A numerical study of the downward mercury flow in a nonuniformly heated round tube under the influence of a transverse magnetic field was carried out using the LES method. The problem was solved in a wall-conjugate formulation with modeling a contamination layer (contact electrical resistance) on the inner surface of the tube wall. The calculations were carried out at the following values of the Reynolds, Prandtl, Grashof, and Hartmann numbers: Re = 104, Pr = 0.024, Gr = 6·107, 12·107, Ha = 0–1000. It is shown that the electrical conductivity of the contamination layer has a significant effect on the stability of the flow and the occurrence of abnormally high low-frequency pulsations in the temperature of the liquid and tube wall.

Considerable progress has been made by now in developing mathematical models, algorithms, and available computational tools for simulating heat and mass transfer processes. Advanced approaches yield detailed information on various characteristics of mass transfer in two-phase fluids, in particular during film condensation of vapors. Models developed by various teams are implemented in CFD-codes (ANSYS Fluent, OpenFOAM, Star-CCM+, etc.). To check existing models and select the best one, cross-verification of models and algorithms implemented in various CFD codes and their verification against available and reliable experimental data are needed. In this paper, cross-verification of the VOF (Volume of Fluid) model and the algorithms implemented in the author’s ANES code was carried out against the problem of vapor condensation on a single tube. The calculations were performed using the ANES and ANSYS Fluent CFD-codes. The predictions by the ANSYS Fluent code have been demonstrated to depend on the settings of the algorithms for solving the conservation equation for the liquid volume fraction. Recommendations are presented for setting this code to obtain better agreement of the predictions with experimental data and theoretical relationships. The ANSYS Fluent code was used for two-dimensional simulation of refrigerant-21 condensation in a small tube bundle. Characteristics of the tube bundle (bank) were equal to those of the tube bundle used in the experimental setup of the Institute of Thermophysics, Siberian Branch, Russian Academy of Sciences (tube diameter = 16 mm, transverse tube bundle pitch S_1 = 26 mm, longitudinal tube bundle pitch S_2 = 15 mm). Condensation of saturated vapor having a saturation temperature of T_sat = 333.15 K and arriving at the tube bundle at a velocity of up to 1.2 m/s was studied. The predictions demonstrate qualitative and quantitative agreement with the experimental data.

Numerical modeling of the thermal state of the T283-1600 thyristor with various radiators, on the surface of which boiling of the 3M Novec 649 liquid dielectric occurs, was carried out. Calculations were performed in the “in-house” CFD code ANES. Heat-transfer coefficients for nucleate and transition boiling, as well as critical heat flux, were calculated using the formulas of V.V. Yagov. The change in boiling mode from nucleate to transition was carried out with equal heat flux calculated using the corresponding formulas: approximately 110 kW/m2, which is 17

The inundation of the lower tubes in a tube bundle with condensate formed on the upper tubes is among the major causes of reduced condensation rate. Experimental methods can hardly be used to get detailed information on the inundation effect, since each factor that controls the condensation process should be considered individually. However, CFD modelling using direct interface-tracking methods is an effective tool for analyzing such processes. This paper reports the results of simulation of the saturated propane vapor condensation on the surface of a horizontal tube using the Volume of Fluid method with the modified Lee model. The study presents, for the first time, the characteristics of saturated vapor condensation including the effect of inundation predicted using a 3D model. The interphase surface and the distribution of instantaneous heat transfer coefficients over the surface of the inundated pipe are presented. It has been shown that increasing the temperature of the condensate flow can substantially affect the distribution of the heat transfer rate over the tube surface due to the formation of a thermal entrance region. These results may become a starting point for modelling condensation in a tube bundle that enjoys current interest in practice.

The work is motivated by the need to study the effect of abnormal quasi-periodic high-amplitude temperature fluctuations in a liquid metal flow in pipes and channels, which arise under conditions close to those specific for liquid metal (PbLi alloy, etc.) hydrodynamics and heat transfer in blankets of tokamak fusion reactors. Such temperature fluctuations in the cooling channels of the fusion reactor blanket pose a threat to the strength of the channel walls. The influence of the physical properties of the channel walls and potential fouling of their inner surface on the generation of these fluctuations has not been studied as yet. Therefore, this problem has been numerically investigated for a MHD mixed convection of a liquid metal in a round tube for a downward flow of mercury with the one-side heating condition under a strong transverse magnetic field using conjugate problem statement by the LES (Large Eddy Simulation) method. Numerical simulation was performed for four cases: (i) the effect of thermophysical and electrophysical properties of the steel pipe wall is neglected, (ii) the effect of wall material properties is included, (iii) the effect of the moderate contact electrical resistance caused by a layer of fouling deposits or oxides on the inner surface of the pipe is accounted for, and (iv) the resistance of the thin fouling layer is assumed to be very high. The predictions for the cases (i) and (iv) are in good agreement with the experimental data and the results of the direct numerical simulation. They demonstrate the existence of quasi-periodic abnormal high-amplitude temperature fluctuations in the fluid with a frequency of approximately 0.14 Hz. With a relatively low electrical resistance of the fouling film (case iii), the frequency of high-amplitude temperature fluctuations was considerably lower (0.07 Hz). In the absence of electrical contact resistance on the inner surface of the steel pipe (case ii), high-amplitude velocity and temperature fluctuations were not revealed in the fluid. Thus, it was shown for the first time that the physical properties of a wall and the electrical resistance between the fluid and the electrically conducting wall were responsible for the development of abnormal tem-perature fluctuations in the liquid and the wall and control of their amplitude and frequency. The causes of the revealed effects are discussed.

The results of a numerical analysis of the condensation of R113 freon vapor in a horizontal round pipe with a standard size of 38 × 3 mm in the range of mass velocities from 50 to 150 kg/(m 2 s) were presented. Studies of the features of hydrodynamics and heat transfer in the stratified and stratified wave regimes of condensate flow are still relevant due to their insufficient knowledge. Thus, according to recent data, the heat-transfer intensities in the zone occupied by a stream and in the sections of the inner surface of a horizontal pipe wetted with a thin condensate film are comparable. Therefore, for an adequate assessment of the real contribution of the stream region to the average heat transfer coefficient along the pipe perimeter, the existing methods should be refined. The VOF (Volume of Fluid) method realized in in-house CFD-code ANES was used to simulate a two-phase flow. The intensity of mass transfer was calculated using a modified Lee model in which the relaxation coefficient was determined automatically based on the algorithm proposed by the authors of this work in previous publications. To describe the turbulent transport, a version of Menter’s SST turbulence model was used. Models of mass transfer, turbulent flows of a liquid film and vapor phase, VOF algorithms, and software tools that implement them were verified on experimental data on R113 freon condensation in a downward flow in a vertical pipe. Numerical modeling of condensation processes has been performed and the obtained data have been compared with the results of calculations using various methods recommended in the literature. Information is presented on the distribution of local characteristics along the length and perimeter of the pipe. It is noted that at low values of mass velocity [50 kg/(m 2 s)] at some distance from the pipe inlet, a hydraulic jump occurs, leading to a significant change in the distribution of the vapor void fraction along the length of the channel.

A numerical study of the mixing processes of multicomponent gas flows with the help of static mixers was carried out to reduce the temperature and gas mixture composition inhomogeneities in the fuel pipeline. The literary sources of interest for this work are analyzed. Two types of static mixer are selected: a series of elements from a twisted band and a leaf mixer. For these designs, numerical calculations are made at the specified parameters of mixing gas flows containing methane, hydrogen, and nitrogen. Turbulent flows of the mixture were modeled in a stationary formulation using the equations of conservation of mass, momentum, and energy averaged by Reynolds. Two-parameter models with wall-side functions were used to determine turbulent viscosity. As boundary conditions at the entrance to the static mixer, the fields of the desired variables, obtained earlier by the authors of this article at the exit from the T-shaped mixer with Reynolds numbers (4–6) × 10 6 in the main and adjacent pipes for the supply of fuel mixture components, were set. The analysis of the efficiency of the mixing process using stationary mixers of various modifications was carried out. The fields of the components of speed, temperature, and mass fractions of the mixture at the exits from static mixers were obtained and pressure losses in the structures were determined. The optimal design of the mixer is proposed, which consists of four elements in the form of a 180° plate, each element of which has a length (half-spin period) equal to two diameters of the pipe. Adjacent elements are twisted in opposite directions and adjoin each other at an angle of 90°. It is shown that it is possible in a fragment of the fuel pipeline, including a static mixer and a straight section of the pipe with a length of not more than five diameters, to achieve the required uniformity of the composition and temperature of the fuel mixture in the outlet section of the said fragment.

The processes of film condensation of stagnant and moving vapor on a single tube and various tube bundles were examined in many studies. Nevertheless, the local characteristics of heat transfer and the details of the interaction of the flowing down condensate with a moving vapor flow, which can have a significant effect on the characteristics of the condensation process in tube bundles, are not well understood. The paper presents the results of simulation of the condensation of practically stagnant and of moving saturated vapor on a horizontal cylinder. The mathematical model of a two-phase flow is based on the Volume of Fluid (VOF) method, which is implemented in the in-house CFD-code ANES. The main advantage of the proposed simulation method is that it can capture the interface without any assumptions. The modified Lee model was used to model interfacial mass transfer. An algorithm is proposed for the automatic selection of a constant in this model on the basis of the specified properties of the coolant and parameters of the computational grid. The model was validated against the classical Nusselt solutions for a vertical plate and a horizontal cylinder, known calculating correlations, and predictions obtained using a simplified condensation model proposed by the authors of this paper in previous studies. Information is presented on the drip-off diameters of droplets, the dynamics of heating of subcooled condensate droplets after their drip-off from the tube surface, and the effect of external tube spraying on the condensation rate. The obtained data are compared with the available experimental results.

The purpose of this study is to sample a procedure for numerical simulation and calculation of the processes of mixing in pipes of a T-junction (tee) of natural gas with the so-called “stripped” components, such as methane, hydrogen, and nitrogen, to obtain a mixture that can be used as a fuel at thermal power plants. The specific of fuel gas mixing is high Reynolds numbers of the simulated flows, which can be as high as Re = (5–10) × 10 6 . An analysis is presented of some experimental and modern computational studies of the processes of flow mixing in pipes and T-junctions. It is pointed out that the application of various well-accepted models for eddy viscosity or Reynolds stresses in the numerical simulation on the basis of Reynolds-averaged conservation equations yields a satisfactory agreement with experimental data on mixing flows in a T-mixer only with an unjustified decrease of the turbulent Schmidt (Prandtl) number to the value 0.1 or an increase of the known constant of turbulence models C μ by a factor of 9. It can be concluded that eddy-resolving methods are unsuitable for the investigation of mixing processes in fuel pipeline joints due to high Reynolds numbers and a great length of the main pipe. An analysis of the predictions has revealed large fluctuations in the local ratio of the generation rate of the turbulent kinetic energy to the rate of its dissipation and a sharp decrease in its value averaged over the pipe cross section at a distance of several diameters from the starting point of mixing, which is not characteristic of pipe flows, mixing layers, or jets. An attempt was made to improve the predictive capabilities of the standard k –ε model for developed turbulence, while keeping the turbulent Schmidt number Sc_t and the constant С μ within the substantiated limits. An empirical formula for Sc_t and a modification of the standard k –ε model, which takes into account the variability of С μ according to the Rodi dependence carefully verified against data on various free flows, are proposed. Experimental investigations of isothermal mixing of air flows in a tee mixer, one of which contained tracers in the form of glycerin-based liquid microdroplets, were carried out. The profiles of hydrodynamic characteristics of the flow downstream of the tee were measured by the planar optical SIV method at a distance of 5.5 D from the axis of the pipes' intersection. To verify the modified k –ε model, numerical simulation was performed of the mixing of gases and liquids in a tee mixer, and the predictions were compared with the experiment. The results are presented of the calculation of natural gas mixing in a tee mixer with a methane-hydrogen fraction from petrochemical facilities.

Various literary sources present the results of experiments that were carried out in order to investigate the process of condensation on a horizontal cylinder of a moving steam of freon R-113. These results demonstrate a qualitative disagreement with the trends following from the available theoretical dependencies. The authors of these experimental data indicated some possible reasons for this difference, but a detailed verification of the above assumptions is difficult due to the difficulties in obtaining information about the local characteristics of the condensation process. In this work, the VOF (Volume of Fluid) method is used to simulate the experimental modes of R-113 freon condensation on the surface of a horizontal cylinder from a downward flow moving at a velocity of up to 6 m/s at a pressure close to atmospheric. The Lee model was used to simulate interfacial mass transfer. The selection of its constant was carried out using the algorithm proposed earlier by the authors of this work. Data on changes in the local characteristics of heat transfer during condensation from a moving vapor flow, obtained using the VOF method, are presented. The calculation results are in good agreement with the “unusual” experimental data and confirm the experimentally recorded anomalous (compared to the existing theoretical dependences) increase in the heat-transfer coefficient with an increase in the oncoming flow velocity. It is shown that one of the reasons for the increase in the heat-transfer coefficient is the interaction of the falling condensate film with the vortex structures formed behind the streamlined cylinder. At a certain velocity of the oncoming flow, the falling condensate film is periodically “flooded,” which, in turn, leads to a significant intensification of heat transfer near the lower generatrix of the cylinder. This mechanism is not taken into account in the existing models since, as a rule, it is assumed in them that, after flow separation, the film flows down only due to the action of gravitational forces. A criterion dependence is proposed for determining the boundary of “anomalous” (compared to the theoretical value) heat-transfer intensification.

The study of steam-condensation processes inside pipes of different orientation in space is an urgent task for many industrial applications, including the creation of heat-recovery plants based on the organic Rankine cycle. This paper presents the results of the validation of a mathematical model of a two-phase flow, which is based on the Volume of Fluid (VOF) on experimental data on the condensation of the downward flow of freon R-113 in a vertical round pipe. The data obtained by numerical simulation, both in terms of integral and local characteristics, are compared with experimental data for regimes with mass flux from 26 to 294 kg/(m 2 s), saturation pressures from 10 5 to 3 × 10 5 Pa, and heat flux up to 80 kW/m 2 for pipes with diameters of 9.0, 14.0, and 20.8 mm. The validation results showed the efficiency of the algorithm previously proposed by the authors for determining the relaxation coefficient in the Lee model for calculating condensation inside pipes. The best agreement between the calculations and the experimental data was found when using versions of Menter’s SST turbulence model. Several simplified one-dimensional models of steam condensation inside pipes have been tested. Recommendations on the choice of the computational grid for the studied class of problems are presented. To describe the processes of halon condensation by the VOF method, the characteristic thickness of the liquid film should account for at least ten control volumes (computation mesh cells), and the longitudinal size of the cells should not exceed half the capillary constant. It is shown that it is possible to calculate the heat-transfer characteristics using a coarser grid (with a longitudinal step of up to two capillary constants); however, in this case, waves do not appear on the film surface, which significantly affects the hydraulic characteristics of the flow.