Simulation of aerosol nucleation and growth in a turbulent mixing layer

Abstract

A large-scale simulation of aerosol nucleation and growth in a turbulent mixing layer is performed and analyzed with the aim of elucidating the key processes involved. A cold gaseous stream is mixed with a hot stream of vapor, nanometer sized droplets nucleate as the vapor becomes supersaturated, and subsequently grow as more vapor condenses on their surface. All length and time scales of fluid motion and mixing are resolved and the quadrature method of moments is used to describe the dynamics of the condensing, non-inertial droplets. The results show that a region of high nucleation rate is located near the cold, dry stream, while particles undergo intense growth via condensation on the hot, humid vapor side. Supersaturation and residence times are such that number densities are low and neither coagulation nor vapor scavenging due to condensation are significant. The difference in Schmidt numbers of aerosol particles (approximated as infinity) and temperature and vapor (near unity) causes a drift of the aerosol particles in scalar space and contributes to a large scatter in the conditional statistics of aerosol quantities. The spatial distribution of the aerosol reveals high volume fraction on the hot side of the mixing layer. This distribution is due to drift against the mean and is related to turbulent mixing, which displaces particles from the nucleation region (cold side) into the growth region (hot side). Such a mechanism is absent in laminar flows and is a distinct feature of turbulent condensing aerosols.

An LES-PBE-PDF approach for modeling particle formation in turbulent reacting flows

“…The flow configuration is the same as the one that was recently investigated by Zhou et al [73]. In the context of a flow DNS, these authors specifically considered a moment transformation of the PBE and attained closure by invoking the quadrature method of moments (QMOM).…”

“…In the context of a flow DNS, these authors specifically considered a moment transformation of the PBE and attained closure by invoking the quadrature method of moments (QMOM). Contrary to Zhou et al [73], we confine the attention to the downstream part of the flow domain in which turbulence is fully developed, retrieving inflow conditions from the DNS database. Although the DNS-QMOM results of Zhou et al [73] serve as a reference for comparison with our LES-PBE-PDF predictions below, some reservations as to the significance of this comparison remain, in particular, owing to the QMOM approximation in the DNS and the choice of inflow boundary conditions on part of the LES.…”

“…Contrary to Zhou et al [73], we confine the attention to the downstream part of the flow domain in which turbulence is fully developed, retrieving inflow conditions from the DNS database. Although the DNS-QMOM results of Zhou et al [73] serve as a reference for comparison with our LES-PBE-PDF predictions below, some reservations as to the significance of this comparison remain, in particular, owing to the QMOM approximation in the DNS and the choice of inflow boundary conditions on part of the LES. However, it is emphasized that our primary objective is less to provide a comprehensive validation of the LES-PBE-PDF approach (including the physical closure schemes on micromixing and turbulent transport), but rather to demonstrate the effectiveness of the LES-PBE-PDF method, to indicate its predictive capabilities and to show that our numerical solution scheme based on the method of Eulerian stochastic fields and an explicit adaptive grid discretization in droplet size space is computationally efficient.…”

“…Considering an LES-PBE-PDF Figure 2: Schematic illustration of a spatially developing mixing layer with DBP droplet condensation. Here,û 1 (x 2 ) andT (x 2 ) indicate the mean inflow profiles of axial velocity and temperature, respectively, which Zhou et al [73] applied in the context of a DNS of the present mixing layer. The mean DBP mass fraction inflow profile is similar in shape toT (x 2 ).…”

“…In place of an experimental test case, we selected the DNS-QMOM investigation by Zhou et al [73] as reference mainly since it allowed for the adoption of the same kinetic expressions for the droplet nucleation and growth rates in the LES-PBE-PDF model as Zhou et al [73] had used for the DNS-QMOM analysis. In this way, any uncertainties regarding the droplet formation kinetics could be eliminated.…”