Abstract
Pulsed cavity ringdown spectroscopy (CRDS) is used to develop a novel, ultra-fast, high-sensitivity diagnostic for measuring species concentrations in shock tube experiments. The diagnostic is demonstrated by monitoring trace concentrations of ethylene in the mid-IR region near 949.47 cm⁻¹. Each ringdown measurement is completed in less than 1 µs and the time period between successive pulses is 10 µs. The high sensitivity diagnostic has a noise-equivalent detection limit of 1.08 x 10⁻⁵ cm⁻¹ which enables detection of 15 ppm ethylene at fuel pyrolysis conditions (1845 K and 2 bar) and 294 ppb ethylene under ambient conditions (297 K and 1 bar). To our knowledge, this is the first successful application of the cavity ringdown method to the measurement of species time-histories in a shock tube.
Highlights
Fast-time response diagnostics capable of performing sensitive measurements are invaluable in the study of transient combustion systems such as shock tubes, internal combustion engines, and gas turbines
We have developed a highly sensitive ultra-fast diagnostic based on pulsed cavity ringdown spectroscopy (CRDS) and deployed it for making in situ species time-history measurements in shock tube chemical kinetic experiments
Since absorption-based diagnostics are most commonly used for measuring species concentrations in shock tube experiments, a closer look at Beer-Lambert law provides us the possible routes to increase the sensitivity of these diagnostics: Absorbance = ln(I0 / I ) = P.χ.S(T ).φv (P,T ).L
Summary
Fast-time response diagnostics capable of performing sensitive measurements are invaluable in the study of transient combustion systems such as shock tubes, internal combustion engines, and gas turbines. We have developed a highly sensitive ultra-fast diagnostic based on pulsed cavity ringdown spectroscopy (CRDS) and deployed it for making in situ species time-history measurements in shock tube chemical kinetic experiments. Continuous-wave (cw) laser absorption spectroscopy based diagnostics with microsecond time resolution [7] have extensively been used for measuring species concentrations in shock tube experiments. Sun et al [7,10] utilized OA-ICOS technique to develop sensitive diagnostics for shock tube kinetic studies They were able to achieve high sensitivity by replacing shock tube windows with cavity mirrors. We demonstrate its capability to make fast time-response and sensitive species concentration measurements by recording ethylene concentration time-histories in shock tube pyrolysis experiments To our knowledge, this is the first successful application of the CRDS technique to shock tube kinetic experiments
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