Aspects of oscillating flames

Photography and chemieluminescence from CH radicals have been used to identify the reaction zones and quantify the areas and shapes of kerosene-fuelled flames with swirl numbers of 0.7 and 0.8 and an overall equivalence ratio of 0.25. The air flow was oscillated at a frequency of 350 Hz and the results suggest that the oscillations caused a sequence of vortex rings at the burner exit and that these distorted the reaction zone and increased its area in the near burner region leading to an overall shorter flame. For the swirl number of 0.7, the flame was lifted and the oscillations led to an increase in the average lift off length whereas the higher swirl number caused an attached flame with and without oscillations. The stretch rate, evaluated from the variation of the flame area in time, was higher for the lifted flame suggesting that lift off was caused by local extinction.     

Multiscale laminar flows with turbulentlike properties

By applying fractal electromagnetic force fields on a thin layer of brine, we generate steady quasi-two-dimensional laminar flows with multiscale stagnation point topology. This topology is shown to control the evolution of pair separation (Delta) statistics by imposing a turbulentlike locality based on the sizes and strain rates of hyperbolic stagnation points when the flows are fast enough, in which case Delta(2) approximately t(gamma) is a good approximation with gamma close to 3. Spatially multiscale laminar flows with turbulentlike spectral and stirring properties are a new concept with potential applications in efficient and microfluidic mixing.     

Phase Doppler Anemometer for Measurements of Deterministic Spray Unsteadiness

A method and analysis was developed to quantify the amplitude of deterministic spray unsteadiness based on Phase Doppler Anemometry (PDA), which sampled time‐dependent droplet velocity and size measurements, in order to determine the fluctuations of droplet data rate and number density, which are quantities relevant to fluctuations of droplet concentration. The data processing method of the PDA measurements was assessed in a pulsed spray at a frequency of 20 Hz injected in a swirl‐stabilised burner. Comparisons between quantities relevant to droplet concentration fluctuations, measured by PDA and a light scattering technique, quantified the deterministic spray unsteadiness and agreed to within 15%. The developed PDA approach was applied in the swirl‐stabilised burner to measure the amplitude of deterministic spray unsteadiness of an otherwise steady spray, which was caused by the instability of the atomisation process. The intensity of deterministic fluctuations of droplet data rate and number density, occurring at a frequency range around 600 Hz due to the atomisation process, was quantified to 15% of the corresponding mean value and this spray unsteadiness generated fluctuations on the air and droplet velocity fields. The deterministic spray unsteadiness could survive up to the end of the recirculation zone of the air flow at the burner exit and, therefore, could influence flame stability.     

Estimation of the droplet size spread with the laser induced fluorescence/Mie technique

The laser measurement technique based on the ratio between the laser induced fluorescence (LIF) and the scattered light (Mie) intensities of droplets is presently limited to the evaluation of the Sauter mean diameter of the droplets. The important measurement of the droplet size spread is currently missing. An extension of the LIF/Mie technique for the measurement of droplet size spread is proposed here and is evaluated numerically. The method is based on the imperfect relationships between the scattered light intensity and the droplet surface area or the fluorescent light intensity and the droplet volume, which convey additional information that can be used to evaluate the droplet size spread.     

A Study of Parameters that Influence the Accuracy of the Planar Droplet Sizing (PDS) Technique

A combined theoretical and experimental study of the parameters affecting the accuracy of Planar Droplet Sizing (PDS) measurements is presented. The principle of the PDS technique relies on the assumption that the intensity emitted by a fluorescent dye added to a liquid is proportional to the volume of a resulting droplet during atomisation and that the scattered light intensity is proportional to its surface area, allowing measurement of Sauter Mean Diameter (SMD) by taking the ratio of these intensities. A geometrical optics light scattering approach was extended to calculate the fluorescence intensity emitted by a droplet, in addition to providing the scattered light intensity integrated over the collection aperture. The theoretical approach quantified the influences of scattering angle, refractive index, droplet size and dye concentration on the PDS technique. Experiments with monodisperse droplet streams confirmed the calculations in terms of dependence of the scattered and fluorescence intensities. The limitations of the technique have been established together with an appropriate calibration procedure for application in dense sprays.     

Spatial resolution of a chemiluminescence sensor for local heat-release rate and equivalence ratio measurements in a model gas turbine combustor

The spatial resolution of a Chemiluminescence Sensor, based on focused Cassegrain optics, to detect the location of the reaction zone and heat-release rate in a model gas turbine combustor is reported. The sensor measures simultaneously the chemiluminescent intensities from OH* and CH* excited radicals in flames in order to obtain information on the local flame characteristics. The spatial resolution was evaluated by a combined theoretical and experimental study in laminar and turbulent flames and was supported by detailed chemistry calculations, including the chemiluminescent species, of unstrained one-dimensional flames. The experimental study involved simultaneous measurements of chemiluminescence with the sensor and laser-based reaction rate imaging, using the product of OH and CH₂O radicals obtained from planar laser-induced fluorescence (PLIF), and OH PLIF for the location of the reaction zone. The study quantified the influence of flame shape and dimensions and the direction of traverse of the focal region of the sensor through the flames on the spatial resolution, thereby identifying the limitations and optimising the applicability of the sensor. The sensor was used to obtain local time-dependent measurements of heat-release and equivalence ratio of a reacting mixture, based on the chemiluminescent intensity ratio of OH*/CH*, in a swirl-stabilised model gas turbine combustor and quantified the degree of air–fuel premixedness, probability of reaction and power spectra of pressure and chemiluminescent intensity fluctuations in two unsteady flames.     

Hydrogen abstraction from n-butanol by the methyl radical: high level ab initio study of abstraction pathways and the importance of low energy rotational conformers

Hydrogen abstraction reactions by the methyl radical from n-butanol have been investigated at the ROCBS-QB3 level of theory. Reaction energies and product geometries for the most stable conformer of n-butanol (ROH) have been computed, the reaction energies order α < γ < β < δ < OH. The preference for n-butane to favour H-abstraction at C(β) and C(γ) while, in contrast, n-butanol favours radical reactions at the C(α) carbon is rationalised. Transition state (TS) barriers and geometries for the most stable conformer of n-butanol are presented, and discussed with respect to the Hammond postulate. The reaction barriers order as α < OH < γ < β < δ. This relative ordering is not consistent with product radical stability, C-H bond dissociation energies or previous studies using O[combining dot above]H and HO[combining dot above](2) radicals. We provide a molecular orbital based rationalisation for this ordering and answer two related questions: Why is the γ-channel more stable than the β-channel? Why do the two C(γ)-H H-abstraction TS differ in energy? The method and basis set dependence of the TS barriers is investigated. The Boltzmann probability distribution for the n-butanol conformers suggests that low energy conformers are present in approximately equal proportions to the most stable conformer at combustion temperatures where ?H(3) radicals are present. Thus, the relative significance of the various H-abstraction channels has been assessed for a selection of higher energy conformers (ROH'). Key results include finding that higher energy n-butanol conformers (E(ROH') > E(ROH)) can generate lower energy product radicals, E(ROH') < E(ROH). Moreover, higher energy conformers can also have a globally competitive TS energy for H-abstraction.     

Local curvature measurements of a lean,partially premixed swirl-stabilised flame

A swirl-stabilised, lean, partially premixed combustor operating at atmospheric conditions has been used to investigate the local curvature distributions in lifted, stable and thermoacoustically oscillating CH₄-air partially premixed flames for bulk cold-flow Reynolds numbers of 15,000 and 23,000. Single-shot OH planar laser-induced fluorescence has been used to capture instantaneous images of these three different flame types. Use of binary thresholding to identify the reactant and product regions in the OH planar laser-induced fluorescence images, in order to extract accurate flame-front locations, is shown to be unsatisfactory for the examined flames. The Canny-Deriche edge detection filter has also been examined and is seen to still leave an unacceptable quantity of artificial flame-fronts. A novel approach has been developed for image analysis where a combination of a non-linear diffusion filter, Sobel gradient and threshold-based curve elimination routines have been used to extract traces of the flame-front to obtain local curvature distributions. A visual comparison of the effectiveness of flame-front identification is made between the novel approach, the threshold binarisation filter and the Canny-Deriche filter. The novel approach appears to most accurately identify the flame-fronts. Example histograms of the curvature for six flame conditions and of the total image area are presented and are found to have a broader range of local flame curvatures for increasing bulk Reynolds numbers. Significantly positive values of mean curvature and marginally positive values of skewness of the histogram have been measured for one lifted flame case, but this is generally accounted for by the effect of flame brush curvature. The mean local flame-front curvature reduces with increasing axial distance from the burner exit plane for all flame types. These changes are more pronounced in the lifted flames but are marginal for the thermoacoustically oscillating flames. It is concluded that additional fuel mixture fraction and velocimetry studies are required to examine whether processes such as the degree of partial-premixedness close to the burner exit plane, the velocity field and the turbulence field have a strong correlation with the curvature characteristics of the investigated flames.