<div class="section abstract"><div class="htmlview paragraph">Understanding the filtration mechanisms in partially damaged Diesel Particulate Filters is very important for the design of exhaust systems with efficient On-Board Diagnosis functionality, especially as new threshold limits have been recently applied for particulate mass leakage. Two common types of DPF failure are included in this study, namely rear plug removal and internal failure due to uncontrolled regeneration with excessive deposit loading. Initially, the two respective filters were loaded on the engine bench with particle measurement upstream and downstream, and then they were disassembled and sectioned to study the deposit distribution. The analysis of the second filter revealed several modes of failure that should be expected under real-life conditions such as material accumulation in the inlet channels, substrate melting, and crosswise and diagonal crack development. Moreover, a computational model with the necessary adjustments is used to simulate the loading experiments and interpret the underlying filtration mechanisms. The processed results reveal small effects of temperature and mass flow rate on the filtration efficiency and a comparatively stronger impact of the total deposit loading. The local deposit loading is uniform in the intact segments, while it is non-uniform with a minimum value at the failure location in the unplugged and internally damaged segments. This finding is consistent with the wall flow predicted by the model, whereas some discrepancies of the model can be attributed to a secondary collection mechanism.</div></div>
In the present work, a computational optimization study of thrust bearings lubricated with spatially varying viscosity lubricants is presented, with the main goal of minimizing friction coefficient. In practice, spatial variation of viscosity could be achieved by utiliz ing electrorheological or magnetorheological fluids. The bearings are modeled as twodimensional (2D) channels, consisting of a smooth moving wall (rotor) and a parallel or inclined stationary wall (stator), which can be (i) smooth, (ii) partially textured with rec tangular dimples, and (iii) smooth and partially hydrophobic. The bearings are consid ered to be operated with an ideal lubricant that exhibits different values of viscosity in two distinct regions of the fluid domain: a high viscosity area is considered at the channel inflow, with the viscosity acquiring a reference (low) value farther downstream. The flow field is calculated from the numerical solution of the Navier-Stokes equations for 2D incompressible isothermal flow. The bearing geometry is defined parametrically. Three optimization problems are formulated, corresponding to: (I) a conventional smooth converging slider, (II) a parallel slider with artificial surface texturing at part of the stator surface, and (III) a parallel or converging slider with hydrophobic properties at part o f the stator surface. Here, the geometry parameters, as well as the increased viscos ity value and the corresponding application regime, form the problem design variables. Bearings are optimized for maximum load capacity and minimum friction coef ficient. Optimal solutions are compared against corresponding ones for operation with constant viscosity. It is demonstrated that, by using spatially varying viscosity, a substantial reduction of friction coefficient can be achieved, for all optimization problems considered. This decrease is shown to be a consequence of a sharp pressure rise in the high viscosity regime, resulting in a corresponding rise in load capacity, accompa nied by a less pronounced increase in wall shear stress, and thus in total friction force.
In the present work, a CFD-based optimization study of thrust bearings lubricated with spatially varying viscosity lubricants is presented, with the main goal of minimizing friction coefficient. In practice, spatial variation of viscosity could be achieved by utilizing electrorheological or magnetorheological fluids. The bearings are modeled as two-dimensional channels, consisting of a smooth moving wall (rotor), and a parallel or inclined stationary wall (stator), which can be (i) smooth, (ii) partially textured with rectangular dimples, and (iii) smooth and partially hydrophobic. The bearings are considered to be operated with an ideal lubricant that exhibits different values of viscosity in two distinct regions of the fluid domain: a high viscosity area is considered at the channel inflow, with the viscosity acquiring a reference (low) value farther downstream. The flow field is calculated from the numerical solution of the Navier-Stokes equations for two-dimensional incompressible isothermal flow. The bearing geometry is defined parametrically. Three optimization problems are formulated, corresponding to: (I) a conventional smooth converging slider, (II) a parallel slider with artificial surface texturing at part of the stator surface, and (III) a parallel or converging slider with hydrophobic properties at part of the stator surface. Here, the geometry parameters, as well as the increased viscosity value and the corresponding application regime, form the problem design variables. Bearings are optimized for maximum load capacity and minimum friction coefficient. Optimal solutions are compared against corresponding ones for operation with constant viscosity. It is demonstrated that, by using spatially-varying viscosity, a substantial reduction of friction coefficient can be achieved, for all optimization problems considered. This decrease is shown to be a consequence of a sharp pressure rise in the high viscosity regime, resulting in a corresponding rise in load capacity, accompanied by a less pronounced increase in wall shear stress, and thus in total friction force.
We examine the time-dependent three-dimensional gas-particle flow in an intact wall-flow filter consisting of channels alternatively plugged at each end and a partially damaged filter in which the rear plugs are removed. Our focus is placed on highlighting the differences in the flow pattern and the deposition process between the two geometries. The Navier–Stokes equations are solved for the fluid flow coupled with a Brinkman/Forchheimmer model in order to simulate the flow in the porous walls and plugs. Discrete particle simulation is utilized to determine the nanoparticle trajectories. Using this scheme, we are able to characterize the main features of the flow fields developing in the intact and damaged filters with respect to the Reynolds number and identify those affecting the transport and deposition of particles that have three representative response times. We present fluid velocity iso-contours, which describe the flow regimes inside the channels, as well as in regions upstream and downstream of them. We provide evidence of local recirculating bubbles at the entrance of the channels and after their exit, whereas back-flow occurs in front of the rear plugs of the intact channels. We show that the flow leaves the channels as strong jets that may break up for certain flow parameters, leading to turbulence with features that depend on the presence of the rear plugs. The removal of the rear plugs affects the flow distribution, which, in turn influences the flow rates along the channels and through the walls. We describe the particle trajectories and the topology of deposited particles and show that particles follow closely the streamlines, which may cross the surface of permeable walls for both flow configurations. The distribution of deposited particles resembles the spatial variation of the through-wall flow rate, exhibiting two peak values at both ends of the intact filter channel, and one local maximum near the entrance of the damaged filter channel that is diminished at the exit. We also investigate in detail the particle deposition on the frontal face and indicate that particle accumulation at the edges of the entrance is favored for particles with low response times in flows with high fluid mass rates for both intact and damaged filters. Finally, we examine the filtration efficiency for the defective channels without rear plugs and show that fewer particles are captured as the Reynolds number is increased. A smaller reduction of the filtration efficiency is also predicted with increasing particle size.
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