Showing papers by "Vasudevan Raghavan published in 2015"

Comprehensive numerical modeling of ignition of coal dust layers in different configurations

Abstract: Ignition of coal dusts deposited over sufficiently hot surfaces represents a common industrial hazard. If these surfaces are above a minimum threshold temperature, then the heat transfer from them and the associated chemical heat release can cause spontaneous ignition. This ignition front can be an ignition site for nearby combustibles as well, which depends on the location of ignition and the corresponding surface temperature attained by the coal dust. A comprehensive numerical study of ignition process of coal dust layers in different configurations, by solving the coupled Navier–Stokes and energy equations, provides a clear insight of the thermal field in the coal layer as well as the flow field over the coal surface. Such study will also provide quantitative information about the convective heat transfer coefficient, which varies for different cases. In this study, ignition phenomena of coal dusts deposited over a flat plate, wedges of different angles and a 3D corner, have been analyzed using a comprehensive numerical model. The governing equations are solved using Ansys FLUENT and user defined functions. The model has been validated using experimental results and is used to study the effect of wedge angles and the direction of gravity vector on the ignition process.

Effects of burner configurations on the natural oscillation characteristics of laminar jet diffusion flames

Abstract: In this work, effects of burner configurations on the natural oscillations of methane laminar diffusion flames under atmospheric pressure and normal gravity conditions have been studied experimentally. Three regimes of laminar diffusion flames, namely, steady, intermittent flickering and continuous flickering have been investigated. Burner configurations such as straight pipe, contoured nozzle and that having an orifice plate at the exit have been considered. All burners have the same area of cross section at the exit and same burner lip thickness. Flame height data has been extracted from direct flame video using MATLAB. Shadowgraph videos have been captured to analyze the plume width characteristics. Results show that, the oscillation characteristics of the orifice burner is significantly different from the other two burners; orifice burner produces a shorter flame and wider thermal plume width in the steady flame regime and the onset of the oscillation/flickering regimes for the orifice burner occurs a...

The effects of grids on the near-field evolution of turbulence in a low-Reynolds number jet

Abstract: The near field evolution of a low Reynolds number free jet and associated turbulent fluctuations are investigated experimentally, in the presence of a grid placed at nozzle exit. The jet spreading characteristics without grid are validated first with available data in the literature and the well known features of vortex roll-up and pairing phenomena in the shear layer region are confirmed. The inclusion of a grid at the nozzle exit appears to reduce velocity fluctuations in the near field, but it accelerates the transition to a full-grown turbulent flow field with respect to axial distance, by the introduction of finer scales. Grids also cause mean velocity overshoot in the radial profile by distorting the approach flow in the boundary layer region ahead of the grid. Distinct vortex roll-up and pairing phenomena in the shear layer region could not be observed with grids. However, weak shear mode instabilities originating in the shear layer for 0 ≤ z/D ≤ 3 appear to interact with the potential core, leading to the formation of a “unified shear/preferred mode” instability. These features are dominantly seen for grid-mounted jet flows having large displacement thickness to grid opening ratio. Power spectra indicate a progressive evolution of turbulent flow features with respect to axial distance, starting with the development of inertial sub-range into a −5/3 slope exponent; followed by the establishment of full range of turbulent scales, and the eventual attainment of isotropy. The establishment of inertial sub-range is attributable to vortex-pairing and turbulent fragmentation in free jets and to the introduction of fine scales at the jet inlet in the cases with grids.

Experimental Study of Initial Fuel Temperature on the Burning Rates of Kerosene Pools in Cold Environment

Abstract: Oil spill cleanup in Arctic is usually done by in-situ burning. The fuel oil will have a very low initial temperature as well as cold boundaries. This study focuses on the variation of the mass burning rate of the fuel with varying initial temperatures of the fuel, which is kept in a metal bowl surrounded by ice. Kerosene is chosen as the fuel. The bowl diameters have been varied from 30 mm to around 50 mm. The fuel bowl is kept surrounded in an ice bath and is ignited after it attains a particular temperature. The initial temperature of the fuel is varied from 4 o C to 16 o C. Experiments are carried out for 5 minutes after which the flame is quenched. The results show that with increase in the initial temperature of the fuel the mass burning rate also increases. The mass burning rate also increases as the fuel bowl diameter increases. The surface temperature of the fuel, just after quenching, increases with an increase in the initial temperature of the fuel. The ignition time of the fuel is reduced with increase in initial temperature of the fuel and with increase in bowl diameter. The flame height variations are also recorded.

Evaporation Characteristics of Karanja Bio-diesel and Its Diesel Blends

Abstract: The evaporation characteristics of karanja biodiesel and its blends with diesel measured in a suspended droplet facility are reported in this article. The results have been compared with the evaporation rates of pure single component hydrocarbon fuels, such as n-decane, n-heptane, and neat fossil diesel. Unlike pure hydrocarbons, the evaporation rate for karanja biodiesel is seen to be linear. The evaporation rate decreases as the biodiesel content in the blend increases and its variation tends to be nonlinear particularly at higher temperatures. The karanja and its blends with diesel have a longer heating phase and droplet lifetime compared to diesel.