**2. The peculiarities of atomization of liquid fuels with different physical properties.**

Experimental studies of the features of fuel–air sprays were performed at the Central Institute of Aviation Motors using laser diagnostics setup. The description of the test bench is given in reference [15]. The setup is equipped with instruments for laser measurements of the quality of spraying and the rate of droplets by the light scattering. In this work, the physical studies were carried out using the method of Phase-Doppler anemometry (PDPA TSI, United States). Digital photography was carried out using a Canon XL\_H1 three-matrix color camera-recorder (Japan). As an object of study, a double-channel fuel burner with combined centrifugal-airblast design was chosen [16]. The scheme of the spraying device is shown in Fig. 1. The channels of the nozzles are arranged concentrically. A pressure swirl pilot channel with a low rate of flow and cylindrical outlet nozzle is mounted on the burner axis. The main fuel feed channel is airblast with a ring nozzle. It is placed between the two air swirlers for better atomization of the liquid film and for stabilization of the fuel–air spray. The angles of the vane inclination of inner and peripheral swirlers relative to the axis of the device were 60° and 45°. The outer diameter of the fuel nozzle in a pneumatic atomizer is 22 mm and 1.1 mm in a centrifugal atomizer. A detailed description of the burner is given in [14], where it was tested on various petroleum and alternative fuels.

**Figure 1.** Test object

Figure 1. Test object

1

Foreign companies in recent years (2008-2014) have been intensively studying the possibility of using alternative fuels without the need for modification of aircrafts and engines. The first flight of the airplane on biofuel took place in 2008. The British Airline Virgin Atlantic Airways Ltd is the proprietor of that aircraft. Boeing and its international partners are already working hard to bring biofuels from the testing stage to the manufacturing stage. Boeing 747-8 Freighter and the 787 Dreamliner made the first demonstration of transatlantic and transpacific flights on biofuels in 2011 and 2012 [3]. In May 2014, KLM began weekly flights by an Airbus A330-200 between Queen Beatrix International Airport, in Oranjestad, Aruba, and Amsterdam's Schiphol Airport, Netherlands, using converted cooking oil as aircraft fuel [4]. So far, Russia has not done commercial-scale biofuel production. However, this trend has a great future because of the presence of large sown areas and water surfaces in our country [5]. Within the framework of the International Aviation and Space Salon MAKS-2013 Airbus and Rosteh State Corporation signed a partnership agreement in the field of aviation biofuels in Russia using

One can find a large number of articles devoted to biofuels in the world literature (e.g. [6], [7]), a number of articles in [8]). An overview of current studies of the structure of such fuels as well as the characteristics of the processes of combustion and pollutant emissions in various types of engines is given in [9]. However, vast majority of the work are carried out in relation to internal-combustion engines or diesel engines. Studies on the atomization and combustion of biofuels compared with petroleum fuels in relation to gas turbine engines, as well as designing multi-fuel combustion chambers in the press are virtually absent. Nevertheless, one can see from Table 1 (the physical properties of various fuels corresponding to the Russian and international standards [10- 13]) that the spread in values of the fuel properties is rather wide, especially for viscosity. The present work is a continuation of researches [14] and [15]. In research [14] the design, manufacture and test of individual injectors and the burner as a whole for low-emission combustion chambers of gas-turbine engine or gas-turbine plant have been executed. Results showed that the designed spray unit can be used for different liquid fuels, both for fossil and for alternative fuels. The present work is devoted to the study of the influence of the physical properties of conventional fuels and biofuels on the characteristics of fuel–air aerosols and the combustion process. Fuel spraying was carried out by means of the

**2. The peculiarities of atomization of liquid fuels with different physical**

Experimental studies of the features of fuel–air sprays were performed at the Central Institute of Aviation Motors using laser diagnostics setup. The description of the test bench is given in reference [15]. The setup is equipped with instruments for laser measurements of the quality of spraying and the rate of droplets by the light scattering. In this work, the physical studies were carried out using the method of Phase-Doppler anemometry (PDPA TSI, United States). Digital photography was carried out using a Canon XL\_H1 three-matrix color camera-recorder (Japan). As an object of study, a double-channel fuel burner with combined centrifugal-airblast

only renewable resources.

330 Biofuels - Status and Perspective

developed burner.

**properties.**

kerosene (νF = 1.9 × 10–6 m<sup>2</sup>/s, σF = 25 × 10–3 4 N/m), and (3) a mixture of diesel fuel with rapeseed oil in the ratio of 50 : 50 (νF = 13.7 × 10–6 m2/s, σF = 30 × 10–3 5 N/m), which imitated 6 liquid biofuel. 7 We used three methods of spraying: hydraulic (in which the energy of the liquid is used for 8 spraying), pneumatic (spraying of liquid in the flows of air), and a combined centrifugal-9 pneumatic process, in which liquid spraying occurs due to the of the liquid state's own 10 energy and the energy of air. 11 The first stage in the research on the atomization process is the investigation of the In this section, we studied the atomization at normal conditions for three types of liquid: (1) water (kinematic viscosity ν<sup>F</sup> =1.05 × 10–6 m2 /s, surface tension σ<sup>F</sup> = 73 × 10–3 N/m), (2) kerosene (ν<sup>F</sup> = 1.9 × 10–6 m2 /s, σ<sup>F</sup> = 25 × 10–3 N/m), and (3) a mixture of diesel fuel with rapeseed oil in the ratio of 50 : 50 (νF = 13.7 × 10–6 m2/s, σF = 30 × 10–3 N/m), which imitated liquid biofuel.

2 In this section, we studied the atomization at normal conditions for three types of liquid: (1) water (kinematic viscosity νF =1.05 × 10–6 m<sup>2</sup>/s, surface tension σF = 73 × 10–3 3 N/m), (2)

12 decomposition of liquid fuel films due to the loss of their own stability (hydraulic

13 atomization). Figures 2–6 show the results of this investigation. We used three methods of spraying: hydraulic (in which the energy of the liquid is used for spraying), pneumatic (spraying of liquid in the flows of air), and a combined centrifugalpneumatic process, in which liquid spraying occurs due to the of the liquid state's own energy and the energy of air.

The first stage in the research on the atomization process is the investigation of the decompo‐ sition of liquid fuel films due to the loss of their own stability (hydraulic atomization). Figures 2–6 show the results of this investigation.

The results of this series of experiments allow us to make some assumptions about the mechanism of the liquid film decay into droplets. The film of the fuel is formed as a result of the interflow of swirled liquid streams into a single stream along the length of the swirl chamber and the nozzle of the injector. In this case, one can assert that when the fluid moves through the caves of the injector, its outer layer is decelerated due to the friction with the surface and the velocity components diminish in this layer. This gives rise to the shear stresses along the fuel film thickness. We can assume that with an increase in the velocity, depending on the properties of the fluid and geometrical parameters of the atomizer, the shear of the layers becomes so significant that the outer sublayer is swirled in the opposite direction relative to the velocity vector (the scheme is shown in Fig. 2). At the exit of the nozzle, after sudden

**Figure 2.** Scheme of the formation of waves–vortexes on the surface of the liquid film

Figure3. Development and decay of the waves upon hydraulic spraying of kerosene through an annular nozzle **Figure 3.** Development and decay of the waves upon hydraulic spraying of kerosene through an annular nozzle (pho‐ to) at GF =17.5 g/s, GA = 0

expansion and separation from the surface, the intensity of the vortex increases rapidly and waves are formed. When moving downstream, the wave height above the level of the film increases (see markers *1* and *2* in Fig. 3). The growth of these waves is caused by the fact that swirled formations move in the axial direction with lower velocity than the film and then disintegrate into bundles and individual droplets at a certain moment when capturing additional mass (marker 3 in Fig. 3). Such a vortex structure of the waves is confirmed by a series of images shown in Fig. 4, where we can clearly see that a sufficiently large number of droplets deviate from the direction of the stream outwards from the film at various expiration velocities and different nozzle designs. This can be explained by the disintegration of the swirling roller of the fluid as it moves in the direction backward to the main flow, and the roller (or its components) deviates to the periphery of the burner. 2 The results of this series of experiments 3 allow us to make some assumptions about 4 the mechanism of the liquid film decay into 5 droplets. The film of the fuel is formed as a 6 result of the interflow of swirled liquid 7 streams into a single stream along the length of the swirl chamber and the nozzle of the 8 injector. In this case, one can assert that when the fluid moves through the caves of the 9 injector, its outer layer is decelerated due to the friction with the surface and the velocity 10 components diminish in this layer. This gives rise to the shear stresses along the fuel film 11 thickness. We can assume that with an increase in the velocity, depending on the properties 12 of the fluid and geometrical parameters of the atomizer, the shear of the layers becomes so 13 significant that the outer sublayer is swirled in the opposite direction relative to the velocity 14 vector (the scheme is shown in Fig. 2).At the exit of the nozzle, after sudden expansion and Figure2.Scheme of the formation of waves–vortexes on the surface of the liquid film (photo) atGF=17.5 g/s, GA= 0

15 separation from the surface, the intensity of the vortex increases rapidly and waves are 16 formed. When moving downstream, the wave height above the level of the film increases 17 (see markers 1 and 2 in Fig. 3). The growth of these waves is caused by the fact that swirled 18 formations move in the axial direction with lower velocity than the film and then 19 disintegrate into bundles and individual droplets at a certain moment when capturing 20 additional mass (marker 3 in Fig. 3). Such a vortex structure of the waves is confirmed by a 21 series of images shown in Fig. 4, where we can clearly see that a sufficiently large number of 22 droplets deviate from the direction of the stream outwards from the film at various 23 expiration velocities and different nozzle designs. This can be explained by the 24 disintegration of the swirling roller of the fluid as it moves in the direction backward to the 25 main flow, and the roller (or its components) deviates to the periphery of the burner.

Figure 4. Liquid ejection across an expiration film upon hydraulic atomization of kerosene (a), (b) and biodiesel (c) at GA = 0: (a) cylindrical nozzle, GF = 2.7 g/s; (b), (c) annular nozzle, GF = 12.3 g/s **Figure 4.** Liquid ejection across an expiration film upon hydraulic atomization of kerosene (a), (b) and biodiesel (c) at GA = 0: (a) cylindrical nozzle, GF = 2.7 g/s; (b), (c) annular nozzle, GF = 12.3 g/s

1

Now we consider the dependence of the character of the fuel–air spray on the properties of the atomized liquids. Shown in Figs. 5 and 6 are pictures of the expiration of different fluids from cylindrical (Fig. 5) and annular (Fig. 6) nozzles with the same mass flow rate. The wave height above the level of the film depends apparently on the fluid viscosity because the relative shift of layers of the fuel becomes more difficult with increasing viscosity. Therefore, in Fig. 5a (water) one can easily see high wave formations. In Fig. 5b (kerosene) these formations have an appreciably lower height, and in Fig. 5c (a mixture of diesel and rapeseed oil), they are entirely absent. 1 Now we consider the dependence of the character of the fuel–air spray on the properties of 2 the atomized liquids. Shown in Figs. 5 and 6 are pictures of the expiration of different fluids 3 from cylindrical (Fig. 5) and annular (Fig. 6) nozzles with the same mass flow rate. The wave 4 height above the level of the film depends apparently on the fluid viscosity because the 5 relative shift of layers of the fuel becomes more difficult with increasing viscosity. Therefore, 6 in Fig. 5a (water) one can easily see high wave formations. In Fig. 5b (kerosene) these

7 formations have an appreciably lower height, and in Fig. 5c (a mixture of diesel and

expansion and separation from the surface, the intensity of the vortex increases rapidly and waves are formed. When moving downstream, the wave height above the level of the film increases (see markers *1* and *2* in Fig. 3). The growth of these waves is caused by the fact that swirled formations move in the axial direction with lower velocity than the film and then disintegrate into bundles and individual droplets at a certain moment when capturing additional mass (marker 3 in Fig. 3). Such a vortex structure of the waves is confirmed by a series of images shown in Fig. 4, where we can clearly see that a sufficiently large number of droplets deviate from the direction of the stream outwards from the film at various expiration velocities and different nozzle designs. This can be explained by the disintegration of the swirling roller of the fluid as it moves in the direction backward to the main flow, and the roller

**Figure 3.** Development and decay of the waves upon hydraulic spraying of kerosene through an annular nozzle (pho‐

2 The results of this series of experiments 3 allow us to make some assumptions about 4 the mechanism of the liquid film decay into 5 droplets. The film of the fuel is formed as a 6 result of the interflow of swirled liquid

(photo) atGF=17.5 g/s, GA= 0

Figure3. Development and decay of the waves upon hydraulic spraying of kerosene through an annular nozzle

(or its components) deviates to the periphery of the burner.

Figure2.Scheme of the formation of

to) at GF =17.5 g/s, GA = 0

332 Biofuels - Status and Perspective

film

waves–vortexes on the surface of the liquid

7 streams into a single stream along the length of the swirl chamber and the nozzle of the 8 injector. In this case, one can assert that when the fluid moves through the caves of the 9 injector, its outer layer is decelerated due to the friction with the surface and the velocity 10 components diminish in this layer. This gives rise to the shear stresses along the fuel film 11 thickness. We can assume that with an increase in the velocity, depending on the properties 12 of the fluid and geometrical parameters of the atomizer, the shear of the layers becomes so 13 significant that the outer sublayer is swirled in the opposite direction relative to the velocity 14 vector (the scheme is shown in Fig. 2).At the exit of the nozzle, after sudden expansion and 15 separation from the surface, the intensity of the vortex increases rapidly and waves are 16 formed. When moving downstream, the wave height above the level of the film increases 17 (see markers 1 and 2 in Fig. 3). The growth of these waves is caused by the fact that swirled 18 formations move in the axial direction with lower velocity than the film and then 19 disintegrate into bundles and individual droplets at a certain moment when capturing 20 additional mass (marker 3 in Fig. 3). Such a vortex structure of the waves is confirmed by a 21 series of images shown in Fig. 4, where we can clearly see that a sufficiently large number of 22 droplets deviate from the direction of the stream outwards from the film at various 23 expiration velocities and different nozzle designs. This can be explained by the 24 disintegration of the swirling roller of the fluid as it moves in the direction backward to the 25 main flow, and the roller (or its components) deviates to the periphery of the burner.

**Figure 2.** Scheme of the formation of waves–vortexes on the surface of the liquid film 1

Figure 5. Comparison of the expiration of different liquids at the same mass flow through the cylindrical nozzle without air supply at GA = 0, GF = 5 g/s: (a) water, (b) kerosene, and (c) mixture of diesel with rapeseed oil (50%–50%) **Figure 5.** Comparison of the expiration of different liquids at the same mass flow through the cylindrical nozzle with‐ out air supply at GA = 0, GF = 5 g/s: (a) water, (b) kerosene, and (c) mixture of diesel with rapeseed oil (50%–50%)

=20 g/s. **Figure 6.** The same as in Fig. 5 in the case of an annular nozzle and GA = 0, GF =20 g/s.

1

Figure 6. The same as in Fig. 5 in the case of an annular nozzle and GA = 0, G<sup>F</sup>

Figure 6. The same as in Fig. 5 in the case of an annular nozzle and GA = 0, G<sup>F</sup>

Apart from the expiration velocity from the nozzle, the spray angle mainly depends on the viscosity of the fluid rather than on the surface tension (see Figs. 5, 6). The spray angle decreases with increasing viscosity. An analogous result stems from the experiments [17]. On the other hand, most likely, the remoteness of the point of the film disintegration from the output section of the nozzle depends mainly on the surface tension coefficient. As we can see from Fig. 6, the self-decay of kerosene and diesel-oil mix, with close surface tension coefficients occurs approximately on the same generator length of the film while the self-decay of water (having a considerably higher surface tension), occurs much earlier.

Having studied the waves formed on the surface of liquid films, one can assume the presence of similar effects in the case of mixed fuel–air flow. In reality, oscillations in the fuel concen‐ tration are usually observed upon pneumatic spraying. Their magnitude may change depend‐ ing on the design of the sprayer unit, the injection velocity, and the properties of the ambient medium. In swirled flows in which the regions of the inverse fluid flows are formed, pulsations are observed on both the outer and inner borders of the burner. The formation of these pulsations is similar to the wave formation on the surface of the film (Fig. 2), but at the same time these effects are not related physically. Apparently, these formations occur behind the exit of the air nozzle due to sudden expansion and braking of the layer on the boundary with the external medium. Figure 7 demonstrates the independence of these vortexes on the waves formed on the surface of the film. Figure 7a shows the fuel spraying using a pressure swirl nozzle without air supply. The vortexes caused by the film of the fuel are visible only near the nozzle, but we do not see any pulsations of concentration elsewhere. Figure 7b shows the same injector with an external air swirler. We can easily see large wave formations propagating down through the flow and weakly correlating with the waves in the film.

Figure 6. The same as in Fig. 5 in the case of an annular nozzle and GA = 0, G<sup>F</sup> =20 g/s.

Figure 6. The same as in Fig. 5 in the case of an annular nozzle and GA = 0, G<sup>F</sup> =20 g/s.

**Figure 6.** The same as in Fig. 5 in the case of an annular nozzle and GA = 0, GF =20 g/s.

a considerably higher surface tension), occurs much earlier.

a

a

b

b

c

c

A B Figure 7. Expiration of kerosene through the centrifugal nozzle at GF = 5.5 g/s: (a) without air supply (GA = 0); (b) GA = 40 g/s

A B Figure 7. Expiration of kerosene through the centrifugal nozzle at GF = 5.5 g/s: (a) without air supply (GA = 0); (b) GA = 40 g/s

**Figure 7.** Expiration of kerosene through the centrifugal nozzle at GF = 5.5 g/s: (a) without air supply (GA = 0); (b) GA =

Apart from the expiration velocity from the nozzle, the spray angle mainly depends on the viscosity of the fluid rather than on the surface tension (see Figs. 5, 6). The spray angle decreases with increasing viscosity. An analogous result stems from the experiments [17]. On the other hand, most likely, the remoteness of the point of the film disintegration from the output section of the nozzle depends mainly on the surface tension coefficient. As we can see from Fig. 6, the self-decay of kerosene and diesel-oil mix, with close surface tension coefficients occurs approximately on the same generator length of the film while the self-decay of water (having

Having studied the waves formed on the surface of liquid films, one can assume the presence of similar effects in the case of mixed fuel–air flow. In reality, oscillations in the fuel concen‐ tration are usually observed upon pneumatic spraying. Their magnitude may change depend‐ ing on the design of the sprayer unit, the injection velocity, and the properties of the ambient medium. In swirled flows in which the regions of the inverse fluid flows are formed, pulsations

1

334 Biofuels - Status and Perspective

1

40 g/s

burner diameter upon liquid atomization by three different methods: (a) centrifugal method, GF = 5 g/s, GA = 0; (b) centrifugal–pneumatic method, GF = 5 g/s, GA = 40 g/s; (c) pneumatic method, GF = 20 g/s, G<sup>A</sup> **Figure 8.** Linear distribution of the average droplet diameter over the burner diameter upon liquid atomization by three different methods: (a) centrifugal method, GF = 5 g/s, GA = 0; (b) centrifugal–pneumatic method, GF = 5 g/s, GA = 40 g/s; (c) pneumatic method, GF = 20 g/s, GA = 40 g/s; –● water, –■ kerosene, –▲ mixture of diesel with rapeseed oil (50%–50%)

Figure 8. Linear distribution of the average droplet diameter over the

Now we consider the impact of fluid properties and the related aforementioned phenomena on the dispersity of aerosol when using different methods of atomization (Figs. 8, 9). The first and most studied method of droplet atomization is that of centrifugation (hydraulic atomiza‐ tion). The liquid is fed through a near-axis pressure swirl nozzle without the external air flow (Figs. 8a, 9a). The average velocity of the fuel nozzle outlet of is 19–26 m/s depending on the type of liquid. The centrifugal–pneumatic method of atomization is shown in Figs. 8b and 9b. In this case, the average velocities of the fuel and air have the same order of magnitude. To implement centrifugal–pneumatic atomization, the fluid is fed through a pressure swirl nozzle with the same mass flow rate (5 g/s) as in the first case. Additionally, the air is fed through external swirlers with the total mass flow rate of 40 g/s. The average fuel velocity at the burner outlet is the same as in the first method (19–26 m/s) and on the order of 25 m/s for the velocity of air. In the third (pneumatic) atomization method (Figs. 8c, 9c), the injection velocity of the fluid is smaller than the velocity of air.

When implementing this method, a small part of liquid (3 g/s) is fed through the near-axis nozzle, while the main part (17 g/s) is fed through the annular airblast injector, and air velocity in this case is the same as in the second method. In this case, the average velocity is 3–4 m/s. The radial distribution of the diameter D10 of droplets is shown in Fig. 8, where D10 is the arithmetic mean of the size in an ensemble. This parameter determines the most probable size of droplets in the given region, and it can be used in predicting the engine wake-up mode: the greater the number of small droplets that enters the spark discharge zone, the simpler their evaporation and ignition.

Figure 9 shows the distribution of Sauter's mean diameter D32 of droplets - the ratio of the volume of a droplet to its surface area being averaged over the ensemble at a given point of space. This parameter is important when predicting the efficiency and homogeneity of the fuel burning.

As we can see from Figs. 8a and 9a, the viscosity of the liquid has a significant impact on the dispersity of droplets obtained by the centrifugal method of spraying. Thus, in the case of water having a surface tension three times higher, one can obtain droplets that are even smaller when compared with kerosene, which has a viscosity 1.9 times greater. In this case, a highly viscous mixture of diesel with rapeseed oil does not form waves on the surface of the film and is not sprayed at all (Fig. 5c). In the case of centrifugal–pneumatic spraying, as we can see from Fig. 8b, the most important role is played by the surface tension coefficient. The curves in Fig. 8b are arranged according to a consecutive increase in the surface tension and correspond to kerosene (bottom line), a mixture of diesel and rapeseed oil (middle line), and water (upper line). In Fig. 9b, we can note an insufficient secondary atomization of certain droplets with a large size in the flow of liquid with high viscosity. However, upon closer inspection, we can see that, at the locations of the maxima of concentration, the aforementioned dependence also takes place and it is violated only on the axis of the device near the zone of reverse flows due to insufficient intensity of the air flow. Further fragmentation of the drops of the viscous fluid near the separation zone can be done by aerating the root region of the fuel–air spray.

= 40 g/s; –• water, –■ kerosene, – mixture of diesel with rapeseed

Now we consider the impact of fluid properties and the related aforementioned phenomena on the dispersity of aerosol when using different methods of atomization (Figs. 8, 9). The first and most studied method of droplet atomization is that of centrifugation (hydraulic atomiza‐ tion). The liquid is fed through a near-axis pressure swirl nozzle without the external air flow (Figs. 8a, 9a). The average velocity of the fuel nozzle outlet of is 19–26 m/s depending on the type of liquid. The centrifugal–pneumatic method of atomization is shown in Figs. 8b and 9b. In this case, the average velocities of the fuel and air have the same order of magnitude. To implement centrifugal–pneumatic atomization, the fluid is fed through a pressure swirl nozzle with the same mass flow rate (5 g/s) as in the first case. Additionally, the air is fed through external swirlers with the total mass flow rate of 40 g/s. The average fuel velocity at the burner outlet is the same as in the first method (19–26 m/s) and on the order of 25 m/s for the velocity of air. In the third (pneumatic) atomization method (Figs. 8c, 9c), the injection velocity of the

When implementing this method, a small part of liquid (3 g/s) is fed through the near-axis nozzle, while the main part (17 g/s) is fed through the annular airblast injector, and air velocity in this case is the same as in the second method. In this case, the average velocity is 3–4 m/s. The radial distribution of the diameter D10 of droplets is shown in Fig. 8, where D10 is the arithmetic mean of the size in an ensemble. This parameter determines the most probable size of droplets in the given region, and it can be used in predicting the engine wake-up mode: the greater the number of small droplets that enters the spark discharge zone, the simpler their

Figure 9 shows the distribution of Sauter's mean diameter D32 of droplets - the ratio of the volume of a droplet to its surface area being averaged over the ensemble at a given point of space. This parameter is important when predicting the efficiency and homogeneity of the fuel

As we can see from Figs. 8a and 9a, the viscosity of the liquid has a significant impact on the dispersity of droplets obtained by the centrifugal method of spraying. Thus, in the case of water having a surface tension three times higher, one can obtain droplets that are even smaller when compared with kerosene, which has a viscosity 1.9 times greater. In this case, a highly viscous mixture of diesel with rapeseed oil does not form waves on the surface of the film and is not sprayed at all (Fig. 5c). In the case of centrifugal–pneumatic spraying, as we can see from Fig. 8b, the most important role is played by the surface tension coefficient. The curves in Fig. 8b are arranged according to a consecutive increase in the surface tension and correspond to kerosene (bottom line), a mixture of diesel and rapeseed oil (middle line), and water (upper line). In Fig. 9b, we can note an insufficient secondary atomization of certain droplets with a large size in the flow of liquid with high viscosity. However, upon closer inspection, we can see that, at the locations of the maxima of concentration, the aforementioned dependence also takes place and it is violated only on the axis of the device near the zone of reverse flows due to insufficient intensity of the air flow. Further fragmentation of the drops of the viscous fluid

near the separation zone can be done by aerating the root region of the fuel–air spray.

fluid is smaller than the velocity of air.

evaporation and ignition.

336 Biofuels - Status and Perspective

burning.

diameter upon liquid atomization by three different methods (designations are the same as in Fig. 8). **Figure 9.** Distribution of the Sauter's drop diameters over the burner diameter upon liquid atomization by three differ‐ ent methods (designations are the same as in Fig. 8).

Figure 9. Distribution of the Sauter's drop diameters over the burner

In the pneumatic method of spraying, as we can see from Fig. 8c, the influence of the properties of liquids on the linear size of droplets is nearly absent and all the curves merge into one. This parameter of aerosol is mainly determined by the air flow. Sauter's mean diameter of droplets (Fig. 9c) depends also on the surface tension coefficient. The influence of individual large drops is somewhat smoothened compared with the centrifugal–pneumatic method of spraying.

In the centrifugal–pneumatic method of spraying, the dispersity of droplets averaged over the whole section of the flambeau is the best. However, note that the ratio of the mass consumption of air to the mass flow rate of fuel (AAFR), which, in particular, determines the quality of spraying, equals 8 for this method and 2 in the case of the pneumatic method. Thus, in the case of low gas-turbine engine operating modes and high viscosity fuels like biodiesel or biokerosene, the centrifugal-pneumatic atomization method is optimal, while the pneumatic method is optimal for high operating modes.

1

### **3. The selection of mixed liquid fuel**

For conducting the hot tests, ethanol and mixed biofuel on the basis of aviation kerosene (as most close relating to a turbine engine) have been chosen as alternative fuels. As one can see from Table 1, the combustion value of biofuels (especially ethanol) is significantly lower than that of fossil fuels. Furthermore, the viscosity of vegetable oils is ten times greater than the viscosity of the organic fuel. Therefore, for aircraft engines a blend of biofuels with conven‐ tional aviation fuels is more preferable then pure biofuels. At present, the use of industrially processed aviation biofuels in the Russian territory is not possible. Various versions of a percentage ratio of components of combustible mixtures on the basis of plant oil and ethanol (Table 2 and Fig. 10) have been investigated. Plant oil is necessary as surfactant for ethanol dissolution in the fuel. Aviation kerosene TS 1 or gasoline have been chosen as the main component of the mixture. The optimum ratio of components has been selected.

**Figure 10.** Photos of mixed fuels; signatures correspond to embodiments of the Table 2


**Table 2.** The embodiments of mixed liquid fuels

**3. The selection of mixed liquid fuel**

338 Biofuels - Status and Perspective

1

For conducting the hot tests, ethanol and mixed biofuel on the basis of aviation kerosene (as most close relating to a turbine engine) have been chosen as alternative fuels. As one can see from Table 1, the combustion value of biofuels (especially ethanol) is significantly lower than that of fossil fuels. Furthermore, the viscosity of vegetable oils is ten times greater than the viscosity of the organic fuel. Therefore, for aircraft engines a blend of biofuels with conven‐ tional aviation fuels is more preferable then pure biofuels. At present, the use of industrially processed aviation biofuels in the Russian territory is not possible. Various versions of a percentage ratio of components of combustible mixtures on the basis of plant oil and ethanol (Table 2 and Fig. 10) have been investigated. Plant oil is necessary as surfactant for ethanol dissolution in the fuel. Aviation kerosene TS 1 or gasoline have been chosen as the main

component of the mixture. The optimum ratio of components has been selected.

1 2 3 4

5 6 7 8

9 10 11

**Figure 10.** Photos of mixed fuels; signatures correspond to embodiments of the Table 2

2 Figure. 10 Photos of mixed fuels; signatures correspond to embodiments of the Table 2

Analysis of the samples revealed that the use of different ratios of starting fuel, ethanol, and vegetable oil show results strikingly different from each other. It was possible to obtain wellblended homogeneous mixture only at certain narrow ranges of percentages of components. The embodiment 3 was chosen for further testing, as it showed the optimal ratio of components without settling on the bottom and without stratification. Variant 1, variants 6 and 7, and variant 8, which have also shown good mixing level, are notable for big maintenance of ethanol, kerosene or gasoline (up to 90 %). It is not beneficial in terms of the economic feasibility of introducing a new type of fuel.

For hot tests in the aviation combustion chamber, the mix in a ratio of 40% of kerosene TS-1, 20% of castor oil, and 40% of ethanol has been chosen as the most homogeneous and well mixed without any precipitations and stratifications. Its physical features are: ρ<sup>F</sup> = 850 kg/m3 , νF = 4.7 × 10–6 m2 /s, and σF = 27 × 10–3 N/m.
