Buoyancy induced limits for nanoparticle synthesis experiments in horizontal premixed low-pressure flat-flame reactors

Abstract

Premixed low-pressure flat-flame reactors can be used to investigate the synthesis of nanoparticles. The present work examines the flow field inside such a reactor during the formation of carbon (soot) and iron oxide (from Fe(CO) 5 ) nanoparticles, and how it affects the measurements of nanoparticle size distribution. The symmetry of the flow and the impact of buoyancy were analysed by three-dimensional simulations and the nanoparticle size distribution was obtained by particle mass spectrometry (PMS) via molecular beam sampling at different distances from the burner. The PMS measurements showed a striking, sudden increase in particle size at a critical distance from the burner, which could be explained by the flow field predicted in the simulations. The simulation results illustrate different fluid mechanical phenomena which have caused this sudden rise in the measured particle growth. Up to the critical distance, buoyancy does not affect the flow, and an (almost) linear growth is observed in the PMS experiments. Downstream of this critical distance, buoyancy deflects the hot gas stream and leads to an asymmetric flow field with strong recirculation. These recirculation zones increase the particle residence time, inducing very large particle sizes as measured by PMS. This deviation from the assumed symmetric, one-dimensional flow field prevents the correct interpretation of the PMS results. To overcome this problem, modifications to the reactor were investigated; their suitability to reduce the flow asymmetry was analysed. Furthermore, 'safe' operating conditions were identified for which accurate measurements are feasible in premixed low-pressure flat-flame reactors that are transferrable to other experiments in this type of reactor. The present work supports experimentalists to find the best setup and operating conditions for their purpose.

Initial reaction steps during flame synthesis of iron-oxide nanoparticles

“…A few years later, Hecht et al 12 used Fe-LIF measurements to quantify the iron atom concentration downstream the burner head. In contrast to the previous investigations with a horizontal flow direction leading to unwanted convection, the orientation of the Duisburg reactor is now bottom-up following the recommendations of Weise et al 13 to avoid buoyancy effects. The main reactor chamber has a diameter of 100 mm with four windows enabling optical access.…”

In situ flame chemistry tracing by imaging photoelectron photoion coincidence spectroscopy

“…Given the constraints imposed by the existing iPEPICO spectrometer, the burner chamber had to be aligned horizontally, even though non-negligible buoyancy effects have been shown to affect measurements at large distances (>100 mm) between burner and sampling probe in a similar setup used for nanoparticle synthesis. 14 The effect of buoyancy on measurements close to the burner is small and does not typically change flame chemistry conclusions.…”

Insights into the Mechanism of Combustion Synthesis of Iron Oxide Nanoparticles Gained by Laser Diagnostics, Mass Spectrometry, and Numerical Simulations: A Mini-Review

“…Because the zone of particle formation is usually located further downstream, this additional information is indispensable, as shown in Figure 13. 26 Shortly thereafter, Weise et al 11 demonstrated the impact of buoyancy for different reactor/burner orientations with respect to gravity. Using detailed CFD simulations and the previously postulated model, 26 they were able to attribute discontinuities in the Fe 2 O 3 particle growth measurements exclusively to the flow field distorted by buoyancy (see Figure 14).…”