FOR 5507:  ExRef: Explosion hazards of low global warming potential refrigerants

• Subject Area: Thermal Engineering/Process Engineering
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 497007546

The increasing demand for refrigerating and air-conditioning systems causes a continued rise of the required amounts of refrigerants in Germany. To date, both naturally-occurring and synthetic refrigerants have been used for various applications. The use of many hydrofluorocarbons (HFCs) was severely restricted in 2016 as a result of their high global warming potential (GWP). Due to the often insufficient properties of naturally-occurring refrigerants, such as high operating pressure (CO2), toxicity (NH3), or the release leading to increased fire and explosion hazards (NH3, propane, butane), synthetic refrigerants will continue to play a significant role in the transition of refrigeration and air conditioning technology to refrigerants with low GWP. In addition to difluoromethane, these primarily include unsaturated HFCs, either as pure substances or in mixtures of various unsaturated HFCs. These substances will replace high-GWP hydrofluorocarbons in the European Union by 2050. For a safety assessment of fluorinated refrigerants, they are usually classified according to ISO 817 into different safety classes corresponding to the toxicity and flammability of the refrigerant under consideration. Many of the new fluorinated refrigerants with low GWP are non-toxic and have laminar burning velocities below 10 cm/s. Thus, they are classified as A2L. The low flame speeds and high minimum ignition energies of these substances lead to very large uncertainties in the determination of safety-related parameters based on empirical studies. Therefore, a methodical investigation will be carried out in the proposed research unit to enable the safety assessment of HFCs with low GWP quickly and effectively in the future using a knowledge-based approach. This approach includes automatically generated reaction mechanisms and molecular structure-based burning-velocity models generated by machine learning. For this purpose, the ignition process and subsequent flame propagation must be fundamentally studied to understand in detail the interaction of chemical with transport and flow processes. This includes both the interaction of vortices induced by pressure gradients with the flame detaching from the spark channel and the phase of self-sustaining flame propagation, since loss processes, e.g. due to radiation, as well as buoyancy play a significant role due to the low burning velocities. In addition, the chemical reactions must also be better understood in order to develop suitable reaction mechanisms. This requires the joint and closely linked research of various scientific fundamentals over a medium-term period. Therefore, the proposed research unit comprises six subprojects in order to answer the above-mentioned questions with complementary experimental and numerical investigations in a network.

• DFG Programme: Research Units

Projects

• Automated kinetic modeling of fluorinated refrigerants (Applicant Jocher, Agnes )
• Coordination Funds (Applicant Markus, Detlev )
• Experimental investigation on the oxidation and pyrolysis kinetics of fluorinated refrigerants. (Applicant Shu, Bo )
• Hierarchical modeling concepts for ignition processes of fluorinated refrigerants (Applicant Maas, Ulrich )
• Ignition and flame kernel formation in case of fluorinated refrigerants (Applicant Markus, Detlev )
• Laminar burning velocity measurements of refrigerants under terrestrial and microgravity conditions (Applicant Beeckmann, Joachim )
• Numerical investigation and theoretical description of flame propagation of unsaturated hydrofluorocarbon refrigerants (Applicant Pitsch, Heinz )

• Spokesperson: Dr.-Ing. Detlev Markus