laser imaging of pressure waves and cavitation generated...

18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016

Laser imaging of pressure waves and cavitation generated by diesel fuel injector jets in a model of an injection rate meter

D.Pearce 1,*, Y.Hardalupas2, A.M.K.P Taylor 2 1: Delphi Diesel Systems

2: Dept. of Mechanical Engineering, Thermofluids, Imperial College London * Correspondent author: [email protected]

Keywords: Cavitation, Pressure waves, Cavity Collapse, Shear cavitation

ABSTRACT

In some types of injection rate meter, a Diesel injector delivers fuel into a fuel filled test vessel which may be a very long pipe or a relatively small constant volume chamber: either way, several phenomenon may arise such as the reflection of pressure waves at walls and the generation of pressure waves through cavitation and subsequent collapse downstream of the nozzle. This may make the accurate retrieval of the signal related to the rate of injection itself difficult. A model constant volume chamber with optical access was constructed as a simulacrum to such an injection rate device that allows visualization of such phenomena. So-called “Shadowgraph like schlieren” techniques using a Nd:Yag laser light source were applied to the jet of a commercial diesel injector. Cavitation arose in the turbulent shear layer of the jet associated with strong individual vortical structures. Pressure waves within the test chamber were observed from the collapse of the cavitation bubbles along with other flow structures.

1. Introduction

Diesel engines currently occupy an overwhelming fraction of the heavy duty prime mover market. The next generation of diesel engines are expected to meet increasingly stringent targets for emissions, efficiency and torque delivery. These targets are driven by the increased global demand for more environmentally friendly, cleaner combustion technologies. In Europe, the emissions legislation currently places limits on NOx, CO and particulates for on-highway use vehicles which are defined in (Anon., 2011). The US market has similarly stringent regulations on emissions controls with new efficiency requirements and requirements over the 'life of the vehicle' (Anon, 2015a) as opposed to ‘start of life’ or ‘0km requirements’. To help meet these requirements, engine manufacturers are utilising highly optimised strategies such as multiple injection and fuel injection rate shaping. These collectively challenging demands on engine developers put similarly challenging demands on the fuel injection equipment which is traditionally seen as the primary enabler for improving combustion control.


In order for injection systems manufacturers to develop new products to meet engine requirements, detailed measurements of the injector charactistics must be made which include timing parameters, rate shape and the totalised mass delivered per engine cycle. Considering that a typical full load injection requirement may be 250mg while a typical idle or cold start pilot injection quantity may be less than 5mg means that injection measurement devices must cover a large dynamic range. The instantaneous mass flow rate curve is of particular interest due to increasing optimisation of features such as multiple injections and modified (or variable) rate shape. A number of commercial devices exist to measure injection characteristics which can be classified into the following three basic categories: constant volume, constant pressure or the 'Bosch' rate tube (which operates on the pressure-velocity relation). A description of the constant volume type will be given here but it should be noted that all the above device types involve liquid into liquid injection which forms the basis for the application of the experimental techniques used in this paper.

The constant volume type injection measument device has the injector arranged to deliver fuel into a fuel filled chamber of known volume. This arrangement was first proposed by Zeuch (Zeuch, 1961) and later developed and investigated by others, the first being Takamura (A Takamura et al 1989) who added a built in calibration piston. Other investigators include Bower (Bower & Foster, 1991) who compared the Zeuch to a Bosch tube and Arcoumanis (Arcoumanis & Baniasad, 1993) who refined the analysis techniques used.

With knowledge of the temperature and bulk modulus (or compressibility) of the fuel, the mass flow rate can be derived from the rate of change in pressure of the chamber using the equations

𝐾 = 𝑉×𝑑𝑃𝑑𝑉(1)

𝑑𝑚𝑑𝑡 = 𝜌×

𝑉𝐾×

𝑑𝑃𝑑𝑡 (2)

Equations (1) and (2) show that any factors affecting either the compressibility or the measured change in pressure of the chamber, may alter the calulation of mass flow rate of fuel. If cavitation were present in such a device, the bulk compressibility would be that of a bubbly mixture rather than a pure fluid as is usually assumed. The implicit assumption of a constant bulk compressibility across the entire volume may also be incorrect if the cavitation bubbles are highly localised.

There has been some simulation work to support the hypothesis that cavitation downstream of the nozzle could be affecting the measurements from such a device(Sander,


Fischer, Hartung, Majer, & Lafrenz, 2014) (Pearce, Hardalupas, & Taylor, 2015) , as well as from a similar micro channel geometry (Egerer, Hickel, Schmidt, & Adams, 2014).

The equations above assume that the primary measurement, namely that of dP/dt is related solely to the entry of mass into the constant volume chamber with a pressure transition from before injection (initial chamber static pressure) to after injection (final chamber static pressure). This assumption represents a powerful simplification because strong pressure waves should be expected from the injection process itself due to the extremely rapid nozzle opening time resulting in pressure waves analogus to 'water hammer’. In addition to these strong pressure waves (termed 'primary' waves here) and their reflections, cavitation bubble collapse is known to also generate strong pressure waves (Jean-Pierre Franc & Michel, 2004), termed here 'secondary' waves. These secondary waves affect the measured dP/dt signal as an added 'noise' component that needs to be corrected for. For example if the rate of bubble collapse were known or functionally measureable then the appropriate filter could be empoyed for signal analysis. When one considers that the actual pressure difference across the nozzle and hence mass flow rate may also be varying significantly in time due to injector internal wave dynamics (Sezal, Schmidt, Schnerr, Thalhamer, & Förster, 2009), it can be seen that there results a complicated, multi mode pressure signal.

The use of an optical chamber to visualise the injection of a liquid jet to a liquid chamber in the context of diesel injectors was reported by (Gray, Meckel, & Mannheimer, 1966) and (Yoshikawa, Nishida, Arai, & Hiroyasu, 1988) to make qualitative assessments of spray parameters such as penetration and break up length. Gray et al. also presented evidence to show that cavitation processes are present in the region between the shear layer of the jet issuing from the injector nozzle holes and the quiescent fluid of the downstream chamber. Similar studies of coaxial or co-flow submerged jets such as (Ooi, 1985) have also reported cavitation present in this shear layer. More recent work (Straka, Meyer, Fontaine, & Welz, 2010), has also linked vortical structures formed due to vortex rollup, with cavitation effects. Vortex formation due to this roll up effect of the ejected boundary layer is well known although most studies have looked at the flow over hydrofoils, impellers and similar applications which involve much longer timescales, as well as much larger geometries and much lower static pressures than that of an injection event. Less well understood however is the extent of the low pressure regions formed at the core of such vortices, which is partially due to the difficulty in measuring such flow fields. Authors such as Shariff (Shariff & Leonard, 1992) and Choi et al (Choi, Hsiao, Chahine, & Ceccio, 2009) present a theoretical analysis of such fields while Ran and Katz (Ran & Katz, 1994) show that pressure fluctations in the near field are related to these vortices (and vortex pairing)


although some of these results are contradictory. Arndt (Arndt, 2002) provides an extensive summary of this relationship between cavitation and vorticity.

In this study, the jet from the nozzle would most closely resemble a coaxial, co-flow or Craya-Curtet type flow regime. Revuella et al (Revuella, Martínez-Bazán, Sánchez, & Linan, 2004) examine this flow profile and concluded that because entrainment requirements cannot be generally met, the flow should generate toroidal structures arising from the vorticity at the shear interface. Chahine (Chahine & Genoux, 1983) report on experimental work that generates these toroidal structures in the form of cavitation vortex rings although this is confined to a specific range of the Strouhal number.

Several methods exist to study the flow field near the nozzle (Ooi & Acosta, 1984) but due to the very short timescales for injection (< 1 millisecond) and extremely high velocities (internal nozzle velocity could be 500m/s) the non intrusive schlieren optical technique was chosen. This allows investigation of the effects of both cavitation and local pressure variations such as would be found in vorticies. This technique allowed the visualisation of density gradients and, at least in the absence of significant temperature changes, these gradients correspond to the pressure field.

The schlieren technique has been widely reported for the visualisation of flow phenomena (Merzkirch, 1987; Settles, 2001; Verso & Liberzon, 2015), the technique is based on measuring the angular deflection of parallel light rays through a test medium. The angular deflection is a function of the refractive index change which for most gases, is in turn proportional to the density as described by the Gladstone-Dale equation: and similarly for most liquids. For the liquid used in, the refractive index was a function of both temperature and pressure and care was taken to differentiate between these effects by keeping the temperature essentially constant. To determine the change in refractive index, n and using the coordinate system from Figure 2, the ray curvature is given by the equations e.g. from Settles (Settles, 2001),

𝜕0𝑥𝜕𝑧0 =

1𝑛𝜕𝑛𝜕𝑥 (3)

and 𝜕0𝑦𝜕𝑧0 =

1𝑛𝜕𝑛𝜕𝑦(4)

The deflection is found by then integrating over the path of the rays through the medium as follows:


𝜀8 =1𝑛

𝜕𝑛𝜕𝑥 𝜕𝑧(5)

and

𝜀: =1𝑛

𝜕𝑛𝜕𝑦 𝜕𝑧(6)

whichformsthebasisforschlierentypemeasurements.

SchlierentypemeasurementshavebeensuccessfullycarriedoutbyMaugeretal (CyrilMauger,

Méès,Michard,Azouzi,&Valette,2012)onaplanarversionofanozzle likegeometryusing thesame

calibrationtestoilas inourwork.Their resultswereable tocapturebothcavitation inception (bubble

formation)aswellasthepressurewavescausedbybubblecollapse.Figure1showsanimagefrom(Cyril

Maugeretal.,2012)withthesefeaturesclearlyvisible.Resultsshowingcavitationintheshearlayerofa

microchanneljetwerealsoreportedbyWinklhoferetal(Winklhofer,Kull,Kelz,&Morozov,2001).Inour

work,thenozzleswerecircularandsmall(diameterwas~185μm)andtheensuingjetexpandedinall3

dimensions. Thismeans that the density gradients towhich the schlieren system is sensitive, namely

thoseinthexandyplanes,areexpectedtobesubstantiallyweakerthanwasthecaseforMaugeretal.

Figure 1: Image showing pressure wave from bubble collapse in a planar geometry (Cyril Mauger et al., 2012).


2. Method

The test chamber was constructed to allow optical access from two sides as seen in Figure 2 with a visible window area of 40 x 20mm. In order to support high chamber static pressures and to minimise any possible distortion, the window thickness was 20mm. Two injectors were used in this study, both Delphi F2E (Anon, 2015b) which differed only in the nozzles fitted. Nozzle 1 represented a typical engine application with 7 nozzles holes of approximately 185 µm diameter. Nozzle 2 had only two nozzles holes, 180 degrees apart with a similar nozzle diameter, spray cone angle and conicity as the application nozzle (Nozzle 1). The two hole nozzle was developed in order to simplify the analysis by having the nominal axes of the jets perpendicular to the field of view. The window material chosen was Perspex so as to closely match the refractive index of the fluid, ISO4113 calibration oil. A controllable pressure source, based on a Delphi GL5.0 pump and rail, supplied the calibration oil at a maximum pressure of 2500 bar. Rail pressure control was typically better than ±5 bar with this system. The test chamber was fitted with a Kistler 6052C piezo electric pressure transducer which was synchronously recorded with image acquisition.

The chamber back pressure was controlled via a Swagelok® adjustable pressure regulator and could test at static pressures up to 30 bar. The system temperature was monitored at several locations with K type thermocouples:

• Fuel tank • Fuel inlet to pump • Fuel return to tank • Fuel Backleak • Lube Tank • Lube Return (outlet cooling system)

The system temperture was controlled via orifice valves on the process water supply to two large chiller plates on the return path to the lubrication and fuel circuits. This design maintained both tank temperatures at ~18 °C. During testing, tank temperatures would increase as a function of load at a max rate of 1 °C per hour. Testing was paused whenever the temperature drifted by more than 3 °C.


Figure 2: Test chamber with optical access windows

Initial images using a backlight were taken to verify the timing and triggering hardware configuration, which was done with a National Instruments 7831R FPGA and 2 x 6110E acquisition cards. Images were taken with a 16 bit Andor DH534 intensified CCD camera using a Micro-Nikkor 105mm lens and 2X teleconverter. This arrangement give approx 20µm per pixel resolution.

Figure 3: Nozzle 1 (7 hole nozzle) shown with backlight 400µs after start of injection logic. Image on right has

outlines showing the injector body in red and the jet outline in blue.


The main results were taken using a 'shadowgraph like schlieren', a term that has been adopted from Mauger (C. Mauger, Méès, Michard, & Lance, 2014) in order to differentiate from true schlieren type imaging. In the ‘shadowgraph like schlieren’, no knife edge cut off is used: instead, the ray deflection is measured as the intensity difference between a reference image with no flow and the test image. In order to capture the expected pressure waves and associated phenomenon, an extremely short duration, high intensity light source was required. A Nd:Yag pulsed laser was chosen in order to maintain the required the camera exposure times to adequately “freeze” the motion of the flow. The second harmonic of the laser output (at 532nm) was focused via lens 1 ( see Figure 4) on to a flourescent PMMA target. This target was needed in order to destroy the coherence of the original laser pulse which would not allow schlieren image formation due to speckle (Merzkirch, 1987)(Settles, 2001). The PMMA target also shifted the wavelength of emitted light to be approximately centred at 599nm. The emitted light from this target was passed through a collimating lens, lens 2, to produce a source of parallel beams. To avoid any residual 532nm laser light reaching the camera, a NF533-17 notch filter centred on 533nm was used after lens 2. Images were processed using a custom Matlab® routine where the intensity from a series of non injecting reference frames were first averaged and then subtracted from the images which included injection events.

Figure 4: Plan view of the optical layout


3. Results Figure 3 shows the injection event at 400 !s after the start of injection drive logic (the right

hand image has visual markers added to aid identification: white for test chamber window, red for the injector tip and irregular blue for individual jet sprays). Note this image is from injector 1 and so has multiple jets. The dark area of the jet indicates that that the incident light is being reflected by cavitation bubbles away from the the camera and back towards the source. Since the injected fuel has the same optical qualities as the quiesecent fuel in the chamber and is at the same temperature, images without cavitation would be expected to have no dark areas. These initial results showed cavitation in the shear layer of the jet - dark areas due to scattering of the incident beams.

Initial ‘shadowgraph like’ images show that cavitation clouds were present at all back pressure and rail pressure combinations tested. Images presented here are from a 500bar rail pressure and ~1 bar static chamber pressure. Figure 5a shows cavitation clouds present on Injector 2 (two hole nozzle) at 215 µs after the start of injection logic. Figure 5b shows an expanding pressure wave whose source is a collapsing cavitation bubble in the left jet. Figure 6a and 6b shows two injection events recorded at the same time delay after start of injection logic as in Figure 5 and the clouds are shown to be highly repeatable. Each image in Figures 5 and Figure 6 represent individual injection events and are not otherwise related apart from sharing the same delay from start of injection logic. It can be seen that the pressure waves radiating from collapsing bubbles are also relatively repeatable. Fig 6a also shows that the left jet cloud is almost entirely fluid (rather than vapour bubbles) as it has become semi transparent rather than dark. Figure 6b shows that the cavitation clouds have re-asserted in the left jet and are again opaque indicating turbulent and cavitational flow. These images show that when fully cavitating, the jet extent can be clearly identified (i.e. cavitation occurs along the entire length of the jet). When the jet is not fully cavitating, the extent of the jet is identified by the density variations at the tip.


Figure 5a Left – Nozzle 2 (2 hole nozzle): Cavitation clouds visible on left and right jets, 215µs after injection logic.

5b right – Pressure waves propagating from the collapse of cavitation bubbles from the left hand jet. 500 bar rail

pressure and ~1bar static chamber pressure.

Figure 6a Left – Nozzle 2 (2 hole nozzle): Expanding pressure wave from cavity collapse (note that the left jet is

semi-transparent), 215µs after injection logic. 6b Right – Expanding pressure waves propagating from the collapse

of cavitation bubbles from the left hand jet ( note that Left jet is now opaque). 500 bar rail pressure and ~1bar static

chamber pressure.


In the presence of strong vortical motion in the jet shear layer, such a jet might form a series of cavitation rings briefly before turbulence ultimately destabilises the ring and it collapses. Figure 7 shows what may be such a cavitating vortex ring. It should be noted that this is the only image out of a set of around 50 to show this particular structure: it is more usual to see fully developed cavitation clouds (Fig 6b) or partially developed cavitation clouds (Fig 5b, 6a). Due to the quality of this image, its possible that this has been misinterpreted and is an artefact or other abberation. Further investigation is underway to clarify this phenomenon.

Figure 7 – Possible cavitation vortex rings (200µs after start of injection logic): picture magnified to better show the

left jet


4. Conclusion

Liquid into liquid injection, such as occurs in most Diesel injection rate meters, has been shown to be prone to cavitation in the shear layer. This cavitation extends in space for the length of the observed fuel jet. Vorticies formed though interaction of the jet shear layer with the ambient liquid can promote the formation of cavitation by providing a localised low pressure core. This cavitation might be an impediment to the effective measurement of chamber pressure, such as is found in a Zeuch type device. The assumptions around homogenous bulk compressibility for a such a device would also be violated along with a distorted transient pressure signal. This implies that care may be required when filtering the signal from a Zeuch device to accurately measure the rate of injection due to the extraneous pressure wave acitivity.

4. Acknowledgements

The authors would like to thank Loic Méès for his invaluable support and discussion on the setup and use of shadowgraph like schlieren imaging. Dimitris Touloupis also generously gave his time helping in the lab and it would not have been possible to complete this work without his assistance and perseverance. 5. References Anon. European Regulation 595/2009, 582/2011 (Euro VI - Heavy duty ) (2011). Europe. Anon. (2015a). Cutting Carbon Pollution, Improving Fuel Efficiency, Saving Money, and

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laser imaging of pressure waves and cavitation generated...

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