Μελέτη διατάξεων σταδιακής καύσης

Abstract

Τhis study aims to the assessment and analysis of the flow field produced by the interaction between two axisymmetric jets with different Reynolds numbers. The scheme is considered as a generic staged combustion configuration. In this frame, the identification and control of the mechanisms that dominate the flow field is one of the most significant issues towards the enhancement of mixing, thus the increasing of combustion operational and environmental efficiency. The scope of the study is to characterize and control the mixing process during the interaction and merging of the jets. The study contributes to the understanding of the dominating mechanisms occurring in the flow, through the assessment of the mean and turbulent flow features. The initial conditions used in the experiments diverge from the marginal conditions that refer to the interaction of equal momentum jets and the combining of a “strong” and a “weak” jet. In addition, due to the particularly low, although turbulent, Reynolds numbers used at jets’ exits, the capability of the jets to produce and maintain strongly turbulent structures during and after their interaction is also evaluated. During the experiments included in this thesis, a two dimensional Laser Doppler Anemometry (LDA) was used to measure the main components of the velocity vector (i.e. in the axial and the radial or horizontal directions). A pair of similar optical systems were used to produce two ellipsoidal control volumes and collect the light scattered by tracer particles, that faithfully represent the flow. Axisymmetric turbulent jets constitute the main component regarding the realization of staged combustion configurations based on multiple jets arrangements. Jet flow fields have been extensively studied during the last decades, mainly focusing on the self-similar region, i.e. the region where the mean and turbulent features are dynamically preserved. Recently, most of the studies are related to the identification of divergences arising from the different initial conditions and specific parameters, such as the shape of the orifice, the type of the confinement imposed at the exit, the jet density etc. The effects of these features are briefly summarized within the context of the thesis. In particular, the low Reynolds number effect is under consideration, as the jets recorded in the experiments are compared to typical previous studies devoted to the self-similarity region of jets with significantly higher Reynolds numbers. In the first part of the experiments, a jet of reference is monitored within an area covering the initial development and extends to the boundaries of the self-similarity region. Measurements include axial and radial distributions of the mean and turbulent axial and radial velocity components, turbulent shear stresses and terms that represent the turbulent transport of the Reynolds stresses. The budget of the turbulent kinetic energy is presented at the final measurement station, showing the particular mechanisms occurring in the flow. Measurements on the jets’ interaction field are presented afterwards. The distributions are systematically presented in comparison to the distributions recorded during the sole action of the primary jet Characteristic profiles are presented at locations within the merging and the mixing zones. In the early stages of development, the patterns of both jets can be identified. Within the merging region, besides the absorption of the secondary jet, the measurements indicate a spatial suppression of primary jet’s characteristics. Further downstream, the profiles resemble to those of a standalone jet. Higher values of the mean and turbulent terms are observed while the profiles extend over a wider region, in accordance with the secondary jet’s Reynolds number. Results of the present study demonstrate that skewness and flatness factors can be used as indicators of small scale mixing. The field of the interaction is further analyzed and discussed for a third pair of initial conditions and the proper decomposition of the mean and turbulent profiles using three different types of seeding conditions. In the early stages, the measurements confirm the independent action of the jets, although the tendency of the secondary jets’ pattern to be deformed. Within the merging zone, the distributions referring to the different types of seeding are characterized by humps and increased turbulent features in the secondary jet’s development region. The shape, the intensity and the extent of the humps depends on the seeding type. Most of the characteristics observed in this region are related to the probability density function distributions, which are composed by samples that correspond to tracers emanating from the individual sources. However, within the mixing zone, the profiles are nearly similar for the individual types of seeding, leading to the conclusion that all the samples contributing to measurements have been adapted by the flow field due to the small scale mixing, which is continuously improved further downstream.