We are pursuing research that cuts across a variety of disciplines, including heat transfer, energy systems, fluid mechanics and materials. Problems tend to be pursued from an experimental (as contrasted with an entirely computational or theoretical) approach. Recent projects include studies of droplet combustion; liquid jet impingement with noncircular hydraulic jumps; micro-scale boiling on impulsively heated thin metal films; photothermal deflection spectroscopy to measure thermal properties of material systems including bulk, thin film and superlattice structures; and film boiling with chemical reaction.

Droplet Combustion: Our research on droplet combustion has been directed toward examining the structure of soot formed during combustion and how it is influenced by parameters such as ambient pressure and gas composition (e.g,. mixtures of oxygen, nitrogen and helium), gravity (normal to low gravity) and liquid composition. An emphasis is on the spherical droplet flame in which the droplet and flame are spherical and concentric, and soot aggregates are trapped in a shell structure. The transport process for this situation is "simple" and one-dimensional which is especially attractive to model. Yet, it also has a practical relevance through the limit condition that the spherical flame configuration provides to sub-models that account for the influence of convection. We have provided data and new insights for this burning situation that includes the role of pressure (up to 35 atm) and composition for such fuels as jet fuels (JP8), nonane, hexanol, and commercial grade thermal stability additives (i.e., the widely used JP8+100 in commercial and military transport systems).

Liquid jet impingement with noncircular hydraulic jumps: A liquid jet impinging onto a solid surface is an efficient means to cool and /or coat the surface. Performance is limited by formation of a "hydraulic jump". Under circumstances that are not well understood, the jump shape transitions from the familiar circular shape to a non-circular structure with a complex flow pattern into and downstream of the jump. We are examining the influence of gravity on the jump shape and position. For circular jumps, lowering gravity simply increases the jump radius while preserving the circular shape. For noncircular jumps gravity may influence both the position and shape of the jump which we are in the process of examining in a low gravity facility in our laboratory.

Microboiling:When a solid surface is immersed in a liquid and heated at rates of several hundred million degrees per second, bubble nucleation can occur which may provide a useful mechanical impulse to the liquid that has been harnessed to push or pump liquids on the microscale. Indeed, thermal ink-jet printing technology is based on this process, for example, where the heater itself is of micrometer dimensions and the heating pulse duration is several microseconds. The small length and time scales of this process challenge the metrology for making quantitative measurements. We have recently developed a new visualization method to capture the evolution of bubbles formed from microsecond pulses on microscale thin film heaters (dimensions of several micrometers wide) immersed in various liquids as related to ink jet printing and microscale pumping with a time resolution of nanoseconds and effective framing rates of several hundred million degrees per second. Our current studies are examining the influence of controlling surface wetting through self-assembled monolayers.

Photothermal Deflection Spectroscopy: We are developing a method to measure the thermal diffusivity of material systems, including bulk, thin film and superlattice structures, using a laser-based technique which relies on detecting the deflection of a "probe" laser beam, directed along the material surface, due to the change of the index of refraction of the gas (air) immediately adjacent to the region around which a "heating" laser beam impacts the material. A theoretical model for heat flow in the material is then coupled with an algorithm for matching measured and predicted probe beam deflections using variables of the model (e.g., thermal properties) as 'fitting' parameters. The project involves a combination of theory, experiment, and model-based estimation methods.

Film Boiling with Chemical Reaction: A new platform, based on film boiling, is being developed for chemical processing by catalytic decomposition of organic liquids. The project involves developing a model for conversion of pure methanol to hydrogen and carbon monoxide based on film boiling on the outside of a heated tube immersed in an organic liquid, and constructing an apparatus to illustrate the operational and performance characteristics of the reactor.