ACERC
Abstracts - 1992 |
ACERC
Abstracts - 1992 |
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Thrust Area 4: Turbulent, Reacting Fluid Mechanics and Heat Transfer |
McMurtry, P.A. and Givi,
P.
Chapter 14, Finite Element Methods in Fluids, 8:355, 1992. Funded by
National Science Foundation, NASA Lewis Research Center and ACERC.
The purpose of this chapter is to describe the implementation of the spectral-element technique for simulating a chemically reacting, spatially developing turbulent mixing layer. Some of the important experimental and numerical studies that have been directed at investigating the development, evolution, and mixing characteristics of shear flows are reviewed. These works provide a framework for subsequent discussions on the formulation of the spectral-element method. The mathematical formulation of the physical configuration of the spatially developing reacting mixing layer, together with a rather detailed presentation on the application of the spectral-element for the numerical simulation of mixing layers is given. Results from two- and three-dimensional calculations of chemically reacting mixing layers are discussed. Finally, conclusion and speculation for future related work are given.
Bonin, M.P.
Optical Measurement of Particle Size, Velocity and Number Density in Two-Phase,
Isothermal and Reacting Flows, Ph.D./BYU, April 1992. Advisor: Queiroz
Barron, J.T. and Queiroz,
M.
Heat Transfer in Fire and Combustion Systems, 199:183-188, 1992. Funded
by Air Force Office of Scientific Research and ACERC.
The effects of distributed combustion on acoustic growth rates in a modified Rijke burner has been investigated. Three different particle types were used (25 and 43-µm aluminum, 7-µm zirconium carbide and 10- and 43-µm aluminum oxide) at mass loadings between 0% and 10%. The results indicate that the degree of acoustic driving or damping is a function of frequency of oscillation and the type, size and concentration of particles. Acoustic driving was produced by 25- and 43-µm aluminum, and 7-µm zirconium carbide particles. The greatest amount of driving was produced by the 25-µm zirconium carbide particles. The effects of distributed combustion and particle damping were also found to be strongly dependent on frequency. Aluminum oxide particles at low concentrations (<=2%) caused acoustic driving most probably because of flame catalytic effects. At high concentrations, the acoustic oscillations were damped because of viscous damping effects.
Shirolkar, J.S.; Queiroz,
M. and McMurtry, P.A.
Heat Transfer in Fire and Combustion Systems, 223:97-103, 1992. (Also
presented at the ASME National Heat Transfer Conference, San Diego, CA,
August 1992). Funded by ACERC.
Data from a three-dimensional numerical simulations for a binary single step chemical reaction in a temporally developing turbulent shear layer were used to study the dissipation statistics of a conserved scalar as well as of temperature. Two specific chemical reaction cases were considered: isothermal reaction and chemical reaction with moderate heat release. The average and rms profiles of the mixture fraction dissipation are presented. The study indicated that in both cases the mixture fraction dissipation is related to the reaction rate. The temperature dissipation was found to be lognormally distributed in a region where there was significant chemical reaction. Also features of the temperature dissipation compare qualitatively well with experimental results reported in the literature.
Menon, S.; McMurtry, P.A.
and Kerstein, A.R.
Linear Eddy Mixing of Complex Engineering and Geophysical Flows, 1992
(in press). Funded by US Department of Energy, NASA Lewis Research Center and
ACERC.
To improve predictive capabilities for modern combustion processes, sophisticated models that faithfully represent the physics of turbulent mixing and reaction are required. Models that are presently used to study combustion systems, such as those based on gradient diffusion assumptions, omit important aspects of the subgrid mixing process. More sophisticated methods based on probability density function (PDF) transport equations treat the reaction process exactly, but the molecular mixing submodels that are employed are not fully satisfactory. Although the utility of these models should not be overlooked, there is clearly a need to develop alternative methods for calculating turbulent mixing processes.
In modeling the scalar mixing process, one encounters difficulties not present in modeling momentum transport. These difficulties can be primarily attributed to the interactions among turbulent stirring, molecular diffusion, and chemical reaction at the smallest scales of the flow. A reliable model of subgrid mixing and reaction should therefore include and distinguish among these distinctly different physical processes. To accomplish this, it appears that a comprehensive description of the scalar microfield is needed. This is fundamentally different from momentum transport modeling, in which the main influence of the small scales is to provide dissipation for the large-scale structures. Thus, while the effect of subgrid stresses on the momentum transport can be reasonably treated with various eddy viscosity models, a similar characterization of the subgrid scalar field in terms of an eddy diffusivity is neither sufficient nor correct, since an accurate description of the small-scale dynamics is critical to the overall mixing and the combustion process. As a result of these features of the turbulent mixing process, progress in the development of subgrid mixing models for us in large eddy simulation (LES) has been limited.
These fundamental issues are relevant to both the solution of the Reynolds-averaged flow field (where only the mean motion is of interest) and the solution of the unsteady flow field simulated using LES techniques. In this chapter, the development of a subgrid modeling approach specifically directed at scalar mixing is addressed. The model is based on Kerstein's (1988, 1991) linear eddy model. The emphasis here is placed on premixed combustion applications, although the general formulation has also been applied to turbulent diffusion flames (McMurtry et al., 1991, 1992).
Menon, S.; McMurtry, P.A.
and Kerstein, A.R.
AAIA Journal of Jet Propulsion and Power, 1992 (in press). (Also presented
at the AIAA Annual Aerospace Sciences Meeting, Reno, NV, January 1992).
Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
An advanced gas turbine engine to power supersonic transport aircraft is currently under study. In addition to high combustion efficiency requirements, environmental concerns have placed stringent restrictions on the pollutant emissions from these engines. A dual-stage combustor with the potential for minimizing pollutants such as NOx emissions is undergoing experimental evaluation. A major technical issue in the design of this combustor is how to rapidly mix the hot, fuel-rich primary stage product with the secondary diluent air to obtain a fuel-lean mixture for combustion in the secondary stage. Numerical design studies using steady-state methods cannot account for the unsteady phenomena in the mixing region. Therefore, to evaluate the effect of unsteady mixing and combustion processes, a novel unsteady mixing model is demonstrated here. This model has been used in a stand-alone mode to study mixing and combustion in hydrogen-air nonpremixed jet flames. NOx production in these jet flames was also predicted. Comparison of the computed results with experimental data shows good agreement thereby providing validation of the mixing model. This mixing model has been developed so that it can also be implemented within steady-state prediction codes and thus, may eventually provide an improved engineering design analysis tool.
McMurtry, P.A.; Gansuage,
T.C. and Kerstein, A.R.
Physics of Fluids A., 1992 (in press). (Also presented at the Thirteenth
Symposium on Turbulence, Rolla, MI, September 1992). Funded by US Department
of Energy, Office of Naval Research and ACERC.
The linear eddy-mixing model is used to predict the evolution of a decaying scalar field in statistically steady homogeneous turbulent flow over a wide range of Reynolds and Schmidt numbers. Model results at low Reynolds number and order unity Schmidt number are shown to be in good overall agreement with direct numerical simulations. Results at higher Schmidt and Reynolds numbers reproduce conventional scaling properties of the scalar statistics. Predictions of Schmidt number and Reynolds number sensitivity of the evolution of the scalar concentration probability density function are presented and interpreted.
Chen, C; Riley, J.J. and
McMurtry, P.A.
Combustion and Flame, 87:257-277, 1992. Funded by Office of Naval Research
and ACERC.
This article presents an investigation of the Favre averaging method for turbulent flows with chemical reaction. A set of data from direct numerical simulations of a chemically reacting turbulent mixing layer is employed. Favre-averaged quantities are compared directly with their corresponding Reynolds-averaged values. The gradient transport assumptions in the kappa-epsilon model in Favre-averaged form are also tested. Finally the transport equations for the Reynolds stress and scalar flux of the chemical product are studied term by term. Some Favre-averaged quantities such as u, ˜c, and ˜k are very similar numerically to their Reynolds-averaged values. Other Favre-averaged quantities, however, like ˜e, ˜p, ˜u, and ˜v, are significantly different from their Reynolds-averaged values. The gradient transport models generally work rather well when the mixing layer is in a naturally developing turbulent state, although some important weaknesses are noted. Some significant effects of pressure on they Reynolds stress and scalar flux are exhibited.
McMurtry, P.A.; Cremer,
M. and Kerstein, A.R.
APS Division of Fluid Dynamics, Tallahassee, FL, November 1992. Funded
by US Department of Energy and ACERC.
The linear eddy-mixing model is used to study effects of the turbulence length scale distribution on the transient evolution of a passive scalar in a statistically steady, homogeneous turbulent flow. Model simulations are carried out using length-scale distributions that are respectively wide-banded and narrow-banded, representing mixing at high and low Reynolds number. The two cases found to exhibit qualitative differences in mixing behavior. The low Reynolds number linear eddy results exhibit behaviors not seen in DNS studies, such as trimodal transient scalar pdf's. It is found that asymptotically non-Gaussian scalar pdf's may be obtained both for high and low Reynolds number cases.
The sensitivity of mixing behavior to flow bandwidth is interpreted mechanistically. In particular, a geometrical representation of scalar fields in turbulence based on a lamellar picture is shown to exhibit analogous sensitivity to bandwidth. Under assumptions corresponding to narrow-band mixing, this "Clipped laminar profile" representation reproduces the pdf evolution previously obtained using mapping closure. Thus, the mapping closure solution as well as the DNS is found to be narrow-band in character. This interference is consistent with the absence of length-scale considerations in the mapping closure analysis, and with the good agreement of the mapping closure solution with DNS results.
Dawson, R.W. and Queiroz,
M.
IV Encontro Nacional de Ciencias Termicas (ENCIT 92), Rio de Janeiro,
Brazil, December 1992. Funded by ACERC.
Presented in this paper is an experimental investigation of the effects of fuel droplets on temperature dissipation. For this purpose, temperature dissipation measurements have been completed in a lifted premixed propane flame issuing from a converging nozzle. Hexane droplets with initial diameter of 50µm were then introduced into the core of the flame while dissipation measurements were repeated. Profiles of dissipation show that the evaporating droplets cause a reduction in the temperature gradient at the core of the flame and at the outer shear layer. Deviations from the lognormal distributions in the outer shear layer are shown to be less significant and occur closer to the centerline when droplets are present.
Rasmussen, K.G. and Queiroz,
M.
Int. Journal of Exp. Heat Transfer, 1992 (in press). [Previously presented
at The III Encontro Nacional de Ciencias Termicas (ENCIT 90), Santa Caratina,
Brazil, 1990.] Funded by ACERC.
Gas temperature measurements have been completed in a lifted propane flame with and without droplets issuing from a contoured nozzle at several radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, due to local evaporative cooling effects. Rms temperature profiles with two local extremes at either side of the average reaction zone were observed for 5 < x/D < 20 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jet's fuel-rich, central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, due to an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed to flow field in the core region, to the point of inhibiting a complex vortical structure existent in the gaseous flame.
McMurtry, P.A. and Givi,
P.
AIAA Progress in Engineering Series, 135:257-301, 1992. Funded by National
Science Foundation, NASA Lewis Research Center and ACERC.
The Navier-Stokes equations, along with appropriate conservation equations of energy and chemical species, are generally accepted to provide an "exact" model for most turbulent combustion phenomena of interest. Unfortunately, the complexity of these equations prohibits both their analytic and numerical solutions except under idealized conditions. To provide engineering tools to predict the performance of turbulent combustion systems, it is necessary to resort to approximate analyses in which the effects of turbulence are not treated exactly, but are incorporated by means of turbulence models.
An overview of spectral methods is given, together with a discussion of their implementation in the numerical solution of fluid transport. Various classifications of spectral methods and their convergence properties are described. The "spectral element" method, which constitutes a new category of spectral approximations, is also presented. Its flexibility in dealing with complex flow geometries is highlighted. A review of the recent applications of spectral methods to reacting flow problems is also given. Emphasis is placed primarily on the interpretation of the physical phenomena captured by spectral simulations of reactive flows. There is some mention of spectral simulations of the turbulent mixing of a passive scalar, but only where an explicit connection to chemical reactions has been made. We conclude with an evaluation for the benefits and limitations of spectral methods, and some speculation of their contributions for turbulent combustion research in the future.
McMurtry, P.A.; Menon, S.
and Kerstein, A.R.
Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute,
Sydney, Australia, July 1992. (Also presented at the APS Division of Fluid
Dynamics, Tallahasee, Fl, November 1992 and the AIAA Annual Aerospace
Sciences Meeting, Reno, NV, Janaury 1992.) Funded by U.S. Department of
Energy, NASA Lewis Research Center and ACERC.
A new sub-grid mixing model for use in large eddy simulations of turbulent combustion is presented and applied to a hydrogen-air diffusion flame. The sub-grid model is based on Kerstein's Linear Eddy Model (Comb. Sci. Tech. 60, 391, 1988), which reduces the description of the scalar field to a locally one-dimensional representation. The formulation involves performing separate linear eddy calculations in each cell to describe the small-scale scalar mixing and reaction process. Convective transport across grid surfaces is accomplished by "splicing" events by which linear eddy elements are copied to and from neighboring grid cells based on the grid-resolved velocity field.
The model is first used to predict the mixing of a conserved scalar in a turbulent shear flow. The model correctly predicts the behavior of the pdf of the scalar field. In particular, it displays a non-marching peak at the preferred mixture fraction as the shear layer is traversed. It is then illustrated how a reduced chemical mechanism can be implemented within the linear eddy subgrid model formulation. The model is used to predict NO formation in a hydrogen-air diffusion flame using a reduced chemical mechanism involving nine reactive scalars.
Butler, B.W., Wilson, T.
and Webb, B.W.
Twenty-fourth Symposium (International) on Combustion/The Combustion Institute,
Sydney, Australia, July 1992. Funded by Empire State Electric Energy Research
Corp. and ACERC.
This paper reports experimental measurements of the time-resolved particle temperature fluctuations in an 80 MWe utility boiler. Results are presented in terms of mean and RMS temperature fluctuations, as well as power spectra and probability density functions of temperature. Results show significant variations in mean and RMS temperature with elevation in the boiler; RMS temperature fluctuations are high in the burner planes, decay to low levels just below the boiler nose, then increase again as the combustion products proceed over the nose toward the convective pass. Power spectra of the temperature fluctuations reveal significantly stronger high frequency content in the burner plane than at higher elevations. Preferred frequencies were observed downstream of the superheater pendants, and are believed to be due to vortex shedding from the pendant tubes. Probability density functions reveal that the turbulent fluctuations in particle temperature are not Gaussion in the flame zone, but exhibit very nearly normal distributions after complete burnout.
Lauterbach, D.
Effect of Inlet Geometry on the Mixing of an Incompressible Turbulence Jet
in a Crossflow, M.S./U of U, June 1992. Advisor: McMurtry
Shirolkar, J.S.
An Analytical Study of Particle Dispersion in Dilute, Particle-Laden Reacting
and Nonreacting Turbulent Flows, M.S./BYU, December 1992. Advisor: Queiroz
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