Subproject C06

Polymer materials have become ubiquitous in various industrial sectors, displacing metallic materials, and integrating into our daily lives. However, a significant drawback of polymers lies in their high flammability. The societal and economic significance of researching flame-retardant polymers and understanding fire propagation to enable better fire safety cannot be overstated. The project addresses this critical challenge by focusing on the thermochemical conditions and flow dynamics of partially premixed and non-premixed boundary layer flames. This subproject aims to advance our understanding, particularly investigating the combustion of polymers and the interaction of the resulting flames with phosphorus-based flame retardants synthesized by C08 . The overarching objective is to contribute insights for developing mathematical models in B06 and C07 and provide extensive experimental data for validating numerical simulations in C07.

Through these systematic investigations, we aim to enhance the scientific understanding of flame-retardant polymers, fostering advancements that extend beyond subproject boundaries and contribute to the broader landscape of fire safety research.

This subproject aims to enhance our comprehension of partially- and non-premixed boundary layer flames in a flow parallel to a vertical wall. Leveraging insights from A04 , the subproject investigates the complex interplay of near-wall mixture gradients, flow boundary layers, and thermal near-wall effects.

Building upon this, this project aims to investigate the mode in which flame retardants affect these generic boundary layer diffusion flames, particularly focusing on how phosphorus-based flame retardants act in this flow configuration.

In cooperation with C08 , boundary layer flames arising from burning polymer samples shall be investigated, introducing further complexity through the interconnected combustion process and release of combustible material from the polymer as well as the multitude of involved species. A comparative study of polymer samples with and without flame retardants will focus on the effect of flame inhibition and the coupled effect of fuel release. To incorporate gas-phase and solid-phase flame inhibition effects, the condensed phase of the burnt samples will be analyzed by C08.

Previous research by Geschwindner et al. (2020) focused on the combustion behavior of micrometer-sized polypropylene (PP) particles, comparing neat PP with a formulation containing 10 wt% of a phosphorus-containing flame retardant (PSMP). Utilizing high-speed planar laser-induced fluorescence (OH-PLIF) and thermal decomposition analysis, the study provided insights into the gas-phase activity of flame retardants and their influence on combustion dynamics. Results indicated a shift in the peak reactivity zone and decreased peak intensity for flame retardant-containing particles, as observed in cone calorimeter tests.

In a subsequent study, Steinhausen et al. (2022) investigated partially premixed laminar methane-air flames undergoing side-wall quenching (SWQ). Through a combination of experimental and numerical approaches, the research assessed the effects of injecting dimethylmethylphosphonate (DMMP), a phosphor-based flame retardant, into the main flow near the flame's quenching point. The findings revealed that DMMP addition stabilized the flame further from the wall, leading to a reduction in the local heat-release rate and maximum wall heat flux. This study contributed valuable insights into understanding the combined effects of flame retardants and heat losses in near-wall flames.

Furthermore, Geschwindner et al. (2023) conducted experimental investigations to assess the efficacy and mode of action of flame retardants in polymers. By combining thermal decomposition analysis with optical combustion diagnostics, the study examined heat and mass transport processes in four different PP composites with various flame retardants, including PSMP. Experimental findings using OH-PLIF indicated a decrease in OH signal intensity during combustion for particles with gas-phase activity. The study provided comprehensive insights into polymer chemistry and combustion dynamics in flame-retarded polymers, linking findings across multiple scales and laying the groundwork for understanding flame inhibition mechanisms.

The burner facility used for this project builds upon the previous ambient pressure side-wall quenching burner, developed for subproject A04 . It is further advanced for this subproject, adding gas flow through an exchangeable inlet in the temperature-controlled side wall. Thus, a very small inflow into the boundary layer of the vertical main flow allows for a variable boundary layer flame. Alternatively, the new insert can be replaced with a polymer sample to be analyzed. With this burner, the flow boundary conditions of the main flow and inlet can be varied in a wide range in terms of species, as well as flow conditions.

This setup allows for good optical accessibility, enabling the use of multiple minimally invasive laser diagnostic techniques for multi-parameter studies. These studies allow for an in-depth insight into the coupled thermochemical and fluid mechanical processes. Planar laser-induced fluorescence (LIF) of the OH radical is used to allow for detailed flame visualization. Particle image velocimetry (PIV) is set up to measure the velocity field at the inflow as well as the interaction with the main flow. For this, a new liquid particle seeding method was developed to enable the seeding of very slow inflows below 1L/h for velocimetry. Decay time-based thermographic phosphor thermometry (TPT) is used to analyze the wall temperature in the region of the boundary layer flame. To capture the thermochemical state of the reacting flow the temperature as well as two species concentrations are measured in the gas phase at the same time, by using dual-pump coherent anti-Stokes Raman spectroscopy (CARS). This will be combined with further methods, such as LIF techniques, Laser-induced incandescence, or others, to measure quantities relevant to the process.

Presently the focus of this subproject is on measuring the thermochemical state, particularly of the non-premixed boundary layer flame, to improve the understanding of this complexly coupled process. For this, the CARS system is being commissioned at the current burner facility. CARS in the gas phase will be combined with the wall surface temperature measurement from TPT, providing information on the wall heat flux. By measuring two species concentrations as well as temperature, wall temperature, and the derived wall heat flux, and visualizing the flame at the same time using OH-LIF, the goal is not only to understand the boundary layer flame itself but also to elucidate simplified inhibition effects. This aims to provide phenomenological understanding as well as validation data to subproject C07, to cooperate in building a more complete understanding of the processes and inhibition effects.

The subproject cooperates closely with subproject C07 , directly coordinating investigations, discussing boundary conditions as well as identifying knowledge gaps to be addressed with further investigations. Polymer samples and flame retardants as well as chemical analysis of the pyrolysis process are provided by C08 . This subproject will further cooperate with C08 for experiments with polymer samples and the analysis of the effects of flame inhibition on the condensed phase after combustion. Method transfer on diagnostic techniques as well as experimental setup and boundary conditions will take place with C01 , C02 , A06 , and A05 . In particular, C07 , C06, B02 , B03 , and A06 will attempt to maintain a consistent comparable case. Beyond that, the safety aspects of working with phosphorus-based flame retardants are discussed regularly with C08 , and B04 , and coordinated with B05 and C07 .

• Christopher Geschwindner, Daniela Goedderz, Tao Li, Jan Köser, Claudia Fasel, Ralf Riedel, Volker Altstädt, Christian Bethke, Florian Puchtler, Josef Breu, Manfred Döring, Andreas Dreizler, Benjamin Böhm, Investigation of flame retarded polypropylene by high-speed planar laser-induced fluorescence of OH radicals combined with a thermal decomposition analysis, Exp Fluids 61, 30 (2020), https://doi.org/10.1007/s00348-019-2864-5
• Matthias Steinhausen, Federica Ferraro, Max Schneider, Florian Zentgraf, Max Greifenstein, Andreas Dreizler, Christian Hasse, Arne Scholtissek, Effect of flame retardants on side-wall quenching of partially premixed laminar flames, Proceedings of the Combustion Institute,v Volume 39, Issue 3 (2023), Pages 3745-3754, ISSN 1540-7489, https://doi.org/10.1016/j.proci.2022.07.207
• Christopher Geschwindner, Daniela Goedderz, Tao Li, Johannes Bender, Benjamin Böhm, Andreas Dreizler, The effects of various flame retardants on the combustion of polypropylene: Combining optical diagnostics and pyrolysis fragment analysis, Polymer Degradation and Stability, Volume 211 (2023), 110321, ISSN 0141-3910, https://doi.org/10.1016/j.polymdegradstab.2023.110321