Effect of ecologically safe gas-aerosol mixtures on the velocity of explosive combustion of n-heptane

To solve the problem of effective retardation, researchers propose to use mostly gases, powders and their mixtures, as well as, in some cases, khladons and their mixtures with gases. Given the characteristics of known agents of volumetric fire-extinguishing, they share common shortcomings: devices are rather sizeable, gas storage requires a significant number of cylinders, powders need rather big containers, in which they tend to clod. In addition, it takes too long to supply the above-mentioned substances compared with the velocity of explosion front propagation. Determining an influence of the addition of gas-aerosol mixtures on the velocity of flame propagation throughout stoichiometric n-heptane-air mixture will make it possible to define effective concentrations and ratios of fire-extinguishing aerosol and gases СО 2 and N 2 on the velocity of flame propagation throughout a combustible homogeneous mixture, which will guide towards a more efficient use of gas-aerosol mixtures in order to prevent explosions and fires. Present research shows high effectiveness of influence of the addition of a binary mixture of fire-extinguishing aerosol and gases СО 2 and N 2 on a decrease in velocity of flame of the homogeneous heptane-air mixture. It was experimentally found that the influence of binary mixtures on the stoichiometric n-heptane-air mixture decreases the flame propagation velocity by up to 6.5 times, compared with the original velocity of flame propagation throughout the stoichiometric mixture. Thus, even small addition of binary gas-aerosol mixtures to the homogeneous combustible systems decreases explosion power and prevents the occurrence of detonation in them. Fire-extinguishing concentrations of aerosol and gases in this case decreases considerably due to synergy between them. Determining the effect of binary gas-air mixtures on the velocity of flame propagation throughout homogeneous combustible mixtures allows us to define conditions for effective anti-explosive and fire-retardant protection of sites with the presence of flammable, combustible and explosive media and substances.


Introduction
Modern means of volumetric fire suppression, as it is known, do not always provide adequate fire extinguishing efficiency and retardation. That is why quite often there are cases when a fire and an explosion still occur even under condition of protection of the volume with fire extinguishing gases or powders. In such cases, the explosion front might propagate at the corresponding velocity, which may lead to a partial or complete destruction of the equipment through creation, in this case, of the elevated pressure. It is known that the fire extinguishing concentration of СО 2 for most hydrocarbons is about 21 %, and retarding concentration is about 30 % or higher [1][2][3][4]. For nitrogen, these values are, respectively, for n-heptane (С 7 Н 16 ) 33.6 % [4,5], and retarding concentration of N 2 is 43.9 %, which is 3-4 times as high as the retarding concentration of khladons [5]. In addition, according to the Montreal Protocol [6], СО 2 relates to greenhouse gases, emissions of which are regulated. Often enough, in order to limit the use of СО 2, , it is replaced with nitrogen N 2 as an environmentally friendly fire extinguishing agent. Nitrogen as a fire extinguishing agent is chemically neutral to most substances, cheap and easily available.
Thus, for effective explosion protection, it is required to use enormous amounts of СО 2 or N 2 gases, or khladons, which in most cases are toxic; in addition, their production and usage are banned because they deplete the planet's ozone layer [7].
At present, an alternative to gas extinguishing agents is the binary mixtures of the specified gases with a fire extinguishing aerosol [4], in which (at certain ratios) fire extinguishing and retarding concentrations of СО 2 and N 2 are considerably smaller than their individual fire extinguishing and retarding concentrations.
In this case, a combination of advantages of the aerosol as a clean, efficient and low-cost fire-extinguishing agent, and benefits of gases makes it possible to obtain a super-effective binary mixture with universal fire extinguishing characteristics. The specified mixture will possess high fire extinguishing capacity, will be environmentally clean, quite cheap and have the capability to effectively suppress explosions or decrease the scale of consequences due to a decrease in the velocity of propagation of the explosive flame front. This will prevent or minimize the effects of explosions.
Thus, at present, there is a lack of effective extinguishing agents, which would provide protection against explosions and fires for sites of chemical, light, petrochemical, nuclear, and machine-building industry. In these industries, there are sites with considerable volumes of circulation and storage of flammable, combustible and explosive substances, which in case of explosion or fire lead to catastrophic consequences. Such fires cause significant destructions and contamination of the environment as a result of possible explosions and significant volumes of combustion. Thus, over the past few years, there have been several dozens of catastrophic fires. The most devastating of them are the fires that took place in Kuwait in 1990-1992 at oil wells, oil storage depots, in Hemel Hempstead, Hertfordshire, UK; in the village of Kriachki, Kyiv Oblast, Ukraine, on June, 8, 2015, at the oil depot, etc. All of these fires lasted for up to 7 days, the modern means of fire extinguishing were used -fire extinguishing foam, water with wetting agents, and extinguishing powders. The possibility of initiation and lengthy existence of such fires emphasizes the relevance of the specified topic of research.

Literature review and problem statement
Gas-vapor-air mixtures are known to explode at various velocities of flame front propagation In general, at an explosion of a homogeneous gas-vapor-air mixture, velocity of flame front propagation is 0.5-10 m/s for deflagrational combustion, 7-12 m/s for kinetic combustion, and 200-3000 m/s for detonational combustion [8]. Under certain conditions, namely, when the walls of a reaction vessel are rough or at large volumes of flammable mixture [8], a combustion of gas-vapor-air mixture abruptly passes into detonational. Detonation is extremely dangerous, because flame velocity rises up to several thousand meters per second, and pressure -up to several thousand atmospheres.
At present, gases and powders are mainly used for the retardation and prevention of explosion. Retardation concentration is known to be 1.4 times as high as a fire-extinguishing concentration; the consumption of fire-extinguishing agents (mixture) for retardation will increase, accordingly, by times. At depressurization of volume, consumption may increase even larger and it is not always possible to create the necessary concentration for effective retardation. Given these characteristics, all known means of volumetric fire extinguishing share a common drawback -devices that implement the retardation method with their use are rather sizeable, gas storage requires a significant number of cylinders, and powder storage requires rather big containers. Fire extinguishing aerosol powder in this case requires regular replacement, as it possesses the ability to cake. In addition, it takes too long to supply the above-mentioned substances, compared with a velocity of explosive front propagation. Therefore, these methods have a number of shortcomings that render relevant the subject matter of explosion safety of homogeneous combustible mixtures.
It is known from an analysis of scientific literature [4,5] that binary mixtures of fire-extinguishing aerosol and gases-retardants have a high fire extinguishing and retarding efficiency. In a binary gas-aerosol mixture, their effective extinguishing ratios is from 5 g/m 3 of aerosol and from 2.5 % of СО 2 or N 2 gases. Fig. 1, which shows results of research [9], demonstrates that a fire extinguishing aerosol concentration with the addition of СО 2 rapidly decreases. Thus, when adding СО 2 in a range of concentrations of up to 5 %, fire extinguishing aerosol concentration decreases larger than twice.
The structure of aerosol [9] is known to be constantly changing -from the moment of aerosol formation and its transformation under the influence of flame in the process of extinguishing. In this case, the first to occur is the process of absorption of СО 2 and N 2 gases with the formation of salts K 2 СО 3 , KCL, KHCO 3 , KOH, which provide a fire extinguishing effect. Such structures will have much more efficient fire-retardant effect due to a combined action of the mixture of inhibitors and retardants [10]. The same indicators will be characteristic for the fire extinguishing aerosol, formed during combustion of the aerosol forming mixture that additionally contain gasifiers -DCDA, DFA, which provide for the formation of СО 2 and N 2 gases. This greatly enhances fire extinguishing effectiveness of the aerosol and is in line with results of papers [9][10][11]. Thus, article [9] indicated that the addition of СО 2 gases to the fire-extinguishing aerosol, based on potassium salts, leads to reducing its fire-extinguishing concentration for hydrocarbon fuel from 33 g/m 3 to 12 g/m 3 at СО 2 concentration of only 5 %. In article [10], authors also established a significant decrease in the fire-extinguishing concentrations of aerosol at suppression of a diffusive methane flame. Paper [11] reports a significant retarding effectiveness of the specified mixtures for hexane-air media; at 10 % of СО 2 , it is necessary to add only 8 g/m 3 of the aerosol to provide effective retardation. Therefore, the addition of gases to aerosol considerably increases fire extinguishing effectiveness of a gas-aerosol mixture. Effectiveness of the binary mixture increases dramatically at addition of СО 2 to aerosol, probably due to its large thermal capacity, which can be proved by results of the work, given on the diagram in Fig. 1. As for fire-extinguishing gases, we should note that gases-rarefiers, which include in particular carbon dioxide, nitrogen, argon and their mixtures, are non-toxic and do not produce toxic products of thermal decomposition, do not deplete the ozone layer and are more readily available and cheaper than the combustion inhibitors. One of the first gas fire-extinguishing substances, proposed as an alternative to khladons, was carbon dioxide (СО 2 ) [12].  As for a fire-extinguishing aerosol, its fire extinguishing effectiveness mostly depends on the size of particles. Thus, paper [13] shows that with a decrease in the dimensions of particles from 100 to 20 µm, velocity of flame propagation drops dramatically with the addition of dispersed particles of salt NaCl, K 2 СО 3 , KHCO 3 and others to the propane-air mixture. As authors of [13] assume, this happens due to heat absorption and particles evaporation into the flame.
Article [14] shows that there is a direct relationship between dimensions of particles and fire-extinguishing efficiency, flame propagation velocity, which is manifested due to a possible decomposition of these particles in the flame, which provides retardation and inhibition of combustible mixture and the flame.
Thus, paper [15] argues that by using a mixture of free radicals and СО 2 it is possible to decrease the concentration of fire-extinguishing chemical agents by 3-4 times. The conclusion of the authors [16] indicates that non-combustible gaseous components dilute a combustible system and decrease oxygen concentration. This leads to a decrease in the fire-extinguishing effectiveness of an inhibitor, which, naturally, dramatically decreases the velocity of flame propagation. In article [17], it is indicated that the addition of nitrogen leads to a significant decrease in the fire-extinguishing concentration of hladone 1301, for n-heptane, while maintaining all useful properties of the components. Paper [18] clearly states that the fire extinguishing effectiveness of the mixture of inhibitors and retardants significantly increases due to synergy among them [18][19][20][21][22][23]. Paper [19] presents calculations of flame temperature at addition of inert gases to a mixture of inhibitor and flegmatizer. They showed a decrease in flame temperature, which in turn provides enhancing of inhibiting effect of the mixture of hladone-1301 and СО 2 . Author of paper [20] pointed out to a substantial efficiency of fire-extinguishing aerosol during extinguishing flammable liquids in the open space with a fire-extinguishing supply intensity of 8 g/m 2 •s. Regarding the influence on living organisms, authors of article [21] note that male rats tolerate a 60-minute influence of aerosol without any significant damage. In this case, a fire-extinguishing aerosol remains in suspension in the air for maximum 40 minutes. Accordingly, it is possible to argue about relative safety of the fire-extinguishing aerosol for living organisms, in particular humans. Combined use of СО 2 and N 2 gases along with fire-extinguishing powders is also known. Paper [22] indicates that the combined fire-extinguishing with powder and inert gases also leads to a decrease in fire-extinguishing concentration of the powder. The same is stated in article [23]. In paper [24], authors emphasize that the additional amount of combustion products in the volume will cause an increase in the activation energy of combustible systems by almost 100 times. Addition of СО 2 and N 2 gases results in a decrease in the velocity of laminar hydrogen flame and low pressures. In this case, the addition of СО 2 has greater effectiveness in contrast to N 2 , the impact of which on the flame propagation velocity is not sufficiently known, which is outlined in paper [25] that notes a somewhat larger significant supressing effect of N 2 on flame propagation than that expected. Article [26] also highlights a significant impact of СО 2 on the velocity of propagation of laminar flame under conditions of decreased gravity, and emphasizes considerable fire-extingushing influence of СО 2 under these conditions. Thus, the use of binary mixtures of fire-extinguishing aerosol and gases-retardants for retarding combustible mixtures is effective, but not completely studied, given the fact that almost all factors of influence are implemented in them -inhibition, cooling, dilution and retardation. In particular, a problem of the impact of these mixtures on flame propagation velocity during explosions of homogeneous gasvapor-air mixtures was not sufficiently examined.
To determine characteristics of influence of binary mixtures of fire-extinguishing aerosol and СО 2 or N 2 on the velocity of explosive combustion of a homogeneous combustible mixture, it is most expedient to use a vapor-air heptane mixture. Accordingly, determining a dependence of the velocity of flame propagation throughout a stoichiometric heptane-air mixture (SHAM) on additives and parameters of binary gas-aerosol mixture will make it possible to determine conditions of effective retardation and explosion suppression. Obtaining data on the influence of binary gas-aero-sol mixtures on the velocity of flame propagation throughout a stoichiometric n-heptane-air mixture will make it possible to prevent the emergence of deflagration, kinetic or detonational combustion in homogeneous combustible systems, as well as it will minimize effects of explosions in such cases.
Thus, determining the features of the impact of environmentally safe binary gas-aerosol mixtures on the velocity of explosive combustion of n-geptane will enable us to provide effective fire-retardant and anti-explosion protection of sites of chemical, light, petrochemical, nuclear, and machine-building industry, at which a formation of combustible homogeneous media is possible.

The goal and objectives of reearch
The goal of present research was to provide the minimization of effects of explosions of homogeneous combustible mixtures using ecologically safe binary gas-aerosol mixtures (aerosol and gas СО 2 or N 2 ).
To accomplish the set goal, the following tasks had to be solved: -to establish experimentally the effect of binary gas-aerosol mixtures (aerosol -CO 2 or N 2 ) on a stoichiometric n-heptane-air mixture and to determine flame propagation velocity in it; -to perform a frame by frame analysis and to analyze and identify the features of influence of the addition of binary mixtures of fire-extinguishing aerosol and gases-retardants on an explosion of stoichiometric n-heptane mixtures.

Materials and methods of examining the influence of gas-aerosol mixtures on flame velocity
To determine velocity of flame propagation throughout SHAM under the action of binary gas-aerosol mixtures, we used the following materials and devices: aerosol-forming mixture (AFM), consisting of 20 % of iditol (С 13 Н 12 O 2 ) and 80 % of potassium nitrate (KNO 3 ). AFM is prepared by agitating a respective weight of the charge. Nitrogen -N 2 and CO 2 are 99.96 % chemically pure. Viderecording of the explosive combustion was performed by the photocamera Nikon 1 J4, which can take 3-second videos with a frame rate of 1200 per second at a resolution of 416 x 144 p [28].
The base of the plant (Fig. 2) is a thick-walled glass cylindrical vessel of 0.5 l capacity with a powerful electrical ignition source, located inside the cylinder. The upper and lower parts of the cylinder are closed with lids. On the underside lid, there is source of ignition 2, the top lid has a opening, covered with rubber cork 4, and a spiral igniter of AFM 5. The glass housing of cylinder 1 is fixed on the bench on two vertical racks 6. The fasteners of the glass cylinder make it possible to rotate it in the vertical plane. The rubber cork contains a gas pipe, by which gas is fed. Distance from the source of ignition to the top wall of the cylinder is 135 mm, to the bottom wall -80 mm.
Ignition of the batch of AFM was performed by coil electrical igniter, the amount of gas was measured using a piston measurer with a volume of 100 ml. The experiment was carried out as in the following way: the cylinder was heated up to 50 о С. Then the batch of AFM was combusted in the volume of the cylinder and appropriate amount of nitrogen N 2 , or СО 2 was added, after that, an appropriate amount greater completeness of the mixture combustion. At an introduction into the mixture of additions of gas-aerosol with a concentration of aerosol from 2.5 g/m 3 and concentration of СО 2 8 %, we observed a decrease in the flame velocity, as shown in diagram (Fig. 4), from 8.5 m/s up to 1.8 m/s. When adding more than 4 g/m 3 of aerosol and 3 % of gas, flame propagation velocity decreased more intensively and, in some cases, the flame front did not propagate throughout the entire volume of the mixture.
Thus, a substantial decrease in the original velocity of flame propagation throughout stoichoimetric heptane-air mixture from 8.45 m/s to 1.8 m/s or less is the result of the combined effect of the binary gas-aerosol mixture. The combined effect of gas-aerosol mixture involves cooling with aerosol and gas, inhibition and dilution of the homogeneous mixture with the specified components.

Conclusions
1. It was established experimentally that the influence of binary gas-aerosol mixtures on the stoichiometric n-heptane-air mixture decreases flame propagation velocity by up to 6.5 times compared with the original flame propagation velocity. In this case, we found dependence of velocity of flame propagation throughout stoichiometric n-heptane-air mixture on concentrations and ratios of components of binary mixtures -aerosol and CO 2 or N 2 , which is in the fact that within a range of aerosol concentrations from 8 g/m 3 to 2.2 g/m 3 and of СО 2 gas from 16 % to 8.2 %, there occurs a maximum decrease in flame propagation velocity. For the nitrogen-aerosol mixture, these values correspond to the concentration range of aerosol from 15 g/m 3 to 6.5 g/m 3 and of N 2 from 20 % to 8.5 %. Maximum flame propagation velocity, which corresponded to the minimum concentration ratios of aerosol and gas, was in both cases about 2 m/s. 2. A frame by frame video recording analysis was performed of the explosions of stoichiometric n-heptane-air mixtures, and special features of effect of the addition of binary mixtures of fire-extinguishing aerosol and gases-retardants were revealed. They are in the fact that with the addition of СО 2 from 8.2 % and of aerosol from 2.5 g/m 3 , or hitrogen from 8 % and aerosol from 6 g/m 3 , there were cases of incomplete flame propagation throughout the entire volume of combustible mixture. In this case, the flame propagation velocity had minimum values of 0.5-3.5 m/s at the start of flame propagation, then it dramatically increased after depressurization of the volume of the explosive cylinder.