LAMINAR FLOW

FLOW- the quantity of fluid i.e. gas or liquid passing a point in unit time.

F= Q/T

Where-
F- Mean flow
Q- Quantity (mass or volume)
T- Time.

LAMINAR FLOW

In Laminar flow-fluid moves in a smooth manner and there are no eddies or turbulence.

The flow is greatest at the centre, being about twice the mean flow.

As the side of the tube is approached the flow becomes slower, until it becomes zero at the wall.

In order to drive fluid thru the tube a pressure difference, must be present across the ends.

Laminar flow has got a CONICAL VELOCITY PROFILE.


HAGEN-POISEUILLE equation.

Laminar flow is not the only type of flow occurring in anaesthesia. App., breathing systems, airways and circulation.

Laminar flow may change to TURBULENT FLOW if a constriction is reached which results in the fluid velocity increasing.

Fluid no longer exists in smooth manner, but, swirls in eddies and the resistance is higher than for the same laminar flow.

Turbulent flow has a FLAT VELOCITY PROFILE.

Although bronchi and smaller air passages are smaller than the trachea, air flow thru them is slower so laminar flow is usual in the LOWER RESP. TRACT.

Laminar flow-VISCOSITY.

Turbulent flow-DENSITY.


ROTAMETER

Anaesthetists require a continuous of gas flows in the anaesthetic machine and this is provided by a VARIABLE ORIFICE FLOWMETER.
This is often referred to by the trade name ROTAMETER.

A BOBBIN is supp. In the middle of a glass or plastic tube which is tapered.

As the gas flow increases, the bobbin rises in the tube and the clearance round the bobbin increases.

In other words, there is a variable orifice around the bobbin which depends on the gas flow.

The pressure across the bobbin remains CONSTANT because it gives rise to a force which balances the force of gravity on the bobbin.

The increase in area of the annular orifice as the bobbin rises reduces the flow resistance at higher flows and so the pressure across the bobbin stays constant, despite the flow increase.


VENTURI PRINCIPLE

The entrainment of a fluid thru the side arm into an area of low pressure caused by a constriction in a tube is the basis of the venturi principle.
It is called the BERNOULLI EFFECT.

Some equipment may include a tube with a constriction in which the cross section gradually decreases and then increases –this is known as VENTURI.

This principle is employed in gas mixing devices, including fixed performance oxygen devices, suction equipment and scavenging equipment.