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Mechatronics > Fluid Systems > Flashcards

Fluid Systems Flashcards

1
Q

What is a fluid system

A

A system using confined pressurized fluid with a transmitted force to generate work done.

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2
Q

Purpose of a fluid system.

A

To transmit power is primary function. lubricant, cooling also

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3
Q

Types of Fluid systems

A

Hydraulic - liquid (usually oil)
Pneumatic - Inert gas/ air

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4
Q

Fluid system pros

A

High bandwidth
No complex system E.g no gears
Smooth & compact
No wear/less breakage
High speed/force/power
Can be finely controlled/no slack
Uniform & flexible

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5
Q

Fluid system cons

A

Can leak at seals/connections
Needs heavy/noisy pump
Cavitation = leads to loss of precision
Contamination = filtration needed
Chemical action = corrosion a
Fluid needs to be positively confined in system

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6
Q

What type of variable is pressure?

A

Across

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7
Q

Absolute vs Gauge vs Differential pressure

A

Absolute = measured in respect to perfect vaccum.
Gauge = measured in respect to atmospheric pressure
Differential = difference of pressure between two specified points

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8
Q

Pressure equation

A

P = force/area

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9
Q

Pressure conversions

A

1 psi = 6895 pa
Patm = 101325 pa

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10
Q

What type of variable is Flow?

A

Through

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11
Q

Volumetric vs Mass vs Velocity flow (3 definitions of flow)

A

Volumetric = measures
volume of flow passing point per unit time.
Mass = measures mass of flow passing point per unit time
Velocity = measures linear speed of fluid per unit time

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12
Q

Flow conversions

A

1 gpm = 15850 m^3/s = 0.264 Ipm
1 m^3/s = 0.0000631 gpm = 0.0000167 Ipm

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13
Q

Flow equations

A

Q = A x V
V = avg velocity

Qm = m./ρ
m. = mass rate
ρ = density

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14
Q

Flow: Hydraulic vs Pneumatic

A

Hydraulic = generally treated as incompressible (density is constant)
Pneumatic = mass flow rate (Qm) is used as flow variable

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15
Q

Flowmeters

A

Contact = restricts flow, used in careful systems where small pressure drop matter
Contactless

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16
Q

Power equation

A

power = P x Q
P = pressure
Q = flow rate

Efficiency = power output/ power input

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17
Q

Power conversions

A

1 watt = 746 hp = 0.293
1 hp = 0.00134 watt
1 Btu/hr = 3.413 watt

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18
Q

Power definition

A

Rate at which work is done.
Work done = amount of force needed for object to move set distance.

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19
Q

Density definition

A

How close particles are packed in a substance.
Mass per unit volume.

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20
Q

Density Equation

A

ρ = m/V
=mass/ Volume

Note: Temp affects density but not mass

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21
Q

Specific gravity definition

A

Used to determine relative lightness of material compared to water. Relative density.

Note: both density and specific gravity are independent of size.

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22
Q

Specific gravity equation

A

SG = ρsubstance/ ρwater
p = density

<1 = lighter than water
1> = heavier than water

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23
Q

Viscosity definition

A

Resistance to flow.

Note: Temp affects viscosity, as temp in increases viscosity decreases

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24
Q

Viscosity in hydraulic systems

A

Needs to be compromise

If viscosity too high = difficult to push through pipes/fitting = loss of mechanical efficiency

If viscosity too low = fluid leaks by internal seals = loss in volumetric efficiency

25
Q

Dynamic/Absolute viscosity definition

A

Resistance to flow/shear of fluid. Measured by placing fluid in between two plates + shearing.

26
Q

Dynamic viscosity (μ) equations

A

τ = F/A = μ ΔV/Δy

SI units (Pa-s) but more common unit id cP (centipoise)

1cP =0.001 Pa-s

Dynamic viscosity of water at 20c is 1 cP

27
Q

Kinematic viscosity definition (ν)

A

Kinematic viscosity is the dynamic viscosity measured with respect to density. Ratio of the two.

Can be measured by by the time it takes to flow through a capillary.

28
Q

Kinematic viscosity equation (ν)

A

ν = μ/ρ
μ = Absolute/dynamic viscosity
ρ = density

SI units (m^2/s) but more common unit id cSt (centistoke)

1 m^2/s = 1.0 x 10^6 cSt

Kinematic viscosity of water at 20c is 1 cSt

29
Q

Bulk modulus definition

A

The pressure needed to cause a given decrease in volume of a fluid. “Springiness of fluid.”

30
Q

Bulk modulus equation

A

β =ΔP/(ΔV/V)
ΔP = pressure change
(ΔV/V) = change in volume/original volume.

Typical oil will decrease 0.5% for every 1000psi increase

31
Q

Pascals law

A

In a confined fluid at rest, pressure acts equally in all directions and acts perpendicular to the walls.

P= F/A

P1 x V1 = P2 x V2

32
Q

Pascals law: Static pressure

A

Static fluid pressure doesn’t depend on shape, total mass or surface are of liquid.

Pfluid = F/A = m.g/A —> p = m/V
Pfluid = ρVg/A —> ρgh
P = Patm + Pfluid

33
Q

Boyles Law

A

In a closed container with a given number of molecules as the volume decreases particle per unit volume increases = more collisions = greater pressure.

Note temperature and
mass must be constant.

34
Q

Boyles law equation

A

P ≈ 1/V —> P1V1 = P2V2

35
Q

Charles law

A

If pressure is constant, fluid expands when heated. When temp rises, molecules move faster and collides more, with more force. To keep the mass and pressure constant, volume must increase

36
Q

Charles law

A

V ≈ T —> T1/V1 = T2/V2

ΔV = V2 - V1 = V1 x (T2 - T1)/ T1

V2 = V1 + ΔV = V1 + V1/T1 (T2 - T1)

37
Q

Gay Lussacs Law

A

If the volume is kept constant during temperature rise = results in the following formula for pressure increase:

P1/T1 = P2/T2 = P3/T3 = constant

38
Q

General gas equation

A

For a given mass of gas, pressure and volume divided by absolute temp is constant.

(P1 x V1)/ T1 = (P2 x V2)/ T2

39
Q

Bernoullis principle

A

Within a flowing fluid, increase/decrease in speed occurs simultaneously with the increase/decrease in pressure. Increase in speed = decrease in pressure.

When a fluid goes through a narrow space = goes faster

40
Q

Bernoullis Principle Equation

A

P1 + 1/2Pv1^2 + Pgh1 = P2 + 1/2Pv2^2 + Pgh2 =

P1 = pressure energy
1/2PV1^2 = Kinetic energy
Pgh1 = Potential energy

For actual rather than ideal flow

P1/Pg + V1^2/2g + h1 * Ha = P2/Pg + V2^2/2g + h2 + He + Hl

Hl = energy lost
He = Heas of energy ecxtracted
Ha = Head energy by pump

41
Q

Flow velocities

A

At low velocities flow = smooth and uniform

At high velocities flow = turbulent

42
Q

Flow velocity & Reynolds number

A

Laminar flow Re < 2300
Turbulent flow Re > 4000
Transitional flow 2300 < Re > 4000

43
Q

Fluid energy loss

A

Flow of fluid through hoses, pipes, fittings etc can result in energy losses due to:

Internal fluid friction
Friction against wall
Orifice drag

Higher friction = efficiency loss

44
Q

Reynolds number equation

A

Re = ρVD_h/μ = VD_h/ν

For non circular pipes:

D_h= 4A/S

V = fluid velocity
D_h = Hydraulic diameter
S = perimeter

45
Q

Reynolds’s number flow

A

High are - Inertia predominant force, inertia promotes turbulent flow
Low Re - Viscosity predominant force, viscosity promotes turbulent flow

46
Q

Reynolds’s number definition

A

Re defines fluid flow and relates viscosity, density and fluid velocity to size. (Non dimensionless ratio of inertia/ viscous forces)

47
Q

Pressure losses

A

When fluid is pumped through a system, certain amount of energy is lost due to friction.

Fluid particles rub against pipe = frictional loss.

Rate of shear and heat generated are greatest near the wall + this is where most of the energy transfer occurs

48
Q

Major losses definition

A

Occurs when the fluid flows through pipes, hoses, tubing etc and is calculated for the length of the pipe

49
Q

Minor losses

A

Occur at valves, fittings, bends, enlargements, contractions, orifice. Converted to loss through equivalent length of pipe

50
Q

Major losses equation

A

hf = f L/D V^2/2g —> ΔP = f ρL/2D V^2

L/D = ratio of pipe
V^2/2g = velocity of head
f = friction factor
V^2 = avg flow velocity
D = diameter
L = conduit length
ΔP = pressure drop

51
Q

Major losses: Friction factor

A

Laminar flow Re <2100/2300

f = 64/Re

Turbulent flow

smooth pipes

f = 0.316/Re^0.125

Rough pipes (approximation)

f= 0.25/ [log10(ε/3.7D + 5.74/Re^0.9)]^2

52
Q

Minor losses - K values explained

A

Pressure drops as fluids undergo sudden expansion/contractions/ flow through pipe fittings, valves, & bends

The pressure loss associated w Bernoullis equation + defined as no. of velocity heads lost due to friction

Velocity heads: energy associated w fluid velocity.

When friction some energy lost and k values represent how much energy lost for that component.

53
Q

Minor loss - K values equation

A

hf f = K(V^2/2g) —> ΔP = K (ρ/2)V^2 = K (ρ/2A^2) Q^2

54
Q

K values - Enlargement/Reduction

A

Enlargement

K = (1 - (D1/D2)^2)

Reduction

K = 0.5(1 - (D1/D2)^2)

Fittings + bends

K = ft (L/D)

ft = friction factor in turbulent range
L = Length of fitting
D = Inside diameter of fitting

55
Q

Minor losses - Equivalent length

A

Minor losses are independent of Reynolds number and can be described as the loss through equivalent length of a straight pipe.

Don’t need to remember following:

hf = f (L/D) (V^2)
hff = K (V^2/2g)

hff = hf so —-> L = D (K/f)

56
Q

Minor losses - C coefficients

A

3rd type of pressure loss = comes from flow of fluid through destructed orifices & short tube & some fittings

57
Q

Minor losses - C coefficients equation

A

Theoretical velocity of free stream emitted horizontally from bottom of a tank = V = sqrt(2gh)

Friction losses are incorporated as discharge coefficient of velocity so:

V= cd (sqrt(2gh)) —> Q = ACd(sqrt(2gΔh) = ACd(sqrt (2ΔP/ρ)

58
Q

Fluid question equations to remember

A

Re (Reynolds no.) = VD/ν
V = Q/A where A = (D/2)^2 x π
ν = kinematic viscosity

Pressure drop =
ΔP = f x ((ρxL)/2D ) xV^2

Or

ΔP = 1/2ρ( v1^2 x v2^2)

Mechatronics (4 decks)

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