Supercritical Water Flow Research Articles

Experiments on grain size segregation in bedload transport on a steep slope

Sediment transport in mountain and gravel-bed-rivers is characterized by bedload transport of a wide range of grain sizes. When the bed is moving, dynamic void openings permit downward infiltration of the smaller particles. This process, termed here ‘kinetic sieving’, has been studied in industrial contexts, but more rarely in fluvial sediment transport. We present an experimental study of two-size mixtures of coarse spherical glass beads entrained by turbulent and supercritical steady water flows down a steep channel with a mobile bed. The particle diameters were 4 mm and 6 mm, and the channel inclination 10%. The spatial and temporal evolution of the segregating smaller 4 mm diameter particles was studied through the introduction of the smaller particles at a low constant rate into the large particle bedload flow at transport equilibrium. Particle flows were filmed from the side by a high-speed camera. Using original particle tracking algorithms, the position and velocity of both small and large particles were determined. Results include the time evolution of the layer of segregating smaller beads, assessment of segregation velocity and particle depth profiles. Segregation resulted in the progressive establishment of a quasi-continuous region of small particles reaching a steady-state penetration depth. The segregation dynamics showed a logarithmic time decreasing trend. This evolution was demonstrated to be dependent on the particle streamwise shear rate which decays downwards exponentially. This result is comparable to theories initially developed for dry granular flows.

Numerical investigation of the effect of surface roughness on flow and heat transfer characteristics of single sphere particle in supercritical water

Supercritical water fluidized bed is a novel gasification reactor which can achieve efficient and clean utilization of coal. The rough surface of particle produced during grinding and thermochemical conversion processing will deeply affect supercritical water-particle two-phase flow and heat transfer characteristics. In this paper, fully resolved numerical simulation of supercritical water flow past single rough sphere particle with the Reynolds number ranging from 10 to 200 was carried out to investigate the effect of surface roughness. The simulation results show that as roughness increases, the separation bubbles generated in the dimple enhance the flow separation but has no significant effect on the drag coefficient. Particle surface-average Nusselt number decreases with an increase of roughness and surface enlargement coefficient due to the isolation effect at low Re and local separation bubbles in the dimple at high Re. Furthermore, the effect of surface enlargement coefficient on heat transfer efficiency factor for supercritical water near the critical point is greater than that under constant property condition and has a higher dependence on Re.

Numerical analysis of diameter effects on convective supercritical water flow in a vertical round tube

Numerical investigation was performed to analyze the diameter effects on heat and mass transfer of supercritical water in a vertical round tube. The scaling of diameter ranges from the several millimeters to less than a millimeter. Both the enhanced and deteriorated regimes of heat transfer were discussed in detail, especially from the aspect of flows in the boundary layer. The shear stress transport model was implemented with enhanced wall treatment. The results indicate that the diameter can affect the supercritical water flows significantly, as it changes the heat and mass transfer within the boundary layer. In enhanced heat transfer, the heat transfer performance improves as the diameter decreases. Additionally, the peak heat transfer coefficient is closely related to the pseudo-critical temperature in the buffer layer. In deteriorated heat transfer, the unexpected increase in wall temperature becomes suppressed sufficiently and even eliminated with decreasing tube diameter. The changes in diameter redistribute the flow patterns adjacent to the wall and primarily influence the buoyancy in the boundary layer; this is regarded as the most important factor affecting the heat transfer deterioration. The simulation findings may further enrich the knowledge of supercritical fluids and provide theoretical guidance on its applications.

Study on the Movement and Deposition of Particles in Supercritical Water Natural Circulation Based on Grey Correlation Theory

The movement and deposition of particles that occur during their natural circulation in supercritical water exercise an important impact on the safe and stable operation of a supercritical water reactor (SCWR). When supercritical water flows in pipelines, a large number of corrosive particles may be generated due to pipeline corrosion or the purity of the fluid itself. The presence of particulate matter affects the heat transfer efficiency of the pipeline, increasing flow resistance and easily promoting heat transfer deterioration. ANSYS-CFX numerical analysis software was used to simulate the natural circulation loop of supercritical water, and micron particles were added in the initial flow field. The effects of heating power, particle concentration and particle diameter on particle deposition were obtained. Through this analysis, it can be concluded that the heating of the pipeline has a certain inhibitory effect on the deposition of particles. The rise in both initial particle concentration and particle diameter serve to reinforce the deposition of particles in the heating section. Depending on the degree of influence, the contributory parameters to particle deposition include particle diameter, particle concentration and heating power in turn.

Transformation of lignin under uniform heating. I. Gasification in a flow of water vapor and supercritical water

The paper presents the research results on the lignin gasification in a flow of water vapor and supercritical water (SCW) at uniform heating (1.5 K/min) of the tubular reactor to 928 K. It was revealed that the highest yield of thiols and H2S occurs at T < 613 K; at that, the amount of sulfur in the thiols is by ≈ 3.5 times greater than in the H2S. The temperature dependence of the CO2 yield has a minimum within the range of 703–748 K that results from the superposition of two following processes: exhaustion of O-containing groups in lignin, and acceleration of the steam reforming and water gas shift reactions with increasing temperature. The latter is indicated by a sharp increase in the yield of H2 at T > 748 K. The maximum yield of phenols is observed at 658–703 K. It is shown that in consequence of SCW gasification of lignin, a carbonized microporous residue is formed. According to X-ray diffraction analysis, burkeite, Na6(CO3)(SO4)2 is the major mineral component of the lignin SCW conversion residue.

Numerical investigation on drag coefficient and flow characteristics of two biomass spherical particles in supercritical water

Supercritical water fluidized bed (SCWFB) is a novel reactor which can achieve efficient and clean gasification of biomass. The study of supercritical water (SCW)-solid two-phase flow characteristics in SCWFB is of great significance for biomass conversion and optimization of the reactor. However, the work on the drag coefficient of single biomass particle and biomass particle cluster has limitations to describe the interactions between particles accurately. In this paper, two-particle model was used to study the drag coefficient and flow characteristics of supercritical water flow past biomass particle cluster in the range of 10 < Re < 200. The simulation results show the interactions between particles can transmit larger distances perpendicular to the flow field at low Re. Drag coefficient of particles is mainly affected by the wake zone generated behind the upstream particles and nozzle effect between particles. Nozzle effect which is significant at high Re will promote flow separation and make form drag and friction drag increase. And the wake zone generated behind the upstream particles has the opposite effect.

Analysis of heat transfer of supercritical water by direct numerical simulation of heated upward pipe flows

Direct numerical simulation (DNS) was performed to investigate the turbulent heat transfer of supercritical water in an upward pipe with and without buoyancy. The heat transfer deterioration mechanisms were comprehensively studied by analyzing the effects of large thermophysical property variations, flow acceleration and buoyancy. It is shown that the large property variations near the wall severely impair the heat transfer in supercritical water especially for forced convection. The flow acceleration induced by thermal expansion shows an obvious suppression on turbulence but does not affect the heat transfer significantly due to the quick developing of thermal boundary layer. A small buoyancy first impairs the heat transfer and then improves the heat transfer due to the turbulence attenuation and recovery. With buoyancy increasing, the turbulence recovery gets earlier and stronger, and the wall temperature is significantly reduced due to the large buoyancy production of turbulent kinetic energy (TKE) in supercritical water flows. The instantaneous vortical structures based on Q criterion also clearly show the process of turbulence attenuation and recovery. In addition, the simulation using two eddy-viscosity turbulent models is compared to DNS data. The results show that the failed prediction of wall temperature and Nusselt number by those turbulent models is possibly due to the bad results of TKE, production of TKE and turbulent Prandtl number (Prt).

Effect of internal helical-rib roughness on mixed convection flow and heat transfer in heated horizontal pipe flow of supercritical water

Horizontal flow of supercritical fluids is very common in industrial facilities involving heat transfer. The serious inhomogeneity in the thermal field remains as one of the major concerns need to be addressed in horizontal supercritical flow. This work aims to evaluate the effectiveness of internal helical-rib roughness in improving heat transfer of horizontal supercritical flow. A well-validated, three-dimensional numerical model was employed to solve the turbulent mixed convection. Results show that helical ribs easily inhibited buoyancy effect in horizontal flow. Under condition of weak buoyancy influence, helical ribs hardly improved local or integral heat transfer as a result of barely modified flow field. In heavily buoyancy-affecting mixed convection, strong interaction between natural convection and rib-induced swirl flow was observed. The flow field continued evolving along the flow direction, which resulted in a more uniform thermal field and significant improvement in heat transfer. Variation trend of the location where heat transfer was the worst was identified. Finally, it was concluded that though internal helical-rib roughness is less effective in horizontal flow than in upward flow, it can still significantly relieve the serious thermal inhomogeneity and heat transfer deterioration in horizontal smooth pipes at Grq/Grth > 200.

Green synthesis and characterization of metal ions-mixed titania for application in dye-sensitized solar cells

The objective of the present study is to synthesize various metal ions mixed TiO2 nanoparticles using a continuous hydrothermal synthesis pilot reactor for dye-sensitized solar cells (DSCs). In the pilot plant, aqueous solutions of the metal salts are mixed with a flow of supercritical water (450 °C and 24.1 MPa) in a confined jet mixer for continuous synthesis of metal ions-mixed nano-titania. Characterization of the particles was made using Brunauer–Emmett–Teller technique for specific surface area, powder X-ray diffraction analysis and transmission electron microscopy for identification and crystallite size, X-ray photoelectron spectroscopy for surface analysis and infrared spectroscopy for distinct group identification. Following the already existing procedures and using the titanates synthesized, dye-sensitized cells of 1 cm2 area were assembled and their photovoltaic parameters were evaluated under standard test conditions. The power conversion efficiencies (η %) for 40 mol % Zn2+, 5 mol % Zr4+ and 10 mol % Zn4+ titania were obtained to be 4.8, 4.95 and 4.9, respectively. The promising efficiency results from a greener and large-scale production of nano-titania is a step forward towards commercializing DSC technology.

Pore scale CFD simulation of supercritical carbon dioxide drainage process in porous media saturated with water

ABSTRACTDuring the geological carbon sequestration, the short-term behavior of CO2 is controlled mainly by two-phase flow of supercritical carbon dioxide (ScCO2) and water in saline aquifers. A better understanding of ScCO2-water two-phase flow in the porous media under the geological conditions will improve predictions of the long-term CO2 storage reliability. In this paper, the computational fluid dynamic (CFD) simulation method was used to study the flow and distribution of ScCO2 and water in the porous media at the pore scale. It was found that as the contact angle decreases and the surface tension increases, the relative permeability of water increases and the relative permeability of carbon dioxide decreases. As the viscosity ratio increases, the relative permeability of carbon dioxide increases, but the relative permeability of water does not change significantly.

Coupled supercritical CO2 dissolution and water flow in pore-scale micromodels

Dissolution trapping is one of the most important mechanisms for geological carbon storage (GCS). Recent laboratory and field experiments have shown non-equilibrium dissolution of supercritical CO2 (scCO2) and coupled scCO2 dissolution and water flow, i.e., scCO2 dissolution at local pores/pore throats creating new water-flow paths, which in turn enhance dissolution by increased advection and interfacial area. However, the impacts of pore-scale characteristics on these coupled processes have not been investigated. In this study, imbibition and dissolution experiments were conducted under 40 °C and 9 MPa using a homogeneous/isotropic hexagonal micromodel, two homogeneous elliptical micromodels with low or high anisotropy, and a heterogeneous sandstone-analog micromodel. The four micromodels, initially saturated with deionized (DI)-water, were drained by injecting scCO2 to establish a stable scCO2 saturation. DI water was then injected at different rates with logCa (the capillary number) ranging from −6.56 to −4.34. Results show that bypass of scCO2 by displacing water is the dominant mechanism contributing to the residual CO2 trapping, triggered by heterogeneity in pore characteristics or pore-scale scCO2-water distribution. Bypass can be enhanced by pore heterogeneity or reduced by increasing transverse permeability, resulting in relatively low (<2% of CO2 solubility) or high (9–13% of CO2 solubility) dissolved CO2 concentration in displacing water. The overall dissolution of residual scCO2 increases with decreasing Ca, and approaches to their solubility at low Ca value with sufficient residence time. This main trend is similar to a capillary desaturation curve that represents the relationship between the residual saturation and Ca. Spatially, dissolution initiates along the boundary of bypassed scCO2 cluster(s) in a non-equilibrium manner, and the coupling of water flow and dissolution occurs which fragments the bypassed scCO2 clusters and enhance scCO2 dissolution.

Investigation on Heat Transfer Properties of Supercritical Water in a Rock Fracture for Enhanced Geothermal Systems

Supercritical water (SCW) has shown promise as a working fluid to extract heat from hot dry rock (HDR); however, fundamental research on its heat transfer characteristics in HDR fractures is still required. A 2D heat transfer model that considers the variable thermophysical properties was updated to numerically investigate the effects of mass flow rate, thermal reservoir temperature, and fracture aperture size on the heat transfer characteristics of SCW flow through a single HDR fracture. The heat transfer performance of SCW and supercritical CO2 (scCO2) was compared under the same conditions. The results indicate that the heat transmission performance of SCW is superior to scCO2 at high temperature and high pressure. It is essential to synthesize the thermal reservoir temperature and pressure, site conditions, and heat transmission fluids during HDR development.

SPECIFICS OF THE FLOW OF SUPERCRITICAL WATER UNDER CONDITIONS OF MIXED CONVECTION

A study of the specifics of supercritical water flow under conditions of mixed convection was made on the basis of CFD modeling. The results of comparison of the numerical solutions obtained in the presence and absence of the buoyancy forces are given. It is shown that the influence of Archimedean forces on the flow structure is localized in the central region of tube length. The data of computer modeling of supercritical water flow pattern with the variation of heat flux q on the tube wall are obtained. The dependence of the localization of mixed convection zone from the value of q is established. The change of the local Richardson number in the context of the effect of the localization of buoyancy forces action is analyzed. For the conditions under consideration, the dependence of turbulent transfer intensity on the effect of Archimedean forces is investigated. It is shown that this action causes oppression of turbulent transport, which is most significantly manifested near the wall of the tube.

Numerical Simulation of Supercritical-Water Flow in Concentric-Dual-Tubing Wells

Summary Much work has been performed on the modeling of saturated/superheated-steam flow in wellbores. At present, the study on supercritical-water (SCW) flow in wellbores, especially concentric-dual-tubing wells (CDTWs), is very limited. In this paper, work was performed on modeling of SCW flow in CDTWs. First, a comprehensive mathematical model comprising a pipe-flow model, supercritical-fluid model, and heat-transfer model is established. In the model, the heat exchange between the integral joint tubing (IJT) and annuli is taken into consideration. Numerical solutions of SCW flow in CDTWs were obtained with a straightforward numerical method. Then, sensitivity analysis was conducted. The following results were found: SCW in annuli (with a higher temperature) releases thermal energy to SCW in the IJT, which causes increase of temperature in IJT. As a result, SCW density in the IJT has a decrease. The density gradient near the wellhead increases with increasing of injection rate. When the injection temperature in the IJT is larger than that in annuli, SCW density increases with well depth near the wellhead.

Analysis of grid spacer effects on the flow and heat transfer of supercritical water flow in an inner sub-channel of a SCWR based on the second law of thermodynamics

In this paper, a numerical investigation was performed to understand the effects of grid spacers on the thermal hydraulic performance of supercritical water flows in the inner sub-channel of a tight, hexagon rod bundle. The Speziale-Sarkar-Gatski Reynolds stress model was verified, compared with experimental data and then used to simulate the heat transfer of supercritical water flows in a sub-channel with grid spacers, including a standard grid spacer and a split-vane type grid spacer. The entropy generation analysis method was used to analyze the effects of the angle of the split-vanes downstream of the grid spacer. It was found that the weakened heat transfer in the narrow gap region was observed downstream of the standard grid spacer, whereas it did not appear downstream of the grid spacer with split-vanes, indicating that the split-vanes have significant capacity to improve heat transfer. A comparison of various angles of split-vanes showed that small angles have lasting and stronger intensity of secondary flow than larger angles at the cost of a moderate pressure drop. The second law analysis illustrates the entropy generation behavior downstream of the grid spacer with split-vanes of different angles. Entropy generation analysis under various conditions was conducted to gain further insight. Based on these results, the optimal angle of split-vanes can be achieved.

Numerical study of flow and heat transfer of supercritical water in rifled tubes heated by one side

Deep understanding of flow and heat transfer of supercritical fluids in rifled tubes with non-uniform heating is of great importance for the safety of modern supercritical steam power equipment. In this paper, the heat transfer of supercritical water flowing in a vertical one-side heated rifled tube was numerically investigated. The differences of flow and heat transfer between the circumferentially non-uniform heated rifled tube (NRT) and uniform heated rifled tube (URT) were investigated. The distributions of temperature field, thermophysical properties, axial velocity and turbulent kinetic energy were analyzed. The mechanism of heat transfer improvement of NRT was comprehensively revealed. Results showed that the maximum inner wall temperature in NRT (Tiw,N,max) was 10 K lower than that in URT for the simulation conditions. Both radial and circumferential heat transfers were improved in NRT due to the swirling flow. Under high q/G (ratio of heat flux to mass flux) conditions, the buoyancy effect was weakened in NRT comparing to that in URT. Flow turbulence in boundary layer of y+ > 30 was intensified in NRT, which was crucial for supercritical fluid heat transfer. The positions of Tiw,N,max were not always at the midpoint of the heated side. And a prediction method for Tiw,N,max was proposed.

ОСОБЛИВОСТІ ЗМІНИ ТЕПЛОФІЗИЧНИХ ВЛАСТИВОСТЕЙ НАДКРИТИЧНОЇ ВОДИ ПІД ЧАС ТЕЧІЇ В КРУГЛИХ ТРУБАХ, ЩО ОБІГРІВАЮТЬСЯ

Для процесу течії надкритичної води в каналі, що обігрівається, проведено дослідження особливостей просторового розподілу фізичних властивостей води, зумовлених їх істотною температурною залежністю, характерною для надкритичних середовищ у ділянці псевдофазового переходу. На основі комп'ютерного моделювання розв'язано задачу тепломасоперенесення у вертикальній круглій трубі під час висхідного руху в ній води надкритичних параметрів. Показано, що характер розподілу властивостей теплоносія у поздовжньому перетині труби певною мірою визначається рухом в ньому фронту псевдофазового переходу. Проаналізовано вплив величини густини теплового потоку, що підводиться до стінки труби, на положення границь початку і кінця зони псевдофазового переходу і на пов'язані з цим особливості просторової зміни фізичних властивостей теплоносія. Наведено результати СFD моделювання з розподілу уздовж довжини труби, що обігрівається, таких властивостей надкритичної води, як густина, динамічна в'язкість, коефіцієнт теплопровідності і питома теплоємність. Виконано зіставлення особливостей розподілу зазначених властивостей, розрахованих за температурою на стінці труби і на її осі. Досліджено характер зміни питомої теплоємності по радіусу труби і проаналізовано вплив рівня теплового потоку, що підводиться, на положення максимуму теплоємності в різних поперечних перетинах каналу.

A brief communication on the effect of seawater on water flow in offshore wells at supercritical state

At present, the study of supercritical water (SCW) flow in wellbores is at the starting stage. In this paper, a simple but useful model is developed to study the effect of seawater on the thermophysical properties of SCW in offshore vertical wellbores. Firstly, based on the momentum and energy balance equations, a flow model describing SCW flow in a tube is established. Then, coupled with transient heat transfer model in seawater and formation, and thermophysical parameters of SCW, a comprehensive mathematical model is established. In order to solve the model, the governing equations are expressed in the form of difference equations. The straight forward numerical method is adopted to solve the model from wellhead to well-bottom. In the process of solving, iterative technique is used to control the calculation accuracy. Finally, type curves of SCW flow in offshore wellbores and sensitivity analysis are discussed. Results show that (a) the flow of seawater results in a rapid decline in the temperature/enthalpy of SCW in wellbores. (b) Heat loss is the dominant factor of physical parameter distribution in wellbores when the injection rate is relatively small. (c) Heat loss has an obvious influence on temperature drop when SCW is sparse in volume. (d) The SCW pressure decreases with increasing of injection temperature.

A model for predicting thermophysical properties of water at supercritical state in offshore CDTW

Thermal methods have been widely used for heavy oil recovery. In this paper, a mathematical model is proposed for estimating the key water properties in offshore concentric dual-tubing wells (OCDTW) at supercritical state.Firstly, based on the pipe flow theory, a governing model, comprised of the energy and momentum balance equations, is established for supercritical water (SCW) flow in the vertical tubes. Then, the effect of heat loss to seawater as well as formation is taken into consideration of the mathematical model. The thermophysical properties of SCW are calculated through interpolation method from the pressure-temperature based tables. Besides, the heat exchange between the the integral joint tubing (IJT) and annuli is also considered. Results show that: (a) Seawater and the thermal exchange inside the offshore wellbores are two main factors contributing to the profile change of thermophysical properties in IJT and annuli. (b) Different from the heat and mass transfer characteristics of wet steam flow in wellbore, SCW possesses some unique features. For instance, the pressure is not sensitive to temperature while the temperature is sensitive to heat loss.

CFD MODELING OF HEAT TRANSFER UNDER FLOW OF SUPERCRITICAL PARAMETERS WATER IN VERTICAL BARE TUBES

The results of the CFD modeling of heat transfer under condition of the upstream flow of supercritical water in vertical bare tubes are presented with a change in the wide range of mass flux and heat flux at the channel wall. The results of comparison of experimental and calculated data for different heat transfer regimes - improved, normal and deteriorated - are presented. The results of the CFD modeling relating to the spatial distribution of physical properties of supercritical water are analyzed.