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According to the structural and heat transfer characteristics of soil heat storage, a novel layered slice algorithm is proposed to realize the rapid and accurate solution to the problem. The heat transfer process between the double U-tube heat exchanger and the surrounding soil is taken as an example to analyze its solving performance. The study finds that the layered slice algorithm has higher simulation precision and faster solving speed. Its maximum relative error of temperature is only 0.19%. Compared with the traditional 3D simulation algorithm, it can accelerate about 2.2~2.56 times. At the same time, the layered slice algorithm has an excellent parallel characteristic. Its maximum parallel speedup ratio is more than twice that of the traditional 3D algorithm. Due to the superior solving performance, the proposed algorithm can help the optimization design of the buried-tube heat exchangers. [ABSTRACT FROM AUTHOR]

This paper presents an adaptive coupled volume-of-fluid and level set (VOSET) method based on unstructured grids to simulate incompressible interfacial flows. In this study, a novel high quality refinement algorithm for unstructured grid cells based on level set and surface curvature is first developed to ensure a preset bandwidth of the adaptive region around the interface. Subsequently, the adaptive VOSET algorithm is integrated into a finite volume solver with a large-scale sparse matrix preconditioned bi-stable conjugate method. Finally, the accuracy and robustness of the current method are validated by four classical test cases: the dam break, single bubble rising, a Rayleigh–Taylor instability problem, and a liquid jet problem. It is found that the adaptive coupled VOSET algorithm makes the numerical results near the interface more accurate, especially the curvature and surface tension at corners. Moreover, the adaptive VOSET is more efficient since it reduces the number of grid cells significantly. The present results show that only 4.85%–24.5% of the number of adaptive grid cells and 5.31%–15.93% of the computational time are needed for VOSET compared to that of fixed unstructured grid cells, and thus, it has outstanding properties of both time and computational cost savings. [ABSTRACT FROM AUTHOR]

In this paper, a coupled volume-of-fluid (VOF) and level set (LS) method on general curvilinear grids, combining the advantages of VOF method and LS method is developed for interfacial flow simulations. In this method, an iterative geometric operation is introduced to calculate the level set function on physical domain and computational domain. In particular, a direct construction of the distance function is successfully implemented on the physical domain and thus the numerical error induced by contravariant transformation from the computational domain could be avoided. The level set based continuum surface tension (CSF) model on curvilinear grids is derived to model surface tension accurately, and then the model as well as the proposed coupled volume tracking method is incorporated into an incompressible Navier–Stokes solver. Finally, four benchmark problems including time-reversed single vortex flow, static droplet, dam break and single bubble rising problems are tested on irregular domains to validate the accuracy and robustness of the proposed method to deal with complex domains. The results show good agreement with previous experimental data or numerical results, good characteristics of mass conservation are also observed, indicating that the present method can accurately simulate incompressible two-phase flows. • A coupled volume-of-fluid and level set method on general curvilinear grids is first proposed. • An iterative geometric operation is introduced to calculate the level set function. • A direct construction of the distance function is implemented on the physical domain. • The level set based continuum surface tension (CSF) model on curvilinear grids is derived. • The present method can accurately simulate incompressible two-phase flows in complex domains [ABSTRACT FROM AUTHOR]

The striking predilection of temporomandibular disorders (TMD) in women, especially during gonad-intact puberty or reproductive years, indicates that oestrogen plays an important role in the progression of TMD, but the underlying mechanism remains unclear. In this study, unilateral anterior crossbite (UAC) was used to create temporomandibular joint osteoarthritis (TMJ OA) models in rats, while 17β-estradiol (E 2 ) injections were applied to mimic patients with high-physiological levels of oestrogen. Micro-CT scanning, histological staining and real-time PCR assays were preformed to observe the degenerative changes in the mandibular condylar cartilage and subchondral bone. The results showed that obvious degradation was found in the condylar cartilage and subchondral bone of rats with UAC procedure, including decreased cartilage thickness, loss of extracellular matrix, increased apoptotic chondrocytes and expression of pro-inflammatory and catabolic factors, decreased bone mineral density and increased osteoclast activity. E 2 supplements aggravated the condylar cartilage degradation but reversed the abnormal bone resorption in the subchondral bone induced by UAC. Our results revealed that high-physiological oestrogen plays a destructive role in condylar cartilage but a protective role in subchondral bone at the early stage of TMJ OA. These dual and distinct effects should be given serious consideration in future OA treatments. [ABSTRACT FROM AUTHOR]

We develop a coupled volume of fluid and level set (VOSET) method for unstructured quadrilateral grids to simulate incompressible two-phase interfacial flows in irregular domains. In the method, an analytic piecewise linear interface calculation (PLIC) is first proposed and its solution speed is much faster (about five times) than that of Brent's iterative method; then an iterative geometric operation by which the level set function near interfaces can be calculated, is extended to unstructured quadrilateral grids; moreover, the volume fraction advection is solved by a Lagrangian-Eulerian advection scheme. Finally, the present method is validated by the single vortex flow problem, bubble rising problem and falling droplet problem. Our simulation results show good agreement with those in previous studies. [ABSTRACT FROM AUTHOR]

This paper presents a coupled volume-of-fluid and level set (VOSET) method for unstructured triangular meshes to simulate incompressible two phase flows. In the method, a multi-dimensional advection algorithm, which includes 11 different cases for computing volume fluxes across cell faces, is proposed for the evolution of the volume fraction. An iterative geometric operation is presented to calculate the level set function. Thus, both advantages of volume-of-fluid (VOF) and level set (LS) methods are preserved in this unstructured VOSET method. Finally, the feasibility and accuracy of the current method are validated by three classical test cases – static drop, dam break, Rayleigh-Taylor instability problems. The current simulation results show good agreement with the experimental and numerical results in the literature. [ABSTRACT FROM AUTHOR]

The IDEAL algorithm is an efficient and robust pressure-velocity coupling algorithm, which has been applied in a variety of fluid flow and heat transfer problems. However, the further development of the IDEAL algorithm encounters with two key issues: it is hard to be mastered by other researchers and difficult to be extended to complex engineering problems. In order to overcome these two issues, the IDEAL algorithm is implemented in the world's most widely used open source CFD software - OpenFOAM. In that way, it is convenient for any researcher to employ the IDEAL algorithm to solve complex fluid flow problems. Then, the performance of IDEAL algorithm is analyzed with focus on complex steady-state incompressible fluid flow problems. The results indicate that the IDEAL algorithm is superior to the SIMPLE and SIMPLEC algorithms in convergence and robustness for complex cases. In particular, the IDEAL algorithm can reach convergence, whereas the SIMPLE and SIMPLEC algorithms cannot obtain convergent solution in some cases. This research lays a foundation for a wider application of the IDEAL algorithm in complex engineering problems. [ABSTRACT FROM AUTHOR]

In order to accurately, robustly and efficiently simulate the three-dimensional (3D) interfacial flow problems with large density and viscosity ratio, we should, on the one hand, adopt the accurate and robust interface capturing method, on the other hand, use the efficient solution algorithm for coupling the velocity and pressure. In this paper, a coupled volume-of-fluid and level set (VOSET) method is applied to capture the two-phase interface, while an inner doubly iterative efficient algorithm for linked equations (IDEAL) is used to couple the velocity and pressure. Both of the VOSET method and the IDEAL algorithm were first proposed by the present author and further extended by other authors. Finally, the superiority of the VOSET method combined with the IDEAL algorithm (VOSET + IDEAL) is verified by three interfacial flow problems with large density and viscosity ratio. [ABSTRACT FROM AUTHOR]

We propose a coupled volume-of-fluid and level set (VOSET) method for interfacial flow simulations on unstructured triangular grids. In this method, the volume fraction advection is performed using a Lagrangian–Eulerian remapping algorithm, and the level set function is calculated by a simple iterative geometric operation. The present VOSET method can not only satisfy mass conservation but also predict the surface tension with good accuracy, thus combining the advantages and overcoming the disadvantages of volume-of-fluid (VOF) and level set (LS) methods. Finally, the present method is verified by well known Zalesak's slotted-disk revolution, single vortex flow, dam break and single bubble rising problems. The results illustrate that the present method can accurately simulate incompressible two-phase flows for unstructured triangular grids. [ABSTRACT FROM AUTHOR]

A coupled volume-of-fluid, level set, and smoothed physical parameter (VOF+LS+SPP) method based on FLUENT is used to simulate bubble dynamics in the gravitational and uniform electric fields. Both of the bubble and surrounding medium are assumed to be perfect dielectrics with constant but different permittivities. The effects of electric Bond number, permittivity ratio, Morton number, and Eotvos number on the deformation and rising motion of a single bubble are systematically investigated. Simulation results show that a vertical electric field elongates the bubble along the electric field direction and accelerates the bubble rising. The electric Bond number has a much greater effect on bubble deformation and rising velocity than the permittivity ratio. The bubble behaviors in the electric field are similar for different Morton numbers but totally different for various Eotvos numbers. The influence of electric field on bubble Reynolds number changes a little for different Morton numbers but decreases distinctly with the increase of Eotvos number. [ABSTRACT FROM AUTHOR]

IDEAL proposed by the present author is an efficient segregated algorithm for solving the incompressible fluid flow and heat transfer problems. However, its convergence rate is greatly influenced by the under-relaxation factor. The convergence rate under an optimum under-relaxation factor is dozens of times quicker than that under the most unfavorable under-relaxation factor. To lessen the influence, the IDEAL algorithm combined with a fuzz control method, called IDEAL+FC, is introduced to automatically regulate the values of the under-relaxation factor for accelerating the iteration convergence. Finally, it is demonstrated that IDEAL+FC is superior to IDEAL in terms of convergence rate and robustness. The rapid convergence rate can be achieved by IDEAL+FC even if the initial under-relaxation factor is the most unfavorable value. [ABSTRACT FROM PUBLISHER]

In the field of aerospace, minimum and seal of equipments cause the increase in the thermal loading sharply. Due to the lack of driving force, the performance of conventional condenser deteriorates greatly under the small gravity environment, which leads to reduction in the service life of equipments. In this study, a passive condenser, developed on basis of the phase separation concept, is utilized to improve the performance of the condensation heat transfer under the small gravity environment. As a result of the limitation of experiments, the mechanisms of heat transfer enhancement of the phase separation condenser tube are revealed through numerical simulation based on the volume-of-fluid (VOF) method. The following conclusions could be obtained: (1) A novel phase distribution of 'gas near the tube wall and liquid in the tube core' is formed. The thin liquid film is indeed created after the flow pattern modulation by inserting mesh cylinder. (2) The condensation quantity for single bubble in the annular region increases about 16 times greater than that in the bare tube region in the case of J = 0.0574 m/s and J = 0.0229 m/s. (3) Gas volume fraction affects the parameters of liquid film thickness, bubble length and liquid bridge length. The increase in the gas volume fraction results in the decrease in the evaluation index from 21.56 to 12.82. The evaluation index is defined as the ratio of the condensation quantities per unit tube length of the annular region and the bare tube region. [ABSTRACT FROM AUTHOR]

The main objective of this article is to solve inverse heat conduction problems with the particle swarm optimization method. An enhanced particle swarm optimization (EPSO) algorithm is proposed to overcome the shortcoming of earlier convergence of standard PSO algorithms. The EPSO is used to estimate the unknown time-dependent heat source in complex regions. Numerical experiments indicate the validity and stability of the EPSO method. [ABSTRACT FROM PUBLISHER]

• A new multi-crack initiation criterion of anisotropic rock is established. • Effects of crack relative positions and sedimentary angle on maximum SIF ratio is studied. • Two collinear cracks are always initiated at inner-tips in Mode I (α 1 ≥ 28°) or in Mode II (α 1 < 28°). • Mode II fracture occurs only when the two cracks both have small inclination angles. • The new criterion is verified by uniaxial compression tests of double-cracked shale specimens. Multi-crack initiation prediction of anisotropic rock is very important for designing fracture-networks and improving efficiency of shale-gas exploitation. Current literature is mainly focused on multi-crack interacting stress intense factors (SIFs) but less on multi-crack initiation criterion for anisotropic rock. In this study, a new multi-crack initiation criterion of anisotropic rock is established based on the maximum ratio of Mode I and Mode II SIF to its fracture toughness on the critical crack plane (θ IC or θ IIC), K I M θ IC / K IC M θ IC and K II M θ IIC / K IIC M θ IIC , in order to predict the multi-crack initiation parameters (crack initiation tip, angle and mode). Effects of crack relative position (α 1 , γ , D h and D s) and sedimentary angle (β) on K I M θ IC / K IC M θ IC and K II M θ IIC / K IIC M θ IIC are analyzed for anisotropic rock. Research results show that under uniaxial compression stress, two collinear cracks are always initiated at two inner-tips simultaneously in Mode I (with large crack incline angle, e.g., α 1 ≥ 28°) or in Mode II (with small crack incline angle, e.g., α 1 < 28°). Two vertical cracks (α 1 = 45°, α 2 = 135°, with extension line of one crack passing through the center of another crack) are initiated at only one crack-tip for any sedimentary plane angle (β = 0°∼180°) in Mode I. Two arbitrary cracks are initiated at one crack-tip in Mode II only when the absolute value of two-crack incline angles with respect to horizontal direction are both small (e.g., |α 1 |=10°<28°, |α 2 |=20°<28°). Theoretical prediction results are in good agreement with uniaxial-compression test results of shale and sandstone plates with two arbitrary cracks. The new multi-crack initiation criterion can be further developed to predict the whole multi-crack propagation process of anisotropic rock. It provides a theoretical basis for safety assessment of rock engineering and cracking-network formation of shale-gas hydraulic exploitation. [ABSTRACT FROM AUTHOR]

Two-phase loop systems are important to maintain low temperature in a spacecraft at micro-gravity. It is known that the condenser size at micro-gravity can be one order magnitude larger than that at the earth gravity. This paper explores the effectiveness of the modulated flow technique at micro-gravity for the condensation heat transfer enhancement. A mesh cylinder is suspended in a tube, dividing the tube cross section into annular region and core region. When an intermittent two-phase flow interacts with the mesh screen structure, gas bubbles are prevented from entering the mesh cylinder thus they flow in the annular region to form the thin liquid film on the wall. Liquid can be separated to flow in the core region. Here the bubble dynamics in the bare tube section and modulated flow section are investigated for three different slug flow cases. It is found that the mesh cylinder modulates the slug flows in the bare tube section to form the elongated-ring-slug bubble train in the modulated flow section. Liquid plugs can disappear due to the continuous liquid flow from the annular region to the core region. Thus the two-phases are thoroughly separated with gas flowing in the annular region, forming ultra-thin liquid film thicknesses on the wall at micro-gravity. The modulated flow pattern forms the liquid film thicknesses to be about 1/30–1/10 of those in the bare tube section for the three cases. Based on the modulated bubble dynamics and parameters, the condensation rates per unit wall area ( m ̇ ) can be about 10–100 times of those in the bare tube section for the three cases. The modulated flow pattern and enhanced heat transfer are more obvious when the gas volume flow rates are decreased. This study provides a new approach to enhance the condensation heat transfer for space applications under micro-gravity environment. [ABSTRACT FROM AUTHOR]

Based on the volume-of-fluid (VOF) method in the FLUENT code, many phase-change models have been proposed and applied to simulate evaporation and condensation problems. To further improve the accuracy, in this article a new phase-change model is built using user-defined functions (UDFs). The accuracy of this new phase-change model is verified by two evaporation problems (a one-dimensional Stefan problem and a two-dimensional film boiling problem) and one condensation problem (single steam bubble condensation in subcooled water). The simulation results of this new model show good agreement with the classical analytical or numerical results, proving its accuracy and feasibility. [ABSTRACT FROM AUTHOR]

Purpose – Based on the numerical research on the relationship between the flow pattern transition and the condensation heat transfer in circular microchannels, the purpose of this paper is to bring forward a concept of external separation circular microchannel to regulate and control the flow pattern for enhancing the condensation heat transfer. Design/methodology/approach – The numerical research is based on the volume of fluid method and the vapor-liquid phase change model proposed by the present authors. Findings – By numerical research on the condensation process of water in a general circular microchannel, it is discovered that, with the increase of the inlet velocity and the reduction of the temperature difference between the saturation temperature and the channel wall temperature, the bubble detachment frequency is raised and the water vapor condensation length is extended, representing an exponential growth. Therefore, for the condensation process with low temperature difference and high mass flow rate, it is in urgent need to regulate and control the flow pattern. Originality/value – To prevent the flow pattern in the general circular microchannel converted from annular flow to slug flow and then to bubble flow, this paper brings forward a concept of external separation circular microchannel, which regulates and controls the flow pattern by discharging partial liquid from the annular wall opening. After regulation and control, the flow pattern is converted from original periodic annular flow/slug flow/bubble flow to current stable annular flow. Accordingly, the heat transfer performance is enhanced and the condensation length is lowered remarkably. [ABSTRACT FROM AUTHOR]

• True Mode II fracture toughness (K IIC) of anisotropic rock is measured by shear-box test. • Shear-box test is proved to be an effective method for measuring K IIC of anisotropic rock at any anisotropy orientation angle. • A new physical factors Y I0 and Y II0 are proposed to calculate K I0 and K II0 for shear-box specimen. • When θ = 0–10°, K Iθ is negative, which promotes occurrence of Mode II fracture. • The K IIC of anisotropic shale is increased monotonously as β (β = 0–90°) increased. Fracture toughness of anisotropic shale is an important parameter in shale-gas exploitation technology. Currently available literature is mainly focused on Mode I fracture toughness (K IC) of anisotropic rock under tensile loading. Although there are the cracked straight through Brazilian disk and crack ring disk used to determine Mode II fracture toughness (K IIC) of anisotropic rock under pure shear stress, their fracture trajectories are deviated from the original crack plane and cannot be regarded as the true Mode II fracture. In this paper, shear-box test was firstly adopted to measure K IIC of anisotropic shale. New physical factors Y I0 and Y II0 were proposed to describe effects of both the geometry and the material parameters on stress intensity factors (SIFs) of the original crack plane (K I0 and K II0) and to derive the calculation formulae of SIFs on arbitrary (K Iθ and K IIθ). Calculated results show that K IIθ reaches its maximum absolute value at θ = 0–10° where K Iθ is negative, which promotes occurrence of Mode II fracture. The predicted planes of Mode II fracture agree well with the tested fracture trajectories (amostly along the original crack plane). The K IIC is increased with increase of β (β = 0–90°). K IIC is 3–4 times as large as K IC and can be regarded as the true Mode II fracture toughness of anisotropic shale. The shear-box test is an effective method for measuring K IIC of anisotropic rock. [ABSTRACT FROM AUTHOR]

The passive phase separation concept was proposed to modulate flow patterns for heat transfer enhancement. By the flow pattern modulation, the gas tends to be near the wall and the liquid tends to be in the tube core. Experiment has been performed to verify the fresh idea and the flow pattern modulation mechanism was analyzed qualitatively. This paper focuses on the numerical simulation of the bubble dynamics for a single bubble in the vertical phase separation condenser tube to quantitatively explore the flow pattern mechanism, based on a multiscale grid system and the volume-of-fluid (VOF) method. It is found that: (1) the modulated liquid film thickness can be decreased by 70% compared to that in the bare tube region; (2) the modulated bubble traveling velocity can be doubled, causing the increased liquid velocity and velocity gradient in the annular region to weaken the fluid boundary layer; (3) the significantly increased bubble traveling velocity in the annular region promotes the mass and momentum exchange between the annular region and the core region, and yields the self-sustained pulsating flow in the core region. The above three factors are benefit for the performance improvement of the heat transfer facilities. [ABSTRACT FROM AUTHOR]

The segregated algorithm-IDEAL (inner doubly-iterative efficient algorithm for linked-equations) is an efficient and stable algorithm. In this algorithm, there exist inner doubly-iterative processes for pressure equation, which almost completely overcome two approximations in SIMPLE algorithm. Thus the coupling between velocity and pressure is fully guaranteed, greatly enhancing the convergence rate and stability of iteration process. In this paper, implementation of the IDEAL algorithm on a 3D collocated grid system is conducted. The interlace velocity is calculated by the modified momentum interpolation method (MMIM), by which the converged result is independent of the under-relaxation factor. Finally, five three-dimensional incompressible fluid flow and heat transfer problems are provided to compare the convergence rate and robustness between the IDEAL and three other most widely-used algorithms (SIMPLER, SIMPLEC and PISO). By the comparison it can be concluded that the IDEAL algorithm is more robust and efficient than the three other algorithms. [ABSTRACT FROM AUTHOR]

Hydrogen production from electrolytic water is one of the most crucial techniques for generating green hydrogen. If the flow of hydrogen gas and alkali water in the transportation pipe exhibits slug flow, vibrations may occur, leading to potential accidents. The study of preventing slug flow is significant for pipe design. However, currently, there is no available research on the gas-liquid two-phase flow patterns of hydrogen gas and alkali water. Therefore, this study investigates the gas-liquid two-phase flow patterns in the pipe transporting hydrogen gas and alkali water and provides recommendations to prevent slug flow. Firstly, a numerical model based on the Volume of Fluid (VOF) method is established to simulate the two-phase flow of hydrogen gas and alkali water. Then, the effects of hydrogen gas and alkali water flow rates, pipe orientation, pipe diameter, and pipe length on the gas-liquid two-phase flow patterns are studied. Subsequently, the two-phase flow patterns are analyzed under different pipe layouts, including horizontal-vertical pipes, horizontal-horizontal pipes, horizontal-vertical-horizontal pipes, etc. The research reveals that increasing the pipe diameter, reducing the pipe length and increasing the hydrogen gas flow rate can effectively prevent slug flow in vertical pipes. Additionally, horizontal pipes and horizontal-horizontal pipes are less likely to experience slug flow. Furthermore, the consecutive appearance of two elbows induces slug flow even at higher hydrogen apparent velocities. Extending the length of the pipe between two consecutive elbows can effectively suppress the occurrence of slug flow. The present work can provide an important reference for the design of pipes used in hydrogen production from electrolytic water. [Display omitted] • The flow pattern of hydrogen-alkali water two-phase flow is studied. • Six different structures of pipes are studied, especially those containing elbows. • Two consecutive elbows with short distances can easily induce a slug flow. • Altering flow rates and pipe structural parameters can suppress slug flow. [ABSTRACT FROM AUTHOR]

Gravity plays a crucial role in influencing bubble behavior and heat transfer in flow boiling, and its impact can vary significantly in aerospace settings. Therefore, this study aims to numerically investigate flow boiling heat transfer in a rectangular mini-channel under hyper-gravity conditions, specifically at 12 times the normal gravity. To accomplish this, a coupled volume-of-fluid and level set method is employed, taking into account fluid-solid conjugated heat transfer as well as a nucleus site density model derived from experimental data. By reproducing the flow pattern and heat transfer characteristics under different heat flux and flow rate conditions, the study unveils the effects of hyper-gravity on flow boiling heat transfer. When the flow rate is lower under hyper-gravity conditions, a notable phenomenon occurs wherein numerous bubbles detach from the heating wall and coalesce into a vapor film at the top of the mini-channel due to increased buoyancy. In contrast, under normal gravity, bubbles merge and slide on the heating wall, leading to the formation of a dry patch below. Consequently, hyper-gravity results in a lower wall superheat, and the disparity in average wall superheat between normal and high gravities escalates as the added heat flux rises. Notably, in the hyper-gravity environment, the frequent detachment of bubbles in the middle and downstream sections of the mini-channel leads to an initial increase in wall superheating, followed by a plateau along the flow direction. As the flow rate increases, the inertial force intensifies. However, intriguingly, the discrepancy in flow boiling heat transfer between normal and high gravities does not exhibit a monotonic decrease with the increasing flow rate. This behavior can be attributed to the pressing of more bubbles onto the heating wall under normal gravity, resulting in the formation of dry patches at high velocities. [ABSTRACT FROM AUTHOR]

The interface tends to behave as a circular shape under the effect of surface tension in the two-dimensional two-phase fluid flow. In this study, based on the coupled volume-of-fluid and level set (VOSET) method, an accurate circle-based interface reconstruction (CIR) algorithm for structured meshes is proposed, which features a straightforward implementation procedure. A portion of the standard circle is employed to approximate the curve interface, without the need for complex classification, coordinate transformation, and equation transformation. The radius of this circle is computed simply by the curvature and signed distance at the mixed cell center, and the center of this circle is determined using a straightforward dichotomy method under the mass conservation constraints. Results indicate that the coupled VOSET and CIR (VOSET-CIR) method maintains superior computational accuracy in the signed distance, interface curvature, and dynamic interface reconstruction compared to the coupled VOSET and piecewise linear interface calculation (VOSET-PLIC) method, with accuracy improvements ranging from 34% to 1490% across different test cases. Furthermore, the VOSET-CIR method outperforms the efficient least squares volume-of-fluid interface reconstruction (ELVIRA) and PLIC algorithms in reconstructing random circles and surpasses the quadratic spline based interface reconstruction (QUASI) algorithm in reconstructing random squares. In terms of computational efficiency, except for the initial PLIC algorithm, the time costs of the VOSET-CIR method are substantially lower than those of the ELVIRA and QUASI algorithms, as well as the VOSET-PLIC method. [ABSTRACT FROM AUTHOR]

Plugging and diverting fracturing is a promising technology that aims to enhance the heat extraction efficiency in hot dry rock. The key to the success of this technique is the formation of effective plugging zones in existing fractures. However, given the high temperature and high stress of hot dry rock, the migration and sealing mechanisms of temporary plugging agents in such reservoirs are quite different from those in conventional tight reservoirs. Using the computational fluid dynamics/discrete element method coupled method, this paper numerically investigates the migration mechanism of temporary plugging agents in rough fractures of hot dry rock. First, we construct a model of a rough fracture surface in hot dry rock by performing computerized tomography scanning. Second, we adopt the well-established theory of the joint roughness coefficient to describe the fracture surface roughness. Then a discrete phase model that considers the effect of temperature is constructed to characterize the interparticle interaction of temporary plugging agents. A bidirectional coupling algorithm between the fluid flow in the fracture and the migration of temporary plugging agent particles is adopted. Finally, the effects of key factors such as fracture wall temperature, fracture roughness, injection angle, and injection location on the migration mechanism of granular temporary plugging agents in rough fractures are analyzed in detail. The results show that fracture roughness and temperature have a significant impact on the migration process in hydraulic fractures. When the fracture surface roughness increases by 10.44 as measured by the joint roughness coefficient, the particle force and particle temperature increase by 12.0% and 37.8%, respectively. When the fracture surface temperature increases by 200 K, the particle force and particle temperature increase by 88.2% and 14.4%, respectively. [ABSTRACT FROM AUTHOR]

Bubble nucleation is the initial stage of flow boiling and plays an important role in boiling heat transfer. However, bubble nucleation occurs at a microscopic scale, rendering it challenging for the macroscopic computational fluid dynamics method to realistically simulate this intricate process. In this paper, based on the coupled volume-of-fluid and level set method, a heterogeneous nucleation model is improved and conducted to simulate the subcooled flow boiling in a rectangular mini-channel, considering these conditions both in the presence and absence of a microlayer. The coefficient of the original heterogeneous nucleation model is adjusted across a range from 0.1 to 10.0 times its previous value to establish multiple new nucleation models for illustrating their effects on flow patterns and heat transfer characteristics. For flow boiling without a microlayer, when the coefficient of the original heterogeneous nucleation model is halved, the nucleate boiling intensity upstream of the channel diminishes, resulting in a reduction in the heat transfer coefficient. Nevertheless, this alteration mitigates the formation of slug flow and the appearance of dry patches near the channel outlet, consequently averting a sharp increase in outlet wall superheat. Quantitatively, relative differences of 23.83% and 90.48% in average and local maximum wall superheat are observed, respectively. In contrast, the presence of a very thin microlayer beneath the growing and slipping bubble in flow boiling with a microlayer is notable. This microlayer quickly evaporates, dissipating more than 77% of the input heat flux and substantially expanding the bubble volume. Consequently, under identical wall superheat conditions, the influence of variations in the number of activated bubbles induced by different heterogeneous nucleation models on heat transfer and flow patterns in flow boiling is significantly attenuated. Specifically, when the difference in nucleus site density remains within a tenfold range, the differences in the average and maximum wall superheat are limited to just 16.78% and 33.86%, respectively. Concerning flow boiling in a mini-channel featuring a microlayer, the simulation results verify that large deviations in the activated bubble number have few effects on the flow pattern and wall superheat, greatly reducing heterogeneous nucleation model requirement and promoting the numerical study of flow boiling. [ABSTRACT FROM AUTHOR]

Guide vanes are the key components of axial flow cyclones (AFCs). The structural parameters of these vanes have a significant impact on the separation performance of the AFC. A multi-objective optimization study of guide vanes was conducted using a computational fluid dynamics (CFD) model previously proposed by the present authors, support vector machine (SVM), and non-dominated sorting genetic algorithm-II (NSGA-II). The obtained Pareto optimal solutions demonstrate that the separation efficiency and pressure drop increase as the number and wrapping angle of the guide vane increase; further, they decrease as the outlet angle and width of the guide vane increase. Moreover, the correlation between the separation efficiency and the pressure drop in the Pareto front was regressed to facilitate the design of the guide vane to achieve the desired separation performance. The research results can provide useful guidance for the design and optimization of AFCs. Unlabelled Image • Multi-objective optimization of guide vanes for axial flow cyclone is performed. • A CFD model proposed by the present authors is used to construct the database. • Surrogate model of objective functions is built via Support Vector Machine. • The surrogate model is fed into the NSGA-II to perform the optimization. • Special characteristics of Pareto optimal solutions have been investigated. [ABSTRACT FROM AUTHOR]

In this paper, the bubble nucleation of ultra-thin liquid argon film on a platinum surface is studied by molecular dynamics simulation. In the simulations, the liquid is heated by the middle part and cooled by the two sides of the platinum surface. Observation of bubble nucleation is performed after increasing the temperature of heating zone. The space and time evolution of argon density distribution and the size of bubble are monitored to investigate the nucleation processes above surfaces with and without nanostructures. The results illustrate that nanostructure surface increases the solid-liquid contacting area and the degree of surface temperature non-uniformity, which results in the enhancement of heating intensity and higher rate of nucleation. Comparisons are made among surfaces with different nanostructures, with which it is further found that the heat flux increases with the height of nanostructures. Higher nanostructure reduces nucleation duration. [ABSTRACT FROM AUTHOR]

A single sensible thermal storage system has the disadvantage of poor system efficiency, and a sensible-latent graded thermal storage system can effectively solve this problem. Moreover, the graded thermal storage system has the virtue of being adjustable, which can be adapted to many power generation systems. Therefore, this paper first analyzes the influence factors of the graded thermal storage system's exergy and thermal efficiency. Subsequently, each factor's significance was analyzed using the response surface method, and the prediction model for system exergy efficiency and cost was established using the support vector machine method. Finally, the second-generation nondominated sorting genetic algorithm (NSGA-II) was used to globally optimize the graded thermal storage system's exergy efficiency and cost by Matlab software. As a result, the exergy efficiency was increased by 11.01%, and the cost was reduced by RMB 5.85 million. In general, the effect of multi-objective optimization is obvious. [ABSTRACT FROM AUTHOR]

A two‐dimensional model of the charging process on a heat storage unit in a shell‐and‐tube type latent heat subsystem of a solar power plant with direct steam generation was constructed in this study. The effects of the outer diameter to inner diameter ratio, aspect ratio, phase change material (PCM) thermal conductivity, and heat transfer fluid (HTF) mass flow rate were investigated. Results show that increasing the PCM thermal conductivity, HTF mass flow rate, and aspect ratio of the heat storage unit can shorten heat storage time, but the ratio of the outer diameter to the inner diameter of the heat storage unit has an optimal value of 6 in this problem. Using response surface methodology analysis, the influence of the aspect ratio, outer‐to‐inner‐diameter ratio, PCM thermal conductivity, and HTF mass flow rate on the storage time of the phase change heat storage unit is in descending order. After a genetic algorithm optimization, the storage rate of the heat storage unit increased by 35%. The results of this study can guide the heat storage unit to achieve a better practical application performance. [ABSTRACT FROM AUTHOR]

To simulate bubble dynamic behaviors under an electric field conveniently and accurately, a volume-of-fluid, level set, and smoothed physical parameter (VOF+LS+SPP) method based on FLUENT is first proposed. Compared with the VOF and VOF+LS methods based on FLUENT, the VOF+LS+SPP method has very high precision and the maximum deviation is only 7%. In addition, its simulation results are superior to those results obtained by the front tracking, LS and phase field methods in the literature and almost the same with the data acquired by the VOSET method. Finally, the proposed method is used to investigate the law and mechanism of bubble deformation with different permittivity ratios. [ABSTRACT FROM PUBLISHER]

• A new theory of multi-crack propagation under HM loading is established based on criterion of maximum tensile-shear stress intensity factor ratio. • The new optimization method of double-crack distributions for improving network fracture conductivity of natural gas exploitation has the advantages of high precision and simple form. • The predicted results are verified by the biaxial compressive HM coupling tests of red sandstone specimens with double cracks. Hydraulic fracturing is used for natural gas extraction and its purpose is to form fracture networks. The effective measure to enhance the productivity of hydraulic fracturing is to improve the conductivity of network fractures, which takes crack penetration length as evaluation index. In this paper, a new Mode I and Mode II SIF calculation formula for branch-crack tips of multi-crack was deduced by the complex function and the superposition method. A new multi-crack fracture criterion was established based on the criterion of maximum tensile-shear SIF ratio under hydraulic-mechanical (HM) load. The total double-crack propagation length L was selected to characterize the conductivity of network fractures. An infinite plate contains any double-crack AB (hydraulic crack) with unequal length and CD (natural crack) under hydraulic-mechanical biaxial compression is taken as an example. The effects of single factor (such as relative distance Ds and Dh , relative inclined angle α , crack length a 1 and a 2) on the total length L are calculated and the calculation formula of L is derived by the uniform design method. Take L max as the optimization objective, the optimization model of fracturing parameters was established to obtain the optimization results, which are verified by test results. Results show that peak load decreases (or increases) and decreases (or increases) more when the crack relative inclination angle and relative vertical distance decrease (or increase). The crack growth is restrained by the increase of hydraulic pressure. [ABSTRACT FROM AUTHOR]

Under a low flow rate, gravity may become prominent for bubble behavior and heat transfer of flowing boiling because of the weakness of drag force from liquid, and its effect changes with the inclination angle of the mini-channel but without consensus. In this paper, based on a reasonable nucleus site density model and considering conjugate heat transfer, the coupled volume-of-fluid and level set method is adopted to study the subcooled flow boiling in an inclined three-dimensional rectangular mini-channel (0°–180°) with a characteristic size of 1.0 mm at a low flow rate of 88.8 kg m−2 s−1. The inclination angle is found to have a slight effect on the flow boiling, which is different from the conclusion drawn based on the traditional-macro channel. A bubbly flow appears when a heat flux of 300 kW/m2 is added. An unconventional impact force is proposed, which presses large bubbles to slip along the heating wall, with slight differences in the flow pattern under different inclination angles. When the inclination angle is close to 0°, the upstream small/medium bubbles leave the heating wall under gravitational effects, which is conducive to heat transfer. As the inclination angle approaches 90°, gravity pushes the large bubbles downstream to leave the channel, favoring the rewetting of the dry patches below. These two positive effects fail as the inclination angle approaches 180°, leading to slightly worse overall heat exchange efficiency. However, the maximum differences in the average and local wall superheating of the mini-channel are only 8.4% and 22.5%, respectively, across the range of inclination angles because the flow pattern remains similar under the effect of the impact force. In addition, the effect of inclination angle on flow boiling becomes weaker with the increase in heat flux because of happening of slug flow. [ABSTRACT FROM AUTHOR]

Stratified thermal energy storage (TES) tanks are widely used in thermal power plants to enhance the electric power peak load shifting capability and integrate high renewable energy shares. In this study, a data‐driven surrogate modeling and optimization study of the unequal diameter radial diffuser previously proposed by the present authors is conducted. First, based on the orthogonal experimental design, numerical experiments are performed to generate the performance database. Then, the database is used to establish the data‐driven surrogate model via the support vector machine. Subsequently, the single‐objective optimization and multiobjective optimization of an unequal diameter radial diffuser are conducted using the genetic algorithm. For the single‐objective optimization, the optimal thermocline thickness is 0.829 m when the diameter ratio of the long baffle and the tank is 0.426, the diameter ratio of the short baffle and the long baffle is 0.823, and the distance between the two baffles is 228.51 mm. For multiobjective optimization, the obtained Pareto optimal solutions are obtained. Under the premise of maintaining excellent thermal stratification, the selected Point C can reduce the steel cost by 88.1%. The research results are helpful for designing efficient and economical unequal diameter radial diffusers for TES tanks. [ABSTRACT FROM AUTHOR]

• A comprehensive review on numerical simulations of boiling heat transfer from atomistic to mesoscopic and macroscopic scales is presented. • Critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling are highlighted. • Potential solutions and future research in the field of boiling heat transfer using the MDS, LB and CFD methods are proposed. Boiling is an efficient heat transfer mode with significant potential for thermal management in high-power electronic equipment. However, a comprehensive understanding of the boiling process, which encompasses bubble nucleation, growth, coalescence, slipping, and detachment across various scales, remains challenging. Molecular dynamics simulation, lattice Boltzmann, and computational fluid dynamics methods are popular and powerful tools for investigating boiling heat transfer phenomena at microscopic, mesoscopic, and macroscopic scales. These methods enable researchers to uncover the underlying boiling mechanisms and propose heat transfer enhancement techniques. Therefore, this paper provides a comprehensive review of boiling heat transfer, spanning from atomistic to mesoscopic and macroscopic scales, utilizing these three numerical methods. It addresses critical issues related to nanoscale bubble nucleation mechanisms, pool boiling, and flow boiling, and proposes potential solutions and future researches, supplementing our previous review [Some advances in numerical simulations of multiscale heat transfer problems and particularly for boiling heat transfer, Annu. Rev. Heat Transf., 6 (2022) 217–269]. Besides, by shedding light on the characteristics of these numerical methods in studying boiling heat transfer, this paper aims to foster their development and advance enhanced heat transfer technologies. [ABSTRACT FROM AUTHOR]

The inner doubly iterative efficient algorithm (IDEAL), an advanced fully implicit algorithm for solving the pressure-velocity coupling problem, lacks a comprehensive implementation procedure on commonly used collocated unstructured grids. Therefore, this study provides a thorough demonstration of the fundamental principles, solving procedures, discretizations of momentum and pressure equations, Rhie-Chow interpolation implementation, mass continuity processing, and boundary condition treatment associated with the IDEAL algorithm on a collocated unstructured grid. Additionally, a comparative analysis between the semi-implicit method for pressure-linked equations (SIMPLE) and the IDEAL algorithm is conducted concerning computational efficiency under different grid types and flow conditions. The results indicate a significantly higher computational efficiency achieved by the IDEAL algorithm across all tested cases, attributed to its ability to overcome two fundamental assumptions inherent in the SIMPLE algorithm. The average acceleration ratio for these cases is approximately 3.13 (tSIMPLE/tIDEAL). This study serves as a valuable reference for the further development and application of the IDEAL algorithm in solving flow problems with complex irregular domains discretized by unstructured grids. [ABSTRACT FROM AUTHOR]

The modeling of shell-and-tube phase change heat exchanger for the direct steam generation usually does not consider the effect of the natural convection in the previous studies, which is different from the actual physical process. To overcome this issue, a numerical model considering the natural convection is established in the current work. The condensation process of saturated steam is modeled using the VOF model and Lee model, while the melting and solidification process of phase change material (PCM) is modeled using the enthalpy-porosity method. The heat transfer characteristics of shell-and-tube phase change heat exchanger for direct steam generation are studied, with and without considering the natural convection. The research indicates that the condensing amount of the steam at the outlet increases significantly after considering the natural convection. Accordingly, the mass fraction of the steam at the outlet decreases significantly, which reduces the melting time of the PCM by about 30%. Therefore, considering the natural convection effect is necessary for the numerical simulation of the shell-and-tube phase change heat exchanger for direct steam generation. [ABSTRACT FROM AUTHOR]

Recently, our research group proposed the phase separation condenser tube, in which a mesh cylinder was inserted to form the flow structure of 'gas near the tube wall and liquid in the tube core', significantly enhance the condensation heat transfer. But the bubble leakage towards the core region may worsen the heat transfer enhancement. In order to prevent the bubble leakage, the critical criterion was proposed based on the Young-Laplace equation, considering the inertia force, viscous force and pulsating flow. It was found that the critical criterion depends on the dimensionless parameter G, the We number and a coefficient C. The numerical model was developed in terms of the volume of fluid method to predict the two-phase laminar flow in the phase separation condenser tube. The results show that the bubble leakage takes place at the bubble tip, which is agreed with the experimental observations. The critical curve distinguishing the non-bubble-breaking and bubble-breaking was obtained by comparing the bubble dynamics at different G* and We. The coefficient C was determined. The critical criterion for the bubble leakage is given as $$ G^{*} \times We = \left( {0.22We^{0.99349} - \frac{{G^{*} }}{{4.7 \times 10^{3} \times We^{0.00651} }} + \frac{196.39}{{We^{0.00651} }}} \right) \times \frac{{4{ \cos }\alpha }}{\Delta \varTheta W} $$ , providing the design and operation guidance for the phase separation condenser tube. [ABSTRACT FROM AUTHOR]

Abstract: The flow field modulation concept was proposed. A mesh cylinder was suspended in a tube, dividing the tube cross section into an annular region and a core region. The invention is called the modulated heat transfer tube (MHTT). The numerical simulation was performed in laminar flow regime at constant heat flux boundary condition with water as the working fluid. An equal flow area criterion was proposed for the conversion of 3D to 2D mesh pores. The non-uniform grids link micron scale of mesh pores with meter scale of tube. The results show double-peak velocity distribution over tube cross section. The near wall region has larger velocity and velocity gradient, accounting for the heat transfer enhancement mechanism. For any specific Reynolds number, there is a critical length beyond which heat transfer is deteriorated. Therefore, a set of short mesh cylinders was suspended in the tube. The configuration is called the improved modulated heat transfer tube (IMHTT). It is shown that the IMHTT ensures significant heat transfer enhancement over the whole tube length. This study provides a new enhancement mechanism and tube configuration for heat transfer, having a wide engineering application potential. [Copyright &y& Elsevier]

The solar-driven calcium looping is a very promising high-temperature thermochemical energy storage technology. The calcium-based particles suffers from high thermal stress and is prone to fragmentation in high-flux concentrated solar power systems. Therefore, in this study, calcium carbonate spherical particles with certain hardness were prepared using the extrusion spheronization method, and the real pore structure of calcium carbonate particles was obtained through micro-computed tomography scanning (μ-CT). Next, the displacement and strain of the calcium carbonate particles during the heating process were measured in real-time using optical methods. Integration of displacement vectors in different directions showed that the displacement in the X direction was the dominant direction. Subsequently, numerical simulation was employed to calculate the temperature field and stress field of the calcium carbonate particles with the real pore structure. The results showed that with the increase of heating time, the maximum temperature difference within the particles increased from 6 K to 13 K, and the corresponding maximum thermal stress on the particles increased from 2.13 MPa to 3.87 MPa. Finally, the maximum thermal stress obtained from numerical simulations was compared with experimental results, and the results showed good agreement between the experimental and simulated values, with an error range of 8.93 %–21.64 % and an average error of 16.9 %. The findings are of significant importance for preventing the fracture of high-temperature thermochemical energy storage particles during operation. [ABSTRACT FROM AUTHOR]

Flow boiling stands out as an efficient mode of heat transfer with significant potential for reducing carbon footprint. Based on a coupled Volume-of-Fluid and Level Set (VOSET) method, this numerical study reveals the microlayer and its evaporative heat flux distributions, as well as the effects of microlayer evaporation and depletion on bubble dynamic behavior, flow pattern, wall superheat distribution, and boiling crisis for the flow boiling in a mini-channel. These aspects are firstly comprehensively reported by the numerical method. Results indicate that the rapid evaporation of a thin microlayer positioned beneath elongated bubbles significantly contributes to heat dissipation during flow boiling in a mini-channel. The heat flux through the microlayer surpasses 300 kW/m2, accounting for approximately 42% of heat dissipation at 200 kW/m2. Furthermore, this contribution increases to over 70% as the heat flux exceeds 500 kW/m2. As a result, in comparison to scenarios without microlayers, the presence of microlayers results in an average heat transfer coefficient (HTC) that is 3.97 times larger within the range of 200–500 kW/m2. Additionally, microlayer evaporation delays the happening of flow boiling crisis. Without microlayers, the wall superheat exceeds 180 K at 500 kW/m2, whereas under microlayer influence, the maximum local wall superheat is only 37.09 K at 1000 kW/m2. With further increases in heat flux, the emergence of large dry patches resulting from microlayer depletion induces the beginning of heat transfer deterioration. Finally, the ongoing coalescence of elongated bubbles necessitates a prolonged period for rewetting dry patches at the exit of the mini-channel, triggering the flow boiling crisis at 1300 kW/m2. The study results provide crucial guidance for the development and application of the flow boiling heat transfer technology for the thermal management of data centers. [ABSTRACT FROM AUTHOR]

• The effects of distilled water and nano emulsion on the physical and mechanical properties of hot dry rocks at high temperatures are investigated. • The temperature increase promotes the formation of massive thermal damage fractures in the hot dry rock samples, and the fluid weakens the mechanical strength of hot dry rocks. • The distilled water has a more pronounced effect on weakening the mechanical strength of the samples than the nano emulsion. Hydraulic fracturing is a crucial technology for efficient heat extraction from hot dry rock (HDR). The fracturing process involves the injection of large volumes of cold fluids, which can significantly affect the petrophysical properties of the rock, particularly when temperature differences and fluid-rock interactions are taken into account. However, there is a lack of research on the evolution of HDR petrophysical properties under the influence of different fluids at high temperatures. In this paper, we utilized a high-temperature high-pressure tri-axial loading system, rock spontaneous imbibition instrument, porosity-permeability tester, and thermal conductivity meter, along with comprehensive analysis techniques such as ultrasonic wave velocity monitoring, CT scanning, and nuclear magnetic resonance (NMR), to systematically investigate the effects of distilled water and nano emulsion on the physical and mechanical properties of the HDR at high temperatures. Through comparison with the experimental results of dry samples, comprehensive characteristics of physical parameters for the HDR under the influence of distilled water and nano emulsion are obtained, such as spontaneous imbibition mass and permeability, thermal physical properties, and rock mechanical parameters. The experimental results show that temperature increase promotes the formation of massive thermal damage fractures in the HDR samples, and the fluid weakens the mechanical strength of the HDR. When the effects of temperature and saturated fluids are combined, the hydraulic fracture network in the HDR samples exhibits increased complexity, contrasting with the fracture patterns observed in the heat-treated dry samples. The distilled water has a more pronounced effect on weakening the mechanical strength of the samples than the nano emulsion. Moreover, the analyses of permeability, thermal conductivity, ultrasonic wave velocity, and NMR results are consistent with the results of mechanical experiments. As the temperature rises, the wave velocity of saturated distilled water samples exceeds that of both saturated nano emulsion samples and dry samples, the velocity of the saturated fluid front in the samples increases, and the range of the fluid in the pores expands. Overall, these findings provide valuable experimental evidence and theoretical guidance for better understanding the hydro-thermal-mechanical behaviors of the HDR, which are crucial for the efficient development and utilization of geothermal energy. [ABSTRACT FROM AUTHOR]

The multiphase heat transfer could be enhanced by creating thin liquid film on the wall. The phase separation concept is called due to the separated flow paths of liquid and gas over the tube cross section to yield thin liquid film. Our proposed heat transfer tube consists of an annular region close to the wall and a core region, interfaced by a suspending mesh cylinder in the tube. The heat transfer tube is a multiscale system with micron scale of mesh pores, miniature scale of annular region and macroscale of tube diameter and length. Great effort has been made to link from micron scale to macroscale. The Volume of Fluid (VOF) method simulates air/water two-phase flow for vertical upflow. The three-dimensional system was successfully converted to a two-dimensional one by using three equivalent criteria for mesh pores. The non-uniform base grid generation and dynamic grid adaption method capture the bubble interface. The numerical results successfully reproduce our experimental results. The numerical findings identify the following mechanisms for the enhanced heat transfer: (a) counter-current flow exists with upward flow in the annular region and downward flow in the core region; (b) void fractions are exact zero in the core region and higher in the annular region; (c) the liquid film thicknesses are decreased to 1/6–1/3 of those in the bare tube section; (d) the gas–liquid mixture travels much faster in the annular region than in the bare tube; (e) three-levels of liquid circulation exists: meter-scale bulk liquid circulation, moderate-scale liquid circulation around a single-elongated-ring-slug-bubble, and microliquid circulation following the ring-slug-bubble tails. These liquid circulations promote the fluid mixing over the whole tube length and within the radial direction. The modulated parameters of void fractions, velocities and liquid film thicknesses in the annular region and three-levels of liquid circulation are greatly beneficial for the multiphase heat transfer enhancement. [ABSTRACT FROM AUTHOR]

• Simulation based on the Lee model and enthalpy-porosity method was established. • Simplified two-dimensional axisymmetric model are applied for practical use. • The high calculation efficiency and accuracy of POD method is validated. • POD can quickly provide the field data and predict the unknown conditions. • The applicability and extensibility of POD algorithm are good. A Proper Orthogonal Decomposition (POD) reduced-order model for the latent heat storage process in a direct steam generation solar thermal power (DSG-STP) system is established based on the numerical simulation combined with the Lee model and enthalpy-porosity approach. Then, the computational accuracy and speed of the POD reduced-order model are examined by two unsteady-state cases. The research results show that the POD reduced-order model has good precision and fast computational speed. In the working conditions tested in this study, the relative mean error (RME) of POD-predicted temperature does not exceed 0.1% compared with the finite volume method's results. Meantime, the POD calculation can improve the computational efficiency by hundreds of times (decrease from about 4 h to 45.865 s, about 314 times, for Case A). Thus, the POD reduced-order model has significant engineering value and is a promising means to accurately and efficiently solve the latent heat storage process in the DSG-STP system. [ABSTRACT FROM AUTHOR]

• Food convective drying model with thermo-hydro and mechanical bidirectional coupling is established. • The moving velocity of solid matrix is considered in mass and energy equations. • Shrinkage enhances the mass transfer process. • Shrinkage inhibits the heat transfer process. Food convective drying is a multiphysics problem coupling the fluid flow, heat transfer, mass transfer and shrinkage, on which a series of numerical simulation has been performed. However, the material shrinkage is not coupled properly with the heat and mass transfer process, which is inconsistent with actual drying process and makes the simulation results inaccurate. Furthermore, the effect of shrinkage on heat and mass transfer during the food convective drying process hasn't been fully studied. Taking shiitake mushroom (Lentinula edodes) as an example, a multiphase porous media model with thermo-hydro and mechanical bidirectional coupling (THM) for food convective drying is established in this paper. The accuracy of proposed model is validated by experiment results. Base on the proposed model, the shrinkage process of the shiitake mushroom during drying is analyzed in detail, and the effect of shrinkage on heat and mass transfer is studied by comparing with the thermo-hydro coupling model (TH). Compared with the traditional TH model, the proposed THM model presents a faster decrease of moisture content, which consists better with the experiment results. Also, there is larger evaporation rate with a relative deviation of 29.5% at the peak. Besides, lagging movement of the high evaporation rate area towards the internal region of shiitake mushroom is observed. What's more, the temperature of shiitake mushroom from THM model is lower than that from TH model with the maximum temperature difference of 3°C, and the temperature in the internal region of shiitake mushroom presents a slower rise. The model established in this paper that considers the shrinkage can accurately simulate the food convective drying process and help to deeply understand the mechanism of convective drying. In engineering practice, the proposed model can provide technical support for optimizing the drying process and improving the product quality. [ABSTRACT FROM AUTHOR]

Considering that the dish-Stirling solar thermal power system is unable to run continuously and steadily due to the lack of energy storage capacity, a dish type direct steam generation solar thermal power system with thermal energy storage was proposed. According to the demand of power generation capacity, we designed two kinds of sensible heat storage systems (i. e., two-tank molten salt storage, single-tank thermocline storage) and two latent heat storage system (i.e., two-tank latent heat storage system and single-tank latent heat storage system). Based on the principles of heat transfer, thermodynamics, and economics, the thermal storage performance of four kinds of heat storage systems is compared and analyzed. The results show that the two-tank latent heat storage system has the best heat storage performance, and its exergy efficiency is 77.67%. In terms of economy, the two-tank latent heat storage system has the lowest cost, and the single-tank latent rank the second, and the cost of the two-tank molten salt storage system is the highest. In two aspects of thermodynamic performance and economy, the two-tank latent heat storage system is recommended in the dish type solar power with direct steam generation. [ABSTRACT FROM AUTHOR]

• A comprehensive workflow to model hydraulic fracturing with diverters in naturally fractured formations. • A fully coupled finite element model with consideration of seepage flow, fracture flow, and rock deformation with adaptive insertion of cohesive elements as crosscutting natural fractures. • Spring elements are used to model the temporary plugging behaviour. Both microseismic interpretation and post-frac production analysis have confirmed that initial hydrocarbon production and ultimate recovery in reservoirs are mainly associated with the degree of fracture complexity. Hydraulic fracturing with degradable diverting materials is an effective technology for enhancing fracture complexity. However, the physics behind the effectiveness of this technology is not fully understood at present. In this paper, we first present a comprehensive workflow to model hydraulic fracture by accounting for interactions with numerous crosscutting natural fracture or joint sets, as well as the effect of temporary plugging in opened fractures. This model is a fully coupled seepage flow in porous media, fluid flow in fractures, and rock deformation finite element model with adaptive insertion of cohesive elements as crosscutting natural fracture or joint sets. In addition, spring elements are used to model the temporary plugging behavior that can be automatically activated once the diverters are injected into the opened fractures. Numerical simulations that correspond to the orthogonal and non-orthogonal approach angles have been implemented for the model. Our analysis shows that key factors such as the tensile and shear strength of natural fracture or joint sets, horizontal stress anisotropy, and the position of temporary plugging can play a significant role in the enhancement of fracture complexity and the diversion into natural fracture or joint sets. Adding degradable diverting materials has a particularly noticeable effect on enhancing fracture complexity and helps overcome cohesive resistance at the intersections. This investigation provides new insight into the formation mechanism of fracture networks in naturally fractured formations. [ABSTRACT FROM AUTHOR]

Based on the elementary solutions and new integral equations, a new analytical-numerical method is proposed to calculate the interacting stresses of multiple circular holes in an infinite elastic plate under both remote stresses and arbitrarily distributed stresses applied to the circular boundaries. The validity of this new analytical-numerical method is verified by the analytical solution of the bi-harmonic stress function method, the numerical solution of the finite element method, and the analytical-numerical solutions of the series expansion and Laurent series methods. Some numerical examples are presented to investigate the effects of the hole geometry parameters (radii and relative positions) and loading conditions (remote stresses and surface stresses) on the interacting tangential stresses and interacting stress concentration factors. The results show that whether the interference effect is shielding (k < 1) or amplifying (k > 1) depends on the relative orientation of holes (α) and remote stresses (

Using traditional high‐fidelity numerical simulation to simulate fluid flow in fractured porous media in a real field remains challenging. It involves a large number of degrees of freedom when matrix and fracture equations are solved. To address this challenge, we propose a Galerkin‐free framework to construct a reduced‐order model (ROM) based on the proper orthogonal decomposition (POD). Compared with the typical POD‐based modeling process commonly used in previous studies, the POD‐ROM can be built without performing the Galerkin projection of flow equations onto the low‐dimensional space spanned by the POD basis functions. The numerical integration method was incorporated to obtain the POD time coefficients based on the flow equations solved by the conventional finite volume method. Two complex fracture cases reflecting high‐contrast porous media in a two‐dimensional domain were designed to verify the accuracy and efficiency of the established Galerkin‐free POD‐ROM. Sensitivity analysis of parameters was conducted to examine the adaptability of the ROM. The results illustrate that, compared with the fine‐scale model, the ROM can significantly reduce the CPU time without compromising the quality of the numerical solutions. [ABSTRACT FROM AUTHOR]

• Bubble dynamics during nucleate boiling of FC-72 are studied experimentally and numerically. • The dynamical interface is captured by the three-dimensional VOSET method. • Three dimensional transient thermal response of solid wall during boiling is considered. • The numerical results match very well with experimental data. In this paper, we study the bubble dynamics and heat transfer during a growing and departing bubble of FC-72 on horizontal silicon surfaces experimentally and numerically. In the numerical method, the dynamical interface is captured by the three-dimensional coupled volume of fluid and level set (VOSET) method. The Navier-Stokes equations for the incompressible two-phase flows with surface tension and gravity, along with the continuity and energy equations are solved using the 3D-IDEAL algorithm. The three dimensional solid wall is also coupled into the solver to study the response of a heated solid surface during nucleate boiling. An experiment with working fluid FC-72 was set up to investigate the bubble behavior and heat transfer during the nucleate boiling process. Good agreement is observed between the simulation results and experimental data in prediction of bubble shape during the growth time. It is also found that the departure diameter of saturated FC-72 predicted by the proposed method matches very well with Phan's correlation. The temperature distribution near the bubble and the spatial averaged heat flux are obtained and analyzed. [ABSTRACT FROM AUTHOR]