Development of Combined Dual-Pump Vibrational and Pure-Rotational Coherent Anti-Stokes Raman Scattering (DPVCARS and PRCARS) Systems and their Application to Laminar Counter-flow Flames

Abstract

It is valuable to obtain, experimentally, temperature and concentration information for multiple species in laminar flames to validate and further develop chemical mechanisms which can then be applied in turbulent combustion [1]. This is because underlying flame chemistry and transport properties in combustion environments with different Reynold’s number is similar if the fuels and the oxidizer are the same. Furthermore, according to laminar flamelet theory [2] the mixing layer in turbulent flows can be modeled as diffusion flamelets which can be approximated as non-premixed flames under strain. Laboratory scale, opposed flow burners have been used to stabilize steady-state non-premixed and premixed flames [3,4] for combustion diagnostics. The main advantage of such burners is that the flame is stabilized away from the nozzles which reduces the uncertainty in the heat loss terms in the energy equation thereby, making it much easier to solve for temperature and species concentration. Coherent anti-Stokes Raman scattering (CARS), a non-linear spectroscopic technique, is well suited for obtaining quantitative data in combustion environment. CARS is a four-wave mixing parametric process in which two laser beams called the pump and Stokes beam create a Raman coherence in the medium. A third beam, called the probe beam scatters of the Raman coherence thereby generating the fourth beam called the CARS signal. The Stokes beam is broadband which allows creation of a spectrally wide Raman coherence, thereby permitting single-shot data acquisition. CARS due to it non-linear nature provides species specific, accurate measurements with high spatial and temporal resolution. CARS systems, in comparison with single beam techniques such as spontaneous Raman scattering, are relatively complicated especially for multi-species measurements [5,6]. However, due to its coherent nature, CARS provides signal levels orders of magnitude larger than spontaneous Raman scattering. Therefore, over the years, CARS has been widely applied towards combustion diagnostics [7-8]. CARS can be broadly divided into pure-rotational CARS (PRCARS) and vibrational CARS (VCARS) depending on whether the Raman coherence is generated within pure-rotational or within vibrational-rotation transitions. PRCARS provides excellent temperature sensitivity and precision at lower temperature whereas VCARS provides large signal levels even at high flame temperatures. Several innovative attempts have been made to extend the applicability of the technique by either using multiple colors (DPVCARS) and/or by combining VCARS and PRCARS. Lucht [9] choose the pump and probe frequencies in a way that they could be interchanged to access both N2 and O2 spectra simultaneously. Bengtsson et al. [10] used four beams and spatially arranged them in such a way that the PRCARS and VCARS signals propagated in the same direction. They used a single spectrometer by including 3 more mirrors in it so that the PRCARS and the