1,720,977 research outputs found
Thermal Desalination Through Forward Osmosis Coupled With CO2-Mixture Power Cycles for CSP Applications
Full text linkThis work, performed in the framework of the H2020 EU project “DESOLINATION”, analyses the coupling between CSP plants using transcritical power cycles with CO2-mixtures and an innovative thermal desalination technique based on Forward Osmosis. Calculations are presented for a large scale CSP plant with central tower receiver and direct storage with solar salts in Dubai, adopting the mixtures CO2+SO2 and CO2+C6F6 in the power cycles. The heat rejected from the cycle condenser is recovered directly by the FO plant, where the draw solute is heated up from 40 °C to 76 °C, to allow for the regeneration of the draw solution used in the forward osmosis membrane. The thermo-responsive polymer adopted is PAGB2000, already considered in literature as a promising option. Results show a very effective synergy between the electricity and the freshwater production: high yearly solar to electric efficiencies are possible (around 19%), with a low freshwater specific thermal consumption (around 100 kWhth/m3). The proposed desalination method is more effective than a conventional MED system (with +50% of yearly freshwater produced), while a larger solar field (+28% in surface area) is necessary for a PV+RO plant to produce annually both the energy and freshwater produced by the CSP+FO plants
Experimental study of aqueous foam generation and transport in a horizontal pipe for deliquification purposes
No full textThis work presents and analyses the results of experimental activities aimed at a preliminary characterization of foamy flows for pipeline dewatering, in order to assess whether the addition of surfactants may effectively reduce the liquid holdup in horizontal pipelines. Static tests were run to compare the foam cycle (generation and decay) for three commercial surfactants and to choose the most suitable one. Dynamic tests with the selected product were performed in a 20 m long, 60 mm i.d. Plexiglas® pipe, where a 0.3% wt. solution of surfactant in tap water was pumped after mixing with an air flow at nearly atmospheric pressure and temperature. Superficial velocities ranged between 0.03 m/s and 0.05 m/s for water and between 1.5 m/s and 11.5 m/s for air, which would determine stratified/stratified wavy flows in the case of pure water-air flow, i.e. the benchmark case. Due to the presence of the surfactant, foam formed in the mixing section, which implied a significant change in the flow patterns that were photographically recorded and classified into three main types: plug, stratified wavy and stratified with foam entrainment, as far as the air superficial velocity was increased at constant water superficial velocity. The associated pressure drop, linearly distributed along the pipeline, resulted greater than the benchmark value in all the operating conditions, with a dramatic increase (even more than 100%) for plug flows. On the other hand, the percentage relative difference was found to lower with increasing the air superficial velocity, apart for stratified wavy flows where it seemed to keep constant at about 3.3%. Finally, a theoretical model for stratified flows was used to relate the pressure drop to the void fraction in order to get at least an approximate indication of the liquid load reduction due to the surfactant addition, which ranged between 6% and 39%
Experimental study on coalescer efficiency for liquid-liquid separation
Full text linkThe global community acknowledges water demand and accessibility as major challenges impacting human well-being. Forward Osmosis (FO) desalination coupled with concentrate solar power might represent a promising solution to combine water production with renewable sources. This work assesses the performance of a liquid-liquid separator (coalescer), an important component of the FO process, when using a polymeric thermo-responsive draw agent (PAGB2000). Experimental characterization of the coalescer is carried out for different regeneration temperatures (from 50 to 80 °C), residence time, draw concentration (from 0.30 to 0.60) and metal meshes. The separation efficiency of the coalescer can be as high as 95% for high residence time and regeneration temperatures (> 70 °C). Eventually, an analytical expression of the coalescer efficiency as function of the main operating parameters is proposed both to support desalination plant design and to enable understanding its applicability beyond its original context
Two-phase stratified flow in horizontal pipes: A CFD study to improve prediction of pressure gradient and void fraction
Full text linkImproved understanding of flow regime effects on design-influencing engineering quantities is of primary importance. This work is focused on the numerical prediction of pressure gradient and void fraction in a horizontal pipe where gas-liquid stratified flow is present in different operating conditions. The problem was modeled with unsteady, multiphase CFD (computational fluid dynamics) simulations. Volume Of Fluid (VOF) method was used as multiphase model. To define the numerical methodology, this study provides details on the influence of discretization grid and turbulence model on the simulation accuracy. It shows that mesh density on pipe cross-section is the most important grid parameter to focus on. Different turbulence models are required depending on the gas velocity and on its turbulence flow regime. Transition SST is able to model all the operating conditions but Realizable k-ε is adopted to further increase the accuracy of the results. A general underestimation of the pressure gradient is reported with an average error of − 6.43 % and − 16.21 % for a liquid superficial velocity of 0.04 m/s and 0.06 m/s respectively. Comparison with two-fluid 1D models shows that CFD simulations are the most accurate tools for predicting the pressure gradient at gas superficial velocities lower than 1.3 m/s. The implementation of a drift flux model shows a good agreement between experimental results and CFD simulations concerning void fraction estimation. CFD results are also used to underline the physical phenomena limiting the performance of 1D models
Horizontal Stratified Air–Foam–Water Flows: Preliminary Modelling Attempts with OLGA
Full text linkWater accumulation is a major problem in the flow assurance of gas pipelines. To limit liquid loading issues, deliquification by means of surfactant injection is a promising alternative to the consolidated mechanical methods. However, the macroscopic behavior of foam pipe flow in the presence of other phases has barely been explored. The goal of this work was to propose an approach to simulate air–water–foam flows in horizontal pipes using OLGA by Schlumberger, an industry standard tool for the transient simulation of multiphase flow. The simulation results were compared with experimental data for 60 mm and 30 mm ID (Inner Diameter) horizontal pipelines. Preliminary validation for two-phase air–water flow was carried out, which showed that correct flow pattern recognition is essential to accurately reproduce the experimental data. Then, stratified air–foam–water flows were investigated, assuming different models for the foam local velocity distribution. Foam rheology was considered through the Herschel–Bulkley model with the yield stress varying in time due to foam decay. The results showed good agreement for a uniform velocity profile and fresh foam properties in the case of the 60 mm ID pipeline, whereas for the 30 mm ID, which was characterized by significantly higher velocities, a linear velocity profile and 2000 s foam aging provided the best agreement. In both cases, the pressure gradient was overestimated, and the mean absolute prediction error ranged from about 5% to 30%
Bubble Temperature Effect on the Heat Transfer Performance of R449a During Flow Boiling Inside a Horizontal Smooth Tube
Full text linkSince the Montreal Protocol (dated 1987), the reduction of the environmental impact has been one of the main goals in the HVAC sector, which has led to the replacement of widely used fluids with new environmentally friendly ones. Nevertheless, only new fluids with suitable heat transfer features can be used. The refrigerant mixture R449a, one of the fourth-generation refrigerants, was tested during flow boiling inside a horizontal smooth tube. The experiments were carried out at six different mass fluxes G ∈ [175;400] kg·m−2·s−1 and four different bubble temperatures Tb ∈ [2.5;10] ◦C, while the nominal values for inlet and outlet quality were selected as xTi = 0.1 and xTo = 0.9, respectively. The results highlighted that, as the bubble temperature increases, it has an opposite effect on the pressure drop per unit length and the heat transfer coefficient: the former decreases while the latter grows. The comparison between experimental results and the correlations showed that the Zhang and Webb formula provides the best prediction of pressure drop, while the models provided by Bertsch yield the most reliable predictions for the heat transfer coefficient. Nevertheless, for both quantities, other correlations with similar performances are available
Flow pattern characterization and pressure drop measurements of foamy two-phase flows
No full textLAUREA MAGISTRALEQuesto lavoro, svolto presso il laboratorio di termofluidodinamica multifase del Dipartimento di Energia del Politecnico di Milano, presenta i risultati di una campagna sperimentale per la caratterizzazione di un flusso bifase aria-acqua in tubi orizzontali in presenza di tensioattivi, con conseguente generazione di schiuma. L’obiettivo di questo lavoro è la misura e conseguente analisi delle cadute di pressione e caratterizzazione del flusso aria-acqua-tensioattivo. L’analisi proposta è considerabile una integrazione dei lavori di Colombo et al., Fasani e Pari. In particolare, l’analisi si è concentrata su intervalli di velocità superficiali maggiori rispetto a quelli precedentemente analizzati.
In primo luogo, è stato studiato il flusso aria-acqua senza tensioattivo per velocità
superficiali di gas e liquido comprese tra 2.79 < JG < 8.34 m/s e 0.12 < JL < 0.24 m/s: sono state misurate le cadute di pressione e i gradienti di pressione per fissare il quadro di riferimento. Diversi modelli sono stati implementati, quali Lockhart-Martinelli, Müller-Steinhagen-Heck, Taitel-Dukler, Dukler e Beattie-Whalley. Successivamente, è stato studiato il flusso aria-liquido-schiuma: sono stati osservati due principali regimi di moto, un regime a plug alle più basse velocità del gas e un regime stratificato-ondoso a velocità più elevate. La transizione tra questi due moti avviene nell’intervallo JG = 3.15 - 4.63 m/s. Le cadute di pressione sono state misurate sperimentalmente e i gradienti calcolati. Infine, tutti i valori sono stati confrontati con i risultati ottenuti nel caso del flusso aria-acqua e integrati con i risultati ottenuti nei precedenti lavori.This work, carried out in the laboratory of Multiphase Thermal-Fluid Dynamics at
Energy Department of Politecnico di Milano, presents the results of an experimental
campaign aiming at the characterization of air-water-foam flows in a horizontal pipe.
The goal is the measurement and analysis of pressure drop and the characterization
of the air-water-foam flow. The proposed analysis can be considered an integration of the works by Colombo et al., Fasani and Pari. In particular, the analysis is focused on higher gas and liquid superficial velocity ranges with respect to the previous studies.
Firstly, air-water flow without surfactant was studied for velocity ranges between
2.79 < JG < 8.34 m/s and 0.12 < JL < 0.24 m/s. Pressure drops were measured and
pressure gradients were computed to set a reference frame. Five different models were implemented: Lockhart-Martinelli, Müller-Steinhagen-Heck, Taitel-Dukler, Dukler and Beattie-Whalley. Then, air-water-surfactant flow was studied with the identification of two flow regimes: an intermittent plug flow at the lowest superficial gas velocities and a stratified-wavy flow for the rest of the conditions; the transition occurs in the range of JG = 3.15 - 4.63 m/s. Subsequently, pressure drops were empirically measured and the gradients were computed and compared with the values obtained for the reference case of air-water flow. Finally they were integrated with the results obtained in the previous works
Experimental characterization of coalescer operations for desalination plants based on forward osmosis
No full textLAUREA MAGISTRALEL’accoppiamento tra i processi di dissalazione e le risorse di energia rinnovabile può essere una soluzione sostenibile ed efficiente per soddisfare la domanda di acqua nel mondo. In questo contesto, i separatori liquido-liquido, utilizzati principalmente nel settore petrolchimico, sono stati recentemente proposti in diversi impianti di dissalazione al fine di disgregare le particelle di soluto dall’acqua. Alla luce di ciò, lo scopo di questo lavoro è di valutare le prestazioni di un coalescer in diverse condizioni operative, portando a termine due distinte campagne sperimentali. Fondamentalmente, il coalescer è impiegato come separatore meccanico tra l’acqua pulita ed uno specifico draw agent in un processo di rigenerazione ed è stata determinata un’espressione sperimentale della sua efficienza definita rispetto alla temperatura di rigenerazione, al tempo di residenza e alla concentrazione iniziale del draw agent. Il draw agent impiegato è il PAGB2000, un co-polimero termosensibile. In particolare presenta una temperatura critica (LCST) al di sopra della quale il fluido si separa in due fasi distinte: una ricca
di polimeri, molto concentrata, e una più diluita, povera in polimero. In ambedue le campagne sperimentali le prestazioni del coalescer crescono fortemente con la temperatura ed il tempo di residenza, raggiungendo anche valori vicini al 100%. D’altra parte, l’efficienza del coalescer varia leggermente al cambiare della concentrazione iniziale di draw agent. Per questo motivo, in secondo luogo, viene proposta un’espressione dell’efficienza del coalescer non direttamente dipendente dalla concentrazione di polimero iniziale, valutandone la sua affidabilità. I risultati delle due campagne sperimentali rivelano che la temperatura di rigenerazione è il parametro più
influente rispetto le prestazioni del coalescer. Allo stesso tempo, nella seconda campagna sperimentale, delle maglie interne meno qualitative sono state adottate, registrando una massima riduzione di efficienza del 6%. Alla luce di ciò, è stata condotta un’ulteriore analisi ipotizzando una dipendenza dell’efficienza del coalescer rispetto la densità delle maglie interne testate. Ciononostante, ulteriori campagne sperimentali sono necessarie per completare questa espressione. In secondo luogo, al fine di definire la miglior condizione tecno-economico operativa di un impianto di dissalazione proposto e che prevede l’impiego di un coalescer per attuare il processo di rigenerazione del fluido, è stata condotta un’analisi sull’impatto dell’area attiva della membrana a osmosi diretta sulle prestazioni dell’impianto, considerando due distinti scenari in termini di portata di alimentazione. La domanda energetica da fornire per il processo di rigenerazione della soluzione acquosa è soddisfatta tramite
dell’energia termica recuperata all’interno dell’impianto stesso e del calore di scarto da un impianto CSP. Il recovery ratio (RR) ed il consumo di energia termica presentano un plateau nel momento in cui l’area attiva della membrana supera i 300 m^2 a causa della forte diluizione subita dalla soluzione acquosa di draw agent che porta ad una rilevante riduzione della sua pressione osmotica. Il massimo RR ottenuto è del 36.2%, mentre i consumi termici sono di circa 105 kWh/m^3 e quelli elettrici sempre al di sotto dei 3.2 kWh/m^3. In definitiva, l’impianto proposto presenta una forte competitività rispetto gli altri di dissalazione, soprattutto perchè il suo costo specifico di produzione dell’acqua (LCOW) può raggiunge valori minori di 2.5 €/m^3.Coupling the desalination processes with the renewable energy sources (RES) might be a sustainable and efficient way to overcome the world water demand. In this context, liquid-liquid separators, mainly used in the petrochemical sector, were recently proposed in several desalination plants in order to separate solute particles respect water. In light of this, the aim of this work is to evaluate the performances of a coalescer, at different operative conditions, through two distinct experimental campaigns. The coalescer is applied as a separator between fresh water and a specific draw agent during a regeneration process and an experimental expression of the coalescer efficiency with respect to the regeneration temperature, the residence time and the initial draw concentration has been determined. The draw agent used is the PAGB2000, a thermoresponsive co-polymer. In particular, it presents a lower critical solution temperature (LCST) above witch the fluid separates in two distinct phases: a polymer-rich, more concentrated, and a polymer-poor more diluted. In both campaigns, the coalescer performances strongly increase with temperature and residence time, indeed efficiency achieves
also values near to 100%. On the other hand, performances slightly vary changing different initial draw concentrations. For this reason, secondarily, a coalescer efficiency expression without direct dependency on the initial draw concentration is proposed, evaluating its reliability. The results of the two experimental campaigns reveal that the regeneration temperature is the most affecting parameter
concerning the coalescer performances. At the same time, adopting lower quality internal grids in the second campaign, the highest efficiency reduction detected is around 6%. In light of this, an analysis on efficiency dependency with respect to the grids density has been proposed. In second instance, in order to define the best techno-economical operative conditions of a proposed FO plant where the regeneration process is held in a coalescer, an analysis on the impact of the FO membrane area on plant performances is done, considering two distinct scenarios in terms of feed flow rate. The energy duty for draw solution regeneration process is satisfied by means of thermal energy recovered in the plant and waste heat from a CSP. The recovery ratio (RR) and thermal energy consumption present a plateau when
the membrane active area rises over 300 m^2 due to draw solution dilution and so reduction of its osmotic pressure. The maximum RR obtained is 36.2%, while the thermal consumption is around 105 kWh/m^3 and the electricity one is always lower than 3.2 kWh/m^3. Definitively, the plant proposed shares to be highly competitive, overall since its LCOW can reach values also lower than 2.5 €/m^3
Heat transfer estimation in flow boiling of R134a within microfin tubes: development of explainable machine learning-based pipelines
Full text linkThe present study is focused on identifying the most suitable sequence of machine learning-based models and the most promising set of input variables aiming at the estimation of heat transfer in evaporating R134a flows in microfin tubes. Utilizing the available experimental data, dimensionless features representing the evaporation phenomena are first generated and are provided to a machine learning-based model. Feature selection and algorithm optimization procedures are then performed. It is shown that the implemented feature selection method determines only six dimensionless parameters (Sul : liquid Suratman number, Bo: boiling number, Frg: gas Froude number, Rel : liquid Reynolds number, Bd: Bond number, and e/D: fin height to tube’s inner diameter ratio) as the most effective input features, which reduces the model’s complexity and facilitates the interpretation of governing physical phenomena. Furthermore, the proposed optimized sequence of machine learning algorithms (providing a mean absolute relative difference (MARD) of 8.84% on the test set) outperforms the most accurate available empirical model (with an MARD of 19.7% on the test set) by a large margin, demonstrating the efficacy of the proposed methodology
Foam generation and transport in horizontal pipes : an experimental study
No full textLAUREA MAGISTRALEQuesto lavoro, svolto in collaborazione con Eni S.p.A., presso il Laboratorio di Fluidodinamica Multifase del Politecnico di Milano e presso i laboratori Eni di Bolgiano, presenta i risultati di alcune campagne sperimentali volte a caratterizzare il comportamento statico e dinamico di schiume da tensioattivo. Le schiume sono ottenute aggiungendo tensioattivi in acqua o a una miscela di acqua e glicole. Obiettivo del processo è di validare se l’uso di tensioattivi possa effettivamente ridurre il liquid holdup in condotte orizzontali utilizzate per trasportare gas naturale dal pozzo di estrazione al primo centro di raccolta. La parte sperimentale è stata suddivisa in prove statiche e prove dinamiche. Lo scopo delle prime è stato quello di valutare il processo di formazione della schiuma e le variazioni dello stesso in presenza di glicole, andando così a determinare il tensioattivo più adatto. In seguito è stato studiato il comportamento dinamico, particolare attenzione è stata posta sull’effetto concernente la frazione di vuoto. Al fine di stimare la suddetta quantità partendo da misure di cadute di pressione, diversi modelli, presi dalla letteratura, sono stati implementati. Su tutti il modello di Taitel & Dukler ha dato i migliori risultati e in ragione di ciò è stato preso come modello di riferimento. Regimi di moto caratterizzati da plug e slug sono i più efficaci nel ridurre il liquid holdup, raggiungendo percentuali di riduzione fino al 174%. Pertanto successivi studi, che comprendano sia un maggior campo di condizioni operative sia diversi additivi, dovrebbero essere condotti, insieme al miglioramento dell’approccio modellistico in modo tale da tener in considerazione il comportamento specifico delle schiume.This work, carried out in collaboration with Eni S.p.A., both at Politecnico di Milano Multiphase Flow Laboratory and Eni Laboratories in Bolgiano, presents the results of some experimental campaigns aimed at the characterization of static and dynamic foams behaviour. The foams are obtained by addition of surfactants to water or to water/glycol mixtures. The goal of the entire process is to verify if the surfactants usage may effectively reduce the liquid holdup in horizontal pipelines for natural gas transportation from the production well to the first collection centre. The experimental part consisted of static tests and dynamic tests. The aim of the former was to assess the foam formation process and its variations in the presence of glycol, thus enabling the selection of the most suitable surfactants. Then, the dynamic behaviour was investigated with emphasis on the effect on void fraction. In order to estimate such quantity starting from pressure drop measurements, different models, taken from the literature, were implemented. Above all, the Taitel and Dukler model performed at the best and, hence, was adopted as reference. Plug/slug flow regimes appeared the most effective in lowering the liquid holdup, achieving percentage reductions up to 174%. Accordingly, further investigations to explore broader ranges of operating conditions as well as different additives would be recommended, together with a refinement of the modelling approaches to take into account the peculiar behaviour of foams
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