Dr. Shraddha Naik

Flow is defined as the quantity of a fluid i.e. a gas or liquid passing in unit time F = Q / t F=flowQ= quantity of liquid

T=time

Flow is of three types:Laminar flowTransitional flow Turbulent flow

Laminar Flow A fluid flows in a steady manner No eddies or turbulence Present in smooth tubes Velocity is lowFlow is greatest at centre ( 2x mean flow)To draw the fluid , a pressure difference must

be present across the ends of tube.

Laminar flowLaminar flow Reynold’s number< 2000 'low' velocity Fluid particles move in straight lines Simple mathematical analysis possible

Laminar flow

Laminar flow

Determinants of laminar flow Pressure across tube Diameter of tube length of tube Viscosity of tube

All these factors are incorporated in an equation and known as the Hagen- Poiseuille equation

Viscosity Viscosity of fluid also affects the flow of

fluid viscosity increase in following condition - policythemia -Increased fibrinogen level - hypothermia - cigarette smoking - AgeIncreased viscosity leads to increase risk

of vascular occlusion .

Anaesthetic implicationDuring fluid resuscitation, a short wide bore

cannula e.g.14G is superior to a 20G cannula or a central line.

Intubating patients with very small tube increases resistance to flow and thus pressure increases to deliver the same amount of flow through the tube.

Critical velocityThis is the velocity for a given fluid for a

given tube beyond which laminar flow gets converted into turbulent flow.

When velocity of fluid exceeds this critical velocity , the character of flow changes from laminar to turbulent.

This critical velocity applies only for a given fluid in a given tube.

Turbulent flow Reynolds's number > 4000 'high' velocity Particle paths completely irregular Average motion is in the direction of the

flow Cannot be seen by the naked eye Changes/fluctuations are very difficult to

detect. Must use laser. Mathematical analysis very difficult - so

experimental measures are used Most common type of flow.

Onset of turbulent flow Turbulent flow occurs –1. Sharp increase of flow 2. Increase in viscosity or density of the fluid 3. Decrease in diameter of tube

Turbulent flow Laminar flow change to turbulent flow if

constriction is reached Velocity of fluid increases Fluid is no longer in a smooth fashion Swirls in eddies Resistance is higher than for the same

laminar flow . Flow is no longer directly proportional to

pressure

Turbulent flow

Where turbulent flow is seen ?Turbulent flow is present where there is an

orifice, a sharp bend and some irregulararity which may lead to local increase in velocity

Factors affecting flow and pressure during turbulent flow Q α √P

α 1/√ l

α 1/ √ρ Q= flow P = Pressure l = length of tube ρ = density of fluid

All these factors are combined to an index known as Reynolds number

Reynold number = vρ∂ / ŋ

v= linear velocity of fluid ρ = Density η =viscosity d = diameter of tube

Transitional flowTransitional flow Reynolds's number-2000-4000'medium' velocity

Clinical Aspects Of Flow Laminar flow is present in bronchi, smaller

air passage as they are narrower than trachea.

Turbulent flow is present in corrugated rubber tubing .

Sharp bend or angles increase turbulence In quiet breathing , the flow in resp tract is

laminar, while speaking , coughing or taking deep breath turbulent flow tends to occur .

A lining layer of mucus may affect the flow . In circulatory system, bruit and murmur can

be heard due to turbulence of flow.

Variable orifice flowmeterIn a variable orifice flow meter there is

mixture of turbulent and laminar flow and for calibration purposes both viscosity and density is important.

At low flows, gas flow depends on the viscosity of the gas.

At higher flows, gas flow depends on the viscosity of the gas.

Recallibration is required if flow meter is used for a different fluid than for what it was initially desigened.

How to measure the resistance A constant flow is passed through the

apparatus Difference in pressure P1-P2 between the

ends of tube is measured By dividing pressure difference by flow Provided the flow is laminar , resistance is

independent of flow

Resistance= Pressure / Flow R= P/ Q

Resistance

How to measure the resistance?

Bernoulli’s PrincipleDescribes the relationship between the velocity

and pressure exerted by a moving liquid.Applied to both liquids as well as gases.Venturi effect is based on the Bernoulli’s

principle.Venturi effect is entrainment of fluid (gas or

liquid ) due to the drop in pressureWhen a fluid flows through a constriction in the

tube there is reduction in fluid pressure.The fluid velocity correspondingly increases in

order to satisfy the law of conservation of energy.

Applications of Venturi effectVenturi masks used for oxygen therapy.Sander’s jet injector.Nebulisation chambers.Atomizers that disperse perfumes or spray

paints.Water aspirators.Foam fire fighting nozzles and extinguishers.Modern vaporizers.Sand blasters to mix air and sand.Vehicle carburetors.

Venturi

Venturi

Venturi

Venturi oxygen mask