Thermofluids Flashcards
What is Drag Force?
- force that flowing liquid exerts on a body in flow direction
What is Lift Force?
Force acting perpendicular to the flow direction.
- acts against gravity to support the weight of the object in fluid.
- change in flow direction creates lift force in opposite direction.
What is the Drag Coefficient equation?
- what do the V and A stand for.
Cd = Fd / 1/2 p V^2 A
V = Speed of the object moving through the fluid.
A = Reference Area / Frontal Area - area perpendicular to the flow.
What is the Lift Coefficient Equation?
What do V & A stand for?
CL = FL / 1/2 p V^2 A
V = velocity of object moving through the fluid
A = Planform area - Area that would be seen if looking down
(Q13) What are the assumption for: Navier Stokes Questions?
- steady flow (d() / dt = 0)
- incompressible fluid ( p = constant)
- fluid flows in x-direction ( v = 0, w = 0)
- no variation in velocity in x- direction ( u = constant)
- gravity working in -y (gx = 0, gy = -g, gz = 0)
- viscous fluid (backwards y = 0)
(Q13) What are the steps needed to complete a: Navier Stokes Question?
- Sketch diagram.
- List all assumptions.
- Apply continuity equitation.
- Apply momentum equation.
- Solve resulting differential equation by double integration.
- Apply boundary conditions to obtain constants.
- Substitute in constants.
- Calculate for required parameters.
What is a Heat Engine cycle?
- system that converts heat of thermal energy to mechanical energy
- mechanical energy then used to do mechanical work
- bring substance from high temp to low temp
What is the 4 stages of a Heat Engine Cycle
1 - Recieves Heat
2 - Converts part of the heat to work
3 - rejects remaining waste to low temp sink
4 - cycles
Explain the 4 main parts of a heat engine:
1. Heat Source
2. Working Substance
3. Thermodynamic Process
4. Efficiency
- Heat Source - heat engine works between hot and cold reservoirs
- Working Substance - engine using working substance with finite heat capacity that abroad’s heat from hot tank,
- Thermodynamic Process - 1: working substance expands and contracts. 2: heat absorbed from hot and rejected to cold. 3: supplied heat converted to work.
- Efficiency: determines how much useful work it does for given heat.
What are the 4 reversible processes in: carnot cycle
1-2: isothermal heat addition
2-3: isentropic expansion
3-4: isothermal heat rejection
4-1: isentropic compression (repeat)
(Q4) What are the steps to answer a carnot cycle question if given Qh & QL
- calculate thermal eff. n = 1- (Qout/Qin)
- calculate power output. Went,out = n * Qin
(Q5) What are the steps to answer a carnot cycle question if given Th & TL & Qh?
- calculate therm eff. n = 1 - (Qout/Qin)
- calculate heat reject. Qh / QL = Th / TL => QL = Qh / (Th / TL)
what are the 4 stages of a 4-stroke reciprocating engine?
- Intake - Fuel enters, expands, piston moves down
- Compression - Piston moves up, compresses fuel.
- Ignition - Spark ignites fuel, piston moves down, provides useful work.
4- Exhaust - Waste exits, cycle repeats.
Reciprocating Engine Components: What is Intake and Exhaust?
Intake: Draws air-fuel mix in.
Exhaust: Expels waste.
Reciprocating Engine Components: What is TDC and BDC?
TDC - Top Dead Centre: Position of piston that forms smallest volume.
BDC - Bottom Dead Centre: Forms largest volume.
Reciprocating Engine Components: What is Stroke and Bore?
Stroke: Largest distance piston can travel.
Bore: Diameter of piston.
(Q7) Steps to solve Otto Cycle Question.
Max temp and pressure of cycle.
1-2:
- interpolation to find T2, U2.
- table to find vr1.
- vr2 = v1 = v1 / r
P1v1 / T1 = P2v2 / T2 => P2=P1 (T2/T1)(v1/v2)
v1/v2 = r
2-3:
- qin = U3 - U2
-interpolation to fine T3, v3.
-P3v3/T3 = P2v2/T2 => P3=P2(T3/T2)(v2/v3).
v2/v3 = 1
(Q7) Steps to solve Otto Cycle Question.
Net work output. (4-1)
net work = net heat @ cycle 4.
- v4/v3 = r => v4 = r * v3
- interpolate for T4, U4.
- qout = u4 - u1
-wnet = qnet => qin- qout
(Q7) Steps to solve Otto Cycle Question.
Thermal Efficiency
nth = wnet / qin
(Q7)Steps to solve Otto Cycle Question.
MEP
v1 = R * T1 / P1
MEP = wnet / v1 (1 - (1/r))
(Q8) Steps to solve Diesel Cycle Question (Cold air assumptions).
Temp and pressure at end of each cycle.
1-2:
- use table
- v2 = v1 / r
- T2 = T1 (v1 / v2)^k-1. - v1/v2 = r
- P2 =P1 (v1 / v2)^k. - v1/v2 = r
2-3:
- P3 = P2
- P3v3 /T3 = P2v2 / T2 => T3 = T2 ( v3 / v2). - v3/v2 = rc
3-4:
T4 = T3(v3 / v4)^k-1. - v3 = rc * v2, v4 = v1.
P4 = P3 (v3/ v4)^k
(Q8) Steps to solve Diesel Cycle Question (Cold air assumptions).
net work output
net work = net heat transfer
mass of air = m1 - P1 * v1 / R * T1
2-3:
- Qin = m(h3-h2) = m(cp)(T3 - T2)
4-1:
-Qout = m(u4 - u1) = m(cv)(T4 - T1)
wnet = Qin - Qout
(Q8) Steps to solve Diesel Cycle Question (Cold air assumptions).
thermal efficiency
nth = wnet / Qin
(Q8) Steps to solve Diesel Cycle Question (Cold air assumptions).
Mean effective pressure.
MEP = wnet,out / v(max) - v(min)
=> wnet,out / v1 - v2
(Q16) How to solve LMTD counter flow question.
- length of heat exchanger required
- Q = [m * c(p) ( Tout - Tin)] - for water
- Q = [m * c(p) (Tin - Tout)] - for thermal
=> Tout = Tin - (Q / m * c(p)) - change T(Lm) = change T1 - change T2 / Ln ( change T1 / change T2)
=> change T1 = Th,in - Tc,out
=> change T2 = Th,out - Tc, in
- Q = U* A(s) * change T (Lm)
=> A(s) = Q / U * change T (Lm)
==> A = piDL
===> L = A(s) / pi * D
(Q17) How to solve LMTD counter flow question.
- max heat transfer rate
- outlet temp of cold and hot water
- Ch = mh * cp(h)
Cc = mc * cp(c)
Qmax = Cmin (Th,in - Tc,in) - change T(max) = Th,in - Tc,in
- Q = Cc (Tc,out - Tc,in)
=> Tc,out = Tc,in + Q / Cc
- Q = Ch (Th,in - Th,out)
=> Th,out = Th,in - Q / Ch
What is Fouling?
- build up of deposits on heat exchanger surfaces
- performance deteriorates with build up
- effected by velocity and ???
Fouling:
What is Precipitation Fouling.
How do you prevent it?
-build up of solid deposits
- occurs when solubility of compounds is exceed and they crystallise
Prevention:
- process water to remove solids
- treat water to remove solids
Fouling:
What is Corrosion Fouling.
How do you prevent it?
- fouling caused by chemical reactions
— such as oxidation, decomposition, corrosive fluids
Presentation: - fluid treatment
-coating HE pipes
-material selection (plastic pipe instead of metal)
-regular cleaning
-optimise operating conditions - minimise fouling
Fouling:
What is Algae Fouling.
How do you prevent it?
- build up of algae growth
- biological fouling
Presentation: - chemical treatment
- regular cleaning
-filtration
-material selection (anti-microbial properties) - uv sterilisation
Do an Interpolation example!
Q1, Q2 Interpolation, Q3
Q1) a - use table to find vf, b - v=mv
Q2) a - use table to find P, b - x = mg / m(total) ==> v = vf + x(vg - vf) ==> v = m*v
Q3) interpolation equation i cba typing it
Q4) interpolation equation, again cba typing
