Flame-Fluids-Energy Lab

Saha Research Group

Recent Research Focus

Flame Dynamics

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.

Panel A: Stable laminar flame at 1atm with diffusionally neutral (Le=1) mixture. 

Panel B: Darrieus-Landau hydrodynamic cellular instability in diffusionally neutral (Le=1) mixture at high pressure (5atm).

Panel C: Diffusional-thermal cellular instability in Le < 1 flame at 1 atm. 

Turbulent Flame Propagation:

As most of the practical applications of combustion for power generation (land-based gas-turbines), aero-propulsion (aero-engines, rocket engines) and transportation (IC engines) involve turbulent flames, it is a major topic of research in both turbulence and combustion communities. In addition to its fundamental relevance, a successful understanding of the individual and coupled physical and chemical factors that control the propagation and structure of the turbulent flames would also assist the prediction of practically important factors such as, net mixture (fuel) consumption rate, heat release rate, as well as the mechanisms and limits of flame stabilization.

Our research focuses on two aspects of turbulent flames. In the first aspect we aim to understand the underlying physical phenomenon and unify propagation speed of turbulent flames for a wide range of turbulence intensities, mixture conditions and pressures etc. Based on a theoretical model based on G-equation a unified scaling has been proposed, which are being validated for large range of conditions. Of particular interest to this research is the recognition that experimental measurement of turbulent flame speeds needs to span over conditions affected by flame-front instabilities (Darrieus-Landau and Diffusional-Thermal) because even when these instabilities are ostensibly passive, their effects remain omnipresent through large thermal expansion and Lewis numbers, respectively. In this regard we investigate propagation speeds of expanding turbulent flames at high pressure (5-20 atm) and high turbulent Reynolds number (up to 10,000), defined as ReT=u’L_I/n, where u’ is the root mean square velocity, L_I the integral length scale and n the kinematic viscosity, all measured on the unburned side.

The other aspect of research aims to understand the role of molecular diffusion on turbulent flames. On contrary to common belief that the role of molecular diffusion (and Lewis number) diminishes at high turbulent Reynolds number, our studies show strong Le effects on flame propagation even at ReT~10k.

High speed Schileren video of expanding turbulent flame

Normalized flame speed data for nC4-nC8 at different pressure

Droplet Dynamics

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. 

Drop bouncing on a thin liquid film

Top: Side view image; Middle: Bottom view color interferometry; Bottom: Measured gas layer thickness between drop and the liquid film

Mixing and penetration of Drop Impacted on Liquid Film

The post-merging motion of a drop impacted on liquid film is critical for many natural and industrial processes, including IC engines, where sprayed fuel drops often collide with the fuel film deposited on engine and piston walls; and rainfall, when rain drops impact the ocean surface. Since the impacting drop can be of different temperature and contain liquids with different chemical species than the impacted film, it is the motion of the merged drop and the associated mixing which initiates many post-merging thermal and chemical processes. 

In this project we experimentally study on the morphological evolution of a penetrating drop inside the liquid pool after coalescence, using high speed laser induced fluorescence technique. The drop is found to form structures as it penetrates the liquid pool/film. The penetration dynamics ans well as the structure formation is highly affected by the impact inertia, surface tension, viscous loss and film thickness.

Sequence of images depicting the penetration process of a water drop impacted on a deep pool

Drop mixing in a deep pool

Drop mixing in a shallow pool