Home – Home – Jornals – Thermophysics and Aeromechanics 2002 number 4

The paper overviews experimental and numerical studies of flow and heat transfer in a single spherical cavity. Major structural components of the flow pattern in spherical cavities are discussed. Much attention is paid to the formation mechanism of the tornado-like vortex in the separation flow past cavity. Based on a simultaneous consideration of fluid dynamics and heat transfer, hot problems still existing in this research area are outlined.

A retrospective analysis of previous numerical hydrodynamic and heat-transfer studies of vortex flow in the neighborhood of flat-wall dimple is given. For the turbulent flow mode, the approach based on the use of multi-block overlapping computational grids and Menter

To describe the counter-gradient transport of active (heat) scalar in a shear-free thermally strongly stably stratified flow downstream of a turbulizing grid both a completely differential model of transport for the first- and second-order moments and a completely explicit anisotropic algebraic model of gradient diffusion for active scalar were used. The results of computer modelling show that for a weak stratification the both models give a distribution of vertical heat flux in good agreement with measurement data. For a strongly stable stratification the effect of counter-gradient heat transfer is described only by the model of differential transport equation for the vector of turbulent flux of active scalar. In this case the numerical modelling results agree qualitatively with measurement data.

This paper continues the research of a gas-dynamic duct of supersonic heterogeneous devices with the use of a mathematical model developed earlier. Analysis is performed using dimensionless criteria of acceleration/deceleration path. A new criterion was introduced: the ratio of those lengths. This helps to estimate how the nozzle length, cut depth, pressures and temperatures in the pre-chamber and after the shock wave influence the flux of kinetic energy of particles bombarding the target surface (without real calculation of their acceleration (deceleration)). This excludes the particles from process analysis.

The problem of initiation of transition from nucleate to film boiling regime is considered from two statements: stationary and dynamic ones. Analysis of stationary problem allowed determination of relationship between heat load and critical size of a film domain. According to investigation of development dynamics of boiling crisis, at local deterioration in heat transfer, there is some critical size of a film domain, which is determined by a heat load and duration of initiating action. Whereas, at a local temperature increase, the critical conditions are determined only by the amplitude of temperature perturbation and heat power.

Basing on the three-dimensional equations of convective heat and mass transfer the processes of the combustion of a high-ash coal-dust fuel have been modelled numerically in the regions of real geometry (the PK-39 furnace chamber of the Aksuy State Regional Electric Station (SRES)). The distributions of temperature and NO_x concentrations are given over the furnace chamber height. The computed results are compared with experimental data obtained at the SRES. An analysis shows that the modelling and experimental results are in a good agreement, and the distribution of the nitrogen oxides over the torch length and the level of their extension in the atmosphere are close to industrial ones.

The paper is of generalizing character and summarizes in some way the researching cycle devoted to the development of arc generators of the water-steam plasma. The critical analysis of various designs of the plasmatorches intended for water-steam plasma production and their abilities is presented. The main emphasis is put on the plasmatorches wherein the spatial stabilization of the electric arc is carried out with the aid of a steam vortex. The operation conditions for these plasmatorches are discussed in details, and the ways of their realization are offered.

The results of experimental investigations of temperature fields and Joule heating in hollow cathodes in vacuum plasmatorches have been considered. The characteristics of thermal fields on the hollow cathode inner surface have been obtained with the use of the solution of a two-dimensional inverse thermal task. It has been shown that the Joule heating of cathodes may be ignored at cathode thickness of δ > 2^.10^-3 m and discharge currents of (I/d1) < 10⁴ À/m. The role of re-emission processes and plasma pressure in the cathode cavity at parameter gradient formation over the cathode length has been elucidated.

Flow aerodynamics in a cavity of cylindrical electrode of the arc plasmatorch and its erosion were experimentally studied. Dependency between erosion peculiarities and arc current was found out. Criterion relationships, which provide maximal electrode efficiency, were obtained.

The paper presents results of numerical simulation for screening effect of charged dust particles that are generated above the electrode due to its sputtering by plasma flow. Two problems are solved sequentially: calculation of plasma parameters for a diverter of fusion reactor and simulation of nucleation of liquid droplets due to condensation of metal vapour (electrode material). A one-dimensional  hydrodynamic computer code TDF was used for calculation of plasma parameters: temperature, density, material and heat flux along the plasma flow (directed along the magnetic field lines). This simulation also takes into account electric thermal conductivity, ionization, recharging, atom excitation, and plasma recombination. Existence of charged metal droplets and neutral component of plasma due to plasma recombination on the surface was included into this model.  Plasma parameters calculated by TDF are the initial data for kinetic code TAG, which is a stochastic model of non-equilibrium fluctuation stage of metal vapour condensation – during this stage the distribution function of nuclei on their size is formed.