Vasudevan Raghavan

A numerical investigation of the flame characteristics of steady, laminar diffusion flames sourced by methanol wicks, established over vertically oriented parallel surfaces in a mixed convective flow field is carried out. Two configurations are considered; in the first, both surfaces contain methanol wicks facing each other, whereas in the second, only one of the surfaces contains the wick. In both these configurations, the flow of air is upwards and parallel to the surfaces. The effect of wick separation distance and forced convective air flow velocity on the thermal and reactive flow fields, flame shapes, average mass burning rates, as well as on the conditions leading to flame lift-off are brought out. Numerical model that uses a single-step kinetics for methanol-air oxidation and optically thin radiation sub-model, is employed. It is observed that the average mass burning rates are more sensitive to the separation distance at lower air velocities. As separation distance increases, the air velocity leading to flame lift-off increases, before becoming a constant finally. For lower separation distances, the lift-off occurs at a higher air velocity for single fuel wick case. The flow, temperature and species fields are used to explain the predicted trends.

A numerical investigation of the characteristics of laminar co-flow methane-oxygen diffusion flames has been carried out. The temperature and nitric oxide (NO) distributions in unconfined and partly confined flames are studied in detail. Radial confinements of different diameters and with a length of 150 times the fuel jet diameter have been considered to allow atmospheric nitrogen entry only from the top. A numerical model with a 43-step chemical kinetics mechanism and an optically thin radiation sub-model is employed to carry out simulations. The numerical model has been validated using the experimental data available in the literature. The effect of oxygen flowrate on temperature distributions is studied thoroughly. Confined flame extents are compared with the corresponding unconfined flame extents with the help of OH contours. The effect of confinement diameter on temperature and NO distributions is analysed in detail. At low oxygen flowrates, the extents of confined flames are higher than those of an unconfined flame. At a higher oxygen flowrate, the extent of unconfined flame becomes higher. The confined flames are in general hotter than the unconfined flames. However, at the highest oxygen flowrate and for an intermediate confinement diameter, the flame has the lowest maximum temperature. The amount of NO produced in confined flames is higher than the unconfined flames, due to air entrainment from the top of the confining tube, which increases the residence time for nitrogen transport and its oxidation. At the highest oxygen flowrate considered, numerical predictions show that for a given confinement length, there is an optimum confinement diameter which results in a minimum net production of NO among all the flames.

Biogas is an alternative fuel that typically contains around 45% carbon-dioxide by volume, besides methane. Due to the inherent content of carbon-dioxide, it is necessary to study the flame characteristics and stability limits in cross-flow non-premixed burners. In this study, cross-flow non-premixed flames, where biogas is injected through a horizontal porous plate and air is blown parallel to the fuel injector, are studied systematically. In order to increase the stable operating regime, devices such as backward facing steps and cylindrical bluff-bodies are commonly employed. Different step-heights and locations from leading edge of the fuel injector are considered for the cases with backward facing steps. A rectangular cylindrical bluff-body is also used as a flame stabilizing obstacle. Baseline cases are studied without any backward facing step or cylindrical bluff-body. Volume flow rate of biogas is varied from 36 liter per hour to 360 liter per hour. Air velocity is varied in the range of 0.2 m/s to 3.0 m/s. For a given fuel velocity, air velocity is gradually increased in order to record the transition of flame from one regime to another. Flame stabilization is carefully assessed by monitoring the high definition direct flame photographs captured from front and top views, for all the cases. The cases are repeated at least three times to ensure repeatability. Stability maps are plotted as a function of fuel velocity and air velocity for all the cases. For cases with backward facing steps, both step height and its location play an important role in delineating the boundaries of the flame regimes. Parametric variations show interesting features. Bluff-body flames become quite oscillatory and three dimensional at higher air velocities. For this case, stability maps of flames from biogas and pure methane are compared.

Evaporation of stagnant (zero relative velocity) as well as moving spherical droplets of n-dodecane in a zero-gravity and high pressure nitrogen environment is modeled. The non-ideal effects, solubility of ambient gas into the liquid-phase, variable thermo-physical properties, and gas- and liquid-phase transients are included in the model. The model is quantitatively validated using published experimental data. Numerical predictions show that, for stagnant droplets at sub-critical ambient temperatures, the droplet lifetime continuously increases with pressure, while at critical temperature, the lifetime initially increases and thereafter remains almost constant. At super-critical temperatures, the lifetime decreases continuously with increasing ambient pressure and the average evaporation constant shows a local maximum at a particular ambient pressure. In the case of moving droplets, at super-critical ambient temperature, the rate of increase of average evaporation constant with ambient pressure becomes significant as the initial droplet relative velocity increases. For low initial velocities (<1 m/s), the average evaporation constant gradually increases with ambient pressure and subsequently levels-off with further increase in ambient pressure. The droplet lifetime decreases with increase in ambient pressure or initial velocity. Penetration distance of the moving droplets decreases with ambient pressure and increases with initial droplet relative velocity. The mechanisms influencing the differences in evaporation under varying conditions of pressure and temperature are discussed. © 2011 American Institute of Physics.

This paper presents comprehensive numerical simulations of evaporation of droplets constituted of two liquid fuels in high pressure nitrogen ambient under normal gravity condition. A transient, two-phase and axisymmetric numerical model has been used for the simulations. Transport processes in liquid- and vapor-phases have been solved along with interface coupling conditions. Gas-phase non-idealities, solubility of ambient gas in liquid-phase, and pressure and temperature based variable thermo-physical properties in both liquid- and vapor-phases are considered in the numerical model. Phase equilibrium has been estimated using fugacity coefficients of all species in both phases. The range of Weber number has been chosen such that droplet remains almost spherical throughout its lifetime. Simulations have been carried out until the droplet surface regresses to one-tenth of its initial value or when the critical state for the mixture is reached. The numerical model has been quantitatively validated against the experimental data available in literature. The validated model is used to systematically study the evaporation characteristics of suspended n-heptane-hexadecane droplets in nitrogen ambient. The effects of the pressure, temperature, initial liquid-phase composition and forced convection velocity on evaporation characteristics have been discussed in detail.

Stability of flames are affected by fuel properties, geometry of the burner and operating conditions. In this experimental work, first the characteristics of non-premixed flames of Liquefied Petroleum Gas (LPG) and air in cross-flow configuration, where air jet flows perpendicular to the fuel stream, are studied experimentally. Flame transition and stability regimes of non-premixed flames of LPG and air, in a cross-flow burner without and with obstacles, are determined by systematically varying the fuel and air flow rates. Obstacles such as backward facing step and cylindrical bluff bodies are considered. Subsequently, the effects of fuel properties on the stability of flames are analyzed, Flame stability regimes of natural gas (methane) and biogas (methane and carbon-dioxide), measured from a similar burner are available in literature. These have been compared with the stability of LPG flames in terms of power rating of the burner and global equivalence ratio (defined for non-premixed flames).

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 at a higher fuel flow rate. In the natural flickering regime, the dominating frequency of flame flickering remains within a small range, 12.5 Hz to 15 Hz, for all the burners and for all fuel flow rates. The timeaveraged flame length-scale parameters, such as the maximum and the minimum flame heights, increase with respect to the fuel flow rate, however, the difference in the maximum and the minimum flame heights remains almost constant.

Hydrogen is a highly flammable gas and accidental release in confined space can pose serious combustion hazards. Numerical studies are required to assess the formation of flammable hydrogen cloud within confined spaces. In the present study, numerical investigations on the release of helium and hydrogen gases as high-velocity jets and their subsequent distribution inside an unventilated cylindrical enclosure (AIHMS facility) has been carried out as a first step towards numerical studies on hydrogen distribution in confined spaces for safety assessments. Experimental data for jet release of helium at volume Richardson number 0.1 and subsequent distribution has been used as benchmark data. Sensitivity studies on the influence of grid sizes, time-steps and turbulence models are performed. The performance of the validated numerical model is evaluated using statistical performance parameters. Similarity relations are used to determine input parameters for hydrogen jet for corresponding experimental data with helium jets. Finally, the mixing and flammability aspects of hydrogen distribution inside the enclosure are studied using four numerical indices that quantify mixing and deflagration potential of a distribution. It is concluded that the helium experiments can be used for validation of numerical models for hydrogen safety studies and any one of the similarity relationships, viz., equal buoyancy, equal volumetric flow, or equal concentration can be used for assessing the behaviour of hydrogen release and distribution within confined spaces.

The paper presents a numerical model for analyzing vapor-liquid equilibrium of ternary (three-component) system at high pressures. The gas-phase non-idealities and solubility of gas in liquid are considered in the numerical model. The model is useful for studies involving evaporation of liquid at different pressure and temperature conditions, where the interface liquid and vapor compositions are required. At high ambient pressures, ambient gases dissolve into the liquid. Thus, even a single component liquid fuel evaporating in a high pressure ambient gas, effectively behaves like a two-component liquid system. This study considers a ternary system. The numerical model has been validated against the experimental data available in literature. The validated model is used to study the solubility of ambient gas in a binary liquid mixture at high pressures. The effects of pressure, temperature and liquid phase composition on the solubility of gas in liquid have been studied systematically.

Mechanical vibrations introduced in diffusion flame burners significantly affect the flame characteristics. In this experimental study, the effects of axial vibrations on the characteristics of laminar diffusion flames are investigated systematically. The effect of the frequency and amplitude of the vibrations on the flame height oscillations and flame stability is brought out. The amplitude of flame height oscillations is found to increase with increase in both frequency and amplitude of burner vibrations. Vibrations are shown to enhance stability of diffusion flames. Although flame lifts-off sooner with vibrations, stability of the flame increases.