Flameless combustion has been proposed as a technique to increase efficiency and reduce pollutant formation in furnaces, power plants and combustion engines. Despite considerable activity over the past fifteen years, very little research has concentrated on obtaining an understanding of the processes that underlie their successful operation. In particular, the role of thermal radiation in determining the thermal environment and modifying the combustion processes has received only modest attention. The relatively large thickness of the combustion zone at any fuel-oxidizer interface enhances the role of radiation. Moreover, the interaction of fluctuations in temperature, radiation, and velocity fields in a hot, nearly uniform environment characteristic of a furnace operating in a flameless combustion environment has been ignored. This proposal focuses on both these phenomena, using a unique approach involving a new formulation of both problems. The aim is to develop and apply a mathematical, computationally tractable theory to study these processes with a view to producing relatively simple procedures to determine how intense radiation modifies classical ideas of combustion heat release and thermal and velocity fluctuations in a flameless combustion environment.
Intellectual Merit: Existing theories of combustion heat release rely heavily on the notion of a thin flame. Velocity fluctuations in the hot, nearly uniform environment associated with furnace configurations are typically assumed to be caused by turbulence induced by fuel and oxidized jets. This work will present results that do not rely on the thin flame assumption to determine combustion energy release. It will also show how velocity fluctuations can arise in large heated volumes without the need for turbulent flow, due to the influence of thermal radiation.
Broader Impacts: This work will furnish the beginnings of the knowledge required to develop good design tools for efficient, low pollution furnaces and related combustion devices. Furnace design has been aided by the use of computer simulation techniques, particularly those implemented in computational fluid dynamics codes. However, such codes are no better than the physical models upon which they are based. Since turbulent flow models are always incorporated in such codes, there is necessarily a large amount of empiricism in them. Thus, it is not possible to gain a fundamental understanding of the basic principles underlying good flameless furnace design in the absence of knowledge of the phenomena themselves that will be developed by this project.
Radiative flameless combustion is a mode of combustion that promises significant improvements in the energy efficiency of industrial furnaces, and opens the possibility that the production of certain pollutants like NOx can be greatly reduced. The purpose of this grant was to undertake some fundamental studies that shed light on the conditions needed to achieve this type of combustion, and determine the paramters that control thee ondtirions. The work undertaken in this grant has served several purposes. First and most important, it has been shown that radiative flameless combustion can be studied much like any other combustion process, using a mixture of analytical and numerical techniques to set up and analyze a simplified model that contains the essential physics of the problem. The results obtained demonstrate how the phenomena of interest depend on the dimensionless parameters that control flameless combustion. Two canonical problems have been considered; radiative flameless combustion in a spatially uniform strain field and the analogous problem in a planar momentum jet. The analysis of the spatially uniform strain field demonstrated that at a sufficiently small spatial scale, any non-premixed combustion problem reduces to this geometry, an observation made previously for flaming non-radiative combustion problems The radiative flameless combustion problem for a uniform strain field reduces to the analysis of a four parameter system of equations in one spatial dimension. The boundaries of the flameless combustion region in the four parameter space were studied by seeking solutions to the steady state problem and assuming the boundaries to occur where a steady state solution could no longer be found. Some support for both the approach and the results obtained was furnished by Prof. A. Atreya who used data obtained from his furnace experiment at the University of Michigan to demonstrate the consistency between his observed flameless combustion regime and that predicted by the analysis. The analysis of flameless radiative combustion in the planar momentum jet is described in detail for both flaming and flameless combustion. This configuration was selected because it is inherently two-dimensional, and because any furnace will have fuel jets exiting into a large spatial domain that can be reasonably approximated as infinite on the scale of the jet. In contrast with the uniform strain problem, the strain rate is neither prescribed nor uniform, but must be found as part of the solution for the flow field. Similarly, the parameters characterizing both the radiative losses and chemical energy release are multiplied by functions that vary with position along the jet centerline. Despite these complications, there are important similarities with the uniform strain configuration. There are still four dimensionless parameters that complete the mathematical description of the problem, which is now formulated as a system of two-dimensional equations. Here, emphasis has been placed on the changes in jet characteristics induced by radiative losses and slow chemistry in a hot environment. The most important parameters from the viewpoint of controlling furnace operation have been identified, and it is hoped that future work will enable the methodology developed here to be applied to the operation of real furnaces, with consequent improvement in efficiency and pollutant reduction.
