Robert E. Peck

Increased Hot-Plate Ignition Probability for Nanoparticle-Laden Diesel Fuel

The present study attempts to improve the ignition properties of diesel fuel by investigating the influence of adding aluminum and aluminum oxide nanoparticles to diesel. As part of this study, droplet ignition experiments were carried out atop a heated hot plate. Different types of fuel mixtures were used; both particle size (15 and 50 nm) as well as the volume fraction (0%, 0.1%, and 0.5%) of nanoparticles added to diesel were varied. For each type of fuel mixture, several droplets were dropped on the hot plate from a fixed height and under identical conditions, and the probability of ignition of that fuel was recorded based on the number of droplets that ignited. These experiments were repeated at several temperatures over the range of 688-768 degrees C. It was observed that the ignition probability for the fuel mixtures that contained nanoparticles was significantly higher than that of pure diesel.

A numerical analysis of heat transfer and combustion in porous radiant burners

Abstract 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.

Numerical Simulations of Heat Transfer and Fluid Flow Problems Using an Immersed-Boundary Finite-Volume Method on NonStaggered Grids

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.

Flame Stabilization and Multimode Heat Transfer in Inert Porous Media: A Numerical Study

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 cond...

An experimental and theoretical study of porous radiant burner performance

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.

ANALYSIS OF A BILAYERED POROUS RADIANT BURNER

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 indicate 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.

KINETIC MODELING OF FUEL-NITROGEN CONVERSION IN ONE-DIMENSIONAL, PULVERIZED-COAL FLAMES

Abstract A detailed reaction mechanism for converting fuel nitrogen to nitric oxide and molecular nitrogen in gas flames is used to model nitrogen chemistry in fuel-rich coal-dust/oxidizer flat flames. In the devolatilization zone a simplified pyrolysis model is used and a hydrocarbon oxidation scheme supplement the nitrogen reactions. The predicted distributions of nitrogenous species (HCN, NH3, NO and N2) are compared to time-resolved experimental data obtained for two stoichiometrics and coal-types. The model accounts for the radical removal on particle surfaces, and various heterogeneous reactions for reduction of NO are considered. In the devolatilization flame zone the coal-N conversion mechanisms appear to be augmented by physical and chemical processes occurring in the vicmity of the devolatilizing particles. The current analysis supported by other investigations indicate that processes such as formation of fuel-rich volatile clouds and heterogeneous reduction of NO on surfaces of coal-particles a...

Improving the performance of porous radiant burners through use of sub-micron size fibers

Abstract 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.

Modelling of Jet- and Swirl-stabilized Reacting Flows in Axisymmetric Combustors

Abstract Turbulent reacting flows with and without swirl are calculated using fast and finite-rate chemistry models and closure is effected by extending constant-density turbulence models to reacting flows. Three experimental combustor flow fields are compared with the calculations: one is a premixed, opposed-jet combustor and the other two are non-premixed, sudden-expansion combustors with and without swirl. The results indicate that an isotropic eddy viscosity model based on the turbulence kinetic energy (k) and its dissipation rate (e) is sufficient to represent turbulence in non-swirling, reacting flows, whereas an algebraic stress model provides a better overall mean field prediction for reacting flows with swirl. However, flow expansion due to heat release during combustion is only fairly well represented by the submodels used. A finite-rate chemistry model is found to be superior to the fast chemistry approximation in the non-premixed combustor and a two-step global reaction mechanism gives an adeq...

An experimental study of high-temperature, oxidative pulverized coal devolatilization

Flat-flame coal-dust burner and entrained-flow reactor experiments were performed to study the devolatilizationof size-graded Pittsburgh #8 coal samples under rapid heating, oxidizing conditions. The flat-flame burner characterized by high heating rates, high temperatures, and short residence times was used on three different particle-size cuts and two proximate volatiles-to-oxygen equivalence ratios. The entrained-flow reactor characterized by high heating rates, lower temperatures, and longer residence times was used at two different ambient temperatures. Solid samples were collected at different positions and thereby different residence times in the flames and subjected to ultimate and TGA proximate analysis. The results indicate that, in rapid heating systems (10 5 K/s), release of primary volatiles occurred within 5–10 ms. The 10–20- and 30–40- μ m fractions gave approximately the same total volatile yield of 50% and peak temperatures of 1550 K, while the conversion for a 0–75- μ m fraction was 5% higher with measured peak temperatures 100 K higher, indicating in situ combustion of small particles and devolatilization of large particles. Increasing equivalence ratio lowered the temperatures around 100 K, and volatiles yield decreased 5%. Devolatilization kinetics for flat-flame burner (FFB) and entrained-flow reactor (EFR) were fit by the distributed activation energy parallel reaction model using literature data combined with a total volatile yield estimate of 52% and ultimate analysis data from the flame experiments. Thermogravimetric analyzer (TGA) devolatilization profiles of the collected samples and comparisons of characteristic temperature and time of devolatilization to time of diffusion were found to be very useful tools in the analyzing process of the experimental results.