(688e) Heat Transfer Measurements and Modeling in Nano-Aluminum Combustion

The mechanism of nano-aluminum combustion remains a topic of active debate.  Classical combustion theory suggests that as the particle diameter approaches the nanoscale, gas-phase diffusion of oxidizer to the combustion front will no longer be the limiting process.   Surface reaction or diffusion through a growing oxide barrier have been proposed as possible limiting processes.  If a surface process limits the combustion rate, then it is often assumed that the concentration of the gas-phase oxidizer species at the particle surface rises to the freestream value.  With the removal of the concentration gradient, the Reynolds analogy suggests that the thermal gradient will also be negligible, due to extremely rapid heat loss from the particles.   Under these conditions, the particle temperature is not expected to rise above the ambient temperature significantly.

However, recent measurements by our group and others suggest that the particles do indeed exceed the ambient temperature under some conditions.  We find that the degree of overshoot of the temperature above ambient is a sensitive indicator of the thermochemical processes in the vicinity of the particle.  By monitoring both the particle temperature with pyrometery and the burning time with photometry in our heterogeneous shock tube, we generate observables that are very sensitive to the nature of the nano-aluminum combustion model.

For this work, we investigate particles of 18  to 120 nm in size at ambient temperatures of 1500 and 2000 K, in oxygen, carbon dioxide, and water vapor oxidizers.   A very simple spherically symmetric model is used to interpret the results.  The model assumes that some surface process limits the combustion rate, and uses a fully non-continuum approach to model the heat transfer by conduction and radiation.  The two adjustable parameters in the model are a time-averaged oxidizer reaction probability and a time-averaged energy accommodation coefficient (EAC).  The former constant directly gives the burning time, and the combination of the two constants provides the combustion temperature as a function of time.  

The primary conclusion from this study is that temperature and burning times can only be consistent if the EAC of the particle at elevated temperature is of the order of 0.005, which is at least two orders of magnitude smaller than most studies assume.   Values of the EAC in this range for burning metal particles were predicted by Igor Altman, and this work strongly supports his hypothesis.  Time averaged sticking probabilities are found to be of the order of 0.001 to 0.002.