Lecture 2 - Fundamentals

Question 1

what are the terms in the steady flow energy equation

Answer 1

enthalpy + kinetic + potential = heat in - work done

Question 2

how do you convert the SFEE to per unit time

Answer 2

multiply enthalpy, kinetic, and potential terms by mass flow rate which is then equal to heat in per unit time - work done per unit time

Question 3

what are the assumptions of the SFEE

Answer 3

mass flow at inlet is constant in time and equal to the mass flow at outlet (constant mass flow rate)
the properties at any particular point within the open system do not vary with time
any heat and work crossing the boundary does so at a uniform rate

Question 4

simplify SFEE for a turbine or pump

Answer 4

no heat (adiabatic), no change in height, kinetic energy much smaller than enthalpy, 
-w = h2 - h1

Question 5

SFEE for a boiler or condensor

Answer 5

no work is done on/by the fluid, no change in potential, no significant change in velocity
Q = h2 - h1

Question 6

what do a boiler or condensor do

Answer 6

add or remove heat at constant pressure

Question 7

SFEE for a diffuser and nozzle

Answer 7

adiabatic and no work transfer takes place, velocity change is significant
h1 - h2 = 1/2(C2^2 - C1^2)

Question 8

what do a diffuser and nozzle do

Answer 8

convert between enthalpy and kinectic energy

Question 9

What is the 2nd law of thermo

Answer 9

impossible to construct a system that will operate in a cycle by using heat from a single reservoir and do an equal amount of work on the surroundings

Question 10

What does the 2nd law of thermo prevent

Answer 10

heat transferring from cold to hot

Question 11

equation of efficiency

Answer 11

eta = what we want / what we pay for

Question 12

What is entropy

Answer 12

thermodyanmic quantity representing the unavailability of a systems thermal energy for conversion into mechanical work often interpreted as the degree of disorder or randomness in the system

Question 13

What has more entropy a hot or cold cup of cofee

Answer 13

hot, though a hot cup will go to a lower entropy state as cold as heat is transferred to the surroundings leading to an overall increase in entropy

Question 14

entropy must always

Answer 14

increase

Question 15

what is an isentropic process

Answer 15

one in which entropy remains the same

Question 16

on an enthalpy entropy diagram a real process will always

Answer 16

move to the right

Question 17

isentropic efficiency of an expansion

Answer 17

eta = h1 - h2 / h1 - h2s = real/ ideal
where h1 and h2 are the real starting and finishing points
and h2s is the isentropic finishing point

Question 18

what should efficiency always be

Answer 18

less than 1

Question 19

isentropic efficiency of a compression

Answer 19

h2s - h1/ h2 - h1 = ideal / real

Question 20

always remember steam is

Answer 20

not an ideal gas -> must use steam tables as you cannot use ideal gas formula

Question 21

What is the equation of mixture dryness fraction

Answer 21

x = stotal - sf / sg - sf, values can be replaced by enthalpy as well

Question 22

how would you find hg, hf for steam

Answer 22

look in lbot, look at pressure and temperature of saturated steam to find hf and hg for particular situation

Question 23

why is the equation of efficiency for compression and expansion different

Answer 23

(draw diagram of h s graph to explain) as in a real situation under compression more work must be done for a given change in pressure, whereas in an expansion less work must be done

Question 24

in a simple heat engine how can the efficiency be calculated

Answer 24

Q1 - Qs = W
eta = (Q1 - Q2)/Q1 = 1 - Q2/Q1
If reversible
eta = 1 - T2/T1

Question 25

What is entropy equal to

Answer 25

integral from 1 to 2 of dQ/T = s2 - s1 = ds

Question 26

What is entropy a consequence of

Answer 26

consequence of second law of thermodynamics in the same way as internal energy arises from the first law of thermodynamics

Question 27

Draw and label a Ts diagram for steam

Answer 27

compressed liquid region
Saturated liquid line
Critical Point
Saturated liquid vapour region
Saturated vapour line
Superheated vapour region

Question 28

What is there a flat region under the saturation line on steam ts diagram

Answer 28

as heat must be added to convert the steam from liquid to vapour increasing the entropy of the system but not increasing the temperature