Energy & Fuels 2007, 21, 2991-2997
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Investigation on Methanol Spray Characteristics Gong Yanfeng,*,† Liu Shenghua,‡ and Li Yu‡ Engine Performance DeVelopment Section, Engine Department, Research and DeVelopment Center, First Automobile Works, Changchun, 130011, People’s Republic of China, and School of Energy and Power Engineering, Xi’an Jiaotong UniVersity, Xi’an, 710049, People’s Republic of China ReceiVed October 11, 2006. ReVised Manuscript ReceiVed June 28, 2007
The effects of opening pressure, ambient density, and nozzle diameter on penetration length and cone angle of methanol sprays were investigated. The methanol sprays schlieren photographs were recorded by a highspeed charge coupled device (CCD) camera. The methanol spray characteristics were analyzed. The methanol spray penetration length increases with the increase of the opening pressure. The methanol spray penetration and tip velocity decrease quickly with the increase of ambient density; on the contrary, they will increase with the increase of nozzle diameter. Between 12 and 18 MPa, the opening pressure has little influence on the spray angle and the angle remains nearly constant during the whole injection process. The spray cone angle increases with the increase of the ambient density or the nozzle diameter. Compared with diesel, the penetration of methanol is shorter and the cone angle of methanol is larger at the same experimental condition. The temporal evolution law of Hiroyasu was modified according to the experimental data, and the new correlation describes the methanol spay characteristics perfectly.
1. Introduction Diesel engines, due to their dominant advantages of high thermal efficiency, rigid and simple structure, and fuel economy, have played a dominant role in the fields of power. They have been widely used in commercial vehicles and other domestic and industrial applications. However, the pollutants emitted from diesel engines are detrimental to human health and to the ecological environment. The major pollutants emitted from diesel engines are particulate matter (PM) and nitrogen oxide (NOX). Hence the diesel engines industry is under increasing pressures worldwide to find methods to reduce PM and NOX emissions. But it is difficult to reduce NOX and PM in normal diesel engines simultaneously due to the tradeoff curve between NOX and PM.1,2 The diesel industry, driven by strict pollutant emissions regulations and energy crisis, expect to find some new fuels that can substitute for diesel. The idea of using oxygenated fuels as a means of producing cleaner diesel engines was introduced over 50 years ago.3 Consequently, a large number of oxygenates in the form of carbonates, ethers, esters, and alcohols, such as dimethyl carbonate (DMC), dimethyl ether (DME), dimethoxymethane (DMM), and methanol, etc., have been added to diesel or fueled purely on diesel engines.4-8 As an oxygenated fuel, methanol * To whom correspondence should be addressed. Telephone: 86-043185788777. E-mail:
[email protected]. † First Automobile Works. ‡ Xi’an Jiaotong University. (1) Jarrett, R. P.; Clark, N. N. Evaluation of methods for determining continuous particulate matter from transient testing of heavy-duty diesel engines, SAE technical paper 2001-01-3575. (2) Sidhu, S.; Graham, J.; Striebich, R. Semi-volatile and particulate emissions from the combustion of alternative diesel fuels. Chemosphere 2001, 42, 681-690. (3) Choi, C. Y.; Reitz, R. D. An experimental study on the effects of oxygenated fuel blends and multiple injection strategies on DI diesel engine emissions. Fuel 1999, 78, 1303-1317. (4) Liotta, F.; Montalvo, D. The effect of oxygenated fuels on emissions from a modern heavy-duty diesel engine, SAE Technical Paper 932734, 1993.
Table 1. Properties of Gasoline, Methanol and Diesel boiling point, °C autoignition temperature, °C cetane number density, kg/m3 kinetic viscosity at 20 °C, mm2/s octane number latent heat of vaporization, kJ/kg lower heating value, MJ/kg
gasoline
methanol
diesel
∼220 220-260 90 350 44.0
has some attractive features. Some of methanol’s properties are listed in Table 1. It is produced from both fossil and renewable domestic resources, including coal, natural gas, residual oil, and biomass.9 Methanol can be used in neat form as diesel substitute or partially blended with diesel. Huang et al.10 investigated the basic combustion behaviors of diesel/methanol blends based on the cylinder pressure analysis. Their studies showed that increasing methanol mass fraction of the diesel/methanol blends would increase the heat release rate in the premixed burning (5) Wang, Hw.; Zhou, L. B.; et al. Study on emission characteristics of direct injection diesel engine fueled with dimethyl ether (in Chinese). Trans. CSICE 2000, 1 (18), 6-10. (6) Edgar, B. L.; Dibble, R. W.; Naegeli, D. W. Autoignition of dimethyl ether and dimethoxy methane sprays at high pressures, SAE Technical Paper 971677, 1997. (7) Sinha, A.; Thomson, M. J. The chemical structures of opposed flow diffusion flames of C3 oxygenated hydrocarbons (isopropanol, dimethoxy methane, and dimethyl carbonate) and their mixtures. Combust. Flame 2004, 136, 548-556. (8) Hilden, D. L.; Eckstrom, J. C.; Wolf, L. R. The emissions performance of oxygenated diesel fuels in a prototype DI diesel engine, SAE technical paper, 2001-01-0650. (9) Ahouissoussi, N. B. C.; Wetzstein, M. E. A comparative cost analysis of biodiesel, compressed natural gas, methanol, and diesel for transit bus systems. Resour. Energy Econ. 1997, 20, 1-15. (10) Huang, Z.; Lu, H.; Jiang, D.; et al. Combustion behaviors of a compression-ignition engine fueled with diesel/methanol blends under various fuel delivery advance angles. Bioresour. Technol. 2004, 95, 331341.
10.1021/ef0605089 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007
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Figure 1. Experimental system.
Figure 2. Representative spray image.
Figure 3. Contour line of spray image. Table 2. Experimental Conditions injector type hole diameter, mm hole depth, mm opening pressure, MPa jerk-pump speed, rpm plunger diameter, mm ambient gas ambient temperature, °C ambient gas density, kg/m3 ambient gas pressure, MPa
single nozzle 0.25, 0.27, 0.29 0.8 12 ( 0.3, 15 ( 0.3, 18 ( 0.5 1000 8 N2 10 13.0, 25.9, 38.8 1, 2, 3
phase and shorten the combustion duration of the diffusive burning phase. Sato et al.11 have studied the feasibility of a highly efficient, low-NOX, spark-assisted direct injection methanol engine using a combination of charge heating and EGR under light-load conditions. The results showed that the engine’s brake thermal efficiency was improved. It was found that the (11) Sato, Y.; Noda, A.; Sakamoto, T. Combustion control of direct injection methanol engine using a combination of charge heating and exhaust gas recirculation. JSAE ReV. 1995, 16, 369-373.
oxygen concentration in the charge could be reduced by conducting EGR at high rate under high charge temperature, so that effective NOX reduction was possible. The importance of the problem of fuel spray for various applications is well-recognized and has been extensively studied experimentally and theoretically.12 For diesel engines, combustion and emission characteristics are influenced by fuel atomization, nozzle geometry, injection pressure, the shape of the inlet port, and other factors. To improve fuel-air mixing, it is important to understand the fuel spray and atomization formation processes. Spray structure and atomization characteristics of diesel have been investigated by Dennis,13 Ishikawa and Niimura,14 and Farrell et al.15 They reported that the characteristics of fuel spray for the fuel injector obtained by using the shadow graphs and particle image velocimetry at various chamber conditions. Delacourta et al.16 studied the effect of very high injection pressure (up to 250 MPa) on the macroscopic spray characteristics. Precise control over the fuel injection and air-fuel mixing process to form the desired in-cylinder mixture is one of the key issues for a spark-assisted direct injection methanol engine design. One of the most challenging problems is to create and stabilize a suitable mixture in the vicinity of the spark plug. As a matter of fact, methanol vapor concentration and distribution in the combustion chamber directly influence the combustion evolution. To apply methanol to the DI diesel engines, it is necessary to investigate the fundamental characteristics of the spray process. However, there are few investigations about the atomization characteristics of methanol sprays. The objective of this work is to investigate the effects of opening pressure, (12) Sazhin, S.; Crua, C.; et al. The initial stage of fuel spray penetration. Fuel 2003, 82, 875-885. (13) Dennis, L. S. Liquid-phase fuel penetration in diesel sprays, SAE paper 980809. (14) Ishikawa, N.; Niimura, K. Analysis of diesel spray structure using magnified photography and Piv, SAE paper 960770. (15) Farrell, P. V.; Chang, C. T.; Su, T. F. High pressure multiple injection spray characteristics, SAE paper 960860. (16) Delacourta, E.; Desmeta, B.; Besson, B. Characterization of very high pressure diesel sprays using digital imaging techniques. Fuel 2005, 84, 859-867.
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Figure 4. Effect of opening pressure on spray penetration.
Figure 5. Effect of opening pressure on spray velocity.
ambient density, and nozzle diameter on methanol’s macroscopic spray behavior and atomization characteristics using a jerk-pump system. 2. Experimental Apparatus and Procedure 2.1. Experimental Apparatus. To obtain the characteristics of methanol spray, the experiment was carried out at various test conditions. The experimental system was composed of four types of devices, as shown in Figure 1: control and measurement devices; fuel injection system; constant volume chamber and optical devices. A constant-volume chamber and a compressed N2 bottle with a precision pressure reducer allowed us to obtain some different ambient densities. Using quartz windows installed on both sides of the chamber, it was possible to fully observe the spray. A singlehole diesel injection nozzle was used. The diameter of the nozzle hole and the opening pressure are listed in Table 2. Fuel was injected by a jerk-pump system pulled by an electrical motor at the speed of 1000 rpm. A high-speed CCD camera was used to observe the spray phenomena. Camera speed was 10 000 frames/ s. The schlieren photography apparatus used in this experiment consists of an argon lamp, a pinhole, mirrors, and a blade as shown in Figure 1. The computer was used to store the image data; also, it controlled the spray timing by an electromagnetic relay and sent a trigger signal to the CCD camera. To correctly evaluate the pulverization process in the liquid phase, the tests are carried out
at ambient temperature 10 °C, thus limiting possible droplets evaporation. Some experimental conditions are listed in Table 2. 2.2. Image Analyses. One of the photographs taken during the tests is shown in Figure 2. The spray contour line of Figure 2 was shown in Figure 3. S represents the penetration length. The spray cone angle θ was obtained from the following equation: θ ) 2 arctan
(Sr)
(1)
Uncertainties in the penetration and cone angle data are due to uncertainties in the opening pressure, ambient density, etc. There is an inherent error in S and r since the spray edges seen in any spray image are never perfectly sharp. The overall uncertainties in S and r were obtained from these individual uncertainties. Using standard uncertainty analysis, the uncertainty in penetration and cone angles reported in this work would not exceed 5%.
3. Results The characteristics of diesel penetration length and spray cone angle were studied by many authors, and some correlations were proposed to represent the penetration. In this paper, the spray characteristics of methanol were measured for different opening pressures, ambient densities, and nozzle diameters.
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Figure 6. Effect of opening pressure on spray angle.
Figure 7. Effect of ambient density on spray penetration.
3.1. Influence of the Opening Pressures. The spray penetration lengths are plotted versus time in Figure 4. The ambient density is 38.8 kg/m3, and the nozzle hole diameter is 0.25 mm. At the exit of the nozzle hole, the spray velocity increases with the increase of the nozzle opening pressure, as shown in Figure 5. As a result, a higher opening pressure leads to a longer fuel penetration than that demonstrated in Figure 4. On the other hand, the spray velocity decreases very quickly with the increase of the spray penetration for the ambient gas’ resistance to the spray. The penetration length curves also become flat with the increase of the time. It can be seen from Figure 4 that the opening pressure has a little effect on the penetration length. When the opening pressure increased from 12 to 18 MPa, the penetration length only increases about 4.6%. The effect of opening pressure on cone angle is also evident from Figure 6. At the lowest opening pressure, 12 MPa, the spray cone angle lies between 15.5 and 19.3°. At a greater injection pressure, 15 MPa, the range is from 16.6 to 19.5°. When the opening pressure is increased to 18 MPa, the range increases 17.2-19.8°. From the data curves, it could be concluded that the opening pressure has little influence on the spray angle and the angle remains nearly constant during the whole injection process.
The density and viscosity of methanol are lower than that of diesel, as listed in Table 1. So, compared with diesel, the penetration velocity of methanol is slower, the penetration length of methanol is shorter, and the spray cone angle of methanol is larger at the same test conditions as shown in Figure 4 and Figure 6. 3.2. Influence of the Ambient Densities. Figures 7-9 show the effects of ambient densities on spray evolution characteristics. The nozzle hole diameter is 0.27 mm. The opening pressure is 15 MPa. The ambient densities are 13.0, 25.9, and 38.8 kg/m3, respectively. Figure 7 and Figure 8 show that the spray penetration and tip velocity decrease quickly with the increase of ambient density. When the density increases from 13.0 to 38.8 kg/m3, the maximum decrease percentage is about 38 and 48%, respectively, for spray penetration and tip velocity. The spray cone angles are plotted versus spray time at different ambient densities in Figure 9. The three plots are for three different ambient densities. Figure 9 demonstrates that the spray cone angle widens with an increase in ambient density. The spray angle of methanol is wider than that of diesel such as that shown in Figure 6 at the same experimental conditions. 3.3. Influence of Nozzle Diameter. The nozzle l/d ratio plays an important role in the fuel spray evolution. To investigate
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Figure 8. Effect of ambient density on spray velocity.
Figure 9. Effect of ambient density on spray cone angle.
the effect of the nozzle diameter on methanol spray characteristics, the nozzle hole length is kept at 0.8 mm and the nozzle hole diameters are selected as 0.25, 0.27, and 0.29 mm. The nozzle l/d ratios are 3.2, 2.96, and 2.76. Figures 10-Figure 12 show the spray evolution under the fixed ambient density and opening pressure. It can be concluded that the penetration tip velocity, penetration length, and spray angle will increase with the increase of nozzle diameter. Compared with the diesel spray characteristics, the penetration length and velocity of methanol are smaller and the spray cone angel is larger as shown in Figure 10 and Figure 12. 3.4. Numerical Analysis. The penetration length S is one of the spray characteristics most described in the literature. Hiroyasu and Arai17 experimentally established a temporal evolution law of this characteristic as eq 2, which seems to satisfy many authors. Since their first study, the measurement techniques used have improved and the various trials carried out by several authors seem to consolidate the validity of this law. The injection pressure was not picked in the experiment. But the curves of methanol penetration length have the same trend as diesel shown in Figure 4, Figure 7, and Figure 10. When (17) Hiroyasu, H.; Arai, M. Structures of fuels sprays in diesel engines, SAE paper 900475.
some coefficients were amended, eq 2 would be valid for methanol.
{
x ( )
2∆P t < tb t Fl S) ∆P 0.25 0.5 0.5 2.95 d t t > tb Fg 0.39
tb ) 28.65
(2)
Fld
xFg∆P
The empirical equation suggested by Hiroyasu and Arai for the spray angle of diesel is expressed by the following equation:
θ ) 83.5
() ( ) ( ) l d
-0.22
d d0
0.15
Fa Fl
0.26
(3)
When replacing 83.5 by 64.9, eq 3 can describe the diesel spray angle of this experiment. The main differences between methanol and diesel are density and viscosity. Selecting a new parameter to replace 83.5 in eq 3, the new equation will be suitable to describe methanol spray angles. On the basis of the
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Figure 10. Effect of nozzle diameter on spray penetration.
Figure 11. Effect of nozzle diameter on spray velocity.
Figure 12. Effect of nozzle diameter on spray cone angle.
experimental data, we select the constant as 68.8 for methanol. Figure 13 shows the comparison of the mean measured spray angle and those calculated from the new equation for diesel and methanol. It can be seen From Figure 13 that the modified relations can describe our experimental results perfectly. But,
an important consideration to be taken into account is that the correlation is applicable to ambient temperature condition as this experiment. The spray evolution may be different at higher temperatures, and the correlation might have to be suitably modified.
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Figure 13. Measured and calculated spray angle.
4. Conclusions The transient characteristics of methanol sprays were investigated under different opening pressure, ambient density, and nozzle diameter in this work. From the experimental data, we could conclude the following: (1) Between 12 and 18 MPa, penetration length and tip velocity increase with the increase of opening pressure. But the effects were not evident. For the 0.27 mm diameter nozzle and 38.8 kg/m3 ambient density, the penetration length only increases about 4.6% when the opening pressure increased from 12 to 18 MPa. The opening pressure has little influence on the spray angle. (2) With the increase of ambient density, the penetration length and tip velocity decrease and cone angle widens quickly. (3) The penetration tip velocity, penetration length, and spray angle will increase with the increase of nozzle diameter. (4) Compared with diesel, methanol penetration is shorter and the cone angle is larger under the same experimental conditions.
(5) Replacing 83.5 in the law of Hiroyasu by 68.8 and 64.9 to form new correlations, then they can describe the cone angles of methanol and diesel correctly in this experiment. Acknowledgment. The authors thank the National Basic Research (973) Program of China (Approval No. 2001CB209206) for financial support.
Nomenclature d ) nozzle hole diameter S ) spray tip penetration θ ) spray cone angle d0 ) sack chamber diameter of nozzle Greek Symbols Fg ) gas density of the environment Fl ) fuel density ∆P ) differential pressure between the entry and the exit hole EF0605089
