Project
Numerical investigation and theoretical description of flame propagation of hydrofluorocarbon refrigerants
Figure 1: Normalized temperature, Θ, for different times of R-1234yf cases with three humidity level. Each figure size is set as along y-axis and along r-axis directions, where
is the spherical volume equivalent radius. In response to the escalating threat of climate change, the global community is progressively phasing out hydrofluorocarbon (HFC) refrigerants with high global warming potential (GWP). This shift necessitates the adoption of nextgeneration HFC-refrigerants like 2,3,3,3 tetrafluoropropene (R-1234yf), difluoromethane (R32), and trans-1,3,3,3-tetrafluoropropene (R-1234ze(E)), which have a lower GWP but are mildly flammable. Addressing these safety hazards requires a fundamental understanding of their combustion properties, particularly the laminar burning velocity (LBV). The LBV is a critical parameter for characterizing the reactivity and flammability of fuels and their mixtures, playing a vital role in developing combustion models and validating chemical kinetic mechanisms. For hydrocarbon flames, extensive literature exists on LBV estimation under various conditions. However, the slow-burning nature of HFC-refrigerants, often with flame speeds less than 10 cm/s, presents unique challenges in accurate LBV measurement due to the influence of buoyancy and radiation. Consequently, there’s a dearth of comprehensive data and predictive models for LBVs in HFCs, which significantly impedes the ability to use these refrigerants safely in technical applications. This project aims to bridge this knowledge gap by developing robust methods for better understanding and predicting the combustion properties and safety parameters of HFC refrigerants by conducting high-fidelity numerical simulations that fully account for buoyancy and radiation effects. This methodology will enable a holistic analysis of HFC flames, offering accurate insights into flame structure and encompassing all relevant physical effects. The research will address key questions, including defining LBV in buoyancy and radiationaffected flames, the impact of Markstein number effects on flame evolution and safety assessment, and the prediction of quantitative structure-property relationships for the safety assessment of existing and future refrigerants. This comprehensive approach is poised to significantly advance the understanding of HFC refrigerants, paving the way for their safer and more effective use.
Project Details
Project term
February 16, 2024–March 12, 2025
Affiliations
RWTH Aachen University
Institute
Institute for Combustion Technology (ITV)
Project Manager
Principal Investigator
Methods
For the simulations within the scope of this project, the Fortran code CIAO is used. It is a semi-implicit finite difference code, employing the Crank-Nicolson time advancement scheme and an iterative predictor-corrector method. The Poisson equation for pressure is solved using the multi-grid HYPRE solver. Momentum equations are discretized with a second-order scheme. For the species and temperature equations, the convective term is discretized using a fifth-order WENO scheme, and the diffusion operator is handled with second-order central differences. The advancement of temperature and species equations employs Strang’s operator splitting. The time integration of the chemical source terms for finite rate chemistry simulations uses a time-implicit backward difference method, implemented in the CVODE solver of the SUNDIALS suite.
Results
During the project, a database of 15 two dimensional simulations of laminar premixed R32 and R-1234yf with air under three levels of humidity was generated. The key findings reveal that, despite the higher unstretched laminar flame speed in lean mixtures, these are more prone to quenching. In contrast, despite their lower unstretched laminar flame speed, rich flames demonstrate enhanced stability, and especially at elevated humidity levels they demonstrate sustained combustion, posing potential fire safety concerns. These behaviors are closely linked to Markstein effects, primarily influenced by the fuel Lewis number, which is significantly greater than unity. To accurately analyze these effects, an approach is introduced by separately determining the Markstein numbers for positive and negative components of curvature and strain rate. It is found that Markstein effects due to positive curvature are the dominant factor, leading to flame inhibition under lean conditions, while Markstein effects due to strain rate have a minor influence. In rich flames, the relatively moderate Markstein number for positive curvature allows for sustained flame propagation. The response of the Markstein numbers in lean and rich mixtures to air humidity also differs significantly. With increasing humidity, the Markstein number for positive curvatures is reduced in rich cases, facilitating sustained flame propagation. In contrast, in lean conditions, increased humidity has no significant influence on the Markstein numbers. From a fire safety perspective, the findings highlight the limitation of using the highest unstretched flame speed across different fuel-air equivalence ratios in dry air for assigning the flammability class of fluorine-rich hydrofluorocarbon refrigerants because this may overlook for example fire risks associated with rich R-1234yf mixtures in humid air. This result underscores the need for fire safety standards to incorporate the combined effects of radiation, buoyancy, nonunity Lewis numbers, and humidity to more accurately evaluate flame sustainability and potential fire hazards.
Additional Project Information
DFG classification: 404 Fluid Mechanics, Technical Thermodynamics and Thermal Energy Engineering
Software: CIAO, FlameMaster
Cluster: CLAIX
Publications
Z. Li, H. Chu, L. Gregory, R. Glaznev, J. Beeckmann, M. Gauding and
H. Pitsch.
Flame Propagation of Refrigerant R-1234yf (CF3CFCH2) in Humid Air: A DNS Study. Proceedings of the Combustion Institute (accepted), Germany, 2025.