Home – Home – Jornals – Thermophysics and Aeromechanics 2024 number 2

In the presented work, the process of heat and mass transfer inside an original design nozzle for a catalytic reformer of diesel fuel in a low-mass-flux mode is investigated by direct numerical simulation using Open FOAM open-source code. The main goal of a new nozzle design is to increase the rate and degree of fuel evaporation, as well as to improve the mixing characteristics of diesel fuel with superheated water vapor before the reaction mixture passes through the catalyst. Inside the nozzle, there are two regions where flows with opposite swirl directions are created; this leads to a strong velocity shear inside the nozzle, intensifying the mixing processes. Simulations were carried out in the Eulerian-Lagrangian formulation, taking into account the processes of evaporation of fuel droplets. The simulation results show that the flow at the outlet of the nozzle has a good uniformity of the mixture composition and provides a high degree of fuel evaporation at the early stages of flow development.

The present study is aimed at searching for mechanisms of wave interaction in a supersonic boundary layer on a flat plate with the flow being distorted by a streamwise disturbance generated on a sharp leading edge by a pair of weak shock waves. The space and time distributions and the frequency-wave spectra of oscillations and their wave characteristics in the linear and weakly nonlinear phases of wave train development in a nonhomogeneous boundary layer under conditions of a fixed power of the local source of controlled disturbances are analyzed. Based on experimental results, possible variants of wave interaction in a supersonic boundary layer on a flat plate in the region of the longitudinal wake generated by an N-wave are proposed.

Numerical simulation of the final viscous stage of degeneration of a momentumless turbulent wake behind a sphere in an isotropic turbulent flow is performed using a modified e ~ ε turbulence model. The laws of degeneration are consistent with those for a momentumless turbulent wake at absent turbulent background.

The effect of icing on the kinematic and power parameters of the working element of a wind generator blade was experimentally studied using a modified laser Doppler anemometry method. Arctic conditions were modeled in a specially designed aerodynamic climatic setup based on an optically transparent plexiglass tube with a square cross-section of 200×200 mm with the following parameters: flow velocity of up to 20 m/s, temperature of up to - 20°C, and relative humidity of up to 90%. The flow was saturated with moisture by fine aerosol generators. Aerosol generator flows were optimized using laser Doppler anemometry. The following parameters were measured: flow velocities at distances of up to 400 mm behind the trailing edge of the blade and power characteristics of the blade element subjected to icing. The effect of icing on aerodynamic and power characteristics is shown.

A method and results of calculating a laminar-turbulent flow past a swept wing under a control action generated by a source of blowing or suction from the model surface are presented. Pioneering results on the influence of three-dimensional blowing and suction sources on stability of the boundary layer on a swept wing and distributions of N-factors of various mechanisms of the laminar-turbulent transition aimed at changing its position are obtained by using the LOTRAN 3 software package.

Numerical computation of aiding mixed convection and heat transfer characteristics in a channel with a baffled heated wall is carried out in this work. The equations of mass, momentum and energy, alongside the boundary conditions, are solved by the finite volume formulation using the open source OpenFOAM^® code. Simulations are accomplished under different parameter combinations, including the Reynolds number, Grashof number, and baffle dimension. The results are presented in terms of streamlines, isotherm contours, Nusselt number, and friction factor. The results obtained revealed that the flow translates from steady to unsteady state at a relatively low value of Reynolds number. The unsteady flow behaviour contributes to improve heat transfer by disturbing the near-wall region. The augmentation of velocity and baffle dimension leads to a notable heat transfer enhancement; however, this enhancement is less sensitive to the heating intensity augmentation.

The influence of a thin layer of phase change material (PCM) on the thermal characteristics of the outer wall of a building made of lightweight thermal insulation material was studied numerically. Changes in temperature and heat flux density were analyzed for various locations of a PCM layer in the wall. It is shown that the use of a thin paraffin layer 4 mm thick in a wall made of foamed polyurethane 100 mm thick can reduce the amplitude of heat flux fluctuations on the inner surface of the wall in the summer from 2 to 13 times, depending on the PCM location. The greatest reduction is achieved when installing the PCM in the central area of the wall. Calculations show that when using a PCM in the walls of buildings made of light thermal insulation materials, a positive effect, associated with a 6-8 hour delay in the maximum heat flux entering the room relative to the maximum daily value of the outside air temperature, is observed in the summer.

The paper describes the results of an experimental study of the influence of control surfaces and fuselage on the structure of a separated flow around a model of a small-size unmanned flying vehicle with a straight leading edge of the wing. The use of oil-soot visualization and hot-wire anemometry shows that the separation region location depends on the attitude of control surfaces, while the presence of a fuselage leads to reduction of the critical angle of attack.

Supersonic underexpanded air jet modeling based on the QGD algorithm is carried out. The process of jet flow evolution and the formation of unsteady flow regions are investigated to obtain a good qualitative and quantitative agreement of the results with experimental and simulation data, known from the literature. The QGD algorithm is shown to enable studies of the structure of shock-wave cells in the initial region of the flow, and to describe the general character of the turbulent jet.

The paper presents the experimental results on unsteady nucleate boiling on the heater surface with rapidly increasing surface temperature. Analysis of the results of high-speed video recording with a frequency of 180,000 frames per second and a spatial resolution of 5.5 μm per pixel shows that the input data for existing models of heat transfer during nucleate boiling must be refined to take into account the existence of cluster and pulsating bubbles. It has been established that bubbles, interacting through the exchange of momentum, heat and steam mass, accelerate the activation of neighboring vaporization sites, so the clusters of bubbles can form at the initial stage of covering the heater surface with steam. The main characteristics of isolated, cluster and pulsating bubbles have been studied for the wall overheating from 0 to 14 K above the temperature of nucleation onset and flow underheating from 23 to 103 K.

This paper presents the study of permeability of helium through walls of hollow glass silica microspheres, which can be used as membranes for gas flow. The study was performed in a special setup for measuring the kinetic sorption curves for helium at given pressure and temperature. A mathematical model based on a mono-dispersion distribution was used for approximating the experimental data. The data was obtained for the temperature range 21.5 -110.0 °С. The helium permeability of microsphere walls and the activation energy for helium sorption by microspheres were defined for this temperature range.

Constructing models of thermophysical characteristics of polymer composite materials in the curing process is a highly pressing task that is substantiated in this paper. A mathematical model of hot curing of polymer composites based on a thermosetting resin in a mold is presented. Based on experimental temperature dependences of thermophysical characteristics measured under different conditions, modeling dependences of the volume heat capacity and thermal conductivity of fiber, fabric and granular polymer composites on the degree of cure and resin fraction during curing are obtained, with their distinctive feature being the replacement of the properties of a porous reinforcing filler with those of the cured resin in the model. The results of experiments and calculations are presented. The proposed models of thermophysical characteristics increase the accuracy of process modeling and calculation of optimal temperature-time curing cycles.

The study is aimed at clarifying and revealing the basic principles of the effect of superheated steam and its parameters on the content of solid carbon particles (soot) in intermediate and final combustion products when burning liquid hydrocarbon fuel. Using a laboratory atmospheric atomizing burner, it was determined that there is a significant amount of solid carbon particles at the base of the burner flame. When heated air is used instead of steam, an increase in soot content by ~ 75% is observed. The analysis of the flame glow intensity in a narrow ultraviolet band also showed that in air the glow values are ~75% higher than when using superheated steam. At the same time, it has been established that the soot content in the final combustion products is affected only by the parameter of the dynamic impact of a jet, which determines air ejection from the environment both into the gas generation chamber and into the external flame region.

The paper presents the results of experimental studies on the effect of multi-walled carbon nanotubes (MWCNTs) additives on the viscosity and rheological characteristics of drilling emulsions based on mineral oil. The formulations of typical drilling fluids containing 65 % hydrocarbon phase were modified with nanotubes. The mass concentration of nanotubes in emulsion varied from 0.1 to 0.5 %. The formulation and method of preparing stable drilling emulsions with MWCNT additives have been proposed. The rheology of drilling emulsions modified with MWCNTs was studied. The dependency of rheological characteristics on the nanotubes concentration was obtained. In general, MWCNT additives can significantly alter the rheological characteristics of drilling hydrocarbon emulsions at lower concentrations compared to the additives in the form of spherical nanoparticles. This is very important for their practical use in industry. The optimal concentration of MWCNTs for controlling the rheological properties of drilling emulsions is about 0.25 wt. %.

Considering some limitations of various macroscopic chemical reaction models including the total collision energy (TCE) and general collision energy (GCE) models, the new modification is implemented in the DSMC algorithm for numerical simulation of dissociation of the air along the stagnation line and around a typical hypersonic atmospheric blunt body, STS-2 in non-equilibrium conditions and modified model is compared with others conventional models. Since the TCE and GCE models are dependent on some experimental parameters ( A and B at Arrhenius rate of reaction equation). Also, due to lack of accuracy of the quantum kinetics model, modification version of chemical reaction models is presented as hybrid of modified quantum kinetics (MQK) and modified collision energy (MCE) which this method is able to extract A and B parameters without need experimental background. Accuracy of the current applied chemical model for calculation of flow field characteristics is assessed by comparison of their results with other methods (analytical models and available experimental data). The results indicate that the modification of hybrid model with advantages of independency of the empirical parameters gives more accurate results and provides more accurate solution compared to conventional methods without need A and B constant experimental parameters.

The analysis was performed for input-condition sensitivity of haemodynamics simulation for the zone of abdominal aortic aneurysm. We took three versions of patient-specific configurations of aneurism and computed the haemodynamics with different spatial and velocity profiles at the inlet. Their impact on haemodynamic characteristics was evaluated. At total, the study was performed for three spatial variants (uniform, parabolic and parabolic-and-secondary-flow velocity profiles) and three versions of time behavior of velocity profiles: this produces nine cases for every of three chosen geometries. The study demonstrated that we can neglect the impact of spatial profile for inlet velocity (including the non-coaxial velocity vector components). Meanwhile, the value of reverse diastolic flow is significant for the solution. However, simulation in the zone of abdominal aortic aneurysm does not demonstrate great differences in simulation results for the values of wall shear stress and velocity for the data averaged over a cardiac cycle. For the distribution of the oscillation index of shear, the maximum deviation from the basic solution is about ~ 10 %, which is quite acceptable for clinical applications.