
Re: Liquid mixing equation
12 January 2007 at 1:29pmThe only person's name I have ever heard associated with the time required to blend liquids is Betty Crocker. I have heard, and even referred myself, to the dimensionless blend time as the "Betty Crocker Number."
Dimensionless blend time is essentially the product of actual blend time and the rotational speed of the mixer, along with some empirical geometry factors, as shown in the attached equation for a pitchedblade turbine.
In the turbulent range (low viscosity, waterlike liquids), dimensionless blend time is a constant. For any given geometry or geometrically similar system, the blend time is inversely proportional to the rotational speed. In other words, the time required to achieve a given blend uniformity depends on a certain number of mixer revolutions.
To calculate the blend time, the dimensionless equation is rearranged to calculate time (as shown below).The time is in minutes or seconds depending on whether the rotational speed of the mixer is in revolutions per minute or revolutions per second, respectively. For a typical pitchedblade turbine the dimensionless blend time for 99% uniformity is:
6.34 = Blend time x Rotational Speed x (Impeller Diameter/Tank Diameter)^2.3 x (Liquid Level/Tank Diameter)^0.5
as shown in the attached dimensionless equation. For a straightblade, radial flow turbine, the dimensionless value is 4.8. Although by the blend time value the straightblade turbine takes less time, it takes more power and/or torque than the pitchedblade turbine for the same result. The dimensionless blend time value for a hydrofoil or propeller is 16.4 but the exponent on the impeller to tank diameter ratio is 1.7 instead of 2.3. Simple factors associated with an exponential decay function can be used to estimate times for 95% and 99.9% blend uniformity.