Home – Home – Jornals – Thermophysics and Aeromechanics 2022 number 2

The paper presents a review of the methods of experimental investigations of the Görtler vortices in boundary layers. The proposed models are used in experiments for the analysis of the Görtler instability; the major methods of Görtler vortex generation are described. It has been revealed that spontaneous uncontrolled emergence of the Görtler vortices is caused by the roughness of model surfaces (especially in the area of the leading edge of a plane model) and free-stream disturbances. Effective methods of introduction of controlled stationary and nonstationary disturbances generating the Görtler vortices in the boundary layer are described. The methods of experimental measurements of boundary layer parameters in the presence of the Görtler vortices are presented; the diagnostic capabilities of these methods are demonstrated, and the major scientific results obtained by these methods are reported.

A numerical model of the dynamics of a flat localized region of turbulent perturbations of non-zero buoyancy in a linearly stratified medium has been developed on the basis of the mathematical model, including differential equations for Reynolds stresses transfer and algebraic model of a vector of turbulent scalar flux. The evolution of a heated turbulent spot has been considered. The non-zero buoyancy is the reason of an increase in the geometrical size of the turbulent spot and generation of internal waves of greater amplitude. The generation of the total energy of turbulence is insignificant even in the case when the initial potential energy of a turbulent spot of non-zero buoyancy is comparable to the initial total energy of turbulence in it.

The present study shows that enhancement of mixing efficiency (increasing the total pressure of the mixture) requires expansion of the cross section of the high-pressure gas flow. It is found that the parameters of the mixtures are identical in the cases of distributed and one-step mixing of the perfect gas flows. However, distributed mixing (multistep mixing as a finite-dimensional analog) is more reasonable because it ensures flow control to provide conditions to satisfy the existence of a steady flow in the channel.

The present study shows that enhancement of mixing efficiency (increasing the total pressure of the mixture) requires expansion of the cross section of the high-pressure gas flow. It is found that the parameters of the mixtures are identical in the cases of distributed and one-step mixing of the perfect gas flows. However, distributed mixing (multistep mixing as a finite-dimensional analog) is more reasonable because it ensures flow control to provide conditions to satisfy the existence of a steady flow in the channel.

The paper presents the results of computational and experimental study for parameters of the innovative pulse-mode ejector used as a flow control device for flight vehicles. The ejector comprises a Helmholtz resonator adjusted to a certain resonant frequency. The computational and experimental results revealed the existence of resonant modes in ejector operation providing a higher efficiency. Simulation for internal aerodynamics for the pulse-mode ejector can explain a high efficiency calculated by the high-pressure gas flow rate, as well as advantages/shortcomings in other metrics (compared to the steady operation mode). The margins of efficient operation of this resonance-operating ejector in the metrics of gas flow rate are calculated. The perspectives for using the ejector as a flow control devices are discussed.

In this paper, analytical analysis on g-Jitter induced natural convection flow in microgravity behaviour is theoretically examined in the existence of Lorentz forces in a vertical micro-channel. Effects of velocity slip and temperature jump are also taken into account. A single component of time harmonic g-Jitter is considered. Solutions are obtained for the velocity and temperature profile with a combined effect of oscillating g-Jitter driving force and induced Lorentz force, the latter resulting from an application of a transverse magnetic field. Four limiting cases are carried out based on the solutions. Numerical procedures are under see for various active parameters namely: g-Jitter induced frequency, fluid-wall interaction, rarefaction, and Hartmann number. The results of the research unveiled that, the amplitude of the velocity decreases at a rate inversely proportional to the g-Jitter induced frequency and with increase in the Hartmann number. The induced flow oscillates at the same frequency as the affecting g-Jitter. Furthermore, a magnetic field can be applied to suppress oscillating flows joined with g-Jitter, it is more effective in damping low frequency flows but only has a moderate damping effect on the flow induced by high frequency g-Jitter. Also, the temperature jump condition induced by the effects of rarefaction and fluid-wall interaction parameter plays relevant impact in slip-flow natural convection.

A physical-mathematical model of a laminar axisymmetric flow of a power-law fluid through a sudden pipe contraction under non-isothermal conditions is presented with allowance for dissipative effects. The rheology of the liquid medium is determined by the Ostwald-de Waale law. The effective viscosity is specified as a temperature function. Two options for setting the temperature boundary conditions on a solid wall are considered: the first implies invariable temperature along the pipe wall; and the second assumes a constant temperature value on the wall except for the area in the contraction plane vicinity, where the boundary is exposed to a zero-heat flux. The process is studied numerically using the finite-difference method. The main characteristics of the flow are calculated and visualized. The effect of thermal boundary conditions on the fluid flow structure and local pressure losses is analyzed.

In this paper, multi-objective optimization of geometric parameters of spirally-cross-corrugated (SCC) tubes is carried out using numerical methods, genetic algorithms (GAs), and artificial neural networks (ANNs). First, the turbulent flow is numerically characterized in various SCC tube geometries using a finite volume method with the realizable k-ε turbulence model. In this approach, the heat transfer coefficient and friction factor f in tubes are calculated. First, two parameters (corrugation pitch-to-diameter ratio ( PR = p / D ) and corrugation depth-to-diameter ratio ( DR = e / D )) are examined in a turbulent flow regime that affects the strength of quadruple longitudinal vortex flows and thermal characteristics. At the final step, using the obtained polynomials for neural networks, multi-objective genetic algorithms (NSGA II) are employed for Pareto based multi-objective optimization of flow parameters in such tubes. This analysis considers two conflicting parameters, f Re and Nusselt number Nu with respect to three design variables, Reynolds number (Re), values of PR and DR . Some interesting and important relationships between the parameters and variables mentioned above emerge as useful optimal design principles involved in the heat transfer of such tubes through Pareto based multi-objective optimization. Such important optimal principles would not have been obtained without the use of a combination of numerical techniques, ANN modeling, and the Pareto optimization.

The oscillating flow of a viscous incompressible fluid in a rigid circular pipe with local constriction has been studied numerically. Fluid viscosity and density of liquid, channel diameter and constriction, as well as amplitude-frequency characteristics of the flow rate are identical to those of the blood flow in the human popliteal artery with stenosis. Reynolds number in the area of stenosis is Re » 5×10³, dimensionless pulsation frequency is Sh = 0.43×10^-3, and normalized constriction of the channel is d = d / D = 0.4. The flow under consideration is characterized by the fact that during one oscillation period the fluid flow rate changes direction four times. This contributes to laminar-turbulent transition when the jet discharges from a contraction throat into the main flow. Qualitative features and quantitative parameters of pulsating flow have been determined including distribution of friction stresses along the channel wall.

A semi-analytical study is performed to examine transient Taylor-Dean flow in a composite annulus within two concentric cylinders partially filled with porous material. In the present model, the circumferential flow is set up as a result of azimuthal pressure gradient as well as the rotation of the two concentric cylinders. The equation governing the flow is rendered non-dimensional and transformed into ordinary differential equation using the well-known Laplace transform technique. The solution is then transformed back to the time domain using the Riemann-sum approximation (RSA) approach. The solution of the steady-state of the present model including the implicit finite difference (IFD) is also computed to validate the result obtained from the RSA approach. It is important to note that the surface resistant force can be controlled by choosing suitable values of β .

Coal mined from the surface or underground is rarely suitable for direct use and requires preparation based on physical and/or chemical methods of removing certain components in order to improve the quality of coal to the required level. High humidity leads to a decrease in the energy efficiency of boiler, so the process of fuel drying is a necessary step in coal preparation, and reducing energy costs for moisture removal is an urgent task. One of the ways of moisture reduction is coal treatment with microwave radiation. As compared with other methods, microwave drying has the following features and important advantages: 1) volumetric heating; 2) selective heating (there is no energy absorption in dried areas); 3) low thermal inertia. However, this method requires high power inputs and there is a need to find energy-efficient processing regimes. In the present work, a drying model has been constructed, the optimal regime of layer drying has been found using the finite element method at the example of brown coal from the Talovsky deposit, and energy costs have been determined.

This study aims to investigate the effect of gas radiation on laminar free convection flow within a square cavity with internal heat generation. The cavity walls are isothermally cooled whereas the inner body is kept constant at a higher temperature. An important finding from this analysis is that radiative contribution plays a major role in the acceleration of the vortexes, providing a homogenizing effect on temperature fields. It is also shown that the optimal level of energy efficiency is achieved if the heater is located at the center of the lower part of the enclosure and when the inclination angle of the enclosure is of π /4.

One of the promising ignition technologies is the plasma thermochemical preparation of pulverized coal for combustion using plasma-fuel systems (PFS). This technology allows increasing the efficiency of fuel application and improving the environmental performance of thermal power plants, as well as eliminating fuel oil, used traditionally to ignite boilers and stabilize combustion of a pulverized coal flame. This paper presents the numerical results on ignition of a pulverized coal flame in a PFS. The plasma-fuel system is designed for oil-free ignition of boilers and stabilization of flame combustion and this is a pulverized coal burner equipped with a plasmatron. In addition to plasmatron electric power and ash content in coal, one of the main operating parameters of PTS, which ensures fuel ignition, is concentration of coal dust in the air mixture, which can be varied over a wide range. The conditions of fuel mixture ignition in the PFS were determined for three above-mentioned operating parameters of PFS using the PlasmaKinTherm program, which combines kinetic and thermodynamic methods for calculating the processes of motion, heating, and thermochemical transformations. The calculations were performed for a cylindrical PFS with a diameter of 0.2 m and a length of 2 m. The coal consumption was 1000 kg/h. The conditions of fuel mixture ignition in the PFS were studied depending on plasmatron power (20-100 kW), coal concentration in the fuel mixture in the range from 0.4 to 1.8 kg of coal per 1 kg of air, and also for three different values of coal ash content (20, 40, and 70%). The main regularities of the process of plasma thermochemical preparation of fuel for combustion have been revealed.