Lecture 2 - Fundamentals Flashcards

1
Q

what are the terms in the steady flow energy equation

A

enthalpy + kinetic + potential = heat in - work done

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2
Q

how do you convert the SFEE to per unit time

A

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

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3
Q

what are the assumptions of the SFEE

A

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

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4
Q

simplify SFEE for a turbine or pump

A
no heat (adiabatic), no change in height, kinetic energy much smaller than enthalpy, 
-w = h2 - h1
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5
Q

SFEE for a boiler or condensor

A

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

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6
Q

what do a boiler or condensor do

A

add or remove heat at constant pressure

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7
Q

SFEE for a diffuser and nozzle

A

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

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8
Q

what do a diffuser and nozzle do

A

convert between enthalpy and kinectic energy

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9
Q

What is the 2nd law of thermo

A

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

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10
Q

What does the 2nd law of thermo prevent

A

heat transferring from cold to hot

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11
Q

equation of efficiency

A

eta = what we want / what we pay for

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12
Q

What is entropy

A

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

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13
Q

What has more entropy a hot or cold cup of cofee

A

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

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14
Q

entropy must always

A

increase

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15
Q

what is an isentropic process

A

one in which entropy remains the same

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16
Q

on an enthalpy entropy diagram a real process will always

A

move to the right

17
Q

isentropic efficiency of an expansion

A

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

18
Q

what should efficiency always be

A

less than 1

19
Q

isentropic efficiency of a compression

A

h2s - h1/ h2 - h1 = ideal / real

20
Q

always remember steam is

A

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

21
Q

What is the equation of mixture dryness fraction

A

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

22
Q

how would you find hg, hf for steam

A

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

23
Q

why is the equation of efficiency for compression and expansion different

A

(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

24
Q

in a simple heat engine how can the efficiency be calculated

A

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

25
Q

What is entropy equal to

A

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

26
Q

What is entropy a consequence of

A

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

27
Q

Draw and label a Ts diagram for steam

A
compressed liquid region
Saturated liquid line
Critical Point
Saturated liquid vapour region
Saturated vapour line
Superheated vapour region
28
Q

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

A

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