flow, viscosity and density Flashcards
what is the difference between density and viscosity?
density = mass per unit volume. usually measured as kg/m3. describes how densely packed a substance is. measured in Rho (p)
viscosity = measure of the tendancy of a fluid to resist flow i.e. how thick/sticky a fluid is. measured in Poise (u)
how does temperature and pressure affect density and viscosity?
Increase in temperature, causes particles to have more energy to move furhter apart hence reduces density of a liquid/ gas
increase in temp
- increases viscosity of a gas
- decreases viscosity of a liquid.
increase in pressure, causes particles to be pushed closer and therefore increases density of a liquid or gas.
increase in pressure
- increases viscosity of a liquid
- descreases viscosity of a gas
define flow
Flow is a measure of the volume of a fluid passing a point per unit time.
this fluid can be gas or liquid.
e.g. L/min
what are the different types of flow?
Laminar flow:
* particles move in straight lines, past one another.
* velocity highest in the middle , slower towards the edges
* viscosity is more important than density in determining flow
* can be predicted/calculated using Hagen Poiseulle
Turbulent flow:
* disorganised, unpredictable, chaotic movement of particles
* can flow in eddie currents (swirls)
* no longer follow hagen poiseulle
* instead flow is proportional to root pressure difference and r^2 and inversely proportional to root lenght and root density of the fluid.
* hence density is more important than viscosity
state the hagen poiseulle equation
FLOW = ΔPπr4 / 8nl
n= viscosity
l = length
what determines if flow is laminar or turbulent?
Reynolds number is a dimensionless number than can help predict if flow is laminar or turbulent
Re = p x D x V / u
p = density, D = diameter, V = velocity, u = viscosity
<2000 = laminar
>2000 = turbulent
turbulent flow also more likely at constrictions
what is a newtonian fluid?
A newtonian fluid is one whos viscosity is only affected by temp and pressure and not by sheer forces applied e.g. stirring.
most are non-newtonian e.g. paint - reduced viscosity with stirring
water is a newtonian fluid
what is the specific gravity?
the density of a substance compared to either air (for gas) or water (for liquids and solids). substance density/ water or air density.
density of air = 1.2kg/m3
density of water = 1kg/m3 at 4 degrees
hence is >1 then more dense
what is the critical velocity of flow?
This is the velocity at which flow will turn from laminar to turbulent.
e.g. in a 9mm ET tube this is 9L/min. below this, likely to be laminar.
what is the bernoulli principle?
states that the kinetic energy + potential energy of a system is always equal to a constant and thus..
At a constant flow rate, when there is a narrowing in a pipe, the velocity must increase to maintain this flow.
Due to 1st law of thermodynamics an increase in kinetic energy (velocity) will be associated with a drop in potential energy (pressure)
hence at the narrowing pressure reduces with this increased velocity
what is the venturi effect?
venturi effect is an application of the bernoulli principle when applied to fluids flowing through pipes
how does the venturi mask work
venturi mask
fixed performance O2 delivery devices using bernoulli principle/venturi effect
* 100% O2 flows through venturi device
* there is a narrowing
* this causes velocity to increase and pressure to drop (bernoulli)
* the drop in pressure results in entrainment of air from surrounding
* total flow rate increased
* 100% O2 diluted by air
* the ratio of entrained air depends on the holes (venturi valves) and rate of O2 flow.
* each valve is coloured to state the %O2 it will deliver at a given flow rate.
state examples of when the bernoulli principle/ venturi effect is seen in practice…
venturi
jet ventilation
nebuliser
what is the entrainment ratio and how is this calculated?
the ratio of entrained flow / driving flow
e.g. 9:1 means 9x more normal air than 100% O2
worked out as ratio calculations based on this ratio and flow rate.
e.g. 4L/min O2 flow means 40L/min in total with entrained air as well
0.21 x 36 + 1 x 4 = 28%
how can critical flow in an ET tube be estimated?
usually related to its diameter e.g. 9mm is 9L/min , 8mm is 8L/min
peak inspiratory flow is usually higher than this so will promote turbulent flow but at normal tidal breathing laminar
what is the Coanda effect?
This describes the tendency of a stream of fluid to follow a convex surface than to follow line of its original course.
the stream of high velocity flow will create area of low pressure around this stream
without any surface this will result in ambient air being drawn in.
If the fluid is near a surface there is no ambient air to be drawn in and instead the lower pressure draws the fluid towards the surface as there is an imbalance of forces acting on the liquid.
how does a nebuliser work?
nebuliser
* O2 flows through a constriction
* high velocity, drop in pressure
* liquid droplets drawn in towards stream of O2
* impacts the anvil and is split into smaller droplets that can travel further into patients airway
how does the shape of aircraft wing help it to lift?
shape means that the air takes a longer path on top. however flow is equal thus velocity must be higher ontop and therefore drop in pressure above this.
pressure difference below top and bottom helps lift.
how is the coanda effect used in the fluid logic ventilator penlon nuffield? what are the advantages?
the air flows through a narrow constiction and then is presented with 2 limbs.
there is a switch to push the fluid to either of these limbs depending on part of ventilation cycle.
opening the valve in one direction allows ambient air to flow in and push the stream to wards the further away limb and vice versa
advantage = does not rely on moving parts, more reliable and robust.
