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Fluid flow and measurement
1.
2. Introduction
Definition of Flow
Types of Flow
Factors Affecting Flow
Clinical Applications
Conclusion
3. A fluid is a state of matter (or matter- in-
transition) in which its molecules move freely
and do not bear a constant relationship in
space to other molecules
Thus it has the ability to take up the shape of
its container
4. Fluids are
Liquid
e.g. blood, i.v. infusions
Gas
e.g. O2 , N2O
Vapour (transition from liquid to gas)
e.g. N2O (under compression in cylinder), volatile inhalational
agents (halothane, isoflurane, etc)
Sublimate (transition from solid to gas bypassing liquid
state)
Dry ice (solid CO2), iodine
5. Flow is defined as the quantity of fluid (gas, liquid,
vapour or sublimate) 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)
6. There are two types of flow:
Laminar flow
Turbulent flow
7. Smooth, steady and orderly flow of fluid in a tube
All the fluid molecules move in a straight line
Therefore they move in parallel layers or laminae with
no disruption between the layers
Velocity of flow is greatest in the axial stream (centre
of the tube). It becomes progressively slower as the
layers move to the periphery
Axial stream velocity is twice the mean flow velocity
Velocity of the layer in contact with the wall is virtually
zero
9. Fluid does not move in orderly manner
The fluid molecules become more disorganized
They form swirls and eddies as they move down the
pressure gradient in haphazard manner
There is increased resistance to flow as the eddy
currents interfere with each other
Therefore greater energy is required for a given flow
rate, compared to when the flow is laminar
12. Pressure: flow is directly proportional to the pressure
difference across the tube
Q ∞ ∆P
Radius: flow is directly proportional to the fourth power
of the radius (or diameter) of the tube
Q ∞ r4, or Q ∞ d4
Length: flow is inversely proportional to the length of the
tube
Q ∞ 1/l
Viscosity: flow is inversely proportional to the viscosity of
the fluid
Q ∞ 1/η
13.
14. The relationship between pressure and flow is
linear within certain limits
As velocity increases, a critical point (or critical
velocity) is reached where flow changes from
laminar to turbulent
Beyond this point, flow is proportional to the
square root of pressure gradient
15.
16. This number is calculated from an equation that
incorporates the factors that determine the critical
point
Reynolds’ number = vρr or vρd
η η
v = velocity of fluid flow
ρ = density of fluid
r = radius of tube
d = diameter of tube
η = viscosity of fluid
Reynolds number does not have any associated unit
It is a dimensionless number
17. if Reynolds’ number exceeds 2000, flow is likely to be
turbulent
a Reynolds’ number of less than 2000 is usually
associated with laminar flow
18. Viscosity (η) is the property of a fluid that causes it to
resist flow
It is a measure of the frictional forces acting
between the layers of fluid as it flows along the tube
η = force x velocity gradient
area
Unit of viscosity is pascal second (Pa s)
19. Viscosity of a liquid decreases with increased
temperature, while viscosity of a gas increases with
increased temperature
From Hagen-Poiseuille equation, the more viscous a
fluid is the lesser the flow. This however applies to
laminar flow and not turbulent flow, where flow is
dependent on the density of the fluid
20. Density (ρ) is defined as mass per unit volume
Unit of density is kilogram per meter cube
(kgm-3)
Density is an important factor of fluid in turbulent
flow through a tube, in which flow is inversely
proportional to square root of density
21. In a tube, the length of the fluid pathway is greater than
the diameter
diameter
length
In an orifice, the diameter of the fluid pathway is
greater than the length
diameter
length
22. As the diameter of a tube increases, the Reynolds
number increases. Eventually if the diameter of the
tube increases enough, it will exceed the length of the
tube. We then call this an orifice
Flow through a tube is laminar and hence dependent
on viscosity (provided that the critical velocity is not
exceeded)
If the flow is through an orifice it is turbulent and
dependent on density
23. The flow rate of a fluid through an orifice is
dependent upon:
the square root of the pressure difference across
the orifice
the square of the diameter of the orifice
the density of the fluid (flow through an orifice
inevitably involves some degree of turbulence)
24.
25. There are two types
Variable orifice (fixed pressure change) flowmeters
e.g Rotameter, peak flowmeter
Variable pressure change (fixed orifice) flowmeters
e.g. Bourdon gauge, pneumotacograph
26.
27.
28. At low flows, the bobbin is near the bottom of the tube
and the gap between the bobbin and wall of the
flowmeter acts like a tube (diameter < length)
Gas flow is laminar and hence the viscosity of the gas is
important
As flow rate increases, the bobbin rises up the
flowmeter and the gap increases until it eventually acts
like an orifice (diameter > length)
At this point the density of the gas affects its flow
29.
30.
31. This useful clinical instrument is capable of measuring
flow rates up to 1000 L per min
Air flow causes a vane to rotate or a piston to move
against the constant force of a light spring
This opens orifices which permit air to escape
The vane or piston rapidly attains a maximum position
in response to the peak expiratory flow
It is held in this position by a ratchet
The reading is obtained from a mechanical pointer
which is attached to the vane or piston
32. Accurate results demand good technique
These devices must be held horizontally to minimize the
effects of gravity on the position of the moving parts
The patient must be encouraged to exhale as rapidly as
possible
33. Bourdon gauge is used to sense the pressure change
across an orifice and is calibrated to the gas flow rate
It uses a coiled tube which uncoils as pressure increases
A system of cogs converts uncoiling of the coil into
clockwise movement of the needle over a calibrated
scale
These rugged meters are not affected by changes in
position and are useful for metering the flow from gas
cylinders at high ambient pressure
34.
35. Measures flow rate by sensing the pressure change across a
small but laminar resistance
Uses differential manometer that senses the true lateral
pressure exerted by the gas on each side of the resistance
element and transduce them to a continuous electrical
output
It is a sensitive instrument with a rapid response to
changing gas flow
It is used widely for clinical measurement of gas flows in
respiratory and anaesthetic practice
However, practical application requires frequent calibration
and correction or compensation for differences in
temperature, humidity, gas composition and pressure
changes during mechanical ventilation.
36.
37.
38.
39. Resistance to breathing is much greater when an
endotracheal tube of small diameter is used
Flow is significantly reduced in proportion to the fourth
power of the diameter
changing the tube from an 8mm to a 4mm may reduce flow by
up to sixteen-fold
Therefore the work of breathing is significantly
increased
Over time, a spontaneously breathing patient becomes
exhausted and soon becomes hypercapnic due to
reduced respiration
40.
41. In anaesthetic breathing systems, the following can
cause turbulent flow, making the work of breathing
greater
› a sudden change in diameter of tubing
› irregularity of the wall
› acute angles at connections
› Unnecessary long circuits
Thus, breathing tubes should possess smooth internal
surfaces, gradual bends and no constrictions
They should be of as large a diameter and as short a
length as possible
42. Heliox is a mixture of 21% oxygen and 79% helium
Helium is an inert gas that is much less dense than nitrogen
(79% of air)
Heliox much less dense than air
In patients with upper airway obstruction, flow is turbulent
and dependent on the density of the gas passing through it
Therefore for a given patient effort, there will be a greater
flow of heliox (density = 0.16) than air (density = 1.0) or
oxygen alone (density = 1.3)
However, heliox contains 21% oxygen – it may be of lesser
benefit in hypoxic patient
43. Humidification, in addition to its other benefits, makes
inspired gas less dense
This may be of benefit by reducing the work of
breathing
44. For a given fluid, with the same pressure applied to it,
flow is greater through a shorter, wider cannula
Thus they are preferred in resuscitation
45.
Flow is principally laminar
There is a possibility of turbulence at the
junction of the vessels or where vessels
are constricted by outside pressure
Here turbulence results in a bruit which is
heard on auscultation
46.
47. As fluid passes through a constriction, there is an increase
in velocity of the fluid
Beyond the constriction, velocity decreases to the initial
value
At point A, the energy in the fluid is both potential and
kinetic
At point B the amount of kinetic energy is much greater
because of the increased velocity
As the total energy state must remain constant, potential
energy is reduced at point B and this is reflected by a
reduction in pressure
48. In the Venturi tube, the pressure is least at the site of
maximum constriction
Subatmospheric pressure may be induced distal to the
constriction by gradual opening of the tube beyond the
constriction
49.
50.
51.
52.
53. describes a phenomenon whereby gas flow through a
tube with two Venturis tends to cling either to one side
of the tube or to the other
used in anaesthetic ventilators (fluidic ventilators), as
the application of a small pressure distal to the
restriction may enable gas flow to be switched from
one side to another
Editor's Notes
Clinical Applications, especially as it relates to ANAESTHESIA
∆ = pronounced “delta”
Velocity gradient is equal to the difference between velocities of different fluid molecules divided by the distance between molecules
These masks, also termed high air flow oxygen enrichment (HAFOE) devices, provide a constant and predictable inspired oxygen concentration irrespective of the patient's ventilatory pattern. This is achieved by supplying the mask with oxygen and air at a high total flow rate.
gradient. This results in water being drawn up through the tube and broken into a fine spray as it comes in contact with the high-speed gas jet