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Abstract This paper reports the effect of inlet flow turbulence intensity on the combustion instability characteristics in a backward facing step combustor. The inlet turbulence intensity is varied by a turbulence generator. Unsteady pressure measurements and OH* chemiluminescence images are recorded over a wide range of operating conditions at different inlet turbulence intensities. The study shows an early onset of instability at low turbulence level, i.e., higher turbulence postpones the onset of instability to higher Reynolds number Re and/or higher equivalence ratio Φ. The early onset of instability in the Re and Φ parameter spaces is due to the change in system parameters such as flame speed and size of the recirculation zone downstream of the step at different turbulence levels. Further, the onset is characterized as subcritical bifurcation. At low Re , the hysteresis zone width is small for low turbulence levels and it is large at higher turbulence levels; and at higher Re , the hysteresis width remains constant at all turbulence levels. Investigation of instability characteristics reveals that there are momentary slippages from limit cycle orbit into brief silent regimes in an intermittent manner. The frequency of occurrence of the momentary silent regimes increases with reduction in turbulence, indicating that higher turbulence helps in maintaining the system in a stable limit cycle orbit. High-speed chemiluminescence imaging reveals the necessity of the vortex rollup in the recirculation zone to grow up to the top wall by dilatation from the heat release for the onset of instability. Considerations of the effect of turbulence on both the flame speed and the recirculation zone size together explain all the observed bifurcation trends. These results suggest that inlet flow turbulence should not just be considered as background noise. The turbulence effects on both the flame and flow should be considered in predicting the instability characteristics. [ABSTRACT FROM AUTHOR]

Examines the acoustic radiation from a rectangular wall mounted cavity in a confined supersonic flow. Maintenance of the free-stream Mach number; Numerical simulations of the unsteady two-dimensional flow; Determination of the dominant frequencies of the acoustic radiation.

Abstract: The heat release response of a two-dimensional (2-D) co-flow diffusion flame is theoretically investigated for the mass fraction fluctuations of reactant concentration at the inlet boundary and for time-varying spatially uniform flow velocity field in the domain. The present work is an extension of the Burke–Schumann steady flame model to an oscillatory situation, but in a 2-D framework. The governing equation of the problem is the scalar advection equation for the Schwab–Zel''dovich variable. An exact solution is found in terms of an infinite series for the case of mixture fraction fluctuations at the inlet boundary. The cases of time-varying uniform flow velocity field and a combination of velocity and mixture fraction fluctuations are investigated numerically. The temperature and heat release rate of the flame are thermodynamically calculated utilizing the mixed-is-burnt approach. The main results of the paper are the response functions. The nonlinearity in the calculation of the heat release rate and the convective nonlinearity due to velocity fluctuations result in the generation of higher harmonics in the response function for a given sinusoidal excitation. Therefore, the response function is decomposed and obtained for each of the significant harmonics. The results show that, in general, the response function decreases with increase in the excitation frequency as reported with premixed flames in the literature. However, in the present case, the decrease occurs when the excitation time-scale is less than the diffusion time-scale. Interesting flame shape variations such as flame clip-off, flipping between overexpanded and underexpanded conditions, and flame wrinkling are observed in the case of mixture fraction oscillations. [Copyright &y& Elsevier]

In the present work, the nano-Al2O3 powder was produced by the wire explosion technique. The shape and size of the particles were measured using Transmission Electron Microscope (TEM) studies. The Scanning Electron Microscope (SEM) studies were carried out to confirm that the particles are of sub-micron size. The compositions of the material were characterized through the Energy Dispersive Angle X-ray Analysis (EDAX) results. The Wide Angle X-ray Diffraction (WAXD) and the Fourier Transform Infra-Red (FTIR) results confirmed the nanosize powder formed by the process as crystalline γ-Al2O3 powder. It is identified that the local temperature of the medium at the time of formation of nanopowder decides the phase characteristics of the powder material. The mechanism of formation of nanopowder by wire explosion technique was explained in detail. [Copyright &y& Elsevier]