STRING CAVITATION IN FUEL INJECTOR

Transcription

1 STRING CAVITATION IN FUEL INJECTOR Raditya Hendra Pratama 1 *, Akira Sou 1 1 *Graduate School of Maritime Science, Kobe University, Japan, 119w29w@stukobe-uacjp It has been pointed out that cavitation occurs in a nozzle of a fuel injector for a diesel engine and affects the injected spray In the nozzle, a so-called string cavitation, which appears almost along the nozzle axis, has been observed However, the mechanism and condition of its formation as well as its effects on injected spray are not well understood Furthermore, the effects of injector needle lift on string cavitation and injected liquid jet are not clarified Due to the difficulty in the visualization of cavitation flow in a tiny nozzle with an extremely high speed and high pressure, visualization of cavitation in a scaled-up nozzle is conducted to understand how it forms and affects the atomization of the injected liquid jet Cylindrical nozzles with an asyetric inflow, which represent multi-hole nozzle, is used in order to reveal the effects of nozzle geometry on string cavitation and liquid jet deformation Effect of needle lift is also examined by changing the gap between needle wall and its seat High speed camera is used to capture and to clearly observe the sequence of string cavitation As a result, we obtained a cavitation regime map which takes needle lift as a major parameter Formation of a string cavitation and a swirling hollow cone liquid skirt was observed in cylindrical nozzle with low needle lift Lower needle lift enhances string cavitation and increases jet angle On the other hand, cavitation and a liquid jet at high needle lift show same behaviors with those observed in a syetric single hole nozzle Effect of upper-volume width was also examined and the result shows that string cavitation occurs only at very low needle lift These results suggest that string cavitation is induced by an instability of the lateral inflow into a nozzle Keywords: String cavitation, Diesel fuel injector, Visualization, Instability, Lateral inflow 1 INTRODUCTION It has been pointed out that cavitation occurs in a nozzle of a liquid injector, eg, a fuel injector for a diesel engine, and affects the injected spray [1] Hiroyasu et al [2] clarified through a visualization of a syetric scale-up nozzle of 4 in diameter that development of cavitation from the inlet to near the exit of a nozzle, which is often called super cavitation [3], enhances liquid spray atomization Following the finding, visualizations of cavitation in real size syetric nozzles were carried out [3-4] Sou et al conducted a high-speed visualization and LDV (Laser Doppler Velocimetry) measurement of turbulent cavitation flow in two-dimensional (2D) syetric nozzle [5, 6] and a syetric cylindrical nozzle [7] to clarified that cavitation clouds are shed from the tail of a recirculation zone filled with cavitation film, and a strong turbulence at the trace of the cavitation cloud induces a large deformation of a discharged liquid jet The above cavitation can be classified into recirculation flow-induced-cavitation (RIC) [], in which flow separation occurs at the inlet To understand complex cavitation flows in real fuel injectors, observations of cavitation in nozzles with an asyetric inflow from upstream [9-11], with tilted angles [9-11], with a complex upstream geometry [12], and with real fuel injector geometries [12-15] have been carried out In some of these nozzles a so-called string cavitation, which can be classified into helical flow-induced-cavitation (HIC), has been observed [, 12, 14-15] However, the effects of injector geometry on cavitation pattern and injected liquid jet are not clarified due to the difficulty in the visualization of cavitation flow in a tiny nozzle with an extremely high speed and high pressure It should be noted that the upstream geometry of a fuel injector changes with the motion of a needle valve In this study, visualization of cavitation in a single cylindrical nozzle of liquid injectors with a syetric and an asyetric transverse inflow at various needle lifts and a discharged liquid jet is carried out to understand string cavitation in fuel injector and its effects on fuel spray Scale-up acrylic nozzles of 4 in diameter D and in length L (L/D = 4) are used to clearly capture the cavitation High speed camera is used to capture string cavitation inception Effects of an asyetric inflow velocity V, the gap Z between a needle valve and its seat of fuel injectors and the upstream channel width (later will be called as upper-volume width) on cavitation are examined 2 EXPERIMENTAL SETUP Figure 1 shows the schematic of experimental setup A filtered tap water at 3ºK was pumped through a valve, a flow meter, and a transparent cylindrical nozzle into ambient air at room temperature The valve was adjusted to control the liquid flow rate, which was measured using one of the flow meters with a proper range and depending on the needle lift condition, and by knowing the liquid flow rate and nozzle area, we calculated the mean liquid velocity V flowing through the nozzle Pressure gauge was installed at the upstream of the nozzle to measure injection pressure Still and high speed images of cavitation and jets were taken using a high speed camera (Phantom V211) with 5 fps in shutter speed for random sampling imaging, and 2, 6, or 1 fps for high speed imaging, and a metal-halide lamp as the backlight source Figure 2(a) shows the schematic of a single hole nozzle and Fig 2(b) shows the schematic of a Valve-Covered-Orifice (VCO) nozzle Liquid fuel flows into a nozzle with a syetric inflow in single hole nozzle and an asyetric transverse inflow is fed through the gap between needle valve and the seat into VCO nozzle It should be noted that liquid fuel flows into a nozzle asyetrically in any multi-hole nozzle, such as sac type and mini-sac type injector We made an acrylic model injector as shown in Figs 2(c), (d) to examine the effects of the syetric and asyetric inflow on cavitation which occurs inside of it as well as the discharged liquid jet The principal dimension of the cylindrical nozzle is 4 in diameter D and in length L (L/D = 4) The gap Z between the needle valve and the seat in asyetric test nozzle was varied (Z = 2, 4,, 12,, 2, 24, 2 and 32, ie, Z/D = 5, 1, 2, 3, 4, 5, 6, 7 and ) to investigate the effects of needle lift on cavitation inside the nozzle and the jet In addition, upper-volume width was also varied: wide upper-volume (3 ) and narrow upper-volume (15 )

2 well known as the most preferred cavitation regime because the liquid spray atomization is enhanced which leads to the improvement in combustion quality However, when V 135 m/s, cavitation switches to hydraulic flip which suppress the liquid jet deformation to a very narrow jet with a glassy structure, which is not preferred in engines Figure 1 Schematic of experimental setup V [m/s] = (a) Single hole nozzle (b) Valve-Covered-Orifice (VCO) nozzle V [m/s] = Figure 3 Cavitation in syetric nozzle and liquid jet (c) Syetric nozzle (d) Asyetric nozzle Figure 2 Single hole nozzle, VCO nozzle, and test nozzles 3 RESULTS AND DISCUSSIONS Figure 3 shows the images of cavitation in syetric nozzle and liquid jet There is no cavitation for V 64 m/s and the liquid jet is wavy Cavitation starts to appear at nozzle entrance at V = 14 m/s and the liquid jet is still wavy When V is increased up to 127 m/s, cavitation exceeds the middle of nozzle length, and the liquid jet deforms wider than that at lower V The cavitation length almost reaches the nozzle exit at V = 131 m/s and the liquid jet deforms drastically This phenomenon, so called super cavitation, is Figures 4-6 show the images of cavitation in asyetric nozzles wide upper-volume and the jet Figure 4 shows cavitation and jet at high needle lift, that is Z/D = Figure 5 shows cavitation and jet at medium needle lift or considered as transition between high and low needle lift, that is Z/D = 3 Figure 6 shows the image of cavitation and jet at low needle lift (Z/D = 5) As shown in Fig 4, no cavitation appears at low velocity With an increase in V (V 112 m/s), a recirculating-flow-induced cavitation (RIC) starts to appear at the nozzle inlet With a slight increase in V, a long cavitation extends to near the nozzle exit (127 m/s V 135 m/s) which known as super cavitation An increase in V which exceeds 143 m/s results in hydraulic flip with no reattachment in the nozzle wall so that the separating flow goes straight through nozzle exit As we can see in the Fig 4, the cavitation forms are similar with those in syetric cylindrical nozzle shown in Fig 3 However the right end of the cavitation is longer than the left end due to the inflow from right upstream The liquid jet also deforms asyetrically The right side of the liquid jet always gives the wider angle than the left one As reported in our previous research [5], the longer cavitation gives a wider liquid jet angle as well as liquid deformation due to stronger vortices This may be a reason why the right interface takes wider angle than the left one

3 V [m/s] = V [m/s] = Figure 4 Cavitation in the asyetric nozzle at wide upper-volume and high needle lift (Z/D = ) and liquid jet Cavitation in asyetric nozzle with medium needle lift (Z/D = 3) is shown in Fig 5 A slightly twisted thick "string cavitation" [, 12-15] appears along the axis of the cylindrical nozzle from the needle valve to the exit of the nozzle At very high V (V 119 m/s) results in a hybrid cavitation, ie a string cavitation and RIC occur simultaneously where the string cavitation occurs along the nozzle axis with a slight tilt, and a short RIC occurs at the right side of the nozzle inlet Hybrid cavitation indicates that a separation flow into the nozzle is still remaining even the helical inflow is dominant The liquid jet angle tends to increases with liquid velocity V due to the centrifugal force acting on a swirling hollow cone liquid skirt Low needle lift condition at wide upper-volume is represented by Z/D = 5, whom is shown in Fig 6 As shown in the figure, same trends, ie, a string cavitation and a liquid skirt are observed in the low needle lift condition Due to the lower needle lift and resulting a higher transverse inflow velocity at Z/D = 5, a string cavitation easily appears at very low V, and jet angle tends to be wider than the higher Z/D because the string cavitation has a larger diameter so that the effective flow area is narrower which result in larger centrifugal force of the helical flow Note that the string cavitation below the needle valve wall is asyetrically slanted toward the left side due to the asyetric transverse inflow The trend implies that we should understand and predict string cavitation before the development of a new fuel injector with an asyetric inflow and low needle lift Pictures of cavitation and liquid jet at narrow upper-volume, various liquid velocities V and needle lifts to diameter Z/D are shown in Figs7- Figure 7 shows the image of cavitation and jet at high needle lift, that is Z/D = 4, while Fig shows the image of cavitation and jet at low needle lift, that is Z/D = 5 Same as wide upper-volume, we also classify the needle lift to diameter ratio condition at narrow upper-volume into two categories, ie high and low needle lift conditions, based on the cavitation regime map which is derived from the whole images observation V [m/s] = V [m/s] = V [m/s] = Figure 6 Cavitation in the asyetric nozzle at wide upper-volume and low needle lift (Z/D = 5) V [m/s] = Figure 5 Cavitation in the asyetric nozzle at wide upper-volume and medium needle lift (Z/D = 3)

4 formation of string cavitation For low needle lift, string cavitation appears even at very low velocity On the other hand in the case of high needle lift RIC is formed at high velocity, which is observed in nozzles with a syetric inflow [2, 5] V [m/s] = V [m/s] = V [m/s] = Figure 7 Cavitation and liquid jet at narrow upper-volume, high needle lift (Z/D = 4) In this discussion, high needle lift condition at narrow upper-volume is represented by Z/D = 4, which is shown in Fig 7 As shown in the figure, the cavitation form and liquid jet structure are similar with that in cylindrical nozzle at wide upper-volume, ie the length between the right side cavitation always gives a slight longer cavitation than the left one Besides the cavitation, the liquid jet also deforms asyetrically In this case, the right side liquid jet always gives the wider angle than the left one Low needle lift condition at narrow upper-volume is represented by Z/D = 5 and is shown in Fig As shown in the figure, similar trend like low needle lift condition at wide upper-volume is observed, ie, a string cavitation which easily appears at very low V and is asyetrically slanted toward the left side due to the asyetric transverse inflow However, liquid jet structure is totally different with that in wide upper-volume, that liquid skirt is not observed even at string cavitation regime Instead, the liquid jet deforms like that in at high needle lift condition, but with wider jet angle Figure 9 shows the cavitation regime map which is derived from the whole images observation The result shows that there are four cavitation regimes, ie no cavitation, recirculation flow induced cavitation (RIC), hydraulic flip and string cavitation By using cavitation regime map, we can easily predict the cavitation regime at certain velocity and Z/D condition As mentioned in the beginning part of results and discussions, we defined the needle condition into two categories, ie high and low needle lifts based on the cavitation regime map result As shown in the map, there are border lines which separate each cavitation regime In the case of wide upper-volume, one line indicates a significant difference between Z/D 4 and Z/D 3 Hence, we defined Z/D 4 as high needle lift condition while Z/D 3 as low needle lift condition In other hand, one line indicates a significant difference between Z/D 2 and Z/D 1 in the case of narrow upper-volume The map clearly indicates that low needle lifts enhances the helical flow and V [m/s] = Figure Cavitation and liquid jet at narrow upper-volume, low needle lift (Z/D = 5) (a) Cavitation regime map (wide upper-volume) (b) Cavitation regime map (narrow upper-volume) Figure 9 Cavitation regime maps Figure 1 shows the illustration of liquid jet measurement The liquid jet angle was measured at times diameter (32 ) downstream of the nozzle exit The distance is determined by considering the deformation that has not well deformed for shorter distance and is too much affected by ambient air for further distance

5 An intersection point can be found between the liquid core and horizontal line drawn at 32 downstream of the nozzle exit The angle between the imaginary line of the nozzle wall extension and the line between nozzle exit edge and intersection point, was measured The measurement has been done for left and right jet angles while the total jet angle is the sum of them (c) Total jet angle Figure 11 Measured jet angles (wide upper-volume) Figure 1 Liquid jet angle measurement As shown in Fig 11, the jet angle increases with velocity V or with decreasing Z/D For low and medium needle lift (Z/D = 5, 1, 2, 3) as shown in Fig 11(a), the jet angle tends to increase by the increase of velocity V, especially when string cavitation appears, due to the increase in the centrifugal force As shown in Fig 11(b), for high needle lift condition (Z/D = 4, 5, 6, 7, ) which is represented by Z/D = 5 &, jet angle slightly increases by the increase of velocity V at no cavitation regime, significantly increases at RIC regime especially when cavitation length almost reach the nozzle exit (super cavitation), and suddenly drops when the cavitation turns into hydraulic flip There is a distinctive characteristic of internal flow pattern for Z/D = 3 & 4, that is not steady due to the unsteadiness of the helical flow in the nozzle and upper volume, which plays an important role in liquid jet characteristics The results also show that the left jet angle has a slight wider jet angle than the right one at low needle lift, whereas at high needle lift, right jet angle has a slight wider jet angle than the left one especially at super cavitation due to the asyetric inflow Discharge coefficient Cd is defined in Eq (1) as a function of velocity V, fluid density ρ, and pressure difference ΔP As shown in Fig 12, discharge coefficient tends to decrease with the increase of velocity V for Z/D = 3, 4, 5, 6, 7, Due to the nearly same results for Z/D = 5, 6, 7,, the trend line of discharge coefficient nearly the same as well For Z/D = 5, 1, 2, the discharge coefficient tends to be constant due to a nearly constant string cavitation diameter From the graph, we can understand that Z/D = 5 gives the lowest discharge coefficient which means a thick string cavitation occupy the nozzle flow area, and is confirmed by the string cavitation diameter measurement which is explained later in this paper From the graph we can easily predict the discharge coefficient in between Z/D = 5 and Z/D = by generating an imagination trend line use the interpolation method It is also possible to predict the required injection pressure at given liquid mean velocity and flow rate C d = ρv2 2 P (1) Figure 12 Mean liquid velocity versus discharge coefficient graph (wide upper-volume) (a) Low needle lift Figure 13 Mean liquid velocity versus string cavitation occurrence probability graph (wide upper-volume) (b) High needle lift In Fig 13, the probability of string cavitation appearance for different needle lifts is plotted against velocity V The plot clearly shows that a string cavitation is formed even at very low velocity in

6 the case of Z/D = 5, 1, 2 Whereas it alternately change with no cavitation and RIC in the case of Z/D = 3 From the high speed images with long period observation we can understand that a string cavitation occurs in stable state for the case of Z/D = 5, 1, 2 so that its occurrence probability reaches 1% even at low velocity (V > 12 m/s at Z/D = 5, V > 16 m/s at Z/D = 1, V > 32 m/s at Z/D = 2) Whereas, in the case of Z/D = 3, it is alternately change to no cavitation at V < 112 m/s and rarely change to RIC at V > 112 m/s String cavitation diameter ratio D SC * is the ratio of string cavitation diameter D SC to nozzle diameter D, defined by Eq (2) and is illustrated in Fig 14 String cavitation diameter is measured at 1 from the nozzle inlet Figure 15 shows the effect of mean velocity on string cavitation diameter ratio for low, in which the string cavitation occurs in the nozzle The string cavitation diameter does not increase significantly with V As shown in the graph, the lower Z/D gives the higher string cavitation diameter ratio, which means a thicker string cavitation occurs at low Z/D due to a strong helical flow D SC = D SC D (2) ms Figure Formation of thin string cavitation (wide upper-volume, Z/D = 3, V = m/s) Figure 17 shows an example of the transition from a thin string cavitation to a thick string cavitation At t = ms, thin string cavitation already existed from inlet of nozzle to the middle of the nozzle and near the nozzle axis At t = 1 ms a large cavitation string start to form near the nozzle exit, and it spreads into upstream direction along the thin string near the axis Once a thin string cavitation is replaced by a thick one, it takes a long time until it starts to shrink and disappear Figure 14 String cavitation diameter measurement ms Figure 17 Formation of thin and thick string cavitation connected together (wide upper-volume, Z/D = 3) Figure 1 shows the shedding of string cavitation and its disappearance process As shown in the high speed image series in Fig 1, another structure of cavitation, ie RIC appears at t = 2 ms and starts the string cavitation shedding When the RIC moves into the string cavitation core at t = 3 ms, the complicated cavitation structure formed and the cavitation shedding starts to take place It takes a long time (1 ms) until the cavitation completely disappears from the nozzle Figure 15 Mean liquid velocity versus string cavitation diameter ratio graph (wide upper-volume) As shown in Fig 13, string cavitation occurrence probability at Z/D = 3 lies between zero and one and depends on mean liquid velocity in the nozzle Internal flow pattern sometimes is not steady due to the unsteadiness of the helical flow in the nozzle, which plays an important role in liquid jet characteristics As a result, a thin string cavitation sometimes occurs especially at the beginning of (thick) string cavitation occurrence and sometimes the thick string cavitation occurs without initiation of thin one Figure shows the high speed images of thin string cavitation formation at Z/D = 3 and V = m/s As shown in the picture, the formation of thin string cavitation starts to appear in the upper part of nozzle and shrink and disappear in it The motion shows that spring cavitation is mainly made of vapor ms Figure 1 Thick string cavitation shedding (wide upper-volume, Z/D = 3) Effect of string cavitation on liquid jet is described in Fig 19 When the thick string cavitation starts to occur inside the nozzle, liquid jet angle is widen due to a swirling motion which results in a wide liquid skirt The liquid jet turned to liquid skirt with a narrower angle when the thick string cavitation occupies the nozzle to reduce flow rate The transformation of liquid jet to liquid skirt is very important in fuel injection and atomization since its structure is completely different each other

7 ms Figure 19 Cavitation and liquid jet (wide upper-volume, Z/D = 3) 4 CONCLUSIONS Visualization of cavitation inside scale-up cylindrical nozzles of 4 in diameter D with an asyetric inflow at various needle lifts Z was carried out in the present study As a result, we observed the formation of a string cavitation and a swirling hollow cone liquid skirt at low and medium needle lifts (Z/D = 5, 1, 2, 3) The lower needle lift enhances thick string cavitation and increases jet angle On the other hand, a Recirculation-flow-Induced-Cavitation (RIC), which cavitation patterns are often similar to those in syetric nozzles, and a largely deformed liquid jet appear at high needle lift (Z/D = 4, 5, 6, 7, ) Note that a string cavitation and RIC appears simultaneously at Z/D = 2, 3 and very high mean liquid velocity At narrow upper-volume, helical flow only exist at very low needle lift, where string cavitation occurs at Z/D = 5, 1 At Z/D 2 the string cavitation does not occurs and RIC takes place The result suggests that a special attention has to be paid when we use a fuel injector with low and medium needle lift, which results in string cavitation in a nozzle and formation of a swirling liquid skirt "Cavitation in a Transparent Real Size VCO Injection Nozzle," Proc ICLASS 23, Collicott, S and Li, H, "True-scale True-pressure Internal Flow Visualization for Diesel Injectors," SAE Paper, , Soteriou, C, Andrews, R, Torres, N, Smith, M and Kunkulagunta, R, "Through the Diesel Nozzle Hole - A Journey of Discovery II," Proc ILASS-Europe 21, Chaves, H, Miranda, R and Knake, R, "Particle Image Velocimetry Measurements of the Cavitating Flow in a Real Size Transparent VCO Nozzle," Proc Int Conf on Multiphase Flow (ICMF 27), S4_Mon_C_6, Andriotis, A, Gavaises, M and Arcoumanis, C, "Vortex Flow and Cavitation in Diesel Injector Nozzles", J Fluid Mech, Vol 61, , 2 15 Hayashi, T, Suzuki, M, and Ikemoto, M, "Visualization of Internal Flow and Spray Formation with Real Size Diesel Nozzle", Proc ICLASS 212, 212 REFERENCES 1 Bergwerk, W, "Flow Pattern in Diesel Nozzle Spray Holes", Proc Instn Mech Engrs, Vol 173, No 25, , Hiroyasu, H, Arai, M and Shimizu, M, "Break-up Length of a Liquid Jet and Internal Flow in a Nozzle", Proc ICLASS-91, , Chaves, H, Knapp, M, Kubitzek, A, Obermeier, F and Schneider, T, "Experimental Study of Cavitation in the Nozzle Hole of Diesel Injectors Using Transparent Nozzles", SAE Paper, Paper No 9529, , Badock, C, Wirth, R, Fath, A and Leipertz, A, "Investigation of Cavitation in Real Size Diesel Injection Nozzles", Int J Heat Fluid Flow, 2, , Sou, A, Hosokawa, S, and Tomiyama, A, "Effects of Cavitation in a Nozzle on Liquid Jet Atomization", Int J Heat Mass Transfer, Vol 5, Iss 17-1, , 27 6 Sou, A, Hosokawa, S and Tomiyama, A, "Cavitation in Nozzles of Plain Orifice Atomizers with Various Length-to-Diameter Ratios," Atomization and Sprays, Vol 2, Iss 6, , 21 7 Sou, A, Ilham, M M, Hosokawa, S and Tomiyama, A, "Ligament Formation Induced by Cavitation in a Cylindrical Nozzle," J Fluid Sci Technol, Vol 3, No 5, , 2 Sou, A, Raditya Hendra Pratama, and Tomisaka, T, "Cavitation in a Nozzle of Fuel Injector", Proc CAV212, No 4, Ganippa, L C, Bark, G, Andersson, S and Chomiak, J, "Comparison of Cavitation Phenomena in Transparent Scale-up Single-Hole Diesel Nozzles," Proc CAV21, A95, 21 1 Miranda, R, Chaves, M, Martin, U and Obermeier, F,