Variation and Modelling of Starting Torque in Industrial Agitators

Sammanfattning

This study aims to find a general solution for describing the starting torque of an industrial agitator. Different setups of agitators are tested, and the torque is measured during start-up. Two propellers are tested together with two different induction motors in air, water and pulp. Using data from the tests, Computational Fluid Dynamics-simulations and a statistical method for analysing factors, Design of Experiments, the starting characteristics are investigated. The special pattern of the measured torque curve for the agitator was found to be similar to the pattern of a measured torque curve for an induction motor. Three areas of interest were found: the initial torque peak, the late torque peak, and the continuous torque. The initial torque peak is found to be purely mechanically induced by the agitator’s moment of inertia and the rotating speed or acceleration. The continuous torque is induced by purely fluid forces acting on the propeller, as the late torque peak is assumed to be as well. Through trial-and-error, models are developed to estimate the maximum torque with propeller specific models using the propeller’s moment of inertia and rotating speed; the initial torque peak using Newton’s second law for rotation; and for some cases the late torque peak where the continuous torque is a factor. Although, three models have been developed, the goal of the project was not reached. The models need to be developed further in order to fit all agitators. There are many questions remaining about the late torque peak: why it is greater than the initial torque peak for some propellers and accelerations, and how to model it in a general matter for all propellers.