Damköhler Number Similarity for Static Flame Stability in Gaseous-Fueled Augmentor Flows

Abstract

Afterburners (or augmentors) are used to increase thrust in aircraft engines. Static flame stability, or the robustness to flame blowoff, is an important metric in the performance assessment of combustion in aircraft engine afterburners, where bluff-body-type flame holders are typically used to stabilize the flame. The design of such flame holders is complicated by the operating conditions, which involve flows at high speed, high temperature, and low pressure. In this paper, experimental and computational studies of Damköhler (Da) number similarity are presented with relevance to augmentor flame stability. The Da number describes the ratio of flow and chemical time scales. Hence, as long as a reference Da number is kept constant, similar characteristics of static stability should be expected of the bluff-body stabilized flame at low and high speeds. Flame stability in high-speed vitiated flows could then be studied at low speed if the chemical time scale is reduced, for instance by lowering the oxidizer flow temperature. However, each chemical reaction has its own Da number and not all can be kept constant at the same time. Since different stabilization mechanisms are governed by different chemical reactions, it is not necessarily clear what the relevant Da number is. Here, a consistent method for defining the Da number is provided based on the analysis of the modeled governing equations. Numerical simulations are performed for three different velocities with the inflow temperature adjusted to keep the Da number constant. Results are validated by comparison with experimental PIV data and the reported flame liftoff height. For the same characteristic Da number and constant momentum ratio between the fuel jet and the vitiated cross flow, the three flames show similar mean features for the recirculation zone and the flame shape. The flow field is found to exhibit von Kármán vortex shedding with the same Strouhal number for all cases. The average nondimensional flame liftoff height is also found to be the same for all cases. These results suggest a method to properly define the Da number to test augmentor stability features in low-speed test facilities under the conditions of similarity.

Keywords:

• Augmentor flows
• Bluff body stabilized flames
• Flamelet approach
• Large eddy simulation
• Non-premixed turbulent flames

ACKNOWLEDGEMENTS

Funding for this work by the Air Force Office for Scientific Research is gratefully acknowledged. The authors thank Drs. Barry Kiel, Jeff Lovett, and Marios Soteriou for advice and helpful discussions.

Notes

Note that a low Mach number scaling was used for the pressure, which implies that the Mach number is low enough that compressible effects are unimportant. This is the case for the conditions considered here. However, the assumption that the effect of compressibility can be neglected in the assessment of static flame stability in augmentors needs future validation.