Growth Mechanisms of Nanostructured Titania in Turbulent Reacting Flows

Abstract

Titanium dioxide (titania) is used in chemical sensors, pigments, and paints and holds promise as an antimicrobial agent. This is due to its photoinduced activity and, in nanostructured form, its high specific surface area. Particle size and surface area result from the interplay of fluid, chemical, and thermal dynamics as well as nucleation, condensation and coagulation. After nucleation, condensation, and coagulation are the dominant phenomena affecting the particle size distribution. Manufacture of nanostructured titania via gas-phase synthesis often occurs under turbulent flow conditions. This study examines the competition between coagulation and condensation in the growth of nanostructured titania. Direct numerical simulation is utilized in simulating the hydrolysis of titanium tetrachloride to produce titania in a turbulent, planar jet. The fluid, chemical, and particle fields are resolved as a function of space and time. As a result, knowledge of titania is available as a function of space, time, and phase (vapor or particle), facilitating the analysis of the particle dynamics by mechanism. Results show that in the proximal region of the jet nucleation and condensation are the dominant mechanisms. However once the jet potential core collapses and turbulent mixing begins, coagulation is the dominant mechanism. The data also shows that the coagulation growth-rate is as much as twice the condensation growth-rate.

A coupled CFD-Monte Carlo method for simulating complex aerosol dynamics in turbulent flows

“…Comparing the results obtained with the present modified LMC-PDF method and PBSM (Hounslow et al 1988), an excellent agreement can be observed, which validates the reliability and computational efficiency of this modified LMC-PDF method. The wider PSD with increasing Re j number is also in accordance with the previous research studies by Reade and Collins (2000) and Garrick (2015), which reveals the effect of turbulence on PSD. By increasing Re j numbers, turbulence induced mixing is greatly enhanced, which leads to an enhanced coagulation process.…”

“…However, as the Re j number further increases from 4800 to 12,800, the normalized particle number density also increases when compared with Re j number of 4800 at the same axial position. These results indicate that coagulation is weakened while nucleation is enhanced when Re j number increases from 4800 to 12,800, which is consistent with the findings in Garrick (2015) that too high injection velocity causes the delay of turbulent mixing and in turn reduces coagulation rate as mentioned above.…”

“…However, the particle number density at the peak is so high that coagulation process takes place, which reduces the particle number density to a basically dynamic equilibrium between the increasing factor (nucleation) and decreasing factor (coagulation) of particle number density. Although coagulation is not the dominant process in the four studied cases, it is shown in Figure 14 that once the Re j number is larger than 4800, the normalized particle number density increases due to the delay of turbulent mixing at high injection velocity (Garrick 2015). As for the case with Re j number of 12,800, the low particle number density near the outlet of the aerosol reactor may be caused by the dead zone between the second and the third partitions of the aerosol reactor where those particles are trapped.…”

“…With Re j number increases from 4800 to 12,800, a transition from coagulation-dominant mechanism to competitive co-existence of coagulation and nucleation can be reasonably concluded from Figure 6. According to Garrick (2015), nucleation is the dominant process until the collapse of the jet potential core and turbulent mixing begins in the proximal region of the jet, where coagulation becomes the dominant process and leads to a broader PSD. Although increasing Re j number may lead to stronger turbulence in order to enhance coagulation, high velocity of jet also causes the delay of turbulent mixing in which nucleation becomes dominant in the proximal region of the jet.…”

Decahedral-shaped anatase titania photocatalyst particles: Synthesis in a newly developed coaxial-flow gas-phase reactor

“…It must be pointed that scaling up of the experimental setup could result in increase in Reynolds number and finally in higher PSH values. It can also cause an acceleration of titania particles coagulation process [45]. Therefore, further study on development of reactor construction will consider this issue.…”