Chendhil Periasamy

Trajectory and Characteristics of Buoyancy and Momentum Dominated Horizontal Jet Flames From Circular and Elliptic Burners

Relative effects of buoyancy and momentum on the characteristics of horizontally oriented circular (Circ) and elliptic (E) burner flames in a quiescent environment over a wide range of jet exit velocities are presented. The major axis of the elliptic burner was oriented horizontally and vertically (referred to as Emaj and Emin flames, respectively). Propane was used as fuel and a small amount of hydrogen was piloted to attach flames to the burner. Global flame characteristics such as flame dimensions, centerline trajectory, emission indices (EI) and radiative fraction, and in-flame transverse concentration and temperature profiles were measured. At a jet exit Reynolds number (Rej) of 2000, based on the area-equivalent diameter of the burner, the flame characteristics were affected by the burner geometry and its orientation. Also, the vertical dimension of the burner exit dictated buoyancy effects. At Rej=12,500, the influence of burner geometry or its orientation was negligible. Elliptic burner flames exhibited lower liftoff and blowout velocities than circular burner flames. Furthermore, the flame stability and nitric oxide emissions were not much affected by the orientation of elliptic burner. Although the elliptic burners produced higher EINO at lower jet exit velocities, the variation in EINO among three burners (Circ, Emaj, and Emin) was insignificant at higher velocities. Some effects of buoyancy on EICO were observed at lower jet exit velocities and the EICO was the lowest for the burners with largest buoyancy flux. Elliptic burner flames produced greater peak flame temperature than the corresponding circular burner flames under most conditions.

Spray Impingement and Evaporation Through Porous Media

This paper presents an experimental study on spray impingement and fuel evaporation processes in porous media. Aviation-type kerosene (Jet-A) was used as the fuel and an open-cell, silicon carbide coated, carbon-carbon ceramic foam was used as the porous medium. The fuel was sprayed into a coflowing, preheated air environment using an air-blast atomizer. A Phase Doppler Particle Analyzer (PDPA) was used to measure Sauter mean diameter (SMD), liquid mass flux, and axial velocity of the droplets. The porous medium was resistively heated to simulate heat feedback from the combustion zone. An organic vapor analyzer, based on catalytic oxidation, was used to measure the concentration of kerosene vapor downstream of the porous medium. The temperature and flow rate of coflow air were held constant, while the fuel flow rate, and hence, the overall equivalence ratio, was varied from 0.3 to 0.6. Higher liquid mass flux was recorded away from the spray core region, due to the swirling action of atomizing air. Surface temperature measurements of porous media revealed the uniformity of thermo-electrical properties of the medium. Vapor concentration measurements with combustion heat feedback (a heat input of 33 kW/m2 ) increased the average vapor concentration by 63% and 43% than that of no heat feedback case for 0.3 and 0.6 equivalence ratios respectively.Copyright

NUMERICAL MODELING OF EVAPORATION PROCESS IN POROUS MEDIA FOR GAS TURBINE APPLICATIONS

A simplified numerical model for analyzing the evaporation processes in porous media for gas turbine applications has been developed. Evaporation of a pointwise -injected kerosene spray in a carbon -carb on porous medium is considered. The computational model consists of a two -dimensional domain of dimensions 20.32x4.04 cm. A control -volume based discretization method is adopted to solve the governing equations. The porous medium offers resistance to th e flow of air -fuel mixture and is modeled as a momentum sink. Non -Darcy flow in porous medium is considered and the viscous and inertial contributions are evaluated using a modified Ergun equation. The transient and conduction flux terms in the energy eq uation are modified to account for the heat transfer in porous medium. Energy feedback from combustion porous media is also simulated using a source term. The effects of porous medium temperature, fuel flow rate, air inlet temperature and porous medium g eometry on the evaporation of spray have been analyzed. For the size under consideration, a porous medium heat source of 642 W is required to achieve 97 % complete evaporation for an air inlet temperature of 473 K. The concentration of fuel vapor is foun d to be higher in the core region due to the nature of point injection. Simulations using different flow rate conditions show that a stronger heat source, in turn higher energy feed back, is required to attain complete vaporization. Approximately a 62 % stronger heat source is required when the fuel flow rate is increased from 0.24 to 0.48 mg/s. The increase in air inlet temperature is found to accelerate the evaporation process. At higher air inlet temperatures (573 K), the fuel is vaporized as soon as it gets injected. Evaporation characteristics are not found to vary much with porous medium geometry, as the porous medium is modeled as a momentum sink. Thermal effects of porous media are found to be more dominant in this study.

Laminar Burning Velocity of Synthetic Gas Premixed Flames Near Extinction Conditions

Synthetic gas premixed flames have received considerable attention in the past. Recent interest in lean premixed combustion requires the fundamental combustion properties at fuel-lean conditions. This paper presents an experimental study on the laminar burning velocity of synthetic gas premixed flames at close to extinction conditions. CH4-H2 and CO-H2 fuel blends at different mixture compositions simulating coal-derived synthetic gases were investigated. A water-cooled nitrogen-stabilized flat flame burner with a diameter of 6 cm was employed for this purpose. The planar laser induced fluorescence technique was employed to precisely measure the flame width. Hydroxyl radical (OH) concentration was used as the marker of reaction field. A pulsed Nd:YAG laser was used in conjunction with an optical parametric oscillator to generate a vertical laser sheet at a wavelength of 283.5 nm. An intensified CCD camera was used to capture the fluorescence signals from OH radicals. For each fuel blend mixture composition, the lean blowout condition was first determined. All measurements were then taken at this condition. For CH4-H2 fuel blends, the burning velocity near extinction varied between 6.88 cm/s and 10.13 cm/s. Burning velocities of CO-H2 fuel blends near extinction varied between 4.36 cm/s and 6.27 cm/s.

Modeling Liquid Spray Evaporation in Heated Porous Media With a Local Thermal Non-Equilibrium Model

The situations such as rapid evaporation, and significant heat generation/convective heat transfer, typically encountered in liquid-fueled porous media combustors, warrant the use of local thermal non-equilibrium models. Knowledge of fuel vaporization and mixing is important to understand the combustion characteristics. In this paper, a two-energy equation model is presented to account for the non-equilibrium between the solid and liquid phases. In this approach, two energy equations for solid and gas phases were solved. Kerosene fuel, issued from an air-blast atomizer, was injected on to a heated porous medium. Governing equations were applied on a 2-D axisymmetric, computational domain of 20.3 cm × 2.5 cm. Computer simulations were conducted using a commercial code Fluent 6.0. Heat transfer from combustion porous medium was simulated by setting a volumetric heat source in the porous region. Accordingly, the peak temperatures in porous media varied from 473 K to 590 K. Axial temperature profiles within the porous media were obtained with equilibrium and non-equilibrium models. Results indicated that the equilibrium models slightly underpredicted the peak temperature. Using non-equilibrium models, radial profiles of kerosene vapor concentration were obtained at different axial locations and the results showed that the thermal effects of the porous medium dominated in the evaporation process. Numerical results were also compared with available data and the agreement was found to be good.Copyright

Lean Extinction Characteristics of Kerosene Spray Flames in Porous Media

This paper presents an experimental study of lean extinction characteristics of kerosene spray flames in porous media. Two silicon carbide coated carbon-carbon foam porous media with 8 and 25 pores per centimeter both with 87% porosity were used in the experiments. The first (evaporation porous medium) was used to enhance the vaporization rate of liquid fuel. The second (combustion porous medium) was used to stabilize flame on the surface or interior of the porous medium. Aviation grade kerosene was injected upstream of evaporation porous medium with an air-blast atomizer. A Damkohler number-based analysis was developed to study the extinction behavior. Preheating residence time was varied by changing the location of the injector and coflow air velocity. Results show that in interior combustion mode the flame was completely contained within the porous medium and heated it to bright yellow glowing temperature. As fuel flowrate was decreased towards lean extinction limit, a weak transient blue flame appeared on the downstream surface, and immediately extinguished. In surface combustion mode, a decrease of fuel flow rate resulted in a flame partially lifted from the porous medium surface. A further decrease of fuel flow rate resulted in flame extinguishment and a fully lifted flame could not be stabilized. A minimum value of Damkohler number was required to initiate interior combustion. Axial temperature profile and surface temperature uniformity in porous media were also examined near extinction conditions.

A Parametric Simulation of the Evaporation in Liquid-Fueled Porous Burners

This paper presents a computational parametric study of evaporation processes in liquid-fueled, simulated porous media burners using a two-energy equation model. The effects of porous medium heat source, porous medium structure, fuel flow rate, and air inlet temperature on evaporation characteristics were determined. Predicted steady-state axial temperature profiles within the porous media and radial vapor concentration profiles at 5 cm downstream of the porous medium are presented. Vapor concentration results showed a strong dependence on porous medium temperature, which, in turn, depended on the strength of the heat source and the effectiveness of heat transfer between porous medium and coflow air. Simulations with different porosities demonstrated that the peak vapor concentration decreased as porosity increased. The peak vapor concentration dropped by 42 % when porosity was increased from 0.5 to 0.87. Under higher fuel flowrate conditions, the extent of completeness of evaporation decreased, showing that much stronger heat source was needed to maintain the complete evaporation. When the coflow air temperature was increased, the peak vapor concentration was found to increase and the vapor concentration spread more radially.Copyright

Aspect Ratio Effects on Partially Premixed Flames From Elliptic Burners in Coflow

In this paper, partially premixed flames of propane-hydrogen blends from elliptic burner geometries in coflow environment have been experimentally studied. Two different elliptic burner geometries with aspect ratios (AR) of 3:1 and 4:1 were used. A circular burner with the same discharge area as that of the elliptic burner was employed for comparison. Measurements were taken at stoichiometric and three other equivalence ratios. Global flame characteristics such as visible height, emission indices, and flame radiation were measured. Flame structure data such as transverse profiles of inflame concentrations of combustion products and local flame temperature were also measured at three axial locations in the flame. Results indicate that elliptic burner flames were shorter, more radiating, and produced lower NO and CO emissions than the corresponding circular burner flames. Results from the inflame measurements of NO and CO were in good agreement with the corresponding global data. Further, the 4:1 AR elliptic burners exhibited a twin-jet flame structure at fuel-rich conditions. The twin-flame structure was evident from the inflame measurements of temperature and combustion species. This study suggests that the combination of elliptic burner geometry and coflow reduces NO and CO emissions from combustion systems, which could potentially lead to cleaner environment.Copyright

A Computational Study of the Evaporation Characteristics of an Air-Blast Atomized, Kerosene Spray in Porous Media

The evaporation characteristics of an air-blast atomized kerosene spray in porous media in a 2D-axisymmetric coflow environment were studied numerically. A swirling primary air stream with varying intensity was used to aid the atomization process. The effects of non-Darcy flow in porous medium were modeled using a modified form of Ergun equation. Local thermal equilibrium between the fluid mixture and porous medium was assumed. Conductive and transient heat flux terms in the energy equation were modified to include the effective thermal conductivity and thermal inertia of the solid region respectively. The effective thermal conductivity was defined as the volumetric average between solid and fluid media. First, the temperature characteristics of the porous medium, arising from different source terms, were obtained. Complete vaporization of kerosene was achieved when the maximum temperature of the porous medium was at 590 K. The effects of porous medium temperature, primary air swirl number, fuel flow rate, and secondary (coflow) air inlet temperature on vaporization were analyzed. For all cases, kerosene vapor concentration profiles at five different axial locations in the domain (0.08, 0.12, 0.13, 0.14, and 0.19m from the nozzle) were obtained. An increase in secondary air inlet temperature from 373 K to 473 K increased the completeness of evaporation from 94% to 97%. When the swirl number was increased from 0.14 to 0.34, the peak vapor concentration was reduced by 31% and more vapor spread radially. The porous medium temperature was found to be a crucial factor in obtaining the complete vaporization of the spray.Copyright