Dr Murphy Flashcards
Approx how may kPa and how many bar are in 1 atmos?
100kPa
1bar
Give gauge pressure in terms of absolute and atmospheric pressure
Absolute pressure minus atmospheric pressure
What is meant by a reversible process?
Both the fluid and the surroundings can always be restored to initial conditions
On a diagram how is a reversible and how is an irreversible reaction shown?
Reversible solid line
Irreversible dotted line
Give the first law of thermodynamics in an equation
Qin - Wout =ΔU
Give the non flow energy equation
Qin- Wout = ΔU
Give the steady flow energy equation (simplified)
Q’in - W’out = ΔKE’ + ΔPE’ + ΔH’
Where ‘ represents dot {first differential wrt time}
Give the equation for m dot
=CAρ
Give an equation for h
h=Pv+u
Describe a perfect gas
Theoretically, ideally behaved gas.
Temp vastly in excess of evaporation point.
Give boyles law and describe what happens to the graph as temp increases.
At constant temp: Pv=constant.
As temp increases curve moves further from origin
Give Charles’s laws and the combination if the 2. What is the constant in the combination?
At constant P: v/T=constant.
At constant v: P/t=constant.
Therefore Pv/t=constant=R=gas constant
Give the equation of state
PV=mRT
Give the equation for specific heat capacity.
Q=mCΔT
Give the equations for the 2 specific heat capacities multiplied by the change in temp.
Cv(T2-T1)=u2-u1
Cp(T2-T1)=h2-h1
What does Cp equal in terms of Cv?
Cp=R+Cv
What does isochoric mean?
Constant volume
What does isobaric mean?
Constant pressure
What does isothermal mean?
Constant temp
What does adiabtaic or isentropic mean?
No heat loss
What is work done equal to?
Area under Pv curve. So W=integral of Pdv
Give the efficiency of a heat engine.
1-Qout/Qin
What is a heat engine?
A thermodynamic system to which net heat is transferred and from which net work is produced.
What is a heat pump?
A system that removes heat from a low temperature body and delivers heat to a high temp body and receives external energy as work or heat from surroundings
Give the general equation for coefficient of performance
COP=desired product/required input
What is Q on a temperature entropy diagram?
Area under curve. Q=integral of Tds
Give a condition for ΔS
ΔS greater than or equal to 0
On a Ts graph which is steeper: constant vol or constant pres? Why?
Constant vol is steeper. Q=CvΔT, Q=CpΔT. Cp>Cv. For const pres there will be a volume change so work done so more heat required for same ΔT. So for the same ΔT more work is done for constant pressure process so must have greater Δs so curve shallower so goes further along graph.
What is the significance of the Carnot efficiency?
The max poss efficiency for any heat engine operating between 2 thermal energy reservoirs
What does the Carnot cycle look like on a Ts graph?
Rectangle
What are the four stages in the Carnot cycle?
1-2) isothermal heat addition
2-3) isentropic exampansion
3-4) isothermal heat rejection
4-1) isentropic compression
Give the Carnot efficiency
η=1-Tl/Th=Wout/Qin
Give some assumptions of the Otto cycle.
-closed system so use NFEE
-perfect gas
Assume compression and expansion are adiabatic
Give the 4 stages of an Otto cycle.
1-2) isentropic compression
2-3) const vol heating
3-4) isentropic expansion
4-1) const vol cooling
Which diagram do we use to represent the Otto cycle? Why?
Pv. Can measure P. Know where piston is so know volume
What is the compression ratio?
Realationship between 2 volumes. V1/V2
Give the four stages of a diesel cycle.
1-2) isentropic compression
2-3) const pres heating
3-4) isentropic expansion
4-1) const vol cooling
What is the dual cycle?
Combination of Otto and diesel to give a cycle most similar to that in real life.
Give the five stages of the dual cycle.
1-2) isentropic compression 2-3) const vol heating 3-4) const pres heating 4-5) isentropic expansion 5-1) const vol cooling
Give the efficiency of the dual cycle.
η=1-Qout/Qin=1-Cv(T5-T1)/(Cv(T3-T2)+Cp(T4-T3))
Give the efficiency of the joule/brayton cycle.
η=Wnet/Qin=(T4-T5)/(T3-T2)
What is the latent heat of vaporisation?
The heat required to turn saturated water completely into saturated vapour
Which diagram do we use for joule/brayton? And how may stages are there?
Ts. Five stages
Giv ethe first law of thermodynamics.
Conservation of energy. Energy cannot be created to destroyed.
Give the second law of thermodynamics
Heat will naturally migrate from a high temp to a low temp
