Role of screen plate design in the performance of a rotor impact mill in fine grinding of biomass

Abstract The role of rotor impact mill screen plate design in biomass grinding has attracted limited interest. This study aimed to clarify the effect of operational parameters and various screen designs on the fine grinding of Sphagnum moss. Contoured screens having forward (rasp) and backward (inverse rasp) inclined trapezoidal apertures of nominal sizes 0.2, 0.3, 0.4, and 0.5 mm were studied. A smooth screen plate having circular perforations (0.5 mm) was used as a reference. Flow phenomena were modeled with Computational Fluid Dynamics (CFD) using 2D geometry for the screen apertures. Product particle size and capacity were mostly dependent on the equivalent diameter of the screen apertures together with rotor frequency and also on the screen design, while the aspect ratio was solely a function of particle size. Among the screens having nominal sizes of 0.3, 0.4, and 0.5 mm, the smooth screen produced the smallest particle size; followed by the rasp screens and the inverse rasp screens. The smooth screen had higher capacity than the rasp or inverse rasp screens, and a narrower operating range. In the case of 0.2 mm apertures, higher capacity was achieved with the inverse rasp than with the rasp screen. The net energy consumption (SEC) of grinding with the rasp screens was at a lower level than with the inverse rasp screens (or the smooth screen). No difference was seen in the case of the smallest size apertures, however. The results, supported by CFD modeling, indicate that the incidence angle of particles does not explain the passage of particles through screen apertures. The passage is affected by the flow patterns above and within the apertures, and the pressure difference over the screen plate. The eddies within the apertures reduce their effective open area, resulting in product particles much smaller than the aperture size. When the aperture inclination is toward the tangential flow (rasp), an eddy is also generated in front of the aperture that turns the flow perpendicular to the screen surface within the aperture. In contrast, when the aperture inclination is against the tangential flow (inverse rasp), an eddy is generated above the aperture that guides the flow smoothly into the aperture, although the flow has to make a U-turn first. A design of inverse rasps seems to be beneficial when very small apertures are used, as it appears that the eddies formed above the apertures prevent blocking by dislodging accumulated particles. The low SEC with rasp screens does not seem to make the passage of particles through apertures easier, but it does enable more efficient size reduction because of the ability of rasps to serve as a grinding track

A novel method for automated SCR catalyst uniformity measurement

Abstract Exhaust after treatment systems are becoming more complicated with the ever-stricter emission regulations. To meet the regulation limits, a tightly packed high-performance system is required. In selective catalytic reduction (SCR) systems, ammonia (NH³) uniformity is a key performance indicator. In this work, a low flow intrusion automated device was designed to measure NOₜ reduction from the SCR catalyst outlet. A topological surface was reconstructed from the measurement data to obtain the shape of the distribution. Before the actual measurements, the method was theoretically tested with distribution extracted from computational fluid dynamics (CFD) simulations. The results show that in stable engine load conditions, uncertainty of the measurement was low, approximately 1.1% from the measured value. In the case of high uniformity, a satisfactory result can be obtained with 10 to 20 measurement positions, whereas in the case of low uniformity, the number of positions should be increased. The distribution shape between the measurements was consistent throughout the measurements

Implementation, demonstration and validation of a user-defined wall function for direct precipitation fouling in ANSYS fluent

Abstract In a previous paper (Johnsen et al., 2015) and presentation (Johnsen et al., 2016), we developed and demonstrated a generic modelling framework for the modelling of direct precipitation fouling from multi-component fluid mixtures that become super-saturated at the wall. The modelling concept involves the 1-dimensional transport of the fluid species through the turbulent boundary layer close to the wall. The governing equations include the Reynolds-averaged (RANS) advection-diffusion equations for each fluid species, and the axial momentum and energy equations for the fluid mixture. The driving force for the diffusive transport is the local gradient in the species’ chemical potential. Adsorption mechanisms are not modelled per se, but the time-scale of adsorption is reflected in the choice of Dirichlet boundary conditions for the depositing species, at the fluid-solid interface. In this paper, the modelling framework is implemented as a user-defined function (UDF) for the CFD software ANSYS Fluent, to act as a wall boundary condition for mass-transfer to the wall. The subgrid, 1-dimensional formulation of the model reduces the computational cost associated with resolving the fine length-scales at which the boundary-layer mass transfer is determined, and allows for efficient modelling of industry-scale heat exchangers suffering from fouling. The current paper describes the modelling framework, and demonstrates and validates its applicability in a simplified 2D heat exchanger geometry (experimental and detailed CFD modelling data by Pääkkönen et al. (2012, 2016)). By tuning the diffusivity, only, good agreement with the experimental data and the detailed CFD model was obtained, in terms of area-averaged deposition rates