Preferential Concentration Of Droplets Research Articles

Effects of eddy shocklets on the segregation and evaporation of droplets in highly compressible shear layers

Numerical studies of droplet-laden spatially developing shear layers are conducted with a high convective Mach number (Mc = 1.0), in which a high-order hybrid weighted essentially nonoscillatory scheme is used for the turbulence as well as shock capturing. The evaporating droplets are tracked as point mass in the Lagrangian manner, and the two-way coupling between the flow and droplets is considered by adding the source terms to the governing equations of the gas-phase. This research focuses on the preferential concentration of droplets and the interactions between droplets and eddy shocklets in the shear layers with high flow compressibility and analyzes the underlying mechanisms of momentum and thermal response behaviors of droplets to eddy shocklets. The segregation of droplets exhibits the strongest spatial preference in the highly compressible shearing vortices, and droplets tend to accumulate as stripes behind the shocklets, associated with the coherent structures. The high flow compressibility results in the strong spatiotemporal variations of pressure and temperature, and the distributions of the expansion zones with low temperature and the compression zones with high temperature occur alternately in the shear layer. The relaxation response behaviors of the droplets to the change of momentum and thermal features of the surrounding carrier gas result in the delay of droplet evaporation in the high temperature region and then the enhancement of droplet-vapors in the low temperature region. The present observations can be ascribed to the interactions between the dispersed droplets and the distributed eddy shocklets in the shear flows with high compressibility.

Evolution of flame kernel in one eddy turnover of high-speed droplet laden shear layers

A high-speed droplet-laden reacting shear layer is modeled numerically to analyze the dynamics of flame kernel for better understanding ignition and extinction processes in spray combustion. The droplet-laden shear layer, separating a hot-air stream from a monodisperse spray-laden air stream, is modeled by the Eulerian-Lagrangian approach, in which the continuous phase is governed by the compressible Navier-Stokes equations together with species transport equations and the discrete droplets are tracked by the Lagrangian method. The convective flux terms of the conservative equations are solved by an adaptive central-upwind WENO scheme. Two-way coupling interactions consider exchanges of mass, momentum, and energy between the carrier-gas fluid and the liquid-fuel spray. The shear layer convective Mach number is specified as 0.4 in the present study, and hence, large scales of motion dominate both the fluid mixing and the droplet dispersion. The preferential concentration of droplets is observed. Auto-ignition kernels occur in the high-strain regions where sufficient fuel vapors distribute near the hot stream-boundary. The incipient chemical reaction consumes the available fuel vapors through a diffusion flame combined with a thin premixed flame, as a lean deflagration propagates across the mixing layer towards the spray. The present results unveil two different types of extinction behavior depending on the local strain rate and the available fuel for the reaction kernel. The latter type of extinction originates when the separated flame kernel is entrained into the core of an adjacent vortex but droplets cannot be entrained in the eddies to produce gaseous fuels for the chemical reaction due to the preferential segregation effect. The dynamics of ignition and extinction in the shear flows depend on the droplet dispersion, the droplet evaporation rate and the diffusion of fuel vapor.

Modulation of spray droplet number density and size distribution by an acoustic field

Multiple interactions may occur when a poly-disperse spray is exposed to an acoustic field. In the context of spray combustion instabilities, acoustic agglomeration, the formation of a droplet number density wave and the modulation of the droplet size distribution are interesting effects. A droplet number density wave, i.e. preferential concentration of droplets in space, may result from size-dependent, one-way momentum coupling between the acoustic field and the spray. The modulation of the droplet size distribution, which has been evidenced in the experimental work of Gurubaran and Sujith (AIAA 2008-1046), is thus a consequence of the droplet number density wave formation. In the present work, the mechanisms that produce these effects are simulated and analyzed in depth by means of computational fluid dynamics. The spray is modeled with both Lagrangian (particles mass-point approach) and Eulerian (continuous phase approach) descriptions. The particular Eulerian method used is a variant of the presumed density function method of moments, which allows to account for the effects of poly-dispersity, in particular the size-dependence of particle velocity. Both the Lagrangian and Eulerian models are validated against experimental data for spray dynamics and spray response to an acoustic field.

On preferential diffusion of binary component liquid droplets evaporating in a two-phase mixing layer

Numerical simulations of a two-phase binary component droplet laden temporally developing mixing layer are conducted in order to investigate the influence of preferential diffusion of liquid species. A multicomponent evaporation model based on classical rapid mixing vaporization and Raoult’s law is applied to the Lagrangian description for individual droplets. Results are first presented for single isolated binary component droplets under the same conditions used in the final mixing layer simulations in order to illustrate the models behavior and ability to account for preferential vaporization of the more volatile species. Single droplet simulations are performed for a variety of binary component species pairs having varying properties; including heptane, decane, hexane, hexane–trichloroacetane (TCA), and water. The results illustrate the importance of the latent heat of vaporization, in addition to the traditionally cited boiling temperature, in determining the correct relative volatilities of the liquid species. The results illustrate that species can exhibit strong preferential diffusion effects even with equal boiling temperatures when their latent heats vary substantially. A Lagrangian droplet model is then coupled with the compressible form of the continuity, momentum, energy and species transport equations governing the carrier gas phase. High resolution simulations are conducted of a two-dimensional temporally developing mixing layer with one stream laden with binary component evaporating droplets. Preferential vaporization is found to significantly affect the evaporated species concentrations and distributions within the mixing layer. Evaporative flow saturation is observed, wherein the laden stream becomes saturated before the evaporation is complete. Resulting species concentration distributions within the mixing layer are determined by the coupled effects of preferential vaporization of the more volatile species, and by preferential concentration of droplets within the flow.

Digital-Imaging Measurement of Diesel Fuel Spray Using Double-Pulsed Laser-Induced Fluorescence. 2nd Report. PIV Measurement of Internal Structure of Diesel Fuel Spray.

This paper reports PIV measurement of diesel fuel sprays injected from a single-hole nozzle at high-injection pressure comparable to operating conditions of commercial diesel engines. The injection pressure ranges from 30-70 MPa. The fuel is injected into a non-combusting atmosphere, which is charged up to 2.0 MPa. A laser-induced fluorescent technique is employed to visualize internal structures of fuel sprays formed by densely distributing droplets. A direct cross-correlation PIV technique is applied to the spray images in order to extract droplet velocity information. Unique large-scale structures, called 'branch-like structures', are observed and shown to be associated with active vortical motions of droplets, which appear to be responsible for the mixing between droplets and surrounding gas. Preferential concentration of droplets is found to occur by the centrifugal force exerted by the vortical motions.