Spray characteristics and controlling mechanism of fuel containing CO₂

Abstract

This paper presents studies of spray characteristics and controlling mechanism of fuel containing CO₂. Using diesel fuel containing CO₂ gas, experiments were conducted on diesel hole-type nozzles and simple nozzles. The steady spray and transient spray characteristics were observed and measured by instantaneous shadowgraphy, high-speed photography, phase Doppler anemometry (PDA) and LDSA respectively. The effects of CO₂ concentration in the fuel, the injection pressure, the nozzle L/D ratio, surrounding gas pressure and temperature on the atomization behavior and spray pattern were evaluated. The results show that the injection of fuel containing CO₂ can greatly improve the atomization and produce a parabolic-shaped spray; and the CO₂ gas concentration, surrounding gas pressure, temperature and nozzle configuration have dominant influences on spray characteristics of the fuel containing CO₂. New insight into the controlling mechanism of atomization of the fuel containing CO₂ was provided.

Keywords

spray characteristics / fuel atomization / fuel containing CO₂

Introduction

With the ever-increasing interest in both pollution control and fuel savings, a great deal of attention has been focused on every aspect of the fuel combustion process. Fuel injection and atomization is one area of major interest. The problem of particular concern is the provision of small droplets of fuel that will mix and evaporate rapidly so as to promote a cleaner burning. The injection of fuel containing dissolved gas, which is quite different from the conventional pressure atomization and the twin-fluid atomization, involves dissolving a gas into a fuel prior to injection. Upon injection, the gas is expected to come out of the solution, forming a gas phase within the fuel which has been supposed to improve the fuel atomization and combustion, and may possibly develop into a new combustion technique [1,2]. In order to obtain a better understanding of the atomization mechanism of the injection of fuel containing CO₂ for the possible application to engine combustion, the author and his collaborators carried out a series of studies on the atomization behavior of the fuel containing CO₂, using laser based diagnostics [3-11]. The steady spray characteristics of fuel containing CO₂ were observed and measured by instantaneous shadowgraphy, phase Doppler anemometry (PDA) and LDSA (an optical technique based on a narrow-angle forward scattering theory) respectively. The effects of CO₂ concentration in diesel fuel, the injection pressure and the nozzle L/D ratio were evaluated. Furthermore, the transient spray characteristics under different surrounding gas conditions were observed by means of high-speed photography. The effects of the injection pressure, surrounding gas pressure and temperature on the atomization pattern and transient spray behavior were examined. As a result, new insight into the controlling mechanism of atomization of the fuel containing CO₂ was provided.

Steady spray characteristics of fuel containing CO₂

Variation of spray pattern and Sauter mean diameter (SMD) with gas concentration is shown in Figs. 1 and 2. It can been seen that in the case of diesel nozzle, with no CO₂ gas, as shown in Fig. 1(a), the spray forms a typical sharp cone-shaped pattern with a spray core surrounded by clouds of atomized droplets. At a relatively low CO₂ gas concentration of 2.4% mass fraction, as shown in Fig. 1(b), the spray is also cone shaped, but, compared with Fig. 1(a), the spray core is rather thick and its surface looks fairly smooth. The resulting droplet size is larger than that of the fuel without CO₂ gas, which is remarkable in the relatively lower injection pressure region. This is the phenomenon called a negative effect of the injection of fuel containing CO₂. At a medium CO₂ concentration of 8.4% mass fraction, the dense central core persists and becomes thicker, but there occurs a noticeable change in the spray pattern. Numerous small satellite droplets form a spray mantle and the spray angle becomes wider, as shown in Fig. 1(c). At a large CO₂ concentration of 14.25% mass fraction, as shown in Fig. 1(d), the spray rapidly expands immediately upon leaving the orifice and characteristically forms a parabolic-shaped pattern, which is called flash atomization. The atomization occurs almost right at the nozzle exit. The spray angle is maximized, and the SMD is minimized. This phenomenon is recognized as a beneficial effect of the injection of fuel containing CO₂ gas. Thus, in terms of atomization, two completely contrary effects of the injection of fuel containing CO<Subscript>2</Subscript>gas, namely, a beneficial effect and a negative effect were found. It is indicated that for a certain nozzle configuration there exists a transition concentration of the gas necessary for good atomization. The beneficial effect is obtained when the gas concentration exceeds the transition concentration.

Figures 3-5 illustrate the radial and axial distribution of droplet size and droplet velocity of the fuel containing CO₂. It can be observed that with the increase in CO₂ gas concentration, the atomization is greatly improved. It is of interest to note that in the case of CO₂ gas concentration of 21.32%, the droplet size becomes almost uniform at the downstream of 44 mm from the nozzle exit. The axial velocity near the nozzle exit of the fuel containing CO₂ is larger than that of pure diesel fuel. With the increase in CO₂ concentration, the axial velocity increases due to the decrease in the density of fuel containing CO₂ within the orifice. With the increase in axial distance, the axial velocity of fuel containing relatively large amount of CO₂ decreases sharply, which are caused by good atomization and fine droplet size. Figure 5 further demonstrates that when flash atomization occurs, the radial velocity near the nozzle exit of the fuel containing CO₂ is much larger than that of pure diesel fuel. The sudden expansion immediately upon leaving the orifice of the fuel containing CO₂ is verified.

Figures 6 and 7 reflect the effect of the L/D ratio on the spray pattern, the spray angle, and the SMD as in simple nozzles. At a large CO₂ concentration of 14.25% mass fraction, the beneficial effect of the injection of fuel containing CO₂ gas is obtained in the region of larger L/D ratio. The spray angle is extremely large and the SMD is small. However, for a small L/D ratio of 4, flash atomization cannot be realized. The fuel emerges as a smooth and almost cylindrical fuel jet immediately near the nozzle exit. The spray angle is very small, and the SMD is slightly larger than that of the fuel without CO₂ gas. This suggests the large dependence of atomization improvement on the nozzle L/D ratio.

Transient spray characteristics of fuel containing CO₂ gas

In this work, the diesel fuel containing CO₂ gas prepared in a fuel vessel was compressed by the high-pressure nitrogen and was injected into the pressure vessel at a constant injection pressure of 5 and 11 MPa, respectively. The spray characteristics was observed and measured using a hole-type diesel nozzle with an orifice diameter of 0.3 mm by means of high speed photography under different surrounding gas conditions, including room temperature and atmosphere, p = 0.1 MPa, T = 293 K, ρ = 1.20 kg/m³; room temperature and high pressure, p = 0.74 MPa, T = 293 K, ρ = 8.64 kg/m³; and high temperature and high pressure, p = 2.02 MPa, T = 800 K, ρ = 8.64 kg/m³.

Figures 8 and 9 exhibit respectively the spray characteristics of fuel containing CO₂ at constant injection pressures of 5 and 11 MPa under different surrounding gas conditions. It can been seen that at the atmospheric condition, the spray rapidly expands immediately upon leaving the nozzle orifice and characteristically forms a flash-boiling spray pattern with unusual large spray angle, spray width and good atomization as depicted in Fig. 10. The vigorous vaporization and separations of CO₂ gas from the fuel on the part of spray tip especially at the low injection pressure can be observed, as revealed in Fig. 8. At the surrounding gas condition of room temperature and high pressure, with the increase in surrounding gas pressure and density, the spray angle decreases and the spray forms a sharp cone-shaped pattern with poor atomization, which is quite different from conventional diesel spray. At the condition of high temperature and pressure, with the increase in surrounding gas pressure and temperature, although visible spray width decreases due to vaporization at a high temperature, large initial spray angle and saw-toothed spray boundary can be observed especially in the case of low injection pressure, which is considered to be caused by flash vaporization and separations of the CO₂ from fuel. In terms of spray penetration, it is worth noticing that spray penetration and velocity is slightly larger in the case of high pressure and room temperature than in the case of atmospheric condition and high pressure, high temperature condition, as given in Fig. 11.

According to previous investigations of conventional diesel spray characteristics, as is known, diesel spray shape and penetration largely depend on surrounding gas density. With the increase in gas density, spray angle increases and penetration decreases. In this work, it can be found that the fuel containing CO₂ never follows this principle and there is a different controlling mechanism. In the case of fuel containing CO₂, the flash boiling phenomena has a dominant influence on the spray pattern, which is dependent on surrounding gas temperature, pressure and CO₂ concentration. The spray pattern further determines the spray shape and the penetration.

The experimental result further suggests that the injection pressure has an influence on the spray pattern and shape of fuel containing CO₂. It can be seen that the spray boundary is more saw-toothed at the injection pressure of 5 MPa than the injection pressure of 11 MPa. This indicates that the flash boiling phenomena and the gas separation due to supersaturation are more vigorous at the low injection pressure, which is believed to be caused by shorter equilibrium time from injection pressure to back pressure.

Factors determining the atomization of fuel containing CO₂

Due to complex molecular formula of diesel fuel, in order to reduce the problem to simpleness, N-dodecane was used to calculate the phase change process of CO₂-n-dodecane binary system. Figure 12 presents the variation of the phase change with the mole fraction X_CO2 of CO₂, pressure and temperature. The locus of critical points corresponding to each X_CO2 was estimated according to the methods of Leach [12] and Huber [13], and the two phase region was determined by the corrected equation of Peng [14]. It can been seen that the phase change of the binary system largely depends on the mole fraction of CO₂, pressure and temperature. Figure 13 further displays the injection process of fuel containing CO₂ gas. On the basis of theory of flash boiling, when the fuel containing CO₂ is injected into atmosphere or high temperature and pressure surrounding gas, the fuel will pass through the saturated liquid line or saturated vapor line of CO₂-fuel binary system, the flash boiling phenomena occurs in the spray, vigorous vaporization and separation of CO₂ from the fuel leads to a great improvement of atomization with unusual large spray angle and spray width, and furthermore results in a decrease in spray penetration. When the fuel is injected into room temperature and high pressure surrounding gas, if the fuel does not pass through the saturated liquid line, flash boiling does not occur. The CO₂ gas comes out of the fuel depending on the degree of supersaturation and residence time. Due to the short penetration time of the spray, the separation of CO₂ gas controlled by Henry’s rule is insufficient to create the expansive force in the spray. On the contrary, the existence of gas phase within the fuel will result in a suppression of atomization, leading to a decrease in spray angle, spray width and finally an increase in spray penetration.

Therefore, it is found that for a certain nozzle configuration and a certain CO₂ concentration, surrounding gas pressure and temperature have dominant influences on flash atomization of the fuel containing CO₂.

The discharge coefficient was further measured to evaluate the flow properties and estimate the quantity of CO₂ gas separated from the fuel within the nozzle orifice. Figures 14 and 15 show the effects of CO₂ gas concentration and nozzle L/D ratio on discharge coefficient. It is worth noting that the flash atomization and the parabolic-shaped spray pattern are associated with a large drop in Cd. This suggests that inside the orifice a substantial amount of CO₂ gas comes out of the fuel, which may be expected to create a compressible two-phase flow that leads to a sudden expansion immediately upon leaving the orifice. The resulting expansive force overcomes liquid cohesion and surface tension and is believed to account for the parabolic-shaped spray pattern of fuel containing CO₂. On the other hand the sharp cone-shaped spray with a fairly smooth and thick liquid core, that is deterioration of the atomization, is related to a slight decrease in Cd. This indicates that a comparatively small amount of CO₂ gas is separated from the fuel within the orifice, which seems not sufficient enough to produce the expansion force. Moreover, the existence of a gas-phase within the fuel results in a suppressing of the atomization, leading to a negative effect of fuel containing CO₂. The result shows that in the case of fuel containing CO₂, the flash separation of a substantial amount of CO₂ from the fuel within the nozzle orifice is critical.

On the basis of the observations and measurement [3,4,15], it is found that the fuel passes through the nozzle orifice, the orifice flow usually exhibits two characteristic patterns—the flow with contraction and reattachment, and the flow free from the wall, which cause a big difference in the pressure distribution inside the orifice, as shown schematically in Fig. 16. For the pattern with the flow with contraction and reattachment, as shown in Fig. 16(a), at point 2 in the contraction region, the pressure drops sharply to a local minimum (fuel vapor pressure). There exits a low-pressure region. For the pattern with the flow free from the wall, as shown in Fig. 16(b), the pressure gradually drops to atmospheric pressure inside the orifice.

For the case of sharp-edged orifice entrance, the orifice flow pattern depends to a large extent on the nozzle L/D ratio. When L/D is small, the flow is contracted near the edge of the orifice entrance and is not reattached to the wall of the orifice, that is, the flow free from the wall. Therefore, the orifice pressure is almost near to atmospheric pressure. The decrease of Cd is much smaller than that in the case of a larger L/D ratio, as shown in Fig. 15. This indicates that even though the CO₂ gas concentration is large, only a comparatively small amount of CO₂ gas is separated from the fuel within the orifice. Consequently, the flash atomization will not occur. When the nozzle L/D ratio is large, the flow reattachment within the orifice will occur. Therefore, the orifice pressure drops to the fuel vapor pressure in the flow contraction region. In this case, Cd decreases sharply. This indicates that inside the orifice a substantial amount of CO₂ gas comes out of the fuel, which is caused by the strong vacuum and the sudden exceedingly supersaturated fuel inside the nozzle orifice. As a result, the parabolic-shaped spray is formed.

The result shows that the pressure distribution within the orifice plays an important role in the separation of CO₂ gas from the fuel inside the nozzle and determination of the atomization effect of fuel containing CO₂ gas. Therefore, it is necessary that the designed nozzle configuration should have a favorable orifice flow pattern to create a strong vacuum, which will lead to flash separation of CO₂ gas from the fuel inside the orifice.

It can be further deduced that in the case of fuel containing CO₂, the two-phase flow within the orifice exhibits two patterns. When the L/D or CO₂ concentration is small, the flow surrounded with CO₂ gas bubbles will be freed from the wall, as shown in Fig. 17(a), which will make the jet more smooth and weaken the disturbance at the liquid-gas interface, resulting in a deterioration of atomization. When there exist a favorable orifice flow pattern and a sufficient gas concentration, the orifice will be choked with CO₂ gas bubbles, as shown in Fig. 17(b), which leads to sudden expansion immediately upon leaving the orifice.

Conclusions

1) The injection of fuel containing CO₂ gas can greatly improve the atomization and produce a parabolic-shaped spray. The spray angle is maximized, and the SMD is minimized and almost uniform.

2) The CO₂ gas concentration, surrounding gas pressure, temperature and nozzle configuration have dominant influences on flash boiling atomization of the fuel containing CO₂. These factors will determine the atomization behavior and spray pattern.

3) In the case of transient spray, the spray characteristics of fuel containing CO₂ are quite different from conventional diesel spray in spray shape and penetration.

4) The injection pressure has an influence on the spray pattern and shape of fuel containing CO₂. The flash boiling phenomena and the gas separation due to supersaturation are more vigorous at the low injection pressure, which is believed to be caused by shorter equilibrium time from injection pressure to back pressure.

5) The concept of injection of fuel containing CO₂ could be attractive in producing more efficient, clean engine and find use in a wide range of application.

References

[1] Karasawa T, Kurabayashi T, Saba H. The effect of compulsorily dissolved gas on the atomization and combustion of liquid fuel. In: Proceedings of the 10th Conference on Liquid Atomization, Japan, 1982, 25–30

[2] Solomn A S P, Chen L D, Faeth G M. Investigation of Spray Characteristics for Flashing Injection of Fuels Containing Dissolved Air and Superheated Fuels. NASA Contractor Report 3563, 1982

[3] Huang Z, Shao Y M, Seiichi S, Hisao N, Takao K, Tamotsu N. Atomization behavior of fuel containing dissolved gas. Atomization and Sprays, 1994, 4: 253–262

[4] Huang Z, Shao Y M, Siichi S, Hisao N, Takao K. The orifice flow pattern, pressure characteristics, and their effects on the atomization of fuel containing dissolved gas. Atomization and Sprays, 1994, 4(2): 123–133

[5] Huang Z, Shao Y M, Siichi S, Hisao N, Takao K. The role of orifice flow pattern in fuel atomization. In: Proceedings of the 6th International Conference on Liquid Atomization and Spray Systems, Rouen, France, 1994, 86–93

[6] Huang Z, Shao Y M, Seiichi S, Hisao N. Controlling mechanism and resulting spray characteristics of fuel containing dissolved gas. Journal of Thermal Science, 1994, 3(3): 191–199

[7] Huang Z, Shao Y M. A New fuel atomization concept using dissolved gas. Journal of Shanghai Jiaotong University (Science), 1995, 5(1): 33–39

[8] Huang Z, Zhang S Y, Zhang L F. Evaluation of the effect of injection of fuel containing dissolved gas (IFCDG) on diesel combustion and emissions. Journal of Combustion Science and Technology, 1996, 2(4): 299–306

[9] Qiao X Q, Huang Z. LDV measurements of drop velocity in flash boiling spray. In: Proceedings of ICLASS, Pasadena, CA, USA, 2000

[10] Huang Z, Yasuhiro D. Transient spray characteristics of fuel containing dissolved CO₂. In: The 6th Annual Conference on Liquid Atomization and Spray Systems—Asia. Busan, Korea, 2001

[11] Xiao J, Qiao X Q, Huang Z, Fang J H. Study of droplet size and velocity of fuel containing CO₂ spray by means of PDA. Chinese Science Bulletin, 2004, 49(11): 1195–1199

[12] Leach J W, Chappelear P S, Leland T W. Use of molecular shape factors in vapor-liquid equilibrium calculations with the corresponding states principle. AIChE Journal. American Institute of Chemical Engineers, 1968, 14(4): 568–576

[13] Huber M L, Hanley H J M. The corresponding-states principle: dense fluids. In: Millat J, Dymond J H, de Castro C A N, eds. Transport Properties of Fluids (Ch.12). Cambridge: Cambridge University Press, 1996

[14] Peng D Y, Robinson D B. A new two-constant equation of state. Industrial & Engineering Chemistry Fundamentals, 1976, 15(1): 59–64

[15] Takao K, Masaki T, Kazuhiro A, Shiga S, Toshio K. Effect of nozzle configuration on the atomization of a steady spray. Atomization and Spray, 1992, 2(4): 411–426

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg