Fysische transportverschijnselen I125practicum

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Fysische transport-verschijnselenI125-practicum

• Fase-verandering en Warmte-overdracht
• Acad. Jaar 2003-2004
• Eerste ingenieursjaar Bio-ingenieur

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Phase Change Heat Transfer -

• Why Freezing?
• Dont care if its alive or dead - Food
  processing, 7-8 of all foods, 15 billion
  industry.
• Kill - destruction of undesired tissue
• Bring back to life later -Cryopreservation
• Cryosurgery advantages
• Low bleeding
• Good aesthetic results
• Minimal use of anesthetics
• Short period of recovery
• Low cost

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Freezing and Thawing

• Freezing of pure water
• Freezing process
• Property changes
• Freezing of solutions and biomaterials
• Temperature profiles and freezing time
• Biomaterials, considering mushy region
• Biomaterials, approx. soln. for pure liquid

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Change in properties during freezing

• Density ?
• Thermal conductivity
• Specific heat
• Dielectric loss
• Thermal diffusivity
• Youngs Modulus

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Freezing makes a biomaterial harder and brittle
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4
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Young's
1
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Modulus
(MPa)


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0
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0


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2
0
0






0


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5
0







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3
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Temperature (C)
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Penetration of freezing front

• Assumptions
• unfrozen part at freezing temperature Tm
• frozen part in pseudo-steady state with constant
  thermal conductivity and approximately a linear
  T-gradient
• sharp freezing front

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Principles of simple equation

• Heat transfer in frozen layer
• pseudo-steady state
• approximately linear gradient and a
• constant thermal conductivity
• Latent heat of fusion at freezing front

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By integration final equation
With prescribed surface temperature Ts
With convection b.c. (T? and h)
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Latent heat

• Biomaterials and soils
• water has the most important latent heat so
• with ? the latent heat (333 kJ/kg for water)
• and (mf) the mass fraction of water in the
  material (bio- or soil)

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Problem Hamburger freezing
Datta, 2002 pag 143)
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Problem frost penetration in soil
(Datta, 2002 pag 144)
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Fysische transport-verschijnselenI125-practicum

• Warmte-overdracht
• door straling
• Acad. Jaar 2003-2004
• Eerste ingenieursjaar Bio-ingenieur

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Electromagnetic Radiation

• No medium required for transmission
• Magnitude of radiation depends on temperature
  level
• Quality of radiation depends on temperature
  level
• Speed is that of light (3108 m/s)
• Thermal radiation is 0.1-100 microns, this
  includes Infared
• Visible (0.38-0.76 microns)

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Wave Hitting a Surface -
Reflected
Incident
a b
Surface
ab, specular a?b, diffuse
Absorbed
Transmitted
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Albedo
Albedo Fraction of radiation reflected from soil
Zie fig 8.7 in Datta, 2002 pag 150
dry
Vegetation (0.25)
Water (0.05/loodrecht)
wet

• Important for energy balance of the soil/
• vegetation

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Emissivity
Zie fig 8.13 in Datta, 2002 pag 156

• E ? Eb (0 lt? lt1)
• When ? lt 1, it is called a gray surface.
  
• All real surfaces are gray
• Emissivity varies with
• Surface condition
• Direction
• Temperature

Emissive power (real body)
Emissivity
Emissive power (black body)
Water hoge emissiviteit
Aluminium lage emissiviteit
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How much radiation can be incident on Earth?
Energy conservation gives
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Radiative exchange

• Configuration or View Factor (F1-2) is defined
  as F1-2 Fraction of the radiation leaving
  surface 1 which is intercepted by surface 2.
• Net exchange between two black bodies is q1-2
  A1 F1-2 ? (T14 - T24)
• Net exchange between one surface and other black
  surfaces in an enclosure is
• q1 ?Ai Fi-j ? (Ti4 - Tj4)
• Net exchange between two surfaces in a two
  surface enclosure (gray surfaces) is

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Concept viewfactor
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View factor between aligned flat surfaces
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ProblemSunbathing
?
emissivity
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Absorption and transmission in biomaterials
Biomaterial Slab
F0
FF0e-x/?
F
x
?
63 of the energy has been absorbed in the
distance x ?.

• At x ?, FF0e-1 0.37 F0

• ? measures how good the absorption is...
• Infared few microns
• Microwave few centimeters
• Radio Frequency hundreds of centimeters

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Radiation

• Solar Radiation
• Behaves like an ideal body at 5800 K
• Water vapor absorbs at longer wavelengths
  (infared)
• Ozone absorbs in ultraviolet

Earths Surface Radiation Eearth ?earth ?
T4earths surface ?earth 1 for water,
vegetation Tearths surface 14C
Radiation from outside atmosphere into
space Teffective (as seen from space)
-18C Thus, heat is trapped by the atmosphere
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Spectral distribution of solar radiation
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Atmospheric Emissions

• H2O, CO2 in atmosphere emit radiation
• It is the only source of radiation at night

Vapor pressure at 1.5 m above ground
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Radiation exchange of a leaf with the sky and
ground
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Problem 8.12.9
Datta, 2002 pag 170
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Problem Datta 8.12.6 (human radiation
(Datta, 2002, pag 170)
Ommit wavelength parts !!!!!