Characteristics of the Solid Volume Fraction Fluctuations in a CFB

In the paper, the fluctuation characteristics of the solids volume fraction in a CFB are evaluated from measurements and Eulerian-Eulerian CFD simulations. In both cases, similarly large fluctuations are observed in the intermediate voidage range whereas dense and dilute suspension regions are more uniform, as expected. The frequency distributions of solids volume fraction are classified to represent three different suspension density regimes: dilute, dense and intermediate “bimodal” regimes

TIME-AVERAGED MODELLING OF CFBS – ANALYSIS OF THE TERMS IN THE MOMENTUM EQUATIONS

The features of the terms in the time-averaged momentum equation for the solid phase are examined for three CFB units ranging from a laboratory-scale unit to a small CFB boiler. The CFD simulations were based on the Eulerian granular model of the Fluent software. The time-averaged terms calculated during the time dependent simulations are analysed and conclusions on their relative importance are drawn. An analysis of the terms in the vertical solids momentum equation in the central region of a CFB is presented

Analysis of fluctuations in velocities, voidage and gas concentration in cfb conditions

The purpose of the present study is to evaluate the possibilities to estimate the fluctuation characteristics of gas concentrations in a CFB from the characteristics of velocity and voidage fluctuations. In CFB combustion, the local concentrations of reactant gases strongly fluctuate which affects the reaction rates. In CFD modeling, the effect of turbulent mixing on reaction rates is commonly taken into account by means of mixing descriptions based on fluctuation time scales. For fast reactions the mixing rate can be the rate limiting step. Similar to gas phase reactions, for heterogeneous combustion of solid fuel, fluctuations in gas concentrations should also be taken into account. One estimate for gas species mixing is obtained by analyzing velocity fluctuations on the basis of which mixing time scales can be determined (1). This approach does not account for the actual correlation between velocity and concentration fluctuations. In a combustion chamber, air enters from riser bottom while combustible gases are released in dense suspension from fresh fuel. There are regions of high oxygen concentration and regions with high concentration of volatiles. The paper shows by means of a simplified CFD simulation how the characteristics of the concentration fluctuations are related to the fluctuations in velocities and voidage. In the simulation, air enters from riser bottom and CO that reacts with oxygen is added to dense suspension. The time-averaged fluctuation characteristics show a similarity to velocity fluctuations and that the effect of the uneven distribution of gas components should be accounted for in the modeling. REFERENCES 1. J. Peltola, S. Kallio, Estimation of turbulent diffusion coefficients in a CFB on basis of transient CFD simulations, Proceedings (Vol. 2) of 21st International Conference on Fluidized Bed Combustion, 2012, pp. 595 – 60

Työterveys- ja työturvallisuustoiminta pelastuslaitoksissa

Tämän tutkimuksen tavoitteena oli selvittää pelastuslaitosten työterveys- ja työturvallisuustoiminnan nykytila sekä kehittämistarpeita. Tutkimuksessa on kartoitettu pelastuslaitosten riskinarvioinnin, työterveyshuollon ja varhaisen välittämisen tilaa. Lisäksi tutkimuksen tavoitteena oli edistää pelastushenkilöstön terveyttä ja toimintakykyä ylläpitävien toimintamallien jalkautumista pelastuslaitoksiin

Comparison of Simulated and Measured Flow Patterns: Solid and Gas Mixing in a 2D Turbulent Fluidized Bed

Results from experiments done in a 2D turbulent fluidized bed cold model are presented. Experiments were conducted to study flow patterns, particle mixing in the bed and spreading of gas blown from one side into the bed. 2D simulations of the experiments were carried out using the Eulerian multiphase models of the Fluent and MFIX CFD softwares