1,720,990 research outputs found
Numerical simulation of oil-water two-phase flow in a horizontal duct with a Venturi flow meter
Full text linkThe progressive depletion of on-shore and light-oil reserves is forcing an increased use of transitional and heavy oils, which implies new challenges both during the extraction and the transportation. Focusing on the latter, a technique to reduce the pressure drop is water injection in the oil stream to create the so-called core annular flow (CAF), a flow regime with an oil core enveloped in a water annulus wetting the pipe wall, so that the apparent viscosity of the mixture is considerably reduced. Behaviour of CAF in ducts with non-uniform sections is still under research. This work is devoted to a CFD investigation about the pressure drop, pressure gradients, velocity profiles and in situ volume fractions in a duct including a Venturi flow meter. Unsteady RANS simulations were carried out using the Volume-Of-Fluid interFoam solver of OpenFOAM. Numerical results were experimentally validated for oil superficial velocities in the range 0.25-0.75 m/s and water superficial velocities in the range 0.44-1.10 m/s and comparisons between different approaches and sensitivity analyses were performed. Satisfactory agreement was found for the pressure drop and pressure gradients, and also for the in situ volume fraction with respect to the predictions of the Arney correlation
Liquid holdup measurement for gas-liquid stratified flows by means of resistive probes and image processing
Full text linkFlow patterns exert a fundamental influence on the behaviour of multiphase flows, and they must be brought into play when dealing with their modelling. This is usually done by means of summarizing quantities as the phase holdups and the interfacial area concentration. Many techniques have been designed during the years to measure them, among which the use of probes relying on electrical resistance is one of the simplest and less expensive. While having these points of strength, resistive probes are intrusive devices. This work is therefore devoted to a comparison between liquid height (and derived quantities) measurements - for stratified and stratified-wavy air-water flows - performed using a conventional resistive probe and by means of an image-based technique. Validation of the latter was performed using computer-generated flow images. Then, an experimental campaign was carried out for flows with liquid superficial velocities in the range 0.03 ÷ 0.06 m/s and gas superficial velocities in the range 0.77 ÷ 2.31 m/s. Results showed that the two methods give answers within very few percent of difference, which is more than satisfactory in this field. The results are also in good agreement with some of the most credited literature models and correlations
Analisi sperimentale della sostituzione del R134a con R1234yf, R1234ze(E), R450A e R513A in una pompa di calore acqua-acqua di piccola taglia
Full text linkNel presente lavoro sono illustrati i risultati sperimentali relativi alla sostituzione del refrigerante R134a con le più comuni alternative a basso impatto ambientale in una pompa di calore di tipo acqua-acqua di piccola taglia. I fluidi refrigeranti alternativi considerati sono R1234yf, R1234ze(E), R450A e R513A. Le prestazioni di questi cinque refrigeranti sono confrontate utilizzando un circuito sperimentale che permette di analizzarne il funzionamento in un ampio intervallo di condizioni operative. I risultati ottenuti mostrano che con l’utilizzo dei fluidi alternativi la potenza termica erogata si assesta entro l’intervallo 77% ÷ 99%, mentre il COP si assesta entro l’intervallo 94% ÷ 100% rispetto ai valori ottenuti con R134a. Inoltre, l’incremento della frequenza di rotazione dell’albero del compressore permette di riportare la potenza termica erogata dalla pompa di calore al valore di riferimento, ma induce un’ulteriore riduzione del COP che si assesta entro l’intervallo 82% ÷ 98%.In the present work, the experimental results of the substitution of R134a with R1234yf, R1234ze(E), R450A and R513A in a small water-to-water heat pump are discussed. An experimental set-up that allows analysing the performance of different refrigerants in a broad range of operating conditions is used to carry out the comparison. The obtained results show that the use of any alternative refrigerant leads to the reduction of the heating capacity and of the COP. The former lies within 77% - 99% and the latter lies within 94% - 100% compared with R134a values. Furthermore, the increase of the rotational frequency of the compressor shaft allows bringing the heat pump heating capacity back to the R134a values but causes a further reduction of the COP that lies within 82% - 98% compared with the baseline value
First experimental results of the use of R1234yf and R1234ze(E) as drop-in substitutes for R134a in a water-to-water heat pump
Full text linkIn the present paper, the first results of an experimental analysis carried out to assess the performance of a water-to-water heat pump in which R1234yf and R1234ze(E) are used as drop-in substitutes of R134a are shown. The heat pump is first tested with R134a to establish a baseline performance and, then, is tested under the same working conditions, i.e. under the same water temperatures at evaporator and condenser outlets, with the above-mentioned HFO refrigerants. The results show that the heating capacity and COP of R1234yf system are up to 9.8% and 6.1% respectively lower than those obtained with R134a. On the other side, the use of R1234ze(E) leads to a capacity reduction and a COP reduction respectively up to 23.1% and 2.5%. A second set of tests is then carried out varying the rotational frequency of the compressor shaft in order to set the heat pump heating capacity to the same value found with R134a. The experimental results demonstrate that increases up to 17% and 50% are respectively needed for R1234yf and R1234ze(E), but subsequent reductions of heat pump COP up to 7.38% and 18.11% arise
Ejector refrigeration: perspectives and comparative analysis
Full text linkWithin the broader discussion regarding the decarbonisation of the household sector, ejector refrigeration is attracting a growing attention. This communication contributes to the present day discussion concerning the performances and the perspectives in ejector refrigeration systems. Based on a very large dataset, gathered from the previous literature (encompassing a wide range of system design, operating conditions and refrigerants), this paper proposes a comprehensive comparative analysis. First, the current trends in ejector refrigeration systems, refrigerants and performances are presented. Second, the relationships between ejector performances, refrigerants and boundary conditions (in terms of non-dimensional temperatures, to ensure generality of the proposed analysis) are presented. In conclusion, this paper is intended to provide guidelines for perspective researchers and practitioners interested in selecting suitable ejector-based systems
Assessment of predictive methods for the heat transfer coefficient during flow boiling of R50 and R170 mixtures
Full text linkLiquefied natural gas (LNG) is a binary mixture of methane (R50) and ethane (R170). Proper evaluation of the heat transfer coefficient during flow boiling of such a mixture is very important to design LNG facilities. Pure fluid correlations have been tested with and without the adoption of corrections to account for the lowered performance due to mass transfer resistances arising both in the nucleate boiling and in the convective boiling regimes. In total, the prediction of 13 correlations has been compared with a new data set from the literature. Results show that the correction related to nucleate boiling has the greatest impact though, in general, the adoption of the corrections leads to under predict the data
Methods to characterize effective thermal conductivity, diffusivity and thermal response in different classes of composite phase change materials
Full text linkThe phase change materials (PCMs) used in devices for thermal energy storage (TES) and management are often characterized by low thermal conductivity, a limit for their applicability. Composite PCMs (C-PCM), which combine active phase (proper PCM) with a passive phase with high conductivity and melting temperature have thus been proposed. The paper deals with the effect of length-scale on thermal characterization methods of C-PCM. The first part of the work includes a review of techniques proposed in the scientific literature. Up to now, special focus has been given to effective thermal conductivity and diffusivity at room or low temperature, at which both phases are solid. Conventional equipment has been used, neglecting length-scale effect in cases of coarse porous structures. An experimental set-up developed to characterize the thermal response of course porous C-PCMs also during active phase transition at high temperature is then presented. The setup, including high temperature-heat flux sensors and thermocouples to be located within samples, has been applied to evaluate the thermal response of some of the above C-PCMs. Experimental test results match Finite Elements (FE) simulations well, once a proper lattice model has been selected for the porous passive phase. FE simulations can then be used to estimate temperature difference between active and passive phase that prevents considering the C-PCM as a homogeneous material, to describe it by effective thermo-physical properties. In the engineering field, under these conditions, the design steps for TES systems design cannot be simplified by considering C-PCMs as homogeneous materials in FE codes
Calorimetric determination of wet snow liquid water content: The effect of test conditions on the calorimeter constant and its impact on the measurement uncertainty
No full textThe liquid water content (lwc) of wet snow is a fundamental parameter in determining snow properties like its strength and adhesion force to surfaces. Among the different methods available for the measurement of the lwc, this paper focuses on melting calorimetry: known masses of hot water and snow are mixed into a thermally insulated container, and the lwc is obtained from the difference between the initial hot water temperature and the final mixing temperature; it is a fast and easy to implement method that requires little equipment, making it suitable for both laboratory and field measurements. Tests with “equivalent” wet snow samples whose liquid water content was known in advance were carried out to assess the method, showing that the liquid water content was overestimated by a variable amount that seemed to depend on the test parameters. To account for the heat capacity of the calorimeter, which was found not to be negligible, a constant in terms of equivalent hot water mass (E) was introduced. However, its value also depended on the test conditions. Hence, a correlation between E and the test parameters was found, and the measurements were repeated using a container of a different material, showing a similar behavior. Eventually, a discussion about the effects of choosing different hot water masses and hot water-to-snow mass ratios on measurement accuracy is provided
Machine learning based models for pressure drop estimation of two-phase adiabatic air-water flow in micro-finned tubes: Determination of the most promising dimensionless feature set
Full text linkThe present study is focused on determining the most promising set of dimensionless features and the optimal machine learning algorithm that can be employed for data-driven frictional pressure drop estimation of water (single-phase) and air-water mixture (two-phase) flow in micro-finned horizontal tubes. Accordingly, an experimental activity is first conducted, in which the frictional pressure drop of both water and air-water flows, at various flow conditions, is measured. Next, machine learning based pipelines are developed, in which dimensionless parameters are provided as features and the friction factor (for the single-phase case) and the two-phase flow multipliers (for the two-phase case) are considered as the targets. Next, the feature selection procedure is performed, in which the most promising set of features, while employing a benchmark algorithm, is determined. An algorithm optimization procedure is then performed in order to choose the most suitable algorithm (and the corresponding tuning parameters) that lead to the highest possible accuracy. Moreover, the state-of-the-art physical models are implemented and the corresponding accuracy, while being applied to the experimentally obtained dataset, is determined. It is demonstrated that only 5 dimensionless features are selected (among 23 provided features) in the obtained pipeline developed for the estimation of the two-phase gas multiplier (in the extraction procedure of which, the single-phase friction factors are determined only using the Reynolds number and two geometrical parameters). Therefore, the latter procedures notably reduce the complexity of the model, while providing a higher accuracy (MARD of 6.72% and 7.05% on the training and test sets respectively) compared to the one achieved using the most promising available physical model (MARD of 15.21%). Finally, through implementing the forward feature combination strategy on the optimal pipeline, the contribution of each feature to the achieved accuracy is shown and the trade-off between the model's complexity (number of features) and the obtained accuracy is presented. Thus, the latter step provides the possibility of utilizing an even inferior number of features, while achieving an acceptable accuracy. Moreover, since these pipelines will be made publicly accessible, the implemented models also offer a higher reproducibility and ease of use
Saturation temperature effect on heat transfer coefficient during convective boiling in microfin tubes
Full text linkThe growing attention on environmental aspects puts severe constraints on HVAC technology, mainly involving the working fluids and energy efficiency. Both are related to the main limiting factor of HVAC system: the heat transfer process, which, frequently, involves boiling and condensation. To provide suitable tools for the HVAC system design, it is necessary to gather information on the heat transfer characteristics of the new refrigerants. Particular interest is focused on R1234ze(e), because it is one of the viable options to face the R134a phase out. Something similar can be repeated for the low temperature ORC systems using a refrigerant as working fluid. Using a specifically designed test rig, heat transfer coefficient and pressure drop measures were performed during flow boiling of R1234ze(e). The operating conditions were defined by four parameters: the evaporation temperature (5 °C, 35 °C and 45 °C), the mass flux (two values were considered: 160 kg/m2s and 220 kg/m2s), the average thermodynamic quality (which varies between 0.25 and 0.75) and the quality change (which was fixed to 0.2). The uncertainty affecting the pressure drop and the heat transfer coefficient resulted lower than 1% and 5% respectively. The results highlighted that the saturation temperature strongly affects the heat transfer coefficient and the pressure drop: as a consequence of the saturation temperature increase from 5 °C to 45 °C, it was observed up to 40% heat transfer coefficient increase and 90% pressure drop reduction
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