Dave Dickey Forum Moderator 290 Posts
Re: Is there a formula to calculate axial thrust?22 September 2010 at 1:29pm
The axial thrust on a tank of any geometry is in response (and directly opposite) to the axial thrust generated by the impeller on the submersible mixer. In general, the local thrust on the walls of the tank will be small. The primary effect of the axial thrust from the mixer will be on the mixer support.
Axial thrust is a characteristic of the impeller design and a function of the impeller geometry and operating conditions. All geometrically similar impellers will have a characteristic thrust number (similar to a power number), which will be a constant for turbulent conditions. Turbulent conditions are typically represented by an impeller Reynolds number greater than 20,000. Although the thrust number will be nearly constant (less that 2% deviation) down to perhaps a Reynolds number of 2,000. Your solids concentration will only enter the thrust number, with respect density and viscosity in the Reynolds number.
The thrust number is the axial thrust force generated by the impeller divided by fluid density, rotational speed squared and impeller diameter to the fourth power. In consistent units, the thrust number is dimensionless and the value is independent of the units used to calculate it. The value of the thrust number must be developed experimentally or from the impeller geometry, consistent with the type of impeller used on the mixer. That thrust number can be used to calculate an axial thrust, by evaluation with specific conditions for fluid density, impeller rotational speed and impeller diameter. The Reynolds number represented by impeller diameter squared, times rotational speed, times fluid density, divided by viscosity. Also a dimensionless group.
The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.
When we recommend submersible mixers for a circular or rectangular tank containing effluent or sewage, there is a software solution that gives the axial thrust required for the particular tank when we feed in the dimensions of the tank and concentration of solids present. Is there any simple formula to calculate the Axial thrust for a particular tank?
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