Robert P. Lucht

Effect of Aviation Fuel Type and Fuel Injection Conditions on Non-reacting Spray Characteristics of a Hybrid Airblast Fuel Injector

I njector spray characteristics have a significant influence on the combustion performance in a gas turbine engine, including an impact on dynamics, emissions and component life. Furthermore, commercial aviation faces fuel cost, environmental, and energy security challenges that arise from the use of petroleum based jet fuels. Sustainable alternative jet fuels can help address these challenges and need to be characterized in their spray performance. The present work describes the detailed characterization of several alternative fuels using a hybrid airblast atomizer on the basis of spray shape, droplet size and velocity distribution at a range of operating conditions including fuel temperature, injector pressure drop and spray chamber pressure and temperature. The characterization is done using optical patternation, phase Doppler anemometry (dual-PDPA) and high speed back-lit imaging. The measurements obtained as part of this work provide the validation data-set for computational modeling of the spray behavior which forms a critical part of the broader project. The results show a strong influence of the fuel temperature on the spray, with lower temperature (290 K to 240 K) decreasing the atomization quality by 14%, while the effect of fuel injection pressure on the spray is minimal. A large effect of pressure drop across the injector is seen on the spray, with a change from 2% to 6% leading to a decrease in drop size of up to 36%, which can result of a shift in the secondary breakup regime of the spray.

The Application of Stereoscopic PIV in a Liquid-Fueled Gas Turbine Combustor

Pratt, Andrew Charles MSAA, Purdue University, December 2015. The application of Stereoscopic PIV in a Liquid-fueled Gas Turbine Combustor. Major Professor: Robert P. Lucht, School of Mechanical Engineering. Strict regulations on aviation gas turbine engine emissions and fuel consumption have driven the development of new lean burning, efficient gas turbine injectors. In an effort to increase fundamental understanding and support modeling efforts, great advancements have taken place in experimental measurement techniques. Specifically, in the field of laser diagnostics. This work describes the application of high repetition rate stereoscopic particle image velocimetry to a gas turbine combustor operating at representative engine conditions. A motivation and brief background of this research is provided. An introduction to Stereoscopic Particle Image Velocimetry (SPIV) and its development is included with a description of the experimental systems and the challenges associated with acquiring useful data in high pressure and high thermal power. The facility capabilities and test stand capabilities are presented along with the operational configuration for both the experimental and diagnostic systems. Finally, results are presented from two operating conditions, one with combustion and one without. Both 3-component SPIV and 2-component PIV data were collected simultaneously at 6 kHz. The vector fields generated from both techniques are compared both qualitatively and quantitatively.

Dual-pump CARS Measurements in a Gas Turbine Combustor Facility using the NASA 9-Point Lean Direct Injector

Lean direct injection (LDI) has been among the seve ral combustion strategies being studied by NASA Glenn Research Center (GRC) to reduce oxides of nitrogen (NO x) emissions while maintaining high combustion effic iency [1]. The NASA 9-point top-hat LDI assembly is a multiplex fuel injector c ontaining nine fuel injection tips and multiple burning zones that replace one conventiona l fuel injector. The nine injectors are placed in a 3 × 3 square matrix arrangement. We have developed a gas turbine combustor facility (GTCF) at the High Pressure Laboratory in Purdue’s Zucrow Laboratory complex. The GTCF has been modified for optical access and dual-pump coherent anti-Stokes Raman scattering (DP-CARS) measurements have been performed at supersonic cruise conditions. A window assembly has been designed, fabricated, and assembled in the GTCF at Purdue University for advanced laser diagnostic stu dies. The window assembly allows optical access from three mutually perpendicular di rections using a pair of thin and thick fused silica windows on each side. The assembly is cooled using water while filmcooling air is provided for the inside of the thin windows. The thin windows are designed for thermal load while the thick windows a re designed for pressure loading. Combusting flows are studied using the central inje ctor of the aforementioned 9-point lean direct injection (LDI) device. The combustor has been operated using Jet-A fuel at inlet air temperatures up to 725 K and combustor pr essures up to 10.2 atm. DP-CARS temperature and major species concentration measurements have been performed in the GTCF at various operating conditions. An injection -seeded optical parametric oscillator (OPO) is used as a narrowband pump laser source so as to improve the accuracy and precision of the CARS measurements. Spatial maps of temperature and major species concentrations have been obtained in high-pressure LDI flames by translating the CARS probe volume in axial and radial directions inside the combustor rig without loss of optical alignment.

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.

Combustion Measurements in a Counter-Flow Burner and Progress towards Vibrational ERE-CARS Measurements in High-Pressure Flames

Stereo Particle Image Velocimetry measurements are reported in a new counter-flow burner. The measurements indicate that the burner produces axi-symmetric, top hat velocity profiles over the central 8 mm at the nozzle exit. N2 CARS will be employed to obtain temperature profiles in counter-flow methane and hydrogen flames in the near future. We also report the progress towards restoration and upgrade of a high-pressure flame facility. Subsequently, vibrational ERE-CARS measurements of NO concentration will be performed in this facility at pressures up to 10 atm.

Concentration Measurements in CO/N2 and Ar/N2 Gas Mixtures using Femtosecond Coherent Anti-Stokes Raman Scattering

Single-laser-shot concentration measurements in CO/N2 and Ar/N2 gaseous mixtures are performed using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS) with a chirped probe pulse. Measurements are performed in a heated gas cell at temperatures of 300, 600, and 900 K for the complete range of binary gas mixtures. Due to the broadband nature of the ultrafast pulses, Raman transitions of CO (2145 cm -1 ) and N2 (2330 cm -1 ) are probed simultaneously. A theoretical model is developed to extract temperature and concentration measurements from the experimental data. The best agreement between theory and experiment was found for probe-pulse time delays from 0 to 2 picoseconds (ps). Preliminary concentration measurement results are promising when the lasers are tuned to the minor species Raman transition frequency. Measurements performed in Ar/N2 mixtures are used to explore detection limits in environments with strong nonresonant backgrounds. Experimental data is also reported utilizing polarization suppression of the nonresonant background and further analysis will be performed in the future. These measurements will help to determine the effect of resonant and nonresonant gases on temperature measurements based on the N2 spectral shape.

Chirped-Probe-Pulse Femtosecond Coherent Anti-Stokes Raman Scattering for Single-Laser-Pulse Flame Temperature Measurements

Single-laser-pulse temperature measurements are made at 1000 Hz by femtosecond coherent anti-Stokes Raman scattering (CARS) with a chirped-probe-pulse. The temporal decay of the Raman coherence is mapped onto the frequency of the CARS signal.

Temperature Measurements in Flames at 1000 Hz Using Femtosecond Coherent Anti-Stokes Raman Spectroscopy

[Abstract] Single-laser-shot temperature measurements at a data rate of 1 kHz are demonstrated using femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy. The excitation of gas-phase Raman lines with spectral widths of 3 GHz by pump and Stokes beams with spectral widths of 3000 GHz is very efficient provided that the pump and Stokes beams are Fouriertransform-limited. The single-laser-shot measurements were performed by using a chirped probe pulse to map the time-dependent frequency-spread decay of the Raman coherence into the spectrum of the CARS signal pulse. Temperature is determined from the spectral shape of the chirped-probe femtosecond CARS signal for probe delays of approximately 2 picoseconds with respect to the impulsive pump-Stokes excitation of the Raman coherence. Fs CARS spectra with very high signal-to-noise ratios are acquired from laminar flames, forced unsteady flames, and turbulent flames. The fs CARS spectrum is not affected by collisional line shapes in contrast to ns CARS spectroscopy. However, the fs CARS spectrum is affected by the spectrum and phase of the pump, Stokes, and probe beams, and the effect of departures from the assumptions of Fourier-transform-limited pump and Stokes beam and a linearly chirped probe beam are discussed. I. Introduction Single-pulse coherent anti-Stokes Raman scattering (CARS) spectroscopy of gas-phase resonances using femtosecond (fs) lasers is discussed. Femtosecond CARS offers two potential major advantages compared with nanosecond CARS; i.e., CARS as usually performed with nanosecond pump and Stokes lasers. These potential advantages are (1) a significant increase in the signal-to-noise ratio of the CARS signal and (2) the capability of performing real-time temperature and species measurements at data rates of 1 kHz or greater. The potential for real-time measurements at frequencies of interest in turbulent flames is the result of the commercial availability of femtosecond laser systems with pulse energies of a few J to a few mJ and with repetition rates of 1 kHz up to 250 kHz. If techniques for determining single-pulse temperatures and/or concentrations can be developed, time series measurements can be performed in turbulent flames and flows at data rates that are faster than turbulent fluctuation frequencies. The potential for significant noise reduction is a result of the