Analysis of flows driven by a torsionally–oscillatory lid in a fluid-saturated porous enclosure with thermal stable stratification

The convection flow caused by a torsionally–oscillatory lid with thermal stable stratification in an enclosure filled with porous medium is studied in this paper. The governing continuity, momentum and energy transport equations are solved by a semi-implicit projection finite element method. The Grashof number and Reynolds number for ranges 10⁴⩽Gr⩽10⁶, 10²⩽Re⩽10³ are involved in this research for Darcy number 10⁻², 10⁻⁴, 10⁻⁶ and porosity 0.4, 0.6. The results present that the permeability of porous medium deeply dominates the flow field and diminishes the flow strength as the permeability is decreased. The increase in the value of Gr/(Re²·Da) retards the stable stratification from the top surface to extend into the interior domain. The influence of oscillatory frequency is so serious in heat flux variation at some particular frequency corresponding to the resonant frequency. There is an evident resonant frequency at Da=10⁻² condition; however, this phenomenon is not clear for Da⩽10⁻⁴ at different oscillatory frequencies.     

Hybrid central–WENO scheme for the large eddy simulation of turbulent flows with shocks

To provide an effective numerical method for the large eddy simulation (LES) of turbulent flows with shocks, a hybrid scheme is developed in a finite volume framework based on the fourth-order central scheme and the third-order weighted essentially non-oscillatory (WENO) scheme. A total of six easy-to-implement and promising switch functions (SFs) are examined in the hybrid central–WENO scheme for the LES of compressible turbulent flows. Both the dissipation and dispersion of the developed hybrid central–WENO scheme are theoretically confirmed using the Fourier technique. Then, the effectiveness and accuracy of this scheme and the SFs are numerically tested by three problems: decaying compressible isotropic turbulence, inviscid, and turbulent transonic flow over a bump. The numerical results show the developed hybrid scheme, coupled with the SF based on local velocity divergence and pressure gradient, has excellent capabilities of capturing shocks and resolving turbulence.     

Algebraic Stress Model with RNG ε—Equation for Simulating Confined Strongly Swirling Turbulent flows

Strongly swirl flow simulation are still under developing.In this paper,ε equation based on the Renormalization Group theory is used into algebraic stress model.Standard k-ε model,algebraic stress model by Jiang Zhang^[5] and present model (RNG-ASM) are applied simultaneously to simulating the confined strongly swirling flow.The Simulating results by RNG-ASM model are compared to the results by other two model,it is shown that the predictions by this model display reasonable agreement with experimental data,and lead to greater improvement than Zhang‘s ASM turbulence model^[5].     

Momentum interaction in buoyancy-driven gas–liquid vertical channel flows

Drag coefficient correlations for bubbles in buoyancy-driven two-phase flows have generally been derived from data on low-viscosity media and within the bubbly flow regime. In a number of applications, e.g. evaporative crystallizers, there is a need to extend this correlation to higher viscosity flows and slug regimes. In this paper, the momentum interaction in gas–liquid vertical channel flow has been studied experimentally over a wide range of void fractions using a circulation loop facility where the buoyancy is the only driving force for liquid circulation. A model for the drag in gas–liquid buoyant flows has been developed, and is applicable for a wide range of viscosity and void fractions.     

Determinants of global natural gas consumption and import–export flows

Based on the data of the BP Statistical Review of World Energy, this paper constructs the consumption and import–export of natural gas identities. It discusses the drivers of changes in global natural gas consumption and trade flows from 2008 to 2015 using the extended logarithmic mean Divisia index. The results show that differences in the natural gas supply and demand across countries or regions, as well as the distribution of energy between the domestic and international markets, can be better explained when natural gas trade movements are considered. By comparing the supply and consumption increment of natural gas, this study finds that only the energy intensity, economic growth, and demographic effects are consistent with each other. The changes in the impact of other effects mainly depend on storage variations and statistical errors. In addition, the primary drivers of the incremental changes in natural gas consumption vary in different countries. They include production scale, import scale, export scale, consumption structure proportion, energy intensity, economic growth, and population and balance effects. Finally, the consumption competitiveness of the liquefied natural gas significantly improved over the examined period.     

A two-dimensional mathematical model of annular-dispersed and dispersed flows—I

A two-dimensional mathematical model of two-phase flow is presented. The analytical formulation of the model involves the mass, momentum and energy conservation equations for vapour and droplet flows, liquid film and for the wall of the channel, and also a number of subsidiary relations incorporated to close the set of equations. The assumptions invoked are analysed.     

Thermal stability of Ln2NiO4+δ (Ln: La, Pr, Nd) and their chemical compatibility with YSZ and CGO solid electrolytes

Thermal stability and chemical compatibility with electrolyte materials for Solid Oxide Fuel Cells (SOFC) have been studied on Ruddlesden-Popper nickelates Ln₂NiO_4+δ with Ln: La, Pr and Nd. Samples of each composition prepared by three different routes, were characterized by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Different microstructures were achieved by each preparation method. The thermal stability of these compounds was analyzed by annealing all the samples at 700 and 900 °C in air. Pr₂NiO_4+δ (PNO) decomposes after 24 h of annealing at 700 °C confirming its instability at these conditions. Evidences of chemical reaction between La₂NiO_4+δ (LNO) and both Ce_0.9Gd_0.1O_1.96 (CGO) and Zr_0.92Y_0.08O_1.96 (YSZ) were observed at 700 and 900 °C, respectively. No decomposition or chemical reaction with YSZ or CGO were found for Nd₂NiO_4+δ (NNO) below 900 °C. Nevertheless, reactivity with both electrolytes was found for this compound at 1000 °C. The kinetics of these reactions strongly depends on the microstructure of Ln₂NiO_4+δ.     

A two-dimensional mathematical model of annular-dispersed and dispersed flows—II

The results of computation by a two-dimensional mathematical model of a droplet-vapour flow are presented. The predicted two-phase flow characteristics, such as temperature, velocity and phase concentration fields, specific deposition flows, and the temperature of heat emitting surfaces are compared with experimental data. Some specific features of two-phase flow identified in calculations are analysed. The degree of the conservatism of some closing coefficients is numerically investigated. The calculations are performed over a wide range of operational parameters: p = 0.147–13.7 MPa, ρw = 260–3000 kg m⁻² s⁻¹.The predicted results are found to be in reasonable agreement with the experimentally measured data.     

Thermal stochastic collision model in turbulent gas–solid pipe flows

New thermal stochastic particle collision model in gas–solid flow in a riser is developed. The simulation is based on four-way coupling of phases considering inter-particle collision and heat transfer. It is shown that the limitation of excessive computational time in Eulerian–Lagrangian simulation of gas–solid flows for the high loading ratios is eliminated by using the stochastic particle collision model. The simulation results demonstrate that the predictions of the developed thermal stochastic particle collision modem are in good agreement with those obtained by the direct particle collision model and the available experimental data. The new stochastic modeling is used and nearly dense gas–solid flow is simulated for high loading ratios up to eight and the results are presented and discussed.     

Flame–vortex interaction driven combustion dynamics in a backward-facing step combustor

The combustion dynamics of propane–hydrogen mixtures are investigated in an atmospheric pressure, lean, premixed backward-facing step combustor. We systematically vary the equivalence ratio, inlet temperature and fuel composition to determine the stability map of the combustor. Simultaneous pressure, velocity, heat release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When fuel is injected far upstream from the step, the equivalence ratio entering the flame is temporally and spatially uniform, and the combustion dynamics are governed only by flame–vortex interactions. Four distinct dynamic regimes are observed depending on the operating parameters. At high but lean equivalence ratios, the flame is unstable and oscillates strongly as it is wrapped around the large unsteady wake vortex. At intermediate equivalence ratios, weakly oscillating quasi-stable flames are observed. Near the lean blowout limit, long stable flames extending from the corner of the step are formed. At atmospheric inlet temperature, the unstable mode resonates at the 1/4 wavemode of the combustor. As the inlet temperature is increased, the 5/4 wavemode of the combustor is excited at high but lean equivalence ratios, forming the high-frequency unstable flames. Higher hydrogen concentration in the fuel and higher inlet temperatures reduce the equivalence ratios at which the transitions between regimes are observed. We plot combustion dynamics maps or the response curves, that is the overall sound pressure level as a function of the equivalence ratio, for different operating conditions. We demonstrate that numerical results of strained premixed flames can be used to collapse the response curves describing the transitions among the dynamic modes onto a function of the heat release rate parameter alone, rather than a function dependent on the equivalence ratio, inlet temperature and fuel composition separately. We formulate a theory for predicting the critical values of the heat release parameter at which quasi-stable to unstable and unstable to high-frequency unstable modes take place.     

Heat transfer model for gas–liquid slug flows under constant flux

This paper investigates the mechanisms leading to enhanced heat and/or mass transfer rates in two-phase non-boiling slug flows. The problem is analyzed in a minichannel geometry subjected to a constant heat flux boundary. Local Nusselt numbers, obtained using Infrared thermography are analyzed in both entrance and fully developed flow regions. These novel measurements highlight the physics governing slug-flow heat transfer and results indicate that optimized slug geometries can yield up to an order of magnitude heat transfer enhancement. Finally, based on the physics identified, a heat transfer model is developed which is also applicable to similar mass transfer problems.     

Numerical Simulation of Natural Convection in Circular Enclosures with Inner Polygonal Cylinders,with Confirmation by Experimental Results

Numerical computations are performed for the natural convection in circular enclosures with inner polygonalcylinders.The polygon surface and the outer envelope are at constant but different temperatures,A body-fittedcoordinate system is used,The coordinate system is generated via simple algebraic equations.The transformedgoverning equations are discretized on a control volume basis with power-law finite difference scheme.TheSIMPLE-like algorithm is used to deal with the linkage between pressure and velocities.The numerical resultsare compared with the experimental data available in the literature,and the agreement between the numericaland experimental results are very good.     

A fully coupled OpenFOAM® solver for transient incompressible turbulent flows in ALE formulation

In this article, the previously developed single block fully coupled algorithm [1 L. Mangani, M. Buchmayr, and M. Darwish, Development of a Novel Fully Coupled Solver in OpenFOAM: Steady-State Incompressible Turbulent Flows, Numer. Heat Transfer B Fund., vol. 66, pp. 1–20, 2014.[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar],2 L. Mangani, M. Buchmayr, and M. Darwish, Development of a Novel Fully Coupled Solver in OpenFOAM: Steady-state Incompressible Turbulent Flows in Rotational Reference Frames, Numer. Heat Transfer B Fund., vol. 66, pp. 526–543, 2014.[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]] for solving three-dimensional incompressible turbulent flows is extended to resolve transient flows in multiple rotating reference frames using the arbitrary Lagrange–Euler (ALE) formulation. Details on the discretization of ALE terms along with a recently developed extension to the conservative and fully implicit treatment of multi-block interfaces into three-dimensional space are presented. To account for turbulence, the kω???SST turbulence model in ALE formulation is solved using Navier–Stokes equations. This multi-block transient coupled algorithm is embedded within the OpenFOAM® Computational Fluid Dynamics (CFD) library, and its performance evaluated in a real case involving a turbulent flow field in a swirl generator by comparing numerical predictions with experimental measurements.     

Quality control of Li1 + δMn2 − δO4 spinels with their impurity phases by Jaeger and Vetter titration

Powders of lithium manganese oxide phases are synthesised and characterised for possible application in a Li-ion battery. A modified phase diagram on the basis of the manganese oxidation state and content is used to interpret the results. X-ray diffraction, flame atomic absorption spectroscopy, and Jaeger and Vetter titration are performed in order to classify the synthesised powders. The experiments reveal powders of Li_{1 + δ}Mn_{2 − δ}O₄ (0 < δ < 0.2, i.e. 0 < Li/Mn < 0.65), with a spinel structure. At higher Li/Mn ratios mixtures of Li_1.2Mn_1.8O₄ and Li₂MnO₃ are formed. Spinels with small δ-values are favoured since they are more stable than the pure LiMn₂O₄, but they have still acceptable capacity. Also, aluminium-doped spinel materials with the composition Li_{1 + δ}Al_nMn_{2 − δ − n}O₄ (0 < n < 0.3, δ = 0.0645) are synthesised and characterised. However, no improvement in terms of capacity is found.     

Prandtl and capillary effects on heat transfer performance within laminar liquid–gas slug flows

This paper investigates a two-phase non-boiling slug flow regime for the purposes of enhancing heat transfer in microchannel heat sinks or compact heat exchangers. The primary focus is upon understanding the mechanisms leading to enhanced heat transfer and the effect of using different Prandtl number fluids, leading to variations in Capillary number. Experimental work was conducted using Infrared thermography and results are presented in the form of Graetz solution, spanning both the thermal entrance and fully developed flow regions. Nusselt numbers enhancements were observed throughout when data was reduced to account for void fraction. However, the gaseous void was also noted to demonstrate an artificial increase with greater thicknesses of the liquid film, due to higher Capillary numbers. Up to 600% enhancement in heat transfer rates were observed over conventional Poiseuille flow. This was verified through Nusselt number measurements over inverse Graetz number ranges from 10^?4 to 1 and slug length to channel diameter ratios from 0.88 to 32. Varying Prandtl and Capillary numbers caused notable effects in the transition region between entrance and fully developed flows. Significant Nu oscillations were observed for low Pr fluids due to internal circulation within the slug. However, these oscillations are observed to be damped out when higher Prandtl number fluids are employed. The thickness of the liquid film surrounding the gas bubbles is shown to have a significant influence on heat transfer performance. Overall, this study provides a greater understanding of the mechanisms leading to significant enhancements in heat exchange devices employing two-phase gas–liquid flows without boiling.     

Electric system management through hydrogen production – A market driven approach in the French context

Hydrogen is usually presented as a promising energy carrier that has a major role to play in low carbon transportation, through the use of fuel cells. However, such a development is not expected in the short term. In the meantime, hydrogen may also contribute to reduce carbon emissions in diverse sectors among which methanol production. Methanol can be produced by combining carbon dioxide and hydrogen, hence facilitating carbon dioxide emission mitigation while providing a beneficial tool to manage the electric system, if hydrogen is produced by alkaline electrolysis operated in a variable way driven by the spot and balancing electricity markets. Such a concept is promoted by the VItESSE² project (Industrial and Energy value of CO₂ through Efficient use of CO₂-free electricity - Electricity Network System Control & Electricity Storage). Through the proposed market driven approach, hydrogen production offers a possibility to help managing the electric system, together with an opportunity to reduce hydrogen production costs.     

Direct numerical simulation of interphase heat and mass transfer in multicomponent vapour–liquid flows

A Volume-of-Fluid methodology for direct numerical simulation of interface dynamics and simultaneous interphase heat and mass transfer in systems with multiple chemical species is presented. This approach is broadly applicable to many industrially important applications, where coupled interphase heat and mass transfer occurs, including distillation. Volume-of-Fluid interface tracking allows investigation of systems with arbitrarily complex interface dynamics. Further, the present method incorporates the full interface species and energy jump conditions for vapour–liquid interphase heat and mass transfer, thus, making it applicable to systems with multiple phase changing species. The model was validated using the ethanol–water system for the cases of wetted-wall vapour–liquid contacting and vapour flow over a smooth, stationary liquid. Good agreement was observed between empirical correlations, experimental data and numerical predictions for vapour and liquid phase mass transfer coefficients. Direct numerical simulation of interphase heat and mass transfer offers the clear advantage of providing detailed information about local heat and mass transfer rates. This local information can be used to develop accurate heat and mass transfer models that may be integrated into large scale process simulation tools and used for equipment design and optimization.     

Integration of bioenergy systems into UK agriculture–New options for management of nitrogen flows

The large flow of reactive nitrogen (N) through agriculture causes negative environmental impacts, pointing to a need for changes in agricultural practices. At the same time, agriculture is expected to provide biomass to support the increasing demand from the UK bioenergy sector. A high-level aggregated model of the agricultural system in the UK was developed, which maintains the existing level of food and livestock production and at the same time increases N recirculation. Integrating three different bioenergy sub-systems into the agricultural system was an essential component of the model development. Cellulosic bioenergy crops were located in the landscape as vegetation filters to intercept and capture N and thereby reduce N leaching. Efficient collection and digestion of manure produced organic N fertiliser and biogas. Efficient forage production for cattle allowed further cultivation of bioenergy plants. Five implementation scenarios were developed to clarify the contribution of these bioenergy sub-systems to improved N management. The results point to a significant potential for improving the productive use of reactive N and for decreasing N losses to water and air. The interception and recirculation of N presently leaching from arable fields is assessed as the most important option. It is also important to increase recirculation of N in manure and in bioenergy system by-flows. Besides mitigating the environmental impacts of agriculture these measures reduce the requirements for newly synthesised N fertilisers. A systems perspective on N, agriculture, and bioenergy systems facilitates N recirculation and promotes effective N use, reducing the need for additional N inputs.