Volume 25, Nº 2 (2018)

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.

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 po-rosity, data on the displacement of the recirculation flow region into the porous material were 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 pro-perties. The algorithm enables the computation of the values of the shear stress and static pressure acting on the scaf-fold surface. The computations have shown that the necessary shear stress is reached in the proposed rotational biore-actor 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 bio-logical 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.

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 insta-bility 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 criticali-ty to another. These processes are accompanied by 1/f^α–noise (flicker-noise), which is regarded as an objective indica-tor of the boiling crisis. The theoretical results of the work have been confirmed by experimental data of other authors.

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 (0≤φ≤0.10), Rayleigh (10³≤Ra≤10⁵) and Reynolds (1≤Re≤500) 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 φ, Ra, and Re.

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.

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.

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.