Abstract View
Heat Transfer in Nanometer Scale Aerosol Particles
Huan Yang, CHRISTOPHER J. HOGAN, University of Minnesota
Abstract Number: 285
Working Group: Aerosol Physics
Abstract
Aerosol particles are usually considered to be in thermal equilibrium with their surroundings. However, subsequent to condensation, coagulation, and evaporation events, particles, particularly those in the 1 nm size range, can be appreciably hotter (condensation, coagulation) or colder (evaporation) than the surrounding gaseous medium because of exchange between potential and thermal energy brought about by these processes. Collisions with surrounding gas, i.e. conduction, typically bring the particle back to thermal equilibrium; however, this re-equilibration process is often either ignored (assumed instantaneous) in models of nanometer scale particle growth, or assumed to occur at a rate described by complete thermal-accommodation collision models. Here, we utilize molecular dynamics simulations in conjunction with a collision rate theoretical framework to examine the heat transfer rate in the free molecular regime to gold and copper nanoclusters in Helium, Hydrogen, Argon, and diatomic nitrogen gas. We show that the traditionally invoked thermal accommodation coefficient, i.e. a coefficient place in front of the net diffuse heat transfer rate to account for non-accommodated collisions, does not follow from collision rate theory, as the incoming energy from gas molecules (during impingement) is exactly calculable and requires no such correction. Instead, we introduce a thermal reemission coefficient, parameterizing the energy carried away by a gas molecule after collision with a particle in comparison to its expected energy based upon a fully-accommodating collision model. Bounds are easily placed on the thermal reemission coefficient by the fact that heat transfer must occur from the hotter to the colder system (e.g. from a hotter particle to a colder gas). We find that the thermal reemission coefficient is rather insensitive to particle size in the investigated size range, but that it is strongly dependent upon the mass of gas atoms/molecules impinging relative to the masses of the atoms in the particle. For the lightest gases examined, the thermal reemission coefficient adopts a value extremely close to the zero-heat transfer limit, suggesting that in light gases, heat transfer in the free molecular regime occurs extremely slowly to nanoclusters and in a system where nanoclusters form and grow, the assumption of instantaneous thermal equilibrium may not be fully justified. We do not find agreement with the fully accommodating model under any conditions where the temperature difference between the gas and particle is large.