Home – Home – Jornals – Thermophysics and Aeromechanics 2024 number 4

The paper presents the results of an experimental study on coolant flow patterns at the inlet section of a fuel assembly (FA) in the cartridge core of a reactor for an RITM small nuclear power plant. The objective was a study on influence of different inlet elements on the coolant axial velocity distribution. This task was performed by a series of experiments on a scale-up experimental model covering the inlet section components from the calibration washer up to the attachment unit between fuel rods and the diffuser. The model also covers the section of the fuel-element bundle between the absorbing grid and the spacer grid. This study is based on the pneumometric method for several critical cross-sections over the model length. The allocation of measuring points covers the entire cross-section of the model. The coolant flow features are visualized using digital maps for the working medium flow axial velocity in the fuel rod bundle cross-section. The experimental modeling can be useful for optimizing the hydraulic profiling of the components at the inlet of the fuel rod assembly. The set of test data can be applied for validation of the LOGOS CFD software and for adjusting the technique of heat and fluid computations for the core zones under a cell approximation.

Models of the severe-accident SAFR module used to calculate the cladding melt relocation along the fuel pin surface during its melting are presented in relation to severe accidents in fast-neutron reactors cooled by liquid metal. The choice of the basic system of equations and closing relations is presented. The models are validated based on experiments on melting and flow of cladding simulator melts. The error of calculating the cladding mass loss due to its melting and flowing along the fuel pin surface is estimated.

A density and a volumetric thermal expansion coefficient of the LiK₃Pb₄ ternary alloy (12.5 at. % Li; 37.5 at. % K; 50.0 at. % Pb) in the liquid state are measured for the first time. Thermal properties are studied using a gamma-ray attenuation technique in the range from the liquidus temperature T_L = 812 K to 990 K. The LiK₃Pb₄ solid alloy is known to be an intermetallic compound. Its thermal analysis at cooling from the liquid state carried out in this work has shown that the compound is likely formed by a peritectic reaction at 789 K. Based on the experimental results obtained, a table of recommended values for the volumetric properties of the LiK₃Pb₄ melt has been developed in the studied temperature range.

The study presents the results of numerical simulation for a process of generating the ash liquid phase zone in the aircraft engine combustion chamber. The simulation was performed for three operation modes: the aircraft enters an ash cloud while cruise flight, the aircraft exits the ash cloud during climb-out; the aircraft exits the cloud during the flight idle mode (recommended by ICAO). This study was conducted for the case of a dual-flow turbojet engine PD-14 (the mainline aircraft MS -21) under the impact from the Sheveluch volcanic ash in the air. Our simulations revealed that there are significant zones inside the PD-14 engine combustion chamber where the gas temperatures are above the meting point for volcanic ash. The high temperatures zones (creating a risk of volcanic ash melting inside the engine) account for over 54 % of the flame tube volume. Additionally, for the nominal flight mode (climb-out), the volume of ash melt zone exceeds 81 % of the total volume, and the flight idle mode provides only 25.3 % of the volume.

In the regimes of mixed convection, the Czochralski method investigates numerically the effect of a melt layer height on hydrodynamics and convective heat transfer in the range of relative heights of the heptadecane layer 0.1 ≤ H/R_С ≤ 2.0 with a step of 0.1. Calculations were performed using the finite difference method at a fixed rate of crystal rotation corresponding to the Reynolds number Re = 95, with the Grashof, Marangoni and Prandtl numbers Gr = 1215, Ma = 2930, Pr = 40.35 for the ratio of crucible radius to crystal radius R_С/R_S = 1.94. In this range of parameters, the flow has a laminar, steady and axisymmetric character. The evolution of the spatial flow shape, velocity and temperature fields, radial distributions of local heat fluxes, and integral heat transfer is studied depending on the relative heights of the heptadecane layer.

A new model of the phase equilibrium line (PEL) of methane has been developed on the basis of the Clapeyron-Clasius equation and the relations of the renormalization group (RG) theory. In contrast to the known PEL, when describing the density of a saturated liquid ρ⁺, density ρ^- and pressure ρ_s, of saturated methane vapor, a system of mutually consistent equations (CE) including those describing the saturation density line and saturation vapor line is used. These equations have a number of common parameters: critical indices, critical pressure, critical temperature T_c, critical density, as well as a series of coefficients of the average diameter model, d_f coefficient D_2β, complexes D_2β/D _1-α and D_2β/D_τ, calculated within the framework of modern RG theory for asymmetric systems. Based on the proposed approach, a methane saturation line has been developed; its average diameter in a wide vicinity of the critical point is described in accordance with the RG theory by the dependence d_ƒ=D_2βτ^2β+D_{1_α}τ^1_α+D_ττ, where τ = (1 - T/T_c). It has been established that d_ƒ = d_ƒ(T) within the framework of the proposed approach is a strictly decreasing function of temperature in the range from the triple point to the critical point. It has also been found that the derivative of the vaporization heat with respect to temperature, in accordance with the principles of thermodynamics, has a minimum in the vicinity of the triple point. Within the framework of the proposed PEL model, experimental data on ρ⁺, ρ^-, and ρ_s published by R. Kleinrahm and W. Wagner in 1986 are provided with standard deviation 0.0011 %, 0.0072 % and 0.0012 %, respectively, that is, with greater accuracy than the international equations of U. Setzmann and W. Wagner derived in 1991.

The dynamics of droplet interaction with a biphilic surface based on the lattice Boltzmann method with multiple relaxation times (MRT-LBM) is numerically simulated. The biphilic surface is a superhydrophilic circle located on a superhydrophobic plane. The paper describes various aspects of droplet spreading after its impact onto the superhydrophilic spot surface, rebound, and formation of a residual droplet for several sizes of the superhydrophilic region. Three typical regimes of droplet interaction with the biphilic surface are identified: droplet separation from the surface, transitional regime, and adhesion. Moreover, the velocity fields inside the droplet during the entire process of interaction are analyzed.

This paper presents the results of development and experiments with a looped thermosyphon with a microstructured heat transfer surface (3D printed) for the case of a flat evaporator and ethanol coolant. The thermal parameters of this device were tested for the temperature in the range from 20 to 128 °С and for the heat flux ranging from 25 to 530 W and for device filling level of 40 ml (100 % of the evaporator volume). It was found that the heat exchanger with pillar-array surface provides the total thermal resistance about 0.18 K/W (corresponding to the top input heat flux equal to 530 W). The developed micropillar array structure for the inner surface of heat exchanger improves the loop thermosyphon efficiency.

The flows of a viscous liquid film along the outer surface of a vertical cylinder are considered. The study employs a model nonlinear evolutionary equation for film thickness deviation from the undisturbed level. It is valid for describing long-wave perturbations in the case of small fluid flows and large cylinder radii. The branching of spatial wave regimes from the undisturbed flow regime is investigated. Particular attention is paid to special cases when the values of the radii of the cylinders lie in the vicinity of some specific critical points. To study such cases, a model system of equations is obtained from the initial equation. Several solutions of this system are given.

Analytical and numerical solutions for the heating of a composite "hydrocarbon - water" drop with a water microdroplet located in the center of a spherical drop of hydrocarbon are compared. The conjugation conditions are fulfilled at the interface: continuity of temperature and heat flux. At the outer boundary of the droplet, a condition of heat exchange with a hot gas streamlining the droplet is set. The analytical formula is based on the decomposition of the solution in a series of eigenfunctions of the Sturm-Liouville problem. An original conservative difference scheme for the numerical integration of the equations of thermal conductivity inside a composite droplet is constructed to take into account the abrupt change in thermophysical properties at the interface of hydrocarbon-water media. The calculation results obtained using the analytical formula and the numerical integration method are consistent with each other. The numerical scheme includes radiative heat transfer and the effect of evaporation on the heat transfer coefficient. A comparison of the simulation results with experimental data is presented.

High-speed shadow imaging of a gas-droplet flow from a microchannel nozzle device was performed by varying the liquid flow rate from 1 to 50 ml/min and the gas pressure drop from 0.5 to 8 bar. For this purpose, an optical system with a stereomicroscope was assembled to ensure a large depth of field and relatively high resolution. The outflow was studied for two types of nozzles: a three-nozzle device with an internal channel diameter of 200 μm and a custom-made nozzle with a microchannel silicon membrane of 243 μm thickness and a microchannel size of 10×10 μm². Spray angles for a single nozzle and an angle averaged over three nozzles were determined. Dependences of the angles on liquid flow rate for each pressure drop and dependences on pressure drop with varying liquid flow rate were obtained. It is shown that a uniform gas-droplet flow can be organized at the nozzle edge with small droplets using a microchannel membrane.

The results of an experimental investigation on heat transfer and critical heat flux during surface cooling with a dispersed flow of deeply subcooled liquid are presented. The study was carried out using a pressure nozzle with a mass flow rate of water of 24.2 g/s. A record critical heat flux of 13.2 MW/m² was achieved in these experiments. The findings indicate that the onset of boiling within the liquid film formed on the impact surface during spray irrigation leads to a notable reduction in the temperature non-uniformity across the heater.

The paper presents an updated physical-mathematical model for a stationary flow of a water-oil flow through the porous space of a rock core (this space is described as an array of capillary clusters). Here we consider an isothermal statement of problem: the temperature is the key parameters for fluid properties and for the value of pressure drop caused by interaction between the fluid phases. The developed model ensures calculating the relative permeability at different temperatures; this approach is based on standard laboratory data for core testing and on experimental data for single-phase filtration of the fluid at different temperatures (or substituted with appropriate formulas). This model was applied for calculating the relative phase permeabilities at different temperatures for the case of weakly-cemented rock formation. This sample was taken from one of Siberian oil fields with a high viscosity oil. The numerical study was conducted on the effect of temperature on the flow pattern in a variable cross-section capillary channel. Simulation was conducted using the OpenFOAM platform. The temperature-caused change in fluid properties alleviates the intensity of a train flow and promotes the transition of the train flow to the droplet flow.

The paper investigates the possibility to control the two-phase flow of immiscible liquids with a high viscosity ratio of the carrier and dispersed phases in a T-type microchannel, with the dispersed phase being a ferromagnetic liquid, by means of a constant magnetic field. The effects of separation of slugs and microdrops of a ferrofluid, as well as the controllability of the structure of the parallel flow of immiscible liquids, have been found. The obtained results may be used to design microfluidic systems in order to efficiently sort particles, as well as intensify heat and mass transfer processes.

Various approaches to oil film visualization of the flow on the cylinder surface aligned transversely in a supersonic flow in a pulsed wind tunnel are compared. Two types of fine-particle coloring pigments Al₂O₃ and TiO₂ are considered. Two methods of pigment application are used: sprinkling onto the model surface pre-coated with an oil film and applying a mixture of the pigment and oil onto the model surface. The mass fraction of the pigment in the mixture for oil film visualization is varied from 10 to 20%. Photographs of the dynamics of variation of the oil film coating on the model surface are presented, which are taken immediately after application, during the experiment, and upon its completion. Based on the results of the present study, recommendations are formulated on using surface flow oil film visualization with due allowance for specific features of application of this method for experiments in pulsed supersonic wind tunnels.

The present study deals with optical and thermophysical properties of nanofluids based on spherical carbon nanoparticles stabilized in water by sodium dodecyl sulfate. Nanoparticles with the mean diameter of 11 nm are synthesized by means of electric arc spraying in helium at a pressure of 3 Torr. For the concentration of carbon nanoparticles in the nanofluid equal to 0.01%, the extinction coefficient varies from 400 to 200 m^-1 in the wavelength range of 180 - 1100 nm. For mass fractions of nanoparticles within 0 - 0.04%, the viscosity is not found to depend on the concentration. With an increase in the concentration, the thermal conductivity of nanofluids in the same range of concentrations is found to be lower than the thermal conductivity of water by up to 4%.

The paper presents the results of numerical simulation of a round turbulent jet propagation in confined space: the jet at Re = 5.4·10⁴ is supplied into a rectangular cavity with a height to width ratio of 0.16. The URANS and LES calculations reproduce the self-oscillating regime, registered previously in the experiments by Lawson et al. (2005). A significant rearrangement of the flow structure and pressure field occurring at a low-flow jet supply from the narrow side wall allows controlling self-oscillations up to their complete suppression. A map of flow regimes has been obtained for three various positions of the opening, supplying the control jet, depending on the control and primary jets momentum ratio. The calculated data provide a quantitative assessment of the flow controllability by injecting a low-flow jet into the zone of the primary jet propagation perpendicular to its axis.

The paper presents the results of the experimental study of the structure of a liquid microlayer at the base of vapor bubbles during water pool boiling using the method of light-emitting diode (LED) interferometry and a transparent design of the heating surface. Microlayer profiles were obtained at different time moments and the effect of heat flux density on its characteristics was analyzed. It is shown that the applied technique with a fairly simple optical scheme allows obtaining of up-to-date information on the structure and dynamics of the microlayer under bubbles during boiling.

The paper considers experimental data of superfluid helium dynamics in a U-shaped cylindrical channel filled with a backfill of metal balls. An experimental cell is presented, and the results of study are shown in the form of a time dependence for the position of vapor-liquid interface. The differences between the oscillations in a free channel and in confined conditions are discussed. For a channel with a finely dispersed porous structure, the oscillation amplitude significantly reduces and a stationary state of the interfacial surface is possible.

This paper presents the development of active control methods for vortex phenomena in hydro turbines. The flow pattern downstream of a simplified turbine runner was studied under conditions typical of a hydro turbine operating at partial load, which are prone to generating large-scale vortex structures and inducing powerful pressure pulsations. Active control was achieved through the injection of additional air jets into the center of the runner cone. The results of experiments covering velocity distributions, velocity pulsations, and pressure pulsations following the injection of jets are presented. Control jets, regardless of their orientation, successfully suppress pressure pulsations. However, jets oriented radially provide the most effective suppression of vortices and reduce the total flow swirl in the draft tube. The pattern of jet supply directly affects the formation of a recirculation zone downstream of the runner. Experimental data on optimal injection align with previous theoretical estimates based on flow linear stability analysis.

The results of an experimental study of the normal integral emissivity (NIE) of lithium, sodium, and potassium during melting and in the liquid state are presented. The research is realized by the radiation method. The experimental results are compared with the theoretical calculation of the NIE from the Foot approximation and analyzed. The behavior of the main thermophysical properties of metals in the melting point region is generalized for the position of alkali metals in the Periodic Table.