Liquid Fuel

The concentration of methane in liquid fuel was controlled by the dissolving pressure of methane, and the flashing phenomenon produced by the separati...
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Energy & Fuels 2005, 19, 2050-2055

Experimental Study on Flashing Atomization of Methane/Liquid Fuel Binary Mixtures Junqiang Zhang,*,† Deming Jiang,† Zuohua Huang,† Tomio Obokata,‡ Seiichi Shiga,‡ and Mikiya Araki‡ School of Energy and Power Engineering, Xi’an Jiaotong Univisity, Xi’an 710049, People’s Republic of China, and Department of Mechanical Engineering, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Received March 23, 2005. Revised Manuscript Received April 18, 2005

This paper aims to investigate the steady spray characteristics of kerosene and diesel fuel containing dissolved methane. The spray pattern images were captured using a digital camera at the nozzle exit, and the spray droplet sizes were measured using a particle size analyzer (LDSA 1300A) based on the narrow-angle forward-scattering theory. Six types of the straight-hole nozzles with different length/diameter ratios (L/D) were employed in the study. The concentration of methane in liquid fuel was controlled by the dissolving pressure of methane, and the flashing phenomenon produced by the separating dissolved methane was utilized to improve the atomization of the spray. Meanwhile, the parameters, including the spray angle, the Sauter mean diameter, and the discharge coefficient were measured, and the droplet size distribution was fitted using the Rosin-Rammler distribution function. Furthermore, comparison of the spray characteristics between kerosene and diesel fuel with dissolved methane was made. The study reveals that, for a given L/D ratio nozzle, there is the corresponding critical value of methane concentration. When the methane concentration in liquid fuel is larger than the critical value, the liquid fuel atomization is found to improve, whereas below the critical value, the atomization would be suppressed. Moreover, spray angles of liquid fuel containing dissolved methane give smaller values compared to that of pure liquid fuel under lower concentration of methane, but it will increase dramatically when the concentration is over a certain value. Due to low viscosity, the flashing atomization of kerosene can be improved remarkably with the dissolved methane in fuel compared to that of diesel fuel containing dissolved methane in the case of high methane concentration.

Introduction With the ever-increasing interest in environment protection, the restriction on exhaust pollution is getting more stringent for many power systems, e.g., diesel engine, industrial gas turbine, and oil burner, which all involve combustion of fuel sprays. High-quality liquid fuel atomization will improve the combustion and decrease the emission of pollution products. The flashing atomization method has been studied for many years for improving the atomization of liquid jets. Some researchers employed the phase transition of superheated liquids to produce flashing atomization,1-3 and others used the fuel containing dissolved gas to produce the flashing phenomenon for improving atomization. In the latter case, upon injection, the gas dissolved will come out of the blended fuel and form bubbles within * Corresponding author. Tel.: +86-2982663421. Fax: +862982668789. E-mail: [email protected]. † Xi’an Jiaotong Univisity. ‡ Gunma University. (1) Oza, R. D.; Sinnamon, J. F. An Experimental and Analytical Study of Flash boiling Fuel Injection. SAE Paper, 830590, 1983. (2) Park, B. S.; Lee, S. Y. An Experimental Investigation of the Flash Atomization Mechanism. Atomization Sprays 1994, 4 (2), 159-179. (3) Suzuki, M.; Yamamoto, T.; Futagami, N. Atomization of Superheated Liquid Jet. First International Conference on Liquid Atomization and Spray Systems, Tokyo, Japan, 1978.

the liquid fuel due to the rapid reduction of pressure. When growing and reaching a condition where the viscosity and surface tension can be overcome by the expansion force of the separated gas, the bubbles explode and form flashing atomization at the nozzle’s exit, resulting in an improvement in atomization. Solomon et al.4 studied the effect of air dissolved in fuel on spray characteristics. They suggested that, for liquid fuel with a low concentration of air, an expansion chamber was necessary in the injector passage in order to improve atomization. Senda et al.5 and Huang et al.6,7 investigated the phase transition of liquefied CO2 dissolved in diesel fuel to initiate flash boiling and improve atomization of diesel fuel. They concluded that, to improve liquid atomization, the concentration of dissolved CO2 in liquid fuel should be above a certain value. (4) Solomon, 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. (5) Senda, J.; Ikeda, M.; Yamamoto, M.; Kawaguchi, B.; Fujimoto, H. Low Emission Diesel Combustion System by Use of Reformulated Fuel with Liquefied CO2 and n-Tridecane. SAE Paper, 1999-01-1136. (6) Huang, Z.; Shao, Y. M.; Shiga, S. Atomization Behavior of Fuel Containing Dissolved Gas. Atomization Sprays 1994, 4 (3), 253-262. (7) Huang, Z.; Shao, Y. M.; Shiga, S. The Orifice Flow Pattern, Pressure Characteristics, and Their Effects on the Atomization of Fuel Containing Dissolved Gas. Atomization Sprays 1994, 4 (2), 123-133.

10.1021/ef0500742 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/17/2005

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Energy & Fuels, Vol. 19, No. 5, 2005 2051 Table 1. LDSA 1300A Parameters type of laser power/mW wavelength/nm diameter of laser beam/mm focal length of lens/mm range of diameter/µm range of SMD/µm droplet size distribution

Figure 1. Schematic diagram of experimental setup.

All the gases used in foregoing research, however, were unburnable gases, so the power performance of these power systems would be influenced adversely by increasing the fraction of gas dissolved, even though the atomization of the spray was improved. Recently, Gemci et al.8 also examined the flashing atomization of a hydrocarbon solution containing n-hexadecane and nbutane. They suggested that, under appropriate conditions, the presence of a small amount of n-butane can significantly enhance the atomization of n-hexadecane. On the basis of this background, the objective of this study is to dissolve a combustible gas, methane, into kerosene and diesel fuel and to explore further the flashing atomization of liquid fuel containing dissolved gas. For a power system, the atomization of liquid fuel is expected to improve by separating methane, and the emissions could be controlled by methane combustion. This idea can be further explained as follows: the flashing atomization due to methane dissolution would help to produce good quality mixtures of air and fuel, and this would improve the combustion. In addition, for mixtures of methane/liquid fuel and air in a combustion chamber, the liquid fuel would burn first, due to its low autoignition temperature. Its reaction products including soot could be further oxidated in the late methane combustion period (the autoignition temperature of methane is 923 K). As a result, the emission of particles may be reduced. In this paper, activities were first devoted to determining the influence of dissolved methane in liquid fuels on the atomization of steady spray through a group of straight-hole nozzles under atmospheric conditions. The results show great influence on liquid atomization from the parameters, including the fuel properties, the concentration of dissolved methane in the liquid fuel, the nozzle configuration, and the injection pressure. 1. Experimental Apparatus and Method A schematic diagram of the experimental apparatus is shown in Figure 1. The setup consisted of two sections, a system to measure methane concentration and an injection system. Two gas bottles were used in the experiment. One was filled with methane used as the dissolved gas, and the other was filled with nitrogen gas, which was employed to adjust the injection pressure. Liquid fuels and methane were mixed in a high-pressure vessel, as shown in Figure 1, and this vessel also supplied fuels toward the nozzles. The fuel injection system included a nitrogen gas bottle, high-pressure vessel, (8) Gemci, T.; Yakut, K.; Chigier, N.; Ho, T. C. Experimental Study of Flash Atomization of Binary Hydrocarbon Liquids. Int. J. Multiphase Flow 2004, 30, 395-417.

He-Ne laser 2 632.8 10 300 1.8-510 10-150 Rosin-Rammler

high-pressure pipeline, and nozzle. A low-pressure vessel and a water manometer were used to measure the methane concentration. Before measuring spray characteristics of liquid fuels containing dissolved methane, the concentration of dissolved methane was measured. From the high-pressure vessel, a small amount of liquid fuel containing dissolved methane was introduced into the low-pressure vessel of which the volume was known in advance, and the dissolved methane would be separated from the liquid fuel at low ambient pressure. As a result, the pressure increased slightly in the low-pressure vessel. The increase in pressure was measured by the water manometer connected with the low-pressure vessel. According to the equation of state for a gas, the volume of methane separated could be calculated under standard ambient conditions (273.16 K, 101 325 Pa). The amount of liquid fuel in the low-pressure vessel was weighted by use of an electronic balance (MP-3000) and its volume could be calculated through density. Thus the concentration (C) of methane dissolved in liquid fuel could be defined as

C)

Vmethane (mL mL-1) Vfuel

where Vmethane is the volume of dissolved methane under standard ambient conditions and Vfuel is the volume of liquid fuel in which the methane was dissolved. According to Henry’s law, the solubilities of methane in liquid fuels increase with the increase of dissolving pressure, so the concentration could be controlled by the dissolving pressure. Enough time was needed for reaching a uniform mixture of liquid fuels and methane. In this study, the range of dissolving pressure was set from 0.1 to 10 MPa. The liquid fuels containing dissolved methane were compressed by high-pressure nitrogen, passed through the nozzles, and injected into atmospheric environment. Injection pressure (Pinj) was set from 5 to 10 MPa. Six straight-hole nozzles were used in the study, whose diameters (D) were the same value (0.3 mm), and the ratios of hole length to diameter, L/D, were 5, 10, 20, 30, 40, and 50. Droplet size was measured by an LDSA 1300A (Toh-Nichi Computer Application Co.) based on a narrow-angle forward scattering technique, and the Rosin-Rammler distribution function was chosen for data fitting. The parameters set are listed in Table 1. SMD in the table denotes Sauter mean diameter. A digital camera, POWER SHOT 20, was used to take the photograph of the injection spray. The fuels used in the experiment were JIS1 kerosene and JIS2 diesel fuel.

2. Results and Discussions on Kerosene 2.1. Spray Pattern and Angle. Figure 2a illustrates the effect of concentration of methane dissolved in kerosene on the spray patterns. The L/D value of the nozzle is 50 and the injection pressure is 8.5 MPa. The result clearly shows that the dissolved gas does have a significant influence on the fuel spray patterns. For pure kerosene and kerosene containing dissolved methane with concentration of 10.8 mL mL-1, the fuel jets do not extend remarkably at the nozzle exit, and they give clear

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Figure 3. Effect of L/D and methane concentration on discharge coefficient.

Figure 2. Variation of spray pattern and spray angle with methane concentration: (a) spray pattern of kerosene (L/D ) 50, Pinj ) 8.5 MPa) and (b) spray angle.

liquid cores at the center of the sprays. Around each liquid core, a thin layer of drops exists. This suggests that a small concentration has little influence on spray patterns. For concentrations of 21.3 mL mL-1 and above, completely different spray patterns appear at the nozzle exit due to rapid separating of dissolved methane. Moreover, the spray angles increase significantly compared to those of pure kerosene. Figure 2b shows the variation of spray angle versus methane concentration for three nozzles with L/D of 10, 30, and 50. The spray angle was obtained by measuring the width of spray image at a distance 60D downstream of the nozzle exit. At low concentrations, spray angles of kerosene containing dissolved methane give smaller values than those of pure kerosene. For concentration from 10.8 to 21.3 mL mL-1, spray angles will increase rapidly. When concentrations of methane are above 21.3 mL mL-1, spray angles will remain at almost the same value. The results can be explained as follows: The kerosene with a low concentration of dissolved methane is likely to form small bubbles. According to the viscosity expression of two-phase flow with lower void fraction,9 the viscosity will be larger than that of pure kerosene; thus, the radial expansion of liquid fuel would be suppressed when being injected from the nozzle, resulting in the small spray angle. For kerosene with a large concentration of methane, the void fraction is large and large bubbles will form easily, so the two-phase flow with low viscosity will be produced within the nozzle. In addition, the higher the (9) Clement, K. Two-Phase Flow: Theory and Application; Tayor & Francis: New York, 2003; pp 140-141.

concentration of dissolved methane, the more the gas will separate from the blended fuel when fuel is injected from the nozzle. As a result, for the fuel with a large concentration, the expansion force becomes larger, and the liquid fuels are broken up strongly at the exit of the nozzles. The droplets also move outward along the radial direction quickly. Moreover, no liquid cores appeared in the sprays, and uniform and extended atomization patterns are formed. So the kerosenemethane jets behave like the high-pressure gas jet, they expand immediately after exiting the nozzles, and larger spray angles are formed. The results indicate the relationship between the spray angle and the methane concentration. For liquid fuel with the same concentration of methane, spray angles will increase with the increase of L/D value. This would be due to the fact that the liquid fuel containing dissolved methane takes a long residence time inside the nozzles and, before being injected out, produces more large bubbles inside the nozzle. 2.2. Discharge Coefficient. Figure 3 shows the discharge coefficient (Cd) as a function of L/D value and concentration of methane. Fuels are injected into an atmospheric environment under the injection pressure of 8.5 MPa. Cd values are obtained by dividing the actual flow rates by theoretical flow rates. The experiments were conducted under four kinds of dissolved methane concentrations, while L/D values were in the range of 5-50. The Cd of a straight-hole nozzle was determined by the pressure loss along the flow passage and by the available flow area of contracted flow. The result shows that the Cd value decreases with the increase of L/D for each fuel, and this would be due to fuels reattaching inside walls of nozzles. With the increase of L/D value, the pressure loss increases, consequently resulting in the decrease in Cd value. For the liquid fuel with more dissolved gas, the same trend is observed, and the Cd value decreases with the increase of L/D ratio more rapidly. For the same nozzle, the increase of methane concentration in fuel also decreases the Cd value. When the fuel with a large concentration of methane flows through the passage of the nozzle from high-pressure upstream to low-pressure downstream, more gas would be released from the liquid fuel to form bubbles and then the bubbles grow rapidly, resulting in a decrease in Cd. The

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Figure 5. Effect of injection pressure on SMD: (a) L/D ) 10, (b) L/D ) 50. Figure 4. SMD variation with the measuring position: (a) L/D ) 10, (b) L/D ) 50.

higher the concentration of methane is, the larger the bubbles that are formed, and the lower the Cd value will be. 2.3. Variation of SMD with the Measuring Position. The measuring position is set as the axial distance (H) from the nozzle exit to the center of the laser beam. The influence of the measuring position on SMD is examined at the injection pressure of 8.5 MPa. The results for L/D ) 10 and 50 are presented in Figure 4, parts a and b, respectively. It can be observed that SMD values increase with the increase of the axial distance, and this would be due to the behavior of droplet evaporation and dispersion, leaving large droplet to be detected. With the increase of axial distance, the running period of the droplets to the measuring position becomes longer than the lifetime of a large number of small droplets produced in the spray. In addition, it is easier for small particles to disperse than for large droplets in the flow, so at large axial distance, more small particles will disperse outside of sprays and the number fraction of large droplets near the spray centerline increases. The SMD measured in this experiment is the property of droplets included in the volume of the laser beam passing through the spray centerline. Thus, the longer the axial distance of the measuring position, the greater the fraction of large drops the volume of the laser beam contains and the larger the SMD values that are recorded. Since the measuring position influences spray characteristics, except under special circumstances, the experiment was conducted under the constant measuring position of 800 mm from the nozzle exit.

Figure 4 also shows that the dissolved methane has a negative influence on SMD for the nozzle with L/D of 10 but a positive influence for L/D of 50 over all measuring positions from 500 to 1000 mm. According to the literature,2 the flow patterns of two-phase flow inside the nozzle did greatly influence atomization. A previous study showed that annular flow is the best for atomization, succeeding by the slug flow. Bubble flow, however, often suppresses the atomization. One factor relating to the flow patterns is the residence time of fuels inside the nozzle. For the straight-hole nozzle, the residence time increases with the increase of L/D under the same injection pressure, with fully developed bubbles, making formation of the slug flow and the annular flow easy. When the effect for improving atomization exceeds the suppression influence due to the increased friction with the increase of L/D, the atomization would be promoted. The study suggests that the parameter L/D plays a great role in the atomization of liquid fuel containing dissolved methane, as the flow pattern of gas-liquid two-phase flow produced inside the nozzle would be changed by L/D settlement. In this figure, the phenomenon that the SMD values of fuel containing dissolved methane for L/D of 50 are always larger than that of pure kerosene for L/D of 10 may be attributed to the inadequacy of dissolved methane and underdeveloped bubbles. If the blended fuel is heated during the injection process, the bubbles will probably grow quickly and the slug flow and/or the annular flow can be formed easily, consequently improving the atomization. 2.4. Effect of Injection Pressure on SMD. The effect of injection pressure on SMD is shown in Figure 5 for different fuels and L/D configurations. With the

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Figure 6. Effect of L/D and methane concentration on SMD.

Figure 7. Effect of methane concentration on SMD.

increase of injection pressure, SMD values decrease in a similar trend for all fuels. In the case of L/D of 10, as shown in Figure 5 a, all fuels containing dissolved methane have larger SMD values than that of pure kerosene under the same injection pressure. For L/D of 50, as shown in Figure 5b, only the fuel with a concentration of 5.3 mL mL-1 suppresses atomization compared with that of pure kerosene over the whole range of injection pressure. In the case of a concentration of 10.8 mL mL-1, SMD gives a larger value than that of pure kerosene when the injection pressure is less than 7 MPa, it gives a smaller value than that of pure kerosene when the injection pressure is greater than 7 MPa, and the same value is shown for SMD at the injection pressure of 7 MPa. This suggests that, when injection pressure is greater than 7 MPa, the dissolved methane could improve the atomization, whereas below 7 MPa, the atomization will be suppressed. The figure also indicates that the critical concentration for improving atomization will decrease with the increase of injection pressure. For the fuels with methane concentration of 29.7 mL mL-1 and above, dissolved methane would play a positive role on atomization within the experimental conditions, and SMD values are all less than that of pure kerosene in this paper. 2.5. Variation of SMD with L/D. The effect of the dimensionless length L/D on SMD is given in Figure 6 under an injection pressure of 10 MPa. The solid line in the figure represents SMD for pure kerosene. It is shown that SMD will increase with the increase of L/D due to the large friction loss for long nozzles. Similarly, the fuels with a concentration of 5.3 and 10.8 mL mL-1 have the same trends for SMD versus L/D ratio. For concentrations of 29.7, 39.5, and 45.1 mL mL-1, SMD values increase rapidly versus L/D at the initial stage when L/D is less than 20 and then keep almost the same value when L/D is larger than 20. Particularly, in the case of a concentration of 45.1, the SMD value shows a slight decrease with the increase of L/D when L/D is larger than 20. This behavior can be explained as follows: For the fuels with a high concentration of methane, annular and slug two-phase flows will easily occur with the increase of L/D, and the influence for improving atomization in the presence of long residence time exceeds that of suppression for atomization caused by the friction.

The SMD distribution region for pure kerosene and several types of blended fuel is shown in Figure 6. The SMD curve of pure kerosene divides the SMD region into two regions. The below region represents the region for atomization improvement, and the upper region represents the suppression region. The figure clearly shows that the effect of dissolved methane on atomization is related to both the concentration of dissolved methane and the L/D ratio. For the experimental conditions in this paper, the phenomenon for atomization improvement does not appear in the case of L/D less than 10. However, in the case of L/D larger than 20, the dissolved methane can improve the atomization. 2.6. Effect of Methane Concentration on SMD. The variation of SMD versus methane concentration for L/D of 10, 30, and 50 under injection pressure of 10 MPa is presented in Figure 7. The dashed line represents the SMD values for pure kerosene under the same injection pressure and L/D ratio. Due to the influence of friction loss, SMD of pure kerosene will increase with the increase of L/D ratio. For L/D of 30 and 50, there exists a peak value for SMD, and the dashed line intersects the SMD curve at a certain position. The methane concentration at the intersection position suggests the critical value for atomization improvement. When the concentration of dissolved methane is less than the critical value, a negative behavior to atomization will occur; only for the circumstance over the critical value could the positive influence be realized. For L/D of 10, no intersection point will occur, and atomization is suppressed within the experimental conditions, and the short residence time within the nozzle would be responsible for this. Furthermore, it can be seen that the sensitivity of SMD to L/D decreases with the increase of methane concentration. The results can be analyzed as follows. Fuel with a low concentration of dissolved methane is likely to form bubble flow and to have a smaller expansion force; those would give an adverse influence on atomization due to the difficulty to overcome the surface tension. In contrast to this, fuel with a high concentration of dissolved methane could undergo slug or the annular flow with low viscosity, and its strong expansion force would be helpful to overcome the fuel surface tension, resulting in the improvement of atomization.

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50. The results show that the SMD of kerosene is smaller than that of diesel fuel over the whole concentration investigated. Nevertheless, these two types of fuel reveal similar trends of SMD value versus methane concentration. Figure 8b gives the variation of SMD versus L/D value. In the cases of pure liquid fuel, little difference of SMD between kerosene and diesel fuel is observed. However, for fuel containing dissolved methane, the SMD of kerosene is much smaller than that of diesel fuel from L/D of 5-50. For diesel fuel with the same methane concentration as in kerosene, to improve atomization, large L/D nozzles are required. We can also suggest that diesel fuel needs a high concentration of dissolved methane for improving the atomization while kerosene can easily realize atomization improvement through dissolving methane into it. Therefore, for flashing atomization, the action of improving atomization by dissolved methane would be enhanced due to lower liquid fuel viscosity, so the lower viscosity is considered to improve flashing atomization. 4. Conclusions

Figure 8. Comparison of SMD between kerosene and diesel fuel: (a) variation of SMD with methane concentration (L/D ) 50), (b) variation of SMD with L/D.

3. Comparison of Diesel Fuel and Kerosene Results The above sections all deal with kerosene. To make clear the influence of different fuels by dissolving methane on spray characteristics and flashing atomization, the experiment on diesel fuel was also carried out. The main properties that influence spray characteristics are fuel surface tension and viscosity. Although diesel fuel and kerosene have similar surface tension, the viscosity of diesel fuel is far greater than that of kerosene, and this would give their difference on fuel flashing atomization. Meanwhile, the solubility of dissolved gas in liquid fuel is another important parameter to determine the spray characteristics of flashing atomization. At the same dissolving pressure, the solubility of methane in kerosene is larger than that of in diesel fuel, resulting in different spray characteristics of flashing atomization. The comparison in atomization for kerosene and diesel fuel containing dissolved methane is given in Figure 8. The measuring position is at 1000 mm downstream of the nozzle exit, and the injection pressure is 10 MPa. Variation of SMD versus methane concentration is shown in Figure 8a with L/D value of

The study in steady spray behavior was performed for kerosene and diesel fuel containing dissolved methane under different conditions. The main results are summarized as follows: (1) The influence of dissolved methane on fuel atomization depends on fuel properties, L/D value, methane concentration, and injection pressure. For a given nozzle, there exists a critical concentration of dissolved methane. Over this value, atomization of liquid fuel will be improved, while below this value atomization will be suppressed. The larger L/D nozzle will conduce the increase in the residence time of fuels containing dissolved methane inside nozzles and then improve atomization. (2) At a low concentration of methane, spray angle decreases slightly due to high viscosity gas-liquid twophase flow. However, when methane concentration is over a certain value, the spray angle will increase dramatically due to the methane separation and finally remain at almost the same value. (3) Due to the lower viscosity of kerosene, the flashing atomization of kerosene can be improved remarkably by dissolving methane in the fuel compared to that of diesel fuel containing dissolved methane in the case of a high methane concentration. Acknowledgment. This work was supported by the State Key Project of Fundamental Research Plan under grant 2001CB209208, by National Natural Science Foundation of China through grant No.50136040, and by the Research Fund of the Doctoral Program of Higher Education of China through grant 20020698044. EF0500742