Radiationparticipatingmedium

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    RADIATION EXCHANGE WITH EMITTING AND ABSORBING GASES

    So far, radiation heat transfer between surfaces separated by a medium that does

    not emit, absorb, or scatter radiation a non-participating medium that is

    completely transparent to thermal radiation - A PERFECT VACUUM or air at ordinary

    temperaturesand pressures comes very closeGases that consist of monatomic molecules such Ar and He and symmetric diatomic

    molecules such as N2 and 02 are essentially transparent to radiation, except at

    extremely high temperaturesat which ionisation occurs.

    Hence, atmospheric air can be considered to be a non-participating medium in

    radiation calculations

    Gases with asymmetric molecules such as H20, CO2, CO, SO2 and hydrocarbons HnCmmay participate in the radiation process by absorption at moderate temperatures,

    and by absorption and emission at high temperatures such as those encountered in

    combustion chambers.

    Therefore, air or any other medium that contains such gases with asymmetric

    molecules at sufficient concentration must be treated as a participating medium inradiation calculations

    Combustion gases in a furnace or a combustion chamber, for example, contain

    sufficient amounts of H20 and CO2 and thus the emission and absorption of gases in

    furnaces must be taken intoconsideration

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    The presence of a participating medium complicates the radiation analysis

    considerably for several reasons:

    a participating medium emits and absorbs radiation throughout its entire volume.

    That is, gaseous radiation is volumetric phenomena, thus it depends on the

    size and the shape of the body. This is the case even if the temperature is

    uniform throughoutthe medium

    Gases emit and absorb radiation at a number ofnarrow wavelength bands. This is

    in contrast to solids, which emit and absorb radiation over the entire

    spectrum. Therefore, the gray assumption may not always be appropriate

    for a gas even when the surrounding surfaces are gray

    The emission and absorption characteristics of the constituents of a gas mixturealso depends on the temperature, pressure and composition of the gas

    mixture. Therefore, the presence of other participating gases affects the

    radiation characteristicsof a particulargas.

    We will consider the emission and absorption of radiation by H2O and

    CO2 only since they are the participating gases most commonlyencountered in practice (combution products in furnaces and

    combustion chambers burning hydrocarbon fuels contain both gases at

    high concentrations), and they are sufficient to demonstrate the basic

    principles involved

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    Radiation properties of a participating medium

    Consider a participating medium of thickness L . A spectral radiation beam of

    intensity I ,0 is incident on the medium, which is attenuated as it propagates due toabsorption. The decrease in the intensity of radiation as it passes through a layer of

    thickness dx is proportional to the intensity itself and the thickness dx. This isknown as Beers law.

    k Spectral absorption coefficient of the medium (m-1)

    L

    0x

    L

    0x

    dxxI

    xdI

    dxxIxdI

    Absorptivity of the medium is assumed to be

    independent of x

    L

    0,

    L,e

    I

    I

    Radiation intensity decays exponentially according to Beers law

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    Spectral transmissivity of a medium is defined as the ratio of the intensity of

    radiation leaving the medium to that entering the medium

    L

    0,

    L,e

    I

    I

    =1 when no radiation is absorbed and thus the radiation intensityremains constantSpectral transmissivity of a medium represents the fraction of radiation transmitted

    by the medium at a given wavelength.

    Radiation passing through a non-scattering (and thus non-reflecting)

    medium is either absorbed or transmitted. Therefore, + =1, andthe spectral absorptivity of a medium of thickness L is

    Le111

    From Kirchoffs law, the spectral emissivity of the medium is

    Le1

    Note that the spectral absorptivity, transmissivity and emissivity of a medium are

    dimensionless quantities, with values less than or equal to 1. The spectral

    absorption coefficient of a medium (, , ), in general, vary with wavelength,temperature,pressure and composition.

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    For an optically thick medium ( a medium with large value ofL)0ee

    L 11

    1e1L

    9933.01073795.6e;5L35

    An optically thick medium emits like a blackbody at the given

    wavelength

    An optically thick absorbing-emitting medium with no significant

    scattering at a given temperature Tg can be viewed as a black surface

    at Tg since it will absorb essentially all the radiation passing through it

    and it will emit the maximum possible radiation that can be emitted

    by a surface at Tg, which is Eb(Tg)

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    Emissivity of gases and gas mixtures

    Spectral absorptivity of CO2 at 830 K and 10 atm

    for a path length of 38.8 cm

    Shape and width of absorption

    bands vary with temperature and

    pressure, but the magnitude of

    absorptivity also varies with the

    thickness of the gas layer

    Hence, absorptivity values without

    specified thickness and pressure

    are meaningless

    The non-gray nature of properties should be considered in radiation calculations for

    high accuracy. This can be done using a band model, and thus performing

    calculations for each absorption band. However, satisfactory results can be obtained

    by assuming the gas to be gray, and using an effective total absorptivity and

    emissivity determined by some averaging process.

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    Even with gray assumption, the total emissivity and absorptivity of a gas depends

    on the geometry of the gas body as well as the temperature, pressure, and

    composition.

    Gases that participate in radiation exchange such as CO2 and H2O typically coexist

    with non-participating gases such as N2 and O2, and thus radiation properties of an

    absorbing and emitting gas are usually reported for a mixture of the gas with non-

    participating gases rather than the pure gas.

    The emissivity and absorptivity of a gas component in a mixture depends primarily

    on its density, which is a function of temperature and partial pressure of the gas

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    Emissivity at a total pressure P other than P = 1 atm is determined by multipllying

    the emissivity value at 1 atm by a pressure correction factor Cw for water vapour

    atm1,www C

    Note that Cw = 1 for P = 1 atm and thus (Pw + P)/2 0.5 ( a very low concentration ofwater vapour is used in the preparation of the emissivity chart in the figure and thus

    Pw is very low)

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    If the CO2 and H2O gases exist together in a mixture with non-

    participating gases

    wcg

    atm1,wwatm1,ccg CC is the emissivity correction factor which accounts for the overlap ofemission bands. For a gas mixture that contains both CO2 and H2O

    gases, is plotted.

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    The emissivity of a gas also depends on the mean length an emitted

    radiation beam travels in the gas before reaching a bounding surface,

    and thus the shape and the size of the gas body involved.

    During their experiments in the 1930s, Hottel and his coworkers

    considered the emission of radiation from a hemispherical gas body to

    a small surface element located at the center of the base of the

    hemisphere. Therefore, the given charts represent emissivity data for

    the emission of radiation from a hemispherical gas body of radius L

    toward the center of the base of the hemisphere.

    It is certainly desirable to extend the reported emissivity data to gasbodies of other geometries, and this is done by introducing the

    concept of mean beam length, which represents the radius of

    equivalent hemisphere

    CONCEPT OF MEAN BEAM LENGTH

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    Concept of the mean beam length

    Consider a hot, isothermal, non-scattering gas volume radiating toward a black area element

    dA on its surface.

    The spectral heat flux arriving at and absorbed by dA from a volume element is equal to thespectral emission by dV into all 4 directions the fraction intercepted by dA the fractiontransmitted along the path from dV to dA

    Spectral heat flux arriving at dA from all volume elements may be written as

    SV b eScosdA

    dVIq

    244

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    b i i f d i ( l d )

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    Absorptivity of gases and gas mixtures (Hottels procedure)

    Absorptivity of a gas that contains CO2 and H2O gases for radiation emitted by a

    source at temperature Ts can be determined similarly from

    wcg = is determined from the emissivity correction figure at the sourcetemperature Ts. Absorptivity is assumed to be same as emissivity, hence same charts

    may be used.

    gscsw

    45.0

    s

    g

    wc2

    gscsc

    65.0

    s

    g

    cc2

    T/L TP,TT

    TCOH

    T/L TP,TT

    TCCO

    The notation indicates that the emissivities should be evaluated using Ts instead of

    Tg, PcLTs/Tg instead of PcL and PwLTs/Tg instead of PwL.Note that the absorptivity of the gas depends on the source temperature Ts as well

    as the gas temperature Tg. Also, = when Ts = Tg, as expected.The pressure correction factors Cc and Cw are evaluated using PcL and PwL, as in

    emissivity calculations.

    Wh th t t l i i it f t t t T i k th i i

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    When the total emissivity of a gas g at temperature Tg is known, the emissive powerof the gas (radiationemittedby the gas per unit surface area) is given by

    4

    gsggTAE

    If the bounding surface is black at temperature Ts, the surface will emit radiation to

    the gas at a rate of AsTs4 without reflecting any, and the gas will absorb thisradiation at a rate ofgAsTs4,whereg is the absorptivityof the gas.Then the net rate of radiation heat transfer between the gas and a black surface

    surrounding it becomes

    If the surface is not black, the analysis becomes more complicated because of the

    radiation reflected by the surface. But for surfaces that are nearly black with an

    emissivity s > 0.7, Hottel recommends this modification

    4sg4ggssblack,netsgray,net TTA2

    1Q2

    1Q

    The emissivity of wall surfaces of furnaces and combustion chambers are typically

    greater than 0.7, and thus the relation above provides great convenience for

    preliminary radiationheat transfer calculations.

    44 sgggsnet TTAQ Black enclosure

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    Reconsider the cylindrical furnace in the above example. For a wall temperature of

    600 K, determine the absorptivity of the combustion gases and the rate of radiation

    heat transfer from the combustiongases to the furnace walls.

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