Experimental and Numerical Study of Swirling ... - Solid Mechanics

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Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines Summary & Conclusions The aim of this thesis was to study and understand various aspects of the confined swirling flow during the uniflow scavenging processes for large two-stroke marine diesel engines. A description of the working of large two stroke marine diesel engines were given in order to understand the role of scavenging process and its significance on the overall engine performance and efficiency. Different features of uniflow scavenging process in currently operational marine diesel engines were discussed and it was identified that the real engine scavenging process is very complex, scientifically less understood in detail and one of the potential processes for making improvements in order to develop future fuel and environmentally efficient marine diesel engines. Out of the many aspects of complex physics of scavenging process, the focus in this thesis was to study and characterize the confined swirling flow during the scavenging process. The study was conducted by developing a simplified experimental model with air at room temperature and pressure without considering mixing and stratification. The model comprised of a transparent acrylic cylinder fitted to a swirl generator having guide vanes. Features like a movable piston and changing cylinder length were also included. The design is more close to features of engine cylinder and makes it different from the confined swirling pipe flow experimental setups earlier reported in scientific literature. Different experimental techniques were used and CFD simulations were performed and the results were compared with the experimental data. 1.1 Experimental Measurements Two different experiments are conducted using Stereoscopic PIV measurements at different cross-sectional planes. For both the experiments the measurements are conducted at fixed guide vane angle of the swirl generator and two Reynolds numbers in the cylinder i.e. 65,000 and 32,500. For each measurement, nearly 1000 instantaneous PIV snapshots were taken. For the fully open intake port, LDA measurements are also conducted in the inlet section and also at the entrance of the cylinder. The design swirl parameter for the experimental setup is found to be 0.34 and is kept constant for all the measurements. 164 Summary & Conclusions
Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines 1.1.1 Swirling Flow in a Pipe with different Lengths In this experiment the inlet to the measuring cylinder was kept fully opened and the measurements are conducted by changing the length of the cylinder to 8D, 6D and 4D. The results from LDA measurement show that in the inlet section, the wake effect behind the guide vanes reduces significantly before entering the contraction region. The velocity distribution in the inlet section at different circumferential position is not symmetric at a radial distance of 200 mm. However, after the flow passes through the contraction region the variation of the angle between radial and tangential component (along the circumferential positions) is reduced to 1.5°. At an axial position z/D of 0.368, the LDA measurements show a near symmetric distribution of tangential velocity. The PIV results show that the resulting confined swirling flow has a tangential velocity profile similar to Burgers vortex and the axial velocity has a wake-like profile. The tangential velocity profile shows that unlike the theoretical Burgers vortex profile, the region surrounding the inner forced vortex core region is not irrotational. Instead, the region has a very weak vorticity that may possibly be as a result of a radially decaying vortex causing a vorticity transfer from vortex core to the surrounding regions. This result is in accordance with some other experimental results reported earlier in the scientific literature. The swirl decays downstream the flow and the vortex core size increases. This changes the tangential velocity profile towards more forced vortex and the axial velocity to be more uniform by transferring more mass in to the central velocity deficit region. The effect of variation in Reynolds number is mostly observed in the vortex core region where for initial measuring positions the peak value of normalized tangential and axial velocity is found to be higher for low Reynolds number. Further, for axial velocity, the minimum velocity in the velocity deficit region is found to be lower in case of high Reynolds number. However, the axial velocity profile at all measuring positions does not show any reverse flow. The average size of the vortex core at low Reynolds number is observed to be relatively smaller than at high Reynolds number. For first five measuring positions close to cylinder inlet, which are common in cylinder lengths 8D, 6D and 4D, the variation in cylinder length seems to have no significant effect on the normalized tangential and axial velocity profiles for a given Reynolds number. Similar behavior is observed for the positions which are common in cylinder lengths 8D and 6D. For position z 6 (z/D=3.595) being very close to cylinder outlet for cylinder length 4D, a small increase in the tangential and axial velocity is observed due to small diameter outlet pipe. This indicates that the downstream change in cylinder length is not detected by the flow at upstream positions and thus the flow mainly depends on the upstream conditions at the cylinder inlet. The magnitude of radial velocity at position z 1 (z/D=0.963), is almost a factor of 10 lower than tangential and axial velocity and decreases further 165 Summary & Conclusions
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