Fluid or Air Entrainment

Bernoulli's Priniciple or Effect has practical applications in the field of respiratory care in that it allows us to deliver precise oxygen concentration levels with simple devices by mixing oxygen with entrained room air. It also is applied in the process of delivering aerosol treatments with medications. When entrainining air the effect is applied through the use of air injectors and venturi devices.

Air injector

Drop in pressure caused by increased velocity through restriction (jet) pulls in (entrains) additional air through open port.

 

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Influenced by diameter of jet orifice (restriction), and size of entrainment port

 

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B) Fixed jet size – changing size of opening of entrainment port influences amount of air entrained and resulting total flow. Gas velocity and pressure drop remains constant so increasing opening of port allows more air to be entrained, decreasing size of port decreases the amount of air that can be entrained.

C) Fixed entrainment port – changing size of jet diameter - the smaller the opening of the jet the faster the gas velocity, causing a greater drop in pressure, and thus more entrained volume and total flow. The larger the opening of the jet, the slower the gas velocity, less drop in pressure, less air entrained.

 

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Most common device for air or fluid entrainment in respiratory care equipement

Venturi Principle

modification of the Bernoulli effect (Giovanni Venturi)

The pressure that has dropped as the fluid flows through a constriction in the tube can be restored to the preconstriction pressure if a gradual dilation occurs in the tube (an angle of divergence that is less than or equal to 15 degrees) distal to the constriction.

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Venturi device includes a dilation of the gas passage just distal to obstruction, and if the angulation of the funnel is not over 15o, the gas pressure will be restored nearly to its pre-restriction level.

Advantages of a venturi over an air injector:

greater entrainment and therefore higher total flow output

Stable oxygen % even with variations in total flow (as opposed to an injector where velocity would decrease with flow, and cause a decrease in entrainment of air)

Major limitation: Back-pressure (obstruction downstream from Venturi) reduces entrainment - resulting in a higher oxygen concentration than expected.

Used with early ventilators for adjusting oxygen concentrations (FIO2), or decreasing pressures within circuits to facilitate exhalation.