(584a) Efficient Fuel Atomization Via Spontaneous Wave Swirling

An atomizer is common-place hardware used to convert a contiguous liquid into a spray. Such devices are seen in fuel injectors, assisting in the generation of fuel surface area in the presence of air in an engine. Effective energy utilization requires high surface area generation via fine droplets. Within a twin-fluid atomization process, a high-speed co-flowing gas is used to disperse the fuel. More traditional designs utilize a central pipe of liquid met with an annular injection of gas which then excites interfacial instabilities. However, an alternative configuration involves a central flow of hot steam fed with an annular injection of co-axially flowing cold non-Newtonian fuel slurry. Using this inverted design, it has been found that, for a particular novel core-disrupting forced-interaction atomizer, a periodic high-blockage ratio wave forms within the devices’ nozzles in the presence of shear-thinning fuel. During this process, the fuel is both heated and sheared. The interaction between the cold fuel and the hot steam initiates a cycle of blockage and release as the flow of the steam is impacted by the deformation of the fuel. Three mechanisms were identified: wave impact momentum, pressure buildup, and droplet breakaway. The pulsing cycle that accompanies these mechanisms takes place at a frequency of 1000 Hz, defined by successive stretching, bulging, and in culmination, rupturing. Traditional Kelvin-Helmholtz instabilities (KHI) are present within the cycle amplifying the streamwise disturbance, and Rayleigh–Tayler instabilities (RTI) are present providing amplification to instabilities orthogonal to the primary flow direction. Due to the nature of the pulsing waveforms, the process is referred to as Wave-Augmented Varicose Explosions (WAVE). Despite these studies, there has been no prior discovery of the existence of, or the mechanisms which produce, spontaneous axis-switching-swirling (SASS) motion on the pulsing/heating non-Newtonian fuel wave surface. The various causes and manifestations of SASS will be explored herein with the support of a previously validated simulation tool.

As the combination of fuel and steam exits the atomizer, a kind of “hoop stresses” forms over the body of the annular fuel which acts like a stiff (relative to the gas) cylinder (Figure 1a). Imperfections due to axial disturbances (KHI) and azimuthal disturbances (RTI) along the inner surface of the fluid then result in a creasing effect in response to the imposed stresses (Figure 1b). These creases then extend into the flow of steam. These creases can be thought of as waves also, and these new waves are orthogonal to the main pulsing fuel wave moving through the atomizer. KHI and RTI then begin to develop on both sides of the newly emerged orthogonal waves just as they did with the primary fuel wave. The instabilities gradually begin to pull fuel from the orthogonal waves into small fin like protrusions which reach into the central flow of steam. These protrusions will continue to increase in size as well as exhibit dynamics reflective of the state of the instabilities. As time progresses, the fuel protrusions begin to experience a reduction in tangential velocity as they bend about an axis that is aligned with the central nozzle axis. The velocity will eventually pass through a zero point and begin to accelerate in the opposite direction of the original rotation. This process will then be repeated, resulting in the progressive flapping of the protrusions with each change in rotational direction (Figure 1c). The alternating motion of the fully developed protrusions will also increase in both frequency and amplitude as time progresses (Figure 1d). Further, the height at which the protrusions extend into the steam similarly increases (Figure 1e). There being multiple creases and with this being the inside of the cylinder, the result is a bulk alternating tangential velocity of the edges of the cylinder. Those images represent simplified conceptual views, but Figure 1f depicts a sample snapshot strain rate contour plot from the CFD model showing the developed creases with the SASS motion beginning to develop. Finally, Figure 1g illustrates another sample image of a fuel volume with a quadrant removed for easy liquid shape assessment. It is obvious that waves produce creases, and these creases will develop into tangential motions to be outlined in the full presentation.