Showing papers by "Vasudevan Raghavan published in 2016"

Numerical study of hydrodynamics in gas-solid reactors operating within bubbling fluidisation regime

Abstract: Hydrodynamics of dense gas-solid flows is investigated computationally using Euler-Euler methodology. The method used is primarily based on the kinetic theory of granular flow (KTGF) and additionally by incorporating the features of frictional pressure models (FPM). Frictional stresses are accounted when solid volume fraction reaches the frictional packing limit (FPL). Investigations on the effects of bed pressure drop and other gas-solid flow characteristics have revealed that a value for frictional packing limit around 0.61 yields better results. It is also found that the FPM affects the bed hydrodynamics up to a superficial gas velocity of around 1.5 times the minimum fluidisation velocity. The numerical results of bed pressure drop and bed expansion ratio are validated against the corresponding experimental data available in literature. Detailed velocity and voidage profiles are reported along with the contours of solid volume fraction, and velocity vectors of gas and solid phases.

Analytical, experimental and computational investigation of the influence of stoichiometric mixture fraction on structure and extinction of laminar, nonpremixed methane flames and ethane flames

Abstract: Fundamental studies on combustion in laminar, nonpremixed flames are often carried out using conserved scalar quantities. These conserved scalar quantities, represented here by mixture fraction, E, are used as independent variables in activation-energy asymptotic analysis and in rate-ratio asymptotic analysis. These analyses are carried out in the asymptotic limit of large Damkohler number, with chemical reactions presumed to take place in a thin reaction zone, that is located at E = Est. The quantity Est is the stoichiometric mixture fraction. A characteristic diffusion time is given by the reciprocal of the scalar dibipation rate, X. Previous computational studies have shown that the scalar dibipation rate at extinction depends on Est and the maximum flame temperature, Tst. Here a rate-ratio asymptotic analysis is carried out using reduced chemistry to elucidate the influence of Est on critical conditions of extinction. The scalar dibipation rate at extinction was predicted as a function of Est with the mab fractions of reactants so chosen that the adiabatic flame temperature, Tst, is fixed. The predictions of the analysis show that with increasing values of Est, the scalar dibipation rate at extinction first increases and then decreases. To test the predictions of the asymptotic analysis critical conditions of extinction are measured on nonpremixed methane flames stabilized in the counterflow configuration. With increasing values of stoichiometric mixture fraction, the strain rate at extinction was found to increase and the scalar dibipation rate at extinction was found to first increase and then decrease. The predictions of the asymptotic analysis agreed with experiments. A key outcome of the analysis is that with increasing Est the thickneb of the regions where oxygen and fuel are consumed first increase and the decrease. This is responsible for the observed non-monotonic changes in the values of the scalar dibipation rate at extinction with changes in Est. (Less)

Numerical investigation of flame–vortex interactions in laminar cross-flow non-premixed flames in the presence of bluff bodies

Abstract: Flame stabilisation in a combustor having vortices generated by flame holding devices constitutes an interesting fundamental problem. The presence of vortices in many practical combustors ranging from industrial burners to high speed propulsion systems induces vortex–flame interactions and complex stabilisation conditions. The scenario becomes more complex if the flame sustains after separating itself from the flame holder. In a recent study [P.K. Shijin, S.S. Sundaram, V. Raghavan, and V. Babu, Numerical investigation of laminar cross-flow non-premixed flames in the presence of a bluff-body, Combust. Theory Model. 18, 2014, pp. 692–710], the authors reported details of the regimes of flame stabilisation of non-premixed laminar flames established in a cross-flow combustor in the presence of a square cylinder. In that, the separated flame has been shown to be three dimensional and highly unsteady. Such separated flames are investigated further in the present study. Flame–vortex interactions in sepa...

Effect of Downstream Flapping Plate on the Flow Field Characteristics behind a Circular Cylinder

Abstract: Two-dimensional numerical investigations of flow characteristics behind a circular cylinder in the presence of a flapping plate mounted in the wake have been carried out in the present study. The flapping frequency of the plate has been varied in the range from 0.6 to 1.4 times the vortex shedding frequency behind single stationary circular cylinder in the presence of a stationary plate. The gap between the cylinder and the plate is considered to be 1D, 2D and 3D. The maximum amplitude of the tip of the plate has been fixed to be 0.3D (D being the diameter of the cylinder) for each of the frequency ratio for fixed value of Reynolds number (Re) equal to 100. The length of the plate has been chosen equal to cylinder diameter. The dynamic effects and the behaviour of vortex shedding behind the cylinders have been studied elaborately. Effect of fixed amplitude and varying frequency ratio of the cylinder on wake flow behaviour has been presented in the form of vorticity contours, lift, drag and Strouhal number (St) plots. The dynamic forces as well as the flow structure behind the plate are found to be modified in the presence of the flapping plate.