Appropriate mixing system design is a balance between performance and cost. For most mixing systems, all flow in the mixing vessel is induced by the impeller which creates predominantly symmetrical circulatory loops and, on average, does not produce a net horizontal flow impinging against an agitator assembly. However, in some applications a fluid inlet is placed adjacent to an impeller which subjects the impeller to a continuous flow oriented perpendicular to the impeller axis of rotation. Such side flow adversely affects the life of an agitator assembly due to fatigue loading. In a particular commercial waste water treatment mixing application, there was a desire to place an impeller in a high side flow inlet region of a basin which would have necessitated an unreasonably large shaft diameter to prevent premature shaft fatigue failure. Using a combination of CFD flow analysis and fatigue based shaft design; the impeller was placed at an appropriate height to both minimize the fatigue affects of the horizontal inlet flow, as well as to ensure proper mixing. 3 separate CFD studies are presented — The originally requested configuration (impeller next to side flow), impeller situated as high in the vessel as possible (good fatigue life but poor mixing) and the final optimum configuration (acceptable fatigue life and acceptable mixing). Constant Bernoulli side flow forces were computed from time averaged constant flow velocities determined by the CFD studies which allowed the computation of mean and alternating force components whose frequency of application equaled the shaft rotations per minute. A Goodman fatigue analysis approach was utilized.

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