Sensitivity of the Numerical Prediction of Turbulent Combustion Dynamics in the LIMOUSINE Combustor

Abstract

The objective of this study is to investigate the sensitivity and accuracy of the reaction flow-field prediction for the LIMOUSINE combustor with regard to choices in computational mesh and turbulent combustion model. The UMOUSINE combustor is a partially premixed, bluff body-stabilized natural gas combustor designed to operate at 40-80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermoacoustic instabilities is a very time-consuming process. Eor that reason, the meshing approach leading to accurate numerical prediction, known sensitivity, and minimized amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the computational domain. Typically, the structural mesh topology allows using much fewer grid elements compared to the unstructured grid; however, an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter, the studies are extended to combustible flows using standard available ANSYS-CFX combustion models. To validate the predicted variable fields of the combustor's transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under nonreacting conditions. More significant differences are observed in the mi.xing behavior of air and fuel flows. Here, the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow, resolved with the use of the hexahedral mesh, presents better agreement with experimental data and demands less computational effort. Finally, in the paper, the performance of the combustion model for reacting flow is presented and the main issues of the applied combustion modeling are reviewed.

Transient heat transfer between a turbulent lean partially premixed flame in limit cycle oscillation and the walls of a can type combustor

“…As it can be seen, the periodic oscillation of the pressure signal in the time domain appears as a peak frequency of about 232 Hz surrounded by many secondary peaks, the source of these peaks is discussed in Refs. [17,18]. The mode at about 600 Hz which is missing in the CFD prediction corresponds to the structural mode of the combustor.…”

“…That is why the temperature data is not presented here. However, according to previous studies [18,20] on the same configuration, and by keeping all affecting parameters constant, the only effective parameter which may control the frequency of pressure oscillations (i.e. speed of sound and acoustic) is the imposed thermal boundary condition.…”

“…The impact of the meshing technology and sensitivity of the results on the grid have been studied by the authors in Ref. [18] focusing on the fluidonly simulation. All the meshes used in this study are generated using the meshing tool ANSYS Workbench 14.5.…”

Numerical Simulations of Combustion Instabilities in a Combustor with an Augmentor-Like Geometry

“…This mechanism has been observed in another bluff-body-stabilized-flame combustor [19] and in a swirl-stabilized-flame combustor [7]. Another study of the Limousine combustor focuses on the sensitivity of the predictions to the turbulent-combustion modeling [33]. It finds that, of the four models tested, only one reproduces experimental observations within an acceptable error.…”

Acoustic Design Parameter Change of a Pressurized Combustor Leading to Limit Cycle Oscillations

“…Ansys CFX was used as the simulation tool. This simulation tool has previously been used by several researchers to predict combustion instabilities and flame dynamics [49][50][51][52][53]. The burning velocity model (BVM) was used as a combustion model.…”