Three-Dimensional Optical Measurements in an Ethylene Fuelled Model Scramjet Engine

This work documents the development of non-intrusive optical diagnostic methods towards a qualitative study of ethylene flame dynamics in a laboratory scale model scramjet engine. Planar laser Rayleigh scattering (PLRS) and OH based planar laser induced fluorescence (PLIF) have been successfully developed and applied.

Prior to understanding the turbulent flame dynamics due to ethylene combustion in the model scramjet, it is necessary to reveal the role played by turbulent structures in a combustion free environment. Also, shock/ turbulent boundary layers are known to significantly impact unstart dynamics. Hence, PLRS has been chosen to be employed considering its relevancy to the present experimental subject.

Visualizing flow structures in a transient combustion system is a key to establish stable operational regimes. Imaging ground state OH is a proven, simple and cost effective method amongst the LIF based techniques. In addition, these laser based techniques are instantaneous in nature with temporal resolution as high as 10ns.

Flow physics in the scramjet model is complicated due to the interaction of turbulence and flame structures. High intensities of turbulence are expected at such high Reynolds number flows involving combustion. The high strain rates imposed by turbulent structures might, in fact, contribute to flame extinguishment.

In view of turbulence being a 3-dimensional phenomena, there exists a need to visualize the flow profile in a 3-dimensional domain. However, a truly 3-dimensional study is beyond the scope of current research methods. A closer and more accessible alternative would be to apply 2-dimensional flow imaging techniques spanning over multiple planes, provided that the flow shows a quasi-stable behavior. Although optical investigations in the combustor regions have been reported [18], this study, to the best of the author's knowledge, is the first one to cater to the flow field investigation over a significant region beyond the combustor/cavity. Furthermore, the study encompasses multiple planes to achieve a holistic reconstruction of the flow physics.

A unique optical arrangement to aid such a visualization has been developed. The results obtained provide supportive evidence underlining the applicability of these laser based techniques to the present combustion system.