ACERC
Abstracts - 1991 |
ACERC
Abstracts - 1991 |
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Thrust Area 4: Turbulent, Reacting Fluid Mechanics and Heat Transfer |
Boyer, L. and Queiroz, M.
Combustion Science and Technology, 79:1-34, 1991. Funded by ACERC.
Temperature dissipation measurements have been completed in a lifted turbulent non-premixed propane flame issuing from a converging nozzle at several axial stations and along the centerline. The radial and axial components of the temperature dissipation were measured directly. The assumption of log normality is shown to be a good approximation of the temperature dissipation character in the core of the flame out to ro, the radius where the mean temperature is at a maximum. Significant deviations from the lognormal distribution are observed on the outer side of the shear layer. The radial and axial components of the dissipation are reasonably isotropic in the region close to the jet's centerline. However, in direct contrast to the results found in nonreacting flows, detailed comparisons of the radial and axial temperature dissipation profiles in the off-axis region of 0.1 < r/ro < 1.3 indicate that there are definite anisotropic and non self-similar characteristics in the temperature dissipation. Furthermore, joint probability density functions between temperature and its dissipation components have shown that they are uncorrelated on axis, yet both negative and positive correlation existed further outward from ro at all the axial stations measured.
Butler, B.W. and Webb, B.W.
Fuel, 70:1457, 1991. Funded by Empire State Electric Energy Research
Corp. and ACERC.
This paper reports measurements of local gas temperatures and wall incident radiant heat flux in an 80 MWe pulverized coal corner fired boiler. Spatially resolved gas temperatures were measured using a 4 m long, triply shielded suction pyrometer and total wall radiation was determined with an ellipsoidal radiometer. The data include detailed wall radiant heat flux measurements made around the periphery of the boiler at six different elevations. Local gas temperature profiles were measured at four axial positions in the boiler, with special attention to the near-burner region. Boiler operational data and coal proximate, ultimate and particle size analyses are also reported. Local gas temperatures in the boiler reached a maximum of nearly 1800 K near the burners and decayed to 1250 K at a position just above the boiler nose. In the burner plane gas temperatures varied from 600 K near the wall to 1800 K at the center of the flame. Wall incident radiant heat fluxes varied between 440 kW/m² in the near-burner region to 100 kW/m² near the boiler nose. The radiation transport to the wall was observed to vary substantially around the periphery of the boiler, especially in the near-burner regions.
Son, S.F.; McMurtry, P.A;
and Queiroz, M.
Combustion and Flame, 85:51-67, 1991. Funded by ACERC.
Three-dimensional direct numerical simulations were used to study the effect of heat release from a binary, single-step chemical reaction on the statistical properties of a temporally developing turbulent mixing layer. Various statistical moments, probability density functions, power spectral densities, and autocorrelations of a conserved scalar, and the velocity field are presented. Scalar-velocity and pressure-velocity correlations, and joint probability density functions, which are extremely difficult to measure experimentally, were also calculated from the simulations. Many features of the calculated statistics compare qualitatively well with results reported from related experimental studies. Significant changes in the vortex structure occur with moderate heat release, resulting in more diffuse vortices than in the isothermal simulation. Consequently, slower rotation rates of the coherent structures occur with moderate heat release. This effect has previously been shown to be caused by the baroclinic torques and thermal expansion in the mixing layer. The statistics in this study reflect these changes in the vortex structure due to moderate heat release.
Chen, C; Riley, J.J.; and
McMurtry, P.A.
Combustion and Flame, 1991 (in press). Funded by 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 k - 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 are very similar numerically to their Reynolds-averaged values. Other Favre-averaged quantities, however, like and , 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.
Bonin, M.P. and Queiroz,
M.
Combustion and Flame, 85:121-133, 1991. Funded by ACERC.
Parametric, in-situ, particle velocity, size and number density measurements have been made in a full scale, coal burning power plant using an optical diagnostic technique. Available ports in the boiler allowed measurement at three locations above the burner level. Variable test parameters included furnace load, excess air, and burner tilt, using a medium volatile bituminous coal. Higher particle velocities were observed when the boiler was operated at a maximum capacity due to increased air and coal flows. Port-to-port velocity variations were attributed to the rotational nature of the mean flow, changes in gas density with changing gas temperature, and the interaction of the flow with the boiler nose. Measured particle number density profiles were characterized by high values in the small particle size class (< 2 µm), decreasing exponentially with increasing particle size. The measured number density profiles indicated that the combustion process is largely complete at locations 7 m above the burners and that the particles measured consisted primarily of ash, a conclusion that is also supported by the percent carbon-in-ash data. The mass-mean and number-mean particle sizes for all tests varied between 10 and 45, and 0.5 and 0.85 µm, respectively. The characteristic similarity between the particle size distribution of the ash and that of the parent char, previously documented in laboratory scale investigations, was also observed in the present study. Cumulative mass distribution profiles indicated that a significant centrifugal effect is exerted on the condensed phase by the rotating flow. An increase in small particle number density (~ 0.5 µm) was also apparent at lower boiler loads due to changes in the combustion process occurring at these operating conditions, which affect the various modes of ash particle formation.
Dawson, R.W.; Boyer, L.
and Queiroz, M.
Heat Transfer in Combustion Systems, 166:59-67, (R.J. Santoro and J.D.
Felske, eds.), The American Society of Mechanical Engineers, 1991. Funded by
ACERC.
This paper discusses the design of a dual thermocouple probe to measure temperature dissipation statistics in turbulent reacting flows. The probe is described in detail and an analysis is made of its advantages and accuracy, including a parametric study on the effect of wire spacing, sampling frequency, time constant, and cutoff frequency on the calculations of temperature dissipation statistics. Also, measurements performed with the probe in a partially premixed, lifted propane flame are briefly discussed to demonstrate the probe's capability. The measurements reported include mean and rms profiles and normalized probability density function of the lognormal radial component of temperature dissipation, as well as joint statistics between the temperature and its dissipation, represented by joint probability density functions.
McMurtry, P.A.; Menon, S.
and Kerstein, A.R.
Fourth International Conference on Numerical Combustion, St. Petersburg,
FL, November 1991. Funded by US Department of Energy, NASA Lewis Research Center
and ACERC.
A new subgrid modeling technique is described for use in large eddy simulations of turbulent mixing and reaction. In particular, a model for mixing and chemical reactions at the subgrid level is developed based on the Kerstein's linear eddy approach. A unique feature of this model is the separate treatment of turbulent convection and molecular diffusion over all length-scales of the flow. This distinction, which is not retained in existing mixing models, is critical for predicting the small scale mixing process and capturing Schmidt number dependency and the effects of finite rate reaction.
Mengüç, M.P. and Manickavasagam,
S.
University of Kentucky and ACERC Report, August 1991. Funded by Pittsburgh
Energy Technology Center.
The effective radiative properties of pulverized-coal particles have been obtained. Three different experiments have been carried out using narrow-size distribution of Utah Blind Canyon coal. In two of the experiments, the measurements were performed in situ without altering the particle morphology. For this purpose, a test cell was designed to generate a vertically downward coal flow. A CO2-laser nephelometer system was employed to obtain the scattering phase function of pulverized-coal particles at 10.6 µm. An inverse radiation analysis was followed to reduce the experimental data. Also, a Mercury-arc lamp monochromator system was used to measure the effective absorption coefficients of the same coals within the visible wavelength spectrum.
The results show that the "effective" scattering phase function of coal particles are highly forward scattering and show less sensitivity to the size than predicted from the Lorenz-Mie theory. The main reason for this is that the smaller size particles, which are always present in the coal cloud, contribute to scattering and absorption significantly. In addition to this, it was observed that in the visible wavelength range the coal absorption is not gray: at wavelengths about 440 and 550 nm, there is about 10% decrease in the absorption coefficient compared to the rest of the spectrum. This observation is especially important for the two-color pyrometry experiments used to determine particle temperatures.
The two experimental approaches followed in this study are unique in a sense that the physics of the problem is not approximated. The properties determined include all uncertainties related to the particle shape, size distribution, inhomogeneity and spectral complex index of refraction data. From these experiments, it is possible to observe the spectral behavior of coal radiative properties within the visible wavelength spectrum. However, the spectral range considered was not extensive.
In order to obtain radiative property data over a wider wavelength spectrum, additional experiments have been carried out using a Fourier Transform Infrared (FT-IR) Spectrometer. For this purpose, thin pellets were prepared by mixing coal particles with Potassium Bromide (KBr). The spectral measurements were performed over the wavelength range 2.5 to 20 µm. These results were interpreted to obtain the "effective" absorption efficiency factor of coal particles. The results clearly show that the coal/char radiative properties display significant wavelength dependency.
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