Aman Satija

High-repetition-rate three-dimensional OH imaging using scanned planar laser-induced fluorescence system for multiphase combustion

Imaging dynamic multiphase combusting events is challenging. Conventional techniques can image only a single plane of an event, capturing limited details. Here, we report on a three-dimensional, time-resolved, OH planar laser-induced fluorescence (3D OH PLIF) technique that was developed to measure the relative OH concentration in multiphase combustion flow fields. To the best of our knowledge, this is the first time a 3D OH PLIF technique has been reported in the open literature. The technique involves rapidly scanning a laser sheet across a flow field of interest. The overall experimental system consists of a 5 kHz OH PLIF system, a high-speed detection system (image intensifier and CMOS camera), and a galvanometric scanning mirror. The scanning mirror was synchronized with a 500 Hz triangular sweep pattern generated using Labview. Images were acquired at 5 kHz corresponding to six images per mirror scan, and 1000 scans per second. The six images obtained in a scan were reconstructed into a volumetric representation. The resulting spatial resolution was 500×500×6 voxels mapped to a field of interest covering 30  mm×30  mm×8  mm. The novel 3D OH PLIF system was applied toward imaging droplet combustion of methanol gelled with hydroxypropyl cellulose (HPC) (3 wt. %, 6 wt. %), as well as solid propellant combustion, and impinging jet spray combustion. The resulting 3D dataset shows a comprehensive view of jetting events in gelled droplet combustion that was not observed with high-speed imaging or 2D OH PLIF. Although the scan is noninstantaneous, the temporal and spatial resolution was sufficient to view the dynamic events in the multiphase combustion flow fields of interest. The system is limited by the repetition rate of the pulsed laser and the step response time of the galvanometric mirror; however, the repetition rates are sufficient to resolve events in the order of 100 Hz. Future upgrade includes 40 kHz pulsed UV laser system, which can reduce the scan time to 125 μs, while keeping the high repetition rate of 1000 Hz.

Development of a combined pure rotational and vibrational coherent anti-Stokes Raman scattering system

A combined pure rotational coherent anti-Stokes Raman scattering (PRCARS) and vibrational CARS (VCARS) system has been developed. In this system two beams, a broadband beam centered at 607 nm and the frequency-doubled Nd:YAG output at 532 nm is used to generate the PRCARS signal. A second 532 nm beam is used along with the other two beams to simultaneously generate the N(2) VCARS signal using a standard phase-matching scheme.

Development of two-color laser system for high-resolution polarization spectroscopy measurements of atomic hydrogen

We have developed a high-spectral-resolution laser system for two-photon pump, polarization spectroscopy probe (TPP-PSP) measurements of atomic hydrogen in flames. In the TPP-PSP technique, a 243-nm laser beam excites the two-photon 1S-2S transition, and excited n=2 atoms are then detected by polarization spectroscopy of the n=2 to n=3 transition using 656-nm laser radiation. The single-frequency-mode 243 and 656-nm beams are produced using injection-seeded optical parametric generators coupled with pulsed dye amplifiers. The use of single-mode lasers allows accurate measurement of signal line shapes and intensities even with significant pulse-to-pulse fluctuations in pulse energies. Use of single-mode lasers and introduction of a scheme to select nearly constant laser energies enable repeatable extraction of important spectral features in atomic hydrogen transitions.

High speed OH PLIF applied to multiphase combustion (Review)

Multiphase reactive systems can exhibit highly dynamic combustion phenomena that could be better understood by using recently developed high-repetition-rate optical diagnostic and imaging approaches. Here, we present an overview of recent activities using high-speed (5 kHz) OH planar laser-induced fluorescence to visualize and make measurements in several multiphase reactive systems. This technique is used to visualize the dynamically changing OH concentration in the gas phase near the surface of solids, liquids, and gels. In addition to gas-phase OH imaging, condensed phases of various solid propellants, gels, and liquids are found to fluoresce when exposed to the laser radiation centered at 283.2 nm. Simultaneous imaging of condensed phases and gasphase OH radical fluorescence has proven to be particularly useful for various measurements, and several examples are presented.

Dual-band quasi-coherent radiative thermal source

We design, fabricate and characterize the spectral, polarization, angular and temperature dependence of a microstructured SiC thermal infrared source; achieving independent control of the frequency and polarization of thermal radiation in two spectral bands.

Investigation of Gas Heating by Nanosecond Repetitively Pulsed Glow Discharges Used for Actuation of a Laminar Methane-Air Flame

ABSTRACT This article reports on the quantification of the heating induced by nanosecond repetitively pulsed (NRP) glow discharges on a lean premixed methane-air flame. The flame, obtained at room temperature and atmospheric pressure, has an M-shape morphology. The equivalence ratio is 0.95 and the thermal power released by the flame is 113 W. The NRP glow discharges are produced by high voltage pulses of 10 ns duration, 7 kV amplitude, applied at a repetition frequency of 10 kHz. The average power of the plasma, determined from current and voltage measurements, is 1 W, i.e., about 0.9% of the thermal power of the flame. Broadband vibrational coherent anti-Stokes Raman spectroscopy of nitrogen is used to determine the temperature of the flame with and without plasma enhancement. The temperature evolution in the flame area shows that the thermal impact of NRP glow discharges is in the uncertainty range of the technique, i.e., ±40 K.

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

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

Development of Combined Dual-Pump Vibrational and Pure-Rotational Coherent Anti-Stokes Raman Scattering (DPVCARS and PRCARS) System

Coherent anti-Stokes Raman scattering (CARS) [1,2] is a spatially-resolved, time-resolved spectroscopic technique for quantitative measurements in reacting flows [3 – 6]. This work demonstrates a combination of N2/O2/CO2 dual-pump vibrational coherent anti-Stokes Raman scattering (DPVCARS) system and two-beam pure-rotational coherent anti-Stokes Raman scattering (PRCARS) system. It is based on the previous development of combined VCARS and PRCARS system which was used to obtain temperature measurements in non-premixed H2-air flames. The new combined system will be used to measure the temperature profiles and major species concentrations such as N2/O2/CO2 in laminar counter-flow non-premixed (CH4/Air) and partiallypremixed (CH4/H2/Air) flames. The new system is being characterized in H2/Air diffusion flames stabilized over a Hencken burner. CO2 will be added to the oxidizer stream for the system to assess the precision of the system while performing concentration measurements. The new combined system has shown good precision temperature using PRCARS (better than 3%) and N2/O2 mole-fraction ratio (better than 5%) using DPVCARS.

Two Color Polarization Spectroscopy for Measurements of Atomic Hydrogen Concentration using Single-Mode Laser Systems

A two-photon pump polarization spectroscopy probe (TPP-PSP) laser system has been designed for detection of atomic hydrogen (H-atom) in flames. Single frequency mode (SFM) laser output from injection-seeded optical parametric generators (OPG) coupled with pulsed dye amplifiers (PDA) have been used to produce the pump and probe laser beams for TPP-PSP experiment. In TPP-PSP, a 243-nm pump beam excites the 1S-2S two photon transition and the excited atoms in 2S level are probed by polarization spectroscopy between n=2 and n=3 manifolds using a circularly polarized 656-nm pump and a linearly polarized 656-nm probe laser beam. The SFM laser sources for the pump and probe beams allow accurate measurement of TPP-PSP line shapes. Atomic hydrogen was detected at concentrations as low as 11 ppm in atmospheric-pressure, near-adiabatic hydrogen-air flames. The measured atomic hydrogen concentration profiles are in good agreement with equilibrium calculations. The results obtained using SFM laser systems are very encouraging for quantitative measurements of atomic hydrogen in flames.

High-spectral-resolution Two-photon Pump Polarization Spectroscopy Probe Technique for H-atom Detection

We report the development of a two-photon pump polarization spectroscopy probe technique for detection of atomic Hydrogen using two single-mode laser sources. Significant effect of laser parameters on atomic transitions has been observed.