Xilong Yu

Simultaneous measurements of multiple flow parameters for scramjet characterization using tunable diode-laser sensors

This paper reports the simultaneous measurements of multiple flow parameters in a scramjet facility operating at a nominal Mach number of 2.5 using a sensing system based on tunable diode-laser absorption spectroscopy (TDLAS). The TDLAS system measures velocity, temperature, and water vapor partial pressure at three different locations of the scramjet: the inlet, the combustion region near the flame stabilization cavity, and the exit of the combustor. These measurements enable the determination of the variation of the Mach number and the combustion mode in the scramjet engine, which are critical for evaluating the combustion efficiency and optimizing engine performance. The results obtained in this work clearly demonstrated the applicability of TDLAS sensors in harsh and high-speed environments. The TDLAS system, due to its unique virtues, is expected to play an important role in the development of scramjet engines.

Uncertainty in velocity measurement based on diode-laser absorption in nonuniform flows.

This work investigates the error caused by nonuniformities along the line-of-sight in velocity measurement using tunable diode-laser absorption spectroscopy (TDLAS). Past work has demonstrated TDLAS as an attractive diagnostic technique for measuring velocity, which is inferred from the Doppler shift of two absorption features using two crossing laser beams. However, because TDLAS is line-of-sight in nature, the obtained velocity is a spatially averaged value along the probing laser beams. As a result, nonuniformities in the flow can cause uncertainty in the velocity measurement. Therefore, it is the goal of this work to quantify the uncertainty caused by various nonuniformities typically encountered in practice, including boundary layer effects, the divergence/convergence of the flow, and the methods used to fit the Doppler shift. Systematic analyses are performed to quantify the uncertainty under various conditions, and case studies are reported to illustrate the usefulness of such analysis in interpreting experimental data obtained from a scramjet facility. We expect this work to be valuable for the design and optimization of TDLAS-based velocimetry, and also for the quantitative interpretation of the measurements.

Double-end-pumped Nd:YVO4 slab laser at 1064 nm

We demonstrate a high-power laser diode stacks double-end-pumped Nd:YVO4 1064 nm slab laser with a folded stable-unstable hybrid resonator. An output power of 220 W was obtained at the pump power of 490 W with optical conversion efficiency of 44.9%. At the output power of 202 W, the M2 factors in the unstable direction and in the stable direction were 1.7 and 2.3, respectively.

An investigation of electromagnetic wave propagation in plasma by shock tube

This paper presents the electromagnetic wave propagation characteristics in plasma and the change of the attenuation coefficients of the microwave with some parameters by using #800mm high temperature shock tube. In order to get the attenuation of the electromagnetic wave through the plasma behind shock wave, the microwave transmission is used to measure the relative change of the wave power. The experimental results reveal that the dependence of the attenuation coefficients on ne, v, w, and L are in good agreement with the theoretical predictions of electromagnetic wave propagations in the uniform infinite plasma when the plasma layer is thick (such as L/λ ≥ 10). We also measure the electromagnetic wave power attenuate value with and without the magnetic field.

Experimental Study of the Transient Response of Bunsen Flame to Nanosecond Pulsed Discharges

Abstract The transient processes associated with the interaction of a Bunsen flame and nanosecond pulsed discharges (NPD) are explored experimentally with two optical methods. A nanosecond-gated schlieren system is employed to image the shockwave propagation and the hydrodynamic response of the flame to NPD while the time-resolved optical emission spectroscopy measurements are carried out to determine active species and temperature in the plasma region created by the discharges. Therefore, the unsteady process of the interaction of the flame with the discharges is recorded in real-time by the combined measurements. Numbers of experimental evidences for understanding the dynamics of non-equilibrium plasma produced by NPD and performing further numerical simulation are offered.

340 W Nd:YVO4 Innoslab laser oscillator with direct pumping into the lasing level

We present a high power 340 W Nd:YVO4 Innoslab laser end-pumped by 880 nm laser-diode stacks. A maximum output power of 340 W was obtained at the absorbed power of 710 W with optical conversion efficiency up to 47.9 %. At the output power of 300 W, the M-2 factors in the unstable direction and the stable direction were 2.1 and 3.4, respectively.

Laser Diagnostics of Combustion Enhancement on a CH4/Air Bunsen Flame by Dielectric Barrier Discharge

We investigate plasma-assisted combustion for premixed CH4/air Bunsen flames. Dielectric barrier discharge (DBD) is employed to produce non-equilibrium plasma for combustion enhancement. The transient planar laser induced fluorescence (PLIF) technique of CH and OH radicals is used to image reaction zones for enhancement measurement, and the emission spectra of the Bunsen flame are monitored to explore the kinetics mechanism. From the drift of radicals in PLIF images, the quantitative enhancement of plasma on the flame velocities of premixed methane/air flames is experimentally measured, and the data show that the flame velocities are increased by at least 15% in the presented equivalence ratio range. Furthermore, the well analyzed emission spectra of the Bunsen flame (300–800 nm) with/without DBD reveal that the emissions as well as the concentrations of the crucial radicals (like C2, CH, OH etc.) in combustion all are intensified greatly by the discharge. In addition, the appearance of excited spectral bands of N2 and N+2 during discharge indicates that the premixed gas is also heated and ionized partially by the DBD.

Combined TDLAS and OES technique for CO concentration measurement in shock-heated Martian atmosphere

This paper describes the CO concentration and gas temperature distribution measurements behind a strong shock wave in the simulated Martian atmosphere by an optical diagnostic system. The strong shock wave (6.31 ± 0.11 km/s) is established in a shock tube driven by combustion of hydrogen and oxygen. The optical diagnostic system consists of two parts: the optical emission spectroscopy (OES) system and the tunable diode laser absorption spectroscopy (TDLAS) system. For OES system, high temporal and spatial resolution experimental spectra of CN violet system (B2Σ+→X2Σ+, v = 0 sequence) have been observed. Rotational and vibrational temperature distribution along the shock wave is inferred through a precise analysis of high-resolution experimental spectra. For TDLAS system, a CO absorption line near 2335.778 nm is utilized for detecting the CO concentration using scanned-wavelength direct absorption mode. Combined with these experimental results using OES, CO concentration in the thermal equilibrium region is derived. The detected average CO concentration is 7.46 × 1012 cm-3 with the average temperature of 7400 K ± 300 K, which corresponds to the center fractional absorption of 2.7%.

Uncertainty Analysis of Velocity Measured by Diode-Laser Absorption Spectroscopy in a Scramjet Facility

This work investigates the error caused by non-uniformities along the line-of-sight in velocity measurement using tunable diode-laser absorption spectroscopy (TDLAS). Past work has demonstrated TDLAS as an attractive diagnostic technique for measuring velocity, which is inferred from the Doppler shift of two absorption features using two crossing laser beams. However, because TDLAS is line-of-sight in nature, the obtained velocity is a spatially-averaged value along the probing laser beams. As a result, non-uniformities in the flow can cause uncertainty in the velocity measurement. Therefore, it is the goal of this work to quantify the uncertainty caused by various non-uniformities typically encountered in practice, including boundary layer effects, the divergence/convergence of the flow, and the methods (direct absorption vs. wavelength modulation) used to fit the Doppler shift. Systematic analyses are performed to quantify the uncertainty under various conditions, and case studies are reported to illustrate the usefulness of such analysis in interpreting experimental data obtained from a scramjet facility. We expect this work to be valuable for the design and optimization of TDLAS-based velocimetry, and also for the quantitative interpretation of the measurements.

Computed Tomography Measurement of 3D Combustion Chemiluminescence Using Single Camera

Instantaneous measurement of flame spatial structure has been long desired for complicated combustion condition (gas turbine, ramjet et.). Three dimensional computed tomography of chemiluminescence (3D-CTC) is a potential testing technology for its simplicity, low cost, high temporal and spatial resolution. In most former studies, multi-lens and multi-CCD are used to capture projects from different view angles. In order to improve adaptability, only one CCD was utilized to build 3D-CTC system combined with customized fiber-based endoscopes (FBEs). It makes this technique more economic and simple. Validate experiments were made using 10 small CH4 diffusion flame arranging in a ring structure. Based on one instantaneous image, computed tomography can be conducted using Algebraic Reconstruction Technique (ART) algorithm. The reconstructed results, including the flame number, ring shape of the flames, the inner and outer diameter of ring, all well match the physical structure. It indicates that 3D combustion chemiluminescence could be well reconstructed using single camera.