Fluid Systems Flashcards

Question

Dynamic/Absolute viscosity definition

Answer 1

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

Answer 2

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

Answer 3

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.

Answer 4

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

Answer 5

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

Answer 6

β =ΔP/(ΔV/V) ΔP = pressure change (ΔV/V) = change in volume/original volume. Typical oil will decrease 0.5% for every 1000psi increase

Answer 7

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

Answer 8

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

Answer 9

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.

Answer 10

P ≈ 1/V —> P1V1 = P2V2

Answer 11

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

Answer 12

V ≈ T —> T1/V1 = T2/V2 ΔV = V2 - V1 = V1 x (T2 - T1)/ T1 V2 = V1 + ΔV = V1 + V1/T1 (T2 - T1)

Answer 13

If the volume is kept constant during temperature rise = results in the following formula for pressure increase: P1/T1 = P2/T2 = P3/T3 = constant

Answer 14

For a given mass of gas, pressure and volume divided by absolute temp is constant. (P1 x V1)/ T1 = (P2 x V2)/ T2

Answer 15

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

Answer 16

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

Answer 17

At low velocities flow = smooth and uniform At high velocities flow = turbulent

Answer 18

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

Answer 19

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

Answer 20

Re = ρVD_h/μ = VD_h/ν For non circular pipes: D_h= 4A/S V = fluid velocity D_h = Hydraulic diameter S = perimeter

Answer 21

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

Answer 22

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

Answer 23

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

Answer 24

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

Answer 25

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

Answer 26

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

Answer 27

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

Answer 28

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.

Answer 29

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

Answer 30

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

Answer 31

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)

Answer 32

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

Answer 33

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)

Answer 34

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)

Fluid Systems Flashcards

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