Thermodynamics II: 1st Law of Thermodynamics

1. Thermodynamics II: 1st Law of Thermodynamics

2. Objectives • Comprehend the principles of operation of various heat exchangers • Understand boundary layers • Comprehend the First Law of Thermo • Comprehend the basic principles of open/closed thermo systems • Comprehend thermo processes

3. Heat Exchangers • Def’n: device used to transfer thermal energy from one substance to another • Direction of Flow • -> Parallel: not used by Navy • -> Counter: more efficient; used by Navy • -> Cross: used extensively • Number of passes (single or multiple)

4. Heat Exchangers • Type of Contact • Direct: mixing of substances; pour hot into cold • Indirect/surface: no direct contact; some thin barrier used • Phases of Working Substance • liquid-liquid: PLO cooler • liquid-vapor: condenser • vapor-vapor: radiator in home steam-heat

5. Heat Exchangers • Boundary layer/film: w/in pipes or channels of fluid flow, the fluid adjacent to the wall is stagnant • -> local temp increases • -> DT metal decreases • -> amount of heat transfer decreases • -> reduced efficiency & possible damage • Try to minimize film by adjusting flow or increasing turbulence

6. Heat Exchangers • Should be made of materials that readily conduct heat & have minimal corrosion • Maximize surface area for heat transfer • Minimize scale, soot, dirt, & fouling -> reduces heat transfer, efficiency, & causes damage

7. First Law of Thermodynamics

8. First Law of Thermodynamics

9. First Law of Thermodynamics • Principle of Conservation of Energy: • 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)

10. First Law of Thermodynamics • General Energy Equation • Energy In = Energy Out, OR • U2 - U1 = Q - W (or u2 - u1 = q - w) • Where: • U1 = internal energy of system @ start • U2 = internal energy of system @ end • Q = net thermal energy flowing into system during process • W = net work done by the system

11. Thermodynamic System • 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 • May be open (mass can flow in/out) or closed (no flow of mass out of boundaries)

12. Thermodynamic Processes • Def’n: any physical occurrence during which an effect is produced by the transformation or redistribution of energy • Describes what happens within a system • Two classifications: non-flow & steady flow

13. 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) • Energy In = Energy Out • Q - W = U2 - U1 • Example: Piston being compressed

14. Steady Flow Process • Process in which the working substance flows steadily and uniformly through some device (i.e., a turbine) (open system) • Assumptions (at any cross section): • Properties of fluid remain constant • Average velocity of fluid remains constant • System is always filled so volin = volout • Net rate of heat xfer & work performed is constant

15. Processes - Flow Work • 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 • Flow work = pressure x volume (PV)

16. Processes - Enthalpy • Enthalpy: the total energy of the fluid due to both internal energy & flow energies • Represents the “heat content” or “total heat” • Enthalpy (H) • H = U + PV (in ft-lb, BTU, or Joules) • h = u + Pv (divide by lbm)

17. Questions?