基于横纹管氨水溶液垂直管外降膜吸收性能的变化规律

针对三根不同尺寸规格的横纹管,通过实验研究提高其吸收效率,选择吸收性能最优的管型。实验结果表明,随着压差和管内冷却水流速的增大,溶液吸收氨气的量增加,管外溶液传质系数增大。当管外氨水溶液喷淋密度从小变大时,光滑管和横纹管的传质系数均先增加后减小,过程中出现了最大值。该系列实验的结果表明横纹管比相同工况的光滑管有更高的强化传热和传质能力,当溶液喷淋密度为479.6kg/(m?h)时,横纹管比光滑管的传质系数增大了97.8%。三组实验中均发现一个共同的规律,横纹管的传质系数随凹槽尺寸变化而改变,2号横纹管表现出更强的吸收氨气能力。该管凹槽宽度与横纹管外径的比值为0.0814。通过对凹槽内溶液流动模型分析,比较了溶液通过凹槽时掺混和涡流的流动形态,得出了不同尺寸规格横纹管吸收性能差异的一个原因。     

Effect of long undulated bottoms on thin gravity-driven films

Summary We carry out a perturbation analysis for steady gravity-driven film flow over undulations of moderate steepness that are long compared to the film thickness and study the linear stability of the flow in the framework of Floquet analysis. The effect of geometric nonlinearities on the instability becomes relevant for moderate bottom variations. We find that the critical Reynolds number for the onset of surface waves is higher than that for a flat bottom. At higher inclination angles, the theoretical results are in good quantitative agreement with experiment. At inclination angles where the flat part of the undulation is close to being horizontal, the basic solution for the steady flow fails to describe the flow in the flat part, and the linear stability analysis overestimates the critical Reynolds number.     

Nonisothermal flow in a conic liquid trap under conditions of atmosphere-free space

A nonisothermal film flow is considered, which is formed on the inside surface of a conic liquid trap, to the inlet of which a homogeneous flow of monodisperse droplet medium is delivered under conditions of atmosphere-free space. Asymptotic models of steady film flow are constructed and investigated for the conditions of inertial deposition of droplets under the assumption of small relative thickness of film and of the effect of secondary droplets formed upon deposition on the film surface. For a slow flow, the shape of the surface and the parameters of the film are found analytically. A parametric numerical investigation of distributions of velocity, temperature, and film thickness is performed for the general case. The parameters of flow in the inlet section of the offtake channel are found, and the conditions are determined which are required for maintaining a steady-state mode of operation of the drip pan.     

Thermocapillary deformation of a locally heated liquid film moving under the action of a gas flow

We propose a two-dimensional model of a steady laminar flow of a liquid film in a channel in the presence of a cocurrent gas flow. An analytical solution for the problem of temperature distribution is obtained for a linear flow velocity profile. The linearized problem of thermocapillary deformation of the film surface caused by local heating at a constant heat flux is solved. It is established that a thermocapillary bump is formed in the region where a thermal boundary layer emerges on the film surface. Additional perturbations, decaying in the upstream direction, can be present on the free surface in front of the bump. A criterion determining this effect is found.     

Wettability control of micropore-array films by altering the surface nanostructures

By controlling the surface nanostructure, the wettability of films with similar pore-array microstructure can be tuned from hydrophilic to nearly superhydrophobic without variation of the chemical composition. PA1 pore-array film consisting of the horizontal ZnO nanosheets was nearly superhydrophobic. PA2 pore-array film consisting of growth-hindered vertically-aligned ZnO nanorods was hydrophilic. The influences of the nanostructure shape, orientation and the micropore size on the contact angle of the PA1 films were studied. This study provides a new approach to control the wettability of films with similar pore-array structure at the micro-scale by changing their surface nanostructure. PA1 films exhibited irradiation induced reversible wettability transition. The feasibility of creating a wetted radial pattern by selective UV irradiation of PA1 film through a mask with radial pattern and water vapor condensation was also evaluated.     

The effect of substrate wettability on the breakdown of a locally heated fluid film

The effect of the equilibrium contact angle of wetting on the dynamics of the dry patch propagation and on the critical heat flux upon the breakdown of a water film that is heated locally from the substrate side is studied experimentally. The equilibrium contact angle is varied from 27° ± 6° to 74° ± 9° (with no changes in the thermophysical properties of the system) through the use of different types of surface grinding. The studies are performed for three flow modes: (a) a fluid film that freely flows down along a substrate with an inclination of 5° to the horizon, (b) a film that moves along a horizontal substrate under the influence of hydrostatic pressure, and (c) a static film on a horizontal substrate. It is found that the substrate wettability has a significant effect on the dry patch propagation rate and its final size in all these cases, but has almost no effect on the threshold heat flux at which the breakdown of a film occurs.

     

Closure relations for shallow granular flows from particle simulations

The discrete particle method (DPM) is used to model granular flows down an inclined chute with varying basal roughness, thickness and inclination. We observe three major regimes: arresting flows, steady uniform flows and accelerating flows. For flows over a smooth base, other (quasi-steady) regimes are observed: for small inclinations the flow can be highly energetic and strongly layered in depth; whereas, for large inclinations it can be non-uniform and oscillating. For steady uniform flows, depth profiles of density, velocity and stress are obtained using an improved coarse-graining method, which provides accurate statistics even at the base of the flow. A shallow-layer model for granular flows is completed with macro-scale closure relations obtained from micro-scale DPM simulations of steady flows. We obtain functional relations for effective basal friction, velocity shape factor, mean density, and the normal stress anisotropy as functions of layer thickness, flow velocity and basal roughness.     

Liquid film and droplet flow behaviour and heat transfer characteristics of herringbone microfin tubes

Cross-sectional liquid flow rate distribution of vapour liquid two phase flow of R123 in different herringbone microfin tubes has been measured. Droplet and liquid film flow rates are calculated with the measured data and assumptions for droplet distribution and slip ratio. Heat transfer coefficients of evaporation and condensation in herringbone microfin tubes have been measured using R22. Heat transfer enhancement mechanism by the herringbone microfins is discussed by using the measured data and numerically obtained cross-sectional flow field of a single phase flow. Flow rate of thin liquid film flowing on tube sides is affected by the helix angle and fin height. Larger helix angle and higher fin give thinner film. Liquid film flow rates in tube top and bottom are higher than tube sides. Droplet flow rate is increased with increase of helix angle and fin height, although the effect of fin height is not as pronounced as helix angle. Droplet radial mass velocity to tube side walls is increased with helix angle.     

On the asymptotic analysis of surface-stress-driven thin-layer flow

It has been demonstrated experimentally that thin liquid layers may be applied to a solid surface or substrate if a temperature gradient is applied which results in a surface tension gradient and surface traction. Two related problems are considered here by means of the long-wave or lubrication theory. In the first problem, an improved estimate of the applied liquid coating thickness for a liquid being drawn from a bath is found through asymptotic and numerical matching. Secondly, the theory is extended to consider substrates that are not perfectly wetted but exhibit a finite equilibrium contact angle for the coating liquid. This extension incorporates the substrate energetics using a disjoining pressure functional. Unsteady flows are calculated on a substrate of nonuniform wettability. The finite contact angle value required to stop stress-driven flow is predicted and the resulting steady profiles are compared with experimental results for several values of the applied stress.     

Effects of Power Density and Post Annealing Process on the Microstructure and Wettability of TiO2 Films Deposited by Mid-frequency Magnetron Reactive Sputtering

The relationship of ＂preparation parameters-microstructures-wettability＂ of TiO2 films was reported. In this work, TiO2 films were deposited onto glass and silicon substrates by using mid-frequency dual magnetron sputtering technique at ambient temperature with various power densities and deposition time. After deposition, the films were heat treated at different annealing temperatures. X-ray diffraction （XRD）, Raman spectroscopy, and field-emission scanning electron microscopy （FE-SEM） were utilized to characterize TiO2 films. The wettability of the films was evaluated by water contact angle measurement. The phase transition temperature of TiO2 films depended on the power density. It was demonstrated that wettability was strongly structure dependent and the film with the thickness of 610 nm （the power density was 2.22 W/cm^2） showed the lowest contact angle （8°）. It can be concluded that smaller crystallite size, the rutile phase with （110） face being parallel to the surface, and tensile stress favored the hydrophilicity of the TiO2 films.     

Coning during withdrawal from two fluids of different density in a porous medium

The steady response of the interface between two fluids with different density in a porous medium is considered during extraction through a line sink. Supercritical withdrawal, or coning as it is often called, in which both fluids are being withdrawn, is investigated using a coupled integral equation formulation. It is shown that for each entry angle of the interface into the sink there is a range of supercritical solutions that depend on the flow rate, and that as the flow rate decreases the cone narrows. As the magnitude of the entry angle increases this range of flow-rate values decreases to a narrow range as the entry becomes vertical. Only one branch of solutions (that with horizontal entry) has the property that the interface levels off at a finite height, and this is investigated as a separate branch of solution.     

Unsteady laminar flow over a rough surface

A model is developed for the unsteady laminar flow of athin fluid film over a substrate with roughness of the same order as the film height. The limits of large and small surface resistance and small surface-roughness are investigated and it is shown that at leading order the classical parabolic form for the velocity profile is retrieved in all cases. Empirical expressions for the depth-averaged velocity and the ratio of the average to maximum velocities are investigated and shown to agree with the present theory under certain conditions. The method is verified by comparison with experiments for steady uni-directional flow over a surface of known roughness.     

Study of the aging process of corona discharge plasma effects on low density polyethylene film surface

A study of the durability of corona discharge plasma effects on a polymer surface was investigated in this work. We used the corona discharge plasma technique to modify the wettability properties of low density polyethylene (LDPE) film and evaluated the influence of relative humidity and temperature on the aging process with three different storage conditions. The effects of the aging process on the plasma-treated surface of LDPE film were quantified by contact angle measurements, Fourier-transformed infrared spectroscopy, and X-ray photoelectron spectroscopy. The results obtained with these techniques have allowed us to determine how the aging process promotes changes in the plasma-treated surface by decreasing its wettability and taking place a remarkable hydrophobic recovery process.     

The effect of micro-scale surface treatment on heat and mass transfer performance for a falling film H2O/LiBr absorber

The objectives of this paper are to develop a new method of wettability measurement, to study the effect of micro-scale surface treatment on the wettability across horizontal tubes and to apply it for numerical analysis of heat and mass transfer in a H₂O/LiBr falling film absorber. Three types of tubes with roughness are tested in a test rig. Inlet solution temperature (30–50 °C), concentration (55–62 wt.% of LiBr) and mass flow rate (0.74–2.71 kg/min) are considered as key parameters. Reynolds number ranged from 30 to 120 by controlling the inlet mass flow rate. The wettability on the roughened tubes was higher than that for the smooth tubes. The wettability decreased linearly along the vertical location but was proportional to the solution temperature and mass flow rate. The experimental correlations of the wettability for the smooth and the roughened tubes were developed with error bands of ±20 and ±10%, respectively. These are used for the heat and mass transfer analysis of absorbers with micro-scale hatched tubes.     

Effect of tube diameter on boiling heat transfer and flow characteristic of refrigerant R32 in horizontal small-diameter tubes

This study investigated the effect of tube diameter on flow boiling characteristics of refrigerant R32 in horizontal small-diameter tubes with 1.0, 2.2, and 3.5 mm inner diameters. The boiling heat transfer coefficient and pressure drop were measured at 15 °C saturation temperature. The effects of mass velocity, heat flux, quality, and tube diameter were clarified. The flow pattern of R32 for adiabatic two-phase flow in a horizontal glass tube with an inner diameter of 3.5 mm at saturation temperature of 15 °C was investigated. Flow patterns such as plug, wavy, churn, and annular flows were observed. The heat transfer mechanisms of forced convection and nucleate boiling were similar to those in conventional-diameter tubes. In addition, evaporation heat transfer through a thin liquid film in the plug flow region for low quality, mass velocity, and heat flux was observed. The heat transfer coefficient increased with decreasing tube diameter under the same experimental condition. The fictional pressure drop increased with increasing mass velocity and quality and decreasing tube diameter. The experimental values of the heat transfer coefficient and frictional pressure drop were compared with the values calculated by the empirical correlations in the open literature.     

水平管外沸腾强化换热的数值模拟与场协同分析

应用FLUENT软件对制冷剂R134a在光管和横纹槽管水平管外沸腾传热进行三维数值模拟,得到其饱和泡状沸腾过程中体积含气率的分布规律,并比较它们的换热系数。结果表明横纹槽管外侧能够很好地强化沸腾传热。此外,还通过改变边界条件分析质量流量、热流密度的变化对横纹槽管管外沸腾换热系数的影响。最后应用场协同理论,从局部换热角度分析其强化机制。研究表明,横纹槽管水平管外沸腾换热得到强化的原因是其凹槽前后的速度场与温度梯度场之间夹角较小,协同程度更好。     

Wave characteristics of falling liquid film on a vertical circular tube

A falling film with waves on a vertical circular tube has been analyzed using the integral approach. A theoretical model of free surface deflection has been developed. The nonlinear equation obtained in the present work is similar to the deflection equation of a wave on a flat plate. It becomes exactly the same as the wave equation on a flat plate in the case of an infinite radius. This study shows that the wave characteristics depend on the parameters such as wave number, dimensionless wave velocity, tube radius and Reynolds number. As the tube radius decreases, the intensity of the wavy processes increases. The velocity distribution of the falling film was investigated in the present work. The cylindrical model appears to be more appropriate over the Cartesian model when the film thickness to tube diameter ratio is large. Calculated values are in good agreement with other published data.     

Cross-sectional and axial flow characteristics of dry granular material in rotating drums

Cross-sectional and axial flow behaviors of dry granular material in rotating drums are closely related to the dynamic characteristics and velocity distributions between the surface layer and bed material. In this study, both 2D and 3D dry granular flow patterns in horizontal rotating drums are experimentally investigated with flow imaging analysis. A dimensionless flow parameter combining the effects of Froude number, relative particle size and volume filling is proposed in this study, which controls the flow characteristics in a rational drum such as dynamic angle of repose, thickness of the flowing layer, relative free surface velocity, and the shear rates in the flowing layer. The dimensionless granular temperature exhibits linear distribution in the flowing layer, being maximum at the free surface and being negligible at the interface in the rolling regime. The measured shear rate of the plug flow departs from drum angular velocity under the wall slip conditions when the drum surface is smooth. Due to the existence of axial convection and lateral surface profile, the mass flux in the flowing layer is always less than that of the plug flow in the 3D granular flows based on sidewall particle images. One the other hand, the mass flux in the flowing layer is always equal or greater than that of the plug flow in the 2D granular flows. 2D granular flows exhibit higher angles of repose and surface velocities than those of the 3D granular flows at the same volume fillings.     

Effect of motion of a local heat source on thermocapillary deformation of a thin liquid film flowing down under the action of gravity

The influence of a moving local heat source on the structure of flow in a thin liquid film flowing down on an inclined substrate under the action of gravity has been theoretically studied. Two-dimensional steady-state and conjugated hydrodynamic heat transfer problem has been solved in a long-wave approximation. The characteristics of flow are compared for various regimes: from the liquid film flowing down on a vertical surface with an immobile heat source to the behavior of a horizontal liquid layer under the action of a moving heat source. It is shown that changes in the flow velocity profile related to an increase in the velocity of the heat source motion and a decrease in the substrate slope under other equal conditions (constant flow rate, film thickness, and heat release) lead to a sharply increased thermocapillary deformation of the liquid film.