Flow Flashcards
What is the density of air at sea level?
1.2kg/m3
If the specific gravity of a gas is over 1, what does this mean?
It is more dense than air
(Gas specific gravity = gas density / air density
How do we measure the specific gravity of solids and liquids?
We compare it to water instead of air
What is a Newtonian fluid?
One where the viscosity is only effected by temperature and pressure, not by movement etc.
What happens to a Newtonian vs a non-Newtonian fluid when it is put under shear stress (e.g. stirred)?
Newtonian fluids still flow, whereas non-Newtonian will deform.
What are some examples of Newtonian fluids?
Water and gas
How do we calculate laminar flow?
Q= (pi deltaP r4) / 8nl
N = viscosity
What components of the HP eqn. is turbulent flow directly and inversely proportional to?
Directly proportional to r2 and square root of delta P
And inversely proportional to length and density
What kind of flow occurs along a smooth tube?
Laminar flow with increased velocity at the centre, due to the walls offering greater resistance
Where does turbulent flow occur in a vessel / tube?
At an orifice, which is a tube with a diameter greater than its length
What does a rotameter provide in terms of pressure and orifice?
Constant pressure with a variable orifice
How is a rotameter constructed?
A tapered glass tube (slimmer at the bottom) with a bobbin in it. Gas flow from the bottom via a needle valve.
What happens to flow along the length of a rotameter and how does this alter how the bobbin moves?
Gas flow at the bottom causes pressure to lift the bobbin until gravity matches this and causes it to float. Near the base, the bobbin is close to the side of the tube, so length is greater than diameter (between the bobbin and side of the tube), so flow is laminar. As the bobbin climbs higher is moves further from the wall of the tapered tube, so the diameter of the gap exceeds the length of the bobbin, causing turbulent flow.
(Physics 1 page 7 - diagram)
In a rotameter, where do viscosity and density have more of an effect?
Density has more effect on the turbulent flow at the top and viscocity has more effect on the turbulent flow at the bottom
If we increase the density of the gas in a rotameter, what happens to the rotation of the bobbin?
Less rotation
What are two examples of constant orifice, variable flow meters?
Pneumotachograph and simple pressure gauge
What happens in a pneumotachograph?
Laminar flow through small diameter tubes, resistance causes a pressure drop proportional to flow as in HP eqn. Meter reads this across a small section of tube. Needs calibrated for all different gasses.
When are simple pressure gauges used?
If upstream flow is constant e.g. on a cylinder
What type of meters in terms of orifice and pressure are peak flow meters?
Constant pressure, variable orifice
How does a vane flow meter work?
Exhalation causes vanes to move and uncover increasingly larger orifices. Vanes are held by a spring to make opposing pressure. The size of the orifice revealed is shown be the ratchet (red dial pointer outside).
What other types of flow meter are there other than peak flow and vane meters?
Watersight, bubble, thermistor and ultrasonic
State the: boiling point, melting point, critical temperature, critical pressure and saturated vapour pressure at 20 degrees for oxygen.
Temps in Celsius and pressures in bar.
-183
-219
-118
50
1.4
State the: boiling point, melting point, critical temperature, critical pressure and saturated vapour pressure at 20 degrees for N2O.
Temps in Celsius and pressures in bar.
-88
-91
36.5
72
50.8
State the: boiling point, melting point, critical temperature, critical pressure and saturated vapour pressure at 20 degrees for CO2.
Temps in Celsius and pressures in bar.
-79
-57
30
73
57
