ICLASS- Aug.7-Sept.1,, Kyoto, Japan Paper ID ICLASS-3 EFFECTS OF CAVITATION IN A NOZZLE ON LIQUID JET ATOMIZATION Akira Sou 1, Maulana Muhaad Ilham, Shigeo Hosokawa 3 and Akio Tomiyama 1 Assistant Professor, Kobe University, sou@mech.kobe-u.ac.jp Graduate Student, Kobe University, ilham@cfrg.scitec.kobe-u.ac.jp 3 Associate Professor, Kobe University, hosokawa@mech.kobe-u.ac.jp Professor, Kobe University, tomiyama@mech.kobe-u.ac.jp ABSTRACT Cavitation in two-dimensional (D) nozzles and liquid jet were visualized using high-speed cameras to investigate the effects of on liquid jet under various conditions of the and Reynolds numbers σ and Re. Liquid velocity in a D nozzle was measured using a Laser Doppler Velocimetry to examine the effects of on the flow in the nozzle and liquid jet. As a result, the following conclusions were obtained: (1) the in the D nozzles and the liquid jet can be classified into the four regimes: ( regime, liquid jet regime) = (no, wavy jet), (developing, wavy jet), (super, spray) and (hydraulic flip, flipping jet), () liquid jet behavior depends on regime, (3) and liquid jet near the nozzle exit are not strongly affected by Re but by σ, and () ligament formation at liquid jet interface is induced by the collapse of cloud near the exit. Keywords: Cavitation, Liquid Jet, Visualization, Cavitation Number, Reynolds Number, Nozzle, Ligament, Turbulence, LDV 1. BACKGROUND It has been pointed out through a number of experimental studies [1-] that takes place in the nozzle of fuel injectors for Diesel engines and affects the characteristics of a discharged liquid jet. Hiroyasu et al. [1] showed that liquid jet atomization was promoted when extended from the inlet to the exit of a nozzle. Since the shapes of the nozzles used in most of the previous studies have been cylindrical [1-], it has been difficult to measure liquid velocity, turbulence intensity and radial distribution of bubbles in nozzles. Hence, several studies have been carried out using two-dimensional (D) nozzles for visualization of detailed behavior [-8]. Recently some attempts have been made to measure the velocity in the nozzle, which might play an important role in liquid jet atomization [9-1]. However the mechanism of atomization enhancement by has not been clarified yet. In the present study, in D nozzles and liquid jet were visualized using a digital camera and high-speed video cameras under various conditions of Reynolds and numbers to examine the effects of the two dimensionless numbers on and liquid jet and to investigate the atomization enhancement mechanism by. Liquid velocity in a D nozzle was measured by using a Laser Doppler Velocimetry (LDV) to investigate the effects of cavitating internal flow on liquid jet atomization.. EXPERIMENTAL SETUP.1 Liquid Injection System Schematic of experimental setup is shown in Fig. 1 [13-18]. Filtered tap water or light oil was discharged from D nozzles into ambient air by the plunger pump through the cylindrical air separation tank of 9 in inner diameter and 8 in height. As shown in Fig., the D nozzle consists of two acrylic flat plates and two stainless steel thin flat plates, by which sharp-edges were formed at the inlet of the nozzle. To examine the effect of nozzle size on the and liquid jet characteristics, two nozzles of different sizes were used. The width W N, length L N and thickness D N of a nozzle were, 1 and 1, and those of the half scale nozzle were, 8 and., respectively (L N / W N = ). This nozzle shape enabled us to measure not only streamwise but also lateral component of the liquid velocity by LDV. Power Source Stainless steel flat plate Fig. Gas-Liquid Separation Tank Flash Lamp Fig. 1 Plunger Pump D Nozzle Digital Camera or High Speed Camera Experimental setup 3 Sharp edge (a) front view L N =1 W N = (b) side view Acrylic plate D N =1 D Nozzle (W N =, L N =1, D N =1)
Table 1 Mean velocity, V N [m/s] Experimental conditions (T L =9K, W N =) Reynolds number, Re Cavitation number, σ 11. 1.7 1. 1.7 1. 8.9 1..78 17. 8.9 17. 7. 19. 7. 19. 78. Cavitation in nozzle No Developing Super Hydraulic flip Liquid jet Wavy jet Spray Flipping jet. Imaging and LDV Systems Images of and liquid jet were taken by using a digital camera (Nikon D7, 38x pixels) and a flush lamp whose duration was 1 µs [1,17]. Time evolution of in the nozzle and liquid jet were visualized using either an ultra high-speed digital video camera (nac IMACON, 1 x 98 pixels, exposure time t EX = ns, frame rate = 1, fps) [13,1,17] or a high-speed digital video camera (Redlake Motion Pro HS-1, 3x18 pixels, frame rate =, fps, shutter speed = µs). Streamwise and lateral velocities in the larger nozzle were measured using an LDV system (DANTEC, X series) [-17]..3 Experimental Conditions Experiments were conducted at various conditions of the number σ and the Reynolds number Re, which are defined by the following equations. P P σ = (1) a v. ρ LVN where P a is the atmospheric pressure, P v the vapor saturation pressure, ρ L the liquid density, V N the mean liquid velocity in the nozzle, ν L the liquid kinetic viscosity. These parameters were controlled by the changing liquid flow rate, fluid properties and nozzle size. Typical experimental conditions are listed in Table 1 (water, T L = 9K, W N =). The concentration of dissolved oxygen (DO) in water was 9. mg/l when the water temperature T L was 91K. 3. RESULTS AND DISCUSSION 3.1 Flow Regimes Flow regimes in the nozzle and liquid jet near the nozzle exit are suarized in Fig. 3 and Table 1 (water, T L =9K, W N =) [1,17]. When σ>1., bubbles were not observed and liquid jet took the form of "wavy jet". For.7<σ<1., bubble clouds appeared in the upper half of the nozzle. We defined this type of the as "developing ". In developing, liquid jet near the exit was wavy jet. For.<σ<.7, zone extended from the inlet to just above the nozzle exit. We named this regime "super ". In super, liquid jet atomization was enhanced, i.e., ligaments and droplets appeared ("spray") and the spray angle increased. Further decrease in σ (σ<.) resulted in the formation of "hydraulic flip", in which liquid flow separated at one inlet edge and did not reattach to the side wall. Figure shows the distribution of mean liquid velocity in a nozzle of in width [-17]. We confirmed that appeared within the separated boundary layer (SBL). The distribution of mean velocity was not uniform just below the zone, i.e. the reattachment point of SBL, however it became almost uniform near the nozzle exit in no or developing regimes. V N W Re = N () ν L in a nozzle no developing super hydraulic flip 1 1 σ=1.7 Re= σ=1.7 Re= σ=.9 σ=.78 σ=. σ=. Re=8 Re= Re=7 Re=7 liquid jet wavy jet spray flipping jet Fig. 3 Images of in D nozzle and liquid jet (water, T L =9K, W N =, t EX =1 µs)
(a) σ =1.7, Re= (b) σ =.9, Re=8 (c) σ =.78, Re= (d) σ =., Re=7 Fig. Mean liquid velocity distributions in a D nozzle (W N =, T L =91K) 3. Effects of σ and Re on Cavitation and Liquid Jet The Reynolds number Re increases with T L since ν L of water decreases with increasing T L, while σ does not change a lot by varying T L. To examine the influences of σ and Re on and liquid jet, observations were conducted under various T L conditions [18]. Regimes of and liquid jet are shown in Figs. (a) and (b), respectively. The spray regime region on the liquid jet regime map corresponds to the super region on the regime map, which means that the formation of super enhances liquid jet atomization. Since the transition from developing to super does not strongly depend on Re but on σ, the transition from wavy jet to spray also depend on σ. [ 1 ] Reynolds number Re [ 1 ] Reynolds number Re 1. 1... 1. 1. number σ (a) regime map 1. 1.. no cav. dev. cav. sup. cav. hydr. flp.. 1. 1. number σ (b) liquid jet regime map wavy jet spray flip. jet Fig. Effects of σ and Re on and jet (W N =) The effects of Re and σ on normalized length L cav * (=streamwise length of zone L cav / nozzle length L N ) are shown in Figs. (a) and (b), respectively. The values of L cav * in the cases of the nozzle of in width, the half scale nozzle (W N =) and light oil are plotted in the figure. Regime transition in the cases of the half scale nozzle and light oil showed the same trends as that in the nozzle, and L cav * did not depend on Re but on σ, though the effects of dissolved gases, turbulence and sharpness of the nozzle inlet edge were not taken into account in the definition of σ. L cav * L cav * 1..8.. T L =93K T L =33K T L =313K. T L =333K W N =. light oil.. 1. Re 1.. [ 1 ] (a) effects of Re on L cav * 1..8.... 1 σ (b) effects of σ on L cav * T L =93K T L =33K T L =313K T L =333K W N = light oil Fig. Effects of Re and σ on length L cav *
Mean lateral velocity V [m/s] 1-1 Spray angle (deg.) 1 1 spray angle Fig. 7 Spray angle θ T L =93K T L =33K T L =313K T L =333K W N = light oil....8 1 L cav * Fig. 8 Effects of L cav * on spray angle θ σ=.78 σ=. - 1 1 Horizontal location x [] Horizontal location x [] (a) mean velocity (b) turbulence velocity Fig. 9 Lateral velocities above the nozzle exit (y=) RMS of lateral fliuctuation velocity v' [m/s] 3 1 σ=.78 σ=. Spray angles θ were measured from the recorded images as shown in Fig. 7, and plotted against L cav * in Fig. 8. When the value of L cav * is about.8-.9, i.e. in super, spray angle increases as shown in Fig. 8. The mean lateral velocity V and the RMS of lateral fluctuating velocity v' (turbulence velocity) near the exit (distance from the inlet y=) are shown in Fig. 9 to examine the effects of on turbulence velocity and liquid jet. In developing (σ=.78) the value of V near the exit was close to zero and v' was small. On the other hand, v' in super (σ=.) was large and the value of V at x>1. was positive, i.e., the mean flow directed toward the side wall. The lateral flow and strong turbulence in super could be the dominant mechanisms to increase the spray angle and initiate atomization. 3.3 Effects of Super Cavitation on Ligament Formation Cavitation behavior near the nozzle exit in super regime was visualized using an ultra-high speed camera (t EX =ns, 1,fps). Images of the collapse of cloud are shown in Fig. 1 (W N =, σ=.9). The collapse of clouds took place intermittently. The collapse might cause a high turbulence velocity near the exit, and therefore affects the ligament formation. To examine the relation between the collapse of cloud and ligament formation in super regime, and liquid jet interface were visualized simultaneously with a high frame rate (, fps). Since the refractive index of the acrylic plate of the nozzle is higher than that of air, an acrylic plate was placed between the liquid jet and the camera to match the optical distances. In the high frame rate imaging, the image size was limited to 3 x 18 pixels. Hence, the acrylic plate was tilted to capture both and jet interface within a narrow region shown in Fig. 11. High speed camera (a) top view Fig. 11 D nozzle Tilted acrylic plate Liquid jet D Nozzle Visualized region Tilted acrylic plate (b) front view Visualization of and liquid jet 3.1 1. 1 (a) t= µs (b) t=3 µs (c) t= µs (d) t=9 µs (e) t=1 µs Fig. 1 Collapse of clouds above the nozzle exit (σ=.9, Re=8,. µm/pixel, t EX = ns)
Flow Side wall Nozzle Side wall Liquid jet 1 3 ms.ms V N 1.ms 1.ms inlet Nozzle exit Fig. 1 Simultaneous visualization of and liquid jet interface (σ=.1, 3 x 18pixels,, fps) V N Jet y=. y=8 y=13 1 Horizontal location x [] y=. y=. y=13 1 Horizontal location x [] y=. y=8.83 y=13 y= 1 Horizontal location x [] y=. y=8 y=13 y= 1 Horizontal location x [] (a) σ=1.7 (b) σ=.9 (c) σ=.78 (d) σ=. no developing developing super Fig. 13 RMS of streamwise fluctuating velocity
Time series images of and liquid jet are shown in Fig. 1 (σ=.1, 3 x 18pixels,, fps). The arrows in the figure represent the paths moving with mean liquid velocity in the nozzle V N. A ligament was formed when a trace of collapse came out with the velocity V N from the nozzle. The RMS of streamwise fluctuating velocity is plotted in Fig 13. As shown in the figure, strong turbulence appeared just below the collapse point (y=. for σ=.9, y=8.83 for σ=.78, y= for σ=.), and the turbulence decreased as y increased. This indicates that turbulence velocity increases just below the collapse point, and the strong turbulence reaches the nozzle exit only in the case of super. Hence we could confirm that a ligament is formed when liquid with strong turbulence induced by the collapse of cloud near the exit flows out from a nozzle in super regime.. CONCLUSIONS Cavitation in two-dimensional (D) transparent nozzles and liquid jets were visualized using a digital camera, an ultra high-speed video camera and a high-speed video camera. Liquid velocity in the D nozzle was measured using a LDV system. The effects of the number σ and the Reynolds number Re on in the nozzle and liquid jet were examined by varying the liquid flow rate, fluid properties and nozzle size. Cavitation in the nozzle and ligament formation at liquid jet interface were simultaneously visualized using a high-speed camera to examine the mechanism of atomization enhancement by. As a result, the following conclusions were obtained: (1) Cavitation in D nozzles and liquid jet are classified into the following regimes: (1: no, wavy jet), (: developing, wavy jet), (3: super, spray) and (: hydraulic flip, flipping jet). () Liquid jet atomization depends on regime, i.e., ligament formation and spray angle depend on length L cav *. (3) Cavitation and a liquid jet near the nozzle exit are not strongly affected by Re but by σ. () Ligament formation is induced by the collapse of cloud near the nozzle exit. NOMENCLATURE L cav * dimensionless length (=L cav / L N ) L N length of D nozzle [m] Re the Reynolds number T L liquid temperature [K] V N mean liquid velocity in a nozzle [m/s] W N width of D nozzle [m] Greek Symbols θ spray angle [deg.] σ the number ACKNOWLEDGEMENT The authors would like to express their thanks to Mr. Shinji Nigorikawa and Mr. Tatsutoshi Maeda. This study was supported by a Grant-in-Aid for Scientific Research (# 1817) of the Japan Society for the Promotion Science (JSPS). REFERENCES 1. Hiroyasu, H., Arai, M. and Shimizu, M., "Break-up Length of a Liquid Jet and Internal Flow in a Nozzle", Proc. ICLASS-91, pp.7-8, 1991. 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