Flow is defined as the quantity of a fluid i.e. a gas or liquid passing in unit time F=Q/t F=flow Q= quantity of liquid T=time
Flow is of three types: Laminar flow Transitional flow Turbulent flow
Laminar Flow A fluid flows in a steady manner No eddies or turbulence Present in smooth tubes Velocity is low Flow is greatest at centre ( 2x mean flow) To draw the fluid , a pressure difference must be present across the ends of tube.
Laminar flow Laminar flow Reynold’s number< 2000 low velocity Fluid particles move in straight lines Simple mathematical analysis possible
Determinants of laminar flow Pressure across tube Diameter of tube length of tube Viscosity of tube
All these factors are incorporated in an equationand 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 - Age Increased viscosity leads to increase risk of vascular occlusion .
Anaesthetic implication During 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 velocity This 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 Reynoldss 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 –2. Sharp increase of flow3. Increase in viscosity or density of the fluid4. 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
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 duringturbulent 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 indexknown as Reynolds number Reynold number = vρ∂ / ŋ v= linear velocity of fluid ρ = Density η =viscosity d = diameter of tube
Transitional flow Transitional flow Reynoldss 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 flowmeter In 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
Bernoulli’s Principle Describes 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 pressure When 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 effect Venturi 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.