Showing papers by "Vasudevan Raghavan published in 2017"

Effect of ullage on burning behavior of small-scale pool fires in a cavity

Abstract: Experiments have been conducted to study the effect of ullage height on steady mass burning rates in methanol pool flames in a cavity. Two burner diameters are used. At low ullages, the flame dynamics are found to be effective in altering the mass burning rates. From baseline case with almost zero ullage, as the ullage is increased, mass burning rate decreases. It produces a local minimum at a given ullage based on the burner internal diameter. After this point, the mass burning rate increases with increasing ullage and reaches an almost uniform value. Numerical simulations are used to complement the results of the experimental study. Low ullage cases have been simulated using a validated numerical model that uses global single step chemistry, partial equilibrium for carbon-dioxide oxidation and optically thin approximation based radiation model. An axisymmetric domain has been employed. Even though the mass burning rates have been over-predicted by the numerical model, the variation trend has been captured quite well. Results from the numerical model reveal that for very low ullage, flame is phenomenally steady and mass burning rate is higher as the diffusion flame anchors around the rim. As the ullage is increased, a transient flame is seen to anchor around the rim and due to increased flame stand-off, the mass burning rate decreases. When the ullage is further increased, due to axial flapping of the flame that partially covers the burner, oxygen is transported into the burner, causing a recirculation pattern within the burner and partial premixing of fuel vapor and oxygen. As a result, the mass burning rate increases.

Spatio-temporal structure of vertically spreading flame over non-planar PMMA surfaces

Abstract: Burning of PMMA slabs with non-planar surfaces is presented in this paper. Non-planar surfaces have been produced using steps fabricated in transparent thermally thick PMMA slabs. The step size is kept constant and three step locations have been used in this study. Either the plate portion or the step portion of the slab is ignited, but keeping the ignition location at the bottom. Concurrent flame spread over stepped slabs is analysed in detail. Time-dependent %mass loss, surface regression, and OH*, CH* chemiluminescence, and planar laser induced incandescence of soot and planar laser induced fluorescence of polyaromatic hydrocarbons (PAH) are determined. The results indicate that the main influence of the step at any location has been to increase the mass loss rate and therefore, the burning rate. The flow fields are quite different between the step ignition and the plate ignition cases. As a result, secondary flame zones are observed in the wake zone downstream of the step for step ignition cases and the flames influenced by stagnation point flow field are observed for plate ignition cases. The influences of step size for step ignition and location of step for plate ignition cases are also studied. The OH*/CH* maps show their overlap region as that of high heat release rates. The PAH precursor to soot is spatially mapped relative to the latter. Although the plate-injection cases are sooty, LII indicates otherwise because of poor mixing in a forward-facing step leading to substantial sooting above the sample outside the field of view. On the other hand, the effect of step recirculation zone in promoting mixing and suppressing soot is evident from the results.

Method and apparatus for waste combustion

Abstract: A toxic waste incinerator is capable of enhanced combustion of hazardous waste (oil contaminated sand, human waste, garbage, etc.) utilizing immersed non-combustible and thermally conductive objects for increasing heat feedback from the flames to the unburned fuel, while air inlets are used to optimize the air entrainment rate to enhance the burning efficiency. The burning rate of a fluidic mass such as a sand-oil mixture is enhanced using immersed conductive objects (copper rods) which enable rapid heat-up of the flame exposed to the upper surface of the rod and transmits heat back into the sand. Consequent conduction of heat to the porous media through the lower portion of the immersed rod significantly increases vaporization and therefore the burning rate. Incineration may be performed on a transient, exigent basis as with hazardous waste and oil spills, or as part of a permanent fixture for receiving an ongoing waste stream.

Mathematical Modelling of Coal Gasification Processes

Abstract: Coal is by far the most commonly employed fuel for electrical power generation around the world. While combustion could be the route for coal utilization for high grade coals, gasification becomes the preferred process for low grade coals having higher composition of volatiles or ash. Indian coals suffer from high ash content-nearly 50% by weight in some cases. Instead of transporting such high ash coals, it is more energy efficient to gasify the coal and transport the product syngas. Integrated Gasification Combined Cycle (IGCC) plants and Underground Gasification of coal have become attractive technologies for the best utilization of high ash coals. Gasification could be achieved in fixed beds, fluidized beds and entrained beds; faster rates of gasification are possible in fluidized beds and entrained flow systems, because of the small particle sizes and higher gas velocities. The media employed for gasification could involve air/oxygen and steam. Use of oxygen will yield relatively higher calorific value syngas because of the absence of nitrogen. Sequestration of the carbon dioxide after the combustion of the syngas is also easier, if oxygen is used for gasification. Addition of steam can increase hydrogen yield in the syngas and thereby increase the calorific value also. Gasification in the presence of suitable catalysts can increase the composition of methane in the product gas. Several competing heterogenous and homogenous reactions occur during coal major heterogenous reaction pathways, while interactions between carbon monoxide, oxygen, hydrogen, water vapour, methane and carbon dioxide result in several simultaneous gas-phase (homogenous) reactions. The overall product composition of the coal gasification process depends on the input reactant composition, particle size and type of gasifier, and pressure and temperature of the gasifier. The use of catalysts can also selectively change the product composition. At IIT Madras, over the last one decade, both experimental and modelling work has been undertaken to investigate the gasification characteristics of high ash Indian coals and compare the yield with those of high grade Australian and Japanese coals. A 20 kW capacity entrained flow gasifier has been constructed and the gasification characteristics have been studied for Indian coals for different particle sizes, system pressures and air flow rates. The theoretical model incorporates the effects of Knudsen diffusion, devolatilization and various heterogenous and homogenous kinetic steps as well as two-phase flow interactions involving the gaseous and particle phases. Output parameters such as carbon conversion, cold gas efficiency and syngas composition have been compared for different grades of coals under a wide range of operating conditions. The model developed for the entrained flow gasifier predicts the gasification characteristics of both Indian and foreign coals well. Apart from the entrained flow gasifier, a bubbling bed gasifier of 100 kW capacity has also been studied. A pilot plant for the gasification of Indian coals has been set up for this capacity and its performance has been investigated experimentally as well as theoretically at different air and steam flow rates. Carbon conversion efficiency of more than 80% has been achieved.