CHAPTER 3 Energy Transfer by Mass Flow Work or Energy (energy needed to maintain continuous flow)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3-5 FIGURE 3-48 Schematic for flow work. W=F∙L=PA∙L=P∙V so w = Pv

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3-6 FIGURE 3-51 The total energy consists of three parts for a nonflowing fluid and four parts for a flowing fluid. flow energy, Pv, is part of the energy of a flowing fluid. enthalpy, h Usually ignored (small) so energy transferred into a system by a stream is mθ. Rate?

Mass flow Most can be considered steady flow and one dimensional So use:

Heat Transfer 3 Types Conduction –Microscopic work –Follows Fourier’s Law: –k t is thermal conductivity

Heat Transfer Convection –Between surface and fluid –Must have conduction also –h is convection heat transfer coefficient –experimentally determined and depends on conditions

h = 25 – 250 W/m 2 ∙K h = 2 – 25 W/m 2 ∙K

Heat Transfer Radiation – needs no medium –can happen when two bodies are even separated by a cooler medium. –Kirchhoff’s Law emissivity = absorptivity –Good adsorber is good emitter

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 3-58 The absorption of radiation incident on an opaque surface of absorptivity . 3-7