An empirical model for refrigerant flammability based on molecular structure and thermodynamics

Abstract

Screening methods for refrigerant blend flammability using metrics that can be easily calculated are of great interest to the refrigerant industry. Existing flammability metrics such as heat of combustion are not adequate for hydrofluorocarbon blends. Alternative metrics are needed that can be used to assess the flammability of refrigerant blends without requiring time-consuming experimental measurements. In this work we study the combination of the maximum adiabatic flame temperature and the fluorine-substitution ratio as metrics for characterizing the flammability of refrigerant blends. The combination of these metrics yields an estimate of the flammability class of refrigerants (both blends and pure fluids) containing hydrofluorocarbon and hydrocarbon components. The calculations of adiabatic flame temperature are carried out with the open-source chemical kinetics software package Cantera using a mechanism available in the literature. 2. FLAMMABILITY METRICS The flammability of a refrigerant is classified by ANSI/ASHRAE Standard 34 (ASHRAE, 2016) and ISO Standard 817 (ISO, 2014) based on its heat of combustion, lower flammability limit, and laminar burning velocity. The classes range from "1" (fluids exhibiting "no flame propagation") to "3" ("higher flammability"-fluids with a heat of combustion greater than 19 MJ/kg or a lower flammability limit less than 0.10 kg/m 3). Fluids of "lower flammability" are assigned to class "2." Class 2 fluids have a heat of combustion less than 19 MJ/kg and a lower flammability limit greater than 0.10 kg/m 3. There is a further subclass "2L" for class 2 fluids that also meet the additional condition of a maximum burning velocity less than 10 cm/s. Flame propagation and the lower flammability limit are determined by the test method specified in ASTM E-681 (ASTM, 2015), with slight modifications. Although the distinct flammability classes of 1, 2L, 2, and 3 might suggest that there is a clear boundary between "flammable" and "nonflammable" refrigerants, flammability is, in fact, a continuum (Williams, 1974). While methane, for example, is clearly flammable and carbon dioxide is clearly nonflammable, many other fluids are somewhere in between. What constitutes "flammable" is a function of the test method and test conditions. The ASTM E-681 test (as modified for refrigerants) is carried out in a 12 L glass flask with premixed fuel and air, with ignition provided by an electric spark. The criteria for "flame propagation" is that the flame move upwards and outwards from the spark, extend to the walls of the flask, and subtend an angle equal to or greater than 90˚, as measured from the point of ignition. Thus, a refrigerant exhibiting a weak flame with a flame angle less than 90˚ in the E-681 test would

Low-GWP alternative refrigerant blends for HFC-134a :

“…Considering that there is significant variability in the test approaches that can affect the quantitative results 21,22 , it was useful to select a contractor likely to provide a test approach consistent with the existing database. This was important since the present test results are being compared to flammability predictions from our earlier work 1,14 , which was developed based on the existing ASHRAE Standard 34 database.…”

“…This yields the definition 𝜆 ̃= 𝜆 𝑘 𝐵 𝜌 𝑁 2/3 √𝑘 𝐵 𝑇/𝑚 (A. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] As the number density 𝜌 𝑁 = 𝜌𝑁 𝐴 goes to zero at constant temperature, the value of 𝜆 ̃ diverges, but this divergence can be repaired by taking advantage of the result noted by Rosenfeld for inverse-power-law (IPL) fluids of very low but finite density that scaled transport properties are proportional to (𝑠 + ) −2/3 , which allows for the definition of a term that breaks the zero-density divergence:…”

“…The first is to assess experimentally the flammability of the candidate blends predicted to be non-flammable (as determined by ASHRAE Standard 34 12 ) in the previous limited-scope project. In the preliminary work, an empirical model of flammability based on the adiabatic flame temperature and the fluorine to hydrogen ratio of the reactants was used to create a flammability index 14 and rank a list of candidate blends with regard to their flammability. All of the candidate blends selected for further study in the MBHP or ECU were predicted to be non-flammable.…”

Low-GWP alternative refrigerant blends for HFC-134a

“…The classes range from "1" (fluids exhibiting "no flame propagation") to "3" ("higher flammability"). For the refrigerant blend flammability assessment, we developed a new method [11] based on two parameters that can be readily evaluated for a given mixture: the adiabatic flame temperature Tad, and the F-substitution ratio F/(F + H). The adiabatic flame temperature is the temperature reached by a reacting mixture of fuel (e.g., refrigerant) and oxidizer (e.g., humid air) that undergoes an exothermic (heat-releasing) reaction to its most stable equilibrium products, under adiabatic conditions.…”

“…The F-substitution ratio F/(F + H) is the ratio of the number of fluorine atoms to the sum of fluorine and hydrogen atoms in the reacting mixture of air with refrigerant. This approach is based on historical work showing the influence of the adiabatic flame temperature on hydrocarbon flammability limits, and the influence of fluorine-containing species on the combustion chemistry of hydrocarbon flames [11].…”