Recent advances in laser absorption and shock tube methods for studies of combustion chemistry Progress in Energy and Combustion Science (2014) Authors: R.K. Hanson, D.F. Davidson Mechanical Engineering Department, Stanford University Reviewed by: Elia Distaso April 30, 2015

Contents Introduction Shock tube diagnostic methods and experiments Recent shock tube developments Driver inserts Constrained Reaction Volume strategy Recent laser absorption developments Temperature measurement using CO2 laser absorption Multi-species laser absorption diagnostics Isotopic methods Conclusions

I. Introduction I. III. II.

Principles of shock tube operation Introduction Principles of shock tube operation I. III. II.

Introduction Principles of shock tube operation I. III. II. Conventional shock tube Tailored shock tube Tailored driver gases optimize the reflected-shock-contact surface, increasing the test time Tailored test time > 10 ms I. III. II.

Introduction Types of experiments I. III. II. Shock tubes are used to acquire combustion kinetics data in three types of experiments: ignition delay time measurements, species time-history measurements, elementary reaction rate constant measurements. I. III. II.

II. Shock tube diagnostic methods and experiments III. II.

Recent shock tube developments Driver inserts Pressure profiles behind reflected shock waves in conventionally-operated shock tubes exhibit gradual rises, typically in the 1-3%/ms regime. Small increases in temperature can significantly shorten the measured ignition delay time. It is possible to generate near-constant pressure test conditions by slightly degrading the performance of the shock tube using inserts in the driver section. Driven Driver Insert I. III. II.

Recent shock tube developments Constrained Reaction Volume strategy   Conventional shock tube Constrained Reaction Volume The entire driven section is filled with test gas. Only a small part of the driven section nearest the end wall is filled with reactive test gas mixture. I. III. II.

Recent shock tube developments Constrained reaction volume strategy Conventional shock tube Constrained Reaction Volume Conventional filling exhibits large pressure change The CRV data can be accurately modeled using a constant-pressure constraint I. III. II.

Laser absorption developments Temperature measurement using CO2 laser absorption There has not previously been a simple method to achieve highly-precise fast time-response measurement of the temperature in shock-tube tests. By taking advantage of the large CO2 absorption strength near 2.7 mm, reflected shock temperatures could be measured within 0.5-1.0% with a sensor bandwidth of 50 kHz. The reflected shock temperature in this experiment has a standard deviation of 3 K, equivalent to an uncertainty of 0.32%, I. III. II.

Laser absorption developments Multi-species laser absorption diagnostics Laser absorption diagnostics provide a sensitive and non-intrusive method to measure species concentration time-histories in shock tube experiments. The extension of this method to multi-wavelength, multi-species detection has enabled the development of kinetics databases comprised of species-time-histories for reactants (fuel), transient radical species, and stable intermediates. Results from a multi-wavelength study of methyl formate pyrolysis I. III. II.

Laser absorption developments Isotopic methods Laser absorption can also be used to measure elementary reaction rate constants behind reflected shock waves., e.g. pseudo-first-order determinations of OH + stable species rate constants: OH is generated by the rapid high-temperature decomposition of tert-butyl-hydroperoxide (TBHP) to OH, CH3 and acetone. OH from TBHP Tert-butanol There are situations where secondary reactions can influence the observed removal rate. Using isotopic 18O it is possible to distinguish the 16OH derived from the TBHP precursor and the 18OH from the tert-butanol by labeling the OH radical on the tert-butanol. OH from Tert-butanol I. III. II.

III. Conclusions I. III. II.

Summary and future directions New operational protocols in shock tube experiments can provide well-characterized near-constant-pressure test conditions, even during energetic reaction events, for test times of 50 ms and longer. Longer constant-pressure test times will allow for the validation and refinement of negative-temperature-coefficient (NTC) ignition processes. The CO2 laser absorption diagnostic for T(t) provides unique capability to test the ability of detailed reaction models to accurately capture heat release rates. Laser absorption diagnostics can provide quantitative high-bandwidth measurements of time-histories during these experiments for a wide range of species. With these methods, a range of new kinetics target data can be acquired. I. III. II.

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