1 Introduction

Combustion of petroleum products is a complicated physical–chemical process that involves the formation of a combustible mixture of vapor and air above the surface of the liquid and its ignition at some distance from it. Compared to ordinary combustible fires, hydrocarbon fires are a magnitude greater in intensity. The fire extinguishing effect is achieved when the fuel temperature is lowered below its self-ignition or flash point temperature. In this case a disturbance of thermal equilibrium occurs in the burning area. As a result, the spontaneous and continuous course of these reactions becomes impossible and the combustion process stops. The condition of flame extinguishment is determined by equation as:

$$\left\{\begin{array}{c}{q}_{HG}={q}_{HA}\\ \frac{d{q}_{HG}}{d\tau }=\frac{d{q}_{HA}}{d\tau }\end{array}\right\}$$
(1)

where \({q}_{HG}\) is the heat released during the combustion of a liquid hydrocarbons, \({q}_{HA}\) is the heat losses.

The extinguishment of burning liquid hydrocarbons by water is complicated by insufficient heat removing from the burning area, water ingress under the surface of burning liquid and splashing water droplets owing to convection currents. The extinguishment time of liquid hydrocarbon flames depends to a great extent on both the rate of heating and rate of extinguishing agent vaporization.

To increase the effectiveness of extinguishment process, special additives such as salts or nanoparticles may be possible to use. The additives raise heat conduction and decrease surface tension of the base fluid resulting in its more intense vaporization [1]. Even low concentrations of carbon nanoparticles in water make it possible to change the properties of nanofluid compared to the base fluid, affecting the processes of heat and mass transfer. The type of nanoparticles, the way they are synthesized, processed, and stabilized also affect the processes (Table1).

Table 1 The Thermophysical Properties of Aqueous Suspensions with Carbon Nanoparticles
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This work is concerned about the effect of concentration of carbon nanoparticles—astralenes—on the physical properties of fire extinguishing agent and the extinguishment time of liquid hydrocarbon flames.

2 Materials and Methods

Distilled water and carbon nanoparticles—astralenes—were used to produce an extinguishing agent. The astralenes were provided by the “Scientific and Technical Center of Applied Nanotechnology” (Russia, St. Petersburg) [7] and were used as received. They were produced by vaporizing graphite anodes in an electrical arc to form soot-like material with structured carbon fraction exceeding 99 wt%.

The composition of the structured carbon fraction was characterized by an intensive G-band in the Raman spectrum in the frequency range 1570–1640 cm−1 (Fig. 1a). The Raman spectra of the powdered samples were recorded with a Ntegra-Spectra device (532 nm excitation laser, 600/600 grating).

Figure 1
figure 1

Raman spectra (a) and SEM image (b) of astralene powder

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Astralene nanoparticles consist of curved graphite layers with 0.336 nm distance between graphene layers. The diameter of such nanoparticles varies between 20 nm and 150 nm with an average pore size of about 20–60 nm (Fig. 1b). The astralene powders are in the format of large agglomerates in the order of 0.5–3 microns [8]. Also, the samples are characterized by a high thermal stability.

A typical process for extinguishing agent preparation involves dispersing an astralene sample with a 0.05–1.50 vol.% concentration into distilled water at 20 °C and standard atmospheric pressure. The treatment was carried out for 30 min with an “L063-2.5” InLab ultrasonic homogenizer (2.5 kW, 22 kHz).

Photos of samples of fire extinguishing agents are shown in Fig. 2. Suspensions of astralenes retain their stability for 2–3 h. During this time, nanoparticles are aggregated in the liquid.

Figure 2
figure 2

Photo DW (a), suspensions at the content of 0.05 vol.% (b), 0.2 vol.% (c), 0.5 vol.% (d), 1.0 vol.% (e), 1.2 vol.% (f); 1.5 vol.% (g) astralenes

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The study of the fire-extinguishing properties of suspensions was carried out on a laboratory installation, which was a metal box 1 with dimensions 0.5 × 0.5 × 0.5 m (Fig. 3). The extinguishing agent was put into a 5 L tank 3. The pressure of 0.2–0.3 MPa was generated with an air compressor. A flexible rubber tube 7 (10 mm inner diameter) and a spray nozzle 2 (2 mm diameter) were used for feeding of extinguishing agent. The extinguishing agent flow rate was determined by weighing the tank 3 before and after the experiment.

Figure 3
figure 3

Laboratory installation for determining the extinguishment efficiency, diagram (a) and photo (b): 1—metal box; 2—spray nozzle; 3—tank with the fire extinguishing agent; 4—gasoline pan; 5, 6—thermocouples; 7—piping

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The motor gasoline with the octane number of 95 was placed in the pan 4 (0.2 m diameter), ignited and held for 60 s to be further extinguished. The flame was extinguished with the sprayed astralenes suspension. The temperature of burning gasoline in the tank 3 was measured by a thermocouple 5.

The extinguishment time (τ) was fixed at the moment of complete combustion cessation. At least five measurements were taken for each astralene concentration. The data were averaged and compared with the results obtained for distilled water [9, 10].

The size of fire extinguishing agent droplets was estimated by analyzing photos of suspension with the Image Analysis P9 program. For analysis the suspensions were sprayed on a hydrophobic surface with dimensions of 100 × 100 mm.

To determine the latent heat of vaporization (r) a laboratory autoclave with a working chamber volume of 0.002 m3 were used. The measurements were carried out at a maximum excess pressure of 1.0 MPa and 180 °C by the method described in [11]. Prepared astralene suspension with volume of 0.002m3 was placed in an autoclave for measuring the saturated vapour pressure at 100 °C.

The latent heat of vaporization value \(r\) was determined as follows [12]:

$$r=\frac{R}{\mu }\cdot\frac{\mathrm{ln}\left(\frac{{P}_{1}}{{P}_{0}}\right)-\mathrm{ln}\left(\frac{{P}_{2}}{{P}_{0}}\right)}{\frac{1}{{T}_{2}}-\frac{1}{{T}_{1}}}$$
(2)

where R is the universal gas constant, \(\mu\) is the molar mass of water; \({T}_{1}, {T}_{2}\), \({P}_{1}\) and \({P}_{2}\) are respectively the saturated vapour temperature and pressure, which were measured in the autoclave; \({P}_{0}\) is the atmospheric pressure.

The surface tension coefficient of the suspension (h) was defined by the Pendant drop method [13]. The laboratory installation included (a) a glass tube of 50 ml with a tip radius of 0.7 mm secured to the support, (b) a drop collection vessel, and (c) an electronic balance to determine the mass of fluid coming out.

The agglomerate sizes in the suspension were determined by the method of atomic-force microscopy (AFM) with the “Ntegra Spectra” device [14]. The measurements were done on nanofluids of different astralene concentrations from 0.125 vol.% to 1.0 vol.%. To evaluate the structure, the samples were placed on a mica substrate and evaporated at atmospheric pressure and 70 °C–80 °C for 10 min.

Evaluation of nanostructures distribution uniformity in DW was carried out by measuring the permittivity of nanofluids by the parallel plate capacitor method [15]. At a given astralene concentration, nanofluids were placed between the condenser plates followed by measurement of the capacitance. The permittivity value was defined as:

$$\varepsilon =\frac{C\cdot d}{{\varepsilon }_{0}\cdot {S}_{d}}$$
(3)

where \(C\) is the measured capacitance, \(d\) is the gap between the parallel conductive plates, \({S}_{d}\) is the area of parallel conductive plates, \({\varepsilon }_{0}\) is the electric constant.

The relative change in the permittivity values of nanofluids was found out from Eq. (4):

$${k}_{M\varepsilon}={\varepsilon}_{m}/{\varepsilon}_{n}$$
(4)

where \({\varepsilon }_{\mathrm{m}}\) is the measured value of nanofluid permittivity, \({\varepsilon }_{\mathrm{n}}\) is the measured value of distilled water permittivity.

3 Results of Experimental Studies

The studies have shown (Fig. 4), that when the flame was extinguished with sprayed DW, combustion stopped with only the complete burnout of gasoline. It can be seen that the extinguishment time decreases by 75–80% with increasing astralene concentration to 0.5 vol.%. At the concentration greater than 0.5 vol.% the extinguishment time increases by 10–15%. The flow rate of extinguishing agent was about 0.85 \(\mathrm{l}/(\mathrm{sec}\cdot {\mathrm{m}}^{2})\) for plain water and 0.8–1.0 \(\mathrm{l}/(\mathrm{sec}\cdot {\mathrm{m}}^{2})\) for nanofluid.

Figure 4
figure 4

The extinguishment time of liquid hydrocarbon flames as a function of astralene concentration

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In all experiments the gasoline temperature increased from 25 °C to 55–60 °C over 10 s after the ignition of gasoline. When DW was fed into the burning area, an increase in the temperature of the liquid in the model fire source was observed to 95 °C for 15–20 s after the start of extinguishment. During the next 40–45 s the temperature decreased to 65–80 °C with simultaneous cessation of combustion (Fig. 5a). For the suspension with 0.05 vol.% astralene concentration, there was a more intense increase in the temperature of the burning liquid to 95–98 ºC during 5–10 s after the start of extinguishment. During the next 50–55 s the burning liquid temperature reduced to 70 °C and combustion stopped (Fig. 5b). For suspensions at astralene concentration of 0.2–1.5 vol.%, the temperature rise of the burning liquid from start of extinguishment to combustion cessation did not exceed 10 °C (Fig. 5c–g).

Figure 5
figure 5

source during extinguishment with: DW (a), suspensions at the content of 0.05 vol.% (b), 0.2 vol.% (c), 0.5 vol.% (d), 1.0 vol.% (e), 1.2 vol.% (f); 1.5 vol.% (g) astralenes

Changes in the temperature of the liquid in the model fire

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The calculations by Eq. (2) showed (Fig. 6a) that when astralene concentration of increases to 0.5 vol.%, the latent heat of vaporization increases to 20%. Further concentration increasing to 1.5 vol.% results in a 20% decrease in the latent heat of vaporization. At the same time, the heating rate of suspension to the boiling point increases to 50% reaching a plateau at the content of 1.0 vol.% astralenes (Fig. 6b).

Figure 6
figure 6

Effect of astralene concentration on the latent heat of vaporization (a) and the heating rate of suspension to the boiling point

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The surface tension was found to increase with increasing of astralene concentrations (φ) from 0.05 vol.%. At concentration of 1.0 vol. % astralene, the suspension surface tension increases to 10–25% compared with DW (Fig. 7). Further increasing of astralenes content does not affect the surface tension of nanofluid.

Figure 7
figure 7

The surface tension of nanofluid as a function of astralene concentration

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Figure 8 shows photos of droplets of extinguishing agents sprayed on the hydrophobic surface from the feeding spray nozzle (see Fig. 2).

Figure 8
figure 8

Photos of droplets of extinguishing agents sprayed from the feeding spray nozzle: DW (a), suspensions at the content of 0.05 vol.% (b), 0.2 vol.% (c), 0.5 vol.% (d), 1.0 vol.% (e), 1.2 vol.% (f); 1.5 vol.% (g) astralenes

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A relatively uniform distribution of water droplet sizes from 0.9 mm to 1.4 mm was characteristic for DW. For suspensions containing 0.05 vol.% astralenes an average droplet size was mostly smaller than that for DW and ranged from 0.5 mm to 0.8 mm. For suspensions containing 0.2 and 1.5 vol.% astralenes droplet sizes were larger compared to DW and increased with increasing the concentration from 0.9 mm to 2.4 mm.

The results of studying the nanofluid solid residue by atomic force microscopy reflect two trends caused by an increase in astralene concentration (Fig. 9): (a) the change in the size of astralene agglomerates, and (b) the change in the distance between the agglomerates. With a lower content (less than 0.5 vol.%), the distance between agglomerates decreases, while their size increases (Fig. 9a–c). However, with a higher content (more than 1.0 vol.%), there is a sharp enhancement of agglomerate sizes with simultaneously increasing the distance between them (Fig. 9d–f).

Figure 9
figure 9

AFM images of nanofluids on a mica substrate at different astralene concentration: 0.05 vol.% (a), 0.2 vol.% (b), 0.5 vol.% (c), 1.0 vol.% (d), 1.2 vol.% (e), 1.5 vol.% (f)

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The measurements of nanofluid permittivity (ε) revealed that its values reduced up to 2 times for nanofluids containing 0.2–0.5 vol.% astralenes. Further concentration enhancement leads to a growth in the values of ε on average 23% compared to the DW. At the concentration of astralenes from 1.0 vol.% to 1.5 vol.%, nanofluid permittivity becomes comparable to the data for DW (Fig. 10).

Figure 10
figure 10

Effect of astralene concentration on the permittivity of nanofluids

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4 Discussion

Studies presented in [16] showed that carbon nanoparticles are deposited in flammable liquids and don't create a protective film on its surface.

Consequently, in order to describe the physical mechanism of combustible liquid extinguishment with astralene suspensions, it is necessary to take into account the thermal processes that occur when the extinguishing agent is supplied into the burning area. The heat state of the burning liquid can be described using the heat equation:

$$c\rho \frac{dT}{d\tau}=\lambda \frac{{d}^{2}T}{d{x}^{2}}+cm\frac{dT}{dx}$$
(5)

where \(c\), \(\rho\), \(\lambda\)—are respectively the heat capacity, density and heat conduction of combustible liquid; \(T\)—is the temperature; \(\tau\)—is the time; \(x\)—is the coordinate; \(m\)—is the mass burnout velocity of combustible liquid [1].

On the surface of the liquid, the boundary condition for the exponential temperature profile is defined as:

$$\lambda \frac{dT}{dx}=Jr-cm({T}_{b}-{T}_{\infty })$$
(6)

where \(J\)—is the flow rate of extinguishing agent; \(r\)—is the latent heat of vaporization of extinguishing agent; \({T}_{b}\)—is the boiling point of combustible liquid; \({T}_{\infty }\)—is the ambient temperature.

The critical flow rate of extinguishing agent, \({J}_{k},\) depends on droplets size, \({D}_{a}\):

$${J}_{k}=155{m}^{2}\sqrt[3]{\frac{m{D}_{a}}{v\rho}}{\left[\frac{\rho {T}_{b}}{{\rho}_{a}({T}_{m}-{T}_{0})}-2\right]}^{-2}$$
(7)

and the latent heat of vaporization of extinguishing agent, \(r\):

$${J}_{k}=cm({T}_{c}-{T}_{e})/r$$
(8)
$${T}_{e}={T}_{c}-\frac{R{T}_{c}^{2}}{{E}_{a}}$$
(9)

where \({T}_{m}\)—is the average surface temperature of combustible liquid; \({T}_{0}\)—is the ambient temperature; \(v\)—is the kinematic viscosity of combustible liquid; \({\rho}_{a}\)—is the density of extinguishing agent; \({T}_{c}\)—is the combustion temperature of liquid; \({T}_{ext}\)—is the extinguishment temperature of liquid; \(R\)—is the universal gas constant; \({E}_{a}\)—is the activation energy [17].

The theoretical extinguishment time of combustible liquid \({\tau }_{t}\) with sprayed water is determined as follows:

$${\tau }_{t}=-\frac{4}{3}\frac{\lambda \rho }{c{m}^{2}}\left[\frac{{J}_{k}}{{J}_{f}}+ln\left(1-\frac{{J}_{k}}{{J}_{f}}\right)\right]$$
(10)

where \({J}_{f}\)—is the actual flow rate of extinguishing agent [17].

The experimental data on the latent heat of vaporization, the average diameter of droplets and the flow rate of extinguishing agent, as well as reference data on physicochemical properties of motor gasoline [18], were used for calculation of the theoretical extinguishment time. The results of the calculations generally confirm the experimental data obtained: at astralene concentrations of 0.2 and 0.5 vol.%, the theoretical extinguishment time has a minimum value. At astralene concentration of 1.0–1.5 vol.%, the extinguishment time increases, but remains less than for DW (Fig. 11).

Figure 11
figure 11

The theoretical extinguishment time of liquid hydrocarbons as a function of astralene concentration

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Due to the experimental and calculated data, there is a reason to suppose that the extinguishment time of liquid hydrocarbon flames results from heat transfer intensification in the convection burning area during nanofluid evaporation. This effect can be explained by enhancement of the specific heat of vaporization of nanofluid, which, in turn, is caused rising surface tension with an increase in the concentration of carbon nanostructures, as previously reported [13, 19]. A slight increase in the temperature of the burning liquid when using the nanofluid with astralene concentration greater than 0.2 vol. % indicates that a large part of the extinguishing agent evaporates in the convection burning area.

The physical mechanism of heat transfer in nanofluids is related to (a) Brownian motion of carbon nanoparticles, (b) the formation of a highly thermally conductive layer at the solid–liquid phase boundary and (c) the ballistic thermal transport inside nanoparticle agglomerates [3]. The transport characteristics of nanofluids largely depend on the mutual distribution of carbon nanostructures and the distances between them. It determines a significant change in the properties of nanomaterials with a low concentration of astralenes due to van der Waals interactions, the energy of which depends on the distances between nanoparticles, \(R\), and is proportional to \({R}^{-6}-{R}^{-7}\) [20, 21].

The AFM-data shows, that when the concentration of astralenes increases to 1.0 ol.% or more, the sizes of agglomerates appreciable increase and transport characteristics of nanofluid deteriorate. In addition, this effect is also confirmed by data on changes in the permittivity of nanofluids [22].

5 Conclusions

Aqueous suspensions of astralenes are fire extinguishing agents of predominantly cooling and diluting effect. When nanofluid is injected, the droplets are heated to the boiling point with subsequent evaporation and cooling of the burning area. If there is enough water vapor in the burning area, the flame will be extinguished. Thus, the enhancement of the latent heat of vaporization leads to an increase in the amount of heat energy removing from the burning area.

The extinguishment time enhancement at the increasing of astralene concentration is probably caused by the intense agglomeration of nanoparticles. The agglomeration contributes to the deterioration of transport characteristics of nanofluid [22] and decreasing in its evaporation rate in the burning area.

From the data obtained, it can be concluded that it is possible to increase the fire extinguishing efficiency of aqueous suspensions with low concentrations (up to 0.5 vol.%) of astralenes. The enhancement of astralene concentration to 1.0 vol% does not seem appropriate, since it leads to the deterioration of the transport characteristics of nanofluid. As a result, the rate of nanofluid evaporation in the burning area decreases.

It should be taken into account that aqueous suspensions of carbon nanoparticles without any surfactants are not stable for a long time [22]. Therefore, in the future, various methods, including integrated approaches of astralenes disaggregation, should be investigated in order to find the most optimal techniques and modes for obtaining stabilized dispersions.

This paper was financially supported by the Ministry of Education and Science of the Russian Federation on the program to improve the competitiveness of Peoples’ Friendship University of Russia (RUDN University) among the world’s leading research and education centers in the 2016–2020 and by the grant for the project “SUSTECH” ID KS1253 of the Europian program of crossboard cooperation.