Home – Home – Jornals – Thermophysics and Aeromechanics 2019 number 5

The study is aimed at determining the position of the laminar-turbulent transition in subsonic and transonic two-dimensional boundary layers with the use of a novel software package LOTRAN 2.0 developed by the authors. The package is based on the e^N -method and employs numerical data of numerical simulations of the laminar flow performed by standard gas-dynamic software systems based on the Reynolds-averaged Navier-Stokes equations. As an example, the flow past a flat plate is considered. The good agreement of the computed and experimental data on the laminar-turbulent transition location is demonstrated. New data on the laminar-turbulent transition in the boundary layer on a flat plate for transonic flow regimes are obtained.

In the present paper, we report on the results of a study that was performed as part of the program for developing a method for calculating the flow and the mass-flow-rate coefficient in narrow cylindrical bleed channels under conditions of impingement of a thick boundary layer. For setting the boundary conditions at the inlet boundary of the computational domain, we used the pressure and temperature profiles obtained at the outlet boundary of the simulation domain for the flow around the forebody of a hypersonic air inlet. Numerical modeling of the 3D flow around that forebody was carried out in the range of Mach numbers from 3 to 6 at an angle of attack of 6°. The distributions of static pressure calculated along the symmetry plane and in the transverse direction were compared with experimental data. A satisfactory agreement between the calculated and experimental data is shown. The simulation of the flow was carried out within the framework of the Reynolds-averaged stationary Navier-Stokes equations using the k-w SST model of turbulence. The obtained distributions of flow parameters and flow streamlines have allowed us to perform an analysis of specific features of the flow structure on the forebody of the model air inlet and in a boundary-layer bleed channel. The calculations showed that, in the examined range of Mach numbers, a sonic flow in boundary-layer bleed channels could be realized to ensure a maximum rate of the mass flow of air through the channels.

On the basis of the Reynolds-averaged Navier-Stokes equations closed by the shear stress transport model with the consideration of streamline curvature, turbulent flow acceleration in the channel with inclined single-row oval-trench dimples, earlier found in the laminar regime, has been revealed. It is shown that the velocity in the flow core increases up to 1.4 times when dimples are densely packed. The influence of the dense arrangement of dimples on abnormal intensification of the separated flow in the stabilized area of the channel has been assessed. Absolute friction in the backflow zone in the dimple is found to grow up to 5.5 times compared with friction at the smooth channel wall.

The flow structure in a high-speed gas jet released from the nozzle of a promising liquid-fuel spray-type burner is studied experimentally and theoretically. The velocity distribution in a single-phase gas flow is measured by means of particle image velocimetry (PIV) at various operating parameters. Using the one-dimensional isentropic flow approximation, the gas-dynamic parameters are estimated for the characteristic operating regimes of the burner with supply of superheated water steam, as well as with air supply. With the use of the ANSYS Fluent CFD package, two-dimensional numerical simulation of aerodynamic structure of air stream from the nozzle is carried out. The results are compared with the measured velocity field and the flow pattern, typical of a supersonic underexpanded jet, visualized by the direct shadow photography method near the nozzle orifice.

A thermodynamic description of the Ranque effect is given. Inclusion of expansion and compression into the energy exchange process provides a rational explanation to the effect observed. These works are performed by small portions of the gas in polytropic processes. Rotational motion of the gas and the presence of irreversibility in the expansion and compression processes are responsible for energy separation. The mathematical model is constructed with the use of conservation laws and polytropic expansion. Optimization calculations are performed for various values of the efficiency and Mach number of the exhausting hot gas. A method of working process optimization hat can be used in practice is proposed. The efficiency of the expansion work is also determined in the course of optimization.

The paper presents the results of an experimental study of energy production in the Couette -Taylor heat generator with independent rotation of cylinders: the system is applied to solving the problem of direct conversion of wind energy into thermal energy. The system consists of two nested multicylinder rotors. The regimes for two counter-rotating rotors are studied. The study is focused on the rotor drag torque and the heat power of the generator as a function of the relative angular velocity of two rotors at a fixed viscosity of the working liquid or as a function of the working liquid viscosity at a steady relative angular velocity of two rotors. Representing of this multicylinder design of the heat generator to a form of a single equivalent annular channel between two rotating cylinders allows generalization of experimental data for the law of the drag torque and specific heat power as a function of the Reynolds number. This generalization offers a possibility of developing engineering methods for calculating the thermal parameters of various systems for fluid heating.

In this study, the effects of different thermo-chemical models on the macroscopic parameters of the flow behind a strong shock wave have been examined. The effect of the geometric average temperature proposed by Park and the effect of the electronic energy are also presented, and two CVD vibration-dissociation coupling models including those of Park and Kuznetsov are also examined and used for comparison. The Park93 chemical kinetic model with 11 species and 49 elementary reactions was used to describe the non-equilibrium air chemistry. The energy exchange model between translational and vibrational modes is described by the Landau-Teller formula, where the species relaxation time is based on the Millikan-White formula including Park’s high-temperature correction. The theoretical model consisting of the Euler equations supplemented with the equation of molecular vibration and the equations of chemical kinetics using a two-temperature model (translational-rotational temperature and vibrational-electron-electronic temperature) is discretized by a finite difference scheme. Good agreement is found for the relaxation zone between the present results and those obtained by Panesi for the two trajectory point (1634 s and 1643 s) for the FIRE II reentry capsule.

The results of numerical simulations of a jet discharging from a narrow slit at low Reynolds numbers Re are presented. A comparison with the data of laboratory experiments is performed, and it is shown that steady numerical solutions with increasing Re transform into unsteady solutions with self-excitation of sinusoidal instability. Lateral oscillations are superimposed at the inlet section, and their influence on the jet behavior is studied.

The theoretically calculated dependence of the sizes of the main and satellite droplets on the dimensionless wave number of the perturbation of the surface of the Newtonian fluid jet is proved experimentally. It is also shown that an increase in the fluid viscosity significantly changes the dynamics of development of capillary waves in the jet and the mechanism of formation of satellite droplets.

Results of a pioneering experimental study of the cold spray method used for spraying aluminum traces onto the surface of a brittle erodible material (like brick) are reported. The spraying is performed using a sonic nozzle. Particle velocities near the axis of the two-phase jet are measured, and an approximating formula is derived to describe the dependence of the particle velocity on the stagnation pressure and temperature of the accelerating air. Interaction of the two-phase jet with the substrate surface is studied, and it is shown that an increase in the gas stagnation temperature transforms the process of substrate erosion a coating formation process. Parameter values that can be recommended as working ones for practical spraying are identified.

The preparation of a titanium composite powder containing high-melting TiN nanoparticles using an electric-arc plasma torch is studied. Using an Evo MA15 electron microscope (Carl Zeiss) and a DRON-4.0 diffractometer, both the surface morphology and the internal structure of powder particles are investigated, an X-ray phase analysis of these particles is performed, and a distribution map of the elements both over the surface of the obtained composite particle and over its volume is constructed. It is shown that the nanoparticles are uniformly distributed over the entire volume of titanium particles and are completely clad with Ti. The nanostructured powders obtained in this way and used as nanomodifying additives increase the homogeneity of the distribution of the high-melting elements introduced into the melt, protect the nanoparticles from the direct interaction with ambient medium, increase their wettability, and prolong their shelf life. These factors improve the efficiency of metal and alloy nanomodification.

The paper describes a theoretical study on the influence of filtration of a mixture of gaseous products of pyrolysis and water vapor on the dynamic of ignition of coal-water fuel particles under conditions of high temperature heating. The developed mathematical model offers simulation of the process of ignition of coal-water fuel droplets under conditions close to the furnace space (characterized by intense radiation-convective heating) in typical boilers. Comparisons of experimental data (found previously) and simulation data on the ignition delay time ( t _ign ) demonstrate good compliance. Simulation results show that filtration of water vapor and volatiles is a significant factor (influence up to 40 %) affecting the characteristics and conditions of ignition of coal-water fuel droplets. The higher velocity of the vapor-gas mixture flow through the porous structure of the fuel particle results in a longer ignition delay. The effect of using the “simplified” model of filtration heat transfer on the prognostic estimates of coal-water fuel ignition is analyzed. It is demonstrated that using of rather simple models for filtration heat transfer does not bring any significant errors in calculating the ignition delay time.

Ice layer melting on a vertical substrate heated by a radiation source in the form of a halogen lamp is simulated numerically in a single-phase formulation of the Stefan problem. Ice is presented in the form of a clear, non-scattering, selectively absorbing material with two spectral bands of volume absorption. The computational model takes into account the selective nature of the radiation source. The analysis of calculation results shows the predominant role of incident radiation in the formation of the density field of resultant radiation flux in the medium. Satisfactory agreement between the calculations and experimental data is obtained.

Various issues of increasing the thermodynamic efficiency of generation of produced energy carriers, such as water, hydrogen or compressed air are considered. The proposed technology, called multi-generation, is based on the creation of energy complexes consisting of generation facilities and consumers. The task of generation facilities is to manufacture, along with the traditional energy carriers, such as electricity and heated liquid, other produced energy carries and useful products. In the case of separate generation, they would have been generated either at consumers or at targeted enterprises. The advantages of the multi-generation technology implementation are shown for individual generation facilities and consumers, as well as for the energy supply system as a whole. The change in the specific fuel consumption for electricity and heat production is taken as a criterion for evaluating the thermodynamic efficiency for a separate generation facility. For the power supply system, the criterion is the absolute and relative changes in the exergy efficiency of generation of all produced energy carriers. Formulas for comparative calculation of efficiency for combined and separate generation at accepted evaluation criteria are derived. The accepted conditions and results of calculations of changes in the efficiency of generation of produced energy carriers at the transition from separate to combined generation for the energy complex consisting of the T-100-130 steam turbine unit combined with vapor compression and ammonia-water absorption refrigerating machines are presented.

Results of simulations of air injection into a liquid-filled (water or liquid lead) closed vessel with a high pressure difference are presented. The simulations reveal a big difference in processes of air volume formation and evolution of its boundaries during air injection into air or liquid lead. A comparison of periodic pressure pulsations in the gas volume (simulation results) with pressure pulsations described by a quasi-stationary 1D model demonstrates conservatism of the simplified model. The results of destruction of gas cavities with pressure pulsations for water and liquid lead are compared. Other conditions being identical, the gas volume in water decays faster than that in lead.

The density and thermal expansion of the solid and liquid Inconel 718 alloy are measured by the dilatometer method and gamma-ray attenuation technique over the temperature range 293.15-1730 K. The density change during the solid-liquid phase transition is directly measured. A comparison of the obtained results with literature data is carried out.