An investigation of combustion instability in aircraft-engine reheat systems

The principal objective of this study was to examine experimentally the effects of upstream temperature, velocity, gutter blockage, tailpipe length, and main and pilot fuel flows, on the form of combustion instability encountered in aircraft reheat systems which is sometimes referred to as 'buzz'. Tests were carried out at atmospheric pressure for upstream temperatures of between 200 and 500°C, and upstream velocities ranging from 140 to 200 ft/sec. Three values of stabilizer blockage were employed, namely 25, 30 and 35%. The tailpipe length was varied between 9 and 45 inches. Auto-correlation techniques were used in the frequency analysis of the buzz waveforms. It was found that a certain minimum tailpipe length is necessary in order to produce buzz which is then strengthened as the tailpipe length is increased. Buzz also becomes more pronounced with an increase in gas velocity but stabilizer blockage appears to have no discernible effect … [cont.]

Solar gravitational energy and luminosity variations

Due to non-homogeneous mass distribution and non-uniform velocity rate inside the Sun, the solar outer shape is distorted in latitude. In this paper, we analyze the consequences of a temporal change in this figure on the luminosity. To do so, we use the Total Solar Irradiance (TSI) as an indicator of luminosity. Considering that most of the authors have explained the largest part of the TSI modulation with magnetic network (spots and faculae) but not the whole, we could set constraints on radius and effective temperature variations (dR, dT). However computations show that the amplitude of solar irradiance modulation is very sensitive to photospheric temperature variations. In order to understand discrepancies between our best fit and recent observations of Livingston et al. (2005), showing no effective surface temperature variation during the solar cycle, we investigated small effective temperature variation in irradiance modeling. We emphasized a phase-shift (correlated or anticorrelated radius and irradiance variations) in the (dR, dT)-parameter plane. We further obtained an upper limit on the amplitude of cyclic solar radius variations, deduced from the gravitational energy variations. Our estimate is consistent with both observations of the helioseismic radius through the analysis of f-mode frequencies and observations of the basal photospheric temperature at Kitt Peak. Finally, we suggest a mechanism to explain faint changes in the solar shape due to variation of magnetic pressure which modifies the granules size. This mechanism is supported by our estimate of the asphericity-luminosity parameter, which implies an effectiveness of convective heat transfer only in very outer layers of the Sun.Comment: 17 pages, 2 figure, 1 table, published in New Astronom

Superconductivity and short range order in metallic glasses Fex_{x}Ni1−x_{1-x}Zr2_{2}

In amorphous superconductors, superconducting and vortex pinning properties are strongly linked to the absence of long range order. Consequently, superconductivity and vortex phases can be studied to probe the underlying microstructure and order of the material. This is done here from resistance and local magnetization measurements in the superconducting state of Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ metallic glasses with $0\leq x \leq 0.6$. Firstly, we present typical superconducting properties such as the critical temperature and fields and their dependence on Fe content in these alloys. Then, the observations of peculiar clockwise hysteresis loops, wide double-step transitions and large magnetization fluctuations in glasses containing a large amount of Fe are analyzed to reveal a change in short range order with Fe content.Comment: 8 pages, 7 figure

Mott transition, antiferromagnetism, and unconventional superconductivity in layered organic superconductors

The phase diagram of the layered organic superconductor $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Cl has been accurately measured from a combination of $^{1}$H NMR and AC susceptibility techniques under helium gas pressure. The domains of stability of antiferromagnetic and superconducting long-range orders in the pressure {\it vs} temperature plane have been determined. Both phases overlap through a first-order boundary that separates two regions of inhomogeneous phase coexistence. The boundary curve is found to merge with another first order line related to the metal-insulator transition in the paramagnetic region. This transition is found to evolve into a crossover regime above a critical point at higher temperature. The whole phase diagram features a point-like region where metallic, insulating, antiferromagnetic and non s-wave superconducting phases all meet.Comment: 4 pages, 6 figures, Revte