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Energy Transfer By Heat, Work, and Mass

Energy Transfer By Heat, Work, and Mass. Cengel & Boles, Chapter 3. Energy Transfer. Energy transfer to/from closed systems Heat ( Q ) Work ( W ) Energy transfer to/from open systems (control volumes) Heat ( Q ) Work ( W ) Mass flow. Heat.

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Energy Transfer By Heat, Work, and Mass

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  1. Energy Transfer By Heat, Work, and Mass Cengel & Boles, Chapter 3 ME 152

  2. Energy Transfer • Energy transfer to/from closed systems • Heat (Q) • Work (W) • Energy transfer to/from open systems (control volumes) • Heat (Q) • Work (W) • Mass flow ME 152

  3. Heat • Heat (Q) is the transfer of energy due to a temperature difference • a system w/o heat transfer is an adiabatic system • SI units: kJ • Heat rate, , (kJ/s or kW) • Heat per unit mass, q = Q/m • Sign convention: • Q > 0: heat transferred to system from surroundings • Q < 0: heat transferred from system to surroundings ME 152

  4. Heat Transfer Modes • Conduction • transfer of heat through a material due to random molecular or atomic motion; most important in solids • Radiation • transfer of heat due to emission of electromagnetic waves, usually between surfaces separated by a gas or vacuum • Convection • transfer of heat between a solid surface and fluid due to combined mechanisms of i) fluid conduction at surface; ii) fluid flow within boundary layer ME 152

  5. Conduction Heat Transfer • Fourier’s law of conduction: ME 152

  6. Convection Heat Transfer • Newton’s law of “cooling”, or convection: ME 152

  7. Radiation Heat Transfer • Stefan-Boltzmann law of radiation (between a small surface A of emissivity e and large surroundings): ME 152

  8. Work • Work (W) is the energy transfer associated with a force acting through a distance: • Work rate or power • Work per unit mass, w = W/m • Sign convention • W > 0: work done by system on surroundings • W < 0: work done on system by surroundings ME 152

  9. Types of Work • Moving boundary (compression/expansion) work • Shaft work • Spring work • Electrical work • Other forms; work associated with: • Acceleration • Gravity • Polarization • Magnetization • Solid deformation • Liquid film stretching ME 152

  10. Moving Boundary Work • Associated with a volume change of a fluid system (aka compression-expansion work) ME 152

  11. Moving Boundary Work, cont. • Expansion: dV > 0, Wb > 0 • Compression: dV < 0, Wb < 0 • Work processes on P-V diagram: ME 152

  12. Moving Boundary Work, cont. • Special cases: 1) if V = constant, Wb = 0 2) if P = constant, Wb = P(V2-V1) 3) if PVn= constant (known as a polytropic process), (see pp. 135-136 for derivation) ME 152

  13. Shaft Work • Associated with a rotating shaft ME 152

  14. Spring Work • Associated with the extension or compression of a spring; if spring is linear, then force obeys Hooke’s law, ME 152

  15. Electrical Work • Associated with the motion of electrons due to an electromotive force ME 152

  16. Work and Heat • Both are energy transfers • Both are path-dependent functions • P and V are properties, because • Q and W are path functions, because ME 152

  17. Conservation of Mass • “Mass can neither be created nor destroyed” • mass and energy can be converted to each other according to Einstein’s E=mc2, but this effect is negligible except for nuclear reactions) • For closed systems, this principle imposes m = constant since mass cannot cross the system boundary • For control volumes, the mass entering and leaving the system may be different and must be accounted for ME 152

  18. Mass and VolumeFlow Rates • Mass flow rate: fluid mass conveyed per unit time [kg/s] where Vn = velocity normal to area [m/s]  = fluid density [kg/m3] A = cross-sectional area [m2] ME 152

  19. Mass and VolumeFlow Rates, cont. • For most pipe flows,  = constant and the average velocity (V) is used: • Volume flow rate is given by ME 152

  20. Conservation of Mass Principle - Control Volume • Net mass transfer during a process is equal to the net change in total mass of the system during that process where i = inlet, e = exit, 1 = initial state, 2 = final state • in rate form: • In fluid mechanics, this is often referred to as the continuity equation ME 152

  21. Steady-Flow Processes • Steady-flow or steady-state – a condition where all fluid and flow properties, heat rates, and work rates do not change with time. • mathematically: • applied to mass balance: ME 152

  22. Steady-Flow Processes, cont. • Conservation of mass during a steady-flow process: • If control volume is single-stream (i.e., one inlet, one exit), then ME 152

  23. Incompressible Flow • If  = constant, then the mass flow is considered incompressible • for steady-flow: • for single-stream, steady-flow: ME 152

  24. Total Energy of a Flowing Fluid • A flowing fluid contains internal, kinetic, and potential energies: • Fluid entering or leaving a control volume has an additional form of energy known as flow energy, which represents the work required to “push” the fluid across a boundary: ME 152

  25. Total Energy of a Flowing Fluid, cont. • The total energy of a flowing fluid (on a unit-mass basis, ) becomes • Using the definition of enthalpy (h), ME 152

  26. Energy Transport by Mass • Amount of energy transport: • Rate of energy transport: ME 152

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