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Thermodynamics II: 1st Law of Thermodynamics

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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

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Heat Exchangers Def’n: device used to transfer thermal energy from one substance to another Def’n: 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)

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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

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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

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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

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First Law of Thermodynamics

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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 (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 (thermo)

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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

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Thermodynamic System Def’n: a bounded region that contains matter (which may be in gas, liquid, or solid phase) Def’n: 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)

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Thermodynamic Processes Def’n: any physical occurrence during which an effect is produced by the transformation or redistribution of energy Def’n: 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

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Non-Flow Process Process in which the working fluid does not flow into or out of its container in the course of the process (closed system) Process in which the working fluid does not flow into or out of its container in the course of the process (closed system) 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

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Steady Flow Process Process in which the working substance flows steadily and uniformly through some device (i.e., a turbine) (open system) Process in which the working substance flows steadily and uniformly through some device (i.e., a turbine) (open system) 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 xfer & work performed is constant Net rate of heat xfer & work performed is constant

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Processes - Flow Work Def’n: mechanical energy necessary to maintain the flow of fluid in a system Def’n: 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)

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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 (divide by lbm) h = u + Pv (divide by lbm)

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