Vasudevan Raghavan

Laminar flame propagation on a horizontal fuel surface: Verification of classical Emmons solution

This work analyses the classical Emmons (1956) solution of flat plate laminar flame combustion on a film of liquid fuel. A two-dimensional (2D) numerical model developed for this purpose has been benchmarked with experimental results available in the literature for methanol. In the parametric study, numerical predictions have been compared with Emmons classical solution. The study shows that the Emmons solution is valid in a range of Reynolds numbers where flame anchors near the leading edge of the methanol pool and the combustion zone is confined around the hydrodynamic and thermal boundary layers. However, in cases of low free stream velocities the combustion zone is beyond the boundary layer zone and the Emmons solution deviates. In cases of very high free stream velocities, the flame moves away from the leading edge and anchors at a location downstream. The Emmons solution is not applicable in this case as well. For the fuel considered in this study (methanol), accounting for thermal radiation, employing an optically thin radiation model, allows better agreement between experimental and numerical temperature profiles but does not affect the mass burning rates.

Investigation of Combustion Characteristics of Biodiesel and Its Blends

Experiments employing porous sphere technique are carried out to investigate the combustion characteristics of biodiesel produced from karanja oil and its blends with diesel and gasoline. Biodiesel or its blend is transpired to porous sphere surfaces, where it burns. Porous spheres are placed in an air stream blowing with a uniform velocity at atmospheric pressure and temperature under normal gravity conditions. At low air velocities, a flame envelops the entire sphere. Fuel feed rates are recorded for various sphere sizes and air velocities. Overall, the visible flame shape is analyzed with the help of digital flame image. At a particular air velocity, the flame will not envelope the sphere, but establish near the rear region of the sphere and the same is recorded for different sphere sizes. Theoretical correlations are proposed for burning rate as a function of effective Reynolds number and for Damköhler number as a function of Froude number.

A numerical study of quasi-steady burning characteristics of a condensed fuel: effect of angular orientation of fuel surface

A numerical study of laminar diffusion flames established over a condensed fuel surface, inclined at several angular orientations in the range of –90°⩽θ⩽+90° with respect to the vertical axis, under atmospheric pressure and normal gravity environment, is presented. Methanol is employed as the fuel. A numerical model, which solves transient gas-phase, two-dimensional governing conservation equations, with a single-step global reaction for methanol–air oxidation and an optically thin radiation sub-model, has been employed in the present investigation. Numerical results have been validated against the experimental data from the present study. Thereafter, the model is used to investigate the influence of angular orientation of fuel surface on its quasi-steady burning characteristics. Results in terms of fuel mass burning rate, flame stand-off distances, temperature field, velocity profiles and oxygen contours have been presented and discussed in detail. It is observed that orientation angles in the range of –45°⩽θ⩽ –30° (fuel surface facing upwards), yield the maximum mass burning rates. The flame anchoring location near the leading edge of the fuel surface, normal gradient of fuel vapor mass fraction at the surface and oxygen contours have been used to explore this unique behavior. Based on the numerical results, a theoretical correlation to predict the mass burning rate as a function of fuel surface orientation is also proposed. Furthermore, a discussion on the differences in the structure of laminar diffusion flame established over fuel surface as a function of its angular orientation is included.

Numerical simulation of laminar co-flow methane–oxygen diffusion flames: effect of chemical kinetic mechanisms

Laminar co-flow methane–oxygen flames issuing into the unconfined atmosphere have been studied. A numerical model, which employs different chemical kinetics sub-models, including a skeletal mechanism with 43 reaction steps and 18 species and four global reaction mechanisms (two 2-steps and two 4-steps mechanisms), and an optically thin radiation sub-model, has been employed in the simulations. Numerical model has been validated against the experimental results available in literature. The numerical predictions from the global kinetic mechanisms have been compared with the 43-steps mechanism predictions. At all oxygen flow rates, the predictions of the distributions of temperature, mass fractions of CH4, O2 and CO2 by the 2-steps mechanisms are closer to 43-steps mechanism. The overall distribution of H2O predicted by 2-steps mechanisms is close to that of 43-steps except for the maximum value. Especially at higher oxygen flow rates, the modified 2-steps mechanism predicts these quantities much closer to those predicted by the 43-steps mechanism. Further, the 2-steps mechanisms predict location of the reaction zone accurately. However, they can just give an idea of overall CO distribution in terms of the axial and radial locations within which CO will almost be consumed, but not its maximum value in the domain. The 4-steps mechanisms predict the trend of variation of these quantities quite reasonably. However, they under-predict the location of the reaction zone. At higher oxygen flow rates, the predictions by 4-steps mechanisms becomes better, especially in the prediction of maximum CO and H2O. Over all, the modified 2-steps mechanism can be recommended for reasonable and economical predictions of oxy-rich methane flames.

A theoretical correlation for the Nusselt number in direct contact evaporation of a moving drop in an immiscible liquid

Numerical solutions for the Nusselt number during the direct contact evaporation of a moving drop in a stagnant column of immiscible liquid are presented. The effect of bubble growth rate on the radial component of drop velocity is taken into account in the analysis and the Nusselt number is found to be a function of Peclet number, Jakob number and vapour open angle. A comparison of theoretical and experimental correlations for the Nusselt number shows good agreement. The analysis also yields information on the temperature profile and the thickness of the thermal boundary layer surrounding the evaporating drop.ZusammenfassungEs werden numerische Lösungen für die Nusselt-Zahl während der Verdunstung eines bewegten Tropfens, der in direktem Kontakt mit der umgebenden ruhenden Säule aus unmischbarer Flüssigkeit steht, mitgeteilt. In der Berechnung wird der Einfluß der Blasenwachstumsrate auf die radiale Komponente der Tropfengeschwindigkeit berücksichtigt. Es wird festgestellt, daß die Nusselt-Zahl eine Funktion der Peclet-Zahl, der Jakobs-Zahl und des „Öffnungswinkels des Dampfes“ ist. Ein Vergleich der theoretischen und experimentellen Beziehungen für die Nusselt-Zahl zeigt gute Übereinstimmung. Die Berechnung enthält auch Informationen über das Temperaturprofil und die Dicke der thermischen Grenzschicht um den verdampfenden Tropfen.

On the Estimation and Validation of Global Single-Step Kinetics Parameters of Ethanol-Air Oxidation Using Diffusion Flame Extinction Data

In this study, based on the extinction characteristics of nonpremixed flames, global single-step kinetics parameters are estimated for ethanol oxidation in air. Using a quasi-steady heterogeneous experimental setup, the air velocity at which an envelope flame surrounding the sphere transits to a wake flame that burns in the rear region of the sphere, termed as the transition or the extinction velocity, were obtained in a previous study. These extinction velocities for different sphere sizes are employed to estimate the single-step kinetics parameters. The estimated global single-step kinetics has been employed in a numerical model and extinction characteristics of opposed-flow ethanol-air diffusion flames have been predicted and validated against the available experimental results in literature.

Numerical investigation of laminar diffusion flames established on a horizontal flat plate in a parallel air stream

Numerical investigation of laminar diffusion flames established on a flat plate in a parallel air stream is presented. A numerical model with a multi-step chemical kinetics mechanism, variable thermo-physical properties, multi-component species diffusion and a radiation sub-model is employed for this purpose. Both upward and downward injection of fuel has been considered in a normal gravity environment. The thermal and aerodynamic structure of the flame has been explained with the help of temperature and species contours, net reaction rate of fuel and streamlines. Flame characteristics and stability aspects for several air and fuel velocity combinations have been studied. An important characteristic of a laminar boundary layer diffusion flame with upward injection of fuel is the velocity overshoot that occurs near the flame zone. This is not observed when the fuel is injected in the downward direction. The flame standoff distance is slightly higher for the downward injection of fuel due to increase in displacement thickness of boundary layer. Influence of an obstacle, namely the backward facing step, on the flame characteristics and stability aspects is also investigated. Effects of air and fuel velocities, size and location of the step are studied in detail. Based on the air and fuel velocities, different types of flames are predicted. The use of a backward-facing step as a flame holding mechanism for upward injection of fuel, results in increased stability limits due to the formation of a recirculation zone behind the step. The predicted stability limits match with experimentally observed limits. The step location is seen to play a more important role as compared to the step height in influencing the stability aspects of flames.

Thermal analysis of the vortex tube based thermocycler for fast DNA amplification: Experimental and two-dimensional numerical results

In this article, experimental and numerical analyses to investigate the thermal control of an innovative vortex tube based polymerase chain reaction (VT-PCR) thermocycler are described. VT-PCR is capable of rapid DNA amplification and real-time optical detection. The device rapidly cycles six 20μl 96bp λ-DNA samples between the PCR stages (denaturation, annealing, and elongation) for 30cycles in approximately 6min. Two-dimensional numerical simulations have been carried out using computational fluid dynamics (CFD) software FLUENT v.6.2.16. Experiments and CFD simulations have been carried out to measure/predict the temperature variation between the samples and within each sample. Heat transfer rate (primarily dictated by the temperature differences between the samples and the external air heating or cooling them) governs the temperature distribution between and within the samples. Temperature variation between and within the samples during the denaturation stage has been quite uniform (maximum variation a...

A theoretical analysis of direct contact evaporation in spray columns

Abstract This article deals with a theoretical analysis of direct contact latent heat transfer between two immiscible liquids in a counterflow spray column. The non-dimensional parameters governing the evaporation process are identified and a study of the effects of the variation of these parameters is made. The longitudinal dispersion in the continuous phase is taken into account in the analysis. The predicated column heights required for complete evaporation compare favourably with the available experimental data. The theoreatical model also predicts the temperatue profile of the dispersed phase along the column.

Numerical investigation of laminar cross-flow non-premixed flames in the presence of a bluff-body

A knowledge of flame stability regimes in the presence of cylindrical bluff-bodies of various dimensions is essential to design non-premixed burners. The reacting flow field in such cases is reported to be three-dimensional and unsteady. In the literature, only a few experimental investigations with limited measurements are available. Therefore, in this work, a detailed numerical study of laminar cross-flow non-premixed methane–air flames in the presence of a square cylinder is presented. The flow, temperature, species and reaction fields have been predicted using a comprehensive transient three-dimensional reacting flow model with detailed chemical kinetics and variable thermo-physical properties, in order to get a good insight into the flame stabilisation phenomena. Further, analyses of quantities such as local equivalence ratio, cell Damköhler number, species velocity, net consumption rate of methane, which are not easily obtained through experiments even with detailed diagnostics, have been carried out. The influence of the flow field due to varying inlet velocity of the oxidiser, in the presence of the bluff-body, on flame anchoring location has been analysed in detail. Local equivalence ratio contours obtained from non-reacting flow calculations are seen to be quite useful in analysing the mixing process and in the prediction of flame anchoring locations when the flames are not separated. Cell Damköhler number has been calculated using cell size, species velocity of the fuel, which is a derived quantity, and the net reaction rate of the fuel. The flame zone, which is customarily inferred from the contours of temperature, CO and OH, is also shown to be predicted well by the contour line corresponding to a Damköhler number equal to unity. The net reaction rate of CH4 and the net rates of two dominant reactions, which consume methane, show clearly the variation in the flame anchoring locations in these three cases. Further, the three-dimensionality of these flames are analysed by plotting the mean temperature contours in y–z planes. Finally, the unsteadiness in the separated flame case is analysed.