Ajay K. Agrawal

Combustion of hydrogen-enriched methane in a lean premixed swirl-stabilized burner

The combustion characteristics of a premixed, swirl-stabilized flame were studied to determine the effects of enriching methane with hydrogen under fuel-lean conditions. The burner consisted of a center-body with an annular, premixed fuel-air jet. Swirl was introduced to the flow using 45-degree swirl vanes. The combustion occurred within an air-cooled quartz chamber at atmospheric pressure. Flame stability and blowout maps were obtained for different amount of hydrogen addition at several fuel-air flow rates. Gas probe measurements were obtained to demonstrate reductions in CO concentration with hydrogen addition, without adversely affecting the NO x emissions. The flame structure near the lean stability limit was described by direct luminous photographs and planar laser-induced fluorescence measurements of the OH radical. Results show that the addition of a moderate amount of hydrogen to the methane/air mixture increased the peak OH concentration. Hydrogen addition resulted in a significant change in the flame structure, indicated by a shorter and more robust appearing flame. The observed trends concur with the strained opposed premixed flame analysis using RUN-1DL. The computations revealed that enriching the methane with hydrogen increased the strain resistance of the flame as well as the OH levels in the flame.

Schlieren analysis of an oscillating gas-jet diffusion flame

Abstract Flow structure of a flickering gas-jet diffusion flame was investigated using quantitative rainbow schlieren deflectometry. The fuel was pure hydrogen exiting from a burner tube at 43 m/s. Angular deflection data were obtained across the whole field of color schlieren images taken at a spatial resolution of 0.14 mm and a temporal resolution of 60 Hz. These line-of-sight data were analyzed to determine power spectra and flame flicker frequency at different operating pressures to scale the effects of buoyancy. The measurements were tomographically inverted to obtain the refractive index and, hence, temperature distributions assuming chemical equilibrium in the flame. The oscillating flow is described quantitatively in terms of temporal evolution of the temperature field. The flame structure is also described statistically by mean, root-mean-square (RMS), and probability density function profiles of temperature. Results reveal global oscillations in the flow field of the flame. The oscillations were stronger in the outer flow as compared to those at the flame surface. The flame was clipped-off at a downstream location, where the outer vortical structures intensified and entrained cold fluid toward the center region.

THREE-DIMENSIONAL RAINBOW SCHLIEREN TOMOGRAPHY OF A TEMPERATURE FIELD IN GAS FLOWS

We present quantitative rainbow schlieren deflectometry with tomography for measurements of temperature in three-dimensional gas flows. The schlieren apparatus with a continuously graded spectral filter of known transmissivity was used to create color schlieren images of the test media. These images at multiple viewing angles were used to infer beam deflection angles by the medium. The deflection data were used with a tomographic technique to reconstruct the refractive index and thus the temperature field. The temperature distributions obtained by the rainbow schlieren tomography agreed with those measured by a thermocouple probe. This research demonstrates that tomography can be used with full-field schlieren deflectometry to measure quantitatively temperature in asymmetric gas flows. The technique could be used to obtain related properties such as pressure, density, and gas composition.

Experimental study of surface and interior combustion using composite porous inert media

Combustion using silicon carbide coated, carbon-carbon composite porous inert media (PIM) was investigated. Two combustion modes, surface and interior, depending upon the location of flame stabilization, were considered. Combustion performance was evaluated by measurements of pressure drop across the PIM, emissions of NO x and CO, and the lean blow-off limit. Data were obtained for the two combustion modes at identical conditions for a range of reactant flowrates, equivalence ratios, and pore sizes of the PIM. Results affirm PIM combustion as an effective method to extend the blow-off limit in lean premixed combustion.

Combustion Performance of Biodiesel and Diesel-Vegetable Oil Blends in a Simulated Gas Turbine Burner

Recent increases in fuel costs, concerns for global warming, and limited supplies of fossil fuels have prompted wide spread research on renewable liquid biofuels produced domestically from agricultural feedstock. In this study, two types of biodiesels and vegetable oil (VO) are investigated as potential fuels for gas turbines to generate power. Biodiesels produced from VO and animal fat were considered in this study. The problems of high viscosity and poor volatility of VO (soybean oil) were addressed by using diesel-VO blends with up to 30% VO by volume. Gas chromatography/mass spectrometry, thermogravimetric analysis, and density, kinematic viscosity, surface tension, and water content measurements were used to characterize the fuel properties. The combustion performance of different fuels was compared experimentally in an atmospheric pressure burner with an air-assist injector and swirling primary air around it. For different fuels, the effect of the atomizing airflow rate on Sauter mean diameter was determined from a correlation for air-assist atomizers. Profiles of nitric oxides (NO x ) and carbon monoxide (CO) emissions were obtained for different atomizing airflow rates, while the total airflow rate was kept constant. The results show that despite the compositional differences, the physical properties and emissions of the two biodiesel fuels are similar. Diesel-VO fuel blends resulted in slightly higher CO emissions compared with diesel, while the NO x emissions correlated well with the flame temperature. The results show that the CO and NO x emissions are determined mainly by fuel atomization and fuel/air mixing processes, and that the fuel composition effects are of secondary importance for fuels and operating conditions of the present study.

Quantitative measurements in an unsteady flame using high-speed rainbow schlieren deflectometry

Rainbow schlieren deflectometry was integrated with a high-speed digital imaging system to quantify the scalar flow structure of flames at a high temporal resolution. The schlieren image data were processed to determine the temperature field across the whole field assuming chemical equilibrium in the flame. First, shapes of steady hydrogen jet diffusion flames estimated from the schlieren technique were compared with those obtained by direct visualization. Next, the schlieren technique was applied to flame experiments at a jet exit Reynolds number of 300, which produced periodic oscillations or flame flicker. Rainbow schlieren images taken at acquisition rates of 30, 60, 125, 250, 500 and 1000 frames per second were analysed to construct the instantaneous temperature distributions across the whole field, the vortex convection velocity profiles, and the mean and root-mean-square temperature profiles. Results demonstrate that high-speed rainbow schlieren deflectometry is effective in quantitatively describing the transient flow structure of unsteady flames.

INVESTIGATION OF A MINIATURE COMBUSTOR USING POROUS MEDIA SURFACE STABILIZED FLAME

Abstract In this study, heat recirculation in an annulus around the combustor is utilized together with the flame stabilized on the surface of a porous inert media (PIM) made of silicon-carbide coated carbon foam. The effectiveness of the proposed concept is demonstrated experimentally for methane combustion in chamber volume of 0.364 cm3 and overall system volume of 1.5 cm3. Experiments were conducted for reactant flow velocities varying from 0.25 to 1.0 m/s in the equivalence ratio range of 0.50 to 0.80. Measurements include carbon monoxide and nitric oxides concentrations and product gas temperatures at the combustor exit, and temperature profiles on the exterior surface. A computational fluid dynamics (CFD) model incorporating the physics of conjugate heat transfer, radiation heat transfer, flow and heat transfer in the PIM, and heat release by combustion is developed to predict the thermal performance. Results show excellent agreement between measured and computed temperature profiles at different reactant flow rates. The CFD analysis is used to identify important thermal pathways within the system. Finally, a modified design is presented and analyzed computationally. The modified combustion system design achieves a significant reduction in the heat loss as compared to the baseline design tested experimentally.

Abel inversion of deflectometric measurements in dynamic flows

We present an Abel-inversion algorithm to reconstruct mean and rms refractive-index profiles from spatially resolved statistical measurements of the beam-deflection angle in time-dependent, axisymmetric flows. An oscillating gas-jet diffusion flame was investigated as a test case for applying the algorithm. Experimental data were obtained across the whole field by a rainbow schlieren apparatus. Results show that simultaneous multipoint measurements are necessary to reconstruct the rms refractive index accurately.

Combustion Performance of Liquid Biofuels in a Swirl-Stabilized Burner

Fuels produced from renewable sources offer an economically viable pathway to curtail emissions of greenhouse gases. Two such liquid fuels in common usage are biodiesel and ethanol derived from soybean, corn, or other food crops. In recent years, significant effort has been devoted to identify alternate feedstock sources and conversion techniques to diversify the biofuels portfolio. In this study, we have measured emissions from flames of diesel, biodiesel, emulsified bio-oil, and diesel-biodiesel blends. Experiments are conducted in an atmospheric pressure burner with an air-atomized injector and swirling primary air around it to replicate typical features of a gas turbine combustor. Experiments were conducted for fixed air and fuel flow rates, while the airflow split between the injector and the coflow swirler was varied. Results show a significant reduction in emissions as the fraction of total air fed into the atomizer is increased. Blue flames, reminiscent of premixed combustion, and low emissions of nitric oxides and carbon monoxide were observed for all fuel blends. In general, the emissions from biofuel flames were comparable or lower than those from diesel flames.

Abel inversion of deflectometric data: comparison of accuracy and noise propagation of existing techniques

Abel inverse integral to obtain local field distributions from path-integrated measurements in an axisymmetric medium is an ill-posed problem with the integrant diverging at the lower integration limit. Existing methods to evaluate this integral can be broadly categorized as numerical integration techniques, semianalytical techniques, and least-squares whole-curve-fit techniques. In this study, Simpsons 1/3rd rule (a numerical integration technique), one-point and two-point formulas (semianalytical techniques), and the Guass-Hermite product polynomial method (a least-squares whole-curve-fit technique) are compared for accuracy and error propagation in Abel inversion of deflectometric data. For data acquired at equally spaced radial intervals, the deconvolved field can be expressed as a linear combination (weighted sum) of measured data. This approach permits use of the uncertainty analysis principle to compute error propagation by the integration algorithm. Least-squares curve-fit techniques should be avoided because of poor inversion accuracy with large propagation of measurement error. The two-point formula is recommended to achieve high inversion accuracy with minimum error propagation.