Numerical simulation of ash particle impaction in tube bundles - A case study as a basis for a CFD based ash deposit formation model in convective heat exchangers

A CFD model considering the relevant processes during ash deposition in biomass combustion plants is being developed as efficient engineering tool. Additionally, a finite volume based CFD heat exchanger model has been developed, since it is not possible to resolve all the length and time scales of turbulent flow in the heat exchanger tube bundles of the convective boiler sections. A numerical case study based on unsteady RANS with a Reynoldsstress turbulence model and Lagrangian tracking of particles was performed in order to investigate the influence of the most relevant design and operation parameters on the deposition of coarse fly ash particles in heat exchanger tube bundles of water tube boilers and to link the deposit formation model with the heat exchanger model. Typical values for inline tube bundle geometries of evaporators and superheaters (pitch and diameter), flue gas velocities and corresponding Reynolds numbers, Stokes numbers and corresponding particle diameters, flow angles as well as flue gas temperatures were varied. The results showed that the particle mass fraction impacted on the tubes not only depends on the Stokes number of the particles, but also on the Reynolds number. Moreover, the impaction rates depend on the flow angle, on the pitch as well as on the flue gas temperature. With these results, a lookup table for the impaction rates as a function of the influencing values investigated will be created for each tube row and implemented in the CFD based heat exchanger model in order to provide a link to the deposit formation model. With this comprehensive engineering tool under development, the prediction of ash deposit formation will then be possible in the whole boiler including the convective section.