On The Use Of Endwall Emission As A Shock-Tube Ignition Diagnostic

Chemiluminescence emission from exited species such as OH∗ or CH∗ can be a convenient and effective diagnostic for monitoring ignition delay times in shock-heated mixtures. Ideally, the ignition delay time obtained from the radical-species emission signal should agree with ignition delay time as obtained from the pressures trace. Under ideal shock-tube conditions, ignition behind the reflected shock wave occurs first at the endwall, so the measurement of endwall pressure is often considered the best way to determine ignition delay time when such an increase in pressure is available. However, the signal-to-noise ratio of data from a pressure transducer mounted in the endwall can be relatively low when compared to that of an emission signal, so the latter technique provides a useful alternative to pressure. The natural implication then is to use endwall emission as an ignition diagnostic since the S/N would be high and it is intended to monitor conditions at the endwall. In the present paper, an analytical model of endwall emission measurements is presented, and recent experimental results are studied to determine whether or not endwall emission can be used to obtain reliable ignition delay times. The results of this work indicate that endwall emission measurements can lead to artificially longer ignition times under dilute conditions when the increase in the radical species can occur over a period on the order of a hundred microseconds or more. This longer apparent ignition time is a result of the integrated effect of the detector seeing ignition occurring at later times down the length of the driven section. Endwall emission should therefore not be used to infer ignition delay times in experiments where there is no significant pressure rise and the ignition event is not abrupt. However, it is shown from experiment and the simple optics model that endwall emission can, in contrast, be employed reliably to measure ignition delay times when the ignition event is abrupt, as in undiluted fuel-air mixtures