This paper represents a review of the recent researches that investigate the behavior of the gas turbulent flow laden with solid particles. The significant parameters that influence the interactions between the both phases, such as particle size, loading ratio and the gas velocity, have been extensively reviewed. Those parameters are presented in dimensionless numbers in which the applicability of studying its effect in terms of all circumstances of the gas turbulent channel flow at different condition is possible. The represented results show that the turbulence degree is proportional to the particle size. It was found that at the most flow conditions even at low mass ratio, the particle shape, density and size significantly alter the turbulence characteristics. However, the results demonstrate that the particle Reynolds number is a vital sign: the turbulence field becomes weaker if particle Reynolds number is lower than the critical limit and vies verse. The gas velocity has a strong effect on the particles settling along the channel flow and as a result, the pressure drop will be affected.
The multiphase flow associated with turbulence is involved into many industrial applications, such as pneumatic conveying [
The response of the particle motion to the carrier phase motion disturbance is expressed by different dimensionless parameters. The first dimensionless relation is defined as the ratio between the response time of the particle momentum change and the time apart from the particles collisions [
in which, this number represents the ratio of fluid inertia to fluid viscosity near the particle’s surface [
Similar behavior is observed as the mass loading ratio increases and the particles concentration along the pipe cross section will be increased [
experiences three stages [
The aim of the present work is regarding the dilute phase turbulent flow. The dominant mechanisms which govern the particles dispersion such as, gravitational settling, drag force and the lift force, will be reviewed. Then the turbulence modulation mechanisms such as kinetic energy exchange, the dissipation rate and the secondary motion induced by the particles, will be reviewed. The two-way coupling flow, in which the turbulence affects the particles motion and vice versa, many parameters such as, particle size, particle shape and mass loading ratio, will influence the particles and the carrier phase interaction. In the context of seeking, to identify the interactions between phases and the dominant parameters, some dimensionless numbers will be illustrated. In the present work, the important results will be presented and divided into two main sections. First one is concerned with the particles dispersion and the second section will describe the turbulent modulation. We believe that, the physical reasoning of the results is essential to achieve a development in the modelling and further analysis for multiphase flow applications.
The effect of the flow turbulence on the particles motion is called turbulent dispersion. The degree of mixing between the phases, the particles behavior and the cross-sectional distribution are strongly related to the particles dispersion effect [
where τp is known as the required time for the particle to accelerate from rest to 63% of the carrier flow velocity [
Whatever, during the transportation of the particles by mean of turbulence or drag force, the particles exhibit changes, such as, settlement and re-suspension, along with the flow that might be clearly predicted using cross-sectional distribution [
The particles behavior during the transportation of the glass beads with a diameter less than 110 µm through a long glass horizontal pipe with the inner diameter of 75 mm has been presented. The air velocity was 12 m/s with the solid loading of 1.6% [
Another experiment was done by three glass beads with the diameter of 0 - 50 µm, 0 - 110 µm and 180 - 300 µm at the section of x = 300 mm along the pipe flow [
In a simulation process of horizontal flow in a pipe with the inner diameter of 30.5 mm [
The drag force has been studied experimentally with pulverized coal particles with the diameter of 125 - 150 µm [
The flow along a horizontal rectangular cross section channel at Reynolds number of 6826 corresponding to the gas velocity of 6.603 m/s has been examined experimentally [
collisions, which are lead to the radial motion of the particles, alter the axial particles fluctuations. For the larger particles, the variation is disordered, as the mass loading increases the fluctuations decrease and then increase at the highest mass loading. However, for small particles, the fluctuations are almost unchanged as the mass loading is varied.
The radial particles fluctuations level directly proportional to the solid mass loading and the particles size [
The particles shape effect is examined at lower mass loading ratios using the horizontal jet that is laden with rod-shaped nylon fibers with dimensions that are the length of 320 µm and diameter of 24 µm [
phase velocity. The particles exhibit a higher velocity than the carrier phase, this behavior is confined to the shear layer out of the jet core region and near to the jet boundaries where the interaction between the jet and ambient has occurred.
A parameter that is called the Particle sphericity (ф) is incorporated in a simulation study [
Another experimental study was performed. Non-spherical quartz particles of 150 µm mean diameter are injected at high Reynolds number [
The volume fraction is defined as the ratio between the space volume occupied by the particles and the volume occupied by particles and carrier phase. The particles interactions are dominantly affected by the volume fraction. In which for spherical particles the interaction has been categorized according to the volume fractions [
The degree of the coupling that is regarded in the modeling process has its influence on the flow configuration and velocity of the particles. A numerical simulation based on both the two-way and four-way coupling is performed [
The gravitational settling influences the particles to cross-sectional concentration. The mean axial velocity of both phases will be influenced by the gravitational settling. In the pre-mentioned investigation [
The particles size effect on the settling is shown in
Another result was observed, as the mass loading ratio increases, the particles concentration near to the bottom surface of the pipe decreases. This effect is a consequence of the enhancement of the inter-particles collisions rate.
Experimental work was performed to illustrate the air pressure drop along the flow that is generated by the settling effect [
starts to increase again as a result of the particles settling in the vicinity of the lower surface of the pipe. The particles settlement is a consequence of that, as the air velocity is reduced its ability to suspend the particles is consequently reduced.
The results of another study show the pressure drop along a horizontal flow of 4 mm particles with air velocity ranges from 15 to 30 m/s and loading ratio ranges from 0.5 to 2.5 [
The presence of the discrete phase alters the carrier phase turbulence; this effect is called the turbulent modulation. Such as that the mean velocity of the carrier phase is affected by the particles hence the turbulence production rate and the mean strain rate will be changed [
The classification of the interaction between the phases is examined [
and the dissipation rate are reduced relative to the corresponding in the single phase flow. For zone B that is characterized by higher Rep associated with the further increase of τp by increasing the particles size.
At Rep ≥ 400, vortex shading appears. These wakes which are generated behind the particles begin to grow at high particle Reynolds number [
The appearance of the vortex that is induced by the particles observed at Rep ≈ 270. Another investigation shows that for spherical particles the vortex shading appears at Rep > 400 [
A characteristic length ratio has been used to express the turbulent modulation [
As mentioned previously, the larger particles are subjected to the higher inertial effect [
For turbulent jet flow laden by particles of different sizes [
The carrier phase exhibits a higher mean axial velocity when it is laden by particles [
At the jet outlet, the following were observed; the mean velocity is decreasing along the streamwise while the turbulence is developing [
Gas jet turbulent flow laden with solid was studied using direct numerical simulation with high Reynolds number [
The turbulent modification is strongly influenced by the particles concentration within the cross section [
The particles size effect has been examined by using the horizontal flow of plastic particles through a 30 mm pipe [
At Rep = 25 the effect of particles size is investigated by the downward jet of air that laden with particles of different sizes [
The axial variation of the turbulence intensity at the jet centreline has been plotted in
decreased. For the large particles, the turbulence intensity is enhanced. This is observed clearly as x/D varies from 5 to 10.
The secondary motion induced by the unbalanced forces which are induced by the particles that are suspended in the core of dilute horizontal flow. The strength of secondary motion, which in form of circulation cells, is proportional to the pipe wall roughness [
As the particle mass increases, with the large value of St, the particles are lagging to respond to the carrier phase fluctuations. As a result, there is a relative motion is generated between the carrier phase and the particles that are dominantly generated by the fluctuations of the carrier phase [
An experimental study illustrates the effect of the particles shape on the turbulence modulation [
Another experimental study was performed and found that for the spherical particles the turbulence intensity is enhanced at the core region of the pipe and while near the pipe wall the turbulence intensity is attenuated [
The mass loading is defined as the ratio between the dispersed phase mass flow rates to the corresponding of the carrier phase [
The turbulence is influenced by the particles even at the low mass ratio of 0.05% [
The effect of small mass loading ratio has been investigated [
A result that was observed during the simulation process of the secondary motion pattern is affected by the mass ratio [
near to the upper half of the pipe due to the high local momentum which is transferred to the gas. However, the particles concentration near to the bottom surface of the pipe is inversely proportional to the mass loading ratio. As a result, more particles will be attended in the core of the flow. This will enhance the collision at the upper half of the pipe and high local momentum is transferred to the gas.
When the volume fraction is located between 10E−6 and 10E−3, the consequence is the two-way coupling [
The aim of the paper is to review of the recent researches that investigate the behavior of the gas turbulent flow laden with solid particles. The significant parameters that influence the interactions between the both phases, such as particle size, loading ratio and the gas velocity, have been extensively reviewed. Those parameters are presented in dimensionless numbers in which the applicability of studying its effect in terms of all circumstances of the gas turbulent channel flow at different condition is possible. The investigations of multiphase flow using the CFD means to show a good validation to the experimental results, thus promise a further development for the analysis of the interactions between phases.
The presented results show that the lag between the carrier phase and the particles is maximum in the core region of the flow and decreases near to the wall. The particles size is an important parameter. Increasing the particles size raises the difference between the particles and the air velocity. The large particles are able to retain their initial velocity through the flow. Furthermore, the maximum velocity of large particles which are located at the pipe center is higher than the local air velocity. The small particles tend to follow the carrier phase motion under the turbulence effect. And the beak of the mean axial velocity of both phases is displaced little above the pipe center. The particles that occupy the upper half of the pipe are faster than those occupy the lower half at a given superficial gas velocity and solid feeding rate. The velocity difference between the upper half and lower half particles, decreases with increasing the gas velocity. For constant gas velocity, as the solid mass loading increases, the velocity differences increase. As the carrier phase would not be able to re-suspend most of the particles that are settled by the gravity action. The pressure drop along the flow is proportional to the carrier phase velocity and the solid mass loading. The higher mass loading leads to that the particles concentration near to the bottom surface of the pipe decreases. The non-spherical particles are exposed to higher drag force and their velocity resembles the carrier phase velocity.
The particle Reynolds number might be used to express the carrier phase inertia effect around the particles. The turbulence field of the carrier phase is reduced if the particle Reynolds number is low and vice versa. The large particles enhance the carrier phase turbulence where the small particles reduce the turbulence. The strength of secondary flow in form of circulation cells is proportional to particles size. In the case of the large particles, four recirculation cells are formed. Two recirculation cells are formed for the small particles. The mean axial velocity of the carrier phase is decreased as the mass loading increases. Carrier phase radial fluctuation velocity is enhanced with the increasing of the mass loading as well as the secondary flow intensity.
The authors gratefully acknowledge the financial support officers from the office of the Tanta University Research Fund. This work was supported by the Tanta University Research Fund under the research grant (code: cod-tu; 03-15-02).
Kabeel, A.E., Elkelawy, M., Bastawissi, H.A.-E. and Elbanna, A.M. (2016) Solid Particles Injection in Gas Turbulent Channel Flow. Energy and Power Engineering, 8, 367-388. http://dx.doi.org/10.4236/epe.2016.812032