Infrared Band Model Research Articles

A Fast, Accurate Method of Computing Near-Surface Longwave Fluxes and Cooling Rates in the Atmosphere

As radiation plays a key role in the determination of the near-surface thermal environment, great accuracy is required in the computation of radiative fluxes, especially because a small error in the fluxes can lead to large errors in estimated cooling rates. A new code that employs a novel numerical scheme for making precise estimates of longwave fluxes and cooling rates near the surface of the earth, for arbitrary surface emissivities, is presented here. The code is a development of the infrared band model of Chou, Ridgway, and Yan. Unacceptable oscillations found near the surface in the cooling rates provided by this code have been overcome through the new numerical scheme. The new code gives results in excellent agreement with available results from Intercomparison of Radiation Codes used in Climate Models (ICRCCM) test cases and with the line-by-line calculations of Clough, Iacono, and Moncet. The code has no restriction on the number of grid points, yields fluxes accurate to a prescribed tolerance, and permits a discontinuity in temperature at the surface (although this is not used in the results presented). The computing times are comparable (for given accuracy) to those demanded by current codes in use elsewhere. It is found that, as surface emissivity $\varepsilon_ {g}$ departs from unity, the cooling rate rises dramatically near the surface, reaching values as high as nearly 40 K $day^{-1}$ at $\varepsilon_ {g}$ = 0.8 in the midlatitude summer atmosphere, and the effect of the surface is noticeable at heights of up to about 1 km. An analysis of spectral distribution shows that, when the surface is not radiatively black, the major contributions to near-surface cooling rates (due to water vapor) come from the two wavenumber bands, 340–540 $cm^{-1}$ and 1215–1380 $cm^{-1}$ (located on either side of the atmospheric window), in which both absorption and radiative flux are significantly high.

Compatibility of infrared band models with scattering

Techniques for the computation of radiative heating from aluminized solid propellant rocket exhaust plumes must account for infrared emission and absorption by hot H2O and CO2 in the presence of strong threedimensional aerosol scattering by micron-sized A12O3 droplets and particles. Radiative heating computations are usually performed over wide spectral intervals using infrared band models. However, no rigorous extension to infrared band models has been proposed for situations with scattering. Indeed, band models and scattering are widely held to be incompatible. Practical engineering calculations of plume radiative heating have therefore proceeded, using various approximations to the transport equation. Although the errors in these approximations are believed to be reasonably small, they have never been quantified because of the lack of rigorous results. In order to help remedy this deficiency, this article develops two different rigorous solutions. Although they are currently restricted to homogeneous media, they nevertheless provide a previously unavailable means of calibrating code performance and assessing the accuracy of various approximation schemes. Some sample applications are provided for the Space Shuttle Solid Rocket Booster.

Doppler shift effects on infrared band models

Band-model calculations typically are employed in the analysis of low-spectral-resolution radiometer, photometer, and spectrometer measurements of hot-gas emission and absorption. Although they are not used to compute the profiles of individual lines directly, nevertheless, band models are sensitive to line shape. This line-shape sensitivity causes the net band-model emission and absorption to be sensitive to Doppler shift effects. This paper reviews the treatment of Doppler shifts on a Voigt profile spectral line, and then generalizes this result to the curve of growth, i.e., equivalent width, for both single-line and infrared band models. Finally, a synthetic spectrum for the COi 4.3-ji band is computed with and without Doppler shifts to illustrate the magnitude of the effect. It is found that band-model computations are unaffected by Doppler shift in the limiting cases of small and large absorber amounts. In the case of intermediate absorber amounts, significant effects are predicted for hypersonic flows.

A temperature-dependent regular infrared band model

A temperature-dependent infrared band model for the absorption or emission of spectral lines that are simultaneously broadened by Doppler and collisional effects is developed. The result is given in the form of an infinite series which converges repidly and is appropriate for efficient computation. It reproduces accurately the spectral line-intensities and profiles for symmetric molecules. The model has the apparent limitation of producing lines of equal spacing. However, easily applied techniques are demonstrated for overcoming this limitation. The model has been generalized to include the Voigt-profile equivalent of the Clough line shape, which is important in the microwave region. Applications are suggested, such as improvements to the LOWTRAN atmospheric transmittance code and to emission and absorption modeling in general.

Uncertainties propagation for combustion diagnostics using infrared band models

The development of matrix equations for an uncertainty propagations analysis of an emission-absorption (E/A) band-model technique is described. The E/A technique is used for the determination of local values of temperature and species concentration from measurements of radiance and transmittance on axisymmetric gas flows. The approach used depends on linearization of the original nonlinear system of equations comprising the solution to the radiative transfer problem for an emitting-absorbing source such as a turbojet or rocket engine exhaust plume. The resultant linear transformation matrix provides a means for determination of the variance-covariance matrix for inverted temperature and partial pressure, based on the experimentally determined variance-covariance matrix of the measured radiance and transmittances. The diagonal elements of the propagated variance-covariance matrix yield the square of the standard deviation of the temperature and partial pressure. Although the equations are developed for uncertainty propagations analysis of the E/A band-model technique, the results are a general property of linear transformations. They are thus applicable to any situation in which the dependence of analysis on measurements can be approximated by a linear relationship. Inversion of radiance and transmittance lateral profiles obtained from a liquid-propellant rocket engine to radial profiles of temperature and partial pressure with the associated uncertainties propagations is included as an illustrative example.

Infrared Band Model Parameters and Emissivity of Nitrous Oxide

Infrared Band Model Parameters and Emissivity of Nitrous Oxide

Applications of infrared band model correlations to nongray radiation

Various correlations for the total absorptance of a wide band are presented. These are employed in two physically realistic problems (radiative transfer in gases with internal heat source and heat transfer in laminar flow of absorbing-emitting gases between parallel plates) to study their influence on final radiative transfer results.