A COMPARISON BETWEEN EXPERIMENTAL AND NUMERICAL ANALYSIS OF A WELLS TURBINE

Wave energy is one of the renewable energy sources with the highest potential. Several pilot plants have been built based on the principle of the Oscillating Water Column (OWC). Among the different solutions that have been suggested, the Wells turbine has gained particular attention due to its simplicity and reliability. The majority of available studies concentrate on the steady operation of the Wells turbine, while only few analyze its performance under an unsteady and bi-directional air flow, as determined by the presence of the OWC system. In this work, experimental and numerical performance of a high-solidity Wells turbine with NACA0015 profiles under the bi-directional flow generated by a hydraulic piston is compared. The numerical simulations have been conducted using commercial CFD software and focus on unsteady predictions, with particular attention to the behavior of the flow upstream and down-stream of the rotor, flow hysteresis between acceleration and deceleration phases and differences between intake and exhaust strokes due to the non-symmetrical configuration of the machine

Aerodynamic Characterization of a Wells Turbine Under Bi-Directional Airflow

Abstract This work deals with the experimental study of the flow in a Wells turbine submitted to an unsteady and bi-directional airflow. The investigations were carried out on an experimental set-up that can simulate the real operating conditions of a wave energy conversion device using a two-dimensional hot-wire anemometer probe to analyse the flow field upstream and downstream of the turbine during its non-stationary operation. In addition to local measurements, the position of the piston that simulates the wave motion, the driving torque and the turbine rotational speed were also measured. These surveys allowed determination of the turbine instantaneous performances by analysing the aerodynamic flow characteristic at mid-span in the blade-to-blade plane downstream of the rotor. The flow distribution was obtained for the phase of inflow and outflow at different values of rotational speed which was kept constant during data acquisition. The results showed asymmetric behaviour for the two phases of intake and exhaust stroke of the piston and during acceleration and deceleration of the flow. The real entity of the hysteresis phenomenon that arose during the phases of acceleration and deceleration of the unsteady flow was evaluated considering velocity distribution in close proximity of the rotor

Some Considerations about the Rotating Cell Structure

Rotating stall instability in axial flow compressors arises when the mass flow is reduced at constant rotational speed. Despite the number of experimental and theoretical work already published in the scientific literature, many questions still remain unanswered. A complete model that could be of help both in the design process and in the modelling process of existing engines is not yet available. A fundamental research program has been carried out at the Department of Mechanical Engineering of the University of Cagliari with the scope of reaching a better understanding of the basic flow field structures and of the global performance in stall. The cinematic structure of the cell during abrupt full 1-cell stall in a two-stage axial flow compressor with IGV has been analysed with the aid of hot-wire anemometer and of a total pressure probe. The results have revealed interesting features about the cell flow structures and their variations along the stalled performance branch of the compressor. The present paper aims at pursuing further the flow analysis including the investigation of upstream flow field. Therefore, the inlet duct has been extended to perform the measurements. The complete flow field measurements will allow to obtain a complete cinematic description of the cell

Numerical Investigation of a Packed Bed Thermal Energy Storage System with Different Heat Transfer Fluids

Abstract This paper presents the results of a numerical investigation on the transient behaviour of a packed bed thermal storage unit using different fluids: oil, molten salt and air. The storage material consists of loosely spherical particles of alumina packed in a reservoir wherein the heat transport fluid flows from the top to the bottom in the charging phase, and in the opposite way in the discharging phase. The process of charge/discharge of the storage system gives rise to a typical temperature distribution along the flow direction defined "thermocline". The main objective of this work is to analyze the temperature distribution along the storage system and the formation of the thermocline for repetitive consecutive cycles, evaluating the progressive reduction of the stored energy in the solid material for every new cycle. The numerical investigation is based on a two-phase one-dimensional modified Schumann model, where thermodynamic properties of the fluid are temperature dependent

A Study of a Packed-bed Thermal Energy Storage Device: Test Rig, Experimental and Numerical Results☆

Abstract This paper presents the experimental set-up built at the DIMCM of Cagliari University to study a thermal energy storage (TES) system based on alumina beads freely poured into a carbon steel tank using air as heat transfer fluid. The system is instrumented with several thermocouples to detect axial and radial temperature distribution as well as reservoir wall temperature. Experimental temperature distribution along the storage system was compared with the numerical ones obtained by a two-phase one dimensional Schumann model. Numerical results show good agreement with the experimental results if thermal properties are considered as temperature dependent and the experimental temperature profile at the top of the bed is used for simulations

Thermo-fluid Dynamic Analysis of a CSP Solar Field Line During Transient Operation

Abstract Concentrating Solar Power (CSP) technology allows to produce high temperature thermal energy from solar radiation. The thermal energy can be converted into electricity or it can be directly used for industrial processes. Most of the available simulation models of CSP plants evaluate the behavior of the solar field in stationary conditions, neglecting transient thermo-fluid-dynamic effects. Nevertheless, the study of the dynamic behavior of the solar field is a very challenging and interesting task and allows obtaining useful information for the design and the effective management strategies of CSP plants. This paper presents a thermo-fluid-dynamic analysis of asolar field line of the CSP plant currently under construction in Ottana, Sardinia (Italy), which uses thermal oil as heat transfer fluid. Dynamics of the system due to solar irradiance variations have been evaluated by using an axisymmetric unsteady 2D numerical model developed in Comsol® to evaluate the oil temperature distribution along the receiver tube for different operating conditions. The results have been compared with those obtained with a simpler, non-stationary one-dimensional model, developed in Matlab® environment. The comparative analysis show very similar results for the two models and demonstrate that the dynamic effects on the temperature distribution along the solar field line are not negligible

Numerical Evaluation of Entropy Generation in Isolated Airfoils and Wells Turbines

In recent years, a number of authors have studied entropy generation in Wells turbines. This is potentially a very interesting topic, as it can provide important insights into the irreversibilities of the system, as well as a methodology for identifying, and possibly minimizing, the main sources of loss. Unfortunately, the approach used in these studies contains some crude simplifications that lead to a severe underestimation of entropy generation and, more importantly, to misleading conclusions. This paper contains a re-examination of the mechanisms for entropy generation in fluid flow, with a particular emphasis on RANS equations. An appropriate methodology for estimating entropy generation in isolated airfoils and Wells turbines is presented. Results are verified for different flow conditions, and a comparison with theoretical values is presented.Regione Autonoma Sardegna (funding for University of Cagliari co-authors

Correction to: Numerical evaluation of entropy generation in isolated airfoils and Wells turbines

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Multipotent stem cells of mother's milk

In recent years the presence of stem cells (hBSCs: human breastmilkderived stem cells) and epithelial progenitors has been demonstrated in mother’s milk (MM). Stem cells present in samples of fresh MM exhibit a high degree of vitality and this makes possible the performance of cell cultures and to evaluate the differentiation capacity of the hBSCs. The most important datum that expresses the enormous potential of the use of MM stem cells is the presence of a cell population capable of differentiating into the three mesoderm, endoderm and ectoderm lines. The small number of studies and MM samples analyzed and the different sampling methods applied suggest standardization in the collection, analysis and culture of MM in future studies, in consideration of the well-known extreme variability of MM composition, also from the standpoint of cells. The analysis of literature data confirms the uniqueness of MM and its enormous potential