Dal Kang

ABSTRACT Fuel-air mixing behavior under the influence of imposed acoustic oscillations has been studied by investigating the response of the fuel mixture fraction field. The distribution of local fuel mixture fraction inside the mixing zone, which is expected to evolve into the local equivalence ratio in the flame zone, is closely coupled to unstable and oscillatory flame behavior. The Experiment was performed with an aerodynamically-stabilized non-premixed burner. In this study, acoustic oscillations were imposed at 22, 27, 32, 37, and 55Hz. Phase-resolved acetone PLIF was used to image the flow field of both isothermal and reacting flow cases and this data along with the derived quantities of temporal and spatial unmixedness were employed for analysis. The behavior of the unmixedness factor is compared with the previous measurements of oscillations in the flame zone. This comparison shows that local oscillations (of order millimeters or smaller) in fuel/air mixing are closely related to the oscillatory behavior of the flame. For each driving frequency, the mixture fraction oscillates at that frequency but with a slight phase difference between it and the pressure field/flame intensity, indicating that the fuel mixture fraction oscillation are likely the major reason for oscillatory behaviors of this category of flames and combustor geometry.

A steady, premixed, rich (phi=1.25), methane/air flat flame at atmospheric pressure with dilution is used to compare LIF and DFWM and evaluate their response to collisional quenching. The dilution is either pure nitrogen or pure carbon dioxide. The dilution is chosen so that the NO levels and temperature are the same for each stoichiometric ratio in the probed zone, while concentrations of quenching species change significantly. Carbon dioxide and nitrogen are, respectively, strong and mild quenchers. In addition, DFWM and LIF signals can be obtained simultaneously ...

ABSTRACT Measurements of fuel mill.wre fraction are made for a jet flame in an acoustic chamber. Acoustic forcing creates a spatially-uniform, temporally-varying pressure field which results in oscillatory behavior in the flame . Forcing is at 22, 27, 32, 37, and 55 Hz. To asses the oscillatory behavior, previous work included chemiluminescence, OH PUF, nitric oxide PUF imaging, and file! mixture fraction measurements by infrared laser absorption. While these results illuminated what was happening to the flame chemistry, they did not provide a complete explanation as to why these things were happening. In this work, the filel mixture fraction is measured through PUF of acetone, which is introduced into the fuel stream as a marker. This technique enables a high degree of spatial resolution of filellair mixture value. Both non-reacting and reacting cases were measured and comparisons are drawn with the results fi'om the previous work. It is found that structure in the mixture fraction oscillations is a major contributor to the magnitude of the flame oscillations. Introduction Combustion instabilities are caused by a coupling between thermo-acoustic and fluid-dynamic conditions present during the combustion processes. Combustion instability is defined here as 'the amplification of acoustic waves' due to the thermo-acoustic coupling between energy release from the combustion process and acoustic waves inside the combustion chamber. The interaction between vortices (mixing), sound (acoustic, or pressure oscillations), and combustion heat release can lead to self-excited oscillations that can cause structural damage. Any unsteadilless in the rate of combustion is a source of sound, generating pressure and velocity fluctuations [2]. Especially, as environmental issues becoming more important, lean premixed combustion schemes are being widely employed, which, while reducing the amount of NOx produced by lowering the flame temperature, have a greater tendency to cause instabilities since the combustion occurs near the lean blow-out limits [3]. A series of theoretical and experimental works [1, 4-6] has been done on this subject in JPC, Caltech . The coupled effects of acoustic forcing with combustion heat release on species concentration was examined by Pun et al. [4,5] with OH-PUF under atmospheric pressure. In figure 1, the flame region is marked as region (1). Phase resolved imaging revealed phase-dependent response of the combustion process under low frequency (22 ~ 55Hz) acoustic excitations. Since this work is closely related to the current work in that both used the same combustion chamber, bumer, and acoustic excitations, it is very interesting and at the same time important to compare the effects of acoustic forcing on the energy release and filellair mixing. Femandez et al. [6] used an infrared laser technique to measure point-wise fuel/air mixing in terms of 'Unmixedness factor' in the same experimental conditions as in the work by Pun et al. Unfortunately, a direct comparison of Pun ' s 311d Femandez's work is not possible because the technique employed by Femandez is time-, but not phase-, resolved. Femandez's did demonstrate that fuel /air mixing within the eductor block is heavily affected by the acoustic excitation . This effort aims to bridge these two studies by producing mixture fraction data that can be directly compared to both Femandez's unmixedness results and to Pun ' s OH PUF results. This work employs single frequency acoustic forcing at five different frequencies, just as the previous efforts, to examine how the local filellair mixing is affected by the imposed acoustic excitation and how this relates to the observed flame behavior. By comparison with earlier work [6], it also becomes possible to understand the evolution of fuel /air mixing. The phase dependence of the mixing process is evaluated by collecting images at random phase values for each frequency . These images are then sorted by phase, with phase defined as a best-fit sinusoid of the acoustic oscillation (as measured by the pressure transducer).