Numerical investigation of low-frequency instability and frequency shifting in a scramjet combustor

The combustion instability in a laboratory-scale direct-connect hydrogen-fueled scramjet combustor is investigated numerically. The numerical simulation has been carried out using a delayed detached eddy simulation (DDES) with a detailed reaction mechanism. The computational framework has high fidelity by applying multi-dimensional high order accurate schemes for handling convective and viscous fluxes. The field data were accumulated up to 100 milliseconds on each case to capture sufficiently the repetitive behavior of low-frequency instability of order of 100 Hz. The numerical results exhibit the formation/dissipation of pressure and shock wave induced by continuous heat release in the combustor. This motion of pressure/shock wave, so-called upstream-traveling shock wave, presents repeated dynamics between isolator and combustor with a period of several milliseconds. With this periodic hydrodynamic characteristic, the upstream-traveling shock wave interacts with the boundary layer and injected fuel stream affecting fuel/air mixing and burning, and finally inducing the combustion instability in a scramjet combustor. Frequency analysis derived major instability frequencies of 190 Hz and 450 Hz in the isolator and combustor for low and high equivalence ratios, respectively. Current numerical results present the underlying flow physics on the shifting of the instability frequency by changing the equivalence ratio observed by the previous experimental studies. The fact that an instability frequency exists homogeneously from isolator to combustor informs that the combustion instability of scramjet engine is the fully coupled flow/combustion dynamics throughout the engine on a macroscopic scale.     

隔离段激波串流场特征的试验研究进展

高超声速推进技术是国际前沿研究, 其中双模态超燃冲压发动机的发展受到极大关注. 作为超燃冲压发动机的重要部件, 隔离段对发动机的性能和高超声速飞行的实现至关重要, 其中所涉及的流动机理问题也极为复杂. 自从高超声速飞行的概念被提出和论证以来, 相关的理论、试验和仿真研究不断取得进展, 但是对其中的机理问题研究仍有待进一步深入. 本文将从试验研究的角度回顾并综述近年来超燃冲压发动机隔离段的研究进展, 结合精细流动测试技术(Nano-tracer Planar Laser Scattering, NPLS)的发展分析了隔离段流场特征, 包括了激波串流场复杂的三维时空结构特点、湍流特性、非线性迟滞运动、不启动流场特征以及激波前缘检测等. 从风洞设备、隔离段设计、测试技术等方面对隔离段的试验研究进行了分类比较和论述, 对今后隔离段试验研究提出了建议.     

Laser induced plasma spectroscopy for local equivalence ratio measurements in an oscillating combustion environment

Equivalence ratios measured with a laser induced plasma spectroscopy (LIPS, also referred as LIBS) are reported in two different setups. First, a small premixed turbulent burner is used to address fundamental issues concerning the LIPS technique. It is shown that hydrogen excitation within the created plasma is the key parameter to measure in order to retrieve correctly equivalence ratio measurements. Results compared with a spark energy classification strategy show better results with excitation classification, as variations in ratio between the different lines come not only from gaseous concentration but also from plasma’s characteristics. Using spectra from 450 to 800 nm allows the determination of two independent emission ratios to improve single shot accuracy. The developed approach is afterwards applied to phase-locked measurements of equivalence ratio in a lean premixed combustor, for which strong thermo-acoustics oscillations exist. This combustor runs with methane-air, preheated at 700 K and with a typical equivalence ratio of 0.50, for which the sound pressure levels of the oscillations are 170 dB. Measurements at the inlet of the combustor reveal strong correlations between fluctuations of the incoming stoichiometry and pressure fluctuations. It is shown that stoichiometry changes within one oscillating cycle of about 3%. Those changes are crucial for the flame dynamics as dealing with very lean mixtures.     

High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: Temperature dependence and importance of the CH3D contribution

The high resolution absorption spectrum of methane in the 1.58 μm transparency window has been recorded at room temperature and at 79 K by CW-Cavity Ring Down Spectroscopy using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers. The achieved sensitivity (α_min ∼ 3 × 10⁻¹⁰ cm⁻¹) has allowed for a detailed characterization of the 6289-6526 cm⁻¹ region which corresponds to the lowest opacity of the transparency window. A list of 6868 and 4555 transitions with intensities as weak as 1 × 10⁻²⁹ cm/molecule was constructed from the recordings at 297 and 79 K, respectively. By comparison with a spectrum of CH₃D recorded separately by Fourier Transform Spectroscopy, 1282 and 640 transitions of monodeuterated methane, CH₃D, in natural abundance in our sample were identified at 297 and 79 K, respectively.The rotational temperature determined from the intensity distribution of the 3ν₂ band of CH₃D (79.3 K) was found in good agreement with the temperature value previously obtained from the Doppler line broadening. The reduction of the rotational congestion by cooling down to 79 K reveals a spectral region near 6300 cm⁻¹ where CH₃D transitions are dominant.The low energy values of the transitions observed both at 79 K and at room temperature were derived from the variation of their line intensities. These transitions with lower energy determination represent 93.9% and 68.4% of the total absorbance in the region, at 79 K and room temperature, respectively. The quality of the obtained empirical low energy values is demonstrated for CH₄ by the marked propensity of the empirical low J values to be close to integers. The line lists at 79 K and room temperature provided as Supplementary Material allow accounting for the temperature dependence of methane absorption between these two temperatures. The investigated region covering the 5ν₄ band of the ¹²CH₄ isotopologue will be valuable for the theoretical treatment of this band which is the lowest energy band of the icosad.     

Kinetics of cation distribution in non-stoichiometric nickel gallate spinel

Cations in the spinel structure are distributed over sites of tetrahedral and octahedral coordination. The cation distribution and its kinetics in non-stoichiometric nickel gallate spinel were studied by high-temperature optical spectroscopy combined with the temperature-jump relaxation technique. It is found that up to 1200 °C the optical absorption spectra of the spinel are dominated by ligand field transitions of Ni²⁺ ions on octahedral sites. The kinetics of cation site-exchange were investigated from 850 to 1100 °C by monitoring the temporal evolution of the absorbance due to Ni²⁺ ions in octahedral sites after sudden changes in temperature. The temperature dependence of cation kinetics shows two regimes, one below and one above about 950 °C with activation energies of about 230 kJ/mol and 60 kJ/mol, respectively. Due to sample preparation, the low-temperature activation energy corresponds to an extrinsic vacancy migration regime. The activation energy of kinetics in the upper temperature regime is discussed in connection with the increasing reactivity of the optical absorber at high temperatures.     

Wavelength modulation spectroscopy for measurements of gas parameters in combustion field

A novel wavelength modulation spectroscopy sensor for studying gas properties near 1.4 μm is developed, validated and used in a direct-connect supersonic combustion test facility. In this sensor there are two H_2O transitions near 7185.60 cm~(-1) and 7454.45 cm~(-1)that are used to enable the measurements along the line-of-sight. According to an iterative algorithm, the gas pressure, temperature and species mole fraction can be measured simultaneously. The new sensor is used in the isolator and extender of the supersonic combustion test facility. In the isolator, the sensor resolves the transient and measured pressure, temperature and H_2O mole fraction with accuracies of 2.5%, 8.2%, and 7.2%, respectively. Due to the non-uniform characteristic in the extender, the measured results cannot precisely characterize gas properties, but they can qualitatively describe the distinctions of different zones or the changes or fluctuations of the gas parameters.     

N(2)O weak lines observed between 3900 and 4050 cm from long path absorption spectra

Previously unobserved nitrous oxide transitions around 2.5 μm are measured by intracavity laser absorption spectroscopy (ICLAS) analyzed by time-resolved Fourier transform (TRFT) spectrometer. With an accuracy of the order of 10⁻³ cm⁻¹, measured positions of 1637 assigned weak transitions are provided. They belong to 42 vibrational transitions, among which 33 are observed for the first time. These data are believed to be useful in particular to monitoring atmosphere purposes.     

Probing dynamics of a phase transition of two-dimensional nano-domains with STM imaging and manipulation

A reversible, temperature-driven structural surface phase transition of Pb/Si(1 1 1) nano-domains is studied with a variable-temperature scanning tunneling microscope (STM). Finite-size effects of the transition are clearly demonstrated. Most importantly, structural fluctuations in the low-temperature phase can be induced by the direct interaction between the tip atoms and the surface atoms. The structural changes reveal dynamics in the low-temperature phase. Amazingly, the largest size of the domains that can be manipulated decreases with decreasing sample temperature.     

Scanned-wavelength-modulation spectroscopy near 2.5 μm for H2O and temperature in a hydrocarbon-fueled scramjet combustor

The design and demonstration of a two-color tunable diode laser sensor for measurements of temperature and H₂O in an ethylene-fueled model scramjet combustor are presented. This sensor probes multiple H₂O transitions in the fundamental vibration bands near 2.5 μm that are up to 20 times stronger than those used by previous near-infrared H₂O sensors. In addition, two design measures enabled high-fidelity measurements in the nonuniform flow field. (1) A recently developed calibration-free scanned-wavelength-modulation spectroscopy spectral-fitting strategy was used to infer the integrated absorbance of each transition without a priori knowledge of the absorption lineshape and (2) transitions with strengths that scale near-linearly with temperature were used to accurately determine the H₂O column density and the H₂O-weighted path-averaged temperature from the integrated absorbance of two transitions.     

Determination of the low energy values of CH4 transitions in the 2ν3 region near 1.66 μm from absorption spectra at 296 and 81 K

The high resolution absorption spectra of ¹³CH₄ were recorded at 81 K by differential absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers and at room temperature by Fourier transform spectroscopy. The investigated spectral region corresponds to the high energy part of the ¹³CH₄ tetradecad dominated by the 2ν₃ overtone near 5988 cm⁻¹. Empirical line lists were constructed containing, respectively, 1629 ¹³CH₄ transitions detected at 81 K (5852-6124 cm⁻¹) and 3481 features (including 85 lines of ¹²CH₄) measured at room temperature (5850-6150 cm⁻¹); the smallest measured intensities are about 3 × 10⁻²⁶ and 4 × 10⁻²⁵ cm/molecule at 81 and 296 K, respectively. The lower state energy values were derived for 1196 ¹³CH₄ transitions from the variation of the line intensities between 81 and 296 K. These transitions represent 99.2% and 84.6% of the total absorbance in the region, at 81 and 296 K, respectively. Over 400 additional weak features were measured at 81 K and could not be matched to lines observed at room temperature. The quality of the resulting empirical low energy values is demonstrated by the excellent agreement with the already-assigned transitions and the clear propensity of the empirical low J values to be close to integers. The two line lists at 81 and at 296 K provided as Supplementary material will enable future theoretical analyses of the upper ¹³CH₄ tetradecad.     

Combustion characteristics of a dual-mode scramjet combustor with cavity flameholder

Combustion characteristics of a laboratory dual-mode ramjet/scramjet combustor were studied experimentally. The combustor consists of a sonic fuel jet injected into a supersonic crossflow upstream of a wall cavity pilot flame. These fundamental components are contained in many dual-mode combustor designs. Experiments were performed with an isolator entrance Mach number of 2.2. Air stagnation temperatures were varied from 1040 to 1490 K, which correspond to flight Mach numbers of 4.3–5.4. Both pure hydrogen and a mixture of hydrogen and ethylene fuels were used. High speed imaging of the flame luminosity was performed along with measurements of the isolator and combustor wall pressures. For ramjet mode operation, two distinct combustion stabilization locations were found for fuel injection a sufficient distance upstream of the cavity. At low T₀, the combustion was anchored at the leading edge of the cavity by heat release in the cavity shear layer. At high T₀, the combustion was stabilized a short distance downstream of the fuel injection jet in the jet-wake. For an intermediate range of T₀, the reaction zone oscillated between the jet-wake and cavity stabilization locations. Wall pressure measurements showed that cavity stabilized combustion was the steadiest, followed by jet-wake stabilized, and the oscillatory case. For fuel injection close to the cavity, a hybrid stabilization mode was found in which the reaction zone locations for the two stabilization modes overlapped. For this hybrid stabilization, cavity fueling rate was an important factor in the steadiness of the flow field. Scramjet mode combustion was found to only exist in the cavity stabilized location for the conditions studied.     

Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (I) The region of the 2ν3 band at 1.66 μm

In a recent contribution, (Gao B, Kassi S, Campargue A. Empirical low energy values for methane transitions in the 5852-6181 cm⁻¹ region by absorption spectroscopy at 81 K. J Mol Spectrosc 2009;253:55-63.), the low energy values of methane transitions between 1.71 and 1.62 μm were derived from the variation of the line intensities between 296 and 81 K. The line intensities at 81 K were retrieved from the high resolution absorption spectrum of methane recorded at liquid nitrogen temperature by direct absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. For the line intensities at 296 K, the values provided by the HITRAN database were used. As a consequence of the relatively high intensity cut off (4×10⁻²⁴ cm/molecule) of the HITRAN line list in the considered region, the lower energy values were derived for only 845 of the 2187 transitions measured at 81 K. In the present work, our line list was extended by the retrieval of many weak line intensities leading to a set of 3251 transitions. The minimum value of the measured line intensities (at 81 K) is on the order of 10⁻²⁶ cm/molecule. In relation with the project “Greenhouse Gases Observing Satellite” (GOSAT), a much more complete line list for CH₄ at 296 K has become available (intensity cut off of 4×10⁻²⁶ cm/molecule). By applying the two temperature method to our line intensities at 81 K and GOSAT intensities at 296 K, the lower energy values of 2297 transitions could be derived. These transitions represent 99.1% and 90.8% of the total absorbance in the region, at 81 and 296 K respectively. This line list provided as Supplementary Material allows then accounting for the temperature dependence of CH₄ absorption below 300 K. The investigated spectral range is dominated by the 2ν₃ band near 6005 cm⁻¹ which is of particular interest for atmospheric retrievals. The factor 2 narrowing of the Doppler linewidth from room temperature down to 81 K has allowed the resolution of a number of 2ν₃ multiplets and improving the line intensity retrievals. A detailed comparison with GOSAT and HITRAN line lists has revealed a number of possible improvements.     

Investigation on the flameholding mechanisms in supersonic flows: backward-facing step and cavity flameholder

Abstract  

As effective devices to extend the fuel residence time in supersonic flow and prolong the duration time for hypersonic vehicles cruising in the near-space with power, the backward-facing step and the cavity are widely employed in hypersonic airbreathing propulsive systems as flameholders. The two-dimensional coupled implicit RANS equations, the standard k-ε turbulence model, and the finite-rate/eddy-dissipation reaction model have been used to generate the flow field structures in the scramjet combustors with the backward-facing step and the cavity flameholders. The flameholding mechanism in the combustor has been investigated by comparing the flow field in the corner region of the backward-facing step with that around the cavity flameholder. The obtained results show that the numerical simulation results are in good agreement with the experimental data, and the different grid scales make only a slight difference to the numerical results. The vortices formed in the corner region of the backward-facing step, in the cavity and upstream of the fuel injector make a large difference to the enhancement of the mixing between the fuel and the free airstream, and they can prolong the residence time of the mixture and improve the combustion efficiency in the supersonic flow. The size of the recirculation zone in the scramjet combustor partially depends on the distance between the injection and the leading edge of the cavity. Further, the shock waves in the scramjet combustor with the cavity flameholder are much stronger than those that occur in the scramjet combustor with the backward-facing step, and this causes a large increase in the static pressure along the walls of the combustor.     

Optical properties of Yb-doped fibers and fiber lasers at high temperature

Recent advances in power scaling of Yb^+ 3-doped fiber lasers to the kilowatt level suggest a need to examine the performance of Yb^+ 3-doped silica at temperatures well above ambient. We report experimental results for the absorption coefficient, emission cross-section, fluorescence lifetime, and slope efficiency of a Yb³⁺-doped large mode area (LMA) silica fiber for temperatures spanning 23 °C-977 °C. To the best of our knowledge these are the highest temperatures to date for which these optical properties have been measured. We find a sharp reduction in the energy storing capability and lasing performance of Yb^+ 3:SiO₂ above 500 °C that coincides with the onset of non-radiative transitions in the excited state manifold (thermal quenching). As the temperature increases from room temperature to 977 °C, absorption in the 1020-1120 nm operating band increases monotonically, concurrent with a reduction in absorption at the 920-nm and 977-nm pumping bands. Conversely, the spectral weight of the emission cross-section shifts from transitions above 1010 nm to those below, with the exception of the 977-nm emission band.     

Absorption spectrum of rubidium and cesium dimers by compact computer operated spectrometer

In this paper we are presenting the visible absorption spectrum of both rubidium and cesium vapor in the 570-870 K temperature range. We used a classical absorption spectroscopy experimental scheme with several new features. The first concerns the use of modern, compact, computer operated spectrometer (Ocean Optics HR4000CG-UV-NIR), which allowed us to record spectra instantaneously resulting in higher signal-to-noise ratio. The second improvement is connected with the use of the all-sapphire cells (ASC) enabling work with a high density of alkali atoms within precisely defined vapor column. In the superheated regime (above 700 K) thermal destruction of dimer molecules clearly distinguishes triplet from singlet transitions.     

The 1.58 μm transparency window of methane (6165-6750 cm): Empirical line list and temperature dependence between 80 and 296 K

The empirical line parameters of over 12,000 methane transitions have been obtained at 80 K in the 1.58 μm transparency window (6165-6750 cm⁻¹) which is of importance for planetary applications. This line list (WKC-80K) was constructed from high sensitivity spectra of normal abundance methane recorded by CW-Cavity Ring Down Spectroscopy at low temperature. The minimum intensity reported is on the order of 5×10⁻³⁰ cm/molecule. High resolution Fourier transform spectra have also been recorded using enriched CH₃D samples at 90-120 K in order to facilitate identification of monodeuterated methane features in the methane line list at 80 K. The CH₃D relative contribution in the considered region is observed to be much larger at 80 K than at room temperature. In particular, CH₃D is found dominant in a narrow spectral window near 6300 cm⁻¹ corresponding to the highest transparency region.Using a similar line list constructed at room temperature (Campargue A, Wang L, Liu AW, Hu SM and Kassi S. Empirical line parameters of methane in the 1.63-1.48 μm transparency window by high sensitivity Cavity Ring Down Spectroscopy. Chem Phys 2010;373:203-10.), the low energy values of the transitions observed both at 80 K and at room temperature were derived from the variation of their line intensities. Empirical lower states and J-values have been obtained for 5671 CH₄ and 1572 CH₃D transitions representing the most part of the absorbance in the region. The good quality of these derived energy values is demonstrated by the marked propensity of the corresponding CH₄ lower state J values to be close to integers. The WKC line lists at 80 K and room temperature provided as Supplementary Material allow one accounting for the temperature dependence of methane absorption between these two temperatures. The importance of the 80 K line list for the study of Titan and other methane containing planetary atmospheres is underlined and further improvements are proposed. The resulting information will advance the theoretical modeling of the methane spectrum in the 1.58 μm transparency window.     

Air-broadening of H2O as a function of temperature: 696-2163 cm

The temperature dependence of air-broadened half-widths are reported for some 500 transitions in the (0 0 0)-(0 0 0) and (0 1 0)-(0 0 0) bands of H₂¹⁶O using gas sample temperatures ranging from 241 to 388 K. These observations were obtained from infrared laboratory spectra recorded at 0.006-0.011 cm⁻¹ resolution with the McMath-Pierce Fourier transform spectrometer located at Kitt Peak. The experimental values of the temperature dependence exponents, η, were grouped into eight subsets and fitted to empirical functions in a semi-global procedure. Overall, the values of η were found to decrease with increasing rotational quantum number J. The number of measurements (over 2200) and transitions (586) involved exceeds by a large margin that of any other comparable reported study.     

Suppression of low-temperature magnetic states in Mn3O4 nanoparticles

We have investigated the low-temperature magnetic properties of Mn₃O₄ nanoparticles using thermodynamic and magnetic measurements. While bulk Mn₃O₄ exhibits three magnetic transitions close to 42, 40 and 34 K, the two lower temperature transitions appear to be absent above 15 K in Mn₃O₄ nanoparticles. The magnetization and spin entropy associated with the ferrimagnetic transition at 42 K is smaller in the Mn₃O₄ nanoparticles than bulk Mn₃O₄, which is consistent with roughly 30-50% of the spins not contributing to the magnetic order. We tentatively attribute this suppression of the lower temperature transitions to a combination of finite size effects and effects arising from amorphous surface spins on the nanoparticles.     

Empirical low energy values for methane transitions in the 5852-6181 cm region by absorption spectroscopy at 81 K

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.62 and 1.71 μm (5852-6181 cm⁻¹) using a newly developed cryogenic cell and a series of distributed feedback (DFB) laser diodes. The minimum value of the measured line intensities is on the order of 3 × 10⁻²⁶ cm/molecule The investigated spectral range corresponds to the high energy part of the tetradecad dominated by the 2ν₃ band for which a theoretical treatment is not yet available. The positions and strengths at 81 K of 2187 transitions were obtained from the spectrum analysis. From the values of the line strength at liquid nitrogen and room temperatures, the low energy values of 845 transitions could be determined. The obtained results are discussed in relation with the previous work of Margolis and compared to the line list provided by the HITRAN database.     

2.1 Tesla permanent-magnet Faraday isolator for subkilowatt average power lasers

A Faraday isolator with one magneto-optical element providing 31 dB isolation ratio for 330 W average power lasers was produced and investigated in an experiment. These remarkable parameters were achieved by increasing the magnetic field and using a [0 0 1] oriented TGG crystal.