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• 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]

• New theory for heat transfer via microbubble mediated solvent phase change. • Prediction for heat transfer coefficient/microbubble phase fraction correlation. • Consistent with analysis of freezing onset for boiled water placed in a freezer. • Inferred HTCs are inversely correlated related to oxygen solubility at onset temp. • Supports assertion that microbubble phase fraction is controlled by initial temp. Microbubbles dispersions in aqueous solutions can be long lived. For instance, 20micron size microbubbles take on the order of a day to rise one meter. Consequently, any currents in a reasonably sized vessel would be expected to entrain such a microbubble dispersion as the buoyant force is exceeded by the inertial force of liquid currents. This paper argues for the advantages of a microbubble dispersion mediated condenser with two benefits. The obvious advantage over fine bubble direct contact heating or cooling is that the microbubble phase, which can be engineered with a throughput of approximately a hectare per second of interfacial area flux per cubic meter of solution volume, should not be limited by heat transfer to and from the liquid and microbubble phase. Rather the limitation will be on the wetted area for heat transfer of the vessel to its heat exchange configuration. The second potential advantage follows from the theory proposed in this paper. Arranging the condenser in the microbubble mediated airlift configuration will introduce additional heat transfer from microbubbles vaporizing hotter water near the central plume and convecting that additional latent heat to the cold wall, which condenses the water vapor and releases the latent heat. This additional convection of latent heat is proposed as an additional source term for heat transport equation, and the magnitude of the effect is shown to be proportional to the phase fraction of microbubbles. This theory is shown to be consistent with analysis of observations of freezing times measured by Mpemba and Osborne [Phys. Educ. 4:172-5, 1969], that infer heat transfer coefficients from fitting Newton's law of cooling. The inferred heat transfer coefficient ratio from the presumed highest microbubble phase fraction to the lowest is ~7.4:1. Whether or not that enhancement level persists to a microbubble condenser in an airlift vessel, the promise of additional heat transfer should be explored. [ABSTRACT FROM AUTHOR]

• A mass transfer rate relaxation method for the distillation is proposed. • The proposed method relaxes the integer stage number into the continuous variable. • The penalty extent of non-integer stage can be manipulated effectively. • Better optimization result is obtained with higher probability due to its characteristic. Determining the optimal state number in distillation column design is a mixed-integer nonlinear programming (MINLP) problem, which is difficult to solve. One strategy to solve this problem is relaxing integer stage numbers into continuous variables and solving an easier nonlinear programming (NLP) problem. However, the existing relaxation model often converges to non-integer stage numbers or poor local optima in practical applications, which motivates this paper to develop a new relaxation model. The proposed relaxation model employs an interpolation function to relax the discrete decision on the existence of a stage by varying interphase mass and heat transfer rate between zero and equilibrium state. By adjusting a shape parameter in the interpolation function, the interpolation function shape and its penalty extent of non-integer stage can be manipulated. The results of three distillation column optimization cases show that the proposed model can effectively avoid converges to non-integer stage numbers or poor optima. [ABSTRACT FROM AUTHOR]

• Flow patterns in helically coiled tubes (HCTs) are classified by visual observation. • Flow-regime map is proposed and mechanisms of flow transitions are presented. • Void fraction for the entire tube cross-section is measured by WMS technique. • New correlations of distribution parameter and drift velocity are proposed. Helically coiled tubes (HCTs) are widely used in industries owing to their compact structure and excellent heat and mass transfer efficiency. Due to the complexity of three-dimensional flow in HCTs, profound knowledge on the flow pattern characteristics is not well documented in existing literature. In this paper, we employ the wire-mesh sensor (WMS) and image tomography technique to measure a series of essential parameters (e.g. flow structures, phase fraction) to explore the flow pattern transition mechanism. According to our observation, bubbly, plug, slug, wavy and annular flow are recognized, and the mechanisms of flow pattern transitions are carefully discussed based on the analysis of the forces. In addition, the variation of void fraction in both temporal and spatial dimensions is discussed in detail. By introducing the drift flux analysis, new correlations of distribution parameter and drift velocity are proposed taking both HCTs structure and flow condition into account. [ABSTRACT FROM AUTHOR]

In this paper, the study of the heat exchange effect on the dynamic behaviour of single-phase natural circulation with internal heat generation is presented. In order to predict natural circulation instabilities, two different methods of analysis are developed and compared. The first approach is a linear analysis in which the governing equations are firstly linearized around a steady-state solution of the system and then treated by means of the Fourier transform. This strategy is adopted to compute, in a semi-analytical way, dimensionless stability maps for different system configurations, highlighting the heat exchange effect on the system dynamics. The second approach consists in numerically solving the nonlinear governing equations and allows investigating some transients of interest. For this purpose, an object-oriented one-dimensional model of natural circulation loops has been developed, and the corresponding results have been compared with RELAP5 and Computational Fluid-Dynamics (CFD) time-dependent simulations. The developed models have been applied to investigate the dynamic behaviour of two loop configurations characterized by large instability regions, namely the Horizontal Heater–Horizontal Cooler (HHHC) and the Vertical Heater–Horizontal Cooler (VHHC). [ABSTRACT FROM AUTHOR]

In this paper, the interfacial heat transfer in spherical and distorted fluid particles, flowing at Re numbers up to 80.0, is studied through the Volume-of-Fluid approach, aiming the development of closure relations for the interfacial heat and mass transfer, in the context of the Two-Fluid model. The Nusselt numbers of spherical particles are compared with the usual correlations presented in the literature to validate the numerical model. From the approach adopted in this work, based on the detailed modeling of the interfacial heat transfer process, it is possible to analyze the local flow structure and thermal field around the fluid particles, providing a better understanding of the effect of the particle deformation on the global heat transfer coefficients. It is shown that the interfacial heat flux distribution is affected by the particles shape, which substantially affects the flow and thermal fields around the fluid particles and, consequently, the total heat transfer rate. In addition, the effect of the increase of the interfacial area density, due to the particle deformation, on the total interfacial heat transfer is analysed. New correlations are proposed for the interfacial Nu or Sh numbers, in single rising fluid particles, which become dependent on the Eötvös ( Eo ) number, and for the correction of the interfacial area density, which should be included in the closure of the interfacial heat or mass transfer terms in the Two-Fluid model equations. It is shown that, as the particles become distorted, the transfer coefficient decreases, but the interfacial area increases, compensating the effect on the total interfacial flow. Nonetheless, for highly distorted particles, the effect of the interfacial area increase becomes dominant and the resulting total interfacial transfer is higher than the case of spherical particles. [ABSTRACT FROM AUTHOR]

The unique properties of ceramic foams make them well suited to a range of applications in science and engineering such as heat transfer, reaction catalysis, flow stabilization, and filtration. Consequently, a detailed understanding of the transport properties (i.e. permeability, pressure drop) of these foams is essential. This paper presents the results of both numerical and experimental investigations of the morphology and pressure drop in 10 ppi (pores per inch), 20 ppi and 30 ppi ceramic foam specimens with porosity in the range of 75–79%. The numerical simulations were carried out using a GPU implementation of the three-dimensional, multiple-relaxation-time lattice Boltzmann method (MRT-LBM) on geometries of up to 360 million nodes in size. The experiments were undertaken using a water channel. Foam morphology (porosity and specific surface area) was studied on post-processed, computed tomography (CT) images, and the sensitivity of these results to CT image thresholding was also investigated. Comparison of the numerical and experimental data for pressure drop exhibited very good agreement. Additionally, the results of this study were verified against other researchers׳ data and correlations, with varying outcomes. [ABSTRACT FROM AUTHOR]

Two-phase flows are regularly involved in the heat and mass transfer in industrial processes. To ensure the safety and efficiency of such processes, an accurate prediction of the flow field and phase distribution by means of Computational Fluid Dynamics (CFD) is required. Nowadays, Direct Numerical Simulations (DNS) of large-scale two-phase flow problems are not feasible due to the computational costs involved. Therefore, the Euler–Euler framework is often employed for large-scale simulations, which involves macro-scale modelling of turbulence, mass and momentum transfer. The research activities at Helmholtz–Zentrum Dresden–Rossendorf (HZDR) focus on general closure models for multiphase flows that are closer to physics and include less empiricism. As part of this effort, an Algebraic Interfacial Area Density model (AIAD) is developed for the morphology detection in the two-fluid Euler–Euler approach. Drag models for free surface flows are often based on experimental correlations, their applicability thus being limited to certain flow regimes. In this paper, a modified free-surface drag model based on local shear stress is investigated that avoids this limitation. For this purpose, the algebraic morphology detection mechanism of the AIAD model is revised. In DNS of free surface flow a dampening of the gas side turbulent fluctuations in the near surface region was found by previous investigators. This effect has also been accounted for in Euler–Euler simulations by means of dampening functions. In this work, the significance of turbulence dampening, in case of free surface flows, is examined quantitatively for the k – ω turbulence model. Model validation is performed with the commercial CFD code ANSYS CFX by means of experimental data of countercurrent, supercritical stratified air–water flow. The revised morphology detection mechanism is seen as an improvement with respect to the detection of sharp interfaces. Satisfactory quantitative agreement is achieved for the modified free surface drag model based on experimental pressure difference, liquid levels and interfacial shear stress. Furthermore, it is demonstrated that turbulence dampening has to be accounted for in the k – ω model to qualitatively reproduce the mean flow and turbulence quantities from the experiment. More CFD grade experimental data is required for further model validation. [ABSTRACT FROM AUTHOR]

• Alternating spout deflection shows satisfactory gas-solid hydrodynamics homogeneity. • Alternating spout deflection shows satisfactory gas-solid heat transfer efficiency and homogeneity. • Spout fluidization has heat core in annulus region due to spout incoherence. Spout deflection has long been regarded as a threat to efficient gas-solid interactions in spout fluidised beds, as it may break the stable symmetrical particle circulation in the spout-fluidisation flow pattern. In this paper, the so-called alternating spout deflection (ASD) is evaluated and compared with the spout-fluidisation (SF) flow pattern in terms of gas-solid hydrodynamics, mixing and heat transfer performance at particle scale by means of a Computer Fluid Dynamic-Discrete Element Method (CFD-DEM) model. The comparisons show in the SF over ~40% particles are in a densely packed state, whereas in the ASD the proportion of particles in a packing state is less than 20%; Then their mixing performance are compared using the improved Lacy index, indicating that the mixing speed and final state of the mixing process are similar. Further, the two patterns are compared in terms of heat transfer. It is indicated that in the ASD flow pattern, the mean particle temperature drops quicker; moreover, the homogeneity of the heat transfer is also better in terms of mean square error of particle temperature, compared to the SF. The underlying mechanism is also explored: in the ASD the hot particle cores can be mitigated due to the spout incoherence phenomena, which is however unavoidable in the SF. This paper corrects the negative view that the alternating spout deflection is not as effective as the spout-fluidisation pattern in terms of gas-solid mixing and heat transfer performance. [ABSTRACT FROM AUTHOR]

• Effect of heat transfer processes on mass transfer during extraction reported. • Conventional and microwave extraction heating rate incrementally effects yield. • Step changes in microwave extraction time only for high dielectric loss biomasses. • Step changes in mass transfer rates achieved above a threshold microwave power. • Temperature-Induced Diffusion experimentally confirmed to drive mass transfer. Solvent extraction is a mass transfer process. In this paper, we investigate the role of heat transfer in solvent extraction: in particular, how the heat transfer properties of the solid and the heating method (conventional heating and microwave heating) drive this mass transfer process. Water-based solvent extraction of pectin from orange peel, apple pomace, mango peel and carrot pulp was carried out. The thermal conductivity and dielectric loss were shown as good predictors of extraction performance, with step change increases in mass transfer rates when microwave processing was applied to biomass with dielectric loss significantly higher than water (e.g. 120 mins reduced to 45 mins for optimal pectin extraction from apple pomace). When the loss factor was lower there was no difference in extraction performance between the two technologies (e.g. carrot pulp extraction time was 60 mins in both cases). Further investigations were carried out at different heating rates for both conventional and microwave extraction in order to decouple the effects of microwave volumetric and selective heating. It was shown that below a certain power threshold (within the range of 100–120 W in these experiments), microwave and conventional extraction are equivalent, while above the threshold, microwaves achieved a step-change in extraction time. These findings are the first experimental confirmation of recent theoretical advances in microwave biomass processing, in which Temperature-Induced Diffusion drives mass transfer. It is also the first paper to allow identification of biomass characteristics that will be most amenable to microwave extraction. [ABSTRACT FROM AUTHOR]

The absorption of steam bubbles in a hot aqueous solution of Lithium Bromide is a key process that occurs in the absorber vessel of a heat transformer system. During the condensation process, their size and shape changes dynamically with time as they rise up through the column of liquid. An understanding of the factors that control the vertical upwards motion of the bubbles is necessary to enable proper design of such units. However, the exact vertical displacement of a bubble moving through a liquid is difficult to predict and becomes much more complex if the bubble is simultaneously collapsing. In this paper, the displacement of steam bubbles collapsing in a concentrated aqueous lithium bromide solution (LiBr–H 2 O) has been quantified experimentally. A simple kinetic model predicting the vertical displacement as a function of time was then developed from elementary force–balance considerations. A key feature of the system is the large variability in the motion of the bubbles arising from extreme fluctuations in their size and shape. Bubble dynamic morphology was modelled with stochastic techniques and the output from this was used in the kinetic model to predict dispersion in bubble displacement with time. While the uncertainty predicted by the stochastic model is shown to be less than that observed experimentally, it nonetheless highlights the importance of this random behaviour during the design of such an absorption column. [ABSTRACT FROM AUTHOR]

Two-phase flow without phase change can radically increase the heat transfer rate in microchannels due to the internal recirculation of the fluids. In this paper, both numerical and experimental studies on the hydrodynamics and heat transfer of two-phase flow without phase change in small channels and tubes are reviewed. These two-phase flows are either made up of gas–liquid or immiscible liquid–liquid slug flows. This review includes a general introduction of the hydrodynamics of two-phase flow in microchannels and shows that there is little agreement between measured and predicted pressure drop. Furthermore heat transfer rates are examined in the form of Nusselt number ( Nu ) correlations based on different flow parameters. Values are compared using a standard flow regimes for two-phase slug flow indicating huge variability (over 500%) in the Nu values obtained from reported correlations. We attribute this to insufficient description and consideration of the flow conditions. Finally a perspective on future research directions in the field is suggested, including control through wettability and the use of novel liquids. [ABSTRACT FROM AUTHOR]