Thermodynamics I: REVIEW How Substances Are Changed By Heat The main changes that substances undergo...
50
Thermodynamics I: REVIEW How Substances Are Changed By Heat The main changes that substances undergo when they are heated are (1) increase in temperature, (2) change of state, and (3) expansion. Latent Heat the amount of heat energy that must be absorbed or released by a given quantity of a substance to bring about a complete change of state in the substance. Sensible Heat?
Thermodynamics I: REVIEW How Substances Are Changed By Heat The main changes that substances undergo when they are heated are (1) increase in temperature,
Thermodynamics I: REVIEW How Substances Are Changed By Heat The
main changes that substances undergo when they are heated are (1)
increase in temperature, (2) change of state, and (3) expansion.
Latent Heat the amount of heat energy that must be absorbed or
released by a given quantity of a substance to bring about a
complete change of state in the substance. Sensible Heat?
Objectives Comprehend the principles of operation of various
heat exchangers Comprehend the principles of operation of various
heat exchangers Understand boundary layers Understand boundary
layers Comprehend the First Law of Thermo Comprehend the First Law
of Thermo Comprehend the basic principles of open/closed thermo
systems Comprehend the basic principles of open/closed thermo
systems Comprehend thermo processes Comprehend thermo
processes
Slide 6
Heat Exchangers Heat Exchangers Defn: device used to transfer
thermal energy from one substance to another Defn: device used to
transfer thermal energy from one substance to another Direction of
Flow Direction of Flow -> Parallel: not used by Navy ->
Parallel: not used by Navy -> Counter: more efficient; used by
Navy -> Counter: more efficient; used by Navy -> Cross: used
extensively -> Cross: used extensively Number of passes (single
or multiple) Number of passes (single or multiple)
Slide 7
Heat Exchangers Type of Contact Type of Contact Direct: mixing
of substances; pour hot into cold Direct: mixing of substances;
pour hot into cold Indirect/surface: no direct contact; some thin
barrier used Indirect/surface: no direct contact; some thin barrier
used Phases of Working Substance Phases of Working Substance
liquid-liquid: PLO cooler liquid-liquid: PLO cooler liquid-vapor:
condenser liquid-vapor: condenser vapor-vapor: radiator in home
steam-heat vapor-vapor: radiator in home steam-heat
Slide 8
Heat Exchangers Boundary layer/film: w/in pipes or channels of
fluid flow, the fluid adjacent to the wall is stagnant Boundary
layer/film: w/in pipes or channels of fluid flow, the fluid
adjacent to the wall is stagnant -> local temp increases ->
local temp increases -> T metal decreases -> T metal
decreases -> amount of heat transfer decreases -> amount of
heat transfer decreases -> reduced efficiency & possible
damage -> reduced efficiency & possible damage Try to
minimize film by adjusting flow or increasing turbulence Try to
minimize film by adjusting flow or increasing turbulence
Slide 9
Heat Exchangers Should be made of materials that readily
conduct heat & have minimal corrosion Should be made of
materials that readily conduct heat & have minimal corrosion
Maximize surface area for heat transfer Maximize surface area for
heat transfer Minimize scale, soot, dirt, & fouling ->
reduces heat transfer, efficiency, & causes damage Minimize
scale, soot, dirt, & fouling -> reduces heat transfer,
efficiency, & causes damage
Slide 10
First Law of Thermodynamics
Slide 11
Slide 12
Principle of Conservation of Energy: Principle of Conservation
of Energy: energy can neither be created nor destroyed, only
transformed (generic) energy can neither be created nor destroyed,
only transformed (generic) energy may be transformed from one form
to another, but the total energy of any body or system of bodies is
a quantity that can be neither increased nor diminished by the
action of the body or bodies (thermo) energy may be transformed
from one form to another, but the total energy of any body or
system of bodies is a quantity that can be neither increased nor
diminished by the action of the body or bodies (thermo) The total
quantity of energy in the universe is constant (broad) The total
quantity of energy in the universe is constant (broad)
Slide 13
First Law of Thermodynamics General Energy Equation General
Energy Equation Energy In = Energy Out, OR Energy In = Energy Out,
OR U 2 - U 1 = Q - W (or u 2 - u 1 = q - w) U 2 - U 1 = Q - W (or u
2 - u 1 = q - w) Where: Where: U 1 = internal energy of system @
start U 1 = internal energy of system @ start U 2 = internal energy
of system @ end U 2 = internal energy of system @ end Q = net
thermal energy flowing into system during process Q = net thermal
energy flowing into system during process W = net work done by the
system W = net work done by the system
Slide 14
Thermodynamic System Defn: a bounded region that contains
matter (which may be in gas, liquid, or solid phase) Defn: a
bounded region that contains matter (which may be in gas, liquid,
or solid phase) Requires a working substance to receive, store,
transport, or deliver energy Requires a working substance to
receive, store, transport, or deliver energy May be open (mass can
flow in/out) or closed (no flow of mass out of boundaries) May be
open (mass can flow in/out) or closed (no flow of mass out of
boundaries)
Slide 15
Closed System Mass is constant Mass is constant Energy is added
as heat from the flame Energy is added as heat from the flame Work
by the system in the turbine Work by the system in the turbine
Energy is removed in the condenser Energy is removed in the
condenser Work on the fluid by Work on the fluid by the pump
Slide 16
Open system 1 2 z Mass enters and leaves the system Energy
enters and leaves the system Give an equation representing the 1 st
law for this open system
Slide 17
Slide 18
Thermodynamic Processes Defn: any physical occurrence during
which an effect is produced by the transformation or redistribution
of energy Defn: any physical occurrence during which an effect is
produced by the transformation or redistribution of energy
Describes what happens within a system Describes what happens
within a system Two classifications: non-flow & steady flow Two
classifications: non-flow & steady flow
Slide 19
Non-Flow Process Process in which the working fluid does not
flow into or out of its container in the course of the process
Process in which the working fluid does not flow into or out of its
container in the course of the process Energy In = Energy Out
Energy In = Energy Out Q - W = U 2 - U 1 Q - W = U 2 - U 1 Example:
Piston being compressed Example: Piston being compressed
Slide 20
Non-Flow Process
Slide 21
Steady Flow Process Process in which the working substance
flows steadily and uniformly through some device (i.e., a turbine)
Process in which the working substance flows steadily and uniformly
through some device (i.e., a turbine) Assumptions (at any cross
section): Assumptions (at any cross section): Properties of fluid
remain constant Properties of fluid remain constant Average
velocity of fluid remains constant Average velocity of fluid
remains constant System is always filled so vol in = vol out System
is always filled so vol in = vol out Net rate of heat transfer
& work performed is constant Net rate of heat transfer &
work performed is constant
Slide 22
Processes - Flow Work Defn: mechanical energy necessary to
maintain the flow of fluid in a system Defn: mechanical energy
necessary to maintain the flow of fluid in a system Although some
energy has been expended to create this form of energy, it still
represents a stored (kinetic) energy which can be used Although
some energy has been expended to create this form of energy, it
still represents a stored (kinetic) energy which can be used Flow
work = pressure x volume (PV) Flow work = pressure x volume
(PV)
Slide 23
Processes - Enthalpy Enthalpy: the total energy of the fluid
due to both internal energy & flow energies Enthalpy: the total
energy of the fluid due to both internal energy & flow energies
Represents the heat content or total heat Represents the heat
content or total heat Enthalpy (H) Enthalpy (H) H = U + PV (in
ft-lb, BTU, or Joules) H = U + PV (in ft-lb, BTU, or Joules) h = u
+ Pv (specific when divided by lbm) h = u + Pv (specific when
divided by lbm)
Slide 24
Mollier Diagram
Slide 25
Questions?
Slide 26
Thermodynamics III: 2nd Law & Cycles It just dont get no
better than this
Slide 27
Objectives Understand types of state changes Understand types
of state changes Comprehend thermodynamic cycles Comprehend
thermodynamic cycles Comprehend the 2nd Law of Thermodynamics to
include entropy, reversibility, & the Carnot cycle Comprehend
the 2nd Law of Thermodynamics to include entropy, reversibility,
& the Carnot cycle Determine levels of output and efficiency in
theoretical situations Determine levels of output and efficiency in
theoretical situations
Slide 28
Processes In addition to using flow/no-flow classifications for
thermo processes, it is helpful to look at what happens to a medium
also In addition to using flow/no-flow classifications for thermo
processes, it is helpful to look at what happens to a medium also
The terms used to describe the process are clues for handling
specific terms in the eneral energy equation The terms used to
describe the process are clues for handling specific terms in the
eneral energy equation These clues help to alter the general
equation to the specific situation we are examining These clues
help to alter the general equation to the specific situation we are
examining
Slide 29
Processes Isobaric: Isobaric: pressure remains constant
throughout process (some pistons) pressure remains constant
throughout process (some pistons) Results in a change in enthalpy
(h) Results in a change in enthalpy (h) q 12 = h 2 - h 1 q 12 = h 2
- h 1 Isometric: Isometric: volume remains constant during entire
process volume remains constant during entire process Results in a
change in internal energy Results in a change in internal energy q
12 = u 2 - u 1 q 12 = u 2 - u 1
Slide 30
Processes Isenthalpic: Isenthalpic: Enthalpy remains constant
Enthalpy remains constant Throttling processes Throttling processes
h 1 = h 2 h 1 = h 2 Isothermal: Isothermal: Temperature remains
constant Temperature remains constant Inside a steam generator
(S/G) or boiler during steady state conditions Inside a steam
generator (S/G) or boiler during steady state conditions Inside a
condenser in a steam plant Inside a condenser in a steam plant
Slide 31
Thermodynamic Cycles Defn: a recurring series of thermodynamic
processes through which an effect is produced by transformation or
redistribution of energy Defn: a recurring series of thermodynamic
processes through which an effect is produced by transformation or
redistribution of energy One classification: One classification:
Open: working fluid taken in, used, & discarded Open: working
fluid taken in, used, & discarded Closed: working medium never
leaves cycle, except through leakage; medium undergoes state
changes & returns to original state Closed: working medium
never leaves cycle, except through leakage; medium undergoes state
changes & returns to original state
Slide 32
Thermodynamic Cycles Cycles are classified according to the
disposition of the working substance and where heating occurs
Cycles are classified according to the disposition of the working
substance and where heating occurs Open/Closed Cycle Open/Closed
Cycle Heated/Unheated Engine Heated/Unheated Engine
Slide 33
Five Basic Elements of all Cycles Working substance: transports
energy within system Working substance: transports energy within
system Heat source: supplies heat to the working medium Heat
source: supplies heat to the working medium Engine: device that
converts the thermal energy of the medium into work Engine: device
that converts the thermal energy of the medium into work Heated:
heat added in engine itself Heated: heat added in engine itself
Unheated: heat received in some device separate from engine
Unheated: heat received in some device separate from engine
Slide 34
Five Basic Elements of all Cycles Heat sink/receiver: absorbs
heat from the working medium Heat sink/receiver: absorbs heat from
the working medium Pump: moves the working medium from the
low-pressure side to the high- pressure side of the cycle Pump:
moves the working medium from the low-pressure side to the high-
pressure side of the cycle Examples: Examples: Closed-the working
fluid is taken in, used and then discarded. (condensing steam power
plant) Closed-the working fluid is taken in, used and then
discarded. (condensing steam power plant) Open working fluid is
taken in, used and discarded. (combustion engine) Open working
fluid is taken in, used and discarded. (combustion engine)
Slide 35
Basic Thermodynamic Cycle HEAT SOURCE HEAT SINK Pump EngineW Q
in Q out Working Substance
Slide 36
Slide 37
Second Law of Thermodynamics Reversibility: Reversibility: the
characteristic of a process which would allow a process to occur in
the precise reverse order, so that the system would be returned
from its final condition to its initial condition, AND the
characteristic of a process which would allow a process to occur in
the precise reverse order, so that the system would be returned
from its final condition to its initial condition, AND all energy
that was transformed or redistributed during the process would be
returned from its final to original form all energy that was
transformed or redistributed during the process would be returned
from its final to original form
Slide 38
Second Law of Thermodynamics Defn 1: (Clausius statement) no
process is possible where the sole result is the removal of heat
from a low-temp reservoir and the absorption of an equal amount of
heat by a high temp reservoir Defn 1: (Clausius statement) no
process is possible where the sole result is the removal of heat
from a low-temp reservoir and the absorption of an equal amount of
heat by a high temp reservoir Defn 2: (Kelvin-Planck) no process is
possible in which heat is removed from a single reservoir w/ equiv
amount of work produced Defn 2: (Kelvin-Planck) no process is
possible in which heat is removed from a single reservoir w/ equiv
amount of work produced
Slide 39
Second Law of Thermodynamics Implications: Implications: A
thermodynamic process will never yield, in the form of work, ALL
the energy supplied to it. A thermodynamic process will never
yield, in the form of work, ALL the energy supplied to it. No
engine, actual or ideal, can convert all the heat supplied to it
into work, since some of heat must be rejected to a receiver that
is at a lower temperature than the source. No engine, actual or
ideal, can convert all the heat supplied to it into work, since
some of heat must be rejected to a receiver that is at a lower
temperature than the source. No thermodynamic cycle can be 100%
efficient No thermodynamic cycle can be 100% efficient
Slide 40
Second Law of Thermodynamics Quick review: Quick review: 1st
Law: Conservation/transformation of energy 1st Law:
Conservation/transformation of energy 2nd Law: Limits the direction
of processes & extent of heat-to-work conversions 2nd Law:
Limits the direction of processes & extent of heat-to-work
conversions
Slide 41
Entropy Defn: theoretical measure of thermal energy that cannot
be transformed into mechanical work in a thermodynamic system Defn:
theoretical measure of thermal energy that cannot be transformed
into mechanical work in a thermodynamic system It is an index of
the unavailability of energy or the reversibility of a process It
is an index of the unavailability of energy or the reversibility of
a process In all real processes, entropy never decreases ->
entropy of universe is always rising In all real processes, entropy
never decreases -> entropy of universe is always rising Entropy
is determined by the quantity of heat in a system which is capable
of doing work. Entropy is determined by the quantity of heat in a
system which is capable of doing work.
Slide 42
Carnot Cycle Second Law states that no thermo system can be
100% efficient, and no real thermal process is completely
reversible Second Law states that no thermo system can be 100%
efficient, and no real thermal process is completely reversible A
French engineer, Carnot, set out to determine what the max
efficiency of a cycle would be if that cycle were ideal and
completely reversible A French engineer, Carnot, set out to
determine what the max efficiency of a cycle would be if that cycle
were ideal and completely reversible
Slide 43
Carnot Cycle All heat is supplied at a single high temp and all
heat is rejected at a single low temp All heat is supplied at a
single high temp and all heat is rejected at a single low temp
Carnot used a simple cycle Carnot used a simple cycle Thermal
efficiency = (T s T r )/T s Thermal efficiency = (T s T r )/T s T s
= absolute temp flows from source T r = absolute temp at which heat
rejected
Slide 44
Thermal Efficiency
Slide 45
Carnot Cycle All heat is supplied at a single high temp and all
heat is rejected at a single low temp All heat is supplied at a
single high temp and all heat is rejected at a single low temp
Slide 46
Carnot Cycle T Source T Sink Pump EngineW Q in Q out Working
Substance
Slide 47
Carnot Cycle Carnot Principle: the max thermal efficiency
depends only on the difference between the source and sink temps
Carnot Principle: the max thermal efficiency depends only on the
difference between the source and sink temps Does not depend on
property of fluid, type of engine, friction, or fuel Does not
depend on property of fluid, type of engine, friction, or fuel T=
absolute Temperature
Slide 48
Carnot Cycle Carnot Principle: the max thermal efficiency
depends only on the difference between the source and sink temps
Carnot Principle: the max thermal efficiency depends only on the
difference between the source and sink temps Does not depend on
property of fluid, type of engine, friction, or fuel Does not
depend on property of fluid, type of engine, friction, or fuel
Example: Example:
Slide 49
An inventor claims to have an engine that receives 100 Btu of
heat and produces 25 Btu of useful work when operating between a
source at 140F and a receiver at 0F. Is this a valid claim? An
inventor claims to have an engine that receives 100 Btu of heat and
produces 25 Btu of useful work when operating between a source at
140F and a receiver at 0F. Is this a valid claim?
Slide 50
Take Aways Know and apply the 1 st and 2 nd Laws Know and apply
the 1 st and 2 nd Laws Give the general equation and define the
terms Give the general equation and define the terms Give the
attributes of a closed system Give the attributes of a closed
system State the assumptions of steady flow process State the
assumptions of steady flow process Define enthalpy and entropy
Define enthalpy and entropy Apply the concepts of isothermal,
isometric, isobaric and adiabatic Apply the concepts of isothermal,
isometric, isobaric and adiabatic Draw and label a Carnot cycle,
including the associated T-s diagram Draw and label a Carnot cycle,
including the associated T-s diagram Define thermal efficiency and
perform calculations. Define thermal efficiency and perform
calculations. Explain why no thermodynamic process is 100 %
efficient Explain why no thermodynamic process is 100 %
efficient