Publications

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Direct numerical simulation of a three-dimensional spatially developing turbulent slot-burner Bunsen flame has been performed with a new reduced methane-air mechanism. The mechanism, derived from sequential application of directed relation graph theory, sensitivity analysis and computational singular perturbation over the GRI-1.2 detailed mechanism is non-stiff and tailored to the lean conditions of the DNS. The simulation is performed for three flow through times, long enough to achieve statistical stationarity. The turbulence parameters have been chosen such that the combustion occurs in the thin reaction zones regime of premixed combustion. The data is analyzed to study possible influences of turbulence on the structure of the preheat and reaction zones. The results show that the mean thickness of the turbulent flame, based on progress variable gradient, is greater than the corresponding laminar flame. The effects of flow straining and flame front curvature on the mean flame thickness are quantified through conditional means of the thickness and by examining the balance equation for the evolution of the flame thickness. Finally, conditional mean reaction rate of key species compared to the laminar reaction rate profiles show that there is no significant perturbation of the heat release layer.

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The influence of thermal stratification on autoignition at constant volume and high pressure is studied by direct numerical simulation (DNS) with detailed hydrogen/air chemistry. Parametric studies on the effect of the initial amplitude of the temperature fluctuations, the initial length scales of the temperature and velocity fluctuations, and the turbulence intensity are performed. The combustion mode is characterized using the diagnostic measures developed in Part I of this study. Specifically, the ignition front speed and the scalar mixing timescales are used to identify the roles of molecular diffusion and heat conduction in each case. Predictions from a multizone model initialized from the DNS fields are presented and differences are explained using the diagnostic tools developed. © 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The influence of thermal stratification on autoignition at constant volume and high pressure is studied by direct numerical simulation (DNS) with detailed hydrogen/air chemistry with a view to providing better understanding and modeling of combustion processes in homogeneous charge compression-ignition engines. Numerical diagnostics are developed to analyze the mode of combustion and the dependence of overall ignition progress on initial mixture conditions. The roles of dissipation of heat and mass are divided conceptually into transport within ignition fronts and passive scalar dissipation, which modifies the statistics of the preignition temperature field. Transport within ignition fronts is analyzed by monitoring the propagation speed of ignition fronts using the displacement speed of a scalar that tracks the location of maximum heat release rate. The prevalence of deflagrative versus spontaneous ignition front propagation is found to depend on the local temperature gradient, and may be identified by the ratio of the instantaneous front speed to the laminar deflagration speed. The significance of passive scalar mixing is examined using a mixing timescale based on enthalpy fluctuations. Finally, the predictions of the multizone modeling strategy are compared with the DNS, and the results are explained using the diagnostics developed. © 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Three-dimensional direct numerical simulation of a spatially developing slot-burner Bunsen flame has been performed. The simulation is aimed at better understanding the dynamics of turbulent premixed flames in the thin reaction zones regime. A reduced chemical model for methane-air chemistry consisting of 13 resolved species, 4 quasi-steady state species and 73 elementary reactions has been developed specifically for the current simulation. Using the new chemical model a lean premixed methane-air flame at preheated conditions and ambient pressure is simulated. The simulation is performed long enough to achieve statistical stationarity. The data is analyzed to study possible influences of turbulence on the flame thickness. The results show that the average flame thickness increases, in agreement with a few, although not unanimous, experimental results.

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Power sources capable of supplying tens of watts are needed for a wide variety of applications including portable electronics, sensors, micro aerial vehicles, and mini-robotics systems. The utility of these devices is often limited by the energy and power density capabilities of batteries. A small combustion engine using liquid hydrocarbon fuel could potentially increase both power and energy density by an order of magnitude or more. This report describes initial development work on a meso-scale external combustion engine based on the Stirling cycle. Although other engine designs perform better at macro-scales, we believe the Stirling engine cycle is better suited to small-scale applications. The ideal Stirling cycle requires efficient heat transfer. Consequently, unlike other thermodynamic cycles, the high heat transfer rates that are inherent with miniature devices are an advantage for the Stirling cycle. Furthermore, since the Stirling engine uses external combustion, the combustor and engine can be scaled and optimized semi-independently. Continuous combustion minimizes issues with flame initiation and propagation. It also allows consideration of a variety of techniques to promote combustion that would be difficult in a miniature internal combustion engine. The project included design and fabrication of both the engine and the combustor. Two engine designs were developed. The first used a cylindrical piston design fabricated with conventional machining processes. The second design, based on the Wankel rotor geometry, was fabricated by through-mold electroforming of nickel in SU8 and LIGA micromolds. These technologies provided the requisite precision and tight tolerances needed for efficient micro-engine operation. Electroformed nickel is ideal for micro-engine applications because of its high strength and ductility. A rotary geometry was chosen because its planar geometry was more compatible with the fabrication process. SU8 lithography provided rapid prototypes to verify the design. A final high precision engine was created via LIGA. The micro-combustor was based on an excess enthalpy concept. Development of a micro-combustor included both modeling and experiments. We developed a suite of simulation tools both in support of the design of the prototype combustors, and to investigate more fundamental aspects of combustion at small scales. Issues of heat management and integration with the micro-scale Stirling engine were pursued using CFD simulations. We found that by choice of the operating conditions and channel dimensions energy conversion occurs by catalysis-dominated or catalysis-then-homogeneous phase combustion. The purpose of the experimental effort in micro-combustion was to study the feasibility and explore the design parameters of excess enthalpy combustors. The efforts were guided by the necessity for a practical device that could be implemented in a miniature power generator, or as a stand-alone device used for heat generation. Several devices were fabricated and successfully tested using methane as the fuel.