Physical Principles of Respiratory Care

I. States of MatterII. Change of StateIII. Gas Behavior Under Changing

ConditionsIV. Fluid Dynamics

Fluid Dynamics

1. Pressure in Flowing Fluids2. Patterns of Flow

Laminar Flow Turbulent Flow Transitional Flow

3. Flow, Velocity, and Cross-Sectional Area4. Bernoulli Effect5. Fluid Entrainment6. Fluidics and the Coanda Effect

3

Fluid Dynamics

The study of fluids in motion is called hydrodynamics.

The pressure exerted by a liquid in motion depends on the nature of the flow itself.

A progressive decrease in fluid pressure occurs as the fluid flows through a tube due to resistance.

4

Patterns of Flow

Patterns of flow Laminar flowfluid moving in discrete

cylindrical layers or streamlinesPoiseuille’s lawpredicts pressure

required to produce given flow using ΔP = 8nl V./ πr4

Conditions that cause laminar flow to become turbulent1. High linear gas velocity2. High gas density3. Low gas viscosity4. Large tube diameter

Patterns of flow Turbulent flowloss of regular streamlines;

fluid molecules form irregular eddy currents in chaotic pattern.Predicted by using Reynold`s number

(NR)NR = v d2r / h

7

Patterns of Flow

Transitional Flow

8

Poiseuille’s Law(only applies to laminar flow)

Flow of fluid through a tube: Driving pressure Resistance

ViscosityLength of the tubeRadius of the tube

9

Poiseuille’s Law

1. The more viscous the fluid the more pressure is required to cause it to move through a given tube

10

Poiseuille’s Law

2. Resistance to flow is directly proportional to the length of the tube

If the length of a tube is increased four times, the driving pressure to maintain a given flow must be increased four times

11

Poiseuille’s Law

3. Resistance to flow is inversely proportional to the fourth power of the radius of the tube

If the inside diameter of the tube is decreased by one half, the driving pressure must be increased 16 times to maintain original flow

12

Poiseuille’s Law

Respiratory Care Application:

ETT

13

Poiseuille’s Law Asthma

14

Pressure in Flowing Fluids

15

Law of Continuity

The speed of flow in a closed system will be inversely proportional to the area of the tubes through which it flows

16

Law of Continuity

If the area of flow is decreased, then the velocity must increase

If the area of flow in increased, then the velocity must decrease

2.54

17

Law of Continuity

18

19

The Bernoulli Effect

As the speed of the fluid increases, the pressure in a fluid decreases

20

21

Venturi Principle of Fluid Entrainment

If the increase in velocity at a constriction is so great that is causes the pressure of the fluid to fall below atmospheric (becoming negative) it can pull another fluid into the primary flow

22

Fluid Entrainment

Respiratory Care Application:

Air injector

23

Fluid EntrainmentRespiratory Care Application:

Entrainment mask

24

Fluid Entrainment

Respiratory Care Application:

Small Volume Nebulizer

25

Fluid Entrainment

Respiratory Care Application:

Large volume jet nebulizer

26

Fluid Dynamics

Fluidics and the Coanda effect Fluidics is a branch of engineering that

applies hydrodynamics principles in flow circuits.

The Coanda effect (wall attachment) is observed when fluid flows through a small orifice with properly contoured downstream surfaces.

27

28

29

Advantages

Operate without moving parts, minimizing maintenance expenses

Generally cost less than electronic counterparts

Don’t break down as often as their electronic counterparts

30

Disadvantages

Not easily interfaced with microprocessors

Not as accurate as their electrical counterparts

Difficult to measure tidal volume because tidal volume exits with source gas