Fluid flow physics and anaesthetic implication
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Transcript of Fluid flow physics and anaesthetic implication
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Dr. Shraddha Naik
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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
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Flow is of three types:Laminar flowTransitional flow Turbulent flow
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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.
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Laminar flowLaminar flow Reynold’s number< 2000 'low' velocity Fluid particles move in straight lines Simple mathematical analysis possible
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Laminar flow
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Laminar flow
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Determinants of laminar flow Pressure across tube Diameter of tube length of tube Viscosity of tube
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All these factors are incorporated in an equation and known as the Hagen- Poiseuille equation
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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 .
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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.
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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.
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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.
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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
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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
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Turbulent flow
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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
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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
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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
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Transitional flowTransitional flow Reynolds's number-2000-4000'medium' velocity
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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.
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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.
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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
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Resistance= Pressure / Flow R= P/ Q
Resistance
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How to measure the resistance?
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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.
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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.
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Venturi
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Venturi
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Venturi
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Venturi oxygen mask
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