Characteristics of Multi-Cavity Trapped Vortex Combustors

Abstract

This research is motivated towards improving and optimizing the performance of AFRL’s Inter-Turbine Burner (ITB) in terms of greater combustion efficiency, reduced losses and exit temperature profile requirements. The ITB is a minicombustor concept, situated in between the high and low pressure turbine stages and typically contains multiple fueled and non-fueled Trapped Vortex Combustor (TVC) cavities. The size, placement, and arrangement of these cavities have tremendous effect on the combustor exit temperature profile. The detailed understanding of the effect of these cavities in a three-dimensional ITB configuration would be very difficult and computationally prohibited. Therefore, a simple but somewhat similar conceptual axi-symmetric burner is used here the design variations of Trapped Vortex Combustor (TVC) through modeling and simulation. The TVC can be one single cavity or can be represented by multi-cavity combustor. In this paper, both single cavity TVC and multi-cavity TVCs are studied. The single cavity TVC is divided into multiple cavities while the total volume of the combustor remains constant. Four combustors are studied: Baseline, Staged, Three-Staged, and Interdigitated TVC. An extensive computational investigation on the characteristics of these multi-cavity TVCs is presented. FLUENT is used for modeling the axisymmetric reacting flow past cavities using a global eddy dissipation mechanism for C3H8-air combustion with detailed thermodynamic and transport properties. Calculations are performed using Standard, RNG, and Realizable k-ε RANS turbulence models. The numerical results are validated against experimental temperature measurements on the Base TVC. Results indicate that the pressure drag is the major contributor to total drag in the Base TVC. However, viscous drag is still significant. By adding a concentric cavity in sequential manner (i.e. Staged TVC), the pressure drag decreases, whereas the viscous drag remains nearly constant. Further addition of a secondary concentric cavity (i.e. Three-Staged TVC), the total drag does not further decrease and both pressure and viscous drag contributions do not change. If instead a non-concentric cavity is added to the Base TVC (i.e. Interdigitated TVC), the pressure drag increases while the viscous drag decreases slightly. The effect of adding swirl flow is to increase the fuel-air mixing and as a result, it increases the maximum exit temperature for all the combustors modeled. The jets and heat release contribute to increase pressure drag with the former being greater. The fuel and air jets and heat release also modify the cavity flow structure. By turning off the fuel and air jets in the Staged TVC, lower drag (or pressure loss) and exit temperature are achieved. It is more effective to turn off the fuel and air jets in the upstream (front) cavity in order to reduce pressure losses. Based on these results, recommendations are provided to the engineer/designer/modeler to improve the performance of the ITB.