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Abstract: Heat management is crucial to polymer electrolyte membrane (PEM) fuel cell commercialization. Numerical modeling is often used to simulate heat transfer in the various components of the cell and specifically the gas diffusion layer (GDL). Due to the porous nature of the gas diffusion layer and its complexity of anisotropy, the effect of the structure on the thermal conductivity is usually taken into account by introducing an effective thermal conductivity. In this study, the effective thermal conductivity of carbon paper diffusion media was estimated numerically. Carbon paper is often used as the GDL in PEM fuel cells due to its ability to efficiently transport electrons, heat and gaseous species. Using the GeoDict code, a realistic three-dimensional pore morphology of carbon paper was used as the modeling domain and the governing mathematical equations were solved using the commercial software package Fluent (6.3.26) and the ThermoDict solver. The geometrical effects on the effective thermal conductivity were investigated for different geometries. It was found that the effective thermal conductivity is highly sensitive to the geometry of the porous material under investigation. The effective thermal conductivity is much larger in the in-plane direction when compared with the value in the through-plane direction. Further, the change of the effective thermal conductivity due to porosity and compression was studied. Finally, correlations for the through-plane and in-plane effective thermal conductivity were developed. [Copyright &y& Elsevier]

• The comment concerns a paper published in Chemical Engineering Science Journal. • The problem is non-similar. However the authors treated the problem as similar. • In non-similar problems one x-dependent parameter is used. The authors used three. [ABSTRACT FROM AUTHOR]

• A simplified two-fluid model is proposed for critical two-phase flow in microchannels. • The model preserves precision while incorporating fewer constitutive relations. • Proposed an enhanced metastable liquid nucleation model, achieving precise predictions of nucleation pressure. • Advance the understanding of critical flow in microchannels. Critical flow phenomena are present in many chemical engineering processes, such as microchannel heat exchangers, modeling of jet pumps, nuclear safety analysis, and various industrial facilities that involve subcooled or two-phase pressurized fluids. This paper presents a simplified, reliable two-fluid model for accurately simulating critical flow in microchannels. This study develops a five-equation two-fluid model by constructing a mixed energy equation based on the assumption of metastable liquid phase and saturated gas phase. The model utilizes an improved metastable liquid nucleation model as the gas–liquid phase transition boundary to obtain a more accurate flashing inception. It also uses fewer interfacial constitutive relation equations to reduce the occurrence of unstable solutions. The improved nucleation model is validated using experimental data from previous researchers, with prediction errors controlled within 10%. Furthermore, the five-equation model demonstrates substantial improvements in numerical convergence compared to its six-equation counterpart. By comparing critical mass flux predictions, along-channel pressure variations, and interfacial transfer terms, the five-equation model effectively mitigates instability issues observed in the six-equation model while maintaining prediction accuracy. As a result, this model boasts a broader range of applicability and excels in accurately predicting critical flow, even under more complex conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • A one-dimensional whole-loop CFB dynamic model was established with the flow properties. • The model explicitly depicts the solids throughput behavior at the cyclone and standpipe. • Factors affecting the dynamic characteristics of CFB boiler under load regulation were discussed. • An optimization on a 135 MW CFB boiler demonstrated a maximum load ramp rate of about 4 %/min. CFB boilers face an increasing significance for load change rate in China. However, there is currently a lack of mechanistic understanding on limitation on fast peak shaving. Based on the mass and energy balance in the full loop, this paper conducts a dynamic model to describe the coupling effect of gas–solid two-phase flow, heat transfer, and combustion processes under load regulation conditions. In particular, the material throughput pattern of cyclone and standpipe are systemically described. Finally, the model was validated by field test data in a 135 MW commercial CFB boiler. On this basis, the impact of factors such as the draining ash strategy and adding/discharging circulating ash on the load regulation characteristics of the boiler were deeply explored, and a comprehensive optimization solution was proposed. The results showed that the boiler's ramp could be maximized to about 4 %/min after adopting this solution. [ABSTRACT FROM AUTHOR]

Highlights • Transport characteristics in porous media are reviewed, including flow and heat transfer. • Transport phenomena of nanofluids are comprehensively summarized, involving heat conduction, convection and radiation. • Transport literatures for the combination of nanofluid and porous medium are classified and concerned. • Phase change heat transfer of nanofluids and porous media are summarized for liquid-gas type and solid-liquid type. • This review involves conduction, convection, radiation and phase change for specified media. Abstract Increasing the heat transfer rate of heat transfer equipment is an ever-lasting topic in thermal engineering. Due to the advantages of light weight, high specific surface, high thermal conductivity, metal foam is a good extending surface for heat transfer enhancement. Nanofluid has a higher thermal conductivity than the traditional base fluid, so it can be used as an efficient heat transfer medium. This paper focuses on various flow and heat transfer modes of nanofluid, metal foam and the combination of the two, with the physical properties of nanofluid and metal foam summarized. The characteristics of flow and heat transfer are introduced. The motivation of this review paper is to arouse the researchers to pay attention to the basic transport understandings for the heat transfer enhancement of nanofluids in porous media. The knowledge reviewed in this paper is useful for improving the performance of compact heat exchangers, and heat sinks for cooling electronics with porous media and nanofluids. [ABSTRACT FROM AUTHOR]

In this paper, droplet impact on a porous surface is experimentally investigated over a wide range of Weber numbers and surface temperatures. Regime transition criteria have been deduced to determine droplet post-impingement behaviour as a function of the Weber number and surface temperature for which a droplet impacting on a porous surface. Based on the energy balance, an analytical model with improved boundary layer description is proposed to predict maximum spreading of droplet following impact on porous surfaces when the effect of heat transfer is negligible. The results of the model indicate that the spreading process after droplet impact on porous surfaces is governed by the viscous dissipation and matric potential. The maximum-spread model predictions agreed well with experimental measurements reported in this paper and the literature over a large range of Weber numbers and different porous surfaces. [ABSTRACT FROM AUTHOR]

This paper deals with the unidirectional growth of a clathrate-hydrate film along a planar interface between liquid water and a hydrate-guest substance in the gas or liquid state, such as methane gas. The paper first discusses the physical or logical flaws of previous hydrate-film growth models, then describes a new model in which the diffusive mass transfer of the guest substance to the front edge of a hydrate film and the conductive heat transfer from the edge are simultaneously solved to yield a solution for the film growth. The solution procedure is so formulated as to adhere to the balance, on the rate basis, between the film growth relevant to the mass flow of the guest substance to the film-front edge and the heat release from the edge resulting from the exothermic hydrate-crystal formation. The paper finally describes the predictions for the hydrate-film growth along the water/methane interface for comparison with the literature data of relevant experimental observations. [ABSTRACT FROM AUTHOR]

Fluid Catalytic Cracking (FCC) is one of the most important conversion processes used in refineries all over the world. It is used for the conversion of heavy oil feed with high boiling temperature to produce gasoline, diesel, propylene and other valuable products. Coke deposits on the catalyst during the catalytic conversion and deactivates it, therefore catalyst is continuously regenerated in the FCC process. The regeneration step is essential as it directly impacts the products yields. The coke combustion also generates NOx and SOx emissions which levels are highly influenced by the bed hydrodynamics, the operation parameters and the reactor configuration, and are important to quantify. For all these reasons, the understanding of the regenerator hydrodynamic and kinetic is essential. This paper presents a study on the coupling of Barracuda™ CFD code with a coke combustion kinetic model developed at IFP Energies nouvelles to simulate an industrial FCC regenerator. Regenerator operating and performance data, including catalyst samples for coke analysis, are acquired on a selected industrial unit to evaluate the model. The results provide useful insight on the regenerator performance characteristics in terms of air distribution, coke burning rate and temperature profile in the regenerator. The steady state flue gas composition and regenerator dense and dilute phase temperatures are well predicted by the CFD simulation. The CFD prediction of the bed density is underestimated compared to the industrial data. The duration required to completely regenerate the catalyst is also estimated from the results. The CFD coupled coke combustion kinetic model presented in this paper enables us to evaluate the influence of the fluidized bed hydrodynamic on the catalyst regeneration in an industrial FCC regenerator. The developed model serves as a useful tool for the evaluation of future technology development in the FCC regenerator. [ABSTRACT FROM AUTHOR]

• A novel valve mechanism with solvent freezing using recirculating jackets was developed. • A wide range of recirculating flow-rates were tested using computational fluid dynamics. • Several prototypes were 3D-printed in a titanium alloy and tested for pressure resistance. • The switching times and dead volumes were measured for a range of heating-jacket temperatures. In this paper, we demonstrate a proof-of-principle of a freeze-thaw valve (FTV) created in a 3D-printed fluidic device. Portions of channels are enveloped by cooling and heating jackets, and a heat transfer liquid is recirculated through the two jackets. A frozen plug is created in selected portions of the target-channel and the heating jacket ensures that a selected temperature is maintained in the rest of the channel. An FTV can be 3D-printed in a wide variety of materials as single piece devices with no moving parts without high resolution requirements of the printing process. Such valves can therefore be incorporated in devices for liquid chromatography or multi-step synthesis process. Computational fluid dynamic simulations of a prototype T-junction piece show the two zones to be well defined at coolant and heating jacket flow-rates greater than 1 mL/min, with power consumptions of 1–3 W. The prototype was printed in Titanium 6Al-4V using selective laser melting and the frozen plug was shown to withstand 20 MPa of pressure. Switching times between states 1 (with a frozen section) and 2 (with both sections thawed) were 0.2–3 min in computational and experimental tests. The scalability of the freeze-thaw system was demonstrated using a multi-gate valve containing 33 junctions without a proportionate increase in operational complexity or switching times. [ABSTRACT FROM AUTHOR]

• CFD simulations for calculating the laminar flow friction factor in helical pipes. • Different helix designs, especially highly curved ones, are investigated. • A new correlation for predicting the friction factor in helical pipes is developed. • Pressure drop measurements in a 3D-printed highly curved helical tube. • Excellent agreement between the correlation predictions and the experimental data. Highly curved helical pipes offer attracting potentialities for intensified mass/heat transfer performances as they generate intense Dean-type vortices. The evaluation of friction factor in such geometries is necessary for assessing the trade-offs between the increase of transfer efficiency and the associated specific energy requirement. Unfortunately, such data are lacking for highly curved helixes, probably due to the difficulty to manufacture these geometries through traditional manufacturing techniques. In this paper, CFD simulations are carried out for determining the laminar flow friction factor in helical pipes, particularly highly curved ones. For an experimental validation of the numerical results, a highly curved helix was built by 3D-printing. Existing correlations are shown to fail for the accurate prediction of the friction factor in highly curved helixes. A new correlation is thus proposed. An excellent agreement is obtained between the experimental pressure drop measurements and the proposed correlation predictions. [ABSTRACT FROM AUTHOR]

• A combined numerical and experimental approach is developed to study carbonization process. • The approach integrates experiments, a DEM model and a CFD model. • The approach is applied to low-rank coal ellipsoidal briquettes in a pilot-scale coke oven. • Effects of briquettes packing structure on carbonization are identified. • Increased particle dropping height and vibration lead to denser packing structure and higher carbonization efficiency. This paper reports a combined numerical and experimental approach to study the coal carbonization process. It is applied to low rank coal ellipsoidal briquettes carbonization in a pilot-scale coke oven for demonstration. The integrated mathematical model integrates a DEM model to simulate the packing process of ellipsoidal briquettes in the oven and a CFD model to simulate the flow and thermochemical behaviours related to the carbonization process. The model is validated against the experimental measurements in the pilot-scale coke oven. The comprehensive in-furnace phenomena in the carbonization process are simulated, in terms of flow, temperature, gas composition, and carbonization characteristics. The simulation results indicate that it is necessary to include the briquettes packing structure evolution in the carbonization modelling for reliably describing the in-furnace phenomena. Then the effects of some briquette packing parameters, including briquette dropping height and vertical vibration, on the evolutions of packing structure and carbonization behaviour are studied. It is indicated that the dense packing structure resulting from higher dropping height and one-dimensional vertical vibration before the carbonization can improve the heat and mass transfers between the gas and bed, and thus can improve the carbonization efficiency. The computational cost of this approach as well as its future application are discussed. This model provides a cost-effective tool for understanding and optimizing the carbonization process of non-spherical low rank coal briquettes. [ABSTRACT FROM AUTHOR]

• Fully resolved simulations of fluid-particle systems. • Surface reaction with significant heat effects. • Temperature-dependent reaction rates determined by the Arrhenius equation. • Simulation parameters adopted from realistic POX reaction. In this paper, an efficient ghost-cell based immersed boundary method (IBM) is used to perform direct numerical simulation (DNS) of reactive fluid-particle systems. With an exothermic first order reaction proceeding at the exterior particle surface, the solid temperature rises and consequently increases the reaction rate via an Arrhenius temperature dependence. In other words, the heat and mass transport is dependently coupled through the particle thermal energy equation and the Arrhenius equation, and they offer dynamic boundary conditions for the fluid phase thermal energy equation and species equation respectively. The fluid-solid coupling is enforced at the exact position of the particle surface by implicit incorporation of the boundary conditions into the discretized momentum, species and thermal energy conservation equations of the fluid phase. Different fluid-particle systems are studied with increasing complexity: a single sphere, three spheres and a dense array consisting of hundreds of randomly generated particles. In these systems the mutual impacts between heat and mass transport processes are investigated. [ABSTRACT FROM AUTHOR]

• The strategies to enhance the properties of MCHSs are summarized in four aspects. • The properties and manufacturing methods of diamond MCHSs with great heat transfer potential are emphatically summarized. • Reasonable suggestions and opinions on the future development of MCHSs are put forward. • The results of comprehensive HTP of MCHSs are summarized. With the high miniaturization and integration of micro-electro-mechanical systems, micro-satellite, lasers, and high-voltage electrical appliances, the heat transfer of electronic equipment is facing severe challenges. The microchannel heat sink is widely used as an effective heat transfer method, which can achieve large heat flux cooling. However, conventional microchannel heat sinks have disadvantages, such as large wall superheat, low boiling critical heat flux, large pressure drop, and poor temperature uniformity. In view of the above problems, people have devoted themselves to designing and improving microchannel heat sinks to improve their comprehensive heat transfer performance, in recent years. In this paper, the latest research achievements and trends of microchannel heat sinks are systematically reviewed and summarized from four aspects: microchannel structure, internal reinforcement structure, surface treatment, and material types, which are beneficial to promote the practical application and commercialization of microchannel heat sinks. To the best of the authors' knowledge, this is the first time to summarize the research progress of enhancing the heat transfer performance of microchannel heat sinks from the perspective of surface treatment and material types. Then, the heat transfer performance and fabrication technology of diamond microchannel heat sink with great heat transfer potential are mainly studied. Based on the reviewed studies, although the combination of various enhanced heat transfer methods can improve heat transfer, the key issue is how to balance the heat transfer efficiency and the pressure drop penalty. Finally, the important research progress of enhanced microchannel heat sinks is objectively expounded, and the rationalized suggestions for the future research direction and research ideas of microchannel heat sinks are presented. [ABSTRACT FROM AUTHOR]

• High-activity 3D printing AlSiMg monolith catalysts were prepared. • High thermal conductivity of AlSiMg increased the heat transfer capacity. • A distributed parameter model with a reactor scale was used. • The activity and thermal conductivity of the catalyst should be optimized synchronously. 3D printing AlSiMg monolith catalysts are more suitable for Fischer-Tropsch synthesis due to their high thermal conductivity and thermally connected nature compared to the rolling FeCrAl monolith catalysts. In this paper, the effects of the catalyst activity and thermal conductivity on the reaction and thermal performance were studied by experiment and simulation. Two catalysts were prepared based on the rolling FeCrAl and 3D printing AlSiMg substrate and were designated as ROL-FeCrAl-MC and 3DP-AlSiMg-MC, respectively. The C 5+ yield of 3DP-AlSiMg-MC reached 0.98 g/(g cat •h), 4.44 times that of ROL-FeCrAl-MC. The simulation results indicated that the catalyst activity should be improved synchronously with thermal conductivity to keep the balance between the reaction heat and the heat transfer capacity. The catalyst activity of 3DP-AlSiMg-MC was 67% higher than that of ROL-FeCrAl-MC. It was suited to the high thermal conductivity of 3DP-AlSiMg-MC while that of ROL-FeCrAl-MC was well below expectations. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach for extrapolating DEM simulations of heat transfer over a long period of time. This method is an extension of a previously published algorithm for granular motion extrapolation, introducing heat transfer. The main idea is to perform a short-term DEM simulation for one period and then apply a conductive heat transfer extrapolation algorithm. This strategy is tested over a pilot-scale rotating drum. The outcomes of standard and extrapolated DEM simulations are compared. The results are very similar while the computational time is reduced by a factor greater than 100. • A new approach is proposed for extrapolating DEM simulations of conductive heat transfer over a long period of time. • It is applied over a rotating drum and can be adapted to any pseudo-periodic granular systems. • The method is accurate with a significant reduction of the computational time. [ABSTRACT FROM AUTHOR]

This paper presents a comparative assessment of several novel synthetic natural gas fixed-bed reactor designs and configurations. The four reactor concepts presented in the paper use the Sabatier reaction between carbon dioxide and hydrogen to produce methane. Due to the exothermic nature of the reaction and its pressure sensitivity, this study emphasizes the ability and potential of the reactor configurations to provide effective cooling, and minimize heat gain and pressure drops, so that the maximum methane yield and carbon dioxide conversion can be achieved. Out of four reactor designs, Concept 1 follows a conventional cylindrical fixed-bed reactor design; however, the bed is separated horizontally to promote inter-cooling. Concept 2 separates the bed into 2 segments vertically to facilitate cooling along the length of the reactor. Concept 3 follows a horizontal configuration in which the catalyst beds are separated into rectangular prisms. The different configurations of Concepts 1 to 3 were designed by varying the reactor dimensions and the number of catalyst beds. Most notably, Concept 4 using a unique helical design provides more effective cooling to the catalyst bed by increasing the surface area and decreasing the diameter of the reactor channel. However, the volumetric flow of this specific reactor model is significantly reduced. Furthermore, the inlet conditions of 200 °C and 30 bar help achieve a CH 4 yield of approximately 85 % when considering Concept 1. [ABSTRACT FROM AUTHOR]

• Subcooled flow boiling heat transfer of water in the porous metal foam tubes was investigated experimentally. • Filling a tube with metal foam could improve the heat transfer and also pressure drop. • The thermal efficiency of metal foam tubes was investigated. • Based on experimental data, a correlation was presented to predict heat transfer in the metal foam pipes. This paper has investigated the effect of different porosities (0.80–0.90) of the metal foam pipes on heat transfer coefficient and pressure drop of subcooled flow boiling of water experimentally. The metal foam pipe with 0.80 porosity has increased the Nusselt number and pressure drop by 41 % and 21 % compared to the 0.90 metal foam pipe, respectively. The heat transfer of water phase change in a porous medium has received less attention due to the complexities of the problem. One of the challenges in this field is the absence of a correlation for predicting heat transfer in metal foams. In this paper, a systematic method based on dimensionless numbers considering the essential parameters, like mass flux, subcooling, and heat flux, was used to provide a correlation for calculating the Nu-number based on 106 experimental points. The mean absolute deviation (MAD) of the results predicted by the new correlation is 8.9 %, and it predicts 95 % of the database with ±20 %. [ABSTRACT FROM AUTHOR]

Highlights • Mechanistic model for swirling annular flow pattern transition was established. • Two physical mechanisms for swirling annular flow pattern transition were revealed and modeled. • Effects of parameters (e.g., hydraulic diameter, working pressure and swirl angle) on the transition boundary was presented. • Generalized correlation of swirling annular flow pattern transition was proposed. Abstract The accurate prediction of flow patterns and their transition is extremely important for proper design, operation and optimization of two-phase flow systems, since the parameters such as pressure loss and heat and mass transfer are strongly dependent on the flow pattern. So far, the non-swirling gas-liquid flow in straight pipes have been widely studied and various mechanisms that lead to flow pattern transition have been clarified and modeled. However, the dynamics of gas-liquid flow under swirling condition are not well understood, and no detailed models are available for the prediction of swirling flow pattern transition. To address this, in our previous work (Liu and Bai, 2018), a visualization experiment aimed at classifying flow regimes in swirling gas-liquid flow was presented and three typical swirling flow regimes, i.e., swirling gas column flow, swirling intermittent flow and swirling annular flow were classified and defined, respectively. As the swirling annular flow can be regarded as a special case of conventional annular flow (i.e., when tangential velocity does not equal zero), in present paper, a mechanistic model for the prediction of the swirling annular flow pattern transition was developed considering its physical interest and great practical significance. Two physical mechanisms that lead to the transition from swirling annular flow to other flow patterns were revealed and modeled, respectively. The model was evaluated against a wide range of swirling and non-swirling experimental data and based on this model, the effects of different parameters (e.g., hydraulic diameter, working pressure and swirl angle) on the boundary of flow pattern transition were presented. Results revealed that the range of swirling annular flow enlarges with the increase of the working pressure and swirl angle but narrows with the hydraulic diameter. Taking these influencing factors into account, a generalized formula for the prediction of the swirling annular flow pattern transition was proposed. Compared with existing empirical correlations for annular flow, the newly developed correlation provided more accurate and reasonable prediction of flow pattern transition for both swirling annular flow and annular flow. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Fully resolved simulations of heat transfer problems in tubular fluid-particle systems. • Handles mixed boundary conditions, i.e. isothermal and isoflux, consistently. • Investigation of the influence of particle sizes and passive particles. • Comparison between two classes of immersed boundary method, i.e. CFM and DFM. Abstract In this paper two immersed boundary methods (IBM), specifically a continuous forcing method (CFM) and a discrete forcing method (DFM), are applied to perform direct numerical simulations (DNSs) of heat transfer problems in tubular fluid-particle systems. Both IBM models are built on the well-developed models utilized in momentum transfer studies, and have the capability to handle mixed boundary conditions at the particle surface as encountered in industrial applications with both active and passive particles. Following a thorough verification of both models for the classical Graetz-Nusselt problem, we subsequently apply them to study a much more advanced Graetz-Nusselt problem of more practical importance with a dense stationary array consisting of hundreds of particles randomly positioned inside a tube with adiabatic wall. The influence of particle sizes and fractional amount of passive particles is analyzed at varying Reynolds numbers, and the simulation results are compared between the two IBM models, finding good agreement. Our results thus qualify the two employed IBM modules for more complex applications. [ABSTRACT FROM AUTHOR]

Highlights • Large-scale HEN synthesis is a first step towards Total Site synthesis. • Two-step MILP/MINLP approach was developed for larges-scale HEN synthesis. • Number of potential matches is significantly reduced, solutions are directed to global optima using MILP TransHEN model. • MINLP model HENSyn use reduced superstructure for HEN synthesis. • Problem with 173 process streams and multiple hot utilities solved. Abstract Although different methodologies for the synthesis of heat exchanger network (HEN) problems have been introduced in the last forty years, there are still significant challenges to be addressed, such as solving large-scale problems. This study focuses on synthesizing large-scale HENs using mathematical programming to achieve near globally optimal solutions based on a two-step MILP/MINLP approach. In the first step a mixed-integer linear programming (MILP) model, TransHEN, is used that obtains a globally optimal solution at selected Δ T min. By utilisation of this model, the most promising matches are selected based on feasibility and viability. The second step entails using the matches selected in the TransHEN of Step 1 in a mixed-integer nonlinear programming (MINLP) model, HENsyn, using a reduced superstructure, to generate a feasible HEN. This study presents also a simultaneous Total Site synthesis with direct heat transfer between processes, and is the first step in the wider project of synthesising an entire Total Site with direct and indirect heat transfer; and is the first step in the wider scope of synthesising an entire Total Site with direct and indirect heat transfer; however, in order to attain this goal, a tool capable of an appropriately handling large number of streams is required. The newly developed procedure has been tested on several case studies, two of which are presented in this paper. For Case study 1 the results obtained lie within the range of best solutions obtained by other authors. Case study 2, consisting of 173 process stream and involving multiple hot utilities, shows the applicability of the developed method to handle large-scale HEN problems. [ABSTRACT FROM AUTHOR]

Highlights • New set of data for interfacial waves in low viscous oil-water flow in inclined pipes is presented. • A new linear correlation is found for relating the mixture velocity and wave velocity. • Performs robust analysis procedure for extracting wave properties. • Core findings are in line with that of reported literature for low viscous oil-water flow in horizontal pipes. Abstract The interfacial wave phenomenon in oil-water flow systems is an important area of research, due to its importance in real world applications, especially in oil-water transportation in petroleum industry. The influence of interfacial wavy flow and transition of flow regimes on pressure drop, hold-up and heat transfer has motivated the research on this topic to enhance the understanding, to ensure the process safety and to improve the process economy. This paper investigates the interfacial oil-water wavy flow in upwardly inclined pipes. The test fluids are mineral oil (viscosity-1.6 mPa s, density-788 kg/m3) and water. The scope of the study covers the upward pipe inclination angles of +3°, +5°, +6°, mixture velocities of 0.2–0.5 m/s, and input water cut (input water volume ratio) 0.1–0.9. Two different flow patterns were observed in wavy flow in upwardly inclined pipes, namely stratified wavy (SW) and stratified wavy and mixed interface (SW&MI). The flow images were recorded using a high-speed video camera through a transparent test section. The image analysis was performed using several Matlab programs to extract wave properties such as wavelength and wave amplitude, as well as the wave speed. It is observed that the interfacial instabilities increase with the increasing mixture velocity and with increasing inclinations. Increased instabilities cause interfacial waves to generate and release droplets, while the turbulent intensity in the oil phase also influence droplet formation. An approximately linear relation between wave velocity and mixture velocity was obtained for the wavy flow and a correlation is presented accordingly. Wave energy manifests itself in the combined potential and kinetic energy. The potential energy via the wave amplitude and kinetic energy via the wave speed and wavelength. The overall energy for nonlinear breaking waves is a major source for generation of interfacial droplets. When the flow velocities are increased at a constant input water cut and at a given pipe inclination, the flow regime transition from SW to SW&MI occurs. Meanwhile, the prevailing wavelength decreases and the wave amplitude increases towards the point of transition from SW to SW&MI. The wavelength and the amplitude reach a critical value and remain constant until droplets start to form and release. Once the onset of drop formation occurs at the SW&MI flow regime, the wavelength starts to increase and the wave amplitude decreases with respect to their magnitudes at the point of transition. For a given velocity range, the mean amplitude increases with increasing inclination and decreases with increasing water cut. There is an inverse relation between wavelength and wave amplitude, which means higher amplitude always results in lower wavelength and vice versa. The wave velocity was calculated independently by two different analysis techniques applied to high-speed video images. One was carried out in space domain and one in time domain from high-speed image sequences. All data points were within the 7% error margin with respect to 1:1 reference correlation line, assuring the accuracy of analysis techniques and the validity of the correlation derived for relating the wave velocity to the mixture velocity. [ABSTRACT FROM AUTHOR]

In many industrial processes, electrostatic charges are inevitable and affect the hydrodynamic behavior and heat transfer ability of chemical equipment. A comprehensive study of the electrostatic effect on bubble behavior, particle fluctuation velocity and heat transfer coefficient in the fluidized bed with a central jet has been evaluated in this paper by Eulerian-Eulerian two-fluid model coupled with electrostatic model and energy model. The simulated voidage profiles at different positions, bubble detachment time and initial bubble diameter are compared with experimental results from the literature without charge. The bubble behaviors including bubble frequency and bubble numbers, combined with particle fluctuation parameters are analyzed in both charged and uncharged system. The electrostatic effect on two kinds of heat transfer coefficients is quantitatively compared, namely bubble to emulsion phase heat transfers based on the gas throughflow velocity and gas-solid local heat transfer coefficient. Simulation results show that electrostatic charges decrease bubble numbers and granular temperature, whereas the averaged heat transfer coefficients are enhanced. Overall, the electrostatic effect on the hydrodynamic and heat transfer characteristics can be revealed. [ABSTRACT FROM AUTHOR]

Suitably designed laminar duct flows admit chaotic advection which, in concert with diffusion, can lead to rapid heat and mass transport and sharpening of the residence time distribution (RTD). Whilst evolution of these distinct scalar fields are strongly related, the exact relationships between these distinct fields is unknown, nor to what extent they can be simultaneously optimised. In this paper we present a unified framework for the simultaneous optimisation of the three scalar fields; RTD, temperature, and mass concentration. This optimisation is performed in terms of the eigenmodes of the advection-diffusion operator, which generalize classical Taylor-Aris axial dispersion. We apply this optimisation framework to a twisted pipe flow (TPF) at Péclet number Pe = 10 5 , and find 47- and 237-fold increases in transverse heat and mass transfer respectively over straight tube flow, along with a 2,000-fold suppression of RTD variance growth. We show that generality of the eigenmode decomposition suggests this framework is universal to all duct flows. [ABSTRACT FROM AUTHOR]

In order to design advanced nuclear reactors with enhanced safety systems such as passive decay heat removal system (PDHRS), a new design of Isolation Condenser (IC) has been proposed. The effect of inclination of condenser tube on sliding bubble dynamics and associated heat transfer has been studied for seven angles of tube inclination α (with respect to vertical direction), in the range 0°≤ α ≤90°. For this purpose, two phase transient 3D CFD simulations using mixture model (based on Euler–Euler approach) have been performed. The model considers different mechanisms such as single phase natural convection, latent heat transfer due to evaporation, transient conduction due to disruption of thermal boundary layer and enhanced liquid convection due to bubble sliding motion (quenching). The transient vapor fraction ( ϵ G ) contours and flow distribution enables to understand the mechanism of bubble formation and bubble sliding motion. The major heat transfer mechanism was found to be the liquid agitation caused by sliding bubbles on the tube surface. The heat transfer contribution due to evaporation was found to be very small because of highly sub cooled ( ∆ T sub = 70 K ) liquid inside the tank. Results show that the heat transfer was found to be maximum for α =75° and minimum for α =30°. The bubble sliding length at the tube top was found to decrease and at the tube bottom was found to increase with an increase in the inclination angle ( α ). The enhanced transfer at α =75° ensures excellent thermal mixing and hence results in reduction in thermal stratification. This paper is in continuation of our earlier two papers which employed 21 l ( Gandhi et al., 2013a ) and 200 l ( Gandhi et al., 2013b ) PDHRS whereas this paper presents the simulation of 10 l and 10,000 m 3 PDHRS. [ABSTRACT FROM AUTHOR]

[Display omitted] • Heat transfer in propane dehydrogenation is analyzed using CFD-DEM. • HSM particles provide heat by particle–fluid–particle conduction. • Heat from convection and radiation can balance the heat of reaction. • Specific heat capacity of HSM can influence the transient heat transfer. The heat transfer process can significantly affect the propane dehydrogenation (PDH) reaction due to its strongly endothermic nature. This paper describes the influence of heat storage materials (HSM) on the heat transfer and reaction process of PDH, using computational fluid dynamics coupled with discrete element method in combination with reaction kinetics and heat transfer models at the particle scale. During the dehydrogenation, the particle temperature and propane conversion gradually decrease due to the endothermic nature until they become relatively stable. It is found that, initially, HSM particles can provide heat to the reaction through particle–fluid–particle conduction and maintain higher temperature and propane conversion. Gradually, HSM particles can take part in the gas–solid convection and then transfer the heat to the catalyst particles by particle–fluid–particle conduction for the endothermic reaction. Eventually, the heat of reaction and the heat from the hot feed by convection and radiation reach a balance. Furthermore, HSM particles with higher specific heat capacity can provide more heat to the reaction, while the thermal conductivity of HSM particles has insignificant effect on the heat transfer process, since the particle–fluid–particle conduction plays a major role in the heat transfer between particles. This study presents comprehensive understandings of the heat transfer process and the function of HSM particles for endothermic reaction. [ABSTRACT FROM AUTHOR]

• Flow boiling heat transfer of water in a coil was experimentally investigated. • Severe inhomogeneity of circumferential wall temperature existed. • The flow pattern of two-phase boiling flow in the coil was divided into four regions. • Influencing factors on CWTI and heat transfer coefficient were explored and analysed. • A new FBHT correlation in the HCT with better accuracy was proposed. Helically coiled tubes (HCTs) have been widely used in chemical industry and nuclear engineering because of its compact structure and excellent heat transfer performance. This paper designed and built an experimental platform to study the flow boiling heat transfer (FBHT) of subcritical water with high temperature and high pressure in an HCT. The results showed that the wall temperature on the inner side is higher and it gradually decreases when moving to the outer side along the circumferential direction. The variation law of circumferential wall temperature inhomogeneity (CWTI) under different pressures, mass velocities, heat fluxes and vapour qualities was obtained. The distribution of circumferential wall temperature varies greatly in the single-phase region while it varies slightly and is relatively uniform in the two-phase boiling region. As the vapour quality increases, the CWTI decreases first, and maintains at a low value when the vapour quality is about 0.0 ∼ 0.9. When the vapour quality reaches about 0.9, the CWTI increases rapidly. When the vapour quality is less than 0.5, the heat flux and mass velocity have little effect on the CWTI. When the vapour quality is larger than 0.5, reducing the heat flux or increasing the mass velocity can effectively reduce the CWTI. In addition, the flow pattern of two-phase boiling flow in the HCT is divided into four regions, namely bubble flow, slug flow, annular flow and mist flow, and the transition vapour quality x t1 = 0.2, x t2 = 0.5 and x t3 = 0.93 are respectively selected according to the sudden abrupt change of the wall temperature or the differential pressure. Finally, the existing FBHT correlations are collected and evaluated and the calculation accuracy needs to be improved for more accurate calculation. Therefore, a new FBHT correlation with better accuracy is proposed for calculating the heat transfer coefficient of two-phase flow boiling in the HCT. [ABSTRACT FROM AUTHOR]

In this paper, we apply a novel immersed boundary method to simulate pore-scale level fluid flow and convective heat transfer in realistic numerically generated open-cell solid foams in a Cartesian computational domain. Five different periodic foam samples of varying porosities ( ε = [ 0.877 , 0.948 ] ) are generated by numerically mimicking the actual foam formation process (minimizing surface area). The step-by-step procedure for generating the periodic foam geometries is presented. The specific surface areas of the generated foams of different porosities are compared with real foam geometries showing a reasonable agreement. The Reynolds number ( Re ) is varied from Re ≈ 0 (creeping flow) to Re ≈ 500 , and finally drag and Nusselt correlations have been proposed. A detailed analysis is presented on the local velocity and temperature field for the fluid-solid interaction in a complex cellular porous medium. [ABSTRACT FROM AUTHOR]

The heat-transfer enhancement of nanofluids has made them attractive and the subject of many theoretical and experimental researches over the last decade. Of the theoretical approaches employed to investigate nanofluid properties, molecular dynamics (MD) simulation is a popular computational technique that is widely used to simulate and investigate thermophysical properties of nanofluids. In this paper, we review and discuss the MD studies conducted on the thermophysical properties of nanofluids, considering the thermal conductivity and shear viscosity as two important factors for the industrial application of nanofluids. In this study, after introducing different MD methods to calculate those parameters, we classify and review various influential effects including the volume fraction of nanoparticles, nanofluid temperature, Brownian motion of the nanoparticles, as well as the nanoparticle shape and size in terms of the thermal conductivity and viscosity of nanofluids. Viscosity has been studied to a lesser extent than the thermal conductivity of nanofluids. In our review, we note the similarities and differences between previous MD reports on nanofluids, and we highlight gaps and potential ideas that may be of interest for future studies. [ABSTRACT FROM AUTHOR]

Sorption Enhanced Steam Methane Reforming (SE-SMR) has been proposed as an efficient novel technology to increase hydrogen yield and reduce the environmental footprint in comparison to state of art H 2 production processes. Sorbent/catalyst materials characterized by stable behaviour over multiple reforming/calcination cycles may ensure to achieve almost stationary operating conditions utilizing a dual fluidized bed system (the reformer and the sorbent regenerator) with a solid circulation loop. Bifunctional, Combined Sorbent-Catalyst Materials (CSCM) are under development to integrate endothermic catalytic reforming and heterogeneous CO 2 sorption in one particle, decrease mass and heat transfer resistances and reduce the solid hold-up in the reactors. This paper deals with the numerical simulation of a pilot scale bubbling fluidized bed SE-SMR reactor by means of a Two-Dimensional Computational Fluid-Dynamic (2D CFD) approach. The hydrodynamic picture is supplemented with a comprehensive Particle Grain Model (PGM) previously developed to describe the kinetics of catalytic and sorption functions, and successfully validated with micro-reactor reactivity tests and multi-cycle thermo-gravimetric sorption tests. The effect of repeated carbonation-calcination steps (the “history” of the granular material) is included in the computation of the reactor performance by utilizing the appropriate size of the sorbent grains in the carbonation rate expression. The numerical results show quantitatively the positive influence of carbon dioxide sorption on the reforming process, at different operating conditions, specifically the enhancement of hydrogen yield and reduction of methane residual concentration in the reactor outlet stream. A preliminary validation of CFD simulations is also carried out utilizing experimental data obtained from a pilot scale bubbling fluidized bed SE-SMR reactor (total bed mass ≈ 14 kg). An estimate is provided for the inward heat flow that would be required to operate the reactor in stationary temperature conditions: it is substantially reduced by the exothermic sorption process and could be satisfied by means of the solid circulation loop connecting the SE-SMR reactor to the high temperature calciner in the whole dual fluidized bed system. [ABSTRACT FROM AUTHOR]

The boiling flow or condensation is widely encountered in many industrial applications for both cooling as well as heating processes. Compact heat transfer devices, such as micro-heat exchangers and evaporators, are extensively used for both cooling as well as heating processes over conventional heat exchangers, such as microelectronic circuits, automobile and aerospace industries, due to high surface area to volume ratio and heat transfer rates, compactness and easy thermal control. For better design of micro- or mini-heat exchangers, a detailed specific knowledge of the multiphase flow and its properties such as the flow pattern during flow boiling, critical heat flux (CHF) and stable operation are very important. This paper provides a state of art review on boiling flow in microchannels since year 2000 till date. Flow patterns formed and the parameters influencing flow pattern transitions, during multiphase heat transfer in micro- or mini-channels, have been reviewed in detail. The flow regimes and flow pattern maps, and modeling approaches considered for boiling flow in micro-channels/devices with various challenges have been discussed. A lot of contradiction between the experimental data has been observed for the analysis of flow regimes and flow pattern maps. Further, the effect of hydrodynamics during flow boiling and CHF on heat transfer coefficient has been discussed in detail. Recently, with the advancement in measurement techniques, the heat transfer measurement technologies have been synchronized with the visualization techniques, which helped in understanding the boiling flow physics in micro- and mini-channels. Therefore, an in-depth understanding of flow patterns and regimes under boiling flow conditions in mini- and micro-channels can be used to predict the boiling heat transfer mechanism, which can be further used for developing better heat transfer models for boiling flow. Further, enhancement in heat transfer coefficient for boiling flow in microchannels, either by using complex microchannel configurations or nanocoating on the microchannel surface, have received attention recently, which have been discussed and analyzed in the present review. Both micro- and mini-channels have number of applications in aerospace, refrigeration and computational systems; therefore further attention is needed for more robust and precise design. [ABSTRACT FROM AUTHOR]

• A reliable two-fluid model of micro-channel critical flows is proposed. • The metastable liquid and bubble nucleation are included. • The reasonable constitutive relations of two-phase flow are selected. • The fluid flow, heat and mass transfer process in a micro-channel are studied. Critical flows are relevant multiphase phenomena in many applications or scenarios for chemical processes. The paper aims to propose a reliable two-fluid model to accurately simulate the micro-channel critical flow. The fluid density of the metastable liquid is determined using an isentropic transformation assumption. The onset of flashing is evaluated by solving conservation equations coupled with a nucleation model. The available constitutive relations (interfacial force and interfacial heat transfer) are compared and evaluated depending on the analysis of the transfer process in two-phase flow regions to seek reasonable constitutive relations. The paper compares the proposed model and separated flow models using available constitutive relations, especially for the pressure profile. The proposed model shows superior performance in critical flow simulations. To further advance the understanding of the critical flow, the fluid flow and transport phenomena in a micro-channel are studied intensively, especially for the cooling of micro-channels. [ABSTRACT FROM AUTHOR]

• Three-dimensional pseudo potential lattice Boltzmann model is built up to study boiling on two-tier hierarchical structured surface. • Mechanism of accelerated bubble departure on hierarchical surfaces with upward-orientation secondary pillar is revealed. • Lateral-orientation secondary structures can deteriorate heat transfer and bring larger flow resistance. • Hierarchical structured surface with upward-orientation secondary pillars achieves best boiling performance at high wall temperature. In this paper, pool boiling on two-tier hierarchical structured surface is investigated with three-dimensional LB method. Two-tier pillar structures with different scales are designed, termed as primary and secondary pillar, respectively. On the hierarchical surface with upward-orientation secondary pillars, the bubble departure is promoted. And, the heat transfer can be improved by increasing the primary pillar spacing and enhancing surface wettability. Increasing the height of secondary pillar is favorable for enhancing capillary wicking, but can also bring greater flow resistance. On the hierarchical surface with lateral-orientation secondary pillars, the heat transfer is limited by the large flow resistance. With the enlargement of primary pillar spacing, the impact brought by flow resistance is moderated, and the heat flux can be enhanced. Finally, the boiling performance regarding the effect of wall temperature is concerned, where the structured surface with upward-orientation secondary pillars show the better boiling performance than other structured surfaces. [ABSTRACT FROM AUTHOR]

Gas-liquid-solid flows are frequently encountered in chemical, petrochemical and biochemical industries. To overcome the heat and mass transfer limitations in trickle bed reactors and bubble slurry columns, respectively, a micro-structured bubble column (MSBC) can serve as an attractive alternative. In a MSBC, wire meshes are introduced to cut the bubbles in smaller bubbles and enhance the surface renewal (and hence gas-liquid mass transfer) rates, by deformation of the bubbles. Earlier (Jain et al., 2013) modeling efforts using the Euler-Lagrange approach to simulate a micro-structured bubble column employed a bubble cutting closure based on purely geometrical considerations. To improve on this ad hoc procedure in this paper we explore the possibilities of Direct Numerical Simulations to gain more insight in this complex phenomenon with the ultimate aim to develop improved closures. A combined Volume of Fluid-Immersed Boundary method was applied to simulate the interactions between bubbles and wire meshes. When the bubbles are aligned with the opening of the wire mesh, cutting of the bubbles is not observed in our simulations, while cutting was expected based solely on geometrical considerations. When the Eötvös number, Eo , is larger than 4, the bubbles are highly deformable and squeeze themselves through the opening of the wire mesh. In addition, the bubble gets stuck underneath the mesh when the bubbles are small ( Eo ⩽ 4 ) and/or the opening is in the wire mesh is small. Almost all bubbles that hit the intersection of two crossing wires get stuck underneath the mesh, except for large bubbles ( Eo = 15 ), which get cut by the mesh. Based on these results, it is concluded that the cutting of bubbles depends on the Eötvös number, the opening of the wire mesh and geometrical considerations. However, the results also seem to indicate that the diameter of the wire mesh will also influence the cutting behavior. [ABSTRACT FROM AUTHOR]

The presence of droplets exerts a strong influence on liquid film flow rate, pressure drop, heat transfer and many other flow characteristics. Due to the complexity of churn flow, profound knowledge on the droplets in this flow pattern is not well documented in the existing literature. In the present paper, we study a gas-liquid two-phase churn flow in a 19-mm-inner-diameter pipe under atmospheric pressure and carefully design a shadow detection technique to capture the high-resolution images of the entrained droplets in churn flow. The images are subsequently binarized using an adaptive threshold. The results indicate that the amount of the entrained droplets is high in the chaotic churn flow regime and gradually decreases during the transition from churn flow to annular flow and finally reaches a minimum around the churn-annular flow transition. The detailed process of the droplet entrainment is discussed. Based on our experimental observations, it is concluded that the large droplets (chunks) are related to the breakdown of slugs and bag breakup mechanism, whereas the smaller droplets can be ascribed to the breakup of chunks, ligament breakup and impingement. A normalized size probability density function is implemented to analyze the effect of the entrainment mechanism on the range of droplet size. The Sauter diameter of the droplets in churn flow is compared with the existing empirical correlations in annular flow and a formula for the entrained droplet size in churn flow is proposed. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer and mixing performances are studied using three-dimensional numerical simulations of fluid-structure interactions. To this aim, a multifunctional heat exchanger/reactor geometry is investigated, consisting of a circular pipe where five arrays of four equally spaced trapezoidal vortex generators are inserted and inclined in a reversed position opposite to the flow direction with an angle of 45° with respect to the pipe wall. A periodic rotation of 45° is applied to the tabs arrays. Two cases are numerically studied: one using flexible vortex generators (FVG) that deform due to fluid forces applied on the structures and the other using conventional non deformable rigid vortex generators (RVG). For the FVG configuration, the tabs oscillate without addition of any external source of energy except that of the fluid flow itself, leading to a passive but dynamic way to perform vortex formation to disturb the flow. Both flow regimes are laminar with a constant Reynolds number of 1500. The flow structures are analyzed using the proper orthogonal decomposition (POD) technique and the effect of tabs oscillation on vortices creation, suppression and dislocation is highlighted. The effect of self-sustained free elastic tabs oscillation on heat transfer and mixing performances is numerically investigated by comparing the FVG with its corresponding RVG configuration. The Nusselt number comparison shows that the free tabs oscillation can improve the overall heat transfer of about 118% with respect to an empty pipe whereas it is about 97% for the RVG study. Finally, to assess the mixing performance, the transport of a passive scalar initially divided into two different concentrations in the pipe is numerically analyzed through the mixing index value. The FVG configuration shows a drastic improvement of the mixture quality at the exit of the pipe with an increase of 195% with respect to the RVG case, leading to much shorter and compact mixers and reactors. [ABSTRACT FROM AUTHOR]

Heat transfer under convective boundary condition is common in heat exchangers. This paper presented the experimental results of air flow and heat transfer through three stainless steel foam filled tubes under convection boundary condition. The air flow velocity inside the tube is relatively high, which varied from 7.0 to 26.0 m/s. The stainless steel foam filled tubes, manufactured using high-temperature metallic sintering technique, are of different pore densities (10, 30 and 70 PPI) but have the same porosity of 0.93. The air pressure drop through the stainless steel foam filled tubes was measured. It was found that the inertial drag is the dominant part of the pressure drop at higher velocity. The pressure drop experimental data under high velocity were compared to the predictions by the correlations obtained under relatively low velocity and great discrepancies have been found. A new correlation for the pressure drop through metal foams under high velocity was presented. The effect of the boundary condition on the heat transfer performance was addressed by the comparison of Nusselt number obtained in the present study with that obtained under constant heat flux boundary condition in the published investigation. It was found that the Nusselt number obtained under convective boundary condition is much lower than that obtained under constant heat flux boundary condition. A new correlation for Nusselt number under convective boundary condition was developed. [ABSTRACT FROM AUTHOR]

A non-invasive experimental technique based on particle image velocimetry and digital image analysis on images acquired with high-speed cameras operating in the visual and infrared wavelengths has been developed. With this, simultaneously whole-field data on the evolution of flow patterns and particle temperature distributions in a gas-fluidized bed with and without liquid injection can be obtained. A dedicated pseudo-2D gas–solid-fluidized bed was constructed and operated with liquid injection via a nozzle spraying onto the fluidized bed. It was found that for proper processing of the data recorded with the high-speed infrared camera, combination with digital image analysis on images acquired from the visual camera is essential. The application of infrared thermography to gas fluidized beds suffers from the effects of interparticle reflections. The paper addresses the calibration procedure in detail and it is shown how to correct for this. The temperature-dependent effect of the setup window in the calibration is evaluated. To demonstrate the potential of the technique, it has been applied to dry fluidization and fluidization with liquid injection. [ABSTRACT FROM AUTHOR]

In this paper, a detailed three dimensional mathematical formulation and a simplified one dimensional model for the numerical simulation of the flow and coupled heat and mass transport in a gas channel coated with adsorbing material are presented. In the three dimensional model, the velocity distribution of the gas flow is obtained by solving the momentum equation. The coupled heat and mass transport phenomena are locally described in both the gas channel and the adsorbent layer, and the concomitant adsorbate adsorption and desorption processes are taken into account. In the one dimensional model, the gas flow is assumed as a plug flow, and the heat and mass transfer across the solid-fluid interface is estimated by empirical transfer coefficients. A comparative study between the detailed three dimensional model and a simplified one dimensional model is carried out. Both model predictions are compared with experimental data available from literature. The heat and adsorbate concentration gradients observed in both radial and circumferential directions indicate that the detailed three dimensional model is desired. The time-dependent variations of temperature and heat flux distributions at the interface between the gas channel and the adsorbent layer justify the use of the more detailed three dimensional model. Three dimensional modelling is essential to obtain accurate predictions for cases where the solid side transport resistances are dominating. [ABSTRACT FROM AUTHOR]

Abstract: This paper explores the through-/in-plane characteristics of water transport in the cathode gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). Theoretical analysis is performed on the non-isothermal two-phase flow under flow channels. A dimensionless group Da (Damkohler number for PEFC operation), defined as the ratio of water generation rate to water vapor-phase removal rate, is formulated to characterize the flow regimes in a PEFC. This group, lumping geometrical parameters and physical properties, compares the water vapor-phase removal capability (via water diffusion and holding capacity) with the rate of water production by the oxygen reduction reaction. We find that this dimensionless group can be used to characterize the non-isothermal, two-phase phenomena: when Da→0, the fuel cell is subjected to single-phase operation; while as Da→∞ we have full two-phase operation. A more precise expression is explored for the dimensionless group at the channel central line, i.e. Da 0: when Da 0>1 the entire cathode GDL–CL (catalyst layer) interface is in two-phase region, whereas part of the interface is free of liquid water for Da 0<1. The latter scenario is the concept that this paper proposes for improving fuel cell water management: the consequent co-occurrence of single- and two-phase flows in the in-plane direction at Da 0<1 is beneficial to avoid severe dryout and flooding. A two-phase transport model, describing the water and heat transport on the PEFC cathode side, is employed to perform a two-dimensional numerical study. Detailed liquid and temperature distributions are displayed. Simulation predictions are in reasonably good agreement with the dimensionless-group analysis. [Copyright &y& Elsevier]

Abstract: The reduction in size of catalytic microreactors results in high heat and mass transfer rates and a significant increase in the surface area to volume ratio. A further increase in the catalytic surface area can be achieved in a scaled-down version of fixed bed reactors. Since micro-fixed bed reactors are often deemed impractical due to their large pressure drops, one could use precisely structured inserts to increase the surface area, enhance mixing and manipulate the flow distribution. Catalytic propane combustion in microreactors with multi-channel and posted inserts, which consist of multiple static structures (walls separating various channels and pillar-like structures, respectively) in the flow channel of a microreactor, is considered in this series of two papers. In this first paper, we present numerical comparison of multi-channel and posted catalytic inserts for non-adiabatic self-sustained propane combustion. The inserts are oriented axially along the flow direction. We show that channel and post microreactors have similar performance for low thermal conductivity of the inserts. The in-line arrangement of the posted structures is preferred over a staggered arrangement because the former provides higher propane conversion and more stable combustion. The role of thermal conductivity of the microreactor wall structure and the catalytic inserts is investigated. The thermal conductivity of the microreactor structure affects the performance of the posts but not the channels; this is contrary to the effect of catalyst insert thermal conductivity where it is vice-versa. The channel microreactor is more stable towards high flow-rate blowout limit, whereas the post microreactor is significantly more stable at the lower flow-rate extinction limit. This results in stable operation of the post microreactor under more fuel-lean mixtures than the channel microreactor. [Copyright &y& Elsevier]

Abstract: This paper develops a predictive model of convective heat transfer and conversion of cupric chloride particles in a fluidized bed reactor of a copper–chlorine (Cu–Cl) cycle of thermochemical hydrogen production. The hydrolysis reaction of particles in the fluidized bed is endothermic and it requires excess steam for complete conversion of cupric chloride solid. The excess steam supply may be used for partial heat supply to the endothermic reaction, and also to avoid defluidization in the bed. To avoid defluidization, the change of gas flow in the bed due to the reaction should be minimized at a given operating condition. The model predicts the maximum possible steam inlet temperature, steam conversion, amount of partial heat supply, and also gas flow rates through the bed to avoid defluidization. The new model presents significant new insight by analyzing the hydrodynamic and mass transport processes, considering the equilibrium limitation on the conversion of cupric chloride solid. The model results indicate that the chemical reaction requires a high mole ratio of steam for complete conversion of cupric chloride particles. The maximum steam conversion is limited by temperature, pressure, and the presence of hydrogen chloride gas. The maximum conversion of steam at 400°C is 3.75% and it requires excess steam of 12.8 moles per unit mole of cupric chloride solid for complete conversion of solid. The heat supply by steam for the reaction, as well as raising the solid feed to the reaction temperature, varies with reaction temperature. The paper also adds significant new insight by analyzing the steam flow requirement in terms of temperature, conversion rate, and quality of fluidization. Additional new results are presented and applications discussed for the Cu–Cl cycle of nuclear hydrogen production. [ABSTRACT FROM AUTHOR]

Abstract: In the last years, there has been a rapid growth of research and development activities on high temperature molten salts for nuclear and non-nuclear applications. The study of their heat transfer characteristics is a key issue in the current development of the molten salt reactor (MSR) that is one of the innovative nuclear reactors proposed by the Generation IV International Forum. The MSR is a sort of circulating fuel reactor (CFR), which adopts a molten halide salt mixture containing the fissile material and playing the distinctive role of both fuel and coolant. In a thermal-neutron-spectrum MSR, the reactor core is composed by a graphite matrix (neutron moderator), through which the liquid nuclear fuel flows and leads to a strong and intrinsic coupling between thermo-hydrodynamics and neutronics. This peculiar feature requires a suitable and qualified multi-physics simulation environment for a proper description of the system (fuel/coolant+graphite) behaviour. With reference to such complex and non-linear system, the present work is intended to give two different but linked contributions, in the perspective of a multi-physics modelling able to accurately describe the synergy of the involved different physical phenomena (e.g., by means of software). [(I)] On one hand, this work is aimed at establishing a useful validation framework for the assessment of computational fluid-dynamics (CFD) analyses of liquids with internal heat generation. For this purpose, an analytic approach for both fluid velocity and temperature fields in a circular pipe surrounded by a solid region has been developed. It is an extension of the approach elaborated for pipe flow in (Di Marcello, V., Cammi, A., Luzzi, L., 2010. A generalized approach to heat transfer in pipe flow with internal heat generation. Chemical Engineering Science 65, 1301−1310), in order to take into account the heat conduction in the solid domain (represented by the graphite matrix in the specific case of interest). [(II)] On the other hand, the paper presents a preliminary investigation of the thermal-hydraulic behaviour occurring in a typical graphite-moderated MSR core channel. The developed analytic approach has been conveniently applied to this case study: (i) by testing the capabilities of to evaluate the heat transfer characteristics and the hydrodynamic behaviour of such system; and (ii) by investigating the applicability of correlations for the Nusselt number to fluids with internal heat generation. For a deeper insight into the numerical solutions provided by COMSOL, a code-to-code comparison has been also carried out, adopting a dedicated CFD finite volume software (i.e., FLUENT®). As a result, a satisfactory agreement has been found between the analytic solution and the numerical computations provided by COMSOL and FLUENT for a wide range of Reynolds numbers. As concerns the Nusselt number evaluation, the correlations usually adopted for molten salts have been proved as unsuitable for the analysed system, whereas the correlation advanced in the above mentioned paper (that explicitly takes account of the internal heat source) appears more appropriate. The results of the present work are thought to be useful because they provide: (i) a “generalized” analytic approach to the heat transfer that is applicable in a more general context; (ii) an insight into the heat transfer characteristics of the considered graphite-moderated MSR core channel; and (iii) a preliminary assessment of computations that is essential in view of the adoption of multi-physics tools (like COMSOL) for more complex and representative simulations of the dynamic behaviour of the molten salt reactor, and more in general of other kinds of CFRs. [Copyright &y& Elsevier]

Abstract: Randomly packed bed reactors are widely used in chemical process industries, because of their low cost and ease of use compared to other packing methods. However, the pressure drops in such packed beds are usually much higher than those in other packed beds, and the overall heat transfer performances may be greatly lowered. In order to reduce the pressure drops and improve the overall heat transfer performances of packed beds, structured packed beds are considered to be promising choices. In this paper, the flow and heat transfer inside small pores of some novel structured packed beds are numerically studied, where the packed beds with ellipsoidal or non-uniform spherical particles are investigated for the first time and some new transport phenomena are obtained. Three-dimensional Navier–Stokes equations and RNG k–ε turbulence model with scalable wall function are adopted for present computations. The effects of packing form and particle shape are studied in detail and the flow and heat transfer performances in uniform and non-uniform packed beds are also compared with each other. Firstly, it is found that, with proper selection of packing form and particle shape, the pressure drops in structured packed beds can be greatly reduced and the overall heat transfer performances will be improved. The traditional correlations of flow and heat transfer extracted from randomly packings are found to overpredict the pressure drops and Nusselt number for all these structured packings, and new correlations of flow and heat transfer are obtained. Secondly, it is also revealed that, both the effects of packing form and particle shape are significant on the flow and heat transfer in structured packed beds. With the same particle shape (sphere), the overall heat transfer efficiency of simple cubic (SC) packing is the highest. With the same packing form, such as face center cubic (FCC) packing, the overall heat transfer performance of long ellipsoidal particle model is the best. Furthermore, with the same particle shape and packing form, such as body center cubic (BCC) packing with spheres, the overall heat transfer performance of uniform packing model is higher than that of non-uniform packing model. The models and results presented in this paper would be useful for the optimum design of packed bed reactors. [Copyright &y& Elsevier]

Abstract: The following part of this paper reviews existing theoretical correlations to predict the behaviour of two-phase (liquid–solid) and three-phase (liquid–solid–vapour) fluidised beds as well as models describing heat transfer coefficients. Moreover, a theoretical correlation is developed to describe heat transfer during boiling in a three-phase circulating fluidised bed. The approach uses earlier work on two-phase (liquid–vapour) flow boiling, two-phase (liquid–solid) fluidised beds and three-phase (liquid–vapour–solid) circulating fluidised beds. The correlation developed is validated against experimental data obtained in Part I of the presented paper. The model''s ability to predict the experimental data has been successfully demonstrated. The developed expression for heat transfer coefficients is written as follows: i.e., When applied to experimentally obtained data for stainless steel particles ( and 2.5mm, ), the correlation is able to predict the experimental data within a 20% maximum deviation. In the above correlation, and are original to this work deduced from experimental observations. [Copyright &y& Elsevier]

Abstract: In flow boiling apparatus, fouling is frequently a problem. Mechanical methods to mitigate fouling include the impact of solids on the deposit to remove it. Solid particles fluidised by the two-phase boiling mixture may accomplish sufficient deposit removal to keep boiling surfaces clean. This results in a self-cleaning fluidised bed boiling heat exchanger. The particles additionally enhance the heat transfer. In this paper, a comprehensive investigation of three-phase fluidised flow boiling in a circulating system is presented. A test apparatus which is a three-phase circulating fluidised bed was built with glass construction to visualise boiling phenomena in a water system. It could also be operated as a two-phase system and this was investigated to provide the basis for comparison and also to verify reproducibility and confirm well-established flow boiling results. For the three-phase system (where steel particles were added), a range of liquid flowrates and heat fluxes was used and three different particle sizes were investigated. Three-phase results were compared with two-phase results. This work is primarily a study of boiling heat transfer enhancement as a result of addition of particles, but the effect of the particles creating a self-cleaning heat exchanger is a significant operational advantage for industrial application. As expected, heat transfer coefficients were higher overall for the three-phase system than for the two-phase system. The onset of nucleate boiling was independent of heat flux and the heat transfer coefficient initially increased with increasing Reynolds number whereafter there was some deterioration of heat transfer. Visualisation of boiling heat transfer phenomena for the three-phase system is also provided which allows mechanistic explanations of the measured phenomena. In Part II of this paper, a correlation is developed, based on boiling heat transfer modelling, to describe heat transfer during boiling in a three-phase circulating fluidised bed. A flow-dependent function is added as is a boiling heat transfer enhancement factor. This correlation is validated against this experimental data and found to show agreement within about 20% and better agreement with the data than an existing correlation. [Copyright &y& Elsevier]

Abstract: This paper deals with the mathematical modelling of a rotary kiln, which is used for the recycling of aluminium waste. This model is mainly based on the coupling between [•] “a bed model” describing the processes occurring within the bed of aluminium waste flowing inside the kiln, [•] “a kiln model” describing heat transfer within the kiln itself, [•] “a gas model” describing processes occurring within the gaseous phase held inside the furnace. The “bed model” is mainly based on the plug flow of particles of aluminium inside the kiln. Mass balances as well as energy balances allow for the prediction of the fraction of the organic material within the particles of aluminium as well their temperature. Relevant equations for the “kiln model” include heat conduction and heat exchange with solid and gaseous materials. The equations for the “gas model” are mainly based on fluid mechanics equations coupled with turbulence, radiation, and combustion. The software will be used in order to solve this last model. In this paper, some insight will be given on the description of the “bed model” and “the kiln model” and on the procedure used for the coupling of these models. Exchange variables as well as solving procedure will also be included. Numerical results will be compared to experimental ones, obtained from the pilot scale rotary kiln at Pechiney''s research centre. [Copyright &y& Elsevier]

Abstract: Understanding of the flow dynamics, chemical kinetics and heat transfer mechanism within micro-combustors is essential for the development of combustion-based power MEMS devices. In Part I, CFD based numerical simulation has been proven to be an effective approach to analyse the performance of the micro-combustor under various conditions. In this paper, numerical simulations are performed to analyse the combustion behaviour in a three-dimensional micro-combustor based on the prototype used in the MIT micro-gas turbine engine. The CFD model of the micro-combustor includes fuel/air flow path, combustion chamber as well as solid walls used to construct the combustor. The simulation analysis includes not only the detailed chemical reactions occurred in the combustion chamber, but also the fluid flow dynamics, heat transfer within the combustor and heat loss to the ambient. The performance of the combustor is evaluated under various fuel/air ratio, flow rate and heat loss conditions. Through such systematic numerical analysis, a proper operation space for the micro-combustor is suggested, which may be used as the guideline for micro-combustor design. In addition, the results reported in this paper illustrate that the numerical simulation can be one of the most powerful and beneficial tools for the micro-combustor design, optimisation and performance analysis. [Copyright &y& Elsevier]

Abstract: Understanding of micro-scale combustion mechanism is very essential to the development of combustion-based micro-power devices, which may supply much higher energy density than the batteries used nowadays. In part I of this paper, Computational Fluid Dynamics (CFD)-based numerical simulations have been performed to study the combustion of premixed hydrogen–air mixture in a series of chambers with same shape aspect ratio but various dimensions from millimetre to micron level. The transition of the combustion phenomena in the chambers from relatively large scale to micro-scale has been studied numerically to investigate the micro-combustion mechanism. The combustion model of premixed hydrogen–air mixture represents the detailed reaction mechanism with 19 reversible elementary reactions and nine species. The effect of various heat transfer conditions at chamber wall, e.g. adiabatic wall, with heat loss and heat conduction within the wall, on the combustion is analysed. These thermal conditions have strong effects on the combustion especially when the chamber dimension goes smaller and the ratio of surface area to volume becomes larger. Both factors, such as larger heat loss through the chamber wall and smaller chamber dimension size, may lead to the thermal quenching of micro-scale combustion. The simulation results also indicate that the stable combustion in a micro-scaled chamber may be sustained through increasing the ratio of flow residence time in chamber to chemical reaction time, and maintaining proper thermal condition. In part II of this paper, the numerical modelling method developed here is applied to analyse the micro-combustion characteristics in a three-dimensional micro-combustor based on the prototype developed by the MIT group for a micro gas turbine engine. [Copyright &y& Elsevier]