RAD3'ppt Absorption

1
RAD3.ppt Absorption Emission of Radiation
by Gases A BRIEF INTRODUCTION
(Chapter in Outline Notes Radiation Heat
Transfer )
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Components of Course What Stage are We Up To?

• Types of exchangers, revision of OHTCs, fouling
  factors.
• Heat exchanger selection.
• Thermal performance analysis (NTUs) for co-
  counter-current exchangers.
• Multi-pass exchangers (ST).
• Condensation boiling.
• Radiation.

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Basic Situation

• Polar molecules like CO2, H2O, most hydrocarbons,
  CFCs, absorb emit radiation non-polar
  molecules like O2, N2 do not
• Effects are concentrated in particular wavelength
  bands (e.g. infra-red for CO2, H2O, most
  hydrocarbons, CFCs)
• Applications furnaces, greenhouse effect

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Outline

• Mechanisms
• Basic applications example

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Mechanisms

• Radiation absorbed gradually as it passes through
  these absorbing gases.

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• Absorptivity emissivity both functions of
  partial pressure (p) path length (L)
• Total effective emissivity ( ) estimated as
  function of temperature (T) (partial pressure
  length) product from Hottel charts (Figures 2.96,
  2.97 in Hewitt et al, p. 146 for example)

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• Gas absorptivity ( ) at gas temperature (Tg)
  subject to radiation from source at temperature
  Ts different to gas emissivity ( ) at Tg,
  since it may absorb emit at different
  wavelengths

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• Gas absorptivityestimated from empirical
  equation (n 0.65 for CO2 0.45 for H2O)-

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• is gas emissivity calculated at surface
  temperature (Ts) using modified path length

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Path Length

• How is path length calculated in furnaces?
• In complex geometries, mean beam length used
  equivalent to path length
• Mean beam lengths for common shapes available
  in tables eg Hewitt et al, Table 2.13 (e.g. box,
  cylinder)

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Furnace Example Use of Radiation Heat Transfer
Charts

• Calculate the gas emissivity of
• 20 mol CO2
• 20 mol H2O
• 60 mol N2
• at 1000oC (1273K), 2 atm, inside cylinder, 10 m
  high 5 m diameter

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Context

• Combustion of
• liquid fuels (e.g. petrol) tend to produce 11
  ratio of H2OCO2
• gaseous fuels tend to produce 21 ratio of
  H2OCO2
• Reaction
• CH4 2O2 to CO2 2H2O

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Mean Beam Length

• Use tables of mean beam lengths for gas-surface
  radiant heat exchange
• This problem involves a right-circular
  cylinder, height 2.0 x diameter
• Characterizing dimension is diameter D (here 5
  m)
• Mean beam length (from Table 2.13, Hewitt et al)
• L 0.73D 0.73(5 m) 3.65 m

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Partial Pressure x Beam Length Product and
Finding Emissivity
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Application Calculation of Total Rate of
Radiation Heat Transfer

• If internal furnace surface maintained at 200oC
  (473K) by external cooling, what is total
  heat-transfer rate to cylindrical wall, assuming
  that its emissivity is unity?
• Gas emits radiation, wall emits radiation back,
  gas absorbs some of this.

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Nett Heat Flux

• Nett flux flux emitted by gas
• - flux absorbed by gas from wall emission

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• known already (0.51)
• Tg given (1273K)
• Ts given (473K)
• constant (Stefan-Boltzmann constant 5.67 x
  10-8 W m-2 K-4)
• Must calculate the gas absorptivity

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Modified Beam Length, Partial Pressure/Length
Product, Equivalent Gas Emissivity at Surface
Temperature

• L L (Ts/Tg) 3.65 (473/1273) 1.36 m

As before, use the chart
(eg Hewitt et al, p146, Fig2.96) at T 473K, we
find
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Gas Absorptivity for Radiation Emitted at Surface
Temperature

• Here, equal partial pressures so n
  (0.650.45)/2 0.55 (n 0.65 for CO2 and 0.45
  for H2O)

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Nett Heat Flux
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Interpretation

• Cold furnace wall.
• Amount of heat from wall absorbed by gas is
  small compared with that radiated from gas to
  wall.
• Accuracy about 10.
• Heat transfer rate?

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Conclusions

• Relevant to furnace calculations greenhouse gas
  calculations
• Basic mechanisms reviewed example covered