Non-Reacting and Reacting Flow Analysis in an Aero-Engine Gas Turbine Combustor Using CFD - 22nd Cliff Garret Turbomachinery Engineering Award

摘要

A gas turbine combustion system is an embodiment of all complexities that engineering equipment can have. The flow is three-dimensional, swirling, turbulent, two-phase and reacting. The design and development of combustors, until recent past, was an art than science. If one takes the route of development through experiments, it is quite time-consuming and costly. Compared to the other two components viz., compressor and turbine, the combustion system is not yet completely amenable to mathematical analysis. A gas turbine combustor is both geometrically and fluid dynamically quite complex. The major challenge a combustion engineer faces is the space constraint. As the combustion chamber is sandwiched between compressor and turbine there is a limitation on the available space. The critical design aspect is in facing the aerodynamic challenges with minimum pressure drop. Accurate mathematical analysis of such a system is next to impossible. However, because of the advent of fast digital computers nowadays it has become possible to model both geometrical and fluid dynamics aspects of a combustor using computers. The advances in modelling techniques have evolved over a period of time. Commercial codes are now available which has made the task comparatively easier. The present study is an attempt to predict numerically the entire flow field from compressor exit to combustor exit. The novelty of this study is in the modelling of all the holes amounting to 1700 in all, with appropriate grid distribution. The inclusion of conjugate heat transfer was a challenge. The appropriate governing equations are solved using RNG k-ε turbulence model for non-reacting flows whereas for reacting flows with heat transfer only k-ε is used. Eighteen million grid cells are employed to incorporate holes, fuel injection, combustion and conjugate heat transfer. The combustion is modeled using prePDF combustion model. It took nearly 2000 hrs of CPU time in a sun server (4 node machine) with the main memory of 128GB. Non-reacting flow analysis of the combustor reveals that size of the holes in dome and flare has little effect on the total pressure loss. However, increase in inlet velocity by 37% increases the total pressure loss by 86% across combustor. An Injection velocity of 75 m/s is found to give a good combustion characteristic inside the flame tube. Therefore conjugate heat transfer study is carried out with this injection velocity. Temperature distribution inside the flame tube and heat transfer through flare and the hot streaks in the liner wall are studied. Flare (without holes) is subjected to higher temperature (2480 K) as it is found to be close to the primary combustion zone. It is also inferred from the study that the presence of holes protects the dome and flare from high temperature due to combustion. Holes in dome and flare are found to reduce the maximum temperature by 350K. The predicted results provide a complete picture of flow field as well as temperature distribution. Pressure drop and pattern factor have been evaluated. Even though this particular study has not been validated, the code has already been validated for a different geometry. It is gratifying to note that the present numerical procedure has predicted reasonable and plausible results. It may be concluded that CFD can be used with reasonable confidence, for the design and development of gas turbine combustion system. However, to enhance the confidence, validation must be attempted in future studies.