Two-phase Simulation Research Articles

Development of a High Heat Flux Electric Joule Heating System for Testing a One-Side Heated Cooling Channel

Systems such as solar thermal systems, chip coolers, beam dumps of neutral beam injectors, and fusion reactor divertors and blankets are subjected to one-side high heat flux. Specifically, high heat flux (10 MW/m2) is applied on the fusion reactor divertor in steady state. The monoblock design in a divertor is limited by the thermal-hydraulic and mechanical stability, which thermal-hydraulic data are required for. The lack of thermal-hydraulic data for the cooling channel causes difficulties for the design of the monoblock and the determination of the thermal-hydraulic condition for the safety and conversion of energy efficiency. To analyze the mechanisms and thermal hydraulics, a high heat flux heating system is recommended for the purpose of testing one-side heated cooling channels. Comparing an e-beam system to a joule heating system, the e-beam system requires higher cost, expertise, sophisticated design, and more power consumption, which delays the development of heat transfer correlations and the mechanisms of fluid motion inside the cooling channel. The production of 10 MW/m2 heat flux using the joule heating method is challenging, since it is limited by the temperature of the heater. In this study, the limitation of the joule heating system was overcome by optimizing the material selection for the heater, the configuration of the system, and the bonding method of the components used in the system. High heat flux testing was conducted and the target heat flux of 10 MW/m2 was successfully implemented, and it showed good reproducibility of the heating system. The reliability of the newly developed heating system was validated by comparing the results of the experiments with correlation-based simulations. The comparison analysis showed that the experimental results from the new heating system are comparable to the single-phase and two-phase correlation-based simulation results within 5% and 1.5% error, respectively. Further, through the comparison of various correlation-based simulations with the experimental results, we conclude that one-side joule heating results are only describable using a one-side correlation, not using correlations developed from uniform heating systems. To optimize the design of the cooling channel in a one-side heating condition in a faster and easier manner, the newly advanced joule heating system will help contribute to the development of the thermal-hydraulic analysis of the cooling channel.

Application of aerating techniques for noise suppression in capillary tubes

A method for suppressing noise in capillary tubes was developed with aerating techniques. The refrigerant R600a was added to the outlet of a throttling capillary from the end of a transition tube between the capillary and evaporator. To investigate the flow characteristic of R600a in the transition tube, a two-phase flow quasi-steady simulation was performed using the mixture model. The Schnerr–Sauer cavitation model was used, and the turbulence model was realizable k − ɛ. Numerical results show that the local pressure and negative mass transfer rate can both be reduced at the downstream of the capillary where bubbles collapse, when bypass capillaries are used in the aerating model. An experimental system was built to measure the noise in a transition tube with/without a bypass capillary. Results show that the noise sound pressure level decreased by 2 dB on average when aerating techniques were used. This research describes the complex mechanism of the throttling flashing process and provides an effective method to suppress the noise in capillary tubes, which will be beneficial to improve occasional sharp noise in a household refrigerator.

Two-phase mixture simulation of the performance of a grooved helical microchannel heat sink filled with biologically prepared water-silver nanofluid: Hydrothermal characteristics and irreversibility behavior

• A novel heatsink with grooved helical microchannels is suggested. • Biologically synthesized water-Ag nanofluid is considered as the working fluid. • First and second law analysis of nanofluid flow inside the heatsink are evaluated. • Heatsink with staggered grooved helical channels has best first-law performance. • Grooving the helical channel entails an increment in the entropy generation. The present paper investigates the influence of using two grooved channel geometries on the hydrothermal performance of a helical microchannel heat sink. The results were compared with those obtained for the plain channel geometry. Based on the results, the highest flow mixing is observed for the staggered grooved configuration. Moreover, the convective heat transfer coefficient enhances by nearly 70% and 17% for the MCHS with the staggered and parallel grooved channels, respectively, as compared to that with the plain channel. The influence of Reynolds number ( Re ) escalation on the convective heat transfer coefficient is insignificant, while a 21% increase is obtained when NF concentration is increased from 0% to 1%. Furthermore, the increase of φ in the grooved configurations is more effective than that in the plain configuration. In addition, the lowest thermal resistance and temperature uniformity factors were obtained for the staggered grooved configuration. Besides, the parallel grooved configuration represents the highest frictional entropy generation rates among the three studied geometries due to the considerable reduction of the fluid flow cross-section area between the groove ribs. The highest heat transfer coefficient and pressure drop are associated with the staggered grooved configuration. For the plain, parallel grooved, and staggered grooved configurations, the increase of Re from 500 to 2000 escalates the NF pressure drop by 2730%, 4420%, and 4460%, respectively. Therefore, the highest performance evaluation criterion of more than unity was obtained for the MCHS with the staggered grooved channels for the studied range of Re (500–2000). The numerical analysis was also performed to evaluate the influence of groove pitch on the hydrothermal performance of the staggered groove channel for the pitch range of 0.15 mm to 0.75 mm. The results demonstrated that the highest PEC is associated with the groove pitch of 0.75 mm.

Research on the characteristics of fluid flow and heat transfer of Al2O3-water nanofluid in an L-shaped chaotic channel

PurposeThis paper aims to investigate the effects of different volume fractions of Al2O3-water nanofluid on flow and heat transfer under chaotic convection conditions in an L-shaped channel, comparing the difference of numerical simulation results between single-phase and Eulerian–Lagrangian models.Design/methodology/approachThe correctness and accuracy of the two calculation models were verified by comparing with the experimental values in literature. An experimental model of the L-shaped channel was processed, and the laser Doppler velocimeter was used to measure the velocities of special positions in the channel. The simulated values were compared with the experimental results, and the correctness and accuracy of the simulation method were verified.FindingsThe calculated results using the two models are basically consistent. Under the condition of Reynolds number is 500, when the volume fractions of nanofluid range from 1% to 4%, the heat transfer coefficients simulated by single-phase model are 1.49%–25.80% higher than that of pure water, and simulated by Eulerian–Lagrangian model are 3.19%–27.48% higher than that of pure water. Meanwhile, the friction coefficients are barely affected. Besides, there are obvious secondary flow caused by lateral oscillations on the cross sections, and the appearance of secondary flow makes the temperature distributions uniform on the cross section and takes more heat away, thus the heat transfer performance is enhanced.Originality/valueThe originality of this work is to reveal the differences between single-phase and two-phase numerical simulations under different flow states. The combination of chaotic convection and nanofluid indicates the direction for further improving the heat transfer threshold.

Controversial turbulent Schmidt number value in particle-laden boundary layer flows

When solid particles, such as sediments, are transported by a turbulent boundary layer flow, the wall-normal turbulent diffusion of particle concentration has been observed to be much higher than the turbulent diffusion of momentum (the eddy viscosity). This is characterized by a turbulent Schmidt number lower than unity. By using the few available existing direct measurements of Reynolds-averaged wall-normal particle flux and our two-phase flow large eddy simulations we demonstrate that the actual turbulent Schmidt number is indeed higher than unity. The misinterpretation may be explained by settling retardation of particles in turbulent flows compared with quiescent water.

Random trilobe packing using rigid body approach and local Gas-Liquid hydrodynamics simulation through CFD with experimental validation

It is quite difficult to measure the local information of the gas/liquid flow inside random packed extrudate catalyst beds in a large scale. Obtaining the local information through computational fluid dynamic (CFD) simulations in such system is also difficult due to the complexity of random particle packing and two-phase flow simulation. An efficient packing scheme was implemented to randomly pack 2917 trilobe particles (10 cm in height) in a 2-inch column to represent the trickle bed reactor (TBR) based on the rigid body approach. The generated geometry was used to define the computational domain for the two-phase hydrodynamics simulation based on the volume of fluids (VOF) approach. This hydrodynamics modelling study is paired with an experimental study using our in-house developed advanced measurement techniques based on optical fiber probes, which allowed to determine local liquid velocity and saturation profiles. The experimental measurements were used for local validation of the implemented model.

Field-scale impacts of long-term wettability alteration in geological [formula omitted] storage

Constitutive functions that govern macroscale capillary pressure and relative permeability are central in constraining both storage efficiency and sealing properties of CO2 storage systems. Constitutive functions for porous systems are in part determined by wettability, which is a pore-scale phenomenon that influences macroscale displacement. While wettability of saline aquifers and caprocks are assumed to remain water-wet when CO2 is injected, there is recent evidence of contact angle change due to long-term CO2 exposure. Weakening of capillary forces alters the saturation functions dynamically over time. Recently, new dynamic models were developed for saturation functions that capture the impact of wettability alteration (WA) due to long-term CO2 exposure. In this paper, these functions are implemented into a two-phase two-component simulator to study long-term WA dynamics for field-scale CO2 storage. We simulate WA effects on horizontal migration patterns under injection and buoyancy-driven migration in the caprock. We characterize the behavior of each scenario for different flow regimes. Our results show the impact on storage efficiency can be described by the capillary number, while vertical leakage can be scaled by caprock sealing parameters. Scaling models for CO2 migration into the caprock show that long-term WA poses little risk to CO2 containment over relevant timescales.

Examining performance of different two-phase fluid distributions simulation methods based on digital rock technology

The accurate calculations of petrophysical properties are essential to evaluate oil/gas reservoirs. As an advanced non-destructive method, digital rock technology has been widely applied in simulating various rocks' petrophysical properties, such as the behaviors of resistivity and acoustic responses. Fluid distributions within the pore space are essential input data for such simulations. However, the 3D fluid distributions simulated by different methods may not be identical, leading to imprecise petrophysical interpretations. Therefore, it is necessary to compare the results of different methods on the same pore space to investigate their advantages and disadvantages. In our research, we used digital rock technology to compare different simulation methods of two-phase fluid distributions. We first adopted a sandstone core and acquired the actual oil and brine distributions in pores based on the micro-computed tomography scan images. Herein, a new simple and fast method was put forward to fix the rough connection between the oil and the brine phases. In the present study, five two-phase fluid distribution simulation methods were investigated including the steady-state pore morphology method, unsteady-state pore morphology method, network modeling method, steady-state lattice Boltzmann method, and unsteady-state lattice Boltzmann method. Then, we applied these methods on the same pore space and analyzed the results. Finally, the calculated resistivity indexes based on these methods were compared. It turns out that although the results of all different methods captured the main distribution characteristics of oil and brine phases, only the steady-state pore morphology method, unsteady-state pore morphology method, and steady-state lattice Boltzmann method provided the most useful two-phase fluid distribution and saturation exponent for our studied sandstone. These results can not only enhance our knowledge of different simulation methods’ performance but also help to choose the best method for a desired petrophysical characterization task.

Numerical investigation of heat transfer enhancement of graphene nanofluid in turbulent flow conditions using the Lagrangian two-phase model

This paper aims to introduce one of the parameters which contribute to heat transfer enhancement of graphene nanofluid to seek the optimal heat transfer system. Numerical analysis has been performed on the turbulent heat transfer of the nanofluid in a horizonal stainless steel tube which is subjected to a constant heat flux at its over surface. To model turbulent flow, the finite volume method with realizable k-ε model and Wolfstein model is employed to calculate the momentum, continuity, energy conservation, and turbulent equation of continuous phase in two-dimension domains (Axially symmetric flow). To assure its reliability, wall functions are compared with the result of numerical analysis, and it gained its reliability. Lagrangian two-phase model is employed to solve the mass, momentum, energy conservation of dispersed phase and model the physical interaction between two phases. In this model, phase interaction occurs to predict the influence of one phase upon the other phase within the two-phase simulation. This paper mainly focuses on this phase interaction as parameter which contributes to heat transfer enhancement since many other papers on this topic use a single-phase flow model and fail to focus on this. The results of the two-phase model are compared with those of the single-phase model. As a result, the Nusselt number (Nu) of the graphene nanofluid is higher than the base fluid and increased as the flow rate as other investigation have shown. Finally, some results reveal changed property, such as velocity distribution, turbulent kinetic energy distribution and it is found that this change influences on heat transfer enhancement. Moreover, the insight that nanoparticle gathering is the factor of this change is also obtained.

ラグランジュ二相モデルを用いた乱流条件下におけるグラフェンナノ流体の熱伝達促進に関する数値解析

This paper aims to introduce one of the parameters which contribute to heat transfer enhancement of graphene nanofluid to seek the optimal heat transfer system. Numerical analysis has been performed on the turbulent heat transfer of the nanofluid in a horizonal stainless steel tube which is subjected to a constant heat flux at its over surface. To model turbulent flow, the finite volume method with realizable k-εmodel and Wolfstein model is employed to calculate the momentum, continuity, energy conservation, and turbulent equation of continuous phase in two-dimension domains （Axially symmetric flow）. To assure its reliability, wall functions are compared with the result of numerical analysis, and it gained its reliability. Lagrangian two-phase model is employed to solve the mass, momentum, energy conservation of dispersed phase and model the physical interaction between two phases. In this model, phase interaction occurs to predict the influence of one phase upon the other phase within the two-phase simulation. This paper mainly focuses on this phase interaction as parameter which contributes to heat transfer enhancement since many other papers on this topic use a single-phase flow model and fail to focus on this. The results of the two-phase model are compared with those of the single-phase model and experimental results. Finally, some results reveal changed property, such as velocity distribution, turbulent kinetic energy distribution and it is found that this change influences on heat transfer enhancement. Moreover, the insight that nanoparticle gathering is the factor of this change is also obtained.

AN INVESTIGATION OF THE EFFECT OF INITIAL BUBBLE DIAMETER ON THE BUBBLE TRAJECTORY IN THE FLOTATION COLUMN CELL USING CFD SIMULATION

The effect of initial bubble diameter on the bubble motion pattern in a flotation column has been studied by the twophase computational fluid dynamics (CFD) method. The two-phase simulations have been done using the volume of a fluid (VOF) model in ANSYS® Fluent® software. The computational field was a square cross-section column with a width of 0.1 m and a height of 1 m into which air was interred as a single bubble from the lower part of the column by an internal sparger. An experimental test has been also performed and the simulated results have been validated using the values obtained for the bubble rise velocity. A comparison of the simulation and the experimental results has confirmed that CFD can predict the bubble rise velocity profile and its value in the flotation column less than 5% relative to the experimental values. Then the simulations have been repeated with a 20% decrease and increase in the initial bubble diameter to investigate the effect of bubble diameter on the bubble flow pattern. The investigations have shown that as the bubble diameter increases, the velocity decreases and the bubble rises in a more zigzag direction as a result of two counter-rotating trailing vortices behind the bubble increasing.

Solid-gas Two-phase Flow Numerical Simulation of Abrasive Grain Spraying Process for Lithium Metal Electrode All-solid-state Battery

Solid-gas Two-phase Flow Numerical Simulation of Abrasive Grain Spraying Process for Lithium Metal Electrode All-solid-state Battery

Fragmentation and atomization characteristics of near-orifice liquid jet under transverse gas film

In order to understand the fragmentation and atomization characteristics of the liquid jet in transverse gas film, a pintle injection element using air and water as simulants is designed. The two-phase flow large eddy simulation and backlight imaging are used to study the liquid-jet breakup process and spray-field dynamic characteristics in the nearorifice area of pinte injection element under the atmospheric environment. The primary fragmentation process of the liquid jet dominated by surface wave is obtained by large eddy simulation, which reveals the establishment process of the spray field in near-orifice area of the gas-liquid pintle injector. After the subsonic airflow leaves the slit, it expands and accelerates into supersonic state. Then the deceleration and pressurization phenomenon occurs once the supersonic airflow passes through the detached bow shock upstream of the liquid jet. The liquid jet bends downstream due to the difference in pressure between upstream and downstream, and the Rayleigh-Taylor (R-T) unstable surface wave appears on the jet windward surface. As the surface wave develops, the penetration of the wave trough by airflow causes the continuous liquid jet to fragment. Proper orthogonal decomposition (POD) method can effectively reconstruct spray snapshot. The POD mode shows that the low-frequency spray oscillation in near-orifice area is caused by the overall expansion/contraction process of the spray field, while the high-frequency one is due to the “impact wave” movement of the liquid block or liquid mist group on the windward side. The latter is produced by the R-T unstable surface wave before the jet breakup, and can be categorized as traveling wave structure. The dimensionless traveling wave wavelength has a power-law relationship with Weber number.

Multi-Variate Optimization of Polymer Electrolyte Membrane Fuel Cells in Consideration of Effects of GDL Compression and Intrusion

This study applies, tests, and compares comprehensive surrogate-based optimization techniques to optimize the performance of polymer electrolyte membrane fuel cells (PEMFCs). Moreover, parametric cases considering four important design variables, i.e., gas diffusion layer thickness (), channel depth (), channel width (), and land width (), are defined using the latin hypercube sampling technique under reasonable constraint conditions. Multidimensional, two-phase PEMFC model simulations are performed to generate the training and test data under these design conditions. Three famous surrogate models, i.e., response surface approximation (RSA), radial basis neural network (RBNN), and kriging (KRG), are employed to construct objective functions for the PEMFC cell voltages operating in the galvanostatic mode, and their accuracies are tested and compared using root mean square error and adjusted R-square. The surrogates are then linked to stochastic optimization algorithms, i.e., genetic algorithm and particle swarm optimization, to determine the optimal design points. A comparative study of these surrogates reveals that the KRG model outperforms the RBNN and RSA models in terms of prediction capability. Furthermore, the PEMFC model simulations at the optimal design points clearly demonstrate that performance improvements of around 56–69 mV at 2.0 A cm−2 are achieved with the optimum design values compared to typical PEMFC design conditions.

Numerical simulation of the PEM fuel cell performance enhancement by various blockage arrangement of the cathode serpentine gas flow channel outlets/inlets

In this paper, performance enhancement of polymer electrolyte membrane fuel cells by changing the outlet/inlet configuration of parallel-serpentine flow field is investigated. The geometrical changes of the channel outlets/inlets are designed to amplify the effects of transverse over-rib convection in the gas diffusion and catalyst layers. A three-dimensional and two-phase simulation of the fuel cell performance with four flow fields of parallel-serpentine, serpentine-baffled, serpentine-interdigitated and serpentine-stepped channel is conducted and the results are presented in the form of polarization curves, contours of velocity, oxygen concentration, liquid water saturation, local current density, and the dissolved water content. The results show that the liquid saturation volume coverage is reduced from 0.832 for the parallel-serpentine flow field to 0.514 for the serpentine-baffled flow field. Also, the regions with high concentration of oxygen was improved by 26.7% between these two cases. Due to the increase in oxygen delivery to the catalyst layer and better water removal on the cathode side, the serpentine-baffled, serpentine-interdigitated, and serpentine-stepped flow channel geometries showed a significant increase in performance compared to the parallel-serpentine case. The highest net power has resulted for the serpentine-baffled case so that at a current density of 1.5 A/cm2, a performance increase of 38.5% was achieved compared to the parallel-serpentine case.

Computational case study on upward two-phase flow in vertical circular pipe based on CFD approach: Multidimensional flow instabilities investigation

In this paper, a numeric scheme made up of the three-dimensional (3-D) Two-Fluid Model (TFM) and the Rensselaer Polytechnic Institute (RPI) wall boiling model for two-phase flow instabilities simulation was validated by comparing its results with the flow boiling and instability experimental data. Then two-phase flow instabilities in a vertical circular channel were studied based on the validated numeric scheme. The flow instability was analyzed by both the Lumped Parameter Approach (LPA) and the Distributed Parameter Approach (DPA). The average parameter oscillation characteristics and local distributions were analyzed. The transient responses of the local conditions during the flow instability were discussed. Besides, the stability of the vertical circular tube channel flow boiling system was studied with the stable map based on the subcooling number (Nsub) versus the phase change number (Npch). This study provides a new feasible scheme for the simulation of flow instability which also actualizes an improved understanding of multidimensional flow and heat transfer characteristics in the flow instabilities phenomenon.

Nucleation mechanism of iron in an external magnetic field

In this work, the solidification of liquid iron with or without external magnetic field was investigated by using two molecular dynamics methods, namely direct cooling and two-phase simulation. The influence of external magnetic field on the solidification is characterized by the critical temperature and radial distribution functions. Our computational results show that under external magnetic field, the solidification point tends to decrease significantly. By further analyzing the diffusion coefficients and viscosity, we attribute the effect to the stronger fluctuation of liquid iron atoms driven by the external magnetic field.

Effects of surface tension on the Richtmyer-Meshkov instability in fully compressible and inviscid fluids

The nonlinear phase of Richtmyer-Meshkov Instability (RMI) is characterized by the development of asymmetric interfacial perturbation structures known as `bubbles' and `spikes', which could potentially be curbed by surface tension. Our work systematically examines such curbing effects of surface tension within a wide range of Weber numbers. Based on numerical results of two-phase compressible simulations, we confirm two existing analytic theories predicting RMI perturbation development respectively at small and asymptotically large Weber numbers, and propose a heuristic criterion identifying the transitional development of RMI at intermediate Weber numbers.

Numerical simulation of water-gas two-phase displacement process in unsaturated granite residual soil

Infiltration of current in unsaturated soil is essentially a two-phase flow problem of water displacing air in the process of infiltration. The accuracy of traditional two-phase flow research methods cannot meet the needs of engineering, and it is not conducive to repetitive research. In contrast, the numerical simulation method of multiphase flow at the meso-scale can better simulate the whole dynamic process of water flooding. Mao Huan, Qren and others have achieved great results in the field of pore meso-scale research, but on the one hand, most of them focus on the study of rock multiphase flow, and there are some differences between the research object and the actual pore structure. On the other hand, the widely used pore network model method cannot directly show the change of particle velocity at any time and cannot present the phase interface movement state. In view of this, in order to reveal the dynamic percolation mechanism of water-gas two-phase displacement of unsaturated granite residual soil, this paper selected undisturbed granite residual soil in Fuzhou as the research object and studied the dynamic characteristics of two-phase displacement of undisturbed soil samples by using industrial CT scanning images and Level Set method. The results show that the Level Set method can properly capture the interface position between two immiscible fluids for meso-scale water-gas two-phase displacement simulation. The water-gas two-phase displacement process has the characteristics of large pore preferential flow, and the ‘low around’ phenomenon can easily appear in the higher ground of porosity roundness in general. The displacement rate is mainly controlled by the tortuosity of the channel, displacement speed is relatively high in the straight and wide channel. There is an obvious phenomenon of ‘preferential passage’, and its seepage time is positively correlated with the first rapid and then slow characteristics, and the maximum and minimum growth rate are 10.77 % and 1.90 %, respectively. The velocity distribution of the pore cross-section is related to the pore structure, and the phenomena of 'reflux' and 'flow around' cause the displacement velocity to drop sharply, whose decreasing degree can reach 21.62 %. The maximum displacement resistance appears at the hole wall, and the narrower the hole, the greater the resistance. The displacement efficiency is directly proportional to the displacement pressure difference, and the initial pressure growth effect is significant (up to 25.49 %, and only 1.47 % later). The research results can provide a theoretical basis for the study of the water migration mechanism of porous slopes, and also can enrich the theoretical basis of rainfall-induced landslides and are helpful in preventing natural disasters.

Second law performance of a novel four-layer microchannel heat exchanger operating with nanofluid through a two-phase simulation

A 3D two-phase analysis is conducted to evaluate the second law features of a counter-flow four-layer microchannel heat exchanger (MCHE) operating with an Al2O3–water nanofluid. The coolant is considered water, while the hot fluid is the nanofluid. Three MCHEs with varied number of channels are examined. This investigation highlights the minimization of the exergy destruction to figure out the conditions in which the amount of degraded energy is lowest. Suspending more nanoparticles diminishes the thermal irreversibility of the nanofluid up to 61.6%, whereas the cold water experiences around 11.7% higher thermal irreversibility. At the small flow rates, increasing the volume fraction causes smaller exergy destruction, while at great flow rates, the trend becomes slightly ascending. The greater flow rate results in the greater thermal and frictional entropy generations. The second law efficiency firstly escalates via increasing the volume fraction but then remains almost constant or even descends somewhat.