Shengteng Hu

Hydroxyl tagging velocimetry in a supersonic flow over a cavity

Hydroxyl tagging velocimetry (HTV) measurements of velocity were made in a Mach 2 (M 2) flow with a wall cavity. In the HTV method, ArF excimer laser (193 nm) beams pass through a humid gas and dissociate H2O into H + OH to form a tagging grid of OH molecules. In this study, a 7 x 7 grid of hydroxyl (OH) molecules is tracked by planar laser-induced fluorescence. The grid motion over a fixed time delay yields about 50 velocity vectors of the two-dimensional flow in the plane of the laser sheets. Velocity precision is limited by the error in finding the crossing location of the OH lines written by the excimer tag laser. With a signal-to-noise ratio of about 10 for the OH lines, the determination of the crossing location is expected to be accurate within +/- 0.1 pixels. Velocity precision within the freestream, where the turbulence is low, is consistent with this error. Instantaneous, single-shot measurements of two-dimensional flow patterns were made in the nonreacting M 2 flow with a wall cavity under low- and high-pressure conditions. The single-shot profiles were analyzed to yield mean and rms velocity profiles in the M 2 nonreacting flow.

STRETCH AND CURVATURE EFFECTS ON FLAMES

To understand how curvature affects the properties of stretched premixed flames including flame temperature, flame speed, and extinction, comparisons among the tubular premixed flame, the opposed jet flame and the one-dimensional planar flame are drawn physically, experimentally and numerically. By comparing the one-dimensional planar flame and the opposed jet flame, we give the physical process that produces the effects of stretch on laminar flame. The most important difference between them is that the former flow field is one-dimensional, i.e. convection is one-dimensional, and the latter has multi-dimensional convection. The interaction of flow divergence and heat and mass diffusion makes the temperature and flame speed of the stretched flame differ from one- dimensional planar flame; and the flow divergence ratio is proportional to Karlovitz number. By extending the analysis to tubular flame, we can see that the geometry of the tubular flame, i.e. curvature, strengthens the convection in axial direction according to one additional term, which is proportional to the square of Karlovitz number, in the expression of the flow divergence ratio. To prove this, numerical calculation for lean H2/air is presented here. Extinction experiments of lean H2/air, CH4/air and C3H8/air flames are also conducted to verify the analysis.

Thermionic electron emission from chemical vapor deposition diamond by nanosecond laser heating

Vacuum thermionic emission behaviors from bare silicon and chemical vapor deposition diamond coated silicon using laser pulse heating are reported. Under vacuum and subjected to a moderate biasing voltage, the bare silicon and diamond coated silicon emitters show pulses of emission current synchronized with the excitations of nanoseconds 532nm Nd:YAG pulsed laser. The peak emission current increased monotonically with the increased in laser pulse intensity. The Arrhenius plot of the emission current versus laser intensity follows Richardson’s thermionic emission equation for the bare silicon emitter. The diamond-coated silicon emitter shows two activation energies: a value same as that of the bare silicon at low laser intensity and transit to a zero value at high laser intensity.

UV Raman Scattering Measurements of a Mach 2 Reacting Flow over a Piloted Cavity

Abstract : UV Raman scattering measurements were made in a Mach 2 supersonic air flow over a cavity piloted with ethylene fuel (C2H4). The wall cavity simulated the pilot region of a scramjet combustor. In the UV Raman system, a 248 nm KrF excimer laser beam (400 mJ/pulse, 20 ns pulse length) was used to excite the Raman scattering in the combustion zone. Raman scattered light in the 254-278 nm spectral region allows measurement of the following molecular species: CO2 (257 nm), 02 (258 nm), N2 (263 nm), C2H4 (268 nm), H2O (273 nm) and H2 (277nm). To avoid damaging the fused-silica windows on the combustion test section: 1) the laser pulse was stretched from 20 ns to 150 ns using two optical delay cavities, 2) a long focal length lens (5 meters) focused the KrF beam to a relatively large diameter (1 mm diameter) and 3) the laser energy was decreased to 100 mJ/pulse. Under these conditions, the high power pulsed laser beam passed through the side fused-silica windows without inflicting damage. Raman scattered light was collected from the top fused-silica and was focused into a 0.32 meter spectrometer.

Curvature Effects in Non-Premixed Tubular Flames

Non-premixed tubular flames are formed by the opposed tubular burner. A detailed structural investigation using the spontaneous Raman scattering technique is conducted on flames with constant curvature using 15% H2 diluted by N2 against air at various stretch rates. The measured temperature and major species concentrations agree well with the numerical prediction. Compared with the numerical results of the opposed-jet flat flame, it is shown that the curvature weakens the effects of preferential diffusion for the flames studied where the curvature is concave towards the fuel stream. Near-extinction nonpremixed tubular flames using different fuels are also studied. Hydrogen flames (Le 1) prefer concave curvature, which is opposite to the hydrogen flames, and cellular structures appear throughout the burner’s operational range. These phenomena show that for flames with Le 1. Curvature has minimal effects on tubular non-premixed flames with Lewis number close to unity. The comparison between experimental and calculated extinction conditions supports the above conclusions.

Simultaneous Temperature , S pecies and Velocity Measurements via Combined Raman Spectroscopy and HTV

*# Non -intrusive molecular flow tagging method s, such as Hydroxyl Tagging Velocimetry (HTV), ha ve shown the capability of providing quantitative velocity informat ion in both room temperature and flames. Benefiting from the instantaneous nature of HTV and the feasibility of single -pulse measurements of Raman scattering with an UV excimer laser, simultaneous velocity, temperature and major species measurem ents can be realized. In the present work , these non -intrusive diagnostics methods are used to obtain quantitative measurements in a lifted, turbulent hydrogen jet diffusion flame . A narrowband KrF excimer lase r tuned to 248. 634 nm is used to measure major species concentration s (H 2, O 2, N2 and H 2O) and temperature is determined by invoking the ideal gas law. After the firing of the KrF e xcimer laser , a 7 x 7 OH grid is generated via single -photon photodissociation of H2O by a ~193 nm ArF excimer laser. A “read” laser sheet from a pulsed frequency -doubled dye laser operating at 28 1.997 is then use d to reveal the grid through fluorescence caused by A 2 � + (v � = 1) � X 2 �i (v � = 0) pumping. Two consecutive images, of a known time delay , are acquired to calculat e the velocity field * through a time -of -flight method.

Cellular Instabilities in Non-Premixed Opposed-Flow Tubular Flames

*† ‡ An experimental study of cellular instability of non-premixed tubular flames was conducted on a newly designed opposed-flow tubular burner. In this study, H2 diluted by CO2, N2, Ar or He was used as fuel and burned against air in a cylindrical opposed-flow configuration. The extinction conditions of the non-premixed tubular flames are determined as function of the flame curvature and initial mixture strength. The cellular striped structure of the flame is imaged. The number of cells and their sizes are summarized. The cellular nature of the opposed flow non-premixed tubular flame is analyzed as a function of the stretch rate, flame curvature, and initial mixture strength. In this study, the multiplicity of the striped structure at certain conditions is also demonstrated.