James Gounder

Turbulent Spray Flames of Acetone and Ethanol Approaching Extinction

This paper presents measurements of mean temperature, axial velocity, turbulence, and droplets fields in pilot-stabilized jet flames of dilute sprays where acetone or ethanol is used as liquid fuel. Laser-induced fluorescence (LIF) is employed to image hydroxyl radical OH which marks the existence of hot regions or reaction zones. Depending on the fuel used, droplets within the flow are delineated by imaging LIF from acetone or Mie scattering. As the jet velocity is increased, the flames gradually approach blow-off in a region further downstream from the stabilizing pilot. It is found that the mean velocity, turbulence, and droplet fields are somewhat similar for both ethanol and acetone flames, and these do not change much with increasing jet velocity, particularly when normalized with the relevant parameters. However, the temperature and reactive fields are varying and undergo departure indicative of nonpremixed to premixed flame behavior depending on the vapor pressure of the fuel and proximity to blow-off. Broad regions of OH as well as breaks in the OH profile marking possible local extinction are observed in the ethanol flames.

Details and Complexities of Boundary Conditions in Turbulent Piloted Dilute Spray Jets and Flames

A good understanding of the details and sensitivities to boundary conditions is essential for the development of reliable predictive tools for turbulent spray flows. This chapter presents a comprehensive account of the boundary conditions relevant to dilute spray jets issuing in a co-flowing stream of air. The flames are stabilized with an annular pilot to prevent lift-off. Radial profiles of the velocity and turbulence fields as well as droplets size distributions and number densities are presented at the jet exit plane of a range of non-reacting and reacting jets of acetone and ethanol fuels. A non-reacting spray jet of mineral turpentine has been used as a base case to validate the liquid flux measurements. Laser-Phase Doppler Velocimetry (LDV/PDA) is employed for the measurements of velocity and droplet fluxes while Mie scattering is used for the high-speed laser imaging of droplets at the jet exit plane. The probability density function of the droplet size distribution measured at the exit plane is shown to be best represented by the Nukiyama-Tanisawa fit. Droplets less than 10 μm in diameter are shown to be adequate representatives of the gaseous flow. Larger droplets tend to exit the nozzle with mean axial velocities lower than those of the gas (and hence negative slip velocities). The rms fluctuations of the axial velocities of large droplets are found to be higher than those of small droplets. High-speed imaging of Mie scattering from sprays at the jet exit plane reveal that this anomaly is due to droplet shedding from the liquid film that develops on the inner wall of the pipe.

From Dilute to Dense Turbulent Sprays: Combustion, Auto-Ignition and Atomization

In line with the general theme of the International Workshop series on Turbulent Combustion of Sprays (TCS), this Chapter addresses relevant issues of turbulent flame structure, auto-ignition and atomization with reference to well-characterized burners that could be implemented by modelers with relative ease. The discussion of turbulent combustion is limited to dilute sprays stabilized on a simple piloted burner. Attention is shifted to the structure of the reaction zones and the challenges of computing chemical composition of flames of different fuels. Another section is dedicated to studying auto-ignition of turbulent dilute spray flames as observed in a hot vitiated co-flow. A common feature to all liquid fuels studied here is the presence of ignition kernels which grow (and sometimes extinguish) to induce flaming combustion further downstream. It is noted that this downstream region is responsible for the bulk of heat release and its local compositional structure depends on the parent fuel.