Fysische transportverschijnselen I125practicum

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Fysische transport-verschijnselenI125-practicum

• Massatransport
• stationair

Acad. Jaar 2003-2004 Eerste ingenieursjaar
Bio-ingenieur
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Dispersion (in feite verzamel-term)

• Spreading from higher to lower concentration
• In a fluid - due to turbulence
• In a porous solid - due to
• Velocity distribution inside a pore variation in
  pore sizes and Tortuosity and branching

• Similar to diffusion treated with analogous to
  formulas
• Dispersion much faster than molecular diffusion

m2 s
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Mechanische dispersie
Microscopische snelheid
Zeer traag
grootte
richting
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Dispersie/diffusie in poreuze materialen
10000
100
gelijk
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Estimation of E-coeff. in porous media.

• Molecular diffusion ( no flow)
• with ? volumetric water content and
• with ? tortuosity (gt1 def.effective length over
  length of medium )
• E-coefficient with flow (small scale Datta,
  2002, Appendix D6 page351)
• with ?Ld a coefficient (gt lt ? pag 351) and
• d the average pore diameter

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Problem 10.dispersioncoeff.
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Turbulentie in Fluidum
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Convective mass transfer coefficient

• cA1 is the concentration of A in the fluid (not
  solid ) but on the surface in equilibrium with A
  in the solid.
• cA2 is the concentration of A in the bulk fluid

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Units of equation 10.33 (pag 215)

• Eenvoudigste oplossing (bemerk de zin in Datta,
  2002, eq 10.33 is not a law but a defining
  equation) er zijn alternatieven mogelijk.
• hm in m/s ( volgens Table 14.2 pag 302)
• A in m2
• NA1-2 in kg/s
• CA1 en CA2 dan kg/m³

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CA1 en CA2 in kg/m³

• CA1 en CA2 dan kg/m³ dikwijls via het
  Mollierdiagramma (Datta, 2002, C9 pag 341) en
  Appendix C8 (pag 340-
• Mollier diagramma geeft MCmollier (in g water/kg
  droge lucht)
• C8 geeft de densiteit ?da (kg/m³) voor droge
  lucht
• C voor formule

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Class A pan
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Problem Evaporation
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Dispersion, Convection, Diffusion

• Bulk flow effect can be separated the process as
  convectiondispersion
• Bulk flow effect cannot be separated as
  dispersion only
• Diffusion is always present but slow

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Convective Mass Transfer

• Movement of mass due to bulk flow of the system.
  This is in addition to diffusive mass transfer
• Forced convection is due to an external force
  such as a fan
• Free convection is due to density differences
• Formula
• complete analogy to heat transfer
• hm includes effect of diffusion
• hm depends on flow geometry, fluid prop., etc.

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Bepaling van h-convectie coefficiënt

• Eerst hm bepalen zoals met h voor warmte
• Volgende week stationair en niet-stationair

Convectie coefficiënt
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Governing Equation for mass transfer(Chapter 11)
Ez

• Same equation gt Same solutions!
• B.C. more complex simultaneous equations

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Steady-state diffusion in a slab (Chapter 12)
Integrating
Boundary Conditions
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Steady-state diffusion in a composite media
x
Same fluid
Same K
Mass Transfer
Heat Transfer
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Steady-state diffusion in composite media
Complication of equilibrium ? (solved by K)
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ProblemDatta 12.6.3
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Steady-state diffusion in a slab with reaction
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Steady-state diffusion in a slab with reaction
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Steady-state diffusion in a slab with reaction
For Finite Size
This is lower than Flux with reaction
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Problem Oxygen diffusion in soil
0.03
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Comparison soil air-water saturated
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ProblemDatta 12.6.7
Oxygen in top is of shoot is 0.199 g/m3