Mid-IR Laser Absorption Diagnostics for Shock Tube and Rapid Compression Machine Experiments

  • Ehson Fawad Nasir

Student thesis: Doctoral Thesis

Abstract

High-fidelity chemical kinetic models for low-temperature combustion processes require high-fidelity data from fundamental experiments conducted in idealized transient reactors, such as shock tubes and rapid compression machines (RCM). Non-intrusive laser absorption diagnostics, in particular quantum cascade lasers (QCL) in the mid-infrared wavelength region, provide a unique opportunity to obtain quantitative, time-resolved species concentration and temperature from these reactive systems. In this work, three novel laser absorption diagnostics in the mid-infrared wavelength region are presented for three different experimental applications. The first diagnostic was developed for measuring CO2 concentration using an external cavity QCL centered in the ν3 fundamental vibrational band of CO2. Absorption cross-sections were measured in a shock tube, at a fixed wavelength for the R(32) line centered at 2371.42 cm-1 (4.217 µm) over 700 – 2900 K and nominal pressures of 1, 5 and 10 bar. The diagnostic was used to measure rate coefficients for the reaction between carbon monoxide and hydroxyl radical over 700 – 1230 K and 1.2 – 9.8 bar using highly dilute mixtures. The second diagnostic was developed for measuring CO concentration using a pulsed QCL centered at 2046.28 cm-1 (4.887 µm) and an off-axis cavity implemented on the RCM. The duty cycle and pulse repetition rate of the laser were optimized for increased tuning range, high chirp rate and increased line-width to achieve effective laser-cavity coupling. A gain factor of 133 and time resolution of 10 μs were demonstrated. CO concentration-time profiles during the oxidation of highly dilute n-heptane/air mixtures were recorded and compared with chemical kinetic models. This represents the first application of a cavity-enhanced absorption diagnostic in an RCM. Finally, a calibration-free temperature diagnostic based on a pair of pulsed QCLs centered at 2196.66 cm-1 and 2046.28 cm-1 was implemented on the RCM. The down-chirp phenomenon resulted in large spectral tuning (∆v ~ 2.8 cm-1) within a single pulse of each laser at a high pulse repetition frequency (100 kHz). The diagnostic for was used to measure the temperature rise during first-stage ignition of n-pentane at nominal pressures of 10 and 15 bar for the first time.
Date of AwardOct 2018
Original languageEnglish (US)
Awarding Institution
  • Physical Science and Engineering
SupervisorAamir Farooq (Supervisor)

Keywords

  • combustion
  • Kinetics
  • Rapid Compression Machine
  • Quantum Cascade Laser
  • Shock Tube

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