Liquid Film Flow Research Articles

Effect of anisotropy and inhomogeneity on the stability of liquid film flowing down a porous inclined plane

We examined the linear stability of a Newtonian liquid film flow past a porous inclined plane. Falling film on inclined permeable planes displays three instability modes: surface, shear, and porous mode. Most of the previous studies have examined the film flow past an isotropic and homogeneous porous medium. We could find only one study that examined the film flow past an anisotropic and inhomogeneous porous inclined plane; however, it focused on parameter regimes where the porous mode remains stable. Here, we explored the parameter regimes where all three modes become unstable and investigated the effect of the anisotropic and inhomogeneous variations in the permeability of the porous medium. The generalized Darcy model is used to describe the flow through the porous medium along with the Beavers–Joseph condition at the fluid–porous interface. We show that there is a switching of dominant instability mode from surface mode to porous mode with variation in anisotropic parameter. Our results clearly show that for a given Darcy number, the surface mode is the critical instability mode for isotropic and homogeneous porous media. However, when an anisotropic porous medium is considered with high wall-normal permeability than the wall-parallel permeability, the porous mode becomes the most unstable mode. We show a similar exchange of dominant instability mode from fluid mode (i.e., surface or shear mode) to porous mode with variations in inhomogeneity parameter. This switching of the most unstable mode from fluid mode to porous mode with variation in anisotropy and/or inhomogeneity in permeability has not been demonstrated in any of the earlier studies in the context of film flows. We also present an energy budget analysis to decipher the mechanism responsible for making the three modes unstable.

Modelling falling film flow: an adjustable formulation

A new two-equation model for gravity-driven liquid film flow based on the long-wave expansion has been derived. The novelty of the model consists in using a base velocity profile combining parabolic (Ruyer-Quil & Manneville, Eur. Phys. J. B, vol. 15, issue 2, 2000, pp. 357–369) and ellipse (Usha et al., Phys. Fluids, vol. 32, issue 1, 2020, 013603) profile functions in the wall-normal coordinate. The dependence on a free parameter $A$ related to the eccentricity of an ellipse serves as an adjustable parameter. The resulting models are consistent at $O(\varepsilon )$ for inertia terms and at $O(\varepsilon ^2)$ for viscous diffusion effects, and predict accurately the primary instability. Appropriate tuning of the adjustable parameter helps to recover accurate predictions for the asymptotic wave celerity of nonlinear solitary waves. Further, the model is shown to capture the closed separation vortices that can form underneath the troughs of precursory capillary ripples.

Research on wet gas separation method based on swirl and ejection cycle technology

Gas-liquid co-production often occurs in the middle and late stages of natural gas production. Efficient wet gas separation is very important in natural gas transportation and measurement. The traditional separation device has a relatively low separation efficiency due to the flow patterns, a new type of pipe separator was designed based on the swirl and ejection cycle technology. A new systematical separation procedure with three main steps was proposed simultaneously. The wet gas was forced to form an annular flow by a swirler. Then, the liquid film flows into the annular gap. The wave layer was introduced into the swirl separation again with the self-circulating ejection system for fine separation finally. Laboratory experiments and Computational Fluid Dynamics (CFD) simulations show that the additional swirl process can decrease the flow pattern influence effectively, and the separation efficiency can increase to more than 90%. The separation efficiency is mainly determined by the gas superficial velocity, and while the velocity of the inlet gas is less than 21 m/s, the separation efficiency is up to 93%. The separation efficiency prediction model was established based on liquid film porosity, and the prediction relative error is less than 10%. The new device, the separation procedure, and the test results can provide constructive technical reference for the real-world pipe separator application.

Two layer thin film flow over a stretching sheet with uniform transverse magnetic field

The objective of the present work is to study the effect of uniform transverse magnetic field on two immiscible thin liquid film flow over a linear stretching sheet. Applying similarity transformations, the guiding Navier–Stokes equations have been transformed into a system of non-linear partial differential equations, which are numerically solved by the finite difference method. The effects of the governing parameters viz. magnetic field parameter Mn, density ratio m, viscosity ratio n, thickness ratio δ and Reynolds number Re1 are analyzed in detail. It is found that the effect of varying Reynolds number is more pronounced on film thinning in presence of uniform transverse magnetic field.

Optimal analyticity estimates for non-linear active–dissipative evolution equations

Abstract Active–dissipative evolution equations emerge in a variety of physical and technological applications including liquid film flows, flame propagation, epitaxial film growth in materials manufacturing, to mention a few. They are characterized by three main ingredients: a term producing growth (active), a term providing damping at short length scales (dissipative) and a nonlinear term that transfers energy between modes and crucially produces a nonlinear saturation. The manifestation of these three mechanisms can produce large-time spatiotemporal chaos as evidenced by the Kuramoto-Sivashinsky equation (negative diffusion, fourth-order dissipation and a Burgers nonlinearity), which is arguably the simplest partial differential equation to produce chaos. The exact form of the terms (and in particular their Fourier symbol) determines the type of attractors that the equations possess. The present study considers the spatial analyticity of solutions under the assumption that the equations possess a global attractor. In particular, we investigate the spatial analyticity of solutions of a class of one-dimensional evolutionary pseudo-differential equations with Burgers nonlinearity, which are periodic in space, thus generalizing the Kuramoto-Sivashinsky equation motivated by both applications and their fundamental mathematical properties. Analyticity is examined by utilizing a criterion involving the rate of growth of suitable norms of the $n$th spatial derivative of the solution, with respect to the spatial variable, as $n$ tends to infinity. An estimate of the rate of growth of the $n$th spatial derivative is obtained by fine-tuning the spectral method, developed elsewhere. We prove that the solutions are analytic if $\gamma $, the order of dissipation of the pseudo-differential operator, is higher than one. We also present numerical evidence suggesting that this is optimal, i.e. if $\gamma $ is not larger that one, then the solution is not in general analytic. Extensive numerical experiments are undertaken to confirm the analysis and also to compute the band of analyticity of solutions for a wide range of active–dissipative terms and large spatial periods that support chaotic solutions. These ideas can be applied to a wide class of active–dissipative–dispersive pseudo-differential equations.

Parametric analysis of interfacial friction factor for liquid film dynamics sheared by turbulent gas flow

Water droplet impingement on a low-pressure steam turbine blade causing erosion has been recognized as a crucial issue. It is essential to elucidate a comprehensive droplet detachment mechanism, not only from the trailing edge but also from the liquid film surface. In the present paper, we investigate the influence of interfacial friction factor against liquid film dynamics on a wall sheared by a turbulent gas flow, including the liquid film thickness, liquid film velocity and entrained droplet detached from liquid surface for both pipe flow and plate flow conditions. We conduct the analyses by using a liquid film dynamics model, recently established, considering the three-dimensional destabilized waves and droplet entrainment from the liquid surface. As a result, the film thickness and velocity greatly depends on the interfacial friction factor. Interestingly, the rate of entrained droplet to initial liquid film has a minimum value when the interfacial friction factor equals to the inverse of the liquid film Reynolds number, while the remaining liquid film flow rate becomes maximum.

Hydraulic jump on the surface of a cone

This paper addresses several aspects of the axisymmetric flow of a liquid film over the surface of a downward-sloping cone. The study is rooted on a validated computational tool the results of which are interpreted with the help of a hyperbolic time-dependent reduced-order model also derived in the paper. The steady version of the model demonstrates the weakening and ultimate disappearance of the circular hydraulic jump as the cone surface transitions from planar to downward sloping. Mathematically, this evolution is reflected in a change of the model's critical point from spiral to node. A significant advantage of the time-dependent model is that, when it is integrated in time, the flow regions upstream and downstream of the critical point are connected. Due to this feature, when a hydraulic jump exists, its position can be sharply captured automatically with a good agreement with Navier–Stokes simulations. Surface-tension effects are properly accounted for and, in steady conditions, are shown to have a marginal effect on the flow, including the position of the hydraulic jump. A correlation is obtained for the jump radius as a function of the flow rate, liquid viscosity, gravitational acceleration and the angle of inclination of the cone surface. In a suitable limit, the model reduces to the optimal two-dimensional, first-order model for liquid film flow down an inclined plane and, in a different limit, it describes an axisymmetric thin liquid film falling down the surface of a vertical cylinder. Some results are also presented for the waves induced by a pulsating jet on the surface of the liquid film and for a jet impinging on the surface of a cone from below.

Annular liquid film thermohydraulic and intermittent dryout in microchannel flow boiling

The annular flow regime in microchannels is commonly described as the steady flow of a vapor core with a surrounding liquid film along the channel length. However, limited studies have suggested temporal discontinuity in the liquid film coverage of the microchannel wall. The lack of experimental tools to directly interrogate the liquid flow characteristics and the local surface heat flux has resulted in proliferation of different hypotheses concerning the physics of this process. Surface de-wetting due to high shear stresses at the vapor–liquid interface and liquid film instability and rupture due to perturbation growth at the vapor–liquid interface have been suggested as the underlaying causes of this phenomenon. In this study, a recently developed microsensor array capable of measuring the surface local heat flux and liquid film thickness and velocity is utilized to decipher thermohydraulic characteristics of the annular flow regime and intermittent dryout with unprecedented details. The studies are conducted in a 600 μm square cross-section microchannel using FC-72 as the test fluid at 70–80% exit vapor quality. The results show that liquid film thinning because of evaporation rather than de-wetting and film rupture is responsible for intermittent surface dryout. Studies at different mass and heat fluxes suggest that for a given surface heat flux, there is a mass flux threshold above which intermittent surface dryout vanishes. Furthermore, the experimental liquid and vapor flow velocities and liquid film thickness are used to calculate the liquid–vapor interfacial shear stress. The results suggest that under the studied conditions, the liquid–vapor interfacial shear stress has little impact on the surface intermittent dryout. This study enhances predictable and reliable design and operation of two-phase heat sinks at high exit vapor qualities.

Thermal characteristics of refrigerant flow boiling in two mini-channel heat sinks of different aspect ratios for battery thermal management

The two-phase refrigerant cooling system has excellent thermal management performance, which can provide good temperature control of lithium-ion batteries. In this paper, the thermal performance of mini-channel heat sinks with different aspect ratios was investigated for a battery module with a small-scale vapor compression refrigeration system. Two mini-channel heat sinks (MiCHSs), one with a narrow channel cross section of 2 mm × 4.5 mm (MiCHS1) and the other one with square channel cross section of 3 mm × 3 mm (MiCHS2), were fabricated with a visualization window and connected in parallel in the system. The experiments were run at different openings of expansion valve (OEV) and heating power. With the increase of OEV, the refrigerant subjected to fixed heating power in the mini-channels experiences different flow patterns of annular flow, slug flow and small-bubble flow successively. The best cooling performance can be achieved in slug flow, corresponding to an optimal OEV. The optimal OEV value increased from 32 % to 55 % with the increase of heating power ranging from 100 W to 200 W. Under the same optimal OEV condition, the heat transfer coefficient and average temperature of MiCHSs increase with the increase of heating power for both MiCHSs. The channel aspect ratio has an effect on the heat transfer performance of MiCHS. Compared with the narrow channels of MiCHS 1 with a large aspect ratio, the flow boiling in square channel of MiCHS2 can achieve a better cooling performance due to the well-confined bubbly flow and thinner liquid film on the base of the mini-channels.

Simulation of Liquid Film Flow in the Channels of a Regularly Structured Packing

Simulation of Liquid Film Flow in the Channels of a Regularly Structured Packing

Instability of liquid film flow inside of a vertical tube in presence of an interfacial surfactant.

Liquid film coating inside a tube becomes unstable to surface-tension-driven Rayleigh-Plateau (RP) instability. The presence of interfacial surfactant reduces the growth rate of RP instability but is unable to completely eliminate it. Further, it is known that the surfactant mode, an eigenmode emerging because of the presence of surfactant, remains stable for such liquid films. However, the above two observations hold true only when the liquid film remains stationary (i.e., in the absence of base flow). Here, we investigate the linear stability of surfactant-laden liquid film inside of the tube with base flow included in the analysis. We observed a complete suppression of Rayleigh-Plateau instability because of the presence of surfactant when the basic flow is included in the analysis. However, the surfactant mode, which otherwise remains stable in the absence of base flow, is found to be unstable when the basic flow is taken into account in the stability analysis. More importantly, depending on the base flow strength, the wave number corresponding to maximum growth rate and cutoff wave number for surfactant mode are greater than unity for a wide variety of parameters. This is unlike the (RP) instability observed for surfactant-covered stationary films where the cutoff wave number is always less than unity. Further, we show that the growth rate observed for this surfactant-mode instability in the presence of sufficiently strong basic flow is significantly higher than the growth rate of RP instability observed for a stationary liquid film. These observations imply that depending on the strength of basic flow, the effect of presence of surfactant is destabilizing for such film flows as opposed to a stabilizing effect observed for stationary liquid films in earlier studies.

Interfacial and rheological properties of long-lived foams stabilized by rice proteins complexed to transition metal ions in the presence of alkyl polyglycoside

HypothesisTransition-metal coordination complexes are hopeful to make advanced structural materials, since the metal-coordination bonds, unlike typical covalent bonds, can regenerate after rupture, allowing for dynamic, tunable, and reversible mechanical characteristics. Integration of metal-coordinate crosslinking in foam material has rarely been reported. ExperimentsWe developed the hydrolyzed rice proteins (HRP) as the building block for amphiphilic transition-metal coordination complexes that could be used to make long-lived foams with high yield stress. Surface properties of the foaming solution were determined using equilibrium and dynamic tensiometers. Structural information of aggregates in the foaming solution was detected by small-angle X-ray scattering (SAXS) and Cryo-Tem. Visualization of liquid flow in the interfacial liquid film was studied by the reflective optical interference technique. Rheological response of liquid foam was characterized by a rheometer with amplitude-sweep and frequency-sweep modes. FindingsIn the presence of transition metal ions, HRP formed a mechanically strong rigid film. In the absence of transition metal ions or the addition of alkyl polyglycoside (APG), HRP was desorbed to produce a mobile film with a detergent state. The two interfacial states could be actively switched based on facile changes in bulk solution composition (metal ions or alkyl glycoside or chelating agent), and the switching between the two states led to the formation of extremely stable foam with high yield stress or the collapse of foam with a significant decrease in yield limit. The transition-metal coordination complexes adsorbed on the surface of the liquid film could increase the elastic modulus of liquid foam by more than an order of magnitude without increasing the viscosity of the foaming solution. We further revealed the origin of foam stability/instability and used other well-characterized proteins to prepare transition-metal coordination complexes to make long-lived foams. The cases described in this work illustrate the universal nature of the strategy, which in principle can be extended to many types of protein.

Hydrodynamics and thermal performance of turbulent falling films through horizontal tube bundles

Turbulent falling films are commonly encountered in the practical operations of most falling film applications, but the relevant numerical study is very scarce. The hydrodynamics and thermal performance of falling films in horizontal tube bundles under the turbulent regime were numerically investigated in this study. The calculation ranges are: film Reynolds number from 3505 to 8034, heat flux of 47.3 kWm−2, inlet liquid temperature of 46 °C, tube diameter of 19.0 mm and liquid distributor height of 6.4 mm. Based on the results of the present numerical model, the Standard k-ω model is more suitable in reproducing the falling film heat transfer profile, as the present numerical model accurately predicted the turbulent falling film heat transfer characteristics. There are various hydrodynamics depending on the bundle arrangement and flow regime, including splashing, bridging, gas entrainment, Coanda effect, and liquid film thickness. The triangular bundle provides the highest heat transfer coefficient, but the square bundle provides the poorest one, and the square and rotated-triangular tube bundles behave the largest and smallest bundle effect, respectively. The bundle effect is generally independent of film Reynolds number, liquid feeder height and tube spacing.

Entropy analysis of unsteady magnetohydrodynamic thin liquid film flow of Maxwell nanofluids with variable fluid properties

The flow and heat transfer phenomena of a viscous fluid in a thin film over an expanding flat surface possesses optimistic applications in a large number of technological processes. Heat and mass transfer rate within the thin film bears an unequivocal factors on the performance of coating process and the chemical nature of the product. This article concerns with the study of unsteady boundary layer magnetohydrodynamic thin film flow of Maxwell nanofluid past over an exponentially stretching surface subject to slip effect on velocity and suction/injection at the boundary. The heat transfer mechanism is influenced by the thermal radiation, temperature dependent thermal conductivity, viscous dissipation of energy and a non-uniform heat source/sink. We have analyzed the entropy generation of the system by paying due attention to the variable fluid properties and film thickness. The system of arising time dependent coupled nonlinear partial differential equations governing the fluid flow are converted into a system of ordinary differential equation by utilizing suitable self-similar transformations. The reduced two-point boundary value problem is then solved with the aid of well known Runge–Kutta–Fehlberg integration technique based shooting method. The mathematical model suggests an extra kinematic boundary condition which is utilized to compute the thickness of the thin film by employing the Newton–Raphson method. The investigation highlights that the arising controlling parameters such as magnetic parameter, radiation parameter, viscosity parameter, thermal conductivity parameter have strong effect on velocity field, temperature field, Nusselt number, and the thickness of the thin film. Moreover, the Bejan number, a measure of thermal irreversibility, is highly affected by the variation of the magnetic field strength, thermal radiation, dissipation of energy, thermal conductivity parameter, viscosity parameter and heat source/sink.

(Invited, Digital Presentation) Optimization of Mass Transport in a Plasma-Liquid Reactor

A considerable amount of research over the past decade has been directed toward the development of plasma based Advanced Oxidation Processes (AOPs) for the degradation of pollutants in wastewater. Despite these efforts and the exceptional efficiencies reported in laboratory-based tests, the direct contact between a non-thermal plasma (NTP) and liquid introduces a number of challenges. Critically, the characteristics of the NTP, and therefore its chemical composition, are directly linked to the electrical properties of the contaminated water, which inevitably varies. Furthermore, the use of short-duration high voltage pulses to generate NTP is considered the most effective approach, yet the generation of such pulses to create uniform large volume NTP is particularly challenging.Given the drawbacks and challenges associated with direct-contact NTP reactors, several recent studies have examined the use of indirect-contact NTP systems for water treatment [1,2]. In particular, the surface barrier discharge (SBD) configuration enables large volume NTP generation in humid air with modest power supply requirements. Unfortunately, a key drawback associated with the use of indirect-contact plasma liquid reactors is the reduced transport of chemical species from the discharge to the liquid. Highly reactive species produced by NTP generated in humid air, such as atomic oxygen and hydroxyl radicals, rapidly react beyond the plasma to form longer-lived and less reactive intermediaries, such as ozone and hydrogen peroxide. To enhance the efficiency of an indirect-contact NTP reactor the transport of longer-lived intermediary species to the liquid must be maximized. One particularly convenient method to achieve this is through the use of a falling film reactor [3], whereby a thin liquid film flows in close proximity to an NTP source. Such reactors have been used extensively in direct-contact NTP studies [4,5], but have not yet been used for the in-direct NTP treatment of contaminated water.This contribution considers the hypothesis that the efficiency of an indirect-contact NTP wastewater treatment processes can rival that of direct-contact treatment process by combining a falling film reactor with a SBD, figure 1. To test the hypothesis the decolorization of indigo carmine dye was examined as a function of plasma operating parameters whilst the generation of chemical species was assessed in both the liquid and gas phase. Analysis of dye decolorization as a function of energy input revealed that complete decolorization of Indigo Carmine could be achieved, figure 2. However, more energy was used at higher discharge powers to obtain total decolorization. To explain the faster and more efficient decolorization of dye at lower power, FTIR was used to quantify gas phase chemical species. Under high power operation it was observed that the gas phase chemistry rapidly transitioned from being ozone dominated to NOx dominated, attributed to process known as Ozone poisoning. It was shown that the shift in gas-phase chemistry is primarily responsible for the reduction in decolorization efficiency under high power operation.Using optimized NTP parameters the decolorization results were compared against several other plasma-based AOP approaches and alternative AOP technologies, Figure 3. The comparison illustrates the G50 (g/kWh) value (i.e., the energy consumed to decompose one gram of pollutant) against the time needed to reach 50 % decomposition (t50). It is clear that the indirect-contact plasma system investigated in this contribution compares favorably with other plasma-based studies, including those using direct-contact systems. It is also clear that the approach offers a higher efficiency compared to alternative AOP technologies, such as UV, UV/ H2O2 [6], H2O2/photo Fenton, UV/TiO2 and Fenton/photo Fenton [7].In summary, the combination of a large-area SBD and falling film reactor provides a convenient means for wastewater remediation without many of the challenges associated with direct-contact plasma-liquid systems. Despite the limited mass-transport of highly reactive species to the liquid layer, the large flux of longer-lived species was found to be highly effective for the efficient degradation of Indigo Carmine.[1] Kumar et al., Frontiers in Physics, vol. 10, 2022.[2] Pavlovich et al., Journal of Physics D: Applied Physics, vol. 46, no. 14, p. 145202, 2013.[3] Krupež et al., Journal of Physics D: Applied Physics, vol. 51, no. 17, p. 174003, 2018.[4] Hama Aziz et al., Journal of Hazardous Materials, vol. 343, pp. 107-115, 2018.[5] Kozakova et al., Plasma Processes and Polymers, vol. 15, no. 6, p. 1700178, 2018.[6] Rodríguez et al., Environmental Engineering Science, vol. 24, no. 3, pp. 363-371, 2007.[7] Palma-Goyes et al., Electrochimica Acta, vol. 140, pp. 427-433, 2014. Figure 1

Laminar planar hydraulic jump in thin film flow of power-law liquids—Experimental, analytical and numerical study

This study explores the physics of laminar planar hydraulic jump of power-law liquids in a horizontal channel through shallow water analysis. The theory is supplemented and validated by numerical simulation performed by the phase-field method in COMSOL Multiphysics. The analysis is applicable for shear thinning as well as thickening liquids and reduces to Newtonian equations when the flow behavior index (n) is unity. The analytical and numerical results are further validated with experimental data of the present study. The steady state free surface profile in conjunction with a modified Bélanger type equation predicts jump location and strength (the ratio of film height just after and before the jump). The analysis provides a modified scaled relationship between the jump location and the liquid flow rate compared to that proposed for inviscid and viscous Newtonian liquids. An order of magnitude analysis expresses the scaled jump location and strength as a function of inlet Reynolds number (Rein), inlet Froude number (Frin), scaled channel length, and n. We find that an increase in the flow consistency index and/or n favors jump formation and the jump shifts toward the channel exit with increasing n for the same channel geometry, Rein and Frin. The influence of scaled channel length is more pronounced for n < 1 as compared to n > 1 while shear thickening liquids are more significantly influenced by Frin at fixed Rein and scaled channel length. The coupled effect of flow parameters on jump characteristics is depicted in the Rein–Frin plane, which denote the range of the existence of different jump types as the function of n.

Comparative study on the numerical reproduction of falling film hydrodynamics based on different surface tension models

Computational fluid dynamics (CFD) modelling is extensively and successfully used for the reproduction of liquid film hydrodynamics. Surface tension significantly influences the numerical reproduction of falling liquid film flow. In this study, the hydrodynamics of the liquid film flow over a horizontal tube and a vertical plate is explored with different surface tension models. The results reveal that the neglect of surface tension leads to quantitative and qualitative distortions in the numerical simulation of the thin liquid film. Both continuum surface force (CSF) and continuum surface stress (CSS) models can produce the hydrodynamic behaviors of the falling film on the horizontal tube and the vertical plate, including pendant droplets, column necking or necking break, thickness of the film, vortex, liquid film detachment, as well as the values and trends of the velocity components. However, the CSF and CSS models produce different transient phase contours and streamlines, as well as different profiles of the time-averaged velocity components of the falling films on the horizontal tube and the vertical plate. Besides, there is no obvious difference between CSF and CSS in the perspective of time-averaged film thickness on a horizontal tube.

Nonlinear dynamics of a liquid film flow over a solid substrate in the presence of external shear stress and electric field

Using approximation techniques, long-wave length framework and boundary-layer, the effects of electrostatic force and induced shear stress on the flow behavior down an inclined solid substrate are investigated. In general case, the considered model accounts in the presence of inertia regime and streamwise viscous diffusion with the influence of normal electric field and an imposed shear stress. Using the Galerkin weighted residual, two coupled evolution equations for the flow rate and film thickness are extracted. In the appropriate limit cases, the evolution equations obtained by previous authors are recovered. The primary instability has been analyzed using the Whitham wave hierarchy framework. In the nonlinear regime, the behavior of solitary waves arose on the surface of liquid film due to the effects of electrostatic force and imposed shear stress throughout three-dimensional dynamical systems. Some bifurcation points are reported. In both extremely viscous and electrogravity regimes, the Benney-like equation is extracted in a new form. By excluding contribution of external shear stress and viscous dispersion parameter, the interesting results of previous authors are recovered. In both weakly nonlinear and inertialess regimes, the bifurcation points of the three dynamical systems are discussed within the Kuramoto–Sivashinsky type equation.

The development of the MHD film flow numerical simulation platform

<p indent="0mm">In this paper, magnetic confinement fusion is introduced focusing on its fluid mechanics relevant to a liquid lithium divertor. The main research problem is the flow stability of a liquid metal film flow under the influence of a magnetic field. Therefore, a linear stability analysis numerical calculation platform and a direct numerical simulation platform are developed to understand the effect of magnetohydrodynamics on the flow stability of thin films. These platforms will be used to reveal the physical mechanism of a magnetic field on the film flow stability and to guide the design and implementation of a liquid lithium divertor.

Visualization experiment and numerical simulation on behaviors of gas-liquid falling film flow on the air-side of two-row plain finned-tube heat exchanger

The frost-free air-source heat pump (ASHP) with integrated liquid desiccant dehumidification is a more promising energy-saving alternative to the conventional ASHP unit. The performance of spray-type finned-tube heat exchanger (FTHE), as one of the key equipment of this frost-free ASHP system, is affected by a combination of various factors including wall wettability, liquid film flow pattern, liquid film thickness, velocity distribution in a liquid film, interface wave, interface velocity and so on. To provide a theoretical basis for the operation, design and optimization of high-performance spray-type FTHE, a visualization experiment system and a three-dimensional (3D) numerical model are employed to study the behaviors of counter-current gas-liquid falling film flow on the air-side of two-row plain FTHE under different parameters in the present paper. According to the experimental results, the flow pattern of liquid film on the finned-tube surface is a mixture flow composed of the rivulet flow and the thin falling film flow, demonstrating that the assumption of the uniform distribution for liquid film thickness proposed in previous studies is not completely consistent with the actual situation. Meanwhile, the numerical results indicate that the transient-average film thickness on the surface of two-row plain finned-tube is significantly 18.3% – 32.5% higher than that on the vertical plate calculated by Nusselt's model. After that, the critical gas and liquid Reynolds numbers of the flow pattern transition for the liquid film are obtained, and it is pointed out that the wall contact angle of 10° is more conducive to the formation of complete film flow on the air-side of the two-row plain FTHE within the fin pitch range of 4.8 – 6.0 mm. The changes of the falling liquid film and gas-liquid interface characteristics are fundamentally attributed to the drag effect of the interfacial shear force. Furthermore, there is a cubic curve relationship between average interface shear force and gas Reg. A quadratic curve relation exists between average wall shear stress and gas Reg. At the same time, the velocity of falling film fluid at the bottom region of laminar flow is reduced, resulting from the inhibiting effect of the interfacial shear force.