Lecture 5

Question 1

SFEE

Answer 1

steady flow energy equation

Question 2

enthalpy

Answer 2

internal energy + work done

Question 3

why are we able to simplify the SFEE to

Q - W = m (h2 -h1) (all with respect to time

Answer 3

as enthalpy is often by 1000 times the largest terms so can ignore kinectic and potential terms especially for gases

Question 4

simplify the SFEE to bernoullis equation

Answer 4

internal energy does not change and heat is not added
use density rather than specific volume (v = 1/density)
treat density as constant
no work input

Question 5

for closed systemm

Answer 5

Q -W =m(u2 -u1)

Question 6

for steady flow thermal systems

Answer 6

SFEE with enthalpy terms

Question 7

for steady flow fluid systems with work

Answer 7

SFEE densities and internal energy removed

Question 8

steady flow fluid systems

Answer 8

bernoullis equation

Question 9

what does a turbine do

Answer 9

extracts KE from flow and turns it into motion

Question 10

draw a diagram of a turbine

Answer 10

take in high temperature and pressure fluid

out goes low temperature and pressure fluid

Question 11

compressor takes

Answer 11

low temperature low pressure fluid and compresses it doing work on the fluid

Question 12

in a powerstation what is the name of compresses that pressurise the water

Answer 12

feed pumps

Question 13

work is needed to drive a compressor or tubine

Answer 13

work is needed to drive a compressor work is extracted from a turbine

Question 14

SFEE simplification for turbines and compressors

Answer 14

adiabatic Q = 0

gases and steam kinetic and potential energy terms are negligible

Question 15

SFEE eqation for turbine and compressor

Answer 15

-W = mass flow rate (h2 - h1)

or workrate = mass flow rate (h1-h2)

Question 16

SFEE wind and water turbines

Answer 16

kinetic energy term and enthalpy term

Question 17

Maximum work out

Answer 17

always from reversible process

Question 18

isentropic is

Answer 18

maximum work possible but never achieved in areal system

Question 19

for ideal gas undergoing isentropic adiabatic process

Answer 19

n = gamma = cp/cv
ideal work is
mass flow rate * cp * (T1-T2x)
cp as open system

Question 20

for steam undergoing isentropic adiabatic process

Answer 20

entropy at the start = entropy at the end
entropy at start know for p and t
can calculate final as entropy will be the same

Question 21

draw diagram of turbine isentropic and real case including equations

Answer 21

W = m (h1 - h2s)
W = eff *m(h1-h2s)

Question 22

for an ideal gas going through turbine how do calculate the change in temperature

Answer 22

T1 - T2 = eff * (T1-T2s)

W = mass flow rate cp(T1 -T2s)

Question 23

in a turbine in comparison to the isentropic temperature what should the final temperature be

Answer 23

exit temperature is higher

Question 24

For steam turbine efficiency

Answer 24

W = m (h1-h2)
h2 = h1 - eff * (h1 -h2s)
find T2 by finding temp with that enthalpy

Question 25

should W be positive for a turbine

Answer 25

yes as turbine extracts work | it is work done by the system

Question 26

should W be positive or negative for a compressor

Answer 26

negative as compressor doing work on the system

Question 27

for a compressor you have to put more

Answer 27

work in than the isentropic amount

Question 28

feedpump is a

Answer 28

compresses liquid water in a powerstation | water incompressible therefore volume cannot change

Question 29

for an isentropic feed pump

Answer 29

ds = 0 and q = 0 so dh =v dp at constant volume dh = v(p2 - p1) = -w

Question 30

graph diagram of a feedpump

Answer 30

see powerpoint

Question 31

nozzle takes

Answer 31

a low velocity flow at high pressure and temperature and it undergoes an expansion increasing KE

Question 32

nozzle takes

Answer 32

a low velocity flow at high pressure and temperature and it undergoes an expansion increasing KE enthalpy of fluid decreases

Question 33

nozzle

Answer 33

undergoes an expansion

Question 34

diffuser

Answer 34

expansion

Question 35

diffuser takes

Answer 35

high velocity flow and slows it down increases the enthalpy of the fluid pressure will increases

Question 36

nozzle and diffuser simplification of SFEE

Answer 36

everything including work term goes apart from speed and enthalpy

Question 37

isentropic nozzle drawing

Answer 37

see powerpoint

Question 38

if nozzle is not 100% efficient what would happen to exit flow

Answer 38

it would be slower but hotter

Question 39

often assume which end of nozzle and diffuser

Answer 39

that velocity is zero wide end of diffuser or nozzle

Question 40

real devices difference

Answer 40

heat transfer through devices | irreversible pressure loss in fluid flow (mostly due to turbulent losses)