Question

Provide comparison for liquid mixing expriments using different impellers and baffles and discuss how this affect the liquid and the power curves and power requirements.

Answer 1

comparison between impeller and baffles:

1. In centrifugal blower experiment highfield baffles and impellers are sheet metal arrangements which are used to move air. The task of Impellers in centrifugal blowers are to create vacuum and baffles direct air from intermediate blower sections to impellers.

2. Tank baffles: these are the devices inserted to the inside of a vessel near the wall of the vessel. Their main advantage is that it reduces the tangential component of velocity of the total velocity generated on the fluid by a mixing impeller. This results in the top to bottom fluid flow within the vessel, which is necessary for most mixing applications. Without baffles, the time to blend materials together increases substantially and the ability to suspend solids off the bottom of the vessel is significantly reduced.

3. Axial impellers: These impellers are used for mixing applications in which effective top to bottom motion in the tank is require. This top to bottom flow is highly effective when placed over the center of a baffled tank. Example of axial flow impellers are: marine impellers, pitched blade impellers, and hydrofoils. Hydrofoil impellers are also known for its high efficiency. They are a popular choice for applications that require a range from general blending to storage tanks.

Diagram of axial flow impeller:

4. Radial impellers: These are designed in 4-6 blades. In this impellers, the fluid moves perpendicularly direction to the impeller. They generate a radial flow pattern which rotate the contents of the mixing tank to the sides of the vessel. To enhance agitation effeciency and minimizing the vortexing and swirling motions in the tank setting up baffles is important. Radial impellers are a good mixing device for low-level applications inside longer tanks based upon the production of higher shear due to the angle of attack.

Diagram of radial flow impeller:

Power Number is a dimensionless parameter used for estimating the power consumed by the agitating impeller.

For a standard cylindrical vessel geometry, the power consumed by the impeller(s) at a specified rotational speed, for a liquid with known density, can be determined from the Power Number correlation.

Power = Power Number x Density x Agitator speed³ x Impeller Diameter⁵

P = N_p ?n³D_a⁵

Where
P = Impeller power, Watts
N_p = Power Number
? = Density of liquid, kg/m3
n³ – Agitator speed measured in revolutions / second
D_a⁵ – Impeller diameters in meters

Power number data for different type of impellers/turbines, under a given set of conditions are documented in various technical literature. Typical values of power number (Np) are:

• Axial flow impellers, such as marine, helical : 0,25 to 0,75
• Mixed flow impellers, such as kidney, pitch: 0,75 to 2,0

Power Number decreases with an increase in Reynolds number.

Correlation between power number and Reynolds number for Rushton turbine, paddle and marine propeller without sparging:

Correlation between power number and Reynolds number for anchor and helical-ribbon impeller without sparging:

Laminar region: The laminar regime corresponds to reynold number (Re)i < 10 for many impellers; for stirrers with very small wall-clearance such as the anchor and helical-ribbon mixer, laminar flow persists until (Re)i = 100 or greater. In this regime the relation is:
Np ? 1/(Re)i
Power requirement:
P = k₁?N_i²D_i³

Turbulent regime: For this regime power number is independent of Reynolds number. Therefore:
P = Np’?N_i³D_i⁵

Transition regime: Between laminar and turbulent flow lies the transition regime. Both density and viscosity affect power requirements in this regime.

According to the given relation different power curves can be drawn:

Typical arrangement for mixing tank:

Baffle arrangements:

Impeller types:

Viscosity ranges for different impellers: