Supercritical Convection Research Articles

REMOTE MONITORING OF RADIATION TEST TEMPERATURE REGIME AT A HIGH-CURRENT ELECTRON ACCELERATOR

Methods for evaluating the radiation resistance of structural materials of nuclear reactors have been developed and research is being carried out in NSC KIPT. As a part of these studies, a supercritical water convection loop with a chamber for irradiation of the tested samples by electron beam of the accelerator LU-10 (beam energy of 8…10 MeV, beam power up to 10 kW) was developed. A pyrometric system based on a DSLR camera (Digital Single-Lens Reflex Camera) has been created for the remote continuous non-intrusive control of the surface temperature of the irradiation cell in the range of 350…550°С. The work presents the results of calibration and use of the developed system in the conditions of a 500-hour run of sample irradiation in the loop.

Thermosolutal Convection of Natural and Anti-Natural Solutions Through an Angled Cavity Under Cross Gradients in Temperature and Concentration

In the current study, cross-temperature and concentration gradients are used to model the in a binary fluid contained in an angled square cavity. Using a method, the , , and conservation equations were numerically solved. The inclined cavity under equal solutal buoyancy and thermal forces was the subject of the study . Since the horizontal components of the thermal and singular volume forces were equal but opposed to one another, an equilibrium solution for this situation that corresponds to the rest state of the immobile fluid is feasible. However, this equilibrium solution becomes unstable above a specific critical value of the , leading to vertical density stratification inside the enclosure. The results are shown using the and as well as the and for the flow intensity. The existence of the commencement of convection is demonstrated in this work, and both natural and anti-natural flow solutions are obtained. Subcritical convection has also been seen for the natural solution when the is more or less than unity. For the start of supercritical and subcritical convection, the number's critical values are identified. As the climbed, so did the flow's intensity and the rates at which heat and mass were transferred. Reducing flow intensity and accelerating mass transfer are the results of raising the . Different flow patterns are shown for an aspect ratio of 4, and the existence interval of the oscillatory solutions is calculated.

Effects of circumferential heat conduction on heat transfer characteristics of supercritical R134a in horizontal tubes

In this paper, simulations of supercritical heat transfer of R134a in horizontal tubes are performed to study the effects of circumferential heat conduction on the heat transfer deterioration, and the abnormal phenomena of higher temperature distribution of non-gravity supercritical flow is explained. The results show the heat transfer deterioration is caused by the impairment of specific heat and heat conduction in the boundary layer while the following recovery of heat transfer is due to the enhancement of thermal conduction and turbulent convection. A dimensionless parameter of Biot number is defined to characterize the thermal resistance ratio of the circumferential conduction to the convective heat transfer. The wall temperature redistribution due to the circumferential conduction affects the supercritical convection in horizontal tubes, and non-gravity supercritical flow may have a higher wall temperature when the Bi number is small. The deterioration of the top surface can be significantly alleviated in a tube with a larger wall thickness or thermal conductivity.

Experimental investigation of near-critical CO2 heat transfer performance in a closed natural circulation loop

The natural circulation system based on supercritical CO2 has attracted attention due to its simple structure and safety compared with the forced circulation system. This work presented an experimental investigation of the heat transfer characteristics of CO2 based on natural circulation. A closed loop natural circulation experimental system was built, which was specially designed for near-critical pressure experiments. The effects of filling ratio, coolant temperature, heat transfer rate and system pressure were carefully discussed. Experimental results illustrated the loop had excellent heat transfer capability at subcritical pressure, but the heating section showed heat transfer deterioration at supercritical pressures. The heat transfer behaviors were analyzed. From subcritical pressure to supercritical pressure in the loop, the heat transfer mechanism transforms from two-phase transition to supercritical single-phase convection. Gas-like layer and insulating layer will form near the wall in the heating section, resulting in a large thermal resistance at the wall for heat to enter the core flow.

Experimental investigation on the temperature and pressure characteristics of supercritical helium for liquid hydrogen pressurisation

Liquid hydrogen is a good energy storage medium for heavy-lift launch vehicles owing to its high energy density. A suitable pressurisation system is required when liquid hydrogen is injected into the combustion chamber from the storage tank. Helium has the advantages of stable properties and low density. Therefore, when it is used for liquid hydrogen pressurisation in heavy-lift launch vehicles, the weight can be greatly reduced. An experimental setup for supercritical helium natural convection heat transfer for liquid hydrogen pressurisation was established in this work, and the temperature and pressure characteristics during the processes of filling with liquid nitrogen, filling with helium, heating, and repeated heating were analysed based on the setup. The results showed that the cooling rate during the cooling process in the experimental cavity filled with helium was mainly dependent on the distance between the measuring point and the inner wall of the cavity, but not on the height. The heating rate in the upper part of the cavity was the fastest during the process of filling with helium as the supercritical helium with higher temperatures accumulated in this region. The pressure in the cavity substantially affected the dynamic characteristics of the temperature and pressure of the supercritical helium during the heating process. The pressurisation efficiency was increased by approximately 4.6 times when the initial pressure was increased by 7 times. The results are helpful for improving the liquid hydrogen energy storage efficiency, as well as realising a safe and efficient pressurisation of liquid hydrogen.

NUMERICAL SIMULATION ON NATURAL CONVECTION AND HEAT TRANSFER OF SUPERCRITICAL WATER IN A SIDEWALL HEATED CYLINDER

In some reactors related to supercritical water technology, such as supercritical water gasification, natural convection is a factor to be considered because of the high thermal compressibility of supercritical water. In this paper, a numerical method is applied to analyze the natural convection and heat transfer characteristics of supercritical water in a sidewall heated cylinder. In order to match the actual situation, an inlet and an outlet are added to the bottom and the top of the cylinder, respectively. The quasi-DNS method is used for numerical simulation, and the thermodynamic properties of supercritical water are calculated by IAPWS-IF97 equations. The flow structure and temperature distribution inside the cylinder are analyzed, and it is found that the flow boundary layer state will affect wall temperature and heat transfer. A heat transfer correlation of supercritical water natural convection is proposed. The Nusselt number has a power relationship with the Rayleigh number with an exponent 0.248, and the average Nusselt number with turbulent boundary layer will be 21.8% higher than that with laminar boundary layer under the present working conditions. Finally, factors affecting wall heat transfer are discussed according to the findings.

Experimental investigation on laminar heat transfer performances of RP-3 at supercritical pressure in the helical coiled tube

The laminar heat transfer characteristics of RP-3 in the liquid phase and supercritical phase were experimentally studied at 5 MPa in the helical coiled tubes. The influences of secondary flow and buoyancy on heat transfer were analyzed under different heat fluxes, mass flow rates and helical diameters. The Nu increased with the increase of curvature. When the heat flux was 150 kW/m2, the mass flow rate was 1 g/s, and the wall temperature was higher than the critical temperature, the Re, Nu, and centrifugal Richardson number increased exponentially along the tube length. By introducing the combined parameters of density, specific heat and Grashof number, the correlations of RP-3 were established in the helical coiled tube under supercritical laminar mixed convection. In addition, the experimental data under different fuels, helical diameters and tube diameters were used to verify the above correlation.

Transient nonlinear Rayleigh–Bénard convection with single-mode initiation

Transient nonlinear convection in a finite Prandtl number fluid heated uniformly from below is studied analytically and numerically. We consider the simple geometry of a square cavity with normal-mode compatible boundary conditions. By design, only the marginal state of convection onset contributes to the initial condition for the two-dimensional supercritical convection. The thermal amplitude and the flow amplitude are taken as two independent initial conditions. There are two ways of initiation: (i) soft start with a very small initial amplitude, leading to early exponential growth. (ii) Kick-started transient convection with relatively large initial amplitudes, by which we perform a small-time expansion for benchmarking. Seemingly complicated transient flow occurs with a kick-start where the initial spin and the initial buoyancy torque are in conflict. However, the intricate spiraling flow decays after a couple of reversals of flow directions, and a steady convection settles. This is due to the strict antisymmetry of the temperature perturbation around the mid-point of the cavity, in combination with the symmetry of the flow field.

A Hysteresis of Supercritical Water Convection in an Open Elongated Cavity at a Fixed Vertical Heat Flux

The supercritical modes of water convection at room temperature in an elongated horizontal cavity, with a ratio of width to height from 1:1 to 5:1 are investigated. The Prandtl number is assumed to be seven. A constant heat flux is set at the upper free and lower solid boundaries, and the side solid walls are assumed to be thermally insulated. Calculations carried out by the finite-difference method for values of the Grashof number exceeding the critical one by up to fifty times have shown that in the considered range of Grashof numbers in moderately elongated cavities, with the aspect ratios of up to 4:1, a single-vortex stationary regime is realized in the supercritical region. In a cavity with the aspect ratio of 5:1 for a wide range of Grashof numbers, a multiplicity of stationary regimes is possible, from onevortex to three-vortex. Hysteresis transitions between them, taking place at a smooth and stepwise change in the Grashof number, have been discovered and investigated.

Supercritical Convection of Water in an Elongated Cavity at a Given Vertical Heat Flux

The supercritical modes of water convection are investigated at room temperature in an elongated horizontal cavityes, with a width-to-height ratios of 2 : 1 and 3 : 1. The Prandtl number is assumed to be equal to seven. A constant heat flux is set at the upper free and lower solid boundaries, and the lateral solid boundaries are assumed to be thermally insulated. Calculations carried out by the finite-difference method for values of the Rayleigh number exceeding the critical one by up to thirty times have shown that in the indicated interval of Rayleigh numbers in both cavities in the supercritical region, a single-vortex steady state is realized

Supercritical convective flows of melt water in an open horizontal rectangular cavity with a prescribed vertical heat flux

The influence of the intensity of heating, expressed by the Grashof number, on supercritical regimes of thermal convection of melt water in a horizontal rectangular cavity with an aspect ratio of two is investigated. On the lateral solid boundaries, the thermal insulation conditions are satisfied, and a constant vertical heat flux is set on the lower solid and upper free horizontal and non-deformable edges. Under conditions when the cavity-average temperature is close to the temperature of water density inversion in the cavity, a state of mechanical equilibrium is possible, when a stably stratified layer is located on top of an unstable stratified layer. For two cases of the position of the horizontal boundary between these layers, the structure of stationary planar supercritical thermal convection is investigated. The calculations were carried out by the finite-difference method on a square grid with 128 nodes along the horizontal coordinate and 64 along the vertical one. Calculations have shown that, with an equal thickness of unstable and stably stratified layers, supercritical convection in the region up to about six supercriticalities has a two-cell structure in the horizontal direction with two (large at the bottom and weaker at the top) vortices in each of the cells. With an increase in supercriticality, this two-cell structure turns into a four-cell structure in a hysteresis manner. For the case when the thickness of the stably stratified layer is three times less than the thickness of the unstable stratified layer, the supercritical convective flow has the general form of a single-vortex cell elongated horizontally. With an increase in the Grashof number up to about a hundredfold supercriticality, it remains generally single-vortex and does not experience bifurcations.

RESEARCH OF THE STATE OF INTERNAL SURFACES OF A SUPERCRITICAL WATER LOOP AFTER A SESSION OF IRRADIATION

Made of austenitic steel at the NSC KIPT, the supercritical water convection loop Loop-1a was running for more than 500 hours in the first experimental session (in 2011). The materials tested in the loop were placed into a stream of water (more than 50 g/s) at a temperature of 350…400°C, a pressure of 23…25 MPa, and were irradiated by an electron beam with an energy of 10 MeV. Sediments that emerged on the inner surface of the loop were examined. The sediment mainly consisted of compounds of calcium and iron mixed with other elements. There is a possibility to increase corrosion induced by radiation due to dislocation damage, hydrogenation of metal and under the impact of active oxygen.

Numerical analysis on supercritical natural convection by lattice Boltzmann method

One of the main factors affecting the heat transfer efficiency of solar collector is that the ordinary fluid in it is in the state of natural convection. Supercritical fluid is expected to improve the heat transfer efficiency of solar collectors due to its dramatic changes in thermal properties, so it is necessary to carry out the research of natural convection of supercritical fluid. Although researchers have made lots of related experimental investigations, heat transfer mechanism of supercritical natural convection is still unclear. In order to clarify its heat transfer mechanism, this article conducts a numerical analysis for supercritical natural convection applying lattice Boltzmann method, which has been proved to be valid and convenient. Considering the influences of temperature difference and pressure on natural convective heat transfer of supercritical fluid are seldom studied, the relevant researches are carried out in the article. The results imply that pressure of supercritical fluid in solar collectors should be less than 11 MPa so that high heat transfer performance can be obtained. Finally, the correlations of average Nusselt number, Rayleigh number, and pressure are fitted for the convenience of heat transfer calculation.

Analysis of supercritical free convection in Newtonian and couple stress fluids through EOS approach

Supercritical natural convection of Newtonian and couple stress fluids about an isothermal cylinder employing the equation of state (EOS) model is addressed in the current numerical investigation. At first, a suitable equation for heat expansion rate based on Redlich–Kwong EOS (RK-EOS), Soave modification EOS (Soave-EOS), Peng-Robinson EOS (PR-EOS) and Virial EOS (Virial-EOS) is obtained under the supercritical region in terms of compressibility parameter, pressure and temperature. The evaluated heat expansion rate values based on these equations of state and local NuX have been validated with practical values and found that the RK-EOS is an appropriate EOS to predict the natural convection characteristics of supercritical fluid (nitrogen) when compared to other models. The governing couple stress liquid circulation equations over a vertical cylinder are computationally solved by employing the finite difference technique. The current numerical study shows that the steady-state and transient velocity fields of a supercritical Newtonian fluid are considerably higher than couple stress fluid and these velocity profiles increase with an increase in reduced pressure and decrease with an increase in reduced temperature. Further, the thermal field is suppressed with magnifying reduced pressure and enhanced with amplifying reduced temperature. Using the present study, it can be concluded that the RK-EOS is considered to be an appropriate EOS to govern the thermal parameters of supercritical Newtonian and couple stress fluids.

Assessment and modification of buoyancy criteria for supercritical pressure CO2 convection heat transfer in a horizontal tube

Buoyancy has significant effects on convection heat transfer of supercritical pressure fluids. This paper numerically investigates the asymmetric heat transfer characteristics of supercritical pressure CO2 in a horizontal tube with taking buoyancy effects into account. The applicability of three buoyancy criteria is validated by the computational results and new thresholds for buoyancy parameters are proposed. The software ANSYS FLUENT is adopted for present work. The computational variable is heat flux ranging from 1 kW/m2 to 22 kW/m2, the computational constants include tube diameter (22.14 mm), inlet temperature (288.55 K), outlet pressure (7.59 MPa) and mass flow rate (0.148 kg/s). The outlet temperatures for all cases are less than the pseudo-critical temperature. Results show that the asymmetric heat transfer does not change with the magnitude of buoyancy and the variation of overall heat transfer performance desynchronizes with the appearance of natural convection. The original thresholds of two buoyancy parameters are small to indicate the onset of buoyancy effect on overall heat transfer performance, the recommended critical values are given in this paper. The other buoyancy parameter is unsuitable to predict the buoyancy effect on overall heat transfer performance.

Lattice Boltzmann simulation for natural convection of supercritical CO2 in an inclined square cavity

Purpose This study aims to show the importance of natural convection of supercritical fluid in an inclined cavity. The heat transfer performance of natural convection can be improved. Design/methodology/approach A model of an inclined cavity was set up to simulate the natural convection of supercritical fluid. The influence of inclined angles (30 to approximately 90°) and pressures (8 to approximately 12 MPa) are analyzed. To ascertain flow and heat transfer of supercritical fluid natural convection, this paper conducts a numerical investigation using the lattice Boltzmann method (LBM), which is proven to be precise and convenient. Findings The results show that the higher heat transfer performance can be obtained with an inclined angle of 30°. It is also presented that the heat transfer performance under pressure of 10 MPa is the best. In addition, common criterion number correlations of average Nusselt number are also fitted. Originality/value These study results can provide a theoretical reference for the study of heat transfer of supercritical fluid natural convection in engineering.

Large eddy simulations of high-heat-flux supercritical CO2 convection in microchannels: Mixed convection and non-uniform heating

Near-critical-point supercritical fluid convection is a promising solution for emerging high-flux thermal management needs because of the high fluid thermal conductivities and specific heats. Supercritical convection has been extensively studied for large diameter channels with uniform heating (4.1 < D < 22.7 mm, 0.05 < q'' < 330 W cm−2). However, limited information is available on transport processes to guide engineering of high-flux compact supercritical heat transfer equipment, which often have non-uniform heating distributions. To address this need, large eddy simulations (LES) are employed to study supercritical CO2 convection in microchannels (750μm×737μm cross-section). First, the simulation approach is validated with published experimental data. Studies are then conducted for horizontal isothermal heated channels at reduced pressure Pr=1.1, mass fluxes G=100-1000kgm-2s-1 (Re=3000-35,700), average wall heat fluxes q''=24-62Wcm-2, and bulk flow temperatures Tbulk=291-317K (inside and outside pseudocritical range). Results are used to assess the applicability of published supercritical convection correlations for microchannel heat exchangers. All available supercritical correlations are found to under-predict heat transfer coefficients at high heat fluxes (q''=58-62Wcm-2). The bottom-to-top wall heat transfer coefficient (HTC) ratios from these cases are used to determine a new criterion for the onset of significant mixed convection effects. At low mass fluxes (G=100kgm-2s-1), this HTC ratio is found to exceed 2.5×. Simulations indicate that, at these conditions, increased heat fluxes lead to reduced HTCs for low and high mass fluxes, but increased HTCs at intermediate mass flux values. Finally, an illustrative case is evaluated to assess the impact of conjugate heat transfer effects in microscale supercritical convection, and highlight limitations of conventional modeling approaches. This study quantifies the accuracy of convection correlations for high-heat-flux microchannel pseudocritical conditions, provides new criteria for predicting the onset of mixed convection in microchannel supercritical flows, and demonstrates the impact of conjugate heat transfer effects in microchannel supercritical heat exchangers.

Evolution of heat flow, hydrothermal circulation and permeability on the young southern flank of the Costa Rica Rift

SUMMARYWe analyse 67 new conductive heat-flow measurements on the southern flank of the Costa Rica Rift (CRR). Heat-flow measurements cover five sites ranging in oceanic crustal age between approximately 1.6 and 5.7 Ma, and are co-located with a high-resolution multichannel seismic line that extends from slightly north of the first heat-flow site (1.6 Ma) to beyond ODP Hole 504B in 6.9 Ma crust. For the five heat-flow sites, the mean observed conductive heat flow is ≈85 mW m−2. This value is approximately 30 per cent of the mean lithospheric heat flux expected from a half-space conductive cooling model, indicating that hydrothermal processes account for about 70 per cent of the heat loss. The advective heat loss fraction varies from site to site and is explained by a combination of outcrop to outcrop circulation through exposed basement outcrops and discharge through faults. Supercritical convection in Layer 2A extrusives occurs between 1.6 and 3.5 Ma, and flow through a thinly sedimented basement high occurs at 4.6 Ma. Advective heat loss diminishes rapidly between ≈4.5 and ≈5.7 Ma, which contrasts with plate cooling reference models that predict a significant deficit in conductive heat flow up to ages ≈65 ± 10 Ma. At ≈5.7 Ma the CRR topography is buried under sediment with an average thickness of ≈150 m, and hydrothermal circulation in the basement becomes subcritical or perhaps marginally critical. The absence of significant advective heat loss at ≈5.7 Ma at the CRR is thus a function of both burial of basement exposure under the sediment load and a reduction in basement permeability that possibly occurs as a result of mineral precipitation and original permeability at the time of formation. Permeability is a non-monotonic function of age along the southern flank of the CRR, in general agreement with seismic velocity tomography interpretations that reflect variations in the degree of ridge-axis magma supply and tectonic extension. Hydrothermal circulation in the young oceanic crust at the southern flank of CRR is affected by the interplay and complex interconnectedness of variations in permeability, sediment thickness, topographical structure, and tectonic and magmatic activities with age.

Linear and nonlinear stability of Soret‐Dufour Lapwood convection near double codimension‐2 points

AbstractIn this study, the Dufour and Soret effects on natural double‐diffusion convection in a horizontal porous layer was studied numerically using FORTRAN 90 programming and analytically near various convection onset thresholds. The porous layer was subject to a uniform heat and mass fluxes on the horizontal walls while the vertical walls were impermeable and adiabatic. The Darcy model along with the Boussinesq approximation was assumed in the problem formulation. The governing parameters of the problem are the thermal Rayleigh number, RT, the buoyancy ratio, N, the Lewis number, Le, the aspect ratio of the cavity, A, and the Dufour, Du, and Soret, Sr, numbers. For a shallow enclosure, the analytical solution was derived assuming zero convection wave number, which is valid near and above criticality. The onset of subcritical, supercritical and oscillatory convection was investigated. Two linear and nonlinear codimension‐2 points were found to exist. Whether the system was subject to constant fluxes and heat and solute, regardless of the aspect ratio of the layer, the subcritical convection behavior remained the same with similarity in the thresholds expressions for subcritical bifurcation.

Analytical and Numerical Study of Soret and Dufour Effects on Double Diffusive Convection in a Shallow Horizontal Binary Fluid Layer Submitted to Uniform Fluxes of Heat and Mass

Combined Soret and Dufour effects on thermosolutal convection induced in a horizontal layer filled with a binary fluid and subject to constant heat and mass fluxes are investigated analytically and numerically. The thresholds marking the onset of supercritical and subcritical convection are predicted analytically and explicitly versus the governing parameters. The present investigation shows that different regions exist in the N-Du plane corresponding to different parallel flow regimes. The number, the extent, and the locations of these regions depend on whether SrDu&gt;-(1+Le2)/2Le2=f(Le) or SrDu&lt;-(1+Le2)/2Le2. Conjugate effects of cross-phenomena on thresholds of fluid flow and heat and mass transfer characteristics are illustrated and discussed.