59th Annual Report on Research 2014

Energetic nanofuels are fuel dispersions containing metallic or metalloid nanoparticles. It has an important application for enhancing combustion performance. Energetic fuel-particle mixtures have better and promising performance than the pure liquid fuel in terms of higher energy density, faster and easier ignition, enhanced catalytic effect, reduced emission, faster evaporation and burning rate and improved combustion efficiency. The physical properties and dynamic behaviors of nanofluids are influenced by the modified physical chemical properties of fuels by the nanoparticles, the dispersive structure of the nanoparticles, and fairly complicated interplay of heat transfer, fluid flow, and particle interactions. Due to the heterogeneous nature, properties of nanofuels are determined by the type of base fuels and the composition, size, shape, concentration, and physical and chemical properties of the nanoparticles. The characteristics of nanofuels can be dramatically different from those of the base fuels and nanoparticles. Therefore, understanding the basic properties of nanofuels is critical in designing, controlling, and optimizing the relevant combustion process. This work is highly interdisciplinary, and the training for graduate students involves thermal-fluid, particulate, surface and interface, instrumentation, and combustion sciences.

This seed project focuses on investigating evaporation kinetics of sessile and suspending fuel droplets containing energetic nanoparticles. The ultimate goal is to have better control of the stability and evaporation or burning rate of nanofuels to enhance combustion efficiency. Because evaporation or gasification of fuel droplets is a critical step in combustion, quantitative analysis of simultaneous fluid flow, heat and mass transfer, colloidal transport, and the phase change phenomena are essential. The theoretical results are validated by two sets of evaporation experiments, including simultaneous measurement of evaporation kinetics and viscoelastic properties of nanofuel at room to slightly elevated temperature using piezo-based quartz resonator, and evaporation of a suspending nanofuel droplet at room to elevated temperature. In the first experiment, the quartz resonator introduces a high-frequency mechanical perturbation in the ultrasound regime to the samples. The advantages are the great spatial precision (on the order of microns) that can be achieved on detecting the moving liquid-vapor interface and therefore the evaporation flux, and simultaneous measurement of viscous or viscoelastic properties of the particle-in-fuel dispersions.

During this first project year we have applied quartz resonator for the precision measurement of evaporation kinetics of single and binary component nanofuels and developed the theoretical models to interpret the impedance data. The attached figure shows the reduction of resonant frequency or frequency shift versus time recorded by evaporating a pure methanol, 1-butanol, and the methanol-butanol binary mixture. The evaporation kinetics is clearly distinguished from each other. Methanol is more volatile and evaporates faster than the pure butanol and the binary mixture. The measurements are assisted by direct optical visualization and size measurement of the droplet. The periodicity of the spikes is consistent with the predicted acoustic resonance. The simultaneous measurement of viscosity and evaporation kinetics is successful. Specifically, the methanol saturation pressure is fifty times higher than 1-butanol, and thus the signal shows a fast followed by a slow evaporation process for the binary case. The change of viscosity was also traced by the impedance measurement. The evaporation flux can be precisely determined by the resonant frequency spikes. The prediction of overall density and viscosity stratification is also successful. These experimental results have been presented in American Physical Society March meeting. The corresponding theoretical modeling and computational results are being prepared along with experimental data for publication.

In previous combustion experiment we found that the size, material, and concentration of the nanoparticles and the addition of the surfactant stabilizer are critical for the evaporation rate and therefore combustion characteristics of nanofuels. In particular, the droplet burning rate in terms of droplet diameter versus time was higher than that of the pure liquid fuel. The burning rate enhancement is significant, and this is an important discovery because faster evaporation/burning indicates less combustion time and higher combustion efficiency, which are desirable for propulsion systems especially in the high-speed limit because the flow residence time in the combustion chamber is relatively short. However, the underlying physics about this evaporating and burning enhancement are not clear to us. One hypothesis is that the addition of nanoparticles could change the latent heat of evaporation (H_fg) of the fuel, thus making it easier to evaporate. Motivated by this, we performed an experiment to measure H_fg of various nanofuels. The results show that the addition of 3wt.% Ag and Fe nanoparticles in ethanol results in a substantial reduction in H_fg (19% and 13% respectively). On the contrary Al addition slightly increases H_fg (3%). The physics behind these experimental observations, concerning the bonding mechanism in the hybrid system, are being explored right now.

The financial support through this ACS PRF seed grant is very much appreciated and acknowledged in all research disseminations, outreach activities, and in new proposals submitted to National Science Foundation. The grant is very helpful for the PI to land on the pilot study and establish new research direction. Research assistant positions have been provided to graduate students at UCONN and Purdue. Two PhD students are currently funded by this grant. The PIs have provided interdisciplinary training, education, outreach activities and research collaboration during the project year, and introduce the research and training materials to students in the graduate and undergraduate courses.