across the upper surface only 3 Chip surface is isothermal 4 The average heat

# Across the upper surface only 3 chip surface is

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across the upper surface only, (3) Chip surface is isothermal, (4) The average heat transfer coefficient for the chip surface is equivalent to the local value at x= L, (5) Negligible radiation.
Problem: Chip Cooling (cont.)From Eq. (2), the surface temperature of the chip is 2-3225C3010W/107 W/mK0.004m42.5 C.sTCOMMENTS:(1) The estimated value of Tavgused to evaluate the air properties is reasonable. (2) How else could chiphhave been evaluated? Is the assumption of Lhhreasonable?0.851/32-620.0269 W/mK10 m/s0.120 m0.040.703107 W/mK.0.120 m16.6910m/ sLh<
Concentration Boundary Layer The Concentration Boundary Layer Features A consequence of evaporation or sublimation of species A from a liquid or solid surface across which a second fluid species B is flowing. A region of the flow characterized by species fluxes and concentration gradients. A region between the surface and free stream whose thickness increases in the flow direction. c Why does increase in the flow direction? c Manifested by a surface species flux and a convection mass transfer coefficient . , A s N  m h What is the heat transfer analog to Fick’s law? A, A A, A, 0.99 s c s C C y C C A A AB ' Fick s law C N D y     A A, AB 0 s y C N D y    A, A, A, s m s N h C C  AB A 0 A, A, / y m s D C y h C C Mass Transfer
Concentration Boundary Layer (cont.) Definitions Term Variable Units Species molar flux kmol/s·m 2 A, s N  Species molar rate kmol/s A, s N Species mass flux kg/s·m 2 A, s n  Species mass rate kg/s A n Species molar concentration kmol/m 3 Species mass concentration (density) kg/m 3 A C A Species molecular weight kg/kmol A M Convection mass transfer coefficient m/s m h Binary diffusion coefficient 1 m 2 /s AB D 1 Table A.8 Mass Transfer
Concentration Boundary Layer (cont.) Convection Calculations Species Molar Flux: A, A, A, s m s N h C C  Species Mass Flux: A A A A A A , n N C   M M A, A, A, s s m n h  Total Transfer Rates: A, A, A, s m s s N h A C C A, A, A, s m s s n h A Average Mass Transfer Coefficient: 1 s m m s s A h dA A h Mass Transfer
Vapor Concentration Species Vapor Concentration or Density At a Vapor/Liquid or Vapor/Solid Interface The vapor concentration/density corresponds to saturated conditions at the interface temperature T s . A, A,sat s s C C T A, A,sat A A,sat s s s T C T M Assuming perfect gas behavior, the concentration/density may be estimated from knowledge of the saturation pressure. A,sat A, A, A s s s s p T C T R M The concentration may also be directly determined from saturation tables . For example, from Table A.6 for saturated water , 1 A, A A, s g s s T C M Mass Transfer
Vapor Concentration (cont.) Free Stream Conditions The free stream concentration/density may be determined from knowledge of the vapor pressure, assuming perfect gas behavior.