Numerical and Experimental Study on the Comparison between

Energy & Fuels 2008, 22, 2851–2860

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Numerical and Experimental Study on the Comparison between Diesel and Dimethyl Ether (DME) Spray Behaviors According to Combustion Chamber Shape Hyung Jun Kim,† Hyun Kyu Suh,† and Chang Sik Lee*,‡ Graduate School of Hanyang UniVersity, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea, and Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea ReceiVed January 30, 2008. ReVised Manuscript ReceiVed April 20, 2008

This paper describes the experimental and numerical investigation on the spray characteristics of diesel and dimethyl ether (DME) fuel according to the combustion chamber shape under ambient pressure injected through the common-rail system. For the experimental study, diesel and DME fuels were injected into combustion chambers with two shapes (narrow and conventional type). The investigation on the spray behavior according to the various injection conditions (injection pressure, duration, nozzle-wall distance, etc.) was conducted by the visualization system utilizing an Nd:YAG laser as the light source. In order to investigate the numerical study, a three-dimension computational grid was created as the shape of combustion chamber and thermodynamic properties of DME fuel were inserted into the KIVA code. Also, the hybrid breakup model combining the primary and secondary breakup was used to analyze the spray characteristics on the evaporation conditions. In the macroscopic characteristics such as the spray developing process and spray tip penetration, the calculated results were compared with the experimental observations and the evaporation distribution and rate were predicted by the numerical method. It was revealed that the calculated results of spray tip penetration agreed with the experimental observations of DME and diesel fuel for the different combustion chambers. The evaporation characteristics such as vapor phase distribution and evaporation rate were investigated in terms of two kinds of fuels and combustion chamber shapes.

1. Introduction Diesel engines have been applied to large vehicles such as heavy duty trucks, buses, and vessels, but recently, passenger car diesel engines have steadily increased by the development of high speed diesel engines. They have many advantages such as high thermal efficiency, excellent performance of fuel consumption, and good durability compared to gasoline engines. However, there are two problems in diesel engines; nitric oxides (NOx) occur through the self-ignition process after mixing evaporated fuel with air in the cylinder, and particle matter (PM) is produced in the fuel-rich regions. Recently, the problem of resource exhaustion has resulted in high petroleum prices and a limit of reserves, and restrictions of exhaust emission are strengthening for the reduction of environmental pollution. In order to satisfy the strict restrictions, methods for the decrease of NOx and PM in the combustion process in the diesel engine and reduction technology of exhaust emissions through an post-treatment device have been developed. Also, it will be necessary to continuously improve the combustion method in engines. Among the various ways, homogeneous charge compression ignition (HCCI) combustion is considered to be the next generation combustion to solve the exhaust emissions. HCCI combustion, self-ignited by compression of premixed fuel and an air-fuel mixture, shows excellent performance of the fuel consumption because it can be ignited * Corresponding author. Phone: +82-2-2220-0427. Fax: +82-2-22815286. E-mail: [email protected]. † Graduate School of Hanyang University. ‡ Department of Mechanical Engineering.

throughout a lean air-fuel mixture. Because the combustion happens at a low temperature, NOx emissions decrease compared to the conventional diesel engine. It is expected that the low partial fuel-rich regions are capable of reducing particulate matter by using premixed air-fuel. On the other hand, it is not only difficult to control the ignition timing and combustion duration due to the self-ignition but also the engine may be harmed by the unacceptable noise and combustion pressure caused knocking in high load conditions. Besides, the wallwetting problem of fuel on the combustion chamber is caused by an injection of fuel at the condition of low combustion temperature and pressure. This can increase incomplete combustion products such as carbon monoxide (CO) and hydrocarbons (HC). Therefore, much research has been done to resolve these problems in HCCI engines. Kim et al.1 investigated the combustion and emission characteristics according to various injection timing in the HCCI engine. They reported that early injection timing in the HCCI engine decreased the NOx and soot emissions by the low combustion temperature and long residence time of the injected fuel. Also, double injection of fuel remarkably increased the combustion pressure and moved up the ignition timing compared to single injection. Lee et al.2 studied the combustion characteristics according to the premixed ratio and exhaust gas recirculation (EGR) ratio in the partial premixed compression ignition (PCCI) engine. They suggested that an increased premixed ratio led to a decrease of the combustion temperature (1) Kim, D. S.; Kim, M. Y.; Lee, C. S. Combust. Sci. Technol. 2007, 179, 531–551. (2) Lee, C. S.; Lee, K. H.; Kim, D. S. Fuel 2003, 82, 553–560.

10.1021/ef8000696 CCC: $40.75  2008 American Chemical Society Published on Web 06/18/2008

2852 Energy & Fuels, Vol. 22, No. 4, 2008

and retardation of the ignition timing. Therefore, the NOx and soot emissions were lower than from the convention diesel engine and the PCCI engine applying the EGR system is effective to decrease NOx emissions. Moreover, the numerical investigation of the HCCI engine has been actively advanced and Lechner et al.3 carried out numerical studies on emission characteristics of various spray angles with early injection timing in the PCCI engine. In their calculated results, NOx and PM emissions can be reduced through early injection timing with narrow angle and the optimum EGR rate. A numerical study using the KIVA code for the combustion characteristics according to the various injection timing in the high speed direct injection (HSDI) engine was conducted by Kim et al.4 They reported that soot emission was decreased by the early injection but NOx and HC emissions can be produced because of fuel deposition on the wall of the piston bowl. Besides these investigations, many attempts have been made to apply alternative fuels such as biodiesel and dimethyl ether to diesel engines. Among various alternative fuels, dimethyl ether (DME) fuel is a promising alternative fuel for the diesel engine because DME fuel is a simple chemical compound with a short carbon chain, which leads to very low particulate matter during the combustion process. This fuel produces low emissions of NOx compared to diesel fuel. DME fuel has not only a high cetane number but also better self-ignitability than diesel fuel in cold start conditions. For that reason, DME fuel has the effect of reducing exhaust emissions in the direct injection diesel engine because DME is an oxygenated fuel and it can be increased tolerance for EGR. Also, much research has been done because DME fuel has recently become known as an environmentally friendly fuel, and a lower cost manufacturing technique was developed for it. Experimental studies of the spray and combustion characteristics in common-rail diesel engines applying DME fuel were conducted by Kim et al.5 In the spray characteristics, the spray tip penetration of DME fuel is shorter than that of diesel fuel under the same injection conditions because of fast atomization characteristics. Also, the combustion characteristics of DME fuel show a lower maximum combustion pressure and shorter combustion duration than diesel fuel. Suh et al.6 investigated the spray characteristics of diesel and DME fuel injected through a high-pressure injection system. They reported that the spray tip penetration and Sauter mean diameter (SMD) of diesel fuel are larger than those of DME fuel under the same injection conditions. Numerical and experimental investigations on the combustion and emission characteristics of DME fuel in a lightduty DI diesel engine were conducted, and combustion characteristics of a lower peak pressure and a shorter ignition delay compared to the diesel fuel were reported by Gui et al.7 However, DME fuel has a low heating value and high (3) Lechner, G. A.; Jacobs, T. J.; Chryssakis, C. A.; Assanis, D. N.; Siewert, R. M. EValuation of a narrow spray cone angle, adVanced injection timing strategy to achieVe partially premixed compression ignition combustion in a diesel engine. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 2005; 2005-01-0167. (4) Kim, M. S.; Reitz, R. D.; Kong, S. C. Modelling early injection processes in HSDI diesel engines. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 2006; 2006-01-0056. (5) Kim, M. Y.; Bang, S. H.; Lee, C. S. Energy Fuels 2007, 21, 793– 800. (6) Suh, H. K.; Park, S. W.; Lee, C. S. Energy Fuels 2006, 20, 1471– 1481. (7) Gui, B.; Chan, T. L.; Leung, C. W.; Xiao, J.; Wang, H.; Zhao, L. Modelling study on the combustion and emissions characteristics of a lightduty DI diesel engine fueled with dimethyl ether (DME) using a detailed chemical kinetics mechanism. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 2004; 2004-01-1839.

Kim et al.

compressibility, so it is difficult to obtain enough injection mass of fuel. Also, it causes leakage along the fuel supply systems because of lower viscosity than diesel fuel and then it must use lubrication for the solution of abrasion. Because of the many different properties compared with diesel fuel, Ryu et al.8 conducted the investigation of comparison between diesel and DME spray characteristics. They suggested that DME fuel has a lower injection rate and shorter injection delay duration than diesel fuel. The aim of this study is to analyze the spray characteristics of diesel and DME fuel in the two types of combustion chamber shapes according to various injection conditions such as injection pressure, injection duration, and the nozzle-wall distance. Also, the wall film formation process and evaporation characteristics after the impingement on the wall are experimentally and numerically investigated. These results will probably provide the basic information to the engine design applied DME fuel. In order to analyze the spray behaviors and evaporation characteristics, the spray tip penetration and evaporation rate for DME and diesel fuel were compared with experimental observations. The calculated evaporation performances in the combustion chamber according to various conditions were compared and analyzed by the numerical method. 2. Numerical Formulation 2.1. Atomization Model of DME Fuel and Created Computational Mesh. Numerical investigations of the thermodynamic characteristics of liquefied DME according to temperature were conducted by Teng et al.9 They developed equations for the commonly used thermophysical properties of liquid DME such as the density, vapor pressure, viscosity, and latent heat based on chemical structure and molecular theory. Also, thermodynamic properties including enthalpy, entropy, bulk modulus, surface tension, etc. were systematically investigated for the form of a diagram through the spray and evaporation characteristics of DME fuel for applications in DME system design and analysis.10,11 On the basis of the calculated properties of liquid DME fuel, a numerical study was conducted by the insertion of fuel library into the KIVA code. In this paper, Figure 4 shows the 3-dimensional computational grid created by the KIVA-PREP program according to the shape of the combustion chamber and it is composed of three parts for the formation of even mesh. Also, a 72° sector mesh considering the 5 nozzle-hole injector was applied to shorten the calculation time, and the total number of cells was about 7000. The hybrid breakup model combining the primary and secondary breakup theory was used to analyze the atomization characteristics of diesel and DME fuel in the combustion chamber. The WAVE model based on the Kelvin-Helmholtz instability, which was caused by the aerodynamic interaction between the liquid jet and gas on the boundary layer, was applied to the primary breakup model. For the secondary (8) Ryu, B. W.; Bang, S. H.; Suh, H. K.; Lee, C. S. Effect of injection parameters on the spray characteristics of DME fuel. Proceedings of ICEF06, Sacramento, CA, November 5-8, 2006; ICEF2006-1512. (9) Teng, H.; McCandless, J. C.; Schneyer, J. B. Thermochemical characteristics of dimethyl ether-An alternatiVe fuel for compression-ignition engines. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 2001; 2001-01-0154. (10) Teng, H.; McCandless, J. C.; Schneyer, J. B. Compression ignition delay (physical + chemical) of dimethyl ether-An alternatiVe fuel for compression-ignition engines. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 2003; 2003-01-0759. (11) Teng, H.; McCandless, J. C.; Schneyer, J. B. Thermodynamic properties of dimethyl ether-An alternatiVe fuel for compression-ignition engines. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 2004; 2004-01-0093.

Comparison between Diesel and DME Spray BehaViors

Energy & Fuels, Vol. 22, No. 4, 2008 2853

Figure 1. Schematics of the spray visualization system.

breakup model, the RT (Rayleigh-Taylor) model suggested by Bellman et al.12 based on the theory of the Rayleigh-Taylor instability was utilized. In the WAVE model of the primary breakup, the maximum growth rate (ΩKH) from the solution of a dispersion equation proposed by Reitz13 and the corresponding wavelength (ΛKH) are determined by the following equations. ΛWAVE ) 9.02r

ΩWAVE )

(1 + 0.45Z )(1 + 0.4T ) (1 + 0.87Weg1.67)0.6 0.5

(0.34 + 0.38Weg1.5)

0.7

⁄[ ] Ffr3 σ

(1)

0.5

(2) (1 + Z)(1 + 1.4T0.6) Under the assumption that the critical radius (rc) decreased during the breakup time (τWAVE), the droplet radius (rn) after breakup can be calculated by using rc ) 0.61ΛWAVE,

τWAVE )

3.726B1r ΩWAVEΛWAVE

r - rn r - rc ) dt τWAVE

(3)

The calculated results of B1 were compared with the experimental results for the optimization of a breakup constant. From these results, the optimum value of the breakup constant B1 was chosen as 40 in this study. Also, in the RT model of the secondary breakup, a maximum growth rate (ΩRT), the corresponding wavelength (ΛRT), and the wavenumber (KRT) can be determined when the surface tension merely is considered and the viscosity of liquid is neglected as given by14

(

b| + |a b|)(Ff - Fg)] 2 [-(|g ΩRT ) F f + Fg 3√3σ KRT )

)

(5)

1.5 0.5

(

-(|g b| + |a b|)(Ff - Fg) 3σ

)

τRT ) Cτ/ΩRT

(8)

rn ) πCRT ⁄ KRT

(9)

2.2. Evaporation Model. Under the assumption of quasisteadiness in the gas phase and droplets having uniform temperature, the evaporation model based on the lumped-body theory16 with KIVA code is included. The Nusslet and Sherwood numbers are calculated by the Ranz and Marshall correlation17 in the transfer of the heat and mass between a spherical droplet and a flowing fluid as expressed by Nu ) 2 + 0.6√RePr0.33

(6)

0.5

(7)

where, b g and b a are the acceleration in the direction of travel by gravity and drag force, respectively. The breakup constant (CRT) affected atomization characteristics set up the optimal value of 0.1 after the comparison of experimental observations. Also, (12) Bellman, R.; Pennington, R. H. Q. Appl. Mech. 1954, 12, 151– 162. (13) Reitz, R. D. Atom. Spray Technol. 1987, 3, 309–337.

(10)

Sh ) 2 + 0.6√ReSc (11) The rate of mass transfer (m ˙ ) from a droplet and mass transfer number (B) is given by the correlation proposed by Frossling18 0.33

m ˙ ) 2πr(FD)airBSh (4)

ΛRT ) 2πCRT/KRT

the breakup time (τRT) of the RT model and the droplet radius (rn) at the end of breakup is defined by15

B)

Ys - Y∞ 1 - Ys

(12) (13)

where (FD)air is the fuel vapor diffusivity in the air and Y∞ and Ys indicate the mass fraction of fuel vapor in the free stream conditions and on the droplet surface, respectively. Also, the temperature of droplets during the evaporation process can be calculated by an iteration method. 2.3. Spray-Wall Interaction Model. The impingement process of droplets on the wall is an important physical phenomenon for the analysis of spray behavior in the combustion chamber because a fuel is directly injected into the cylinder and the droplets impinge on the piston wall before the complete evaporation. When the droplets impinge on the wall, air/fuel ratio, combustion, and emission characteristics can vary according to the impingement conditions. Therefore, a spray-wall (14) Su, T. F.; Patterson, M. A.; Reitz, R. D.; Farrel, P. V. Experimental and numerical studies of high pressure multiple injection sprays. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 1996; 960861. (15) Beale, J. C.; Reitz, R. D. Atom. Spray Technol. 1999, 9, 623–650. (16) Amsden, A. A.; O’Rourke, P. J.; Butler, T. D. KIVA-II: A computer program for chemically reactiVe flows with sprays. Los Alamos Report, 1989;LA-11560-MS, pp 12-20. (17) Ranz, W. E.; Marshall, W. R. Chem. Eng. Prog. 1952, 48, 141(Part I) and 173(Part II). (18) Faeth, G. M. Prog. Energy Combust. Sci. 1977, 3, 191–224.


Kim et al. and 0.5 or 1.0 ms, and the detailed experimental conditions are listed in Table 1.

4. Results and Discussion

Figure 2. Shapes of combustion chamber and measurement of spray tip penetration.

interaction model was used to analyze the impingement process between the droplets and the wall in this study and it applied the continuity, momentum, and energy equation to the wall film cells. This model suggested by Stanton et al.19 consists of the liquid film model and spray-film impingement model. The criterion in determining impingement regimes such as stick, rebound, spread, and splash regime are sorted according to the Weber number of the droplets. Also, the submodel of the wall film was proposed by O’Rourke et al.,20 and the spray-wall interaction including droplet splash and impingement-pressure spreading was used in this paper.

3. Experimental Apparatus and Procedures 3.1. Experimental Setup. In order to investigate the spray characteristics of diesel and DME fuel according to the combustion chamber shape, the visualization system was composed of the common-rail system, light source with Nd:YAG laser, PC devices, and an intensified charge coupled device (ICCD) camera as shown in Figure 1. The common-rail injection system and two highpressure pumps operated by compressed air were utilized for the high pressure injection and a stable pressure of injection. The DME fuel was pressurized to 10 bar in the fuel tank by nitrogen gas, and then, it converts to liquid phase because it has gaseous conditions in the atmosphere. The combustion chamber that can be pressurized up to 40 bar was applied to raise the ambient pressure by nitrogen gas for the conditions of internal combustion chamber under ambient pressure. Also, the wavelength and power of the Nd:YAG laser were determined to be respectively 532 nm and 270 mJ. The frozen images were obtained by an ICCD camera (The Cooke Corp, Dicam-PRO) and an image grabber through a laser sheet method. The test injector used in this study was the diesel injector of a minisac type with five nozzle holes and an injector driver synchronized with a digital delay/pulse generator (Berkeley Nucleonics Corp, Model 555) for the control of fuel injection timing and duration. 3.2. Experimental Procedures. The combustion chamber shapes of two types were applied to investigate the spray characteristics of diesel and DME fuel under ambient pressure. The fuels were injected into conventional and narrow type combustion chambers with 156° and 60° of spray angle, respectively. The detailed combustion chamber geometry is illustrated in Figure 2. Also, diesel fuel was injected at the compression stroke in the direct injection diesel engine and Figure 3a shows the relation of distance from the nozzle tip and the ambient pressure in the cylinder according to the crank angle. As shown in this figure, the injection timing determined two conditions between the 25 bar-4 mm and 35 bar-1 mm (BTDC 20-10°) as indicated with the red dotted line. Figure 3b shows the nozzle-wall distance according to the combustion chamber and the fuels at the distance from the nozzle tip of 1 or 4 mm were injected into two types of combustion chambers. In this study, the injection pressure and duration setup was 40 or 60 MPa

4.1. Spray Behavior in the Combustion Chamber. Figure 5 shows the comparison between the spray images of the fuels obtained by the visualization system in the injection pressure of the 60 MPa and the calculated spray development processes in the narrow type chamber at the condition of 35 bar-1 mm. In this case, the condition of 35 bar-1 mm means 35 bar of gas pressure and 1 mm of nozzle-wall distance between the nozzle tip and piston head. In the numerical results, green points and blue arrows indicate the droplets and a flow in the combustion chamber, respectively. The injected spray is impinged on the bowl surface of the piston and it forms the wall wetting and floating flow in the vicinity of the chamber bowl surface by the collision and splashed spray. In the narrow type chamber, spray of fuels was more affected by the wall impingement and wall film models than the atomization model due to the impingement on the chamber wall immediately after the start of the injection. Therefore, the stick phenomenon of the fuel droplet on the dry chamber wall was occurred immediately after the start of the injection. The rebound and spread regimes was appeared in the high injection velocity around the impact region. The wall film was developed along the chamber wall from the calculated results using the spray-wall interaction model with the splash regime as shown in Figure 5. Also, it is known that the vortices in the stream of the combustion chamber happened after the start of the injection and the droplets after the breakup were floating as the shapes of vortices. The calculated two-dimensional sliced images agreed fairly well with the experimental results as can be seen in this figure. In the experimental observations, it is shown that the spray tip penetration of diesel fuel is greater than that of DME fuel at 0.6 ms and the spray tip penetrations of the two fuels have similar trends at 1.4 ms after the start of the injection. It can be said that the wall film is slowly increased because the breakup of DME spray actively caused by a lower surface tension than that of diesel fuel after the early stage of the injection but the droplets after the breakup rapidly moved with the stream in the chamber. As shown in the experimental image of DME fuel at 1.4 ms after the start of the injection, the spray image becomes dim by the rapid evaporation. Also, it is known that the entire number of droplets decreased to a remarkably lower amount than in the case of diesel as can be seen in the calculated result of the figure. Figure 6 illustrates the comparison between the experimental and calculated spray images of diesel and DME fuels according to time after the start of the injection in the conventional type chamber at the condition of 35 bar-1 mm. As can be seen in this figure, the experimental observations agreed well with the calculated results and fuel spray injected until the spray droplets after the injection arrived at the right bowl wall of conventional chamber shape. After the breakup of injected spray, the spray droplets on the wetting surface of the bowl in the chamber make the floating flow to the upward direction as a type of bowl shape of chamber because of the loss of momentum in the droplet. In the calculated results, a flow in the combustion chamber shows (19) Stanton, D. W.; Rutland, C. J. Multi-dimensional modeling of heat and mass transfer of fuel films resulting from impinging sprays. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 1998; 980132 (20) O’Rourke, P. J.; Amsden, A. A. A spray/wall interaction submodel for the KIVA-3 wall film model. SAE Technical Paper Series, Society of Automotive Engineers: Warrendale, PA, 2000; 2000-01-0271.



Figure 3. Relation of distance from the nozzle tip and pressure in the cylinder and distance from the nozzle tip in the combustion chamber. Table 1. Experimental Conditions injection system fuel nozzle type number of nozzle holes hole diameter (mm) injection pressure (MPa) injection duration (ms) ambient temperature (K) ambient pressure-distance from the nozzle

Figure 4. Schematic of 72° sector computational grid of two types of combustion chambers.

the circular vortices around the upper and lower sides of the impingement point on the right wall of the chamber. The spray tip penetration of diesel fuel is longer than that of DME fuel because it can be postulated that the breakup of DME spray

common-rail diesel, DME mini-sac 5 0.167 40, 60 0.5, 1.0 293 25 bar-4 mm 35 bar-1 mm

occurred because of low surface tension and the droplets of DME fuel evaporated because of the high vapor pressure at the end of spray. Figure 7 shows the comparison between the calculated flow pattern and experimental spray images of the fuels at the ambient condition of 25 bar-4 mm according to the chamber shapes at the injection pressure of 60 MPa and the 0.6 ms after the start of the injection. In spite of 25 bar of ambient pressure and 4 mm of nozzle-wall distance, the spray images and fuel flow of DME and diesel fuels show similar results compared to that of 35 bar-1 mm at the same elapsed time as shown in the image distribution. The spray image of DME fuel at the end of spray was more blurred than that of the diesel fuel because of the activated evaporation of DME spray as can be shown in this figure. In the case of the conventional type, it is observed that the free jet before the impingement on the wall grows rapidly


Figure 5. Comparison between the experimental and calculated spray images according to fuels in the narrow type chamber (35 bar-1 mm).

Kim et al.

Figure 7. Comparison between the experimental and calculated spray images according to fuels and shape of the combustion chamber (25 bar-4 mm, Tinj ) 0.6 ms).

Figure 8. Outer line of spray images after the start of injection according to the chamber shape and fuels.

Figure 6. Comparison between the experimental and calculated spray images according to fuels in the conventional type chamber (35 bar-1 mm).

because of the low ambient pressure and the droplets clustered on the crevice area of the chamber at 4 mm of position. As shown in the crevice region of the conventional chamber, the circular vortices appeared in the internal crevice area in the piston bowl of the chamber. In the middle of the conventional chamber, both results of the spray image and the predicted one show similar behavior of droplets in the spray as shown in circular vortices in the crevice and inner region of the chamber bowl. It may be postulated that an outer spray of DME fuel actively vaporized before the impingement on the wall and then droplets after breakup lose the kinetic energy.

The outer line of the diesel and DME sprays in the experimental results according to the elapsed time after injection in the narrow and conventional type chamber are shown in Figure 8. The spray development process of diesel fuel rapidly makes more progress than that of DME fuel in both narrow and conventional type chambers. In the narrow type chamber, it can be shown that the liquid film of diesel fuel is thinner than that of DME fuel before the 1.0 ms after the start of the injection because the large droplets are not disrupted by a high surface tension of diesel fuel and they stick on the chamber wall. Also, the different shapes of the DME spray compared to the diesel spray at the 1.8 ms can be judged from the occurrence of vaporization at the end of spray according to the passage of time. In the conventional type chamber, the spray development process of diesel fuel is faster than that of DME fuel, and it can be seen that the spray angle and tip penetration of DME spray are wider and shorter, respectively, than those of diesel spray because of a brisk atomization in an outer spray at the early stage of injection. After the impingement of chamber bowl, the spray area of DME fuel was decreased by rapid evaporation


Figure 9. Comparison between experimental and calculated spray tip penetration according to fuels and the nozzle-wall distance in the two types of combustion chamber.

and then the shape of DME spray at the 1.8 ms can be seen in this figure by the gasification at an outer region of spray. The comparison between experimental and calculated spray tip penetration of DME and diesel fuel according to combustion chamber shapes and the nozzle-wall distance was illustrated in the Figure 9. The spray tip penetration of fuels after the impingement on the chamber wall continuously increased in the narrow type chamber as shown in Figure 9a. The calculated spray tip penetration agreed well with the experimental observations and the spray tip penetrations of fuels at the distance from the nozzle tip of 4 mm are longer than those at 1 mm. From these results, it can be suggested that the ambient pressure at the nozzle-wall distance of 4 mm is lower by about 10 bar and the impingement point becomes more advanced than those at 1 mm of nozzle-wall distance. Also, the spray tip penetrations of diesel and DME fuel have a similar trend at the early stage of injection but the increasing rate of spray tip penetration for DME fuel slows a slight increase in accordance with the increase of elapsed time after the 1.0 ms. Figure 9b shows the spray tip penetration in the conventional type chamber according to two fuels and the distance from the nozzle tip. It can be known that the spray at the nozzle-wall distance of 4 mm progressed toward the crevice area and then reached the end of cylinder wall at 0.7 ms after the start of the injection. In the case of the distance from the nozzle tip of 1 mm, it seems that the increasing rate of the spray tip penetration rapidly decreased because of the breakup of droplets impinged on the wall and the wall-wetting problem. Figure 10 illustrates the comparison between spray tip penetration of the two fuels and shapes of combustion chamber


Figure 10. Comparison between experimental and calculated spray tip penetration according to combustion chamber shape and injection pressure.

at the nozzle-wall distance and injection duration of 1 mm and 0.5 ms, respectively. As expected, the spray tip penetration is increased with the increase of injection pressure, which indicates that large air assistance owing to high injection pressure promotes the atomization of spray. Also, the calculated spray tip penetration shows a similar tendency with the experimental observations and the spray tip penetration of conventional type chamber was longer than that of the narrow type chamber. From these results, it may be guessed that the velocity with which the liquid film wetted the bowl of narrow type was lower than that of the free jet before the impingement on the chamber wall of conventional type at the 0.5 ms after the start of the injection. In the narrow type chamber, the spray tip penetration of 60 MPa is longer than that of 40 MPa because the spread and splash regimes were appeared by the increasing Weber number due to the high injection velocity around the impact region. Then, the wall film caused from the injection pressure of 60 MPa was more quickly moved along the chamber wall than that of 40 MPa. As can be seen in Figure 10b, the gap between the spray tip penetration of both 40 and 60 MPa of injection pressure in the spray tip penetration of DME fuel is smaller than that of diesel fuel because the breakup occurred more actively at the injection pressure of 60 MPa than at that of 40 MPa. Figure 11 shows the comparison between experimental and calculated spray cone angle according to the fuel and the nozzle-wall distance in the conventional type chamber before the impingement on the wall. The measurement of the spray cone angle in this study was determined by the average value of the experimental results, and spray cone angles can be


Figure 11. Comparison between experimental and calculated spray cone angle according to the fuel and the nozzle-wall distance in the conventional type chamber before the impingement on the wall.

calculated by the ratio of the radial and axial distance at the most outer spray. The experimental spray cone angles show a similar trend with the calculated results. It can be seen that the spray cone angle at the injection pressure of 60 MPa is narrower than that of 40 MPa due to the increase of the spray tip penetration by the high injection pressure. Also, it can be shown that the fuel spray has over the 20° of cone angle in the early injection; however, the spray cone angles become smaller under 15° with the elapsed time of start of injection. The spray cone angle of DME fuel shows wider than that of diesel fuel because the breakup was promoted by the lower surface tension and drag force between the droplet and air. 4.2. Evaporation Characteristics of the Spray in the Combustion Chamber. Figure 12 shows the calculated overall SMD and total vapor fuel mass according to the fuel and combustion chamber shape. After the start of the injection, the overall SMD shows the decreasing pattern and the overall SMD of diesel fuel has larger than that of DME fuel. The decreasing rate of overall SMD of DME fuel is higher than that of diesel fuel until the 1.0 ms after start of the injection because the droplet breakup actively occurred by the low surface tension of DME fuel. The total vapor fuel mass of DME fuel in the chamber is much higher than that of diesel fuel because the small SMD value promoted the evaporation of fuel and DME fuel has greater vaporization characteristics. Also, it can be shown that the overall SMD in the narrow type chamber has a larger value than that in the conventional type chamber due to the droplet collision and coalescence caused by the fuel impingement on the chamber wall after the start of the injection.

Kim et al.

Figure 12. Calculated overall SMD and total vapor fuel mass according to the fuel and combustion chamber after the start of the injection.

The evaporation characteristic of fuel is important for analyzing the combustion characteristics because it plays a crucial role in the combustion process. Therefore, the numerical investigation of the evaporation characteristics of diesel and DME fuel was performed in this section. Figure 13 shows the calculated vapor distribution of diesel and DME fuels at the injection pressure of 60 MPa according to the combustion chamber shape and the nozzle-wall distance. As is shown in this figure, the vaporization of DME spray occurred remarkably more than that of diesel spray due to the properties of DME such as low surface tension, viscosity, and high vapor pressure compared to diesel fuel. It can be observed that the evaporation distribution of diesel fuel has a similar distribution of the spray development and the vapor cloud of diesel fuel is feebler than that of DME fuel. In the narrow type chamber injected with DME fuel, a projecting shape at the vapor distribution appeared at the impingement point because the severe atomization of fuel spray was caused by a collision with the wall at the early stage of the injection. After the impingement on the wall, it can be seen that the high density of vapor distribution appeared from the wall surface of the bowl because the droplets wetted to the wall along the wall-shape. Then, the evaporation distribution was widely dispersed by the breakup of droplets at the outer region of spray according to the elapsed time after the start of the injection. At the distance from the nozzle tip of 4 mm, the fuel vapor distribution in the narrow type chamber has a larger portion than that at the nozzle-wall distance of 1 mm before the impingement on the wall. It can be expected that the vapor



Figure 13. Calculated vapor distribution of fuels according to combustion chamber shape and nozzle-wall distance after start of injection (Pinj ) 60 MPa).

distribution at the nozzle-wall distance of 4 mm dispersed more widely than that of 1 mm due to the low ambient pressure and the advance of the impingement point in the narrow type chamber. In the case of the conventional type chamber, the vaporization of DME fuel occurred more strongly than that of diesel fuel and the droplets collided with the wall evaporated at the impingement point and chamber bowl. The evaporation distribution at the nozzle-wall distance of 4 mm shows the vapor of fuel in the crevice area because the spray is injected farther from the nozzle tip. Figure 14 illustrates the calculated evaporation rate of diesel and DME fuels in the two types of chambers according to time after the start of the injection. The evaporation rate means that it is the ratio of the injected total mass and the total mass of fuel vapor in the gas, a unit expressed in the percentage. The evaporation rate of diesel fuel was less than one percent and the evaporation rate at the nozzle-wall distance of 4 mm was higher than that of 1 mm due to the low ambient pressure as can be seen in Figure 14a. However, the evaporation rate of DME fuel remarkably shows an increasing trend of the elapsed time after the start of the injection as that seen in Figure 14b. In the evaporation rate in the conventional type at the nozzle-wall distance of 1 mm, it shows that the rate of evaporation growth slowed because of the wall-wetting problem after the impingement on the wall and then increased. Also, the evaporation rate at the nozzle-wall distance of 4 mm is larger than that of 1 mm because the low ambient pressure promoted the vaporization of DME fuel with the vapor pressure of 5 bar. From this point of view, it may be judged that the evaporation performance of DME fuel is the better than that of diesel fuel under the same conditions in this study. 5. Conclusions This work was performed to analyze the experimental and numerical spray characteristics of diesel and DME fuel in the

two kinds of combustion chamber shapes in light of various injection conditions such as the injection pressure, duration, and the nozzle-wall distance. Also, spray images in the combustion chamber obtained by the visualization system and the numerical investigation of the spray and evaporation characteristics were conducted by a numerical method. The experimental results of the macroscopics characteristics such as the spray development process and spray tip penetration were compared with the calculated results. The evaporation performance according to various conditions was calculated and analyzed by the KIVA code. On the basis of the results of this research, the conclusions are summarized as follows. 1. The calculated results of spray image in the combustion chamber agreed fairly well with the experimetal results of DME and diesel spray. In the narrow type chamber at the nozzle-wall distance of 1 mm, the injected spray is impinged on the bowl surface on the piston and it formed the wall wetting and floating flow in the vicinity of the chamber bowl surface by the collision and splashed spray. In the case of the conventional type chamber, the fuel spray injected until the spray droplets arrived at the right bowl wall. After the breakup of injected spray, the spray droplets on the wetting surface of the bowl in the chamber make the floating flow to the upward direction as a type of bowl shape of chamber. 2. The calculated spray tip penetration of DME and diesel fuel shows good agreement with experimental results with conventional and narrow type combustion chambers. The increasing rate of the spray tip penetration at the nozzle-wall distance of 1 mm slowed because of the breakup of droplets and the wall wetting. Also, the gap between the spray tip penetration of both 40 and 60 MPa of injection pressure in the spray tip penetration of DME fuel was smaller than that of diesel fuel. 3. In the evaporation characteristics, the vaporization of DME spray occurred remarkably more than that of diesel spray and


Kim et al.

of DME fuel shows a remarkably increasing trend of elapsed time after the start of the injection. Acknowledgment. This study was supported by the CEFV (Center for Environmentally Friendly Vehicles) of the Eco-STAR project from the MOE (Ministry of Environment, Republic of Korea). Also, this work was financially supported by the Ministry of Education and Human Resources Development (MOE), the Ministry of Commerce Industry and Energy (MOCIE), and the Ministry of Labor (MOLAB) through the fostering project of the Laboratory of Excellency. Also, this work was supported by the Second Brain Korea 21 Project in 2007.

Nomenclature

Figure 14. Calculated evaporation rate of fuels according to combustion chamber shape and nozzle-wall distance after start of injection (Pinj ) 60 MPa).

the vapor distribution at the nozzle-wall distance of 4 mm dispersed more widely than that of 1 mm. Also, the vapor appeared in the crevice area at the nozzle-wall distance of 4 mm in the conventional type chamber. The evaporation rate of diesel fuel was less than one percent but the evaporation rate

B ) mass transfer number B1 ) constant of the WAVE breakup cp ) specific heat CRT ) breakup constant of RT breakup D ) coefficient of diffusion of the vapor through the surrounding atmosphere K ) thermal conductivity of gas KRT ) wavenumber of RT breakup m ˙ ) rate of mass transfer Nu ) Nusselt number () 2.0 + 0.6Re1/2/Pr1/3) Pr ) Prandtl number () µcp/K) r ) droplet radius rc ) critical radius of droplet rn ) droplet radius after breakup Re ) Reynolds number () UrFr/µ) Sc ) Schmidt number () µ/FD) Sh ) Sherwood number () 2.0 + 0.6Re1/2/Sc1/3) T ) Taylor number () ZWe1/2) We ) Weber number () Ur2Fr/σ) Ys ) mass fraction of fuel vapor on the droplet surface Y∞ ) mass fraction of fuel vapor in the free stream condition Z ) Ohnesorge number () We1/2Re) Λ ) corresponding wavelength F ) density (FDair) ) fuel vapor diffusivity τ ) breakup time σ ) surface tension Ω ) maximum growth rate Subscripts WAVE ) WAVE breakup RT ) RT breakup EF8000696

Numerical and Experimental Study on the Comparison between

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