In this study, the interaction of radiation with particle clouds is investigated numerically. Here, a particle cloud is defined as a specified volume filled with polydisperse particles ranging from 10 to 100 µm. As calculating the radiative properties (absorption and scattering) of all individual particles in such a cloud is computationally too intensive for most use cases, representative radiative properties of the cloud are calculated. Typical parameters to describe a particle cloud are the particle size distribution and particle number density. Within the standard approach, the particle size distribution is used to determine the radiative properties of one representative particle, e.g., a mean diameter. The radiative properties for this single representative particle are then linearly scaled by the particle number density to calculate the particle cloud's radiative properties. Thereby, the precise particle-to-particle radiation interaction within the cloud is neglected, and a black box model of the particle cloud is applied.
A recent study based on a numerical investigation with resolved monodisperse particles has shown that this approach only leads to the same radiative properties of the cloud if the particles are distributed perfectly uniform within the particle cloud. Deviations arise when the particles slightly diverge from a perfectly uniform distribution.
In this study, besides the influence of a non-uniform particle distribution, the influence of polydisperse particle groups is investigated. Since different particle size distributions can result in the same mean diameter, different particle size distributions with the same mean diameter are generated using Gaussian distributions with varying standard deviations. Subsequently, the radiation interaction with fully resolved polydisperse particle clouds is calculated. Then the results are compared to fully resolved monodisperse particle clouds and the standard black box approach. Due to the increased number of small particles in the polydisperse approach compared to the monodisperse approach, the probability of radiation to interact with particles increases. Thus, the discrepancy between the black box approach and the polydisperse approach is 22 − 32 %, while the discrepancy between the black box approach and the monodisperse approach is ~37 %.