Frederik Fuest

Ultrahigh laser pulse energy and power generation at 10 kHz

This Letter presents results from a new master-oscillator, power-amplifier pulse-burst laser system demonstrating ultrahigh pulse energies greater than 2.0 J/pulse at 1064 nm with interpulse separations of 100 μs (10 kHz) for burst durations of 100 pulses. Each pulse generates peak powers exceeding 130 MW and an average power of approximately 20 kW is generated over a 100-pulse-burst. Pulse energies decrease by less than 10% over a 100 sequential pulses, demonstrating negligible droop over long-duration pulse trains. Second-harmonic generation of 532 nm with conversion efficiency greater than 50% is demonstrated for 100-pulse-burst durations.

Quantification and Accuracy of a CMOS-Based Raman Scattering Imaging System for High-Speed Measurements in Flames

In this paper we will describe recent work in our laboratory towards the quantification of a high-speed (> 10 kHz) combined 1D Raman-Rayleigh scattering imaging system utilizing CMOS-based cameras. While our previous work has demonstrated the ability to acquire high-speed Raman/Rayleigh scattering images using a pulse burst laser system (Gabet et al., 2010), further study of the acquisition system is necessary for quantitative results. For the majority of high-speed imaging experiments, CMOS cameras are used because conventional CCD cameras cannot operate at sufficiently high acquisition rates to capture the full range of temporal scales and fluctuations in turbulent flows. Unlike CCD cameras, which typically have uniform and linear pixel response, each pixel on CMOS cameras has a unique response which needs to be characterized individually (Patton et al., 2011; Weber et al., 2011). In addition, CMOS cameras are known to exhibit increased levels of noise, particularly when coupled with an image intensifier. Careful examination and calibration of CMOS-based acquisition systems is of particular importance to understand their limitations and accuracy for low-signal applications such as Raman scattering. This paper will focus on quantifying the precision and accuracy of Raman/Rayleigh scattering measurements of major species, temperature, and mixture fraction using our CMOS-based 1D Raman/Rayleigh system in a series of near-adiabatic H2/air flames and turbulent H2/N2 jet flames. A detailed analysis of the spectral response and signal-to-noise ratio (SNR) of major species (H2, N2, H2O, and O2) and temperature is presented. The ability to measure “single-shot” scalar values accurately in turbulent flames is assessed by comparing scalar results in the DLR H3 (50% N2/50% H2 Re=10,000) turbulent jet flame to previous work (Meier et al., 1996). The ultimate goal of our research is to measure the time-varying profiles of all major combustion species and deduce temporally resolved mixture fraction profiles in turbulent combustion environments.

Spatial resolution-preserving retroreflection for gas-phase laser scattering measurements in turbulent flames using a phase-conjugate mirror

This Letter reports on the demonstration of a stimulated Brillouin-scattering phase conjugate mirror (SBS-PCM) for multipass laser-based scattering measurements in turbulent flames. Retroreflective measurements using the SBS-PCM show substantial improvements in spatial-resolution preservation as compared to measurements using a conventional mirror as a multipass reflector. The results using the SBS-PCM indicate an insensitivity to large index-of-refraction gradients and preservation of the original spatial resolution as defined by the laser beam under reacting flow conditions. This approach offers the possibility of increasing signal-to-noise ratios within low-signal gas-phase measurements such as Rayleigh and Raman scattering.

High-Speed 1D Raman/Rayleigh Scattering Imaging in Turbulent H2/N2 Flames.

In this paper we describe the development of a high-speed (10-kHz acquisition rate) combined 1D Raman/Rayleigh scattering imaging system for quantitative measurements in turbulent non-premixed jet flames. We demonstrate the high-speed 1D Raman/Rayleigh technique in a DLR H3 (50% H2/50% N2, Re = 10,000) turbulent jet flame by utilizing the High-Energy Pulse Burst Laser System (HEPBLS) at The Ohio State University. Presented in this paper are simultaneous, temporallyand spatially-correlated measurements of temperature, major species mole fractions (O2, N2, H2, and H20), and mixture fraction. Also presented are visualizations that highlight the utility of the high-speed system. Detailed measurements in near-adiabatic, laminar calibration flames were used to assess the accuracy and precision of the kHz-rate measurements using the high-speed Raman/Rayleigh scattering imaging system. In general, good agreement was found as compared to adiabatic flame calculations over a broad range of temperature and equivalence ratios. Beyond visualization, the new time-resolved measurements allow the determination of temporal auto-correlation functions (and associated integral time scales) for multiple scalars.

Dynamics of Conserved Scalar Mixing and Transport in Gas-Phase Turbulent Jets

Using a next generation pulse-burst laser system mixture fraction data of an axisymmetric propane jet issuing into air was collected at 10 kHz. Planar Rayleigh scattering measurements were taken for Red = 10,000; 20,000; 30,000 at axial locations ranging from x/d = 10 40. The objective of the current work focuses on validation of the kHz-rate measurements and introduction of temporally based data. To validate the mixture fraction measurements, mean values and RMS fluctuations data are compared to previously published literature. Temporal dynamics are examined using qualitative and quantitative means to develope the fullest picture possible. Image sets and point based mixture fraction traces are used to gain qualitative The integral time scale and unmixedness parameter are plotted for a large range of axial, radial and Reynolds number cases. The functional form of integral time scale dependence on Reynolds number, axial location and radial location is discussed.

Development of a High-Energy Pulse Burst Laser System for High-Speed Fluid Dynamics and Combustion Measurements

This paper describes recent advances made in our laboratory in the development of a new high-energy pulse burst laser system (HEPBLS) for turbulent fluid dynamics/combustion and high-speed flow measurements. We discuss new results from HEPBLS demonstrating ultra-high pulse energies greater than 2.0 Joules/pulse at 1064 nm with inter-pulse separations of 100 uf06ds (10 kHz) for burst durations exceeding 100 pulses with small levels of pulse-to-pulse fluctuations and negligible “droop” over the long pulse burst. Second harmonic generation of 532 nm with conversion efficiency greater than 50% is demonstrated for 100-pulse burst durations at repetition rates of 10 kHz and 20 kHz. The utility of the new system is shown through example 10-kHz image sequences of planar Rayleigh scattering-based mixture fraction and temperature imaging in turbulent non-reacting jets and flames, respectively.