I. N. Shcheglov

Deposition of solid particles in laminar boundary layer on a flat plate

Results are given of experimental and theoretical investigation of deposition of small solid particles on the surface of a flat plate under conditions of vertical laminar boundary layer. The present investigation is aimed at qualitatively and quantitatively estimating the effect made by the parameters of two-phase flow of the “gas—solid particles” type and by the adhesive properties of particles and surface on the deposition of particles on the plate surface. The flow velocity is 1.5 and 3 m/s. In so doing, the value of Reynolds number along the plate does not exceed 105. Synthetic corundum powders with average sizes of 12, 23, and 32 µm are used as the dispersed phase of two-phase flow. The mass concentration of particles in the flow is 0.01 kg/m3. A flat plate of stainless steel is used as the object of investigation. The distributions of gas velocity and concentration of particles within the boundary layer are measured using laser optical diagnostics. The number of particles deposited along the plate surface is measured by the gravimetric method. The adhesive properties of the “particle-surface” pair are studied using the centrifugal method of detachment of particles from the surface. Logarithmic-normal dependences of the number of adhesion of particles on the force of detachment are obtained. The hydrodynamic parameters of two-phase flow in the vicinity of the plate surface are calculated using the model of two-phase laminar boundary layer.The mathematical expression is suggested for the calculation of the magnitude of deposition of solid particles along the surface of a flat plate, which includes the special features of hydrodynamics of flow, the adhesive properties of the particles and surface, and the probabilistic pattern of the process of entrapment of particles by the surface.

Application of particle tracking velocimetry for studying the dispersion of particles in a turbulent gas flow

A method for the determination of the dispersion of solid particles in a turbulent gas flow has been presented. This method is based on recording the particle trajectories with a high-speed video camera on separate regions of a flow, located at various distances from a point source of particles, and the subsequent processing of the frames. This method has been used to study the dispersion of solid particles under the conditions of turbulence in a horizontal channel with a rectangular cross section of 200 × 400 mm for a measuring region length of 2 m. Turbulence of the gas flow was generated by means of a grid with square meshes of the size of 16 mm. The average velocity of the gas flow in the measuring region was 5.1 m/s. The dispersion of 36-, 56- and 128-micron glass particles of spherical shape was studied in a region 450 mm long from the point source of particles. It has been shown that the dispersion of these particles in the direction of the action of the gravity force is larger than their dispersion in the perpendicular direction to the gravity force. The results of this study have shown that an increase in the size of particles leads to a decrease in the dispersion at small flight times of the particles (short-time dispersion).

Numerical Modeling of a Two-Dimensional Vertical Turbulent Two-Phase Jet

The results of numerically modeling two-dimensional two-phase flow of the “gas-solid particles” type in a vertical turbulent jet are presented for three cases of its configuration, namely, descending, ascending, and without account of gravity. Both flow phases are modeled on the basis of the Navier-Stokes equations averaged within the framework of the Reynolds approximation and closed by an extended k-ɛ turbulence model. The averaged two-phase flow parameters (particle and gas velocities, particle concentration, turbulent kinetic energy, and its dissipation) are described using the model of mutually-penetrating continua. The model developed allows for both the direct effect of turbulence on the motion of disperse-phase particles and the inverse effect of the particles on turbulence leading to either an increase or a decrease in the turbulent kinetic energy of the gas. The model takes account for gravity, viscous drag, and the Saffman lift. The system of equations is solved using a difference method. The calculated results are in good agreement with the corresponding experimental data which confirms the effect of solid particles on the mean and turbulent characteristics of gas jets.

Grid-generated turbulence in two-phase flow: The effect of parameters of the carrier medium and particles

Results are given of an experimental investigation of the initial region of decay of grid-generated turbulence in a downward two-phase flow of the “gas-solid particles” type. Optical diagnostic methods are used to obtain the distributions of the dynamic parameters of two-phase flow, namely, averaged and fluctuation velocities of gas, and the curves of decay of turbulence along the flow axis are constructed for grids with square meshes sized 4.8, 10, and 16 mm. The investigation results demonstrate that solid particles 700 μm in size have varying effect on the degree of decay of turbulence. In the case of grids with small mesh sizes of 4.8 and 10 mm, the presence of such particles leads to additional generation of turbulence; in the case of a grid with mesh size of 16 mm, vice versa, the particles suppress the turbulence. Investigations reveal that these tendencies become still more pronounced with increasing concentration of particles. In addition, the investigation of the effect of velocity phase slip reveals that the generation of turbulence increases with the difference between phase velocities. In so doing, variation of the pattern of the effect of particles on turbulence is observed for a grid with large meshes, namely, the suppression of turbulence at low values of velocity slip and generation of turbulence with increasing slip. Based on the results of analysis of experimental data, a criterional parameter is suggested, which defines the effect of particles on the turbulence of two-phase flow, i.e., the ratio of the Reynolds number of particle to the turbulence Reynolds number for gas.

Deposition of Fine Solid Particles in Laminar Flat-Plate Boundary Layers

A method for the assessment of the deposition of fine solid particles in a vertical two-phase laminar flat-plate boundary layer is presented. The method is based on a probabilistic approach to the particles deposition and takes into account both the hydrodynamics of the flow past the plate and the adhesive properties of particles and the plate surface. Electrocorundum powders with particle sizes of 12, 23 and 32 μm were used for the investigations. A stainless steel hollow conical shape was used as a prototype surface for the particles deposition. The experimental investigation used the centrifugal technique for the deposition of particles and examined pairs of particles and surfaces for the characteristics of the deposition process. The results exhibited the typical log-normal distributions of the dependence of the deposition/adhesion process. An overall expression for the particles deposition flux was derived. The expression includes the normal to the surface velocity of the particles, the particle mass concentration observed immediately close to the surface of the plate and the surface of the plate. The hydrodynamic properties of the dispersed phase in the vicinity of the plate surface, namely the normal velocity and the particle mass concentration, were calculated by the mathematical model of the flat-plate laminar boundary layer elaborated in [1]. The validation of the proposed method of assessment of deposition was accomplished by comparing the deposition flux calculated along the plate with experimental data obtained in [2]. A small discrepancy between the numerical and experimental results was observed, which may be attributed to neglecting the microphysics of adhesion, such as the influence of the electric charges and humidity during the experiments.Copyright

Experimental Investigation of the Effect of Coarse Particles on Decaying Grid-Generated Turbulence in a Two-Phase Flow

The results of an experimental investigation of the effect of particles on decaying grid-generated turbulence in a downward vertical turbulent gas-particle flow are presented. The dispersed particles were glass spheres with a mean size of 700 μm. Titanium dioxide particles with a mean size of 2 μm were used as the particle-markers modeling the carrier-medium flow. The turbulence was generated by grids with square cells of two sizes (4.8 and 10 mm) and an impenetrability parameter equal to 0.49 at a mean flow velocity equal to 9.5 m/s. The grid Reynolds numbers were 3000 and 6300. The damping of turbulence by the particles, manifested in an increase in the turbulence decay rate (viscous dissipation) and a decrease in the turbulence energy in the power-consuming spectral band, was detected.