Inerting Mechanism and Experimental Research of Combustible Refrigerants

1 Introduction

The environmental damage caused by refrigerants has attracted more and more attention. According to the Montreal Protocol and its amendments, chlorine-containing refrigerants are gradually being replaced by some environmentally friendly refrigerants. HCs and HFCs refrigerants with low GWP (Global Warming Potential) and zero ODP (Ozone Depletion Potential) are the main research directions of alternative refrigerants. However, flammability is one of its main drawbacks as a refrigerant, which limits its wide application. Therefore, the issue of the flammability of refrigerant substitutes and their mixtures has attracted the attention of some international organizations. Studying the influence of flame retardants on the explosion limit of combustible refrigerants is of great significance to the development of environmentally friendly refrigerants with low pollution.

In this paper, the group contribution method is used to analyze the inhibition coefficient of flame retardants to flammable refrigerants, and the calculation formula of z*small inerting concentration of flame retardant refrigerants is obtained by combining with combustion theory. Experiments on the explosion limit of three mixed working fluids, A/R152a, B/R290 and B/R152a, were carried out. 2 Theoretical analysis of explosion limit

2.1 Group contribution method

When atoms form molecules through chemical bonds, there are internal effects such as attraction and repulsion between electrons, which make the atoms different in space. A random, uniform composition, but formed some specific spatial chemical structure, showing certain group characteristics. The different characteristics of these groups remain basically unchanged in different compounds. Using the characteristics of the groups to calculate the physical parameters and performance effects of their constituent compounds, this is the group contribution method.

The basic form of the group contribution method for calculating physical parameters is shown in formula 1:

where: φ is the estimated physical parameter; A is the constant coefficient; n is the number of divided groups; ni is the number of groups i in the substance; Vφi is the contribution value of the i-th group to the physical property. The core of the group contribution method is based on two basic assumptions: the group constitutes the compound The basic unit, the same kind of group has a constant contribution value to a certain physical property in different substances; a certain physical property of a substance is the sum of the contributions of its constituent groups. The group contribution method is used to calculate the inhibition coefficient of different flame retardants on the flame propagation of flammable refrigerants. The inhibition coefficient of halogen flame retardant can be calculated by formula 2:

2.2 z*small inerting concentration calculation

z*small inerting concentration of flame retardant Defined as the concentration of z* small flame retardant at which the flame retardant inerts the flammable refrigerant or mixture into non-combustibles. After the flammable refrigerant is inerted, the flame propagation speed is reduced due to the presence of flame-retardant gas. Scholar Rosser [1] obtained through a large number of experiments and theoretical analysis that if the absolute burning velocity of the combustible flame is less than 5cm/s, it is considered that the flame cannot propagate. Therefore, this article believes that the concentration of the flame retardant when the flame propagation speed of the flammable refrigerant is reduced to 5 cm/s after using the flame retardant is the z*small inerting concentration.

Scholars Fristrom and Sawyer et al. [2-3] according to combustion theory and a large number of experimental data show that when the concentration of halogen flame retardants in the mixture exceeds 0.3%, the flame propagation speed of combustibles is The change has an exponential relationship with the change in the concentration of the halogen compound. As shown in formula 3:

where: Vu is the flame propagation velocity of the mixture when the flammable refrigerant contains flame retardant, m/s; V0 is the flammable When the refrigerant does not contain flame retardant, the z* maximum flame propagation speed, m/s; Cin is the volume percentage of the halogen flame retardant, %; b is the dimensionless coefficient, reflecting the suppression efficiency of the flame retardant, and the resistance The suppression coefficient of the fuel is related, and it is generally calculated through experiments or simulations.

The inhibition coefficient φ of the halogen flame retardant represents the inhibitory effect of the concentration of the flame retardant on the flame propagation Vu of the mixed flammable refrigerant, which can be solved by the following formula:

The dimensionless form of b proposed by Fristrom and Sawyer uses the concentration of O2 in the mixed gas as the dimensionless parameter. At this time, the inhibition coefficient φ is transformed into formula 5:

It is known from the flame propagation related theory that when the combustible gas and air are in the best mixing ratio of z*, that is, when the concentration of the combustible gas reaches the stoichiometric ratio Cst (the volume percentage concentration when the combustibles are just completely burned% ), the flame propagation speed z* at this time is V0. When the concentration of the flammable refrigerant reaches the stoichiometric ratio, with the increase of the flame retardant concentration in the mixed refrigerant, the volume concentration of oxygen in the mixture can be obtained by formula 6:

Combining the above formulas 3 to 6, the relationship between the volume concentration of the flame retardant Cin and the relative flame propagation speed, the suppression coefficient φ, and the stoichiometric ratio Cst of the combustible refrigerant can be obtained, as shown in formula 7:

When the flame propagation velocity of the flammable refrigerant is less than 5cm/s, this article considers that the flammable refrigerant cannot spread and is non-flammable. At this time, Vuu003d5cm/s, when the mixed flammable refrigerant cannot spread, the concentration of the flame retardant is z*small inerting volume concentration Cinmin. It can be solved by Equation 8:

From Equation 8, we can see that the stoichiometric ratio Cst of a known flammable pure refrigerant, and its flame propagation velocity V0 under the stoichiometric ratio Cst On the premise of parameters such as the suppression coefficient φ of the flame retardant, it is possible to estimate the z*small inerting concentration of the flame retardant required by the mixture when the mixed refrigerant is non-flammable. The small inerting concentration value of z* can also reflect the flame retardant effect of the flame retardant. For flammable refrigerants, the smaller the flame retardant Cinmin is, the better the flame retardant effect of the flame retardant.

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