S. R. Gollahalli

Combustion characteristics of hydrogen-hydrocarbon hybrid fuels

A comparative study of the flame structure and characteristics of diffusion flames of the mixture of hydrogen–hydrocarbon (natural gas and propane) hybrid fuel in a slow co-flowing stream of air is presented. The volumetric content of natural gas and propane in the mixture was varied from 0–35%. The burner exit Reynolds number was varied from 150–3000. Measurements include flame length, radiative fraction of heat release, pollutant emission indices and in-flame profiles of composition and temperature. Results indicate: the increase of natural gas or propane in the hydrogen–hydrocarbon mixture increases the flame luminosity and flame length, increases the radiative heat loss fraction and soot concentration, decreases both NO and NOx emission indices, increases the CO emission index, and decreases the peak temperature in the near burner, mid flame and far-burner regions. The changes are quantified in the paper.

Characteristics of hydrogen-hydrocarbon composite fuel turbulent jet flames

The characteristics (flame length, pollutant emission, radiative heat loss fraction, and volumetric soot concentration) of hydrogen–hydrocarbon composite fuel turbulent jet diffusion flames are presented. A correlation of flame length with hydrogen concentration in the fuel mixture is shown. The reactivity of fuel mixture increases with the increase of hydrogen concentration, which ultimately shortens the combustion time, and thereby reduces the overall flame length. Convective time scale decreases with the increase of hydrogen content in the mixture. The measured and predicted flame lengths show a similar trend; however, the predicted values are 1.4 times higher than the measured values. Axial soot concentration decreases, the CO emission index decreases, but, NO and NOx emission indices increase at higher hydrogen concentrations in the mixture.

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.

Diffusion Flames of Gas Jets Issued From Circular and Elliptic Nozzles

Abstract An experimental study of the characteristics of the burning and nonburning gas jets issued from elliptic and circular nozzles in a low velocity concentric air stream is presented. Propane mixed with nitrogen was used as the jet fluid. The diameter of the circular nozzle was 9.5 mm and the elliptic nozzle had the same exit area as that of the circular nozzle and an aspect ratio (major axis/minor axis) of 3:1. The liftoff and reattachment behavior, temperature profiles, concentration profiles of fuel, oxygen, carbon dioxide, carbon monoxide, nitric oxide, and soot, flame radiation, opacity of the flame samples, the profiles of the streamwise component of the mean velocity, and r.m.s. value of the velocity fluctuation in the streamwise direction were measured. The elliptic nozzle flames were found to have lower flame stability, higher temperatures, lower soot concentration, and lower flame radiation than those of the circular nozzle flames. The nitric oxide concentration in the mid-flame region was ...

Burner wake stabilized gas jet flames in cross-flow

ABSTRACT An experimental study of the stability and structure of a propane gas jet flame in cross-flow at jet to cross-flow momentum flux ratio less than unity is presented. When this ratio is 0.17 the flame structure is characterized by three distinct zones (Type II flame). A planar recirculation vortex attached to the burner tube in its wake forms the first zone. A second axisymmetric flame follows the first zone downstream. The junction of the two zones is characterized by a third zone with an intense mixing of jet and cross-flow streams. When the jet to cross-flow momentum flux ratio is 0.02 the axisymmetric part of the flame is extinguished (Type I flame). Temperature and composition profiles at different regions of these flames indicate that much of the Type II flame is akin to a premixed flame and the Type I flame is diffusion controlled.

Combustion Characteristics of Interacting Multiple Jets in Cross Flow

Abstract An experimental study of combustion characteristics and structure of interacting 2, 3, and 5 propane jets in a cross flow is presented. The separation distance between the burners was set at 8, 12, and 16 exit diameters. The ratio of jet momentum flux to cross flow momentum flux was varied over a range of 44-390. Flame length, blowout stability, temperature, radiation emission, and concentrations of CO, NO, and O2 are measured. The effects of number of jets, their separation distance, and their geometrical arrangement relative to cross flow, on flame length, radiation and stability characteristics are examined. The changes in the radial profiles of temperature, and concentrations of CO, O2, and NO are studied as functions of separation distance and ratio of jet momentum flux to cross flow momentum flux of 3-jet flames. The range between the upper and lower stability limits of the jet velocity of lifted flames in cross flow is higher for multiple-jet flames than for single-jet flames. A third limi...

An Experimental Study of the Combustion of Unsupported Drops of Residual Oils and Emulsions

Abstract Experimental measurements of the time required for disruption of unsupported residual oil/water and residual oil/methanol emulsion drops passing through hot combustion gases are presented. The effects of varying the weight fraction of internal phase, initial diameter of the drop, size of internal phase drops, initial temperature of the drop, ambient temperature, and relative velocity between the drop and ambient gases on drop disruption time have been investigated.

Experimental studies on the flame structure in the wake of a burning droplet

Droplets burn in envelope flames at low approach velocities and in wake flames at high velocities. Detailed measurements on pentane burning on a porous sphere have shown that the wake portion of the envelope flame is quite different from the wake flame. The former has a near wake of a few diameters, which resembles a gaseous laminar diffusion flame, followed by a long far wake in which soot burns. The latter has a short near wake which is similar to the flame zone behind a flame holder, except for the lack of a recirculation zone, followed by a short zone of soot combustion. The burning rate for the envelope flame is about three times greater than the wake flame at a slightly higher approach velocity. Peak temperatures are about 1400°–1500°K, located near the droplet in the wake flame, but in the far wake in the envelope flame. Radiation from envelope flames is far greater than from wake flames; peak emittances are in the ratio of about 3.5 to 1. The contribution of soot to droplet flame radiation is negligible in the near wake, and approaches gas radiation in the far wake. Peak soot concentrations in the envelope flame are more than three times greater than in the wake flame.

Structure of a split gas flame

Abstract An experimental study of the effect of the burner exit velocity profile on the liftoff and reattachment characteristics of a gas jet flame is presented. With a set of tubes attached to a contoured nozzle, flat and rounded velocity profiles are generated at the burner mouth. With flat profiles flames liftoff at the burner mouth itself, whereas with rounded profiles extinction begins at higher levels and leads to the formation of split flames . The measured temperature, dynamic head, and concentration profiles at the extinction regions are presented in conjunction with schlieren photographs to discuss the effects of flow structure on the flame stability characteristics.

Concentration measurements of CH and OH radicals in laminar biofuel flames.

An investigation to determine the dominant route of NOx formation in biofuel flames and to confirm the relationship of the NOx increase with iodine numbers in fuels has been presented. This is done through the measurement of the concentration of OH and CH radicals, indicators of the formation of NOx through the Zeldovich and Fenimore mechanisms. A laminar partially pre-mixed flame at an initial fuel equivalence ratio (φ) = 7 was used to minimize the effects of fluid mechanics and isolate the effects of fuel chemistry. Three biofuels, with different iodine numbers, were studied: soy methyl ester (SME), canola methyl ester (CME) and methyl stearate (MS). Planar Laser-Induced Fluorescence (PLIF) images of hydroxyl radicals (OH) and CH radicals were captured with a diagnostic system consisting of a pulsed Nd:YAG laser and an Optical Parametric Oscillator (OPO) with frequency doubler option (FDO) using proper wavelengths. It was found that the population of OH radicals was low in the flames of all fuels, but significant CH radical concentrations were detected in all the flames, with the maximum population occurring in the SME fuel flame. The presence of high concentrations of CH measured in the regions of peak NOx indicate that NOx formation is primarily through the Fenimore mechanism, rather than the thermal mechanism, at this fuel-rich condition. Moreover, the fuel with highest iodine number, SME, produced significantly more NOx because of its tendency to facilitate the production of more soot, C, and CH radicals.