Flammability Metrics for Next Generation Refrigerants

Summary

There is an accelerating worldwide movement towards the use of new refrigerant gases in heating, cooling, and refrigeration equipment. This movement has already begun and is expected to accelerate in the next few years, driven by the Kigali amendment to the Montreal Protocol. The US is a party to this treaty because it is beneficial for US manufacturers: the new replacement working fluids are produced by the patent-holding US companies whereas the legacy refrigerants are produced in foreign countries, subject to supply chain disruptions. The new refrigerants, however, are flammable, and this property is a new concern for manufacturers and building owners. The switch to new refrigerants needs to be done while minimizing first- and operating costs for US businesses and consumers and limiting the probability of accidents. To this end, industry needs the ability to predict the heat transfer, system performance, and fire behavior of the new compounds. Methods exist for the first two, but the understanding of and predictive tools for flammability of these new compounds are not as well developed. In particular, the project will develop predictive tools for and benchmark the flammability of next-generation refrigerants so that this property can be properly controlled, typically by forming blends of base compounds to form new refrigerants that have the desired performance (e.g., coefficient of performance, capacity, toxicity, and flammability) for a given application. 

Description

Credit: Linteris/NIST

Objective
To develop the capability to measure and predict the flammability of next generation refrigerants to (1) allow the ranking of mixtures in terms of their flammability, (2) assist in selection and implementation of the best replacements for legacy hydrofluorocarbon (HFC) refrigerants, (3) provide an understanding of their behavior in a variety of scenarios, and (4) provide input to industry research and standards on refrigerant flammability.

Technical Idea
To meet the simultaneous requirements for good thermodynamic and fluid mechanic performance (coefficient of performance COP and volumetric capacity Qvol), low toxicity, low index of trapping infrared light in the atmosphere, and low flammability, industry is going to use blends of compounds. Since the number of components blended may be more than five, the number of permutations is very large. To find optimum blends, existing methods are used to analytically predict other properties; however, no predictive tool exists to rank flammability.

The laminar burning velocity is a good metric for characterizing flammability because it is a fundamental combustion parameter that can be calculated from first principles; can be measured; is correlated with quenching diameter, lean flame extinction, minimum ignition energy, and overall chemical rate; and is used as a scaling parameter for turbulent flame speeds and as an input to full-scale explosion and flame propagation models. It is also used as a metric in existing and developing industry-consensus codes and standards for refrigerant flammability. Unfortunately, the present methods of measuring the laminar burning velocity in the standards are inaccurate, cumbersome, and do not allow for inclusion of stretch, radiation, and humidity in the air, which are important parameters affecting the burning velocity. NIST has developed an experiment based on a 2-L constant volume combustion chamber that is more comprehensive and more accurate compared to existing methods, primarily due to the more sophisticated data reduction tool, Constant Volume DAta Reduction Tool (CVDART).

The technical idea in the present work is to improve the experimental methods and predictive tools for burning velocity, apply them to new refrigerant blends that have recently been developed, and understand the effects of water vapor on refrigerant flammability. The primary methods for achieving these are through development of improved experimental methods as well as in detailed numerical models of the laminar burning velocity. Comparisons of the experimental results with the numerical predictions, followed by parametric analyses and sensitivity analyses will provide insight into both the experiments and the models, facilitating their improvement. The kinetic models and burning velocity data/predictions can then be used in full-scale models of refrigerant fires to assess their safety.

Research Plan
Two primary tools will be used: the 2-L constant volume combustion device for measuring burning velocity and the detailed chemical kinetic model and associated flame simulation tools. The experiment will be improved in FY26 by making it fully automated to make it more accurate, safer, and faster, so that more data can be collected in less time. With new data, the kinetic model will continue to be improved and extended. New refrigerant classes have recently become of interest to industry and the military for higher temperature refrigeration (e.g., chillers), including four-carbon hydrofluoroolefins such as R-1336yf and R1336mzz(E) and their blends, and well as two-carbon fluorinated alkenes. These will be tested and kinetic models for their reactions will be developed, validated, and used to understand their flammability. Water vapor has been found to have a very strong effect on the flammability for some refrigerants, but not for others. We will continue to collect data for and perform calculations on these effects to understand the reasons and try to develop guiding principles of use to industry.

NIST is collaborating with researchers at five universities to improve the kinetic models, experiments, and data reduction for refrigerant flammability to make them more accurate. Moreover, NIST plays a central role on ISO 817 SC8 (Refrigerants and refrigeration lubricants) as a liaison between the ISO group and the collaborative research team.

NIST continues to have an active role on ASHARE/AHRI project management sub-committees related to refrigerant flammability research, to ensure that the research is comprehensive and meets the needs of industry.