Lecture 2 - Fundamentals Flashcards
what are the terms in the steady flow energy equation
enthalpy + kinetic + potential = heat in - work done
how do you convert the SFEE to per unit time
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
what are the assumptions of the SFEE
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
simplify SFEE for a turbine or pump
no heat (adiabatic), no change in height, kinetic energy much smaller than enthalpy, -w = h2 - h1
SFEE for a boiler or condensor
no work is done on/by the fluid, no change in potential, no significant change in velocity
Q = h2 - h1
what do a boiler or condensor do
add or remove heat at constant pressure
SFEE for a diffuser and nozzle
adiabatic and no work transfer takes place, velocity change is significant
h1 - h2 = 1/2(C2^2 - C1^2)
what do a diffuser and nozzle do
convert between enthalpy and kinectic energy
What is the 2nd law of thermo
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
What does the 2nd law of thermo prevent
heat transferring from cold to hot
equation of efficiency
eta = what we want / what we pay for
What is entropy
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
What has more entropy a hot or cold cup of cofee
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
entropy must always
increase
what is an isentropic process
one in which entropy remains the same
on an enthalpy entropy diagram a real process will always
move to the right
isentropic efficiency of an expansion
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
what should efficiency always be
less than 1
isentropic efficiency of a compression
h2s - h1/ h2 - h1 = ideal / real
always remember steam is
not an ideal gas -> must use steam tables as you cannot use ideal gas formula
What is the equation of mixture dryness fraction
x = stotal - sf / sg - sf, values can be replaced by enthalpy as well
how would you find hg, hf for steam
look in lbot, look at pressure and temperature of saturated steam to find hf and hg for particular situation
why is the equation of efficiency for compression and expansion different
(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
in a simple heat engine how can the efficiency be calculated
Q1 - Qs = W
eta = (Q1 - Q2)/Q1 = 1 - Q2/Q1
If reversible
eta = 1 - T2/T1
What is entropy equal to
integral from 1 to 2 of dQ/T = s2 - s1 = ds
What is entropy a consequence of
consequence of second law of thermodynamics in the same way as internal energy arises from the first law of thermodynamics
Draw and label a Ts diagram for steam
compressed liquid region Saturated liquid line Critical Point Saturated liquid vapour region Saturated vapour line Superheated vapour region
What is there a flat region under the saturation line on steam ts diagram
as heat must be added to convert the steam from liquid to vapour increasing the entropy of the system but not increasing the temperature
