Bio: Vasudevan Raghavan is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topic(s): Combustion & Laminar flow. The author has an hindex of 17, co-authored 130 publication(s) receiving 1051 citation(s). Previous affiliations of Vasudevan Raghavan include University of Nebraska–Lincoln & Indian Institutes of Technology.
Papers published on a yearly basis
Abstract: A fundamental experimental study to determine the burning rates of ethanol and ethanol-blended fossil fuels is presented. Pure liquid ethanol or its blends with liquid fossil fuels such as gasoline or diesel, has been transpired to the surface a porous sphere using an infusion pump. Burning of the fuel takes place on the surface of the porous sphere, which is placed in an air stream blowing upwards with a uniform velocity at atmospheric pressure and temperature under normal gravity conditions. At low air velocities, when ignited, a flame envelopes the sphere. For each sphere size, air stream velocity and fuel type, the fuel feed rate will vary and the same is recorded as the burning rate for that configuration. The flame stand-off distances from the sphere surface are measured by post-processing the digital image of the flame photograph using suitable imaging software. The transition velocity at which the flame moves and establishes itself at the wake region of the sphere has been determined for different diameters and fuel types. Correlations of these parameters are also presented.
Abstract: In this work, experimental and numerical investigations of spheres burning in a convective environment have been carried out. In the numerical simulations, transient axi-symmetric Navier–Stokes equations along with species and energy conservation equations are solved using a finite volume technique based on non-orthogonal semi-collocated grids. A global single step reaction involving two reactants, two products and one inert species together with an Arrhenius rate equation has been used to model kinetics. For the sake of comparison, an infinite rate chemistry model has also been attempted. The density of the mixture has been evaluated from ideal gas equation of state. Thermo-physical properties like thermal conductivity and viscosity have been evaluated using the Chapman–Enskog description of binary gas mixtures. Specific heats and species enthalpies have been evaluated using piece-wise polynomials of temperature. The burning of isolated spherical particles in a mixed convective environment at atmospheric pressure has been simulated for various particle sizes, free-stream velocities and ambient temperatures. The numerical predictions have been compared with experimental results obtained using the porous sphere technique and the agreement is found to be good. Correlations have been developed for the critical Reynolds number at which transition from the envelope to wake flame occurs and also for the mass burning rates at sub-critical or super-critical Reynolds number regimes. It is observed that, at higher ambient temperatures, transition to wake flame is delayed to a higher critical Reynolds number value. The infinite rate chemistry model predicts flame shapes and mass burning rates with reasonable accuracy in the sub-critical Reynolds number regime, but it fails to predict transition to wake flame shape. For analyzing transition phenomena, a finite rate chemistry model is required.
Abstract: This study investigates the interaction of micron-sized coal particles entrained into lean methane–air premixed flames. In a typical axisymmetric burner, coal particles are made to naturally entrain into a stream of the premixed reactants using an orifice plate and a conical feeder setup. Pittsburgh seam coal dust, with particle sizes in the ranges of 0–25 μm, 53–63 μm, and 75–90 μm, is used. The effects of different coal dust concentrations (10–300 g/m 3 ) entrained into the mixture of methane–air at three lean equivalence ratios, ϕ , of 0.75, 0.80 and 0.85, on the laminar burning velocity are studied experimentally. The laminar burning velocity of the coal dust–methane–air mixture is determined by taking high quality shadowgraph images of the resulting flames and processing them using the cone-angle method. The results show that the laminar burning velocity reduces with the addition of coal dust having particle sizes in the ranges of 53–63 μm and 75–90 μm, irrespective of the equivalence ratio values. However, burning velocity promotion is observed for one case with particle size in the range of 0–25 μm at an equivalence ratio of 0.75. Two competing effects are considered to explain these trends. The first effect is due to volatile release, which increases the overall equivalence ratio and thus, the flame temperature and burning velocity. The second is the heat sink effect that the coal particles take up to release the volatiles. This process reduces the flame temperature and accordingly the burning velocity also. A mathematical model is developed considering these effects and it is seen to successfully predict the change of laminar burning velocity for various cases with different dust concentrations and equivalence ratios of the gas mixture. Furthermore, the implication of this study to coal mine safety is discussed.
Abstract: During a severe loss of coolant accident in a nuclear reactor, steam and hydrogen are produced by the oxidation of reactor core and get distributed in the containment. A water spray system is employed to cool the mixture as well as to enhance the mixing of the gases to avoid hydrogen accumulation. This paper presents two-phase numerical simulations of transient vaporization of a moving spherical water droplet. The numerical model considers the variation of thermo-physical properties in both liquid- and vapor-phases, as functions of temperature and species concentrations. Multi-component diffusion and surface tension effects are also considered. The model has been validated using experimental results available in literature for hydrocarbon fuel droplet evaporation. Validated model is used to study the evaporation characteristics of moving water droplets under conditions typically observed in nuclear reactor during a loss of coolant accident. The effects of ambient temperature and hydrogen concentration on the vaporization characteristics are studied thoroughly.
Abstract: Pine oil biofuel, obtained by the distillation of oleoresins of pine tree, has been chosen as a new renewable fuel for its operation in diesel engine. Notably, the viscosity and cetane number of pine oil was observed to be lower than diesel. The motivation for this work stems from the basic notion that less viscous and lower cetane fuels are considered to be fumigated for their successful operation in diesel engine. As such, pine oil biofuel was vaporized and inducted into the engine cylinder through inlet manifold while diesel was sent through main injection system, providing ignition assistance for the pine oil/air mixture. Prior to conducting engine experiments, the evaporation characteristics of pine oil droplet were studied through suspended droplet experiment so as to get better insights on pine oil droplet evaporation at various temperatures. From this study, it was observed that at higher air temperature (150 °C), evaporation of pine oil was more effective than at lower temperatures (100 °C and 50 °C) and therefore, 150 °C was chosen as preheat temperature for engine fumigation study. Thus, as a novel attempt, the fundamental study on pine oil droplet evaporation is subtly coupled with engine studies, and the effect of vaporization of pine oil on engine characteristics was mapped. As an outcome of engine study, the maximum percentage of diesel replaced was noticed to be 36% at 100% load and 60% at 20% load. Significantly, the engine performance such as BSFC and BTE was observed to be improved with the increase in proportion of pine oil injection. Further, combustion of fumigated pine oil has been reported to be better, with 36% injection of pine oil showing 10.3% higher in-cylinder pressure than that for 6% injection of pine oil at 100% load.
30 Dec 2011
TL;DR: This table lists the most common surnames in the United States used to be Anglicised as "United States", then changed to "United Kingdom" in the 1990s.
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Abstract: The demand for petroleum has risen rapidly due to increasing industrialization and modernization of the world. This economic development has led to a huge demand for energy, where the major part of that energy is derived from fossil sources such as petroleum, coal and natural gas. However, the limited reserve of fossil fuel has drawn the attention of many researchers to look for alternative fuels which can be produced from renewable feedstock. Biodiesel has become more attractive because of its environmental benefits and it is obtained from renewable resources. There are four primary methods to make biodiesel: blending, microemulsion, pyrolysis and transesterification. The most commonly used method is the transesterification of triglycerides (vegetable oil and animal fats) with alcohol in the presence of a catalyst. There is a growing interest in using Jatropha curcas L. oil as the feedstock for biodiesel production because it is non-edible and thus does not compromise the edible oils, which are mainly used for food consumption. Non-edible oils are not suitable for human consumption because of the presence of toxic components. Further, J. curcas L. seed has a high content of oil and the biodiesel produced has similar properties to that of petroleum-based diesel. In this paper, an attempt has been made to review the different approaches and techniques used to generate biodiesel from Jatropha curcas oil. The main factors affecting the biodiesel yield, for example the molar ratio of alcohol to oil, catalyst concentration, reaction temperature and reaction time are discussed. Lastly, the environmental considerations and economic aspects of biodiesel are also addressed.
01 Apr 1997
Abstract: A detailed chemical kinetic model has been used to study dimethyl ether (DME) oxidation over a wide range of conditions. Experimental results obtained in a jet-stirred reactor (JSR) at I and 10 atm, 0.2 < 0 < 2.5, and 800 < T < 1300 K were modeled, in addition to those generated in a shock tube at 13 and 40 bar, 0 = 1.0 and 650 :5 T :5 1300 K. The JSR results are particularly valuable as they include concentration profiles of reactants, intermediates and products pertinent to the oxidation of DME. These data test the Idnetic model severely, as it must be able to predict the correct distribution and concentrations of intermediate and final products formed in the oxidation process. Additionally, the shock tube results are very useful, as they were taken at low temperatures and at high pressures, and thus undergo negative temperature dependence (NTC) behavior. This behavior is characteristic of the oxidation of saturated hydrocarbon fuels, (e.g. the primary reference fuels, n-heptane and iso- octane) under similar conditions. The numerical model consists of 78 chemical species and 336 chemical reactions. The thermodynamic properties of unknown species pertaining to DME oxidation were calculated using THERM.
Abstract: The burning characteristics of fuel droplets containing nano and micron-sized aluminum particles were investigated. Particle size, surfactant concentration, and the type of base fluid were varied. In general, nanosuspensions can last much longer than micron suspensions, and ethanol-based fuels were found to achieve much better suspension than n-decane-based fuels. Five distinctive stages (preheating and ignition, classical combustion, microexplosion, surfactant flame, and aluminum droplet flame) were identified for an n-decane/nano-Al droplet, while only the first three stages occurred for an n-decane/micron-Al droplet. For the same solid loading rate and surfactant concentration, the disruption and microexplosion behavior of the micron suspension occurred later with much stronger intensity. The intense droplet fragmentation was accompanied by shell rupture, which caused a massive explosion of particles, and most of them were burned during this event. On the contrary, for the nanosuspension, combustion of the large agglomerate at the later stage requires a longer time and is less complete because of formation of an oxide shell on the surface. This difference is mainly due to the different structure and characteristics of particle agglomerates formed during the early stage, which is a spherical, porous, and more-uniformly distributed aggregate for the nanosuspension, but it is a densely packedmore » and impermeable shell for the micron suspension. A theoretical analysis was then conducted to understand the effect of particle size on particle collision mechanism and aggregation rate. The results show that for nanosuspensions, particle collision and aggregation are dominated by the random Brownian motion. For micron suspensions, however, they are dominated by fluid motion such as droplet surface regression, droplet expansion resulting from bubble formation, and internal circulation. And the Brownian motion is the least important. This theoretical analysis explains the different characteristics of the particle agglomerates, which are responsible for the different microexplosion behaviors that were observed in the experiments. (author)« less
Abstract: The present work reviews the literature concerning the effects of alcohol/diesel blends on the exhaust emissions of diesel engines operating under transient conditions, i.e., acceleration, load increase, starting and transient/driving cycles. Two very promising alcohols are covered in this survey, namely ethanol and n-butanol. The analysis focuses on all regulated exhaust pollutants, i.e., particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC), with results for unregulated emissions, carbon dioxide and combustion noise radiation also included. The main mechanisms of exhaust emissions during transients are identified and discussed, with respect to the fundamental aspects of transient operation and the differing properties of alcohols relative to the reference diesel oil. Based on the published studies up today, summarization of emissions data and cumulative trends are presented, for the purpose of quantifying the alcohol blends benefits or penalties on the regulated emissions during various driving cycles. Particularly for the emitted PM and smoke, a statistically significant correlation with the oxygen content exists (R2=0.85 and 0.95, respectively). A similar correlation holds true for the heavy-duty, engine-dynamometer data of engine-out CO. Finally, a detailed list is provided that summarizes the main data from all studies published so far.