradient along its vertical axis (450°C at the top and 650°C at the bottom). The yields of liquid products and volatile (C1–C9) hydrocarbons are 41.4 and 28.4%, respectively, relative to the weight of tar supplied into the reactor. Methane is the major component (40.5 mol %) of the volatile products, and the liquid products are dominated by oils (74.4 wt %). Deasphaltization and desulfurization of tar conversion products are observed. The average rate of water decomposition calculated from the quantity of O atoms in the conversion products is 0.24 g/min. Use of counter flows of the reactants in combination with a temperature gradient along the reactor axis affords a higher yield of low-boiling hydrocarbons than in the case of SCW pumping through a tar layer.