Embed Size (px)
Citation preview
ICLASS-2006 Aug.27-Sept.1, 2006, Kyoto, Japan
1. INTRODUCTION
A CI engine has an advantage of the thermal efficiency, however, it has the problem of much more NOx and particulate matters in its exhaust gas than an SI engine. In a CI engine, it is able to realize the simultaneous reduction of NOx and soot emission by applying homogeneous charge compression ignition (HCCI) with the early injection during compression stroke. The self-ignition of the homogeneous mixture produced by the advanced injection causes the rapid-lean combustion and reduces NOx and soot emission while it maintains its thermal efficiency as same as that of a conventional CI engine. However, HCCI has the disadvantage of the limited operating range against a conventional CI engine because it shows the severe knocking at high load condition and the unstable combustion at low one. In HCCI, the combustion characteristics and the exhaust emissions predominantly depend on the mixture formation and the chemical reaction process. Many researchers have tried to control the air-fuel mixing process by changing the injection timing or nozzle specification [1,2], and by controlling the chemical reaction pathway in consideration of the fuel molecule [3,4]. Thus, the combustion control in view of both physical and chemical phenomena is required for the wider operating range and the low emission of HCCI engine.
The authors focus on the fuel properties such as density, viscosity, volatility, ignitability and so on. They are the dominant factors for phenomena inside the combustion chamber. The authors have proposed fuel deign approach that is aimed at providing the optimal characteristics of spray and its combustion by controlling the fuel properties [5-9]. It is capable of changing the fuel properties by
mixing two kind of fuel. The point is that they have different physical and chemical property each other. The fuel with two components based on the fuel design makes possible to control combustion in higher degree of freedom for various engine system.
The authors plan to apply the fuel design approach to HCCI combustion for a CI engine which is realized by advanced injection because mixture control of physical side is allowed by fuel spray in direct-injected HCCI (DI-HCCI) engine. This paper describes the possibility of the mixture control using flash boiling spray as the first step. The flash boiling phenomenon makes significant change on the spray structure which is verified in the case of a pintle type injector for an SI engine [10,11]. In this study, the macroscopic characteristics of a diesel spray with flash boiling were investigated experimentally under the HCCI-like condition. The spray was visualized by the technique of laser observation in a constant volume vessel with the ambient condition corresponding to the several crank angle of an actual CI engine. 2. RESEARCH CONCEPT 2.1 Fuel Design Approach
The conception of fuel design approach is summarized in this section. Artificial control of transport properties
The transportation properties of fuel such as density, viscosity, surface tension are changed by mixing two kind of fuel which has the different properties each other. It is possible to allow the available use of the heavy fuel oil or the gaseous fuel which have not been applied to
Paper ID ICLASS06-134 An Experimental Study on Flash Boiling Spray using Two-Component Fuel
under the Condition of Advanced Injection HCCI Yuji NISHIMURA 1, Yoshimitsu WADA 2, Akihiro YAMAGUCHI 3, Jun-Kyu YOON 4,
Jiro SENDA 5 and Hajime FUJIMOTO6
1 Department of Mechanical Engineering, Doshisha University, [email protected] 2 Department of Mechanical Engineering, Doshisha University, [email protected] 3 Department of Mechanical Engineering, Doshisha University 4 Department of Automotive Engineering, Kyungwon College, [email protected] 5 Department of Mechanical Engineering, Doshisha University, [email protected] 6 Department of Mechanical Engineering, Doshisha University, [email protected]
ABSTRACT The experimental study was conducted to investigate macroscopic characteristics of the flash boiling spray with two-component fuel. The final target is the application of this kind of spray to a homogeneous charge compression ignition (HCCI) engine in which the fuel is injected at the early timing. Recent study reported that to maintain the heterogeneous mixture was effective to avoid the severe knocking and the unstable combustion, which were the main factors to restrict the operating range of HCCI combustion. However, the excessive heterogeneity causes the increase in nitrogen oxides and particulate matters that is the main issue of a diesel engine. In this study, the authors propose to utilize the flash boiling spray of two-component fuel as the means to control the homogeneity in the mixture for a direct-injected HCCI engine. The spray characteristics of this kind of fuel with a flash boiling phenomenon were investigated by means of Shlieren and Mie scattering photography. Test fuel was injected into a constant volume vessel at the ambient condition corresponding BTDC of an actual engine. As a result, it is found that the flash boiling phenomenon greatly changes the spray structure, especially at the condition of the relatively low density and the relatively low temperature. As a consequence, the authors conclude that it is available for HCCI to control the mixture formation by usage of the flash boiling spray particularly at the early injection timing.
Keywords: Flash Boiling Spray, Fuel Design, Two-componet Fuel, Direct-Injection HCCI
automobiles automobiles due to its unmanageable quality. Improvement atomization and vaporization of fuel spray When the fuel with low boiling point (B.P.) is dissolved with other with high B.P., the curve of saturated vapor pressure of the high B.P. component shifts to the region of lower temperature and higher pressure. It is marked that the two-phase region appears between both curves, where the gas phase and the liquid phase exists, as shown in Fig.1. As the result, the volatility of fuel with high B.P. is improved. Furthermore, it is able to control the volatility of fuel owing to the mixing fraction. Additionally it is possible to initiate the flash boiling when the state of fuel with two-component attains to the region below the saturated liquid line in the decompression process with a fuel injection. As a result, the drastic atomization and evaporation of fuel spray are actualized. Ignitability control
There is the possibility of the control of the ignition characteristics by mixing two kind of fuel whose octane or cetane number and chemical molecule are different each other. Accordingly, it is able to control the ignition timing, as a consequence, to achieve the control of combustion process.
2.2 Application of Flash Boiling Spray to DI-HCCI
In this study, we apply the formation of two-phase region by mixing the fuel of low B.P. with that of high B.P., to initiate flash boiling easily to a diesel spray injected through a hole nozzle. The flash boiling phenomenon is the phase-change process from liquid to gas, which is yielded by the rapid growth of initiated cavitation bubbles when the ambient pressure decreased below the saturated vapor pressure of liquid. The bubble growth rate of cavitations is subject to the degree of super-heat, ∆T, that is the difference between the initial temperature of liquid and the saturated temperature at the ambient pressure [12], as illustrated in Fig.1. Therefore, the point is that the flash boiling spray in DI-HCCI is able to be controlled by changing the degree of super-heat which is varied due to the following manner. 1) the control of two-phase region with mixing fraction 2) the variation of ambient pressure due to injection timing
The mixture control of DI-HCCI is achieved in high degree of freedom by the flash boiling spray, as mentioned above.
Some prospective advantage of application to a DI-HCCI engine is cited as follows. 1) HCCI with advanced direct injection has the problem
regarding the spray impingement on the wall of piston or that of cylinder liner wall because the fuel is injected into the atmosphere with low temperature and low density. This can be avoided by reducing injection velocity. However, it causes the poor atomization and the poor air entrainment. The flash boiling has the possibility to shorten spray penetration even at high injection velocity, thus, quantity of fuel impinging on the wall is reduced. 2) Kumano et al [13] reported that the preservation of
certain heterogeneity in the air-fuel mixture was effective factor to avoid severe knocking and unstable combustion, both are just the main issue in HCCI. The homogeneity of mixture is controlled by the changing the injection timing. However, over-advanced injection timing causes spray impingement on the piston head and the cylinder liner. On the other hand, injection timing near TDC brings about much generation of NOx and particulate matter. In the case of the flash boiling spray, it is capable of controlling the homogeneity of mixture by varying the degree of super-heat without any changes in injection timing. 3. EXPERIMENTAL SYSTEM AND METHOD 3.1 Experimental Apparatus
Fig.2 illustrates the schematic diagram of constant volume vessel which is used in this experiment. In this vessel, high pressure and temperature atmosphere is able to be prepared. The ambient temperature is controlled by the cartridge heater installed in the vessel. The thermo-couple of K type monitors the temperature in the vessel. Three quartz windows whose visible diameter is 90[mm] are set at three sides of the vessel for the optical access.
3.2 Optical Setup
Simultaneous imaging technique of Schlieren and Mie scattering adopted as the method to take spray characteristics. Fig.3 is the flow sheet of optical setup. The beam of light (λ=488 [nm]) oscillated through an Ar＋ ion laser (2.0 [W]) is expanded to parallel light of 90 [mm] in diameter by passing through a pin hole whose pin diameter is 10 [mm] and two convex lenses (f=40 [mm], 1000 [mm]). aaaaaa
two -phase region
State of chamber
Temperature
Pre
ssur
e
Initial fuel temperature
Mixing fraction ofLow B.P. fuel
less
Temperature
Pre
ssur
e
ΔT
much
flashing process
ΔT: the degree of super-heat
two -phase region
State of chamber
Temperature
Pre
ssur
e
Initial fuel temperature
Mixing fraction ofLow B.P. fuel
less
Temperature
Pre
ssur
e
ΔT
much
flashing process
ΔT: the degree of super-heat Fig.1 Two-phase region of mixed fuel with two
components
200V
200V
Gas in
Gas out
T
t°
N2
1
3
4
7
9
2
5
6
8
1: Oil pump 2: Thermo-couple 3: Pressure gauge4: Injection nozzle 5: Thermo-couple 6: Heating jacket7: Optical window 8: Heater 9: Insulator
T 200V
200V
Gas in
Gas out
T
t°
N2
1
3
4
7
9
2
5
6
8
1: Oil pump 2: Thermo-couple 3: Pressure gauge4: Injection nozzle 5: Thermo-couple 6: Heating jacket7: Optical window 8: Heater 9: Insulator
1: Oil pump 2: Thermo-couple 3: Pressure gauge4: Injection nozzle 5: Thermo-couple 6: Heating jacket7: Optical window 8: Heater 9: Insulator
T
Fig.2 Schematic diagram of constant volume vessel
The light is transmitted the vessel and converged by the other convex lens (f=1000 [mm]). A pin hole is located on focal point of third convex lens to intercept the light refracted by density of fuel spray. Schlieren image visualized in this way is taken with a high speed video camera (exposure time : 2 [μs]) at the speed of 20,000 [f.p.s.]. Mie scattering light generating from the liquid phase of fuel spray is taken with a CCD camera (exposure time : 346 [μs]) which is placed at the perpendicular direction to the parallel light. The gate timing of CCD camera is 2ms after the start of injection when spray is in the quasi-steady state.
The magnified shadowgraph photography was conducted to capture the spray characteristics in the vicinity of the nozzle outlet. A pin hole shown in Fig.3 is not placed in this case. The same CCD camera captures the shadow image during the quasi-steady state of spray. Its spacial resolution is 19.6 [µm/pixe]. The secondary harmonic wave (λ=532 [nm]) of pulsed Nd:YAG laser is employed as the light source to freeze the high speed phenomena of spray at high resolution. 3.2 Experimental Condition Experimental conditions and nozzle specification are listed in Table.1. The ambient condition in the vessel simulates that at several crank angles before TDC in an actual engine whose compression ratio is 16.3. The crank angles selected is 40, 60, 80 and 100 [deg.BTDC], respectively the nitrogen as the ambient gas is chaged in the vessel at the pressure corresponding to given crank angle. The cartridge heater heats the ambient gas up to the temperature at given crank angle. Electronically controlled common-rail system is used for the fuel injection. The diameter of nozzle orifice is 0.20 [mm], the orifice pressure drop is 50.0 [MPa] and the injection duration of 2.50 [ms]. In this experiment, fuel temperature is one of parameter. The fuel is heated by the ribbon heater wrapped around the common-rail and the pipe of fuel supply. The surface temperature of the injector near the nozzle is defined as the initial fuel temperature Tf.
3.3 Fuel Tested
The fuel tested is the mixture of n-tridecane as a high boiling point fuel and i-pentane as a low boiling point fuel. Hereinafter, it is called C13/iC5. The molar fraction, XiC5, of i-pentane is 0.8. n-tridecane/i-pentane mixture is compared with n-heptane (C7) at the part of conditions, because they are different in the volatility and similar in other physical properties such as density, viscosity. All the conditions in the experiment are plotted on
aaaaaaaaaaaaaaaa
pressure-temperature diagram with the line of the saturated liquid at the equal to 0.8 as shown in Fig.4. Table.2 summarizes the properties of n-tridecane, i-pentasne, n-heptane. They are calculated using NIST mixture property database [14]. 4. RESULTS AND DISCUSSIONS 4.1 Spray Characteristics near Nozzle Exit
Tab.2 Properties of fuel tested Fuel
Boiling point
n-C13H28
Density *
Viscosity *
[K]
[kg/m3]
[µPa・s]
i-C5H12 n-C7H16
509 301 372
766
1794
621
225
683
408
Fuel temp. **
Density ***
Viscosity***
Pres. on saturatedliquid line [MPa]
[kg/m3]
[µPa・s]
310
0.105
666
364
345
0.284
633
265
380
0.626
597
202
410
1.09
564
155
435
1.61
533
125
[K]
**: C13/iC5 mixed fuel (XiC5=0.8)***: calculated at 1.61[MPa]
*: calculated at 1.61[MPa], 293[K]
Fuel
Boiling point
n-C13H28
Density *
Viscosity *
[K]
[kg/m3]
[µPa・s]
i-C5H12 n-C7H16
509 301 372
766
1794
621
225
683
408
Fuel
Boiling point
n-C13H28
Density *
Viscosity *
[K]
[kg/m3]
[µPa・s]
i-C5H12 n-C7H16
509 301 372
766
1794
621
225
683
408
Fuel temp. **
Density ***
Viscosity***
Pres. on saturatedliquid line [MPa]Pres. on saturatedliquid line [MPa]
[kg/m3]
[µPa・s]
310
0.105
666
364
345
0.284
633
265
380
0.626
597
202
410
1.09
564
155
435
1.61
533
125
[K]
**: C13/iC5 mixed fuel (XiC5=0.8)***: calculated at 1.61[MPa]
*: calculated at 1.61[MPa], 293[K]
convex lens(f=1000mm)
Ar＋ Laser (λ=488nm)
convex lens(f=40mm)
ND filter
Pin hole
Pin hole
convex lens(f=1000mm)
CCD camera (Exposure time＝346µs)
High speed video camera
Frame rate=20,000f.p.s.Exposure time=2µs convex lens
(f=1000mm)
Ar＋ Laser (λ=488nm)
convex lens(f=40mm)
ND filter
Pin hole
Pin hole
convex lens(f=1000mm)
CCD camera (Exposure time＝346µs)
High speed video camera
Frame rate=20,000f.p.s.Exposure time=2µs convex lens
(f=1000mm)
Ar＋ Laser (λ=488nm)
convex lens(f=40mm)
ND filter
Pin hole
Pin hole
convex lens(f=1000mm)
CCD camera (Exposure time＝346µs)
High speed video camera
Frame rate=20,000f.p.s.Exposure time=2µs
Fig.3 Schematic diagram of optical settings
Tf=
1.5
1.25
1.0
0.75
0.5
0.25
0
Pre
ssur
e [M
Pa
]
320 360 400 440 480Temperature [K]
BTDC [80deg.]
BTDC [60deg.]
BTDC [40deg.]
305[K] 345[K] 380[K] 410[K]
BTDC [100deg.]
Saturated liquid line of XiC5 =0.8
435[K]Tf=
1.5
1.25
1.0
0.75
0.5
0.25
0
Pre
ssur
e [M
Pa
]
320 360 400 440 480Temperature [K]
BTDC [80deg.]
BTDC [60deg.]
BTDC [40deg.]
305[K] 345[K] 380[K] 410[K]
BTDC [100deg.]
Saturated liquid line of XiC5 =0.8
435[K]
Fig.4 Saturated liquid line of test fuel
Table.1 Experimental conditions
Injection equipment Common-rail type
Nozzle hole diameter dn 0.20[mm]
Orifice pressure drop 50.0[MPa]
Injection quantity 22.2[mg]
Simulated Crank angle[deg. BTDC] 406080100
Ambient gas N2
Ambient pressure [MPa] 0.18 0.26 0.44 0.93
Fuel temperature Tf 310, 345, 380, 410, 435[K]
Ambient density ρa [kg/m3] 1.51 1.99 2.96 5.16
Ambient temperature [K] 625515445405
Ambient viscosity [µPa・s] 21.8 23.3 25.6 29.1
Length of hole ln 0.80[mm]
Injection equipment Common-rail typeInjection equipment Common-rail type
Nozzle hole diameter dn 0.20[mm]
Orifice pressure drop 50.0[MPa]
Injection quantity 22.2[mg]
Simulated Crank angle[deg. BTDC] 406080100
Ambient gas N2
Ambient pressure [MPa] 0.18 0.26 0.44 0.93
Fuel temperature Tf 310, 345, 380, 410, 435[K]
Ambient density ρa [kg/m3] 1.51 1.99 2.96 5.16
Ambient temperature [K] 625515445405
Ambient viscosity [µPa・s] 21.8 23.3 25.6 29.1
Orifice pressure drop 50.0[MPa]
Injection quantity 22.2[mg]
Simulated Crank angle[deg. BTDC]
Simulated Crank angle[deg. BTDC] 406080100
Ambient gas N2Ambient gas N2
Ambient pressure [MPa] 0.18 0.26 0.44 0.93
Fuel temperature Tf 310, 345, 380, 410, 435[K]
Ambient density ρa [kg/m3] 1.51 1.99 2.96 5.16
Ambient temperature [K] 625515445405
Ambient viscosity [µPa・s] 21.8 23.3 25.6 29.1
Length of hole ln 0.80[mm]
Tf
310[K]345[K]380[K]410[K]435[K]
50
40
30
20
10Spra
y co
ne a
ngle
θ1
[deg
.]
Degree of superheat ∆T [K]-90 -60 -30 0 30 60 90 120
Tf
310[K]345[K]380[K]410[K]435[K]
Tf
310[K]345[K]380[K]410[K]435[K]
Tf
310[K]345[K]380[K]410[K]435[K]
50
40
30
20
10
50
40
30
20
10Spra
y co
ne a
ngle
θ1
[deg
.]
Degree of superheat ∆T [K]-90 -60 -30 0 30 60 90 120
Fig.7 Spray cone angle θ1 as a function of degree of super-heat ΔT
10 20 30 40 50
1.5
3.0
4.5
6.0
7.5
9.0
Spr
ay a
ngle
θ2
[deg
.]
Spray cone angle θ1 [deg.]
100[deg. BTDC]310Tf [K]= 345 380 410 435
80[deg. BTDC]60[deg. BTDC]40[deg. BTDC]
10 20 30 40 50
1.5
3.0
4.5
6.0
7.5
9.0
Spr
ay a
ngle
θ2
[deg
.]
Spray cone angle θ1 [deg.]
100[deg. BTDC]310Tf [K]= 345 380 410 435
80[deg. BTDC]60[deg. BTDC]40[deg. BTDC]
Fig.8 Spray cone angle θ1 versus spray angle θ2
Fig.5 displays enlarged shadowgraph images near nozzle exit. They are the typical example under the ambient condition equivalent to 80 [deg.BTDC]. The parameter of the upper (a) is the initial temperature of C13/iC5. In the area below nozzle exit, spray spreads to radial direction as initial fuel temperature increases. It suggests that fuel droplets atomized are given the momentum toward the radial direction due to the flash boiling. The lower (b) is the effect of the kind of fuel. The fuel temperature, Tf, is the constant of 410 [K]. In the case of (b), the ambient gas is the mixture of helium gas and argon gas so that the density and pressure of the ambience is nearly equal to these at 80 [deg.BTDC] while maintaining room temperature. By this means, the effect of heating by the ambient gas is able to be ignored. The spray of C13/iC5 spreads due to the increase in the initial fuel temperature as previously described. However, the spray shape of n-heptane is almost the same as that of n-tridecane in spite of its high initial fuel temperature. N-heptane has the similar physical properties except the volatility to that of C13/iC5. Consequently, the result proves that spray spread of C13/iC5 having its high initial fuel temperature is attributed to the degree of super-heat in its flashing process.
Spray cone angle θ1 is determined from shadowgraph images as shown in Fig.6. θ1 is measured near the nozzle exit where the flash boiling is distinguished. The definition of θ1 is described as follows: (1) The shape of spray from the nozzle outlet to 2 [mm] is divided by the area whose width is equal to the hole diameter, dn, into the outer two areas (2) the outer two areas are replaced to the triangle whose area is as same as that of the original and (3) θ1 is obtained by the summation of θ1’ and θ1’’ which are the apex angles of these triangles.
Fig.7 shows the spray cone angle θ1 of C13/iC5 as a function of the degree of super-heat, Δ T. θ1 is approximately depending onΔ T. Namely, the spray characteristics are strongly affected by cubical expansion of the flash boiling phenomenon in the region below nozzle exit. θ1 becomes wider as increase inΔT. θ1 becomes aaaaaaa
smaller at the sameΔT owing to the increase in the fuel temperature, Tf. The trend is caused by the decrease in the fuel viscosity with increase in Tf, as a result, the mean droplet diameter becomes smaller and the vaporaization is promoted much more.
As shown in Fig.6, θ2 is defined as the gradient angle of outer edge of spray in the region from 4 mm to 15 mm below the nozzle exit, where cubical expansion by flash boiling gives no effect on the shape of spray. Fig.8 shows the relation between θ1 and θ2. At the condition corresponding to 80 and 100 [deg.BTDC], θ2 increases in aaaaa
Tf [K]= 345 380 410 435310∆T [K] = -30.7 4.3 39.3 69.3 94.3
0
15
7.5
Axi
al d
ista
nce
from
no
zzle
exi
t [m
m]
∆T [K] = -142XiC5=0.0
69.3XiC5=0.8
3.8C7
0
15
7.5
(a) Effect of initial fuel temperature C13/iC5, XiC5=0.8
Axi
al d
ista
nce
from
no
zzle
exi
t [m
m]
(b) Effect of the kind of fuel, Tf =410 [K]
Tf [K]= 345 380 410 435310∆T [K] = -30.7 4.3 39.3 69.3 94.3
0
15
7.5
0
15
7.5
Axi
al d
ista
nce
from
no
zzle
exi
t [m
m]
∆T [K] = -142XiC5=0.0
69.3XiC5=0.8
3.8C7
0
15
7.5
0
15
7.5
0
15
7.5
(a) Effect of initial fuel temperature C13/iC5, XiC5=0.8
Axi
al d
ista
nce
from
no
zzle
exi
t [m
m]
(b) Effect of the kind of fuel, Tf =410 [K]
Fig.5 Enlarged shadowgraph images taken in the region
close to nozzle tip (80deg.BTDC)
0 to
2 [m
m]
4 to
15
[mm
]
θ1= θ1’+θ1”
nozzlespray
dn
=
A1’ A1”
θ1’ θ1
”
A1’ A1”
θ2’ θ2
” θ2= θ2’+θ2”
Dis
tanc
e fro
m n
ozzl
e ex
it 0 to
2 [m
m]
4 to
15
[mm
]
θ1= θ1’+θ1”
nozzlespray
dn
=
A1’ A1”
θ1’ θ1
”
A1’ A1”
θ2’ θ2
”θ2’ θ2
” θ2= θ2’+θ2”
Dis
tanc
e fro
m n
ozzl
e ex
it
Fig.6 Definition of spray cone angle θ1 and spray angle θ2
0
15
30
45
60
75
90
Axi
al d
ista
nce
from
noz
zle
exit
[mm
]
100 [deg.BTDC]
Tf [K]310 -17.0
∆T [K]
345 18.0380 53.0410 83.0435 108
0.0 0.15 0.3 0.45 0.6 0.75 0.90
15
30
45
60
75
60 [deg.BTDC]
310 -53.2345 -18.2380 16.8410 46.8435 71.8
Time after corrected start of injection [ms]
Tf [K] ∆T [K]
0
15
30
45
60
75
90
Axi
al d
ista
nce
from
noz
zle
exit
[mm
]
100 [deg.BTDC]
Tf [K]310 -17.0
∆T [K]
345 18.0380 53.0410 83.0435 108
0.0 0.15 0.3 0.45 0.6 0.75 0.90
15
30
45
60
75
60 [deg.BTDC]
310 -53.2345 -18.2380 16.8410 46.8435 71.8
Time after corrected start of injection [ms]
Tf [K] ∆T [K]
Fig.11 Temporal change in spray tip penetration as a function of initial fuel temperature
almost proportion to θ1. Thus, spreading characteristics of spray depends on the atomization due to the flash boiling at lower density condition. At the condition relating to 60 [deg.BTDC], θ2 is plotted over the line proportioning to θ1. This tendency indicates that θ2 is expanded by the effect of shear force between ambient gas and the spray because ambient density is comparatively high at 60 [deg.BTDC]. At the condition corresponding to 40 [deg.BTDC], in other words, at the ambience with high density, θ2 is nearly constant in spite of elevating of the initial fuel temperature, Tf. Thus, the effect of ambient density is dominant factor of spray spread at high density ambience. 4.2 Effect of Degree of Super-Heat on Spray Tip Penetration
Fig.9 indicates the definition of the corrected start of the injection. The function is applied to the spray tip penetration of fully developed spray, and the intersection point of the function and the horizontal axis is defined as the corrected start of the injection [15]. Fig.10 shows the aaaaaaaaaa
images of Schlieren photography under the condition of the initial fuel temperature, Tf, equal to 310 [K] and 435 [K] at 0.75 [ms] after the corrected start of the injection. The spray tip penetration becomes shorter and spray spread becomes wider in the case of Tf=435 [K] comparing with these of Tf=310 [K]. The temporal change in the spray tip penetration under the conditions of 60 and 100 [deg.BTDC] is illustrated in Fig.11. In all of conditions, the spray penetrates in proportion to the time at the beginning of the injection duration as well known. Thereafter, it develops in proportion to 1/2th power of the time as well as the typical diesel spray. The penetration rate of the spray decreases as the increase in initial fuel temperature, in other words, the increase in the degree of super-heat. The result demonstrates the flash boiling promotes the mixing of fuel with surroundings. Consequently, the momentum of fuel spray in the axial direction is attenuated. This tendency is obtained clearly at the lower ambient temperature. The obvious effect of flash boiling is not confirmed at the high density condition, in other words, at that closer to TDC because the shear force between ambient gas and the spray was more effective. Namely, it is much available for DI-HCCI to inject the fuel at the early timing since the flash boiling phenomena are more distinguished than those at the later timing of injection. 4.3 Effects of Degree of Super-Heat on Spray Spreading Angle
Spray spreading angle is measured from Schlieren images. The spray spreading angle, θ3, is defined by following equation to evaluate spreading characteristics:
( )13 2 tan 2w lθ −=
where l is the spray tip penetration, and w is the spreading width of spray. l and w are measured on the median-filtered images of Schlieren photography after the subtraction of the background image. Fig.12 shows the temporal change of θ3 at the ambient condition of 60 and 100 [deg.BTDC]. θ3 becomes wider as the increase in the initial fuel
Tf=310[K]
0
45
90
=435[K]
condition of 100deg. 80deg. 60deg. 40deg.
0
45
90
Axi
al d
ista
nce
from
noz
zle
exit
[mm
]
0.75 [ms after corrected start of injection
Tf
Tf=310[K]
0
45
90
=435[K]
condition of 100deg. 80deg. 60deg. 40deg.
0
45
90
Axi
al d
ista
nce
from
noz
zle
exit
[mm
]
0.75 [ms after corrected start of injection
Tf
Fig.10 Image of Schlieren photography
0.5 0.6 0.7 0.8 0.9 1.0Time after image acquisition [ms]
10
0
20
30
40
Spra
y tip
pen
etra
tion
[mm
]
Corrected Start of Injection
Experimental data
Fitting function curve
0.5 0.6 0.7 0.8 0.9 1.0Time after image acquisition [ms]
10
0
20
30
40
Spra
y tip
pen
etra
tion
[mm
]
Corrected Start of Injection
Experimental data
Fitting function curve
Fig.9 Definition of corrected start of injection
0 20 40 60 80Axial distance from nozzle exit [mm]
0.0
8.0
4.0
0.0
2.0
0.04.0
1.5
3.0
Spr
ay w
idth
in ra
dial
dire
ctio
n [m
m]
I/Imax=0.01
I/Imax=0.48
I/Imax=0.71
435K410380345310Tf [K]
0 20 40 60 80Axial distance from nozzle exit [mm]
0 20 40 60 800 20 40 60 80Axial distance from nozzle exit [mm]
0.0
8.0
4.0
0.0
2.0
0.04.0
1.5
3.0
Spr
ay w
idth
in ra
dial
dire
ctio
n [m
m]
I/Imax=0.01I/Imax=0.01
I/Imax=0.48I/Imax=0.48
I/Imax=0.71I/Imax=0.71
435K435K410410380380345345310310Tf [K]
Fig.14 Contour line of normalized intensity I/Imax, at 60 [deg.BTDC]
aaaaaaaaaaa temperature, in other words, the increase in the degree of super-heat, in the case of the lower ambient density of 100 [deg.BTDC]. As a consequence, the effect of the degree of super-heat on θ3 is more pronounced in the lower ambient density as same as the case of spray tip penetration. Fig.13 is the spray spreading angle, θ3, averaged from 2 [ms] to 3 [ms] when the state of the spray shows steady as a function of the density ratio, that is, the ratio of the density of fuel, ρf, to ambient density, ρa. At Tf equal to 310 [K], θ3 decreases as the density ratio increases up to around 325. And θ3 is almost constant when the density is over about 325. The trend is the same as that of a usual diesel spray, which is because it is difficult to initiate mixing of the fuel spray with ambient gas at low ambient condition. The decrease in θ3 under the condition of Tf over 345 [K] becomes gentler because of the effect of the flash boiling spray. At Tf equal to 435 [K] or 410 [K], θ3 retains to be large despite of high density ratio due to the spreading effect by increasing of the degree of super-heat. As above, the effect of flash boiling on the dispersibility of spray is more remarkable in lower density ambience. 4.4 Effects of Degree of Super-Heat on Spray Structure
To evaluate the effect of flash boiling on the spray structure, the distribution of intensity on Mie scattering images is estimated. The information of Mie scattering shows the area where the liquid phase is dominant. Fig.14 shows the contour line of I/Imax where I is the intensity of Mie scattering and Imax is its maximum at the condition of 60 [deg.BTDC]. The abscissa axis indicates the spread width; the horizontal axis does the distance from nozzle exit. In the case of each I/Imax, distribution area of liquid phase is shortened toward upstream side, and widened in radial direction as the degree of super-heat increases. This result suggests that fuel droplets are given the momentum in radial direction by rapid bubble growth with flash boiling; furthermore atomized droplets evaporate promptly. The spread width of Tf equal to 435 [K] is narrow especially in I/Imax=0.71, nevertheless wide in I/Imax=0.48 and 0.01. This result indicates that tenuous cloud of liquid phase is distributed to wide area due to the effect of flash boiling. At high fuel temperature of 435K, the effect of the flash boiling is able to break up the dense cloud of liquid phase even in the central region of spray. In consequence, the evaporation of atomized droplet is promoted immediately, and then the distribution area of high intensity is reduced. 5. CONCLUSION
The main conclusion of this experiment is summarized as follows: 1) In the vicinity of the nozzle outlet, the characteristics of
spray spread depend on the atomization due to the flash boiling phenomena. The trend is more distinguished at lower ambient density.
2) The spray tip penetration becomes shorter and the spray angle becomes wider as the degree of super-heat increases. This tendency is more remarkable as the ambient density is lower.
3) The distribution area of liquid phase is shortened toward upstream side, and spreads wider in radial direction as the degree of super-heat increases. In flash boiling spray, tenuous cloud of liquid phase is distributed to wide area.
4) The flash boiling phenomenon becomes remarkable as the decrease in the ambient density and the increase in the degree of super-heat.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Time after start of injection [ms]
100 [deg.BTDC]
310 -17.0345 18.0380 53.0
410 83.0435 108
60 [deg.BTDC]
310 -53.2345 -18.2380 16.8
410 46.8435 71.80
5
10
15
20
0
5
10
15
20
25S
pray
spr
eadi
ng a
ngle
θ3
[deg
.]
Tf [K] ∆T [K]Tf [K] ∆T [K]
Tf [K] ∆T [K]Tf [K] ∆T [K]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.50.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Time after start of injection [ms]
100 [deg.BTDC]
310 -17.0345 18.0380 53.0
410 83.0435 108
60 [deg.BTDC]
310 -53.2345 -18.2380 16.8
410 46.8435 71.80
5
10
15
20
0
5
10
15
20
25
0
5
10
15
20
25S
pray
spr
eadi
ng a
ngle
θ3
[deg
.]
Tf [K] ∆T [K]Tf [K] ∆T [K]Tf [K] ∆T [K]Tf [K] ∆T [K]
Tf [K] ∆T [K]Tf [K] ∆T [K]Tf [K] ∆T [K]Tf [K] ∆T [K]
Fig.12 Temporal change in spray spreading angle as a function of initial fuel temperature
100 150 200 250 300 350 400 450Density ratio ρf /ρa [-]
Tf=435[K]Tf=410[K]Tf=380[K]Tf=345[K]Tf=310[K]
8
20
18
16
14
12
10
Spr
ay s
prea
ding
ang
le θ
3[d
eg.]
100 150 200 250 300 350 400 450Density ratio ρf /ρa [-]
100 150 200 250 300 350 400 450100 150 200 250 300 350 400 450Density ratio ρf /ρa [-]
Tf=435[K]Tf=410[K]Tf=380[K]Tf=345[K]Tf=310[K]
Tf=435[K]Tf=410[K]Tf=380[K]Tf=345[K]Tf=310[K]
8
20
18
16
14
12
10
8
20
18
16
14
12
10
Spr
ay s
prea
ding
ang
le θ
3[d
eg.]
Fig.13 Spray spreading angle as a function of density ratio
Flash boiling sprayConventional diesel spray
Impingement of unvaporized fuel
Reducing the quantity of fuel impinging on wall
Heterogeneous mixture Homogeneous mixture
Control by the degree of super-heat
Combustionchamber
Flash boiling sprayConventional diesel spray
Impingement of unvaporized fuel
Reducing the quantity of fuel impinging on wall
Heterogeneous mixture Homogeneous mixture
Control by the degree of super-heat
Combustionchamber
Fig.15 Schematic diagram of the strategy to apply the flash boiling spray to a DI-HCCI engine
As above, it is much available to apply the flash boiling to DI-HCCI in which the fuel is injected in early timing, because the effect of the flash boiling are more remarkable in the lower density atmosphere. Fig.15 shows schematic diagram of the strategy that is the application of the flash boiling spray to an actual DI-HCCI engine. It is possible to avoid the impingement of the fuel in liquid phase because of the effect of the flash boiling that shortens the spray tip penetration. Additionally, the homogeneity of mixture can be controlled by the degree of super-heat because the effect of the flash boiling varies the spray structure greatly. ACKNOWLEDGEMENTS
This work was carried out in the Energy Conversion Research Center of Doshisha University which had been supported by Academic Frontier Project for Private Universities: matching fund subsidy from The Ministry of Education, Culture, Sports, Science and Technology, 2003-2007. REFFERENCE 1. Sjoberg, M., Edling, L., Eliassen, T., Magnusson, L. and
Angstrom, H. “GDI HCCI: Effect of injection Timing and Air Swirl on Fuel Stratification, Combustion and Emissions Formation”, SAE Technical Paper 2002-01-0106, 2002.
2. Lechner, G., Jacobs, T., Chryssakis C., Assanis, D. and Siewert, R., “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 2005-01-0167, 2005.
3. Tanaka, S., Ayala, F., Keck, J.C. and Heywood, J.B., “Two-stage Ignition in HCCI Combustion and HCCI Control by Fuel and Additives. Combustion and Flame 132, 2003, 219-239.
4. Kitano, K., Nishiumi, R., Tsukasaki, Y., Tanaka, T. and Morinaga, M., “Effects of Fuel Properties on Premixed Charge Compression Ignition Combustion in a Direct Injection Diesel Engine”, SAE Technical Paper 2003-01-1815, 2003.
5. Senda, J., Ikeda, M., Yamamoto, M., Kawaguchi, B. and Fujimoto, H., “Low Emission Diesel Combustion System
by Use of Fuel Design Concept”, SAE Technical Paper, 99011136, 1999.
6. Senda, J., Kawano, D., Hotta, I., Kawakami K. and Fujimoto, H., “Fuel Design Concept for Low Emission in Engine Systems”, SAE Technical Paper 2000-01-1258, 2000
7. Kawano, D., Senda, J., Kawakami, K., Shimada, A. and Fujimoto, H., “Fuel Design Concept for Low Emission in Engine Systems 2nd Report: Analysis of Spray Characteristics for Mixed Fuels”, SAE Technical Paper 2001-01-0202, 2002.
8. Kawano, D., Senda, J., Shimada, A. and Fujimoto, H., “Fuel Design Concept for Low Emission in Engine Systems 3rd Report : Analysis of Spray Characteristics for Mixed Fuels” SAE Technical Paper 2002-01-0220, 2002.
9. Kawano, D., Senda, J., Wada, Y. and Fujimoto, H., “ Fuel Design Concept for Low Emission in Engine Systems 4th Report: Effect of Spray Characteristics of Mixed Fuel on Exhaust Concentrations in Diesel Engine”, SAE Technical Paper, 2003-01-1038, 2003.
10. Senda, J., Nishikiori, T., Tsukamoto, T. and Fujimoto, H., “Atomization of Spray under Low-Pressure Field from Pintle Type Gasoline Injector”, SAE Technical Paper 920382, 1992.
11. Adachi, M., McDonell, V. G., Tanaka, D., Senda, J. and Fujimoto, H., “Characterization of Fuel Vapor Concentration Inside a Flash Boiling Spray”, SAE Technical Paper 970871, 1997.
12. Stralen, S. V. and Cole, R., Boiling Phenomena, 1, Hemisphere Publishing Co., 1979.
13. Kumano, K. and Iida, N. “Analysis of the Effect of Charge Inhomogeneity on HCCI Combustion by Chemiluminescence Measurement”, SAE Technical Paper 2004-01-1902, 2004.
14. Friend, J. F. “NIST Mixture Property Database Users’ Guide”, 1992.
15. Naver, D. J. and Seibers, L. D., “Effects of Gas Density and Vaporization and Penetration and Dispersion of Diesel Sprays, SAE Technical Paper 960034, 1996.
