Thermodynamics Notes4

Thermodynamics Notes4 - ME 311 THERMODYNAMICS S. Masutani...

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Unformatted text preview: ME 311 THERMODYNAMICS S. Masutani FALL 2007 Specific Volume = v 1/ Volume occupied by a unit mass Textbook uses notation v to signify the volume occupied by a mole (kgmole, lbmole, gmole) of molecules; hence v MW v = Pressure Very familiar concept: force acting over an area 1 more familiar than force 1 kg 1 kg [Pressure] = [N/m 2 ] = [Pa] [lbf/in 2 ] What is pressure? In the previous example, gravitational attraction acting on molecules or atoms of a solid result in a (weight) force that is applied over an area. In a fluid: collisions transfer momentum to wall: F mV = A Impulse/momentum move imaginary surface to interior of box. Pressure is the flux of momentum through this surface/unit time. ) ( lim ' A F P normal A A = A is minimum area ME 311 THERMODYNAMICS S. Masutani FALL 2007 1. For fluid at rest: P is independent of direction; however, it can vary from point- to-point: h P atm Vol = A 1 h A 1 F Assuming t cons g t cons tan ; tan = 1 1 A P hg A F atm + = s o atm atm P gh P A hg A A F P + = + = = 1 1 1 Outside water water air >> 2. Fluid in motion: consider normal and shear stresses; for problems encountered in this course, the thermodynamic pressure can be taken to equal the normal stress. 3. Static and stagnation pressure (moving fluid) Dynamic head Static pressure moving with fluid Stagnation pressure: pressure of fluid brought to rest: 2 2 V P = Scales Absolute pressure (relative to true vacuum) used in thermodynamic relationships. Measurement Typically, measurement devices detected a differential pressure relative to some reference, usually atmospheric. We encounter the terms: P (gage) = P(abs) P atm (abs); P > P atm P (vacuum) = Patm (abs) - P(abs); P > Patm 2 ME 311 THERMODYNAMICS S. Masutani FALL 2007 Example: MANOMETER 3 Force balance: gL P P gLA A P PA atm atm + =...
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This note was uploaded on 05/19/2008 for the course ME 311 taught by Professor Masutani during the Fall '07 term at Hawaii.

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Thermodynamics Notes4 - ME 311 THERMODYNAMICS S. Masutani...

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