Experimental Investigation On Turbulence Modification In A Horizontal Channel At Relatively Low Mass Loadings

Numerous investigations contributed to turbulence modification of particle-laden flow in the past decades. Although some consensuses have been reached, many of the explanations remain amphibolous and even contradict with each other, resulting from the complexity of particle-fluid interactions. To clearly discover the underlying physics of particle-fluid interactions, a series of experiments were performed in a horizontal channel at relatively low particle loadings with laser measurement technique PIV.In order to achieve the near-wall boundary layer measurement of channel flow and two-phase PIV measurement, a super-resolution PIV (SPIV) method was introduced in first part of this paper. This SPIV method combined the advantages of both conventional PIV and PTV methods and a new technique was developed to pick up a particle mask from the sampled images in the particle identification process. The performance was testified with actual sampled flow field images.In the second part, turbulence modification was studied at a Reynolds number 6826 in particle-laden flow. Two sizes (60μm and 110μm) of particles were used in the experiments and the mass loadings ratio was ranging from 5×10-4 to 4×10-2. The results show that particles attenuate the mean flow and enhance the three components of Reynolds stresses to some extent. Particles lag the fluid in the center region but lead the fluids in the near-wall region. The streamwise particle velocity fluctuations are larger than the gas fluctuations for both sizes of particles; the wall-normal fluctuations are smaller in the 60μm particle case but larger in the 110μm particles than those of the gas phase. Particles are dominated by gravity and particle-wall collisions.The observed turbulence modification is somewhat unexpected according to the former documented results, because direct momentum and energy exchange is neglectable at such low mass loadings. To discover the underlying mechanisms, the dissipation rate, turbulent kinetic energy and characteristic length scales of flow are investigated. The results indicate that the presence of particles can accelerate the large flow structures cracking into small ones, accompanied with a more rapid energy transfer; to maintain the turbulence, large structures obtained more kinetic energy from the mean flow, resulting in the attenuation of mean flow. Finally, it can be concluded that the distortion of flow structures is the main mechanism of turbulence modification. Comparison was made between the present result with that in a vertical channel and it is found that the relationship of directions of gravity and the main stream affects the way particle altering flow structures, and the turbulence modification further.