Stirring experiments were conducted in a pilot scale tank with side-entering axial impellers to study the mixing behavior of biogas fermenters. Propeller flows were analyzed using the PIV technique in dependence of propeller geometry in a viscoelastic fluid, modeling the rheology of organic substrates. Moreover, mixing times via decolorization and thrusts using strain gauges were measured, varying propeller geometry, rotational frequency and viscoelastic rheology. Three general propeller flow types caused by the interplay of elastic and inertial forces could be found and categorized with a newly defined axial elasticity number. In addition, a new correlation between axial flow rate and thrust could be derived, using a single proportionality constant for all analyzed propellers. With this relationship, mixing times could also be summarized and correlated. Thus, by the use of a singular proportionality constant and thrust, mixing times could be predicted with good accuracy independently of propeller geometry and mostly rheology below a critical elasticity number. With this correlation, mixing cycles in biogas fermenters can be optimized regardless of the propeller geometries and substrates in use with the potential to save energy from unnecessary stirring.

搅拌实验在带有侧入式轴向叶轮的中试罐中进行，以研究沼气发酵罐的混合行为。使用 PIV 技术根据粘弹性流体中的螺旋桨几何形状分析螺旋桨流动，模拟有机基质的流变学。此外，通过脱色和使用应变仪的推力测量混合时间，改变螺旋桨几何形状、旋转频率和粘弹性流变学。可以找到由弹性力和惯性力相互作用引起的三种一般螺旋桨流类型，并使用新定义的轴向弹性数进行分类。此外，对所有分析的螺旋桨使用一个单一的比例常数，可以推导出轴流率和推力之间的新关系。有了这种关系，混合时间也可以汇总和关联。因此，通过使用单一的比例常数和推力，可以准确地预测混合时间，而不受螺旋桨几何形状的影响，并且主要是低于临界弹性数的流变学。有了这种相关性，沼气发酵罐中的混合循环可以得到优化，而不管使用的螺旋桨几何形状和基质如何，都有可能从不必要的搅拌中节省能源。