Experimental and computational investigation of a new solar integrated collector storage system

The paper discusses a combined experimental-numerical analysis of an innovative solar thermal device to be used as an Integrated Collector Storage (ICS) system providing domestic hot water. In this equipment the collector acts also as a storage unit, without requiring an external vessel. Due to its simple configuration, the ICS device was successfully used in several circumstances, especially in extreme situations such as in post-earthquake tent cities or to reach remote users in Africa. In order to assess the efficiency of this collector, the draw-off process was investigated measuring the value of the mean temperature of the water discharging from the tap as cold water filled the collector. In the present configuration the draw-off is not completely optimised and a detailed analysis was carried out in order to investigate the mixing of cold and hot water in the solar collector during the discharge phase. A series of thermocouples was placed in selected positions around the shield of the collector to investigate the evolution of the near wall temperature. Furthermore, a numerical analysis based on Large-Eddy Simulation (LES) of the mixing process inside the collector was carried out using an open source, in-house, finite-volume computational code. Even if some restrictive hypotheses were made on the thermal boundary conditions and the absence of stratification, the LES results gave interesting findings to improve the collector performance

Unsteady cfd analysis of erosion mechanism in the coolant channels of a rotating gas turbine blade

The two-phase flow in a rotating wedge mimicking the final portion of a blade turbine internal cooling channel is here presented and discussed focusing on unsteady motion and erosion mechanisms. The rotation axis is placed to properly reproduce a configuration with a very strong deviation (90°). The flow field was modelled by using the well known k---f unsteady-RANS model based on the elliptic-relaxation concept. The model was modified by some of the authors to take into account the influence of turbulence anisotropy as well as rotation. The model was applied to the well-established and fully validated T-FlowS code. A systematic comparison of rotating and non-rotating case was carried out to show the influence of Coriolis force on flow and erosion mechanisms. The rotational effects strongly changed the flow behaviour within the channel, affecting both the unsteady flow and the particles trajectories. In the rotating case, there is no recirculation on the tip region; besides, position of the small recirculation regions above each pedestals change. These, and other minor effects, affect the particle motion thus resulting in a different erosion pattern

A geometric characterization of minimal codes and their asymptotic performance

In this paper, we give a geometric characterization of minimal linear codes. In particular, we relate minimal linear codes to cutting blocking sets, introduced in a recent paper by Bonini and Borello. Using this characterization, we derive some bounds on the length and the distance of minimal codes, according to their dimension and the underlying field size. Furthermore, we show that the family of minimal codes is asymptotically good. Finally, we provide some geometrical constructions of minimal codes.Comment: 22 page

Erosion Prediction of Gas Turbine Compressor Blades Subjected to Water Washing Process

Technical BriefsBlade fouling is a relevant problem in turbomachinery applications. It affects both compressors and turbines. In the first case, fouling can be generated by the presence of dust, ashes or brackish air (in offshore applications). In turbines, fouling is mainly generated by residual of combustion process. Blade fouling generally leads to a reduction of the performance due to an increase on profile losses. Here we focus on the fouling due to salt deposition on naval/off-shore applications referring to machines that are part of the fleet of gas turbines manufacturers. In such applications, it is common to introduce on-line washing devices aiming at removing fouling from the early stages of the compressors. The water is sprayed upstream of the first rotor, it impacts on the rotor blades and thus dissolving the deposited salt. However, this procedure possibly leads to blade erosion and/or corrosion. A clear comprehension of the erosion mechanism is the main objective of the present work. To this end, we propose an integrated multi-phase CFD tool. The multi-phase flow is analyzed by adopting a one-way coupling, thus assuming water droplets to be drag by the carrier flow without influencing the main flow. The droplets are dispersed and tracked singularly by adopting a Lagrangian approach. As for the erosion, well-known and widely accepted models are used. The capability of a Lagrangian code, P-Track, developed and validated at the Department of Mechanical & Aerospace Engineering, Sapienza University in Rome, is presented. The code is able to predict the droplets trajectories, as well as to simulate the impact on the solid walls and the erosion mechanism. Simulations were performed using 25 and 100 ?m droplet size. Results, expressed in terms of normalized erosion rate, show the erosion patterns and erosive effect of the two size classes. Erosive capacity is proportional to droplet size, and the most eroded part of the blade is the leading edge, which is in qualitative agreement with measurements

Large-eddy simulation of the aerodynamic and aeroacoustic performance of a ventilation fan

There are controversial requirements involved in developing numerical methodologies in order to compute the flow in industrial fans. The full resolution of turbulence spectrum in such high-Reynolds number flow configurations entails unreasonably expensive computational costs. The authors applied the study to a large unidirectional axial flow fan unit for tunnel ventilation to operate in the forward direction under ambient conditions. This delivered cooling air to the tunnel under routine operation, or hot gases at 400∘C under emergency conditions in the event of a tunnel fire. The simulations were carried out using the open source code OpenFOAM, within which they implemented a very large eddy simulation (VLES) based on one-equation SGS model to solve a transport equation for the modelled (subgrid) turbulent kinetic energy. This subgrid turbulence model improvement is a remedial strategy in VLES of high-Reynolds number industrial flows which are able to tackle the turbulence spectrum's well-known insufficient resolution. The VLES of the industrial fan permits detecting the unsteady topology of the rotor flow. This paper explores the evolution of secondary flow phenomena and speculates on its influence on the actual load capability when operating at peak-pressure condition. Predicted noise emissions, in terms of sound pressure level spectra, are also compared with experimental results and found to agree within the uncertainty of the measurements

Erosion Prediction of Gas Turbine Compressor Blades Subjected to Water Washing Process

Technical BriefsBlade fouling is a relevant problem in turbomachinery applications. It affects both compressors and turbines. In the first case, fouling can be generated by the presence of dust, ashes or brackish air (in offshore applications). In turbines, fouling is mainly generated by residual of combustion process. Blade fouling generally leads to a reduction of the performance due to an increase on profile losses. Here we focus on the fouling due to salt deposition on naval/off-shore applications referring to machines that are part of the fleet of gas turbines manufacturers. In such applications, it is common to introduce on-line washing devices aiming at removing fouling from the early stages of the compressors. The water is sprayed upstream of the first rotor, it impacts on the rotor blades and thus dissolving the deposited salt. However, this procedure possibly leads to blade erosion and/or corrosion. A clear comprehension of the erosion mechanism is the main objective of the present work. To this end, we propose an integrated multi-phase CFD tool. The multi-phase flow is analyzed by adopting a one-way coupling, thus assuming water droplets to be drag by the carrier flow without influencing the main flow. The droplets are dispersed and tracked singularly by adopting a Lagrangian approach. As for the erosion, well-known and widely accepted models are used. The capability of a Lagrangian code, P-Track, developed and validated at the Department of Mechanical & Aerospace Engineering, Sapienza University in Rome, is presented. The code is able to predict the droplets trajectories, as well as to simulate the impact on the solid walls and the erosion mechanism. Simulations were performed using 25 and 100 ?m droplet size. Results, expressed in terms of normalized erosion rate, show the erosion patterns and erosive effect of the two size classes. Erosive capacity is proportional to droplet size, and the most eroded part of the blade is the leading edge, which is in qualitative agreement with measurements

Graphene-Based Membrane Technology: Reaching Out to the Oil and Gas Industry

This paper presents a critical review and the state of the art of graphene porous membranes, a brand-new technology and backdrop to discuss its potential application for efficient water desalination in low salinity water injection (LSWI). LSWI technology consists in injecting designed, adequately modified, filtered water to maximize oil production. To this end, desalination technologies already available can be further optimized, for example, via graphene membranes, to achieve greater efficiency in water-oil displacement. Theoretical and experimental applications of graphene porous membranes in water desalination have shown promising results over the last 5-6 years. Needless to say, improvements are still needed before graphene porous membranes become readily available. However, the present work simply sets out to demonstrate, at least in principle, the practical potential graphene membranes would have in hydrocarbon recovery processes

Axonal plasticity in response to active forces generated through magnetic nano-pulling

Mechanical force is crucial in guiding axon outgrowth before and after synapse formation. This process is referred to as "stretch growth." However, how neurons transduce mechanical input into signaling pathways remains poorly understood. Another open question is how stretch growth is coupled in time with the intercalated addition of new mass along the entire axon. Here, we demonstrate that active mechanical force generated by magnetic nano-pulling induces remodeling of the axonal cytoskeleton. Specifically, the increase in the axonal density of microtubules induced by nano-pulling leads to an accumulation of organelles and signaling vesicles, which, in turn, promotes local translation by increasing the probability of assembly of the "translation factories." Modulation of axonal transport and local translation sustains enhanced axon outgrowth and synapse maturation