CFD builds a more efficient pulp chopper

Simulating a spinning 82-in.-diameter pulper rotor in CFD software let engineers finetune designs for a 25% energy savings without loosing performance. "Many pulping rotors used in the paper industry were developed decades ago and are relatively inefficient," says John Egan, chief technologist at Mason, Ohio. Pulper rotors churn slurries of recycled paper or cardboard into fiber for recycling. "Building and testing physical prototypes would be tedious and expensive. So we used computational fluid dynamics to evaluate and predict energy consumption. It worked, and with a high degree of accuracy," he says. Comparisons to physical experiments show the software's power prediction to be within 3% of the actual value. "We first analyzed existing designs to understand their fluid-flow characteristics, then modeled and evaluated alternate designs on the computer, and made changes until we hit at least a 20% energy savings," says Egan. "Because the churning flow in pulpers is difficult to visualize, engineers do not fully understand how the mechanical components contribute to defibering a slurry. And older designs perform reliably, so there was little demand to improve them," he adds. Egan first developed a simplified rotor-performance model to screen rotor configurations for analysis. It captured the effects of rotor geometry such as diameter and vane-sweep angle. Although simplified assumptions limit prediction accuracy, analyses can still gage relative capabilities of new designs. "Several promising new ' firstcut' designs were developed this way," says Egan. Meshed files were imported into the Fidap CFD from Engineers applied rotation velocities to all rotor surfaces while tub-wall velocities were set to zero. Top-surface normal velocities were also set to zero while tangential velocities were set to frictionless flow conditions. The top fluid surface was modeled as quasi-free to shorten run times. Using water as the model fluid provided performance comparisons within reasonable periods. The so-called mixing-length