POD And DMD Analysis Of Dynamic Characteristics In Swirl-stabilized Dump Combustor

Modal decomposition method of fluid flow is a mathematical technique widely used in engineering field,which is used to extract important characteristics of flow field from the aspects of energy and dynamics.There are many factors that cause the instability of swirl flame in gas turbine combustor and the dynamic characteristics of swirl are the important inducement of lean burning instability.In order to better study the dynamic mechanism of the cold swirl flow field,the transient velocity field of three-dimensional swirl flow is sampled at high frequency and the important characteristics of the flow field are extracted and analyzed by POD and DMD.The main work of this paper is as follows:(1)POD and DMD algorithms are introduced in detail.(2)Establish a flow model around a cylinder to verify the algorithm and prove that the algorithm is compiled correctly.Then explain the physical meaning of algorithm results.(3)Modal decomposition of cold swirl flow field with swirl numbers of 0.7,1.0 and 1.3 is carried out by POD and DMD.Large-scale coherent structures and characteristic frequencies of flow field with different swirl numbers are obtained.The main results and conclusions are as follows:(1)POD analysis shows that with the increase of swirl number,the energy ratio of the average flow field gradually increases,which indicates that the integrity of flow movement is stronger.(2)The eigenvalues of DMD are mainly distributed near the unit circle,which indicates that the swirling flow is a periodic movement.(3)The precessing vortex core is extracted from POD modes and DMD modes under three conditions,which indicates that vortex core precessing occurs in the flow field.With the increase of swirl number,the energy proportion of precessing vortex core gradually increases and the main frequency rises,which becomes an important feature of flow field.(4)Different from the single spiral structure with low swirl number,three flow patterns with high swirl numbers are extracted by POD and DMD.When the swirl number is 0.7,the flow moves toward the upstream of the combustion chamber in a double spiral with a frequency of about 714Hz.When the swirl number is 1.0,there is a double spiral movement trend of about 823Hz in both the inner and outer shear layers.When the swirl number is 1.3,the leading role of precessing vortex core is enhanced,and both the inner and outer shear layers in the flow field have a trend of four spirals around 1900Hz,which is related to the enhanced interaction between the inner and outer shear layers.(5)The movement of precessing vortex core is related to low frequency,while that of spiral vortex is related to high frequency.With the increase of swirl number,the main frequency of precessing vortex core and spiral vortex will rise.The instability of lean premixed swirl combustion is related to the spiral motion of shear layer,precessing vortex core and vortex breakdown.In order to enhance combustion stability,the coupling between the main frequency oscillation and acoustic oscillation should be avoided.The dynamic characteristics of non-reactive flow field provide methodological guidance for swirl combustion stability mechanism.