1,721,006 research outputs found
Application of a physics-based model to predict the performance curves of pumps as turbines
No full textThis paper presents the application of a physics-based simulation model, aimed at predicting the performance curves of pumps as turbines (PATs) based on the performance curves of the respective pump. The simulation model implements the equations for estimating head, power and efficiency for both direct and reverse operation. Model tuning on a given machine is performed by using loss coefficients and specific parameters identified by means of an optimization procedure, which simultaneously optimizes both the pump and PAT operation. The simulation model is calibrated in this paper on data taken from the literature, reporting both pump and PAT performance curves for head and efficiency over the entire range of operation. The performance data refer to twelve different centrifugal pumps, running in both pump and PAT mode. The accuracy of the predictions of the physics-based simulation model is quantitatively assessed against both pump and PAT performance curves and best efficiency point. Prediction consistency from a physical point of view is also evaluated
Prediction of compressor efficiency by means of Bayesian Hierarchical Models
No full textThe prediction of time evolution of gas turbine performance is an emerging requirement of modern prognostics and health management systems, aimed at improving system reliability and availability, while reducing life cycle costs. In this work, a data-driven Bayesian Hierarchical Model (BHM) is employed to perform a probabilistic prediction of gas turbine future behavior. The BHM approach is applied to field data, taken from the literature and representative of gas turbine degradation over time for a time frame of 7-9 years. The predicted variable is compressor efficiency collected from three power plants characterized by high degradation rate. The capabilities of the BHM prognostic method are assessed by considering two different forecasting approaches, i.e. single-step and multi-step forecast. For the considered field data, the prediction accuracy is very high for both approaches. In fact, the average values of the prediction errors are lower than 0.3% for single-step prediction and lower than 0.6% for multi- step prediction
Performance evaluation of a metal foam heat sink for solar hybrid panels
No full textIn the present work a metal foam heat sink is presented, which is suitable for hybrid solar panels. The heat sink is designed to be placed in contact with a layer of photovoltaic cells, removing from them the heat resulting from the solar radiation. It is composed of an aluminum box containing an aluminum open-cell foam, through which a water flow is forced. Some preliminary measurements are presented. In particular the pressure drops between the inlet and outlet water plena and the global heat transfer effectiveness are shown under different conditions
Biomass integrated gas turbine and ORC combined cycle: Layout and performance analysis
No full textStarting from a previously proposed Integrated Gasification Combined Cycle power plant with an externally fired gas turbine and a bottoming heat recovery steam generator, in this paper a new plant scheme is proposed. The main changes concern, on one hand, a plant layout simplification by removing a regenerative heat exchanger, on the other hand, the replacement of the Water Rankine Cycle with an Organic one. The thermodynamic model of the entire system has been developed by means of the Cycle-Tempo software. The gasification process has been supposed to occur at ambient pressure and air is used as gasifying agent. Moreover, considering the small size (below 1 MW) of this Combined Cycle power plant, the new configuration embodying an Organic Rankine Cycle appears to be more suited for the biomass conversion process even though shows a slightly lower conversion efficiency
Minimization of the primary energy consumption of residential users connected by means of an energy grid
No full textIn this paper, a physics-based model is developed to simulate the interaction between residential users and energy systems. The simulation model is coupled with a dynamic programming algorithm which identifies the optimal operation strategy that allows the minimization of the primary energy consumption of three residential users, arranged with different energy system configurations. The reference scenario, which considers that the users employ a domestic boiler for meeting thermal energy demand, while electric energy is taken from the national electric grid, is compared to the CHP scenario, this latter being differentiated by considering shared thermal and electric energy storages and also shared PM. The most suitable energy system configuration is identified by jointly evaluating primary energy consumption, prime mover working hours and thermal and electric energy share of the prime mover itself
Dependency of the slip phenomenon on the inertial forces inside radial runners
Full text linkThe slip phenomenon consists in the deviation of the fluid fl ow (r elative ve locity ve ctor) wi th re spect to th e blade congruent angles and is mainly due to the finite number of blades. For this reason, slip becomes significant in pumps operating as turbines (PaTs) being characterized by a lower number of blades compared to conventional turbines highlighting a shortcoming of these devices in energy recovery applications. Even though this topic has been widely investigated in the past for centrifugal pumps, it has been often neglected for hydraulic turbine applications. As described in the literature, a counter rotating vortex (known as eddy vortex) develops inside each vane of a rotating machine and its effect can be superimposed to the main flow characteristics. Moreover, the relative vorticity magnitude, which is twice the angular velocity under the hypothesis of inviscid, incompressible and irrotational flow, is constant regardless of the number of vanes. In this work, 3D inviscid steady flow numerical simulations of a purely radial impeller with zero-thickness blades, being designed according to a logarithmic spiral law, have been carried out with the purpose of bringing out the relationship between the flow deviation and the number of blades, neglecting viscous effects which could hinder the inertial ones. The results show a local deviation of the streamline downstream of the trailing edge when the flow is not confined by the blades. The effect of the flow deviation has been also evaluated by calculating the hydraulic performance of the runners. Four different runners have been investigated (with 28, 14, 7 and 3 blades) at their design point and rotating at two different angular velocities. This allowed to correlate the deviation with the inertial effect and to propose a least-squares fitting curve. As shown in previous works, the inclusion of the slip correction factor in 1-D PaT performance prediction models enhances their accuracy
Pump as turbine for throttling energy recovery in water distribution networks
Full text linkNowadays, a great effort is increasingly put by the scientific community into a more sustainable energy management, which requires a higher harvesting of renewable energy sources with respect to conventional ones. In the framework of distributed electricity production, Pumps as Turbines (PaTs), i.e. pumps operated in reverse mode, are becoming more and more tempting, being very cost effective with respect to customized hydro turbines. For instance, Water Distribution Networks (WDNs) are equipped by pressure relief valves (PRVs) in order to regulate flow rates and to reduce leakages. The replacement of PRVs with PaTs could be a feasible practice to achieve both an effective pressure control and a throttling energy recovery. The preliminary identification of specific speed of a pump to be used as a turbine is fundamental in order to find the best suitable solution. However, the insertion of a PaT must consider the variability of water demand and pressure patterns. The hydraulic variability in a water distribution network does not permit to define a unique operating point for a PaT and this aspect is a further obstacle for the functional planning of such a system. In this framework, the present work aims at proposing a methodology to find the more suitable PaT for a specific WDN, starting from the analysis of the pressure and flow rate patterns. The methodology is based on the selection of an existing machine from a pump catalogue. Then, knowing its geometrical information, it is possible to predict the characteristic curve of the pump operating as turbine by using a 1-D performance prediction model. The WDN of a town in the Apulia region (Southern Italy) has been used as a case study, in order to select a PaT useful for throttling energy recovery. Finally, a techno-economic evaluation has been carried out
A simplified model for the prediction of energy consumption in large-scale commercial activities
No full textEnergy saving has gained increasing attention because it represents a strategy for the reduction of pollutant emissions and dependence on fossil fuels. Nevertheless, many human activities are still energy intensive, thus requiring suitable energy efficiency measures. For this purpose, Energy Service COmpanies (ESCOs) propose solutions for industrial, commercial and residential activities with the aim to reduce energy consumptions. In terms of energy saving, the contribution of ESCO activity is often estimated as the difference between the measured consumptions and a reference one that would have occurred in the same operating conditions, if no modifications were made. When the ESCO intervention leads to a saving, both ESCO and customer share the earnings; otherwise, the ESCO must pay some penalties. As a result, the remuneration system is affected by the reference consumption modeling. This aspect is crucial especially for supermarkets, whose energy consumption might depend on many external factors, such as: climatic variables, thermal performance of the building, customer attendance, etc. Supermarkets require a consumption model for the estimation of the energy saving that must be based on the data available at the contract signature. In attrition, the saving estimation procedure has to be readily understandable by the customer, who often has little experience in the specific field. In this paper, three alternative models are proposed to perform the consumption prediction on monthly basis on 9 case studies. As a result, the maximum average annual error was around 8%, while other supermarkets often show average errors between 4% and 5%. Finally, robustness of provided guidelines is tested on an expanded case study, confirming the obtained results
CFD analysis of the squeeze film damping mechanism in the first stage of servovalves
Full text linkA fundamental component of two-stage servovalves is the flexure tube, since it both constitutes a low friction pivot for the inner flapper and allows the torque motor to be separated from the hydraulic fluid, thus avoiding contamination particles being trapped inside the torque motor. The inertia of the torque motor armature interacting with the flexure tube stiffness gives lightly damped resonances, which may lead to fatigue failure due to excessive bending under vibration, as well as limiting the position control bandwidth of the main spool. This effect is counteracted by the film of liquid interposed between the flapper and the flexure tube, which is "squeezed" during the flapper motion providing damping. However, the underlying physics of the damping mechanism caused by the squeeze film inside the flapper-flexure tube system is not well-understood, and to date the scientific literature has lacked analyses and investigations aimed at providing insights into this phenomenon. Because of this, the aim of this paper is to develop a reliable CFD model which can help to understand where and how the damping forces are generated during the flapper motion because of the squeeze film. The developed model could be used in further investigations, aimed, for example, at studying the effects of fluid properties and geometric parameters upon the damping factor, in order to achieve more effective designs which can enhance the damping factor in the flapper-flexure tube system of new generation servovalves. This work has been carried out as a collaboration between the University of Bath and the Polytechnic University of Bari, and Moog Controls ltd (Tewkesbury, UK), a world leading manufacturer of servovalves
Performance characterization of a wells turbine under unsteady flow conditions
Full text linkA small prototype of a Wells turbine has been built by means of 3D printing technology and tested in the test rig facility of the Polytechnic University of Bari under steady-state and pulsating flow conditions. The Wells turbine is crossed by an unidirectional air flow rate that is varied from zero to the maximum by precisely controlling the rotational speed of the suction fan. The Wells turbine is a scaled prototype designed to operate in a 1:10 scaled model of a REWEC3 breakwater for ocean application. The Wells turbine characteristics are evaluated in terms of torque coefficient and pressure drop coefficient vs. flow coefficient. A delayed onset of stall can be observed, with a clockwise hysteretic loop, when the turbine experiences large sinusoidal variation of the flow coefficient at high mass flow rates. The variation of the turbine performance under dynamic flow conditions is crucial for a correct design of the Wells turbine
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