Home – Home – Jornals – Thermophysics and Aeromechanics 2018 number 2

The possibility of exerting control over the turbulent boundary layer on NACA 0012 wing section with the aim of improving the wing aerodynamic characteristics in incompressible flow is experimentally examined. Control over the boundary layer was exercised in the range of the angles of attack from -12 to 12° by implementing combined steady-state blowing/suction of air through a finely perforated surface located, respectively, on the windward and leeward side of the wing and presenting part of streamlined surface.

Using the differential turbulence model supplemented by the transport equation for the turbulent heat flux, a numerical study is carried out of the dependence of the Prandtl turbulent number on the molecular Prandtl number, the intensity of the gas injection (suction) through the permeable wall, and the freestream acceleration (deceleration). The air and mixtures of helium with xenon and argon are considered as gas carriers, and mercury, water, and transformer oil are used as liquid carriers. The obtained results of calculations are consistent with the available experimental data for the turbulent Prandtl number and the quantities included in its definition.

Experiments were performed on the influence of distributed injection of a heavy gas (elegas SF₆) into the near-wall region of the supersonic (freestream Mach number М_∞ = 2) boundary layer on its stability in relation to natural disturbances. Heavy gas injection for the case of linear development of disturbance field results in boundary layer stabilization. It was experimentally proved that the elegas injection can suppress disturbances at the frequencies higher than 15 kHz for the tested range of the streamwise coordinate.

Results of an experimental and numerical study of a supersonic flow over a model forward-facing step with a gas-permeable insert of variable porosity installed upstream of the step are reported. The free-stream Mach number was M = 2.0, 2.5, and 3.0, and the Reynolds number, Re = 5·10⁵. The gas-permeable insert was either a section of a perforated plate or a section of a highly porous permeable cellular material. The flow visualization performed using the shadow method, PIV, and a soot-oil film has shown that the characteristic size of the vortical flow region exhibited a profound decrease on increasing the insert porosity. In numerical calculations performed at high values of that porosity, data on the displacement of the recirculation flow region into the porous material were obtained.

Symmetric and asymmetric self-similar flows of a viscous incompressible fluid along a semi-infinite right-angle dihedral corner with a preset streamwise pressure gradient have been considered. Equations describing such flows in the framework of boundary layer approximation have been derived. The asymptotic behavior of solutions of the derived equations far from the corner edge has been theoretically investigated. A new method of computation of these solutions has been developed. Solutions for two types of asymptotic behavior have been obtained.

The technology using for the replacement of damaged tissues the own cells of the patient, which are placed in a three-dimensional frame – scaffold, is promising for solving the problem of the bone tissue regeneration. A new biological reactor of the rotational type, in which the scaffold tissue rotates in a medium for cultivating the cells, was designed for the development of this technique. A numerical algorithm based on the ANSYS program was developed, which enables one to estimate in a new bioreactor the level of the mechanical load on the cells, which affects their pro­perties. The algorithm enables the computation of the values of the shear stress and static pressure acting on the scaf­fold surface. The computations have shown that the necessary shear stress is reached in the proposed rotational bioreactor on the outer side of the inner cylinder (0.002-0.1 Pa) in the range of rotation frequencies 0.083 < f < 0.233 Hz. At the same time, computational results have revealed the presence of an inhomogeneity in the mechanical action distribution along the scaffold tissue, which is due to the appearance of two Taylor vortices with opposite rotation directions in the gap between the cylinders. The experiments on the flow field visualization inside the rotational biological reactor have shown a qualitative agreement of the flow character with computational results. The proposed numerical algorithm may simulate with sufficient accuracy the fluid flow in a real system. The obtained dependencies can be used in practice for creating an optimal microenvironment of the cells cultivated in the biological reactor.

Distribution of liquid velocity and its pulsations were measured in the space between fuel elements of the 7-rod experimental model of the fuel assembly of nuclear power plants at Reynolds numbers of 8000, 16000, and 24000. The influence of the spacer grid on distribution of the axial component of liquid velocity was determined. It is shown that the relative values of liquid velocity and its pulsations do not depend on the Reynolds number and they are determined by the distance from the spacer grid.

Dynamics of a liquid boiling on a heating element surface is investigated for the equivalent problem of dynamics of temperature of the fuel cell obtained in the concept of boiling curve. It has been shown that the structural instability of the potential of boiling curve leads to the need of studying the problem of temperature fluctuations of the surface on which boiling occurs. A theoretical analysis of the Itô's equation for the considered system model has shown that at a certain intensity of external random factors, the system passes from one state of self-organized criticality to another. These processes are accompanied by 1/f ^a– noise (flicker-noise), which is regarded as an objective indicator of the boiling crisis. The theoretical results of the work have been confirmed by experimental data of other authors.

The article presents the experimental dependences of a macro-contact angle and the diameter of a distilled water drop spreading over solid microstructured surface on surface average roughness (Ra) and fluid flow rate (G). It has been found that at changing G from 0.005 to 0.02 ml/s, the contact angle decreases, and at a liquid flow rate over 0.02 ml/s, it increases. With small values of G (0.005-0.01 ml/s), the drop diameter grows throughout the spreading process. In the range of G from 0.02 to 0.16 ml/s at the final stage of spreading, the contact line pinning, i.e., the diameter constancy, has been detected. The hypothesis about the mechanism of the pinning process has been formulated: it is due to the zero sum of all forces acting on the drop (inertia, viscosity, friction, gravity, and surface tension).

In this work, two-dimensional mixed convection and entropy generation of water-(Cu, Ag, Al₂O₃, and TiO₂) nanofluids in a square lid-driven cavity containing two heat sources, have been numerically investigated. The upper lid and bottom wall of the cavity are maintained at a cold temperature T_C, respectively. The governing equations along with boundary conditions are solved using the finite volume method. Comparisons with the previous results were performed and found to be in excellent agreement. The effects of the solid volume fraction, Rayleigh and Reynolds numbers, and different types of nanofluids on the total entropy generation S_t and on entropy generation due to heat transfer S_h are presented and discussed. Moreover, the heat sources positions have an effect on the total entropy generation and Bejan number. It was found that S_t and S_h decrease with increase of f, Ra, and Re.

The influence of a thermal wake due to gas injection and due to a pulsating optical discharge (POD) on the aero-dynamic-drag force of a body in a supersonic air flow with Mach number M = 1.45 are experimentally examined. With the help of a single-component aerodynamic balance, the influence of the injected subsonic jet and the thermal wake produced by POD on the aerodynamic drag of a hemisphere-on-cylinder model was studied. It is shown that the observed aerodynamic-force reduction phenomenon can be made more pronounced by increasing the laser power and pulse repetition frequency, or by decreasing the distance between the model and the pulsating optical discharge. The maximum aerodynamic-force reduction (up to 15%) due to the thermal-wake action was observed at a maximum laser-radiation power of W = 2.3 kW and at a pulse rate of f = 90 kHz. The joint effect due to the argon jet and due to the POD caused an aerodynamic-drag force reduction reaching 30%.

Local features of thermophysical processes in the channels and pre-nozzle volumes of solid-propellant rocket engines with case-bonded charges of different cross-sectional shapes are considered. The influence of the charge shape on the heat exchange in the nozzle bottom is investigated. It is shown that the value of the Nusselt number at a critical point on the multi-nozzle bottom is determined both by the charge channel form and by the geometry of the pre-nozzle volume. By processing the numerical experimental results the criterial dependences for determining the Nusselt number in the areas of local increase of heat exchange intensity are obtained. The obtained dependences are compared with the known empirical formulas [1–4]. It is found that the use of empirical relationships to estimate the Nusselt number leads to incorrect determination of the parameters of heat transfer on the armored surfaces of the charge, the nozzle covers, and the input parts of the submerged rotating nozzle.

A class of nonlinear problems of non-stationary radiation-convective heat transfer under the conditions of microwave action with a small depth of penetration is considered in a forced laminar flow of liquid around a flat plane. The solutions to these problems are obtained using the effective asymptotic procedures at the successive stages of non-stationary and stationary radiation-convective heat transfer on the heat-radiating horizontal plane. The non-stationary and stationary stages of solution are matched using the “longitudinal coordinate-time” characteristic. The solutions constructed on such principles correlate reliably with the exact ones at the limiting values of such parameters as a small and large intensity of external thermal impact, small and large times, etc. The error of solutions does not exceed 1–7 %. As the plate is removed from the leading edge of the plate due to heat radiation, convective heat transfer degenerates from values characteristic of the boundary condition of the second kind to the values characteristic of the boundary condition of the first kind. A strong effect on the nature of variations of the surface temperature and Nusselt number of the complex parameter of microwave and thermal radiation is noted. An important advantage of the developed method for solving this class of external problems is that even before complex calculations it is possible to perform an exhaustive analysis of the fundamental laws of the processes under study. Despite a number of initial sim­plifications, the latter do not significantly affect the accuracy of results, guaranteeing reliable quantitative information. The developed method can also be extended to the regimes of forced convection with linear dependence of physical properties on temperature using transformation of A.A. Dorodnitsyn. To confirm adequacy of the constructed mathematical model, stationary radiation-convective heat transfer under the forced flow around a flat plate was studied experimentally. The results of comparison of the theoretical and experimental data show that they are in a good agreement. This again confirms the effectiveness of the developed method for constructing theoretical solutions to the nonlinear problems of forced convection using the asymptotic procedures.

This paper presents an analytical investigation of steady, fully developed MHD Couette flow of viscous, incompressible, electrically conducting fluid in the presence of radial magnetic field. Exact solutions are derived for the governing energy and momentum equations by taking into account the effects of viscous and Joule dissipations under relevant boundary conditions. The solutions obtained are graphically represented and the effects of various controlling parameters such as Hartmann number and Brinkman number on the temperature profile and consequently the Nusselt numbers are discussed. The significant result from the study is that increase in Hartmann number leads to enhancement on the Nusselt number at outer surface of inner cylinder while the role of Hartmann number is just reverse on Nusselt number at inner surface of outer cylinder. In addition, the Brinkman number has an insignificant effect on the Nusselt numbers when both surfaces are kept at equal temperature.

The paper presents a set of tests with a setup using steam supply into ejector instead of compressed air. Experiments measured the gas analysis data ¾ volumetric concentrations O₂, CO, CO₂, C_nH_m, NO_x, H₂ at different proportions of air and steam. The data are compared with calculations for thermodynamic equilibrium compositions for the reacting mixture С+Н₂О+air performed by “Terra” computer code including the case of air excess (a £ 1). The cal-culations were also compared with available data on gasification output at a high content of ballasting gas. It was demonstrated that in these operation modes, the steam was an inert dilution agent, which did not exclude the outcome of coal gas production with high Н₂ /СО and СО/СО₂ ratios corresponding to different modes of gasification.

Modern thermal power plants are complex technological systems. Therefore, making informed decisions when studying them requires the use of mathematical modeling and nonlinear optimization methods for plant parameter. The most complex task is to solve a mixed optimization problem wherein a part of optimization parameters vary continuously, and the other can take only discrete (integer) values. An effective method is developed to solve a thermal power plant optimization problem with continuous and discrete parameters. The method suggests an iterative procedure for solving continuous nonlinear programming problems and discrete-continuous linear programming problems. For each iteration, we add new constraints obtained by linearizing nonlinear inequality constraints and the objective function of the initial problem to the system of inequality constraints of linear problem. The effectiveness of the proposed method is exemplified by the optimization of a combined cycle power plant with a mixture of working media (the STIG scheme). Design characteristics of heating surface of a waste heat recovery boiler represent the discrete parameters to be optimized. The research demonstrates a considerable reduction in computational effort compared to the branch and bound method.

The structure of a jet flow formed by the combustion products of conical propane-air flame and impinging onto a normally oriented flat cooled surface is studied experimentally. The velocity field is measured by the particle image velocimetry technique. Based on the non-intrusive measurements, formation of a recirculation zone in the flow between the flame cone and surface has been detected for the first time. Mechanism for the observed phenomenon is proposed. Presence of the low-intensity recirculation bubble on the jet axis can explain the effect of a heat transfer decrease near the stagnation point on the surface, observed in the previous studies.