Due to porosity changes and to the nonslip condition at the wall, the non uniform po rosity profiles in packed beds exhibit steep maxima close to the wall. The govern ing equations of energy and mass conserva tion were solved for fixed bed chemical reactors including these profiles. Under the assumption of non uniform flow the problems become two-dimensional also under adiabatic conditions. In all cases the agreement between available experimental data and theoretical predictions based on realistic flow conditions is improved. In particular measured and calculated moving speeds of migrating reaction zones fit to gether very well for adiabatic fixed bed reactors. Also considerable improvements concerning multiple steady states, temper ature profiles and conversion rates were obtained in situations when the reactor was wall cooled. In fixed bed chemical reactor analysis it is commonto assume uniform flow distribution within the bed. The reality however is different. Due to a change of the average porosity near the wall [ 1,2,3],(Figure 1.) -ε=1 at the wall -the flow velocity increases until close to the wall and is reduced again because of the non slip condition (Figure 2.) The artificial flow pro file is described by the Brinkman equation I =-150^ -L75 ρ »2 ψΙ.Α. Α. ( η iiL,àz a e 3 dp 2 9 £ 3 dp r dr 9 dr B.C. Ζ=0 : ρ=ρ 0 ;Γ=0: s 0 ; Γ « R : U = 0;
Flow through heterogeneous landfills that include macropores may occur under Reynolds numbers higher than those where Darcy’s law is valid. Extensions, such as a Forchheimer approach, may be required to include inertial effects. Our aim is developing predictive models for such landfills that are built from the low-level radioactive waste and debris of dismantled nuclear power plants. It consists of different materials, which after crushing result in a spatially heterogeneous distribution of porous-media properties in the landfills. Rain events or leakage, for example, may wash out radionuclides and transport them with the water flow. We investigate here the water flow and consider an inclusion of macropores. To deal with possibly high velocities, we choose the Forchheimer model and, taking different Forchheimer coefficients into account, compare it to the Darcy model. The focal points of the study are (i) the influence of the macropores on the flow field and (ii) the impact of the choice of the Forchheimer coefficient both on the solution and the computational effort. The results show that dependent on their size, macropores can dominate the flow field. Furthermore, Forchheimer coefficients introducing more inertial effects are associated with considerably higher runtimes.
Abstract. When nuclear power plants are dismantled, only a small portion is heavily contaminated with radioactivity and must be stored in a repository. The remaining material, mainly concrete rubble (construction waste), is decontaminated if necessary and can be stored in conventional surface landfills after clearance. The focus of this work is on the modelling of such landfills and the radioactive substances during raining events. The influence of the heterogeneous nature of the construction rubble should also be investigated. The simulation environment DuMux, mainly developed by our institute, is used to compare different modelling approaches. It follows a previous work by Merk (2012). The research work is supported and accompanied by the Federal Office for Radiation Protection (BfS). Parts of the research initiatives of the BfS in the area of clearance of materials with negligible radioactivity are also presented.
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