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.     

Mathematical modeling of falling liquid film evaporation process

A mathematical model of evaporation process from a laminar falling liquid film on a vertical plate of constant temperature is presented. The model is developed with and without interfacial shear stress due to the vapor flow at the liquid film surface. The vapor pressure drop, vapor exit velocity and cooling rate are calculated for different liquid mass flow values. It is shown that lower liquid mass flow produces higher cooling rate. The results also show that the interfacial shear stress has a considerable negative effect on the cooling rate. It is proved that there exists an optimum distance between the plates, which gives the maximum volumetric cooling rate.     

液滴与壁面撞击流动及传热特性研究

采用Level Set-VOF模拟单液滴撞击壁面的铺展行为及液滴初速度、初始直径及液膜厚度对液滴撞壁传热特性的影响。研究表明:液滴初始速度较大,撞击壁面后发生强烈反弹,液滴在表面回缩破碎及铺展破碎能力加强,导致表面传热系数随之增大;随着液滴初始直径增大,液滴铺展破碎的发生,将对表面传热起促进作用;初始液膜越厚,撞击后液滴溅射能力被削弱且在表面铺展趋势延缓,因此不利于热量迅速传递。     

Quantification of flow and retention of oil in compressor discharge pipe

This paper presents a new method to measure the oil retention and oil circulation ratio (OCR) in the compressor discharge pipe based on oil film thickness, oil film average velocity, oil droplet size, oil droplet velocity, and system mass flow rate. Oil flow parameters are quantified based on visualization using high-speed camera and video processing techniques. The estimated oil retention and oil circulation ratio results are compared quantitatively with the results from sampling measurements under different compressor speed and compressor types. The agreement between video results and sampling measurements verify the accuracy of this innovative method, which can also be applied in other annular-mist flow analysis. It also shows that most of the oil exists in film by mass while oil droplets contributes more to the oil mass flow rate because oil droplets travel in a much higher speed.     

Reduction in the critical conditions for instability of a highly charged droplet moving relative to a medium

It is shown that as the flow velocity of an ideal liquid flowing around a charged perfectly conducting droplet of ideal liquid increases, the critical self-charge of the droplet at which instability occurs decreases rapidly. Pis’ma Zh. Tekh. Fiz. 24, 54–57 (July 12, 1998)     

板式升膜蒸发器蒸发换热特性的实验研究

升膜蒸发利用被蒸发溶液在受热沸腾汽化产生的蒸气带动料液上升,形成一个稳定的液膜,从而加快蒸发,提高了传热系数.针对蒸发器液位、换热温差、加热蒸气的流量等因素对板式升膜蒸发传热系数的影响进行了实验研究,实验结果表明随着蒸发器内液位的降低升膜蒸发传热系数明显提高,此外,换热温差、加热蒸气的流量等因素对升膜蒸发换热性能也有明...     

气液双介质喷嘴雾化特性的影响因素研究

目的 气液双介质喷嘴的应用是烟叶加料工艺的关键因素之一,通过采用现有的模拟手段实现加料过程的高精化,为优化实验奠定基础。方法 文中以外混式双介质喷嘴为研究对象,通过构建基于DPM的数值模型,研究不同蒸汽入口压力与针阀位置对喷嘴雾化的气动性能与粒径特性的影响规律。结果 研究表明,喷嘴出口通流面积越大,气流高速区离喷嘴越近,但蒸汽与料液的速度最大值并未有较大变化,此时液滴喷射距离由远变近,喷射半角由小变大。随着蒸汽压力的增大,气流速度以及高速区长度变大,蒸汽压力为0.1 MPa时,气流速度的最大值与蒸汽压力为0.4 MPa时的相差近25%;不同压力下喷射角变化范围较小,为21.5°~23.5°;除此之外,喷雾束变集中,定向性变好,有助于控制其在烟叶表面喷射的均匀性。结论 对于较近的烟叶墙,采用较大流通截面积的喷嘴结构有利于均匀地覆盖烟叶墙,相反,对于较远的烟叶墙,则适合采用较小流通截面积的结构。适当地提高压力有利于提高料液喷洒的均匀性,减少料液的浪费,提高烟丝制备的工艺水平。     

Experimental investigation on the effect of liquid injection by multiple orifices in the formation of droplets in a Venturi scrubber

Venturi scrubbers are widely utilized in gas cleaning. The cleansing elements in these scrubbers are droplets formed from the atomization of a liquid into a dust-laden gas. In industrial scrubbers, this liquid is injected through several orifices so that the cloud of droplets can be evenly distributed throughout the duct. The interaction between droplets when injected through many orifices, where opposite clouds of atomized liquid can reach each other, is to be expected. This work presents experimental measurements of droplet size measured in situ and the evidence of cloud interaction within a Venturi scrubber operating with multi-orifice jet injection. The influence of gas velocity, liquid flow rate and droplet size variation in the axial position after the point of the injection of the liquid were also evaluated for the different injection configurations. The experimental results showed that an increase in the liquid flow rate generated greater interaction between jets. The number of orifices had a significant influence on droplet size. In general, the increase in the velocity of the liquid jet and in the gas velocity favored the atomization process by reducing the size of the droplets.     

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.     

Simulation of a spray scrubber performance with Eulerian/Lagrangian approach in the aerosol removing process

In this study, a mathematical model has been developed to simulate the performance of a spray scrubber in an industrial ammonium nitrate plant. The model is based on the Lagrangian approach for the droplets movement and particle source in cell (PSI-CELL) model for calculating the droplet concentration distribution. Consequently, unlike former research, the emphasis is on the droplet dynamic behavior. In the current study, for approaching a realistic model, a droplet size distribution rather than average diameter, and also liquid film formation rather than uniform and constant droplet flow rate has been applied. Also, the Eulerian method has been used for the calculation of the particles removal efficiency and energy balance has been applied on the gas to estimate the droplet size distribution. In the experimental section, the concentration of particles and their size distribution in both inlet and outlet gas of the studied scrubber has been measured for the validation of the predicted particles collection efficiency. In addition, the temperature of the gas at inlet, outlet and in the middle of the tower has been measured for the confirmation of the predicted droplet size distribution in the tower. A good consistency between the model and data has been observed. After the model is validated, it is used to investigate the various variable profiles such as liquid film, total projected surface area of the droplets, velocity profile of the droplets and some of the other parameters in the spray scrubbers.     

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.     

Heat exchange processes near the boundary of a film boiling nucleus

Results are presented of an experimental investigation of the change in temperature, heat flux to the liquid, and rate of displacement of the isotherms near a film boiling nucleus propagating over a plane surface. The experiment was carried out in a liquid nitrogen bath at atmospheric pressure on the saturation line. The heater was a sapphire plate 1.2 mm thick having a heat transfer surface area of 77×22 mm². The following facts were established: 1) near the boundary of the film boiling nucleus a new heat exchange mechanism takes place caused by the instability of the liquid microlayer; 2) the maximum heat flux to the liquid is considerably greater than the critical heat flux; 3) the vapor film in the film boiling region grows gradually with increasing distance from the boundary, i.e., there is a smooth transition in terms of heat exchange intensity before the equilibrium film boiling level is reached. Pis’ma Zh. Tekh. Fiz. 25, 39–46 (November 12, 1999)     

Numerical analysis of the impact of two droplets with a liquid film using an incompressible SPH method

In this study, a truly incompressible smoothed particle hydrodynamics (SPH) algorithm combined with an effective surface tension model is extended to simulate the dynamic process of multiple droplets impacting on a liquid film in 2D and 3D. This approach uses a pressure Poisson equation to satisfy the incompressibility constraints, and the Navier–Stokes equations are solved in a Lagrangian form using a fractional-step projection method. The mathematical model is first validated by the simulations of several fluid impact phenomena in comparison with those obtained by other numerical methods. Then the interesting phenomena of two 2D droplets impacting successively on a rigid solid/liquid film are numerically predicted and compared with the corresponding experimental results. Next, the fluid mechanics of two 2D droplets impinging simultaneously on a thin liquid film are numerically investigated. The effects of the impact velocity and the two droplets’ horizontal spacing on the collision behavior are discussed in detail. Lastly, the splashing phenomenon of a 3D droplet impacting on a thin liquid film is simulated. All numerical results obtained are in agreement with the available data.     

双介质喷嘴雾化特性数值模拟研究

目的 双介质喷嘴雾化效果直接影响烟卷加料工艺的进一步提升，通过对雾化过程进行数值模拟，方便对雾化特性进行透彻的分析，提升雾化效果。方法 采用数值模拟方法构建两相流连续相流场与DPM离散态双向耦合的数值模型，研究蒸汽压力、液体流量以及双介质喷嘴结构对喷嘴雾化特性的影响。结果 适当增加蒸汽压力，可以在不影响最大流速、颗粒粒径均匀度及颗粒中值粒径的情况下，减小雾化扩散角，小幅度地增加喷射距离，雾化细度变好，进而提高雾化效果。随着有机液流量的增加，雾化扩散角增大，喷射距离增加，雾化粒径均匀度变好，从而使雾化效果变好。液体路通流面积越大喷雾的贯穿距离越小，气路通流面积越大喷雾的雾化扩散角度越大。若需要得到较好的雾化效果，需要保证较小的蒸汽路通流面积，与此同时液路侧保持正常开度。结论 适当地提高有机液流量或者蒸汽压力，以及采用较小蒸汽路通流面积，同时液路侧保持正常开度的结构，有利于提高料液喷洒的均匀性，减少了料液的浪费，提高了烟丝制备的工艺水平。     

4He Films near Monolayer Completion

We study the role played by the substrate–helium interaction in determining whether a ⁴He film is fluid or solid near monolayer completion. In order to glean information concerning the possible fluid-solid transition we calculate the equation of state as well as the behavior of phonons and rotons for a high density monolayer fluid film. By analyzing the behavior of the long-wavelength excitations and the roton gap, we argue that we can infer instability in the fluid phase. We study a model Lennard-Jones two parameter potential for the substrate–helium interaction. We map out in the two-parameter space the separatrix between those weak substrates that can only support a fluid monolayer from those strong substrates that support a solid. Our approach utilizes a combination of information from both variational calculations and also correlated basis function theory to examine in detail the excitation structure in the monolayer liquid as a function of film coverage and substrate potential.     

A comparison of modelling methods for polydispersed wet‐steam flow

Spontaneous nucleation of water droplets in moist air or steam may result in droplet spectra which are complex in shape and which span a broad range of sizes. This is particularly true if the flow is transonic or supersonic with shock waves present, or if an already droplet‐laden flow re‐expands to give secondary or tertiary nucleations. Computation of such flows requires careful modelling of the size distributions if two‐phase behaviour is to be accurately predicted. In this paper, three methods are presented for treating size distributions and growth of the liquid phase in condensing steam: a mixed Eulerian–Lagrangian method, a fully Eulerian method, and a method based on moments of the droplet spectra. These are compared by computing condensing flow within a one‐dimensional supersonic nozzle under conditions that yield very different types of size spectra. Copyright © 2003 John Wiley & Sons, Ltd.     

Rayleigh wave interaction with,and the extension of,microcracks

Intense Rayleigh waves produced by the impact of high-velocity liquid jets on brittle solids were arranged to interact with well-defined surface flaws of dimensions 50 to 200 μm. The extent of crack growth was monitored as a function of distance from the impact site. It was found that considerable crack growth as well as crack branching occurred for cracks parallel to the incident wavefront and little or no growth for orthogonal cracks. The form of the surface wave was monitored using piezoelectric crystals attached to the surface. The results are discussed in terms of recent fracture mechanics analysis of stress-wave interaction with cracks. The significance of this study to strength degradation of brittle bodies subjected to rain-drop impact is pointed out.     

液氢长距离管道输送探讨

讨论了液氢在长距离管道输送中存在着最佳流速,并分析了实际液氢输送过程中的输送状态.对在稳定的大流量转注过程中存在的单相流和两相流两种流动状态分别给出了在不同压力、流速、漏热等情况下温升、蒸气含量的理论计算结果,并在试验的结果上对液氢的流动状态给出了一些看法.     

Flow physics of upward cocurrent gas-liquid annular flow in a vertical small diameter tube

In this paper, a steady RNG k-ε model, in conjunction with enhanced wall treatment method, was applied to the gas core in order to simulate the flow physics of annular two-phase flow. The model incorporated a physical model of wave characteristics and included the liquid entrainment influence on the flow. Based on the simulation results, flow features in the gas core were quantitatively presented and a model of the liquid entrainment mechanism was proposed. In addition, a parametric study was conducted to determine the impact of changing wave velocity, pressure, and gravitational force on the liquid film flow. The results were validated using a large set of experimental data at normal and microgravity conditions. Also, the law of the wall was applied to previously-collected experimental data. Analysis yielded different flow features of the liquid film at microgravity and normal gravity conditions.