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THERMODYNAMICS - CHAPTER 3 ENERGY TRANSFER AND GENERAL ANALYSIS (TOTAL…
THERMODYNAMICS - CHAPTER 3
ENERGY TRANSFER AND GENERAL ANALYSIS
TOTAL ENERGY
OF A SYSTEM
E (kJ)
in the absence of other forms
, E includes internal, kintetic, and potential energies (also other forms)
=
thermal
SEE microscopic energies
HEAT = HEAT TRANSFER
adiabatic processes
= NO
heat transfer
. NOTE: adiabatic process is different from isothermal processes. :<3:
IMPORTANT CHARACTERISTICS OF HEAT AND WORK TRANSFERS
P. 63
mechanical enery e mech
Mechanical energy
of a flowing fluid per unit mass
:<3:
potential energy
OR
pe = g. z (kJ/kg)
z: elevation of the center of gravity of a system relative to some arbitrarily selected reference level
kinetic enery KE
OR kinetic energy per unit mass
:<3:
Rate of mechanical energy of a
flowing fluid
Mechanical energy change of a fluid during incompressible flow per unit mass (kJ/kg)
Rate of mechanical energy change of a fluid during incompressible flow (kW)
remarks for KE and PE
: for
stationary processes
, there is NO change in KE and PE.
In other words
,for the stationary processes ΔE = ΔU :<3:
FIRST LAW OF THERMODYNAMICS
:
While the mechanisms of Energy Transfer, E in and E out
:
E in - E out = (Q in - Q out) + (W in - W out) + (M in - M out)
adiabatic processes: NO heat transfer
closed system: NO mass transfer
For a closed system undergoing a cycle ( a cycle: the final state is identical to the initial state) or Ein = E out, therefore:
W net,out = Qnet, in (or W = Q)
W net, out =
W out - W in
(when sign convention is used)
Q net, in = Q in - Q out
Note the differential form
Energy change of a system
during a process
AND
WITH
MECHANICAL EFFICIENCY OF A DEVICE OR A PROCESS
=
For pumps
w' pump,u: the useful pumping power supplied to the fluid.
electric
magnetic
chemical
nuclear
microscopic forms
Sum of E microscopic =
internal energy
related to molecular structure and degree of molecular activity
Thermal = Sensible + Latent
Internal = Sensible + Latent + Chemical + Nuclear
macroscopic forms
static vs. dynamic forms
static forms
: The total energy of a system, can be contained or stored in a system
dynamic forms
(or
energy interactions
): The form of energy not stored in a system
heat transfer
work
internal
: see microscopic forms
mass flow rate
(kg/s)
Energy flow rate
:
E transfer in a
closed system
HEAT TRANSFER
driving force = temparature difference
Heat transfer per unit mass: q = Q/m (kJ/kg)
Amount of heat transfer Q when heat transfer rate Q' = constant (kJ): Q = Q'.Δt
Q when heat transfer rate is changed with time Q = ∫ t1 to t2 of Q'.dt
adiabatic syst
: no heat with its surroundings Q = 0 :red_flag:
HT mechanisms (slide 11 - chapter 3)
conduction
convection
radiation
WORK
E transfer = force acting through a distance
to RAISE a body
w = change in the potential energy of the body
to ACCELERATE a body
W = change in the kinetic energy of the body
mechanical forms
:
there must be a force acting on the boundary.
– the boundary must move.
Work = Force x Distance OR W = F . s (kJ)
when force is not constant: W = ∫ 1 to 2 of F. ds
Power = work done per unit time W' (kW)
Work done per unit mass w = W/m (kJ/kg)
Shaft work
: A force F acting through a moment arm r
generates a torque T
W sh = F.s with s = (2pi.r) n
as F acts through a distance s
Power transmitted through the shaft
W' sh
= shaft work done per unit time
with n' =
SPRING WORK
When the length of the spring changes by a differential amount dx under the influence of a force F, the work done is
For linear elastic springs, the displacement
x is proportional to the force applied; OR F = k.x (kN) with k: spring constant (kN/m).
The displacement of a linear spring doubles when the force is doubled
W spring = 1/2 k (x2^2 - x1^2)
x1 and x2: the initial and the final displacements
Work done on Elastic solid bars - SLIDE 17
Electrical work - SLIDE 18
non-mechanical forms
Some definitions for a thermodynamic cycle
A process: the final and initial states are identical
AND
∆U = 0 for a
whole
numbers of cycles.
composed of processes that cause the working fluid to undergo a series of state changes through a process or a series of processes
first law for
a closed system
operating in a thermodynamic cycle
∆E cycle = Q net - W net
AND
Q net = W net
OR: Q in - Q out = W out - W in
Energy conversion efficiiencies
Performance = Desired output / Required input
Efficiency of a water heater
total energy of a system on a unit mass
e
= E / m (
kJ/kg
)
OUTLINES:
understanding of the most important points
What is E, e, and E'
what are the forms of E and the three common forms
what is the 1st law of thermodynamics
what is mechanical energy, mechanical efficiency of pumps and turbines
What is the motor efficiency?
EXERCISE
how to define property values of pure substances?
https://www.rose-hulman.edu/~adams1/courses/me301/files/Property_algorithm.pdf
