Dr Murphy

Question 1

Approx how may kPa and how many bar are in 1 atmos?

Answer 1

100kPa

1bar

Question 2

Give gauge pressure in terms of absolute and atmospheric pressure

Answer 2

Absolute pressure minus atmospheric pressure

Question 3

What is meant by a reversible process?

Answer 3

Both the fluid and the surroundings can always be restored to initial conditions

Question 4

On a diagram how is a reversible and how is an irreversible reaction shown?

Answer 4

Reversible solid line

Irreversible dotted line

Question 5

Give the first law of thermodynamics in an equation

Answer 5

Qin - Wout =ΔU

Question 6

Give the non flow energy equation

Answer 6

Qin- Wout = ΔU

Question 7

Give the steady flow energy equation (simplified)

Answer 7

Q’in - W’out = ΔKE’ + ΔPE’ + ΔH’

Where ‘ represents dot {first differential wrt time}

Question 8

Give the equation for m dot

Answer 8

=CAρ

Question 9

Give an equation for h

Answer 9

h=Pv+u

Question 10

Describe a perfect gas

Answer 10

Theoretically, ideally behaved gas.

Temp vastly in excess of evaporation point.

Question 11

Give boyles law and describe what happens to the graph as temp increases.

Answer 11

At constant temp: Pv=constant.

As temp increases curve moves further from origin

Question 12

Give Charles’s laws and the combination if the 2. What is the constant in the combination?

Answer 12

At constant P: v/T=constant.
At constant v: P/t=constant.
Therefore Pv/t=constant=R=gas constant

Question 13

Give the equation of state

Answer 13

PV=mRT

Question 14

Give the equation for specific heat capacity.

Answer 14

Q=mCΔT

Question 15

Give the equations for the 2 specific heat capacities multiplied by the change in temp.

Answer 15

Cv(T2-T1)=u2-u1

Cp(T2-T1)=h2-h1

Question 16

What does Cp equal in terms of Cv?

Answer 16

Cp=R+Cv

Question 17

What does isochoric mean?

Answer 17

Constant volume

Question 18

What does isobaric mean?

Answer 18

Constant pressure

Question 19

What does isothermal mean?

Answer 19

Constant temp

Question 20

What does adiabtaic or isentropic mean?

Answer 20

No heat loss

Question 21

What is work done equal to?

Answer 21

Area under Pv curve. So W=integral of Pdv

Question 22

Give the efficiency of a heat engine.

Answer 22

1-Qout/Qin

Question 23

What is a heat engine?

Answer 23

A thermodynamic system to which net heat is transferred and from which net work is produced.

Question 24

What is a heat pump?

Answer 24

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

Question 25

Give the general equation for coefficient of performance

Answer 25

COP=desired product/required input

Question 26

What is Q on a temperature entropy diagram?

Answer 26

Area under curve. Q=integral of Tds

Question 27

Give a condition for ΔS

Answer 27

ΔS greater than or equal to 0

Question 28

On a Ts graph which is steeper: constant vol or constant pres? Why?

Answer 28

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.

Question 29

What is the significance of the Carnot efficiency?

Answer 29

The max poss efficiency for any heat engine operating between 2 thermal energy reservoirs

Question 30

What does the Carnot cycle look like on a Ts graph?

Answer 30

Rectangle

Question 31

What are the four stages in the Carnot cycle?

Answer 31

1-2) isothermal heat addition 2-3) isentropic exampansion 3-4) isothermal heat rejection 4-1) isentropic compression

Question 32

Give the Carnot efficiency

Answer 32

η=1-Tl/Th=Wout/Qin

Question 33

Give some assumptions of the Otto cycle.

Answer 33

-closed system so use NFEE -perfect gas Assume compression and expansion are adiabatic

Question 34

Give the 4 stages of an Otto cycle.

Answer 34

1-2) isentropic compression 2-3) const vol heating 3-4) isentropic expansion 4-1) const vol cooling

Question 35

Which diagram do we use to represent the Otto cycle? Why?

Answer 35

Pv. Can measure P. Know where piston is so know volume

Question 36

What is the compression ratio?

Answer 36

Realationship between 2 volumes. V1/V2

Question 37

Give the four stages of a diesel cycle.

Answer 37

1-2) isentropic compression 2-3) const pres heating 3-4) isentropic expansion 4-1) const vol cooling

Question 38

What is the dual cycle?

Answer 38

Combination of Otto and diesel to give a cycle most similar to that in real life.

Question 39

Give the five stages of the dual cycle.

Answer 39

``` 1-2) isentropic compression 2-3) const vol heating 3-4) const pres heating 4-5) isentropic expansion 5-1) const vol cooling ```

Question 40

Give the efficiency of the dual cycle.

Answer 40

η=1-Qout/Qin=1-Cv(T5-T1)/(Cv(T3-T2)+Cp(T4-T3))

Question 41

Give the efficiency of the joule/brayton cycle.

Answer 41

η=Wnet/Qin=(T4-T5)/(T3-T2)

Question 42

What is the latent heat of vaporisation?

Answer 42

The heat required to turn saturated water completely into saturated vapour

Question 43

Which diagram do we use for joule/brayton? And how may stages are there?

Answer 43

Ts. Five stages

Question 44

Giv ethe first law of thermodynamics.

Answer 44

Conservation of energy. Energy cannot be created to destroyed.

Question 45

Give the second law of thermodynamics

Answer 45

Heat will naturally migrate from a high temp to a low temp