Cavitation erosion damage of scroll steel plates by high-speed gas working fluid

A steel plate is one of the critical components of a scroll expander system that can experience cavitation micro-pitting while in service. The content of the present paper consists of two distinct but interrelated parts. The first part aims to highlight that the use of Computational Fluid Dynamics (CFD) simulations in conjunction with experimental measurements can constitute a quite promising tool for the prediction of cavitation erosion areas in scroll expander systems. For this purpose a three-dimensional CFD, steady state numerical simulation of the refrigerant working fluid is employed. Numerical results revealed the critical areas where cavitation bubbles are formed. These numerical critical areas are in direct qualitative agreement with the actual eroded regions by cavitation, which were found by microscopic observations across the steel plate on an after use, scroll expander system. The second part of the paper, aims to further investigate the behaviour and the durability of the steel plate of the studied scroll expander system subjected to cavitation erosion by using an ultrasonic experimental test rig. Scanning Electron Microscopy (SEM) and optical interferometer micrographs of the damaged surfaces were observed, showing the nature of the cavitation erosion mechanism and the morphological alterations of the steel plate samples. Experimental results are explained in terms of the cavitation erosion rates, roughness profile, accumulated strain energy, and hardness of the matrix. The experimental study can serve as a valuable input for future development of a CFD numerical model that predicts both cavitation bubbles formation as well as cavitation damage induced by the bubbles that implode on the steels plates

Optimization for time-driven link sleeping reconfigurations in ISP backbone networks

Backbone network energy efficiency has recently become a primary concern for Internet Service Providers and regulators. The common solutions for energy conservation in such an environment include sleep mode reconfigurations and rate adaptation at network devices when the traffic volume is low. It has been observed that many ISP networks exhibit regular traffic dynamicity patterns which can be exploited for practical time-driven link sleeping configurations. In this work, we propose a joint optimization algorithm to compute the reduced network topology and its actual configuration duration during daily operations. The main idea is first to intelligently remove network links using a greedy heuristic, without causing network congestion during off-peak time. Following that, a robust algorithm is applied to determine the window size of the configuration duration of the reduced topology, making sure that a unified configuration with optimized energy efficiency performance can be enforced exactly at the same time period on a daily basis. Our algorithm was evaluated using on a Point-of-Presence representation of the GÉANT network and its real traffic matrices. According to our simulation results, the reduced network topology obtained is able to achieve 18.6% energy reduction during that period without causing significant network performance deterioration. The contribution from this work is a practical but efficient approach for energy savings in ISP networks, which can be directly deployed on legacy routing platforms without requiring any protocol extension. © 2012 IEEE

Numerical investigation of liquid film instabilities and evaporation in confined oscillating slug-plug flows.

An enhanced volume of fluid (VOF)-based numerical simulation framework that accounts for conjugate heat transfer between solid and two-phase flow regions and phase-change due to boiling/condensation, is utilised in order to investigate the effect of flow oscillation amplitude and frequency on the liquid film evaporation and instability formation in slug-plug flows within heated channels, in saturated flow boiling conditions. Various series of parametric numerical simulations are performed, for different values of flow oscillation amplitude and frequency for a variety of working fluids. For one of the working fluids two different channel diameters are also tested. The oscillations in each case are induced by applying an oscillating pressure boundary condition at the inlet of the channel, keeping the pressure constant at the outlet, after an initial period of constant pressure drop between the inlet and the outlet. Capillary ridges that are initiated at the liquid film, in the vicinity of the leading edge of the considered vapour slugs, are identified as a result of the imposed oscillations, which are translated in the form of capillary waves towards the rear end of the bubbles. It is shown that the formation frequency as well as the geometric characteristics of the generated ridges, are directly related to the corresponding frequency and amplitude of the induced flow oscillations. Furthermore, it is shown that in the initial stages of the bubble fate after the application of the oscillations liquid film evaporation is enhanced with the increase of the oscillation amplitude while it degrades as the frequency of the oscillation becomes higher. However, for large oscillation amplitudes and channel diameters, liquid jets penetrate into the elongated bubbles leading in a lot of cases to bubble break-up

New High-Speed a-Si/c-Si- and a-SiC/c-Si-Based Switches

The electrical and optical characteristics of the new high-speed Al/a-Si/c-Si(p)/c-Si(n+)/Al and Al/a- SiC/c-Si(p)/c-Si(n+)/Al optically controlled switches are presented in this paper. These switches exhibit the lowest ever reported values of rise and fall times, for this kind of switches, of about 3ns. They also exhibit a temperature and light reversibly controlled forward breakover voltage (VBF), together with high values of light triggering sensitivity

Joint Optimization of Intra- and Inter-Autonomous System Traffic Engineering

Traffic Engineering (TE) is used to optimize IP operational network performance. The existing literature generally considers intra- and inter-AS (Autonomous System) TE independently. However, the overall network performance may not be truly optimized when these aspects are considered separately. This is due to the interaction between intra- and inter-AS TE, where a solution of intra-AS TE may not be a good input to inter-AS TE and vice versa. To remedy this situation, we propose considering intra-AS aspects during inter-AS TE and vice versa. We propose a joint optimization of intra- and inter-AS TE to further improve the overall network performance by simultaneously finding the best egress points for the inter-AS traffic and the best routing scheme for the intra-AS traffic. Three strategies are presented to attack the problem, namely sequential, nested and integrated optimization. Our simulation study shows that, compared to sequential and nested optimization, integrated optimization can significantly improve the overall network performance by accommodating 30%-60% more traffic demands

The Effect of Hydraulic Diameter on Flow Boiling within Single Rectangular Microchannels and Comparison of Heat Sink Configuration of a Single and Multiple Microchannels

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW/m2 and 150 to 2400 kg/m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms