Pierpaolo Puddu

A detailed analysis of the unsteady flow within a wells turbine

Sea wave energy is one of the main renewable energy resources. Its exploitation is relatively simple and determines a minimum impact on the environment. The system that is most often used for wave energy harvesting is composed of an oscillating water column device together with a Wells turbine. When designing the Wells turbine, its interaction with the oscillating water column system must be taken into account, if the energy collected is to be maximized. The most important interaction phenomenon is the so called hysteresis effect, i.e. the time delay between the piston-like motion of the air water interface and the torque developed by the turbine. This work presents a detailed analysis of the flow within an oscillating water column system, focusing on the differences in performance and in secondary flow structures between acceleration and deceleration, and between the inflow and outflow phases. This analysis demonstrates how the hysteresis between acceleration and deceleration is caused uniquely by compressibility effects within the oscillating water column system, while differences in the flow parameters and secondary structures near the rotor are negligible, if equivalent flow conditions are compared. The effects of the oscillating water column system configuration on the performance are also highlighted.

Experimental investigation of a packed bed thermal energy storage system

In this work experimental investigations on a thermal energy storage system with a solid material as storage media and air as heat transfer fluid will be presented. The experimental test rig, installed at the DIMCM of the University of Cagliari, consists of a carbon steel tank filled with freely poured alumina beads that allows investigations of heat transfer phenomena in packed beds. The aim of this work is to show the influence of the operating conditions and physical parameters on thermocline formation and, in particular, the thermal behaviour of the thermal energy storage for repeated charging and discharging cycles. Better charging efficiency is obtained for lower values of mass flow rate and maximum air temperature and for increasing aspect ratio. A decreasing influence of the metal wall with continuous operation is also highlighted. In conclusion, the analysis focuses on the thermal hysteresis phenomenon, which causes degradation of the thermocline and the reduction of the energy that can be stored by the accumulator as the repeated number of cycles increases.

3D Flow Field Measurement Around a Rotating Stall Cell

Rotating stall is an unsteady phenomenon that arises in axial and radial flow compressors. Under certain operating conditions a more or less regular cell of turbulent flow develops and propagates around the annulus at a speed lower than rotor speed. Recently little work has been devoted to the understanding of the flow field pattern inside a rotating cell. However, this knowledge could be of help in the understanding of the interaction between the cell and the surrounding flow. Such information could be extremely important during the modelling process when some hypothesis have to be made about the cell behaviour.A detailed experimental investigation has been conducted during one cell operation of an isolated low-speed axial flow compressor rotor using a slanted hot wire and an ensemble average technique based on the cell revolution time. The three flow field components have been measured on 9 axial section for 800 circumferential points and on 21 radial stations to give a complete description of the flow field upstream and downstream of the rotor. Interpretation of data can give a description of the mean flow field patterns inside and around the rotating cell.Copyright

Dynamic Behaviour of a Hydrostatic Regenerative Braking System for Public Transport Vehicles

Abstract A mathematical model to study the dynamic behaviour of a city bus equipped with a hydrostatic regenerative braking system has been developed using AMESim software. The hydrostatic circuit comprises a reversible hydrostatic unit and two hydropneumatic accumulators, connected with the conventional powertrain, according to the power assist scheme. The mathematical model of the complete system involves the development of new specific models and the modification of the standard library models with the AMESet editor. Simulations performed with the mathematical model show reductions in fuel consumption of about 20 % for the regenerative braking system under urban driving conditions.

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.

Evaluation of Reynolds Number Effects on the CFD Simulation of Downwind Sails

Experimental approach is still widely used in the design of sails for high performance yacht. Even though the testing sections of some wind tunnel could reach remarkable dimensions, yachts are too big to be experimentally tested into a wind tunnel and scaled model are used instead. In order to enforce consistency between wind tunnel and real operating conditions, Reynolds similarity should be respected, thus both geometric and flow similarity conditions must held. Enforcing Reynolds similarity on a scaled model would result in values of the incoming flow velocity, unbearable for the model structure, so that de facto experiments in wind tunnel cannot respect Reynolds similarity. The actual relevance of this approximation is still an open question and in the present paper CFD will be used as a tool to investigate the matter.

TIME-SPACE EVOLUTION OF SECONDARY FLOW STRUCTURES IN A TWO-STAGE LOW-SPEED TURBINE

The aim of this work is to highlight the unsteady effects related to wake-blade and blade rows interactions, but also the time-space evolution of secondary flow structures in a two-stage low-speed turbine model designed and constructed to perform unsteady measurements with different techniques [1]. In this case attention has been addressed to the analysis of the flow field in the first stage of the turbine model. Measurements are performed with aerodynamic probes downstream of the first stator and using a single slanted hot-wire anemometer downstream of the first rotor. Time-dependent relative flow field downstream of the first rotor (obtained from phase-locked averaging technique) have been reconstructed for different relative positions between stator and rotor blades. From these results the time-dependent secondary flow vectors have been obtained as well. The mean reference flow used to determine the secondary flow structure has been evaluated for each frame by mass-averaged technique. The evolution of the secondary flow structure due to the influence of the upstream and downstream stators on the first rotor has been investigated. The main unsteady effects put in evidence the variation of the intensity and spatial extension of the vortex flow structure.Copyright

Experimental Study on the Operation of an Industrial FC Fan

Centrifugal forward curved (FC) blade fans are widely used in air ventilation and conditioning and are by far the largest fan category of all. Their success is mainly due to low cost, compactness, and low noise operation. Their peak efficiency, however, falls typically in the range 50%-70%, somewhat lower than that of other centrifugal and axial fans. The vast numbers produced and sold worldwide and the cumulative effect of so many low efficiency installations raises the question of whether it is possible to enhance their efficiency. The present paper is concerned with the investigation of the three dimensional flow field in a double inlet centrifugal industrial FC fan commonly used for handling air. An open loop facility has been designed and constructed at the Department of Mechanical Engineering at Cagliari University to house, drive and test the fan. Its operation will be studied with the aid of hot-wire probes.