Timo van Overbrüggen1,*, Jan Dannemann1, Michael Klaas1 ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2012/upload/31_paper_qnisnj.pdf · used to measure the engine speed. Due to the four-stroke

16th Int. Symp on Appl. Laser Techniques to Fluid Mechanics, Lisbon, Portugal, July 09 – 12, 2012

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Holographic particle-image velocimetry measurements in a four-valve combustion engine

Timo van Overbrüggen1,*, Jan Dannemann1, Michael Klaas1, Wolfgang Schröder1

1: Institute of Aerodynamics, RWTH Aachen University, Aachen, Germany, * corresponding author: [email protected]

Abstract The reduction of fuel consumption and pollutant emission define the major goals of engine development. Both are significantly influenced by the mixing process i.e., during the intake and compression stroke. Thus, the density field and velocity distribution of the fuel air-mixture are highly important for the combustion process and hence, for the pollutant emission. To analyze the structure of the three-dimensional velocity field during the intake phase, the non-reacting flow in a four-valve internal combustion (IC) engine is measured at 160° after top dead center (atdc) at 1,500 rpm via volumetric holographic particle-image velocimetry (HPIV). The engine is a one-cylinder test engine with a displacement of 728 cm3 and full optical access. The holographic PIV system measures the flow field in the combustion chamber with a resolution of 0.75 mm. In addition cross-correlation functions and integral length scales are calculated out of the velocity fields. The current results confirm the existence of large scale structures. The tumble vortex and a pair of counter-rotating vortices below the inlet valves are visualized using streamlines and 𝜆2-isosurfaces. Furthermore, a U-shaped propagation of the vortex core of the tumble-vortex can be observed. The measured integral length scales range from 2.5 to 6.1 mm. These length scales are comparable to values reported in the literature. 1. Introduction

The limited fossil energy resources as well as the debate about pollutant emissions like nitrogen dioxide (NOx), carbon dioxide (CO2), and soot brings the reduction of fuel consumption in internal combustion (IC) engines into the focus of public interest. Hence, engine development concentrates on new combustion processes that both decrease fuel consumption and increase the power output of the engine. Amongst other, the so called Controlled Auto-Ignition (CAI) and Homogeneous Charge Compression Ignition (HCCI) play a prominent role in modern engine development. Both are characterized by a high homogenization and exhaust gas recirculation (EGR). Homogenization avoids partial combustion so that soot emission decreases, whereas lower combustion temperatures significantly reduce NOx emissions. The lower combustion temperatures are achieved through the high thermal heat capacity of the recirculated exhaust gas. Due to dethrottling of CAI engines and a faster combustion near top dead center (TDC), fuel consumption is reduced. Lang et al. (2005) reported a maximum fuel reduction of 15% and a reduction of NOx emissions by up to 90 – 99 %. Unfortunately, CAI as well as HCCI are limited in their operation range due to knocking and misfiring (Ma et al. 2001, Urushihara et al. 2001, Bhave et al. 2005). These instabilities can not be controlled by combustion performance measures, e.g., air-fuel ratio measurements. Therefore, they have to be suppressed on a chemical and physical basis. Stapf et al. (2007) state that self-ignition highly depends on the stratification of fuel, EGR, and fresh air. Consequently, the control of the combustion process strongly depends on the thermodynamic conditions of the in-cylinder mixture (Adomeit et al. 2009). These conditions are significantly influenced by the gas flow within the cylinder, especially during the intake and compression stroke. Hence, the large and small scale structures within these strokes strongly influence the mixing and therefore the combustion process (Heywood 1988). An analysis of the temporal and spatial development of the characteristic flow phenomena helps to gain a better understanding of the mixing process. Two-component planar PIV (2D/2C) has become a standard tool for the analysis of the flow field inside an IC engine. Stansfield et al. (2007) measured the crank angle resolved mean flow in the symmetry plane of an IC engine to calculate tumble ratios. They described the angular momentum


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around a specific axis in the combustion chamber for different engine speeds and found a significant change of the fundamental flow structure between 2,500 and 3,500 rpm. Dannemann et al. (2010) performed planar PIV measurements for several crank angles in an unfired one-cylinder four-valve IC engine. They analyzed the three-dimensional structure of the velocity field based on flow field measurements in eight axial planes. The quasi three-dimensional flow field was reconstructed from the two-dimensional velocity fields for crank angles of 80°, 160°, and 240° after top dead center (atdc). Furthermore, the propagation of the flow field between crank angles of 40° and 320° atdc in steps of 20° was discussed for both symmetry planes. The authors showed that the flow inside an IC engine possesses a highly three-dimensional character. Furthermore, large vortical structures, e.g., ring vortices beneath the inlet valves and their temporal development could be shown. Stereoscopic PIV (SPIV) measurements, which are capable of visualizing all three velocity components (2D/3C), were first performed in an IC engine by Calendini et al. (2002). They recorded 35 double images in the center plane between inlet and outlet valves for different crank angles at 1,000 rpm. However, the ensemble averaged measurements were not able to visualize large scale structures like ring vortices or the tumble vortex. Bücker et al. (2011, 2012) performed stereoscopic PIV measurements in a set of planes for several crank angles at 1,500 rpm. They visualized large scale structures (e.g., ring vortices beneath the inlet valves and the tumble vortex) and analyzed the temporal development of the mean kinetic energy in two measurement planes. However, the highly three-dimensional structure of the flow requires the application of a three-component volumetric or quasi-volumetric measurement technique to capture all three velocity components. Holographic particle-image velocimetry measurements in IC engines are rarely reported in literature. The HPIV technique is being developed since the early nineties. First fully three-dimensional velocity measurements were performed by Barnhart et al. (1993) and Zhang and Katz (1994). Konrath et al. (2002) performed HPIV measurements in off-axis configuration inside an IC engine and measured two parallel planes simultaneously with high spatial resolution. However, fully three-dimensional volumetric measurements were not possible due to high image noise ratios. In this study the total velocity field is measured at 160° after top dead center (atdc) by holographic PIV to show the feasibility of the measurement technique and to gain information of the spatial distribution of the large and small scale structures of the in-cylinder flow. The article is organized as follows. First, the optical four-valve research engine is described in detail. Next, the system to perform fully three-dimensional holographic particle-image velocimetry is specified. After introducing the post-processing algorithm the results are analyzed. The discussion of the results starts with a visualization of the whole three-dimensional flow field. Then, the flow field is described by showing several planes inside the three-dimensional flow field. In addition, the integral length scales are discussed.

2. Experimental setup

2.1 Optical engine

The four-valve research engine is based on a Suzuki DR750 motorcycle four-stroke single-cylinder IC engine that is driven by an electrical 55 kW engine (fig. 1), i.e., the engine is not fired. The bore of the engine is D = 105 mm whereas the stroke is 85 mm. This results in a cylinder displacement of V = 728 cm³. To accomplish full optical access, the cylinder of the Suzuki engine has been replaced by a transparent Perspex liner that is placed between the original cylinder head of pent roof geometry and an elongated lower liner that keeps the piston properly aligned. The piston rings are located in the lower iron liner. The clearance between the piston and the Perspex liner of 0.16 mm is sufficient to ensure a free piston movement within the optical liner. Due to the larger top-land crevice volume, the compression rate is reduced from 9.5 to 9. To further enhance the optical access, the camshaft drive chain was displaced by means of an elongated camshaft and an elongated crankshaft, so that an undisturbed observation from all sides is possible. The flat piston crown also consists of Perspex to ensure optical access. The engine is operated at a mean revolution speed of 1,500 rpm without fuel injection and combustion. The typical progression of the engine speed n is shown in fig. 2, showing the


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decay due to compression and the valve lift of the inlet and the exhaust valve. A critical temperature increase of the Perspex liner has to be avoided. Therefore, the engine can only be operated for a short time (t < 120 s). A shaft encoder with a resolution of 1° crank angle provides the crank angle and can be used to measure the engine speed. Due to the four-stroke cycle length of 720°, a reflective sensor scans a coding disk mounted to the camshaft to clearly identify intake, compression, power, and exhaust stroke.

Fig. 1 Optical one-cylinder test engine

The seeding particles for PIV measurements are provided through an additional reservoir connected to the cylinder intake port. Two flaps in front of the intake ports of the cylinder head allow a fast switching between seeded and unseeded air to minimize the particle contamination of the optical liner during the engine start process (Konrath et al. 2002). A few cycles before the measurement, the flaps are released to provide seeded air for the measurements.

2.2 Holographic particle-image velocimetry system

The holographic PIV-setup is based on a system developed by Konrath et al. (2002, 2003) and was enhanced to preform fully three-dimensional flow measurements within the cylinder of the test engine. The system mainly consists of three optical components: the relay lens module containing the holographic film plate, the reference beam module, and the flow field illumination module. A schematic of the HPIV system is shown in fig. 3. In the following, the basic features as well as some modifications of the HPIV system are explained. A detailed description of the system is given by Dannemann et al. (2012). Two single cavity Spitlight600 Nd:YAG lasers with a total laser pulse energy of approx. 2 x 400 mJ are used to record the holograms. Since holographic PIV recordings require a high coherence length to

seeding

transparent liner inlet flaps

electrical engine

cylinder head

mounting frame

1800


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Fig. 2 Progression of engine speed and valve lift curves.

achieve a sufficiently large contrast within the hologram plane, both lasers are seeded using an injection seeder. The resulting coherence length is approx. 1.5 m. To separate the two laser pulses in the reference module, the pulses are slightly shifted, so that both beams have a distance of approx. 7 mm at the separation mirror. This technique avoids the deployment of a pockels cell and thus reduces the complexity of the HPIV system.A beam splitter divides the beams in a low-power reference beam and a high-power object beam. The path lengths of both beams are carefully aligned to enhance the contrast of the interference pattern. The hologram is recorded on a 127 x 102 mm² green sensitive VRPM glass plate by Slavich. To measure all three velocity components simultaneously, the flow field is recorded from two orthogonal sides. For each direction, two lenses and mirrors are used to relay the particle image in front of a single hologram plate, whereas both optical paths have the same length. The illumination direction of the transmitted particle images are ± 15° (horizontal) with respect to the normal of the hologram plate.

3 axis positioning system

hologramm plate

n 𝑛!

inlet outlet

rpm

crank angle [°]

Val

ve li

ft [m

m]

1300

1500

1700 1600

1400

1200 15

10

5

0 0 360 720 1080 1440 1800 2160


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Fig. 3 Schematic of the recording (left) and reconstruction (right) setup of the holographic particle-image velocimetry system The relay module is mounted on a 1,200 x 900 mm2 breadboard and can be rotated by 180° to perform phase conjugate reconstruction. Therefore, the identical geometry during hologram recording and reconstruction is guaranteed and image aberrations due to lens impairments can be neglected. To derive the velocity vectors from the particle images, the particles are illuminated twice within a defined time interval and both images are stored on the same hologram plate. For temporal separation of the single particle images, two different reference beams are used. They illuminate the hologram from ± 5° vertically and 53° horizontally with respect to the normal of the hologram plane. The maximum numerical aperture is set to 0.094 to avoid overlap of direct and phase conjugate images of one of the four recorded images. For reconstruction, a continuous-wave (cw) Nd:YAG laser with the same wave length as the recording lasers is used. To switch between both reference beams, a laser modulator and a polarizing beam splitter are used. A half-cylinder made from Perspex cancels distortions in radial direction as well as astigmatic aberrations due to the original Perspex liner. The cylinder is placed in the reconstruction setup at the position of the original cylinder. For precise reversal of the reference beam angles, the alignment system described by Heflinger et al. (1978) is used. The remaining reference beam aligning error is neglected using the method described by Konrath et al. (2003).

reference beam module

lens

relay lens module

reference beam module

relay lens module

CCD camera with telecentric lens

ND:YAG laser

mechanical shutter

beamsplitter


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2.3 HPIV post processing The holographic images are scanned using an AVT Guppy F-146 CCD camera and a telecentric lens (Vision & Control T50/7). The camera chip has a size of 1024 x 1392 pixels with a pixel size of 4.65 µm. The lens has a magnification of M = 0.704 and an aperture of N.A. = 0.12 resulting in a depth of focus of approximately 0.24 mm. Thus, the lens does not degrade the resolution due to an immoderate integration in longitudinal direction. The setup is mounted on a three axis positioning system (OWIS Limes 120). The image post processing is performed using Matlab routines and fully automatic scanning of each view is realized by LabView and electronic switching of the laser modulator. The images are post-processed using adaptive cross-correlation algorithms, linear window deformation techniques, and vector filters like global velocity filters and local median filters. In a first step, the three-dimensional vector matrices are allocated, and split into n two-dimensional vector maps, where n is the total number of double images. The calculation of the cross-correlation is parallelized and performed on a high-performance computer cluster due to the large amount of double images (e.g. 51612 double images for the engine measurements). The adaptive cross-correlation starts using an interrogation window of 256 x 256 pixels with 50% overlap. Thus, 7 x 10 velocity vectors are obtained from one double image. For the first iteration, no window shifting and deformation is used. However, the quality of the adaptive cross-correlation increases with each iteration step since window shifting decreases the loss of pairs and the window deformation compensates velocity gradients in the interrogation window. After processing all double images, the 2C/2D vector maps are composed to a three-dimensional vector field using the equations given by Raffel et al. (2007). Spurious velocity vectors are filtered and interpolated after every iteration step. If subsequent iteration steps are obtained, the velocity map is smoothed to obtain better results from the adaptive cross-correlation algorithm. The final resolution of the vector fields is a direct function of the scanning distance. The aforementioned window size of 256 x 256 pixels leads to a vector spacing in the final three-dimensional vector field of 0.75 mm. After processing all holograms for a given crank angle, the averaged mean value for all three velocity components is calculated. The averaged mean velocity component in the x-direction 𝑢 is defined by (Heywood 1988; Tennekes and Lumley 1972)

𝑢 = !!

𝑢!!!!! (1)

where u represents the velocity and n the number of vector fields for the defined crank angle. Furthermore, the 𝜆! -criterion is used to identify vortical structures in the flow field (Jeong and Hussain 1995). The quantity 𝜆! is the second eigenvalue of the symmetric matrix M that contains the 3D velocity gradients of the flow field. Negative values of 𝜆! represent vortex cores. In addition, the integral length scale L is calculated using the fluctuating velocity with respect to a variable distance (Pope 2000):

𝐿!"! = !!!"(!,!,!)

𝑅!" 𝑒!𝑟, 𝑥, 𝑡 𝑑𝑟!! (2)

Here 𝑒! represents a vector of the length 1 in a given direction. The quantity R is the two-point correlation function 𝑅!"(𝑟, 𝑥, 𝑡) = 𝑢!!(𝑥, 𝑡)𝑢!!(𝑥 + 𝑟, 𝑡) (3) with 𝑢′ being the fluctuation of u.

3. Results and Discussion


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In the following, the flow field of the measurements at 160° atdc are described and discussed in detail. The actual measurement volume is shown in fig. 4. Due to slightly different beam profiles of the two Spitlight lasers, the sectional plane of the two volumes is somewhat elliptic. Nevertheless, it was possible to analyze a large area of the flow inside the cylinder.

Fig. 4 Illuminated measurement volume for the holographic PIV measurements.

Figures 5 and 6 show the three-dimensional visualization of the flow at 160° atdc. The three-dimensional streamlines are color coded by the absolute velocity and the 𝜆!-contours are used to visualize the propagation of the in-cylinder vortices. Note that for calculating the 𝜆!-contours the velocity was low-pass filtered in the frequency domain to remove the turbulent small scale structures and enhance the visualization of the large scale flow structures. In the upper part of the cylinder, the counter-rotating vortices, which form below the inlet valves, can be clearly seen. Furthermore, the propagation of the tumble vortex can be identified by the streamlines. The lower part of the 𝜆!-contour shows the center of the tumble vortex. The vortex core is U-shaped which is in good agreement with the measurements of Bücker et al. (2011) who showed a similar propagation in their SPIV measurements. The maximum absolute velocities can be found between the two inlet valves and in the bottom center of the cylinder. This is due to the high inflow velocity as well as the roll up of the tumble vortex at the bottom of the cylinder.


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Fig. 5 Three-dimensional visualization of the engine flow at 160° atdc. Streamlines and 𝜆!-contours (red) are shown. The streamlines are color coded by the absolute velocity.

Fig. 6 Three-dimensional visualization of the engine flow at 160° atdc. The streamlines are color coded by the absolute velocity.


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Fig. 7 In plane velocity components (vectors), out of plane velocity components (color code) and streamlines (red lines) at 160° atdc in three x-z-planes (y = 0 mm (a), y = 10 mm (b), y = 20 mm (c)) (left) and three y-z-planes (x = -20 mm (a), x = 0 mm (b), x = 20 mm (c)) (right).


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Figure 7 shows the mean averaged flow inside the cylinder of the test engine at a crank angle of 160° atdc for three x-z planes and three y-z planes, respectively. The velocity fields are color coded by the out-of-plane component. For clarity, only every 13th in-plane velocity vector is shown. The red lines show the two-dimensional streamlines of the flow. Since the flow in the x-z plane (fig. 6, left) is nearly axisymmetric with respect to the y = 0 mm plane, only measurements in the y = 0 mm, y = 10 mm, and y = 20 mm planes are shown. In the y = 0 mm plane the flow enters the combustion chamber through the gaps between the pent roof and the two inlet valves. The flow direction possesses the same inclination as the inlet valves. After entering the combustion chamber, the flow is redirected via the cylinder wall and the piston. Through this, the main structure in the y = 0 mm plane, the tumble vortex, is formed. Its center can be seen at (x, z) = (-10 mm, 50 mm). For the y = 10 mm plane the vortex center is located at (x, z) = (0 mm, 45 mm) whereas the vortex center in the y = 20 mm plane is located at (x, z) = (5 mm, 45 mm). This confirms the aforementioned U-shaped contour of the vortex center. In fig. 7 (right) the x = -20 mm, x = 0 mm, and x = 20 mm planes are shown. The dominant large scale structures are the two counter-rotating vortices below the inlet valves. These vortices can be seen in all y-z planes. They result from the intake flow through the inlet valves and a roll up below the valves. Furthermore, in the measurement plane x = 0 mm a second counter-rotating vortex pair at the bottom of the cylinder with the vortex centers at (x, z) = (-20 mm, 20 mm) and (x, z) = (25 mm, 20 mm) can be seen. In addition, the flow is nearly axisymmetric with respect to the y = 0 mm plane. In all three measurement planes, a strong positive out of plane velocity in the lower part of the cylinder can be seen, whereas the upper part of the flow possesses a negative out-of-plane velocity. This is due to the tumble vortex that was described above. Figure 8 shows the correlation function obtained from the HPIV measurements. It can be seen that Ruu,x and Ruu,y coincide, whereas Ruu,z differs. Nearly the same behavior can be seen for the two-point correlation of the v-component of the velocity. Hence, the flow is not fully isotropic at least for one flow direction. Furthermore, Rww,x, Rww,y, and Rww,z vary, which also shows the non-isotropic behavior of the flow. This is also evident from the corresponding integral length scales which are listed in table 1.

Fig. 8 Two-point correlation function for the HPIV measurements at x = 0 mm, y = 0 mm and z = 50 mm at a crank angle of 160° atdc for the u-(a), v-(b) and w-(c) velocity component.

Table 1: Integral length scales at x = 0 mm, y = 0 mm, and z = 50 mm at a crank angle of 160° atdc.

Ruu,x Ruu,y Ruu,x Rvv,x Rvv,y Rvv,z Rww,x Rww,y Rww,z

Lij,k [mm] 5.4 5.4 1.9 4.8 6.1 2.5 3.6 3.8 6.0


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Conclusion Volumetric holographic particle-image velocimetry measurements have been conducted in the cylinder of a four-valve IC engine at 160° atdc. It was possible to measure nearly the complete velocity field inside the IC engine with a resolution of 0.75 mm such that the large scale structures inside the engine could be resolved. The two counter-rotating vortices below the inlet valves as well as the tumble vortex were shown. The measurements show a U-shaped profile of the tumble vortex core. Furthermore, the integral length scales were resolved. They are in the range of 2.5 to 6.1 mm. Future investigations will focus on measuring more crank angles to increase the temporal resolution of the measured in-cylinder flow. Furthermore, the signal-to-noise ratio as well as the spatial measurement resolution will be enhanced. At resolutions of up to 0.1 mm it is possible to measure the Taylor length scale that is expected to be in the region of 0.6 - 3.5 mm depending on the crank angle (Petersen and Ghandhi, 2011). Moreover, the fully three-dimensional density field of the in cylinder flow will be measured by a tomographic holographic interferometry measurement system. These measurements will be combined with the flow field measurements to analyze the mixing process of the in-cylinder flow in more detail. Acknowledgments This research is part of the collaborative research center SFB 686 which is funded by the German Research Association (Deutsche Forschungsgemeinschaft, DFG). The support of the DFG is gratefully acknowledged. References

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Documents

Timo van Overbrüggen1,*, Jan Dannemann1, Michael Klaas1 ...ltces.dem.ist.utl.pt/lxlaser/lxlaser2012/upload/31_paper_qnisnj.pdf · used to measure the engine speed. Due to the four-stroke