A data reduction scheme for multiple-wavelength, line-of-sight, optical pyrometry applied to pulverized-coal flames that accounts for the presence of soot and its effect on computed temperatures is presented. Results of the application of the technique to laboratory scale flames are reported. Soot temperatures are calculated to be between gas and particle temperatures, with the gas temperature exceeding the particle temperature. From about 1 to about 10 percent of the original coal by mass is computed to be present as soot in the flames with soot concentration increasing with increasing coal concentration.

Basic ignition, evaporation and combustion behavior of liquid fuel droplets containing metal nanoparticles is experimentally investigated. The addition of metal nanoparticles enhances the ignition and evaporation of the liquid fuel droplets, even at low volume fraction of the nanoparticles (0.1% to 0.5%). These enhancements were, however, independent of the amount, type and size of the nanoparticles and limited to ignition and low temperature evaporation conditions. At higher temperatures, evaporation and combustion rates of the liquid droplets were not significantly affected by addition of nanoparticles. The data suggest, therefore, that the primary role of nanoparticles is in enhancing the thermal diffusion during initiation phase (ignition and low-temperature evaporation) where even in very small amounts addition of nanoparticles increases the ignition probability and evaporation rates.

ABSTRACT This article describes the application of the immersed boundary technique for simulating fluid flow and heat transfer problems over or inside complex geometries. The methodology is based on a fractional step method to integrate in time. The governing equations are discretized and solved on a regular mesh with a finite-volume nonstaggered grid technique. Implementations of Dirichlet and Neumann types of boundary conditions are developed and completely validated. Several phenomenologically different fluid flow and heat transfer problems are simulated using the technique considered in this study. The accuracy of the method is second-order, and the efficiency is verified by favorable comparison with previous results from numerical simulations and laboratory experiments.

This paper describes the use of the immersed boundary technique for simulating fluid flow and heat transfer problems over or inside complex geometries. The methodology is based on a fractional step method to integrate in time. The governing equations are discretized and solved on a regular mesh with a finite volume non-staggered grid technique. Several phenomenologically different fluid flow and heat transfer problems are simulated using the technique proposed in this study. The accuracy of the method is second-order, and the efficiency is verified by favorable comparison with previous numerical and experimental results.

inspection of electrical components mounted in aircraft fuel tanks has revealed sporadic evidence of silver oxide deposits formed by corrosion. The present work shows that these deposits can lead to hot surface ignition of flammable Jet-A vapors given sufficient electrical input. A logistic regression of ignition data showed that electrical power was a relatively good predictor of ignition. Temperatures measured at the base of the deposit during ignition testing exceeded the hot surface ignition temperature of jet-A. A thermal model of the process demonstrated that electrical power dissipation alone could produce the observed surface temperatures.The ability of the deposits to dissipate electrical power increased with the number of times the deposit had been exposed to Jet-A and voltage potential. This change in the electrical property of the deposit coincided with changes in the chemical makeup and structure of the deposit. Pristine deposits were primarily silver oxide, while deposits capable of high-power dissipation were primarily disordered and graphitic carbon with some conglomerates of silver-based material.

A new approach based on the finite-difference technique has been developed to study the steady incompressible Navier-Stokes equations in the laminar region, where the domain is partially bounded by a free surface. The nonstaggered fractional step method is used to solve the flow equations written in terms of primitive variables. The physical domain is transformed to a rectangle by means of a numerical mapping technique. The location of the phase boundary is accomplished by means of two methods depending on the surface tension effect: the normal-stress boundary condition or the kinematic boundary condition. We have tested the accuracy and efficiency of the numerical method by solving four different test problems: lid-driven flow in an inclined cavity, film in the absence of gravity, the "stick-slip" problem, and the Newtonian jet swell problem.

The effect of the initial droplet-size distribution (DSD) on spray combustion processes has been studied experimentally. Suitable flames were produced by igniting ethanol sprays issuing from a pipe into a coannular air stream. Three different DSDs were examined for their effect on spray combustion processes, while the initial Sauter mean diameter (SMD or D-32), mixture ratio, and gas/liquid velocities were held constant. Variations in spray/flame structure resulting from changes in the DSD were determined by measuring droplet sizes and velocities, and gas temperature and composition, within the reacting spray held. The results show that more complete combustion occurs in sprays possessing narrower DSDs, even with the SMD held constant. Reductions in the burning efficiency of sprays with wider DSDs is attributed primarily to their greater relative populations of large fuel droplets and to droplet size-segregating effects occurring at the spray periphery, which affect mixing rates. The results establish the DSD as an important indicator of atomization quality and an independent parameter regarding spray combustion processes.

A theoretical study of the heating effectiveness of a composite porous radiant burner (PRB) is conducted. A one-dimensional laminar premixed flame model incorporating a radiatively participating inert porous medium consisting of two layers of different properties is used to describe the heat release/transfer processes. Combined conductive, convective, and radiative heat transfer is considered. The spherical harmonics method with the P-3 approximation is used to model the radiation part. A multistep reaction mechanism for premixed methane-air combustion is employed. A parametric study is carried out to determine the effect of the radiative properties of the two porous layers on burner performance. Calculations indicated that a significant improvement in the radiative output of a PRB can be attained by optimizing the burner properties upstream and downstream of the flame. Generally, the upstream layer should be of lower porosity, shorter length, and higher optical thickness than the downstream layer. Also, the upstream layer should be highly scattering, while the downstream layer should be nonscattering.

The development of an advanced dynamic model for aeroelastic hypersonic vehicles powered by air breathing engines requires an adequate engine model. This report provides a discussion of some of the more important features of supersonic combustion and their relevance to the analysis and design of supersonic ramjet engines. Of particular interest are those aspects of combustion that impact the control of the process. Furthermore, the report summarizes efforts to enhance the aeropropulsive/aeroelastic dynamic model developed at the Aerospace Research Center of Arizona State University by focusing on combustion and improved modeling of this flow. The expanded supersonic combustor model described here has the capability to model the effects of friction, area change, and mass addition, in addition to the heat addition process. A comparison is made of the results from four cases: (1) heat addition only; (2) heat addition plus friction; (3) heat addition, friction, and area reduction, and (4) heat addition, friction, area reduction, and mass addition. The relative impact of these effects on the Mach number, static temperature, and static pressure distributions within the combustor are then shown. Finally, the effects of frozen versus equilibrium flow conditions within the exhaust plume is discussed.

We have studied experimentally the stability and heat transfer characteristics of lean premixed, methane-air flames embedded in a porous layer. The work is directly relevant to understanding the performance and operating behavior of porous radiant burners (PRB). Flame speed and radiant output data were obtained for different stoichiometries and flame locations in porous ceramic foam. The results indicate that stable combustion at elevated flame speeds can be maintained in two different spatial domains: one spanning the upstream half of the porous region and the other in a narrow region near the exit plane. The heat release and radiant output are also found to increase as the flame is shifted toward the middle of the porous layer. A one-dimensional laminar premixed flame model incorporating a radiatively participating inert porous medium was used to describe the test conditions. Calculations using a one-step reaction confirmed the observation of two stable flame regions. The predicted flame speeds and radiant output agree favorably with the experimental trends.

An analysis has been carried out to determine the performance of porous radiant burners (PRB) as a function of fiber size. PRB made with silica or alumina fibers are considered. The radiative properties of the fibers are determined using the electromagnetic wave scattering theory for two different characteristics temperatures— 1000 and 1500°C. The properties are used in a combined-mode heat transfer model to calculate the amount of energy radiated by the PRB. It is found that fibers smaller than the order of 1 μm in diameter produce significantly higher radiant output. For a characteristic temperature of 1000°C, in some cases, the increase in output is as high as 63 and 109% for silica and alumina fibers, respectively. For a characteristic temperature of 1500°C, the corresponding increases are 72 and 150%, respectively.

A numerical study of combustion and multimode heat transfer in porous radiant burners is performed. Burner characteristics such as flame speeds, radiant outputs and efficiencies are investigated using a one-dimensional conduction, convection, radiation, and premixed flame model. The porous medium is assumed to emit, absorb, and scatter radiant energy. Non-local thermal equilibrium between the solid and gas is accounted for by introducing separate energy equations for the gas and the solid phase. Combustion is described by a one-step global mechanism. The effect of the optical depth, scattering albedo, solid thermal conductivity, upstream environment reflectivity, and interphase heat transfer coupling on the burner performance are studied. It was revealed that for maximizing the radiant output the optical depth should be about ten and the flame should be stabilized near the center of theporous medium. Also, low solid thermal conductivity, low scattering albedo, and high inlet environment reflectivity produced a high radiant efficiency.

Abstract This paper presents a numerical analysis of combustion and multimode heat transfer in inert porous media. The work is directly relevant to the understanding of premixed flame stabilization in porous radiant burners. The influence of the flame location, the radiative properties of the porous material, the solid thermal conductivity, and stoichiometry on the flame speed and stability are determined using a one-dimensional conduction, convection, radiation, and combustion model. The porous medium is allowed to emit, absorb, and scatter radiant energy, Non-local thermal equilibrium between the solid and gas is accounted for by introducing separate energy equations for the two phases. Heat release is described by a single-step, global reaction. The results indicate that stable combustion at elevated flame speeds can be maintained in two different spatial domains. Flame propagation near the edge of the porous layer is controlled mostly by solid-phase conduction; whereas, in the interior both solid conduction and radiation heal transfer are important. The radiative characteristics of the porous matrix such as the optical depth and scattering albedo were also shown to have a considerable effect on flame stability Keywords: Premixed flamesheal transferporous media

We have studied experimentally the stability and heat transfer characteristics of lean premixed, methane-air flames embedded in a porous layer. The work is directly relevant to understanding the performance and operating behavior of porous radiant burners (PRB). Flame speed and radiant output data were obtained for different stoichiometries and flame locations in porous ceramic foam. The results indicate that stable combustion at elevated flame speeds can be maintained in two different spatial domains: one spanning the upstream half of the porous are also found to increase as the flame is shifted toward the middle of the porous layer. A one-dimensional laminar premixed flame model incorporating a radiatively participating inert porous medium was used to describe the test conditions. Calculations using a one-step reaction confirmed the observation of two stable flame regions. The predicted flame speeds and radiant output agree favorably with the experimental trends. 20 refs., 4 figs.

Turbulent nonreactive and reactive flows with and without swirl are analyzed, with particular attention given to the flow fields of a gas-fueled nonpremixed swirl-stabilized combustor and a premixed opposed-jet combustor. Local mean flow properties, including velocity, temperature, and major species concentrations, are calculated by solving numerically the governing partial differential equations with associated submodels for turbulence and combustion. The results of the study indicate that the constant-density k-epsilon turbulence model provides a satisfactory representation of the aerodynamics in most practical combustor flows. The exception is the case of jet-stabilized combustor flow, due to the fact that the k-epsilon model cannot replicate the highly dissipative phenomenon found in such flows.