Accurate modeling of radiative heat transfer through combustion gases has received considerable attention in recent years. The spectral line weighted-sum-of-gray-gases (SLW) model was developed based on detailed line-by-line spectral data of gases. A critical element of the SLW model is the absorption line blackbody distribution function (ALBDF). This function was designed to utilize the spectral properties of gases in an efficient and compact manner. However, there are several limitations of the ALBDF in its original form. First, the valid ranges of temperature and pressure are not large enough to include important applications, such as oxy-combustion, where temperatures can exceed 2500 K, and pressurized combustion, where non-atmospheric pressures are expected. In addition, since the original ALBDF correlation was developed, new spectral data have become available which extend the accuracy of the previous work. Finally, it is desirable to be able to represent the ALBDF of CO in addition to H2O and CO2. Improving the SLW model in this manner will make it more generally applicable and ensure greater confidence in its accuracy. Line-by-line absorption cross-section data were generated carefully using a recently released spectroscopic database, HITEMP 2010. The Voigt line profile was implemented, and line wings were included in regions where they maintain a significant contribution. Line-by-line calculation of the ALBDF, total emissivity, and radiative transfer were also performed in order to provide benchmark data and to explore the influence of variable total pressure. It was found that increasing total pressure causes the ALBDF to shift to lower values at a given absorption cross-section, although this change is weaker at increasing temperature. Total emissivity is strongly affected by total pressure changes, although the change is modest if the product of partial pressure and path length is held constant. Increasing total pressure in a layer of gas increases the radiative flux exiting the gas layer; this was also found to be true for both the case of constant layer length and constant mass of radiating material. Efficient representations of the ALBDF were generated. The hyperbolic tangent correlation of Denison and Webb was updated to reflect improved spectroscopic data and to cover a wider range of temperature (400 K = T = 3000 K) and pressure (0.1 atm = p = 50 atm). The correlation was also extended to CO, which had not been correlated previously. Using tabulated line-by-line data directly was also explored, and these data have been made available for H2O, CO2, and CO. Finally, these efficient representations of the ALBDF were successfully validated by comparison with line-by-line calculations and experimental data for both total emissivity and radiative transfer. The latter included comparisons with intensity measurements and a comprehensive combustion simulation implementing the SLW model.

This study explores natural convection heat transfer and fluid flow from a vertical plate in a radiating gas accounting for real gas spectral behavior. Finite volume techniques are used to solve the coupled nonlinear partial differential equations for mass, momentum, and energy conservation, while radiation transfer is modeled using the Discrete Ordinates finite volume finite angle method. Real gas spectral behavior is accounted for using the Rank Correlated Spectral Line Weighted-sum-of-gray-gases method. It is found that gas temperature and velocity are higher in the boundary layer, thickening the thermal and hydrodynamic boundary layers compared to the limiting case of pure convection. Gas species and concentration significantly impact boundary layer development, affecting radiative heating, temperature, velocity, and wall heat fluxes. Wall radiation transport dominates over convective transport. Increasing the wall temperature for the same wall-quiescent surroundings temperature difference increases local radiative heating, temperature, and velocity, and results in higher wall heat fluxes. As Rayleigh number increases, convection gains importance relative to radiation. Higher total gas pressures moderately increase radiative heating, temperature, and velocity, while reducing wall heat fluxes and convective transport. Increased wall emissivity raises radiative heating, temperature, and velocity, while raising wall heat flux and reducing convective flux. It is concluded that the neglect of participating gas radiation effects can result in significant errors in the predicted flow and thermal behavior, and the total transport. These insights advance understanding of radiation-convection interplay in radiating gas scenarios.

Wildland fire research has been extensive and on going since before 1950. The motivation behind this research is to prevent loss of property and lives. In spite of this research, the heat transfer of fuel ignition and flame spread is not well understood. This dissertation seeks to fill gaps in this understanding through modeling and also by experimentation. The effect of water vapor on the transmission of thermal radiation from the flame to the fuel was investigated. The Spectral Line Weighted-sum-of-gray-gases approach was adopted for treating the spectral nature of the radiation. The study reveals that water vapor has only a moderate effect even at 100 percent humidity. Experiments were conducted wherein wood shavings and Ponderosa pine needles in quiescent air were subjected to an imposed radiant heat flux. The internal temperature of these particles was measured and compared to steady-state model predictions. Excellent agreement was observed between the model predictions and the experimental data. Exercise of the model led to the conclusion that ignition of the fuel element by radiation heating alone is unlikely. Time-resolved radiation and convection heat flux were measured in a series of experimental laboratory fires designed to explore heat transfer behavior during combustion of discontinuous fuel beds. Convection heat flux was shown to fluctuate between positive and negative values during flame engulfment, indicating the presence of alternating packets of hot combustion gas and cool ambient air within the flame. Rapid temporal fluctuations were observed in both radiation and convection. Spectral analysis revealed content at frequencies as high as 150 to 200 Hz. Time-resolved radiation and convection heat flux histories were also collected on fourteen controlled burns and wildfires. The data reveal significant temporal fluctuations in both radiation and convection heat flux. Spectral analysis using a Fast Fourier Trans-form (FFT) revealed content as high as 100 Hz using data sets that were sampled at 500 Hz. The role of the higher frequency convective content in fuel thermal response was explored using a one-dimensional transient conduction model with a convective boundary condition. It was shown that high-frequency (i.e., short-duration) convective pulses can lead to fine fuel ignition.

We investigate several aspects of the numerical solution of the radiative transfer equation in the context of coal combustion: the parallel efficiency of two commonly used opacity models, the sensitivity of turbulent radiation interaction (TRI) effects to the presence of coal particulate, and an improvement of the order of temporal convergence using the coarse mesh finite difference (CMFD) method. There are four opacity models commonly employed to evaluate the radiative transfer equation in combustion applications; line-by-line (LBL), multigroup, band, and global. Most of these models have been rigorously evaluated for serial computations of a spectrum of problem types [1]. Studies of these models for parallel computations [2] are limited. We assessed the performance of the Spectral-Line- Based weighted sum of gray gasses (SLW) model, a global method related to K-distribution methods [1], and the LBL model. The LBL model directly interpolates opacity information from large data tables. The LBL model outperforms the SLW model in almost all cases, as suggested by Wang et al. [3]. The SLW model, however, shows superior parallel scaling performance and a decreased sensitivity to load imbalancing, suggesting that for some problems, global methods such as the SLW model, could outperform the LBL model. Turbulent radiation interaction (TRI) effects are associated with the differences in the time scales of the fluid dynamic equations and the radiative transfer equations. Solving on the fluid dynamic time step size produces large changes in the radiation field over the time step. We have modifed the statistically homogeneous, non-premixed flame problem of Deshmukh et al. [4] to include coal-type particulate. The addition of low mass loadings of particulate minimally impacts the TRI effects. Observed differences in the TRI effects from variations in the packing fractions and Stokes numbers are difficult to analyze because of the significant effect of variations in problem initialization. The TRI effects are very sensitive to the initialization of the turbulence in the system. The TRI parameters are somewhat sensitive to the treatment of particulate temperature and the particulate optical thickness, and this effect are amplified by increased particulate loading. Monte Carlo radiative heat transfer simulations of time-dependent combustion processes generally involve an explicit evaluation of emission source because of the expense of the transport solver. Recently, Park et al. [5] have applied quasidiffusion with Monte Carlo in high energy density radiative transfer applications. We employ a Crank-Nicholson temporal integration scheme in conjunction with the coarse mesh finite difference (CMFD) method, in an effort to improve the temporal accuracy of the Monte Carlo solver. Our results show that this CMFD-CN method is an improvement over Monte Carlo with CMFD time-differenced via Backward Euler, and Implicit Monte Carlo [6] (IMC). The increase in accuracy involves very little increase in computational cost, and the figure of merit for the CMFD-CN scheme is greater than IMC. / Graduation date: 2012

A radiation code based on method of lines (MOL) solution of discrete ordinates method (DOM) for the prediction of radiative heat transfer in nongray absorbing-emitting media was developed by incorporation of two different gas spectral radiative property models, namely wide band correlated-k (WBCK) and spectral line-based weighted sum of gray gases (SLW) models. Predictive accuracy and computational efficiency of the developed code were assessed by applying it to the predictions of source term distributions and net wall radiative heat fluxes in several one- and two-dimensional test problems including isothermal/non-isothermal and homogeneous/non-homogeneous media of water vapor, carbon dioxide or mixture of both, and benchmarking its steady-state predictions against line-by-line (LBL) solutions and measurements available in the literature. In order to demonstrate the improvements brought about by these two spectral models over and above the ones obtained by gray gas approximation, predictions obtained by these spectral models were also compared with those of gray gas model. Comparisons reveal that MOL solution of DOM with SLW model produces the most accurate results for radiative heat fluxes and source terms at the expense of computation time when compared with MOL solution of DOM with WBCK and gray gas models. In an attempt to gain an insight into the conditions under which the source term predictions obtained with gray gas model produce acceptable accuracy for engineering applications when compared with those of gas spectral radiative property models, a parametric study was also performed. Comparisons reveal reasonable agreement for problems containing low concentration of absorbing-emitting media at low temperatures. Overall evaluation of the performance of the radiation code developed in this study points out that it provides accurate solutions with SLW model and can be used with confidence in conjunction with computational fluid dynamics (CFD) codes based on the same approach.

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