Abstract
This paper shows the application of T-shaped micromixers for the generation of aerosols with nanoscale droplets by the mixing of a hot vapor–gas mixture with a cold gas. The fast mixing within a T-shaped micromixer leads to a high supersaturation of the vapor and therefore to an instantaneous, homogeneous nucleation and particle growth. Different mixer geometries, mixing ratios, and gas temperatures have been investigated by numerical simulation to yield optimum mixing results over a wide range of operational parameters. Optimized microreactor geometries were designed and fabricated in silicon with Pyrex glass lids. Special attention was paid to thermal insulation and particle deposition at the channel walls. This concerns not only the mixing chip, but also the design of the fluidic mount with only few bends and corners. Initial experimental results for particle deposition and aerosol generation are presented. High temporal temperature gradients up to 106 K/s lead to a rapid condensation and forming of nanosized particles with a mean diameter of 20–50 nm and a narrow size distribution.
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Abbreviations
- A M :
-
cross section area of mixing channel (m2)
- C :
-
friction correction factor (–)
- d h :
-
hydraulic diameter (m)
- d p :
-
particle diameter (m)
- h :
-
specific enthalpy (J/kg)
- L :
-
channel length (m)
- L m :
-
characteristic mixing length (m)
- L m90 :
-
mixing length 90% mixed (m)
- P :
-
momentum density (kg/m2 s)
- Re :
-
Reynolds number (–)
- St :
-
Stokes number (–)
- T :
-
temperature (K or °C)
- ΔT L :
-
spatial temperature gradient (K/m)
- ΔT τ :
-
temporal temperature gradient (K/s)
- u :
-
(fluid) velocity (m/s)
- \({\bar{u}}\) :
-
mean (fluid) velocity (m/s)
- \({\dot{V}}\) :
-
volume flow (m3/s)
- x :
-
spatial coordinate along channel (m)
- Y :
-
mass fraction of vapor in gas (kg/kg)
- \({\alpha _{\dot{V}}}\) :
-
mixing quality (–)
- η:
-
dynamic viscosity (kg/s m)
- λ:
-
heat conductivity (W/m2 K)
- ρ:
-
density (kg/m3)
- ρP :
-
particle density (kg/m3)
- \({\sigma _{\dot{V}\max}(T)}\) :
-
maximal standard deviation of temperature over volume flow (K)
- τ m :
-
characteristic mixing time (s)
- τ m90 :
-
mixing time 90% mixed (s)
- φ:
-
relative saturation (φ = 1 means saturated state) (–)
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Acknowledgments
We gratefully acknowledge financial support from the Federal State of Baden-Württemberg under the research project “Microtechnology-Supported Integrated Processes‘’. We are very grateful to the group of Prof. Nirschl with Mr. Wengeler and the group of Prof. Kasper with Mr. Heim for their assistance and support in experimental investigations.
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Kockmann, N., Dreher, S., Engler, M. et al. Simulation and characterization of microreactors for aerosol generation. Microfluid Nanofluid 3, 581–589 (2007). https://doi.org/10.1007/s10404-007-0149-4
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DOI: https://doi.org/10.1007/s10404-007-0149-4