BRHS /
## Gas Property ModelsThe radiative properties (absorption and scattering coefficients) of the combustion products and enclosure wall emittance are required for the modelling. In an enclosure fire, the gas radiative properties vary considerably from the comparatively transparent entrained air close to the floor to the highly emissive, luminous flames of fire source, and the optically dense ceiling smoke layer. Various models are available to predict the gas radiative properties. The participating media models (see, for example, (TienCL:1982)) currently available for characterizing the flaming and smouldering fires and the resulting combustion products differ in their generality, sophistication, accuracy and computational cost. They are assessed in terms of their ability to predict radiative heat transfer from one-dimensional, idealised representations of the internal structure of buoyant and jet fires. Exact results can be obtained by line-by-line calculations of spectral absorption-emission lines of molecular gases. However, such calculations are useful in the study of radiative transfer in the atmosphere but are not practical for most engineering applications, and are therefore not discussed here. Narrow-band and wide-band models constructed from the spectral lines, and on a simpler level, the gray gas representation of the molecular spectrum can be considered. The simplest treatment for the case of enclosure fire is to consider the gas to be a gray gas of prescribed constant absorption coefficient. ## Narrow Band ModelA well known narrow band model is that proposed by Grosshandler and Modak (GrosshandlerWL:1981), which is based on the statistical model by Goody (GoodyRM:1964) for tri-atomic molecules with equal line strengths within each narrow band region, and with homogeneous effects accounted for through the Curtis-Godson approximation which employs suitable averages along a line-of-sight. For hydrogen flame, the five gas bands of the H ## Wide Band ModelEdwards and Balakrishnan (EdwardsDK:1973) developed a spectral version of exponential wide band model, which is based on the fact that the absorption and emission of radiation by a molecular gas is concentrated in between one and six vibrational bands. Within these bands, the spectral lines associated with rotational modes of energy storage are reordered in wave number space with exponentially decreasing line intensities moving from the band head. The band shape is then approximated by one of the three simple exponential functions, with radiative properties of each absorption band obtained from specified model parameters. ## Grosshandler’s Total Transmittance, Non-Homogeneous (TTNH) ModelThe total transmittance, non-homogeneous (TTNH) model for CO ## Mixed Gray Gas ModelThe most popular mixed gray gas model for modeling combustion products (including soot) from fires is that proposed by Truelove (TrueloveJS:1976), which is based on representing the banded spectra of CO ## Banded Mixed Gray Gas ModelTruelove’s mixed gray gas model, employing one clear and three gray gas representations, can be written in a banded form where, for a given model spectrum, the gray gas weightings are determined as the fractional amount of black body energy in the spectral regions where “gray gas absorption coefficients” exist (ModestMF:1991). Recently, Cumber and Fairweather (CumberPS:1999) have improved the method by incorporating CO and CH Invalid BibTex Entry! << Measuring and Modeling of Radiation in Hydrogen Combustion | Content | Damage by Low Temperature Releases >> |

Page last modified on February 18, 2009, at 02:28 PM