SESSION 3

OPTICAL DIAGNOSTICS AND RADIATIVE TRANSFER

ABSTRACT

A technique was developed for direct measurement of gas temperatures in the range of 2050K-2700K with improved accuracy and reproducibility. The technique utilized the low-emittance of certain fibrous materials, and the uncertainty of the technique was limited by the uncertainty in the melting points of the materials, i.e., ±15K. The materials were pure, thin, metal-oxide fibers whose diameters varied from 601Mm to 400Mm in the experiments. The sharp increase in the emittance of the fibers upon melting was utilized as indication of reaching a known gas temperature. The accuracy of the technique was confirmed by both calculated low emittance values of transparent fibers, of order 0.01, up to a few degrees below their melting point and by the fiber-diameter independence of the results. This melting-point temperature was approached by increments not larger than 4K, which was accomplished by controlled increases of reactant flow rates in hydrogen-air and/or hydrogen-oxygen flames. As examples of the applications of the technique, the gas-temperature measurements were used (a) for assessing the uncertainty in inferring gas temperatures from thermocouple measurements, and (b) for calibrating an IR camera to measure gas temperatures. The technique offers an excellent calibration reference for other gas- temperature measurement methods to improve their accuracy and reliably extending their temperature range of applicability.

ABSTRACT

Based on the analysis of measured data on flow rates, concentrations and temperatures taken during steady: state operation of a lignite-fired 0.3 MW_t Atmospheric Fluidized Bed Combustor (AFBC) test rig, radiative exchange in freeboard of the combustor was modeled by using a well-stirred enclosure model in conjunction with Radiosity-Irradiation Method (RIM). Radiative properties of the particle laden combustion gases were calculated by assuming gray radiation behavior for both particles and gas, and using Leckner's correlations for gas and Mie theory for particles. The accuracy of the model was tested by comparing its predictions with incident radiative heat fluxes measured on the freeboard wall. Comparisons between the incident radiative heat fluxes predicted by the present model and those measured show that this engineering approach can be used with confidence by designers for the particle loadings typically encountered in freeboards of bubbling fluidized bed combustors.

ABSTRACT

A model for predicting radiative heat transfer in coal-fired furnaces is presented. The radiative transfer equation is modelled by the discrete ordinates method for body-fitted coordinates using the S₄-approximation. The gas absorption coefficient is determined by a weighted-sum-of-gray-gases model, the radiative properties of coal and ash particles are derived from the specific area and a mean efficiency factor of the particle cloud. The scattering phase function is modelled by the Delta-Eddington approximation. The entire radiation model is adopted for vector and parallel computers guaranteeing numerical efficiency. It is tested at an idealized squared combustion chamber with black walls filled by an emitting, absorbing medium with constant absorption coefficient. The computations are carried out on several curvilinear non- orthogonal grids using different boundary conditions. On all grids the computed radiative source terms and wall heat fluxes are in very good agreement with the analytic solution. Furthermore, the model is validated at a coal-fired test facility. Apart from the near burner region the computed temperature distribution only slightly deviates from the measured values.

ABSTRACT

Predictive accuracy of discrete ordinate method (DOM) was assessed by applying it to the prediction of incident radiative fluxes on the walls of a gas turbine combustor simulator (GTCS), and comparing its predictions with measurements. Input data utilized for the DOM were measured gas concentration and temperature profiles and inner wall temperatures of the GTCS which is a cylindrical enclosure containing a turbulent diffusion flame of propane and air. Effects of order of approximation (S₄ and S₆) and using uniform and non-uniform gas absorption coefficients for the non-homogeneous medium on the accuracy of the predicted heat fluxes were also investigated. Comparisons show that S₄ approximation is adequate for the prediction of incident wall heat fluxes and the use of an absorption coefficient profile based on measured gas concentrations and temperatures improves the accuracy significantly.

ABSTRACT

Adequate modeling of thermal radiation is an essential tool for the design of real-live combustion systems. Predictive methods solving the radiative heat transfer equation require the values of absorption and scattering coefficients of the participating media. In the present paper, a compromise between accuracy and computational economy is ensured in the evaluation of those coefficients, by using the exponential wide band model for the gaseous components of the mixture. a new curve fitting approach to the Mie theory for intermediate and large particles and power series to represent the Mie coefficients for small particles. Predicted results with those approaches are presented herein demonstrate the proposed models' high accuracy and relatively low computational costs.