Project
Detailed simulations of the nozzle-dependent primary atomization of the SpraySyn burner
Spray Flame Synthesis (SFS) is a promising approach for producing nanoparticles with tailored functionalities. However, the industrial-scale application of SFS fails these days due to an insufficient understanding of the process. The atomization of the precursorladen fuel is a crucial process step in SFS. SFS usually employs coaxial atomization,where a coaxially aligned high speed gas flow disintegrates a low-speed liquid. Experiments revealed that using different nozzle designs to guide the gas flow substantially impacts the final product particles. Consequently, the understanding of how nozzle design features influence atomization is a crucial part of understanding SFS and how to manipulate final particle properties. In this project, the primary atomization of the SpraySyn burner, a standardized burner for researching SFS developed in the Priority Programme 1980 (SPP1980) of the German Research Foundation (DFG), was simulated and compared to a more straightforward nozzle design. In addition, Lagrangian spray simulations of the downstream far-field using the interface-resolved simulations results as a primary breakup model were performed to study the impact of the primary atomization on mixture formation. Both simulations covered large ranges of length scales, resulting in small mesh spacings such that these simulations had to be performed on an HPC system.
Project Details
Project term
October 1, 2021–May 18, 2024
Affiliations
RWTH Aachen University
Institute
Institute for Combustion Technology (ITV)
Project Manager
Principal Investigator
Methods
The interfacial solver of CIAO, based on a highly accurate interface capturing method and a monotonicity-preserving interface flow solver, is utilized for the simulations of primary atomization. The interface capturing method is a conservative three-dimensional unsplit Volume of Fluid method developed recently and provides advantages in terms of computational costs compared to other methods in the literature. The monotonicity preserving interface flow solver, which enables stable computation at arbitrary density ratios, features a hybrid Lagrangian/Eulerian discretization of the convective transport terms and a monotonicity-preserving momentum rescaling. Furthermore, a Ghost-Fluid Method consistent pressure projection with curvature estimates from the generalized height function methodology provides an accurate surface tension treatment. The geometrical operations on non-convex polyhedral required for the interface capturing scheme and the convective momentum transfer are performed using the computationally efficient routines of the Interface Reconstruction Library. The solution is advanced in time using an explicit Euler method. The low-Mach solver of CIAO, which features a Lagrangian spray module including models for heat and mass transfer, is used for the Lagrangian spray simulations of the far field. The low-Mach solver of CIAO employs a central finite-difference discretization for the convective and viscous terms of the continuity and momentum equations. The convective terms of scalar transport equations are discretized with a weighted essentially non-oscillatory scheme. The solution is advanced in time using the Crank-Nicholson scheme. Both solvers use HYPRE to solve the pressure Poisson equation.
Results
This project’s primary atomization simulations were realized for two nozzle geometries and two mesh resolutions. In addition to the complex standardized SpraySyn nozzle design, a straight pipe was investigated as a simple geometry benchmark. A substantial impact of the nozzle geometry was observed, where the described differences apply to both mesh resolutions. For the SpraySyn geometry, the liquid core is shorter compared to the pipe case, resulting in the recirculation cavity being pushed more upstream. In addition to these global breakup characteristics, a substantial difference in the breakup dynamics was observed. The breakup is generally highly unsteady, resulting in substantial fluctuations in the liquid volume flow rate, leaving the simulation domain. The fluctuations vary between approximately 40% and 350% and 10% and 600% of the nominal volume flow rate for the SpraySyn burner and pipe case, respectively. Besides the intensity, the fluctuation frequency differs between the SpraySyn and pipe nozzle, which shows fewer but more intense peaks of the volume flow rate. Size and velocity statistics of liquid structures downstream where breakup is completed were computed and compared. The shape of the size PDF is similar for all cases. However, a strong mesh dependency was observed. The finer mesh leads to smaller structures. On the other hand, the pipe cases consistently show larger droplets for both mesh resolutions. All PDFs reach their maximum around twice the corresponding mesh spacing, and no convergence is visible, even for the large droplets. These observations are in agreement with other studies of atomization simulations. Lagrangian spray simulations of the far-field were performed for each primary atomization case. A consistent impact of the mesh resolution was observed. However, a validation with experimental droplet measurements showed that the simulations for both mesh resolutions are within the experimental uncertainties. Furthermore, it was observed that both the nozzle-dependent breakup dynamics and the mesh resolution for the interface-resolved simulation have a substantial impact on the mixture formation.
Discussion
The primary atomization was simulated for the SpraySyn burner employing a detailed
interface-resolving simulation strategy. The impact of the nozzle geometry and the mesh resolution was investigated. Furthermore, the simulation results were used to model primary atomization in Lagrangian spray simulations of the far-field to study the impact on the mixture formation. Substantial impact of both the nozzle geometry and mesh resolution was found for the primary atomization as well as the mixture formation. Overall, this research highlights the complexity and importance of modeling spray formation for coaxial atomizers. The strong mesh dependency of the interface-resolving simulation should be tackled by future research to increase prediction capabilities further.
Additional Project Information
DFG classification: 404 Fluid Mechanics, Technical Thermodynamics and Thermal Energy Engineering
Software: CIAO, HYPRE library, parIO, Interface Reconstruction Library
Cluster: CLAIX
Publications
Fabian Fröde, Olivier Desjardins, Malte Bieber, Manuel Reddemann, Reinhold Kneer and Heinz Pitsch,
Multiscale simulation of spray and mixture formation for a coaxial atomizer,
International Journal of Multiphase Flow, vol. 182, 104971, 2025
Fabian Fröde, Modeling Spray Flame Synthesis from Injection to Nanoparticle Formation, Phd, RWTH Aachen University, 2025