Study of Unsteady Heat Transfer as a Key Parameter to Characterize Limit Cycle of High Amplitude Pressure Oscillations

Abstract

The objective of the studies presented in this paper is the numerical prediction of unsteady heat flux and pressure fluctuations during the unstable regime of a combustor. The studied laboratory-scale lean partially premixed combustor was built in the LIMOUSINE project, to explore the mechanisms driving thermo-acoustic instabilities in conditions representative of gas turbine combustors. Due to the thermal interaction between hot gases and the colder liner wall, and also the correlation between gas temperature, density and speed of sound, prediction of the transient heat transfer rate is of high importance. In this paper analysis of transient heat transfer is conducted by coupling of fluid flow and solid body (liner) in one computational domain and thereby taking into account the thermal convection with the environment around the combustor and also the heat conduction transients within the liner. Conjugate heat transfer modeling can give access to the transient temperature distribution in the structure of the combustor which is important for the dynamic heat storage. Also this can be used to estimate the thermal stresses and creep strain as required to evaluate the lifetime assessment of the combustor. In this work the commercial CFD code ANSYS CFX is used to solve the problem, in which fluid and solid regions are solved simultaneously with a finite volume approach. In the fluid region, three dimensional compressible Reynolds Averaged Navier-Stokes equations are solved, while for the solid region only the enthalpy conservation equation is solved. To remove any interpolation errors, in all cases the skin (interface) mesh cells for both the fluid and solid are similar in resolution on either side of the interface. By comparing heat release and pressure data available from the measurements it follows that this simulation can give more accurate prediction of the amplitudes of thermoacoustic instabilities as compared to the solution with imposed thermal boundary conditions (such as isothermal). In the latter case the time history of heat accumulation in the solid is predicted incorrectly. Because the spatial scales of the solid temperature profiles are different in case of steady state or transient oscillatory heat transfer, care has to be taken in the meshing in these two situations. When meshing for a transient oscillatory heat transfer case, the solid mesh resolution needs to be adapted to the thermal penetration depth of the surface temperature oscillations. Hence for the transient heat transfer in limit cycle combustion oscillations, the meshing strategy and size of the grid in the solid part of the domain will play a very important role in determining the magnitude for the pressure fluctuations.