DYNAMICS OF FLOW.ppt

DYNAMICS OF FLOW.ppt

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  DR UFOEGBUNAM M.P DEPARTMENTOF ANAESTHESIA UNTH ENUGU
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  INTRODUCTION  Both gasesand liquids are termed fluids  Liquids are incompressible and at constant temperature occupy a fixed volume and conforming to the shape of a container  Gases have no fixed volume but expand to occupy the total space of a container
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  FLOW  Defined asthe quantity of fluids(Gas,liquid or vapour) that passes a point per unit time. A simple equation to represent this is  Flow(F) = Quantity (Q)  Time(t)  Flow is sometimes written as ∆Q (rate of change of a quantity, mass or volume)  Due to the quantities of different fluids that are given to our patients during a routine anaesthesia, flow is an important area of physics to understand
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  Consider for followingquestions  Why are the scales on ratometers (flow meters) non- uniform as flow increases?, why are there different rotameters for different gases  Under what circumstances is Heliox useful and why?  Will there be much difference ventilating through a size 4 endotracheal tube compared to a size 8 endotracheal tube?  How much more fliud can be administered through a 14G cannula than a 22G cannula?
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  Flow of fluids Viscosity is defined as that property of a fluid which causes it to resist flow. The coefficient of viscosity (п) is defined as  (п) = force × velocity gradient/Area  In this context, velocity gradient is equal to the difference between velocities of different fluid molecules divided by the distance between molecules
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  Cont..  The unitsof the coefficient of viscosity are Pascal seconds  Fliuds which obey this formula are reffered to as newtonian fluids and cofficient of viscosity is a constant for each fluid  Some biological fluids are non- newtonian e.g Blood  Viscosity of liquids diminishes with increasing temp. where as viscosity of gas increases with increasing temperature
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  Types of flow Flow can be divided into two different types  Laminar flow  Turbulent flow  A number of different physical characteristics determine whether a fluid obeys the principle of one or the other
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  Laminar flow throughtubes  A tube is a cylideriacal vessel whose length is much more than the diameter  Hagen-Poiseuille’s (1799-1868) law: the flow rate is proportional to the perfussion pressure, also proportional to the fourth power of the radius and inversely proportional to the length and coefficient of viscosity  Q = π(P1 –P2)r4  ή8L  Laminar flow is smooth, streamlined and noiseless
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  Turbulent flow throughtubes  It occurs when flow exceeds a critical value  Turbulence is facilitated by irregularities and corners in the tube.  Conversion from laminar to turbulent flow increases resistance, which itself becomes roughly proportional to flow  The dimensionless Reynolds number (Re) must exceed 2000 for turbulence to occur
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  CONT..  Re =diameter×density×average velocity/Viscosity  Turbulent flow is noisy, less ordered and eddy currents interfere with each other, increasing drag or resistance to flow.  Turbulent flow occurs when fluids flow at high velocity in large diameter tubes and when the fluids are relatively dense  Decreasing the velocity of a fliud leads to turbulent flow
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  Cont..  Measurments intubes have shown that  When the Reynolds number is less than 2000, there is laminar flow  When the Reynolds number is 2000-4000 there is transitional flow ie. A mixture of laminar and turbulent flow  When the Reynolds number is greater than 4000 flow will be turbulent
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  Flow through orifices An orifice is an opening whose length is much less than its diameter. It causes turbulence and the resistance to flow is related to the gas density rather than viscosity  Helium- oxygen mixture (Heliox) may be useful in upper airway obstruction because the mixture is less dense than Oxygen or Air  In Rotameters or flowmeters, the gas flow around the bobbin is laminar at low- flow rates, so viscosity is important. At high flows, the annular aperture around the bobbin resembles an orifice and density is important
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  Venturi principle  Describedin 1797 (Giovanni venturi, an Italian physicist)  When fluid flows through a tube of varying diameter, the pressure is lowest at the point of maximum velocity, because pressure energy drops as kinetic energy increases  Also similarly described by Bernoulli(1700-1782  With suitable flow and a “venturi” cone shaped tube, subatmospheric pressure can easily be produced
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  Cont..  The injectorreplaces the smooth contruction of a venturi tube with a nozzle through which fliud is injected at high velocity. Surrounding fliuds are sucked in with the main stream  Suction apparatus  Entrainment of air or oxygen during therapy  Jet ventilation eg. By Sanders injector during bronchoscopy
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  THANK YOU