Flame-Fluids-Energy Lab

Flame-front Instabilities in Laminar Flames:

Propagating laminar flames are often subjected to various modes of flame-front instability arising from either intrinsic or external sources. Among these intrinsic instabilities, two of the most common modes are hydrodynamic and diffusional-thermal in nature, which induce cellular structure on the flame-front. The presence of these cells increases the flame surface area and as such could significantly augment the propagation speed as compared to that of the smooth flame. Furthermore, through the continuous evolution of these cells, the flame can achieve a state of "self-acceleration". Mechanistically, hydrodynamic instability, also known as Darriues-Landau Instability, sets in through sharp density change across the flame-front and characteristically occurs for either very large or very thin flames. The diffusional-thermal instability, on the other hand, is controlled by the imbalance in the diffusivities of heat and the various species. The imbalance is usually characterized by the Lewis number, Le, defined as the ratio of the thermal diffusivity of the mixture to its controlling mass diffusivity, such that mixtures with Le < 1 favor the onset of the instability, while Le > 1 mixtures are stabilizing.

Bouncing to Merging Transition in Drop Impact

Drop impact on a liquid surface is ubiquitous in many industrial applications spanning from spray coating, inkjet printing, to internal combustion (IC) engines, in which the impact outcome plays a critical role in affecting the subsequent processes and the performance. For example, in thermal spray coating, the efficient merging of the drop with the pre-deposited film is highly desired to ensure a uniform and controlled coating and to minimize loss of the coating material. On the other hand, in IC engines, the fuel is sprayed into the combustion chamber and is expected to burn out inside the chamber. Inevitably, however, some fuel drops, especially the larger ones and/or those within the spray core, can survive the hot environment and make their way to deposit on the engine wall. This is highly undesirable as it leads to pollutant generation because of the lower temperature at the wall. The prevention of the subsequent accumulation of fuel drops on the wetted engine wall is critical for engine operation and can be facilitated by drop bouncing. Thus, it is important to recognize the conditions under which bouncing or merging occurs and to understand the transition criteria for optimized performance. In addition, the liquid properties for the coating materials, printing materials, or fuel can exhibit large variations in the collision response, including its density, surface tension, and viscosity, which should also be accounted for. 

The impact process may not always result in immediate merging of the drop and the liquid surface. In fact, under favorable condition the drop can bounce from the impacted surface. The bouncing to merging transition not only depends on liquid properties and impact speed, but thickness of the impacted liquid pool also plays a critical role. We investigate the non-monotonic transitions in behavior and the underlying physics through experiments and simulations. Apart from analyzing the global behavior of the drop and the liquid pool during the impact process, we also measure the micron-scale interfacial gas-layer profiles trapped between the drop and the liquid surface during the impact process. More recently, our group has also begun experiments to study the role temperature differences will play between the impacting droplet and liquid film.