Home – Home – Jornals – Thermophysics and Aeromechanics 2023 number 2

The paper describes the results of studying the influence of a free-stream disturbance localized in time and space on the structure of the wake behind a symmetric droplet-shaped airfoil in the range of angles of attack from -20° to +20°. It is demonstrated that the existence of such a disturbance leads to suppression of vortex formation at certain angles of attack. Ranges of angles of attack in which the wake structure remains unchanged are found.

This work is devoted to a numerical study of the influence of foreign-gas injection on the stability of compressible boundary layer on a concave surface. The stability calculation was carried out in the framework of the locally parallel linear stability theory. The results of calculations for the reference case without injection showed that Görtler vortices and the second Mack mode exhibited the highest growth rates at studied parameters. It is found that the injection of a heavy gas (with respect to the oncoming gas) leads to an increase in the rate of growth of Görtler vortices and the second Mack mode, whereas the injection of a light gas leads to a decreased rate of growth of these perturbations.

Based on photometric measurements in an argon flow, the influence of large van der Waals clusters on the transverse dimensions of a supersonic under-expanded jet was revealed. In the range of average sizes of formed clusters 330 < 〈S〉 < 6200 part./clust., a model of corrections to the known gas-dynamic empirical dependences, formed in the absence of a condensed phase, was proposed to take into account the features of gas outflow from a supersonic nozzle under developed condensation. The possible reasons for the broadening of gas flows under conditions of developed condensation were established. The proposed correction model was tested in supersonic outflows of condensing gas on a number of supersonic conical nozzles.

An experimental study of the influence of the leading-edge bluntness radius of a plate on the response of boundary layer on this plate to an N-wave at Mach number M=2 was carried out. Three flat-plate models with leading-edge bluntness radii r = 0.05, 0.5, and 2.5 mm were used in the experiments. The oncoming-flow disturbances were created using a generator installed on the sidewall of the test section of the T-325 wind tunnel of ITAM, SB RAS. It is found that behind the N-wave in the oncoming flow there forms an extended region with an increased level of flow pulsations whose spectrum features amplitudes increased both in the low-frequency and in the high-frequency portion of the spectrum compared to the undisturbed free flow. It was shown that, under experimental conditions, the flow non-uniformity generated by the N-wave can exert a greater influence on the laminar-turbulent transition in the boundary layer of a flat plate with an increase in the leading-edge bluntness radius.

The development of instabilities in supersonic perfectly expanded jets exhausting from nozzles with square and rectangular cross sections into an ambient flow is studied based on the numerical solution of the Navier-Stokes equations. The simulations are performed for various Mach numbers of the jet and the ambient flow. The results of numerical calculations make it possible to identify qualitative and quantitative differences in instability development in cases with supersonic and subsonic air flows, as well as characteristic features of the flow for rectangular jets.

This work is a continuation of a cycle of research by the authors aimed at studying the laws of liquid hydrocarbon combustion under steam supply conditions as applied to the development of low-emission burners. When simulating numerically diesel fuel combustion, n-heptane (formula C₇H₁₆) is used as a one-component analogue. Reliable experimental data are required to verify the obtained results. In this work, for the first time, the combustion parameters of n-heptane are experimentally studied when it is sprayed with superheated steam in a new laboratory atmospheric burner with forced air supply to the gas generation chamber. The calculation results are compared with data on diesel fuel. The performed comparison shows that the characters of gas component dependences inside the flame for diesel and heptane differ. This is primarily due to the different densities, viscosities of fuels, their flammability and burning rate (depending, among other things, on the forced supply of air (oxidizer)). At that, a quantitative comparison shows that the values of gas components at the flame boundary are quite close to each other. The effect of ongoing physicochemical processes in the presence of steam (gasification, hydrocarbon splitting, combustible mixture dilution, formation of active OH radicals) on the composition of final combustion products is higher than the effect of fuel properties. It is shown that during the combustion of n-heptane atomized by a steam jet, all the main features characteristic of diesel combustion with steam supply are retained. High completeness of fuel combustion and low level of toxic emissions into the atmosphere are ensured, which satisfies the European standard EN:267. With an increase in the share of steam in the combustible mixture, the tendency to a CO and NO_x decrease remains. The air flow control in the gas generation chamber of the burner allows further regulation of the level of toxic combustion products released into the atmosphere. The dynamics of dependences of CO and NO_x emissions repeats for heptane and diesel with some local differences. Also, the paper analyzes the correctness of n-heptane applicability for the problems of numerical simulation of diesel combustion in a steam jet with the indicated assumptions.

For the first time, experimental studies were carried out on the effect of hydrogen combustion inside a supersonic (М_е= 2) flat-plate boundary layer on the laminar-turbulent transition in this flow. It was found in experiments that hydrogen injection (originating from the streamlined model) and its combustion in a certain sublayer partly stabilizes the layer flow. This result corresponds to the calculated data from S.A. Gaponov: it was shown that the heat supply into the supersonic boundary layer can deter the disturbance growth (i.e., facilitates the stabilization of the boundary layer stabilization). However, in general (compared to the non-combustion flow), the supersonic boundary layer is destabilized by combined hydrogen injection and fuel combustion. This means that the destabilizing effect of hydrogen injection prevails over the stabilizing effect of heat supply to the boundary layer.

The effect of the parameters of a pulsed-periodic side jet on combustion of a hydrocarbon fuel in an axisymmetric channel flow with the Mach number of 1.7 is studied numerically. The Reynolds-averaged Navier-Stokes equations closed with the k-ε turbulence model are solved. Fuel combustion is modeled with the use of one reaction. It is demonstrated that the temperature of the gas generator for the jet produced only a minor effect on the shock wave structure of the flow. The key role belongs to the pressure of side jet injection. A transonic flow structure is obtained.

The paper presents a numerical simulation of a stagnant (suspending) gas slug in the Taylor flow fitted to the experimental conditions. The simulation is based on the unsteady model of k - ω SST (shear stress transport) turbulence. Simulation covers analysis of gas-and-liquid flow parameters in the zones ahead the slug, after the slug and for the liquid film. The study demonstrates a compliance between the experiment results and simulation regarding the liquid film shear stress, the slug nose shape and the film thickness.

The effect of the parameters of a pulsed-periodic side jet on combustion of a hydrocarbon fuel in an axisymmetric channel flow with the Mach number of 1.7 is studied numerically. The Reynolds-averaged Navier-Stokes equations closed with the k-e turbulence model are solved. Fuel combustion is modeled with the use of one reaction. It is demonstrated that the temperature of the gas generator for the jet produced only a minor effect on the shock wave structure of the flow. The key role belongs to the pressure of side jet injection. A transonic flow structure is obtained.

This study seeks to improve the film cooling performance by embedding a film hole in a V-shaped trench. The angle scale that makes up the V has been changed, 25°, 75° and 115°. The three novel designs were compared to rectangular (transverse) trench and conventional cylindrical hole. The main parameters of film cooling, cooling effectiveness, and total pressure loss, were discussed at three blowing ratios, M=0.5, 1.0, and 2.0. Fifteen cases were simulated using Reynolds averaged Navier-Stokes equations closed by the RNG k-ε model. A good agreement is obtained between CFD results with experimental data of the baseline case. The results showed that the use of compact jet in the trenches, whether transverse or V-shaped, enhances the film cooling effectiveness. The use of V-shaped trench helps to reduce the size of the kidney vortices (CRV) and, thus, enhance the film cooling performance. The V2 shaped trench is the most prefer in improving the film cooling efficiency and reducing the total pressure loss.

The paper presents a developed 2D mathematical model for a regenerative heat exchanger designed for a ventilation system operating with a periodic change in airflow direction. This kind of ventilation system saves the heat energy required for heating of domestic premises during a winter season. Results for calculations by a two-dimensional model are compared with one-dimensional modeling and with available experimental data. The authors formulated a definition of energy efficiency in terms of reduction in heat losses. Our calculations demonstrate that the efficiency of a regenerative heat exchanger might exceed 90 %. The numerical simulation was applied for parametric study of this problem; we revealed the influence of the heat exchanger operational and design parameters on the energy efficiency. Numerical experiments found a group of parameters most significant for energy efficiency of ventilation system; recommendations on system optimization were formulated.

The influence of the size of water droplets, temperature, and velocity of a streamlining jet of dry air on the rate of evaporation is considered. Due to the systematic studies using a thermal imager, it is shown that the interface temperature changes spontaneously. It is assumed that the temperature nonuniformity on the surface is caused by the release of vapor nanobubbles and can be characterized by the velocity of their outflow. Empirical regularities on the intensity of bubble release from a droplet are obtained both as a function of time and diameter, and in the criterion form.

The gasification of solid urotropine was experimentally studied at filtering a high-temperature flow of carbon dioxide through it. It was shown that with an increase in the temperature of the filtered gas from 650 to 920 K, the time of urotropine gasification decreased and the average gasification rate increased from 0.38 to 1.25 g/s, leading to an increase in the flow of urotropine gasification products. The maximum achieved value of the mass of urotropine gasification products was 0.8 g per 1 g of incoming gas. In the temperature range of 480-530 K, intensive gasification of urotropine occurred, while the temperature of the gaseous products leaving the reactor remained practically unchanged. The amount of noncondensable gaseous gasification products did not exceed 1% of the initial mass of the sample.

The object of an experimental study is a vapor bubble formed in a subcooled liquid as a result of absorption of laser radiation transmitted into the working volume through a thin optical fiber. Evolution of a bubble is characterized by its rapid growth and collapse with generation of a hot submerged jet. Some features of the process under study are considered in relation to the field of medicine. Normal saline is used as the working fluid. It is shown that under the same conditions (radiation power, optical fiber diameter, and initial temperature of liquid), the dimensions reached by a vapor bubble in saline solution are much smaller than those in pure water. A significant influence of the shape of a fiber tip on the nature of the process under study was revealed.

This paper considers the microwave processing for snow-ice mass comprising the heating and melting stages. The search for basic patterns of these processes aimed to optimization, control and design of stages is based on mathematical models and their implementation using analytical or numerical methods. A nonlinear mathematical model of the two-phase Stefan problem for a layered system of dielectrics was constructed. This approach takes into account the dependences of the medium permittivity and other parameters on the medium temperature and the design of a microwave radiation source.

Thin films of polycrystalline silicon are widely used in semiconductor industry. One of the methods for obtaining such structures on cheap and low-melting substrates is metal-induced crystallization, since the use of a metal (for example, Au) as a catalyst during crystallization of an amorphous semiconductor allows a considerable reduction of annealing temperature. However, the typical duration of metal-induced crystallization is several tens of hours, in contrast to the method of laser-induced crystallization. In the present work, for the first time it is proposed to combine the advantages of the laser-induced and Au-induced crystallization methods. The authors have identified laser-processing modes of thin films of non-stoichiometric silicon oxide (a-SiO_0,1) using nanosecond radiation with a wavelength in the infrared range which ensure the formation of polycrystalline silicon.

In this work, the thermal conductivity of refrigerants systems from three different hydrofluoroolefins including R1234yf, R1234ze (E), and R1233zd(E) were studied using artificial neural network. A total of 4395 data points of liquid and vapor thermal conductivity at several temperatures (241.92 to 344.46) K and pressures (0.068 to 21.73) MPa were used to train and test the model. Five neurons were used in the input layer, fifteen neurons at hidden layer and one was used in the output layer. Bayesian Regulation back propagation algorithm, logarithmic sigmoid transfer function, and linear transfer function were used at the hidden and output layer, respectively. Temperature, pressure, applied heating power; acentric factor and dipole moment were considered as input variables of the networks. The optimal parameters were obtained through the weights searching method. The average absolute relative deviations and correlation coefficient were 1.48 and 0.9998, respectively. This study shows therefore that the artificial neural network model represents an excellent alternative to estimate the thermal conductivity of different refrigerant systems with a good accuracy.

A general analysis of heterogeneous sources of waste heat at thermal power plants, taking into account the advanced world and domestic experience in creating high-power steam compression and absorption thermal transformers, has shown the possibility of significantly reducing heat losses by integrating absorption heat pumps and refrigerating machines of various designs and capacities into the thermal circuits of existing and projected thermal power plants. It is proposed to carry out a comprehensive optimization of the plants by creating intra-plant trigeneration systems. Taking into account the specific operating conditions of steam turbine condensers, cheaper multicomponent LiCl salt-based aqueous solutions, which are not inferior in efficiency to imported LiBr solutions with anticorrosive additives, are offered for the use as working bodies.

The paper presents the results of thermodynamic calculations and experimental studies on plasma processing of spent fuels and lubricants, which showed the prospects of using the plasma-chemical technology for processing liquid industrial wastes with the production of fuel gas and inert mineral material. The comparison of experimental and calculation results showed an acceptable agreement.