thermodynamics i: review how substances are changed by heat the main changes that substances undergo...

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 Latent Heat of Vaporization? Condensation? Fusion? Saturated Liquid/Vapor? Subcooled Liquid? Superheated Vapor? Saturation Temperature? Saturation Pressure?

Thermodynamics II: 1st Law of Thermodynamics

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

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)

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

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

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

First Law of Thermodynamics

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)

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

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)

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

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

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

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

Non-Flow Process

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

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)

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)

Mollier Diagram

Questions?

Thermodynamics III: 2nd Law & Cycles It just dont get no better than this

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

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

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

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

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

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

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

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)

Basic Thermodynamic Cycle HEAT SOURCE HEAT SINK Pump EngineW Q in Q out Working Substance

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

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

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

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

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.

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

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

Thermal Efficiency

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 Cycle T Source T Sink Pump EngineW Q in Q out Working Substance

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

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:

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?

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

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thermodynamics i: review how substances are changed by heat the main changes that substances undergo...

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