Research

Current Research Focus:

Flame Dynamics:

Flame-front Instabilities:

Propagating laminar flames are often subjected to various modes of flame-front instability arising from either intrinsic or external sources. Among the 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 flame propagation speed as compared to that of the smooth flame. Furthermore, the extent of the augmentation could also continuously increase with evolution of the cells over an expanding flame, leading to the possibility of self-acceleration. Mechanistically, hydrodynamic instability sets in through sharp density change across the flame-front and characteristically occurs for either very large or very thin flames, while diffusional-thermal instability 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.

In our study we employ the well-vetted, dual-chamber, constant-pressure apparatus to conduct spark-ignited, expanding spherical flame experiments in enclosed environments of low or elevated pressure to study the evolution of such instabilities. While by increasing pressure and flame temperature we can induce hydrodynamic instability because of reduced flame thickness, we can simultaneously change the mixture composition to manipulate the mixture Le to control the diffusional thermal instability. By scaling analysis, we proposed a unifying mechanism of laminar flame propagation with hydrodynamic instability.

Turbulent Flame Propagation:

As most of the practical applications for power generation, such as gas-turbines, aero-engines and IC engines involve turbulent flames, it is a major topic of research in turbulence an combustion community. In addition to its fundamental relevance, a successful understanding of the individual and coupled physical and chemical factors that result in a turbulent flames and flame speed would assist the prediction of such practically important problems as the net mixture consumption rate, the 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’LI/n, where u’ is the root mean square velocity, LI 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.

Droplet Dynamics:

Drop impact on liquid surface is ubiquitous in many natural and industrial processes. For example, in ink-jet printing the spreading dynamics of the ink droplets deposited on previously landed droplet controls the printing quality. Depending on the printing surface, the viscoelastic properties of the ink and the impacting speed, the achieved printing resolution can significantly deviate from the desired quality. Similarly for internal combustion engines (ICEs), the fuel droplets impacting the previously deposited fuel layer on hot cylinder and piston surfaces result in vaporization patterns that can be significantly different from droplets vaporizing away from these hot surfaces.

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 this (non-monotonic) transitions 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.