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Turbulence Modulation by Non-Spherical Particles

Aalborg Universitet
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This study deals with the interaction between turbulence and non-spherical particles and represents an extension of the modeling framework for particleladen flows. The effect of turbulence on particles is commonly referred to as turbulent dispersion while the effect of particles on the carrier phase turbulence is known as turbulence modulation. Whereas the former is well understood, no commonly accepted explanation has been presented for the latter. Moreover, considerations regarding the influence of shape on the experienced turbulence modulation must be considered as terra incognita. This study encompass an outlook on existing work, an experimental study, development of a numerical model and a case study advancing the modeling techniques for pulverized coal combustion to deal with larger non-spherical biomass particles.<br/>Firstly, existing knowledge concerning the motion of non-spherical particles and turbulence modulation are outlined. A complete description of the motion of non-spherical particles is still lacking. However, evidence suggests that the equation of motion for a sphere only represent an asymptotical value for a more general, but yet unformulated, description of the motion of non-spherical particles.<br/>Secondly, an extensive parametric study concerning the measurement of turbulence intensity in a particle-laden jet compared to that of a clear jet has been undertaken. The effect of three different sizes of spherical particles as well as two distinct non-spherical shapes is measured at different concentrations using laser-optical techniques. Emphasis is put towards developing a method to evaluate the additional influence of shape. Results suggest that non-spherical particles follow the same tendency as that observed for spheres, only with seemingly greater effect for comparable parameters. This is believed to be due to the increase in drag coefficient for increasing aspects ratios.<br/>Thirdly, a numerical model has been theoretically derived from the governing conservation equations for fluid flow, the Navier-Stokes equations, considering the additional influence resulting from the interaction with particles. Validation, using existing measurements as well as those obtained for the particle-laden jet, demonstrate that the new model is able to predict the experimentally observed tendencies and thus represent an improvement compared to existing models. The additional effect of shape is modeled through the modification of the drag coefficient.<br/><br/>Finally, the acquired knowledge is synthesized into the development of techniques to predict the combustion of large non-spherical straw particles in suspension fired power plants which are to replace coal in tomorrow's society. Although the straw particles are significantly larger than coal particles, the larger drag coefficient associated with straw particles ultimately leads to an attenuation of the carrier phase turbulence. Compared to other modeling choices the inclusion of a model for turbulence modulation is found to be influential for the correct prediction of combustion efficiency.

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