Study of Pulsating Flow in three way exhaust manifolds junctions ...

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

Study of Pulsating Flow in three way exhaust manifolds junctions through PIV

C. Nikita*, Y. Hardalupas, A.K.M.P. Taylor

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

Keywords: PIV processing, exhaust flows, pulsating flows, branched ducts, junctions

ABSTRACT

A recent increase of turbocharged concepts for downsized engines for passenger cars led to an increased interest in the utilisation of the available energy in the exhaust gases before and at the turbocharger turbine, since it affects the overall engine efficiency. The highly unsteady nature of the exhaust flow and the complicated manifold shapes aggregate the 3D flow structures affecting the energy availability to the turbocharger turbine. The aim of the present work is to study the temporal evolution of the exhaust flow patterns and specifically the presence and behaviour of flow separation at the walls of a three way exhaust manifold under isothermal pulsating conditions using Particle Image Velocimetry (PIV). The findings indicate that the vectors angle, as the flow enters the junction area from the branch duct, varies depending on the pulse phase. This also affects the formation and shape of the recirculation zone observed at the outlet duct of the junction. The flow separation observed on the outlet duct causes the bulk flow to pass through a vena contracta inside the duct and the flow downstream of this region is associated with pressure losses. Accurate measurement and calculation of these losses, when flow pulsations are present, is useful for the design and development of efficient coupling between engine exhaust manifolds and downstream turbines.

1. Introduction Turbocharged concepts for downsized engines for passenger cars have recently led to an increased interest in the accuracy of the calculations of the available energy in the exhaust gases upstream of the turbine, since this affects the overall engine efficiency. The study of the manifold design, as a way to improve the engine breathing by means of fine tuning of the propagating pulses, was first examined for naturally aspirated engines in the 1970s (Annand & Roe, 1974; Blair & Johnston, 1970). During the same era,


simulating tools that could calculate the pressure level downstream of the exhaust valves were also developed (R. Benson, Horlock, & Winterbone, 1982). The calculations were based on one-dimensional approach of the gas dynamics and were using measured pressure loss coefficients for a variety of manifold configurations and under a spectrum of operating conditions. In later years, a number of visualisation studies also emerged, which were focused on flow patterns developed in ducts of junctions over steady state conditions using the Schlieren method with air as the working fluid (Abou-Haidar & Dixon, 1992; M. Bassett, Pearson, Winterbone, & Clough, 1998). Visualisation of velocity profiles was also made possible for exhaust flows through the LDA technique by (Snauwaert & Sierens, 1987). A limited number of studies also exist for periodic flows (change of bulk flow direction) but these are usually focused on liquids (Karagoz, 1993) or examine the flow in branches of human aortas (Pedersen, Yoganathan, & Lefebvre, 1992). In an internal combustion engine, the exhaust flow is of highly unsteady nature as the blowdown pulses generated from each cylinder propagate through the junctions comprising the manifold. One of the most recent visualization studies under unsteady conditions is that of (Kalpakli & Örlü, 2013)in which secondary vortices are studied after a 90° duct bend by means of PIV at the cross section of the duct. To aid the calculation of pressure losses in junctions of manifolds, shock tube tests are also available in the literature. Some publications involve visualisation techniques, whereas others are limited to pressure measurements only (M. D. Bassett, Pearson, & Fleming, 2003; Bingham & Blair, 1985; Pérez-García, Sanmiguel-Rojas, Hernández-Grau, & Viedma, 2006). In these studies finite waves of different amplitudes are allowed to propagate in three way junctions and the reflections at the junction boundaries are studied in order to develop more accurate 1D models for engine simulation tools. However, the effect of the bulk flow and secondary flow structures and the pressure losses associated with them is not taken into account in the shock tube test studies. A recent work by the authors (Nikita, Hardalupas, & Taylor, 2015) was focused on the evaluation of the calculated pressure losses through an one dimensional method with reference to the results from three dimensional computational fluid dynamics simulations. The conclusions of this work were indicating that there are 3D flow structures that contribute the overall pressure losses observed in three way junctions which cannot be resolved through an one dimensional approach. In the recent years the limitations of 1D models in the calculation of 3D flows is addressed by the


coupling of 1D and 3D algorithms (Onorati, Ferrari, Cerri, & Cacciatore, 2005). The latter algorithms are responsible for the calculation of the flow field in complicated geometrical features such as junctions whereas the rest of the solution process is based on the 1D approach. However, this approach is still computationally expensive. The most common approach is still the inclusion in 1D algorithms of measured pressure loss coefficients from experiments in steady state flows (R. S. Benson, Woollatt, & Woods, 1963; Miller, 1971). For these methods, an assumption has been made that the unsteady flow is a sequence of steady state instances therefore the use of pressure loss coefficients is of acceptable accuracy. The present work aims at identifying the likely differences in the flow field as the flow accelerates and decelerates depending on the phase of the pulse that travels through a three way junction. 2. Experiment overview and Acquisition Method The present work focuses on measuring the flow structure through Particle Image Velocimetry (PIV) and pressure measurements at a T-junction of a 60° branch angle three way junction, where engine exhaust-like pulses propagate. The medium used for the experiment is air at ambient temperature, supplied by a compressor, which can deliver pressure up to 7bar and flow rates up to 1kg/s. The experimental facility consists of two single cylinder heads, which are connected to two pressurised air reservoirs with volume of 30lt. The pulsed air flows enter each branch of the junction by adjusting the timing of the opening of the poppet valves of each cylinder head through a variable indexing plate, as seen in Fig1. This arrangement can replicate the sequence and phasing between two cylinders of different engines allows the study of the influence of different timing schemes between the flow pulses. The poppet valves are driven by an electric motor, which can run at a range of cam speeds from 300-1500rpm, equivalent of 600 to 3000rpm crankshaft (engine) speed. A transparent T junction manifold is attached at the outlet ports of the two reservoirs, with square cross-section (25mm x 25mm) and overall length of 600mm (Fig2.). The required seeding of the flow is generated by two humidifier units, placed within the pressurised air reservoirs upstream of the cylinder heads, which deliver water droplets with diameter up to 3µm. In this way, no additional mass from a ‘seeding’ air flow is entering the junction. The volume of the reservoirs, 30Litres each, selected so that it ensures uniform distribution


of the air flow and attenuation of the rarefaction waves, generated by the opening and closing of the poppet valves, before these reach the seeder units.

Fig 1. Experimental rig layout Fig.2. T60o Perspex junction (top view)

The PIV system consists of a NewWave Nd:YAG double pulsed laser, rated at 120mJ energy, delivering a round beam of 4mm diameter. The laser beam was transformed to a thin laser sheet of approximately 60mm length and 3mm wide, using a combination of a convex and one concave length as on Figure 3a. The laser sheet intersects the transparent manifold on the mid-plane upstream and downstream of the junction area so that the evolution of the flow in the manifold can be studied. The illumination of the seeding particles was captured with a CCD camera (non-intensified LaVision Imager Intense) with a resolution of 1376 x 1040 pixels using a 50mm 1:1.8 lens with manual focus. The real dimensions of the images recorded were 72.9 x 55.1 mm and the resulting spatial resolution was 53!m/pixel. The image processing was achieved through commercial software (LaVision Davis 7.2) using a cross correlation method. A


multi pass refinement process was used for the calculation of the vectors field starting from interrogation windows of 256 x 256 pixels to 8 x 8 pixels with an overlap of 25%.

(a) (b)

Fig. 3 Flow and laser measurement layout (a) and pressure measurement points (b) The triggering of the laser and camera is achieved by providing an external trigger to the DaVis software in tandem with a rotary encoder fitted at the shaft, connected to the cylinder heads. This allows for the laser pulse events to be tuned to a range of cam degrees to study the evolution of the blow down pulse inside the junction. In order to identify the timestamps of interest, pressure measurements were obtained using high sensitivity pressure transducers across the length of the three ducts comprising the junction. The pressure measurement points are shown in Fig. 3b. The time delay between the two laser pulses, for each timestamp, was also altered to account for the range of velocities expected at the different phases of the pulses. For the PIV measurements, the operating point must be selected based on a combination of factors such as the camera resolution, the range of expected velocities in the flow and the quality of the seeded air. It was decided that a rotational speed of the crankshaft at 5Hz would provide good seeding quality for the processing of the recorded PIV images along the whole pulse duration, both the accelerating and


decelerating phases. An overlap of 45 (cam) degrees was also selected as the valve timing profile of interest, Fig. 4, for the generation of the two pulsed flows.

Fig. 4 Valve timing with 45 cam degrees overlap.

The crankshaft encoder is providing the main trigger to the PIV system, once every crank revolution, which is used as the reference timestep t0. Then the laser and camera are triggered accordingly through the DaVis software. The software can also trigger the acquisition using a fixed delay from the reference time (main trigger). The latter option allowed for the image capturing to occur at different times so that the temporal evolution of the flow can be captured. The timesteps of interest are shown in Fig.5 as an overlay to the pressure trace obtained from the sensor 2, Fig. 3b.

Fig. 5 Trigger events for image acquisition .


The overall duration of the pulse was 210ms and the reference time t0 occurred at the decelerating phase of the pulse coming from duct 1. The remaining triggers occur at a delay from the reference time of 20ms (t1), 40ms (t2), 150ms (t3) and 190ms (t4) respectively.

A total of 300 images of the instantaneous flow field were obtained at each triggering event, so that the flow motion of both pulses is captured. Most of the triggering events were attributed to the pulse travelling from the branch duct (duct 1), where the flow is expected to be associated with turbulent flow structures as it interacts with the wall of the main duct (duct 2). The camera was initially placed at the central junction area, so that one characteristic length of all ducts was captured. Then, the camera was moved towards the outlet duct to study the development of the flow downstream of the junction. 3. Results and Findings The evolution of the exhaust like blowdown pulses will be presented in this section in the form of images of the calculated flow field at the times during the pulsed flows indicated above. Figure 6 shows the ensemble average of the velocity magnitude at the given times, during the pulsed flows, which was calculated from the captured 300 images. The RMS of velocity magnitude fluctuations is also presented in the same figure. The temporal sequence of the images in Fig.6 is such that it follows the propagation of the blowdown pulse of duct 1 (branch duct). As seen in Fig. 6, the flow is directed towards the outlet duct while in parallel it forms a recirculation zone, as the flow separates in duct 2. Due to the flow impingement on the wall as it exits the branch duct, a stagnating region is created at one side of the outlet duct. The development of the flow field inside the outlet duct will be discussed below.


Fig. 6 Ensemble average (left), RMS values (center) and corresponding timesteps t4 and

t1 to t3 (right) during the blowdown pulse of duct 1 to the T-junction


A comparison of the velocity magnitude amongst the images of Fig. 6 shows that the latter is stronger at the acceleration phase of the pulse and then decreases up to the point that the rarefaction wave meets the junction area. At this time backflow is observed, moving at about 25% of the velocity measured during the blowdown pulse. It should be noted that the velocity fields of Figures 6b and 6d have been captured under the same pressure disturbance, as compared to the atmospheric conditions, with the former being a compression wave and the latter a rarefaction wave. However, the bulk flow motion traveling down the exhaust valves contributes to the increased velocity magnitudes observed in Fig. 6b. The main reason for the flow motion at Fig. 6d is the rarefaction wave travelling towards the poppet valves. A small stagnating region at the entrance of the branch duct can also be seen at this instance of the pulsed flow. At the central part of Fig. 6, the RMS of the velocity fluctuations are depicted for the examined times during the pulsed flow. As the blowdown exhaust pulses are a highly transient phenomenon, the flow fluctuations at different realisations, leading to the high RMS of velocity fluctuations. These, however, are further increased at the left part of the flow that exits the branch duct. This can be explained by the variation of the main flow direction, as observed in the two examples of instantaneous images of the flow field in Fig.7.

Fig. 7 Velocity angle variation between two images of the instantaneous velocity field for timestep t0


In order to examine how the flow pattern develops downstream of the T-junction area, a set of images was captured in the outlet duct at t0 (main trigger event). The ensemble average of the flow velocity is presented in Fig. 8, side-by-side with the corresponding ensemble of the junction area to give an indication of the complete flow field at the main trigger event. As it can be observed, the recirculation zone formed inside the outlet duct is approximately two characteristic dimensions long and half a dimension wide. This formation directs the flow through a vena contracta, which increases the pressure losses usually in branching duct systems. As it is known, most of the pressure loss calculation relies on the losses occurring due to the flow entering the junction; which accounts as a sudden area expansion. However, these losses need to be added to any further losses occurring downstream in order to obtain an accurate calculation.

Fig. 8 Streamlines of the flow field at the central area and the outlet duct at timestep t0 The partial blockage of the effective duct area can also impair any pressure measurements at the outlet duct. In the case of engines, it would also affect the inlet conditions at the entry of the turbine volute since this is usually placed close to the manifold’s end junction. However, it seems that the recovery of the flow to its coaxial direction is going to be less than what the 10 diameter rule of thumb dictates. This knowledge is important for the acquisition of accurate pressure measurements although manifolds of internal combustion engines are not expected to have enough length available downstream of a junction, mostly due to packaging reasons.


The exhaust pulse on the main duct (duct 2) is not subjected to significant variations when compared to the flow coming from the branch duct. Figure 9 shows the ensemble average for the velocity magnitude during the exhaust of duct 2. The 120° branch angle of the junction, as seen from duct 2, is not a preferable flow path since the flow travels down the manifold. Hence, there is little disturbance to the observed flow field. The peak velocity value is higher than the one seen at flow coming from duct 1 with the RMS of the velocity fluctuations being at the same level.

Fig. 9 Ensemble average (left), RMS values (center) and corresponding timestep t3 (right) during the blowdown pulse of duct 2 in the T-junction

4. Conclusions The current work was focused on the study of the flow field through a T60° junction of finite length, where engine exhaust-like pulses are left to propagate. This was achieved with the use of PIV method along with pressure measurements of pulses, which were generated from a dedicated experimental arrangement using poppet valves and pressurised air as the working medium. Snapshots of the flow were obtained at a range of times during the pulsed flow to cover the evolution of the flow during both accelerating and decelerating phases. The main area of interest was located at the central part of the T junction, including one characteristics length of the long duct, before and after the junction.


The results for the flow pulse coming from the branch duct indicate that there is a recirculation zone formed just downstream of the junction, as expected due to flow impingement on the main duct wall. The latter has also been observed for studies found in the literature for steady flow tests. However, it was also revealed that depending on the phase of the pulse, the velocity angle with which the flow enters the outlet duct differs. This can be attributed to both the accelerating and decelerating nature of the flow as well as the turbulence phenomena inside such flows. It was also observed that there is a back flow at the end of the flow pulse, which is attributed to the rarefaction wave propagating from the end of the outlet duct. The velocity magnitude observed during that phase is lower than the one observed for the same pressure disturbance during the bulk flow motion. At the outlet duct, a recirculation area is formed which is approximately two characteristic lengths long and half a characteristic length wide, blocking almost half of the cross section of the outlet duct. This flow structure creates a vena contracta that forces the main flow to pass through. The divergence part of the vena contracta is associated with pressure losses, which are localised downstream of the junction and not at the junction area. These losses will need to be added to the overall pressure loss calculation for a given junction shape especially when 1D methods are used that are not accounting for 2D or 3D phenomena inside single ducts. The non-uniform flow field will also impair the inlet conditions of a turbocharger in the case of this being fitted at the end of the manifold. Future work of the authors will focus on the quantification of pressure losses and average velocity values during the evolution of the pulses to be compared with calculations of 1D and computational 3D methods. Acknowledgments The authors wish to acknowledge the financial assistance and valuable discussions with their colleagues at Volvo VCC for the current work. This is part of a collaborative research study on pulsating flow in exhaust systems for passenger vehicles.


References

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