Film Velocity Research Articles

Computational and experimental investigation of a swirl nozzle for viscous fluids

Highly viscous flow in a large-scale pressure-swirl atomizer is studied by (1) 3d scale-resolving large-eddy simulations and volume-of-fluid method, and (2) experiments based on laser-Doppler anemometry, imaging techniques and pressure measurements. Here, a low Reynolds number regime (600 ≤ Re ≤ 910) is investigated by varying the mass flow rate of the water-glycerol mixture. The aim of the study is to perform a comprehensive comparison between the simulations and experiments at a parameter range and nozzle geometry relevant for biomass based fuels. We report the inner-nozzle velocity profiles noting good agreement for mean velocities inside the swirl chamber between the simulations and the experiments. Consistent with the earlier work (Laurila et al., 2019), the simulations indicate the flow mode to be laminar with weak or non-existent gaseous core inside the swirl chamber. As revealed by both approaches, liquid film shapes after the nozzle discharge orifice are qualitatively similar, of hollow cone type, and highly unstable. Both approaches indicate linear scaling of the liquid film velocity with the inlet Reynolds number and discharge coefficients to be in the range 0.57–0.64. The experimentally measured mean opening angles are reported to be 45–62∘, while the numerical counterparts show reasonable correspondence with the experiments. The results demonstrate the predictive ability of the present numerical method in swirl injector analysis.

Numerical Investigation on the Thermodynamic Characteristics of a Liquid Film upon Spray Cooling Using an Air-Blast Atomization Nozzle.

This paper developed a three-dimensional model to simulate the process of atomization and liquid film formation during the air-blast spray cooling technological process. The model was solved using the discrete phase model method. Several factors including the thermodynamic characteristics of the liquid film as well as the spray quality with different spray mass flow rates under different spray heights were numerically investigated and discussed. The results show that the varied spray height has little effect on the Sauter Mean Diameter (d32) of the spray droplet, while the thermodynamic characteristics of liquid film including the liquid film height, the liquid film velocity, and the liquid film generation rate are sensitive to the change of the spray height. With the growth of spray mass flow rates, d32, the liquid film generation rate and liquid film height become larger, while the liquid film velocity with different spray mass flow rates has a similar velocity distribution, indicating that the spray mass flow rate has little effect on the liquid film velocity. The average d32 of droplet size shows a sharp drop when sprayed from the nozzle in a short period of time (<1.5 ms), then approaching smoothness, below a value of 40 , the spray status tends to be stable.

Study on Lubrication Characteristics of Metal Liquid Film Based on Electromagnetic-Elastic Mechanics-Hydrodynamics Multiphysics Coupling Model.

In an electromagnetic rail launcher, a metal liquid film is created at the armature/rail (A/R) contact interface. It has a significant impact on electromagnetic launch performance. In this paper, an electromagnetic-elastic mechanics-hydrodynamics multi physics coupling model is established in consideration of the metal liquid film’s own acceleration, magnetic pressure and dynamic changes in film thickness. Based on this model, the lubricating characteristics of magnetic pressure and fluid pressure distribution, film thickness distribution and velocity distribution of the metal liquid film were studied. When the velocity of the metal liquid film is very fast, and the magnetic pressure is reduced, it may fail to maintain stability and rupture, which may be an important reason for the transition. Finally, this paper analyzes the lubrication effect of the metal liquid film, and points out that when we want strictly to control the muzzle velocity, the lubrication effect of the metal liquid film must be considered.

Liquid film transport around Taylor bubble in a microchannel with gas cavities

In Taylor flow, the low velocity in liquid film around the bubbles limits the mass transfer between the bubble body and liquid phase. To overcome this deficiency, a cross-junction Microchannel with gas Cavities (MGC) is designed and processed. The restricted inert nitrogen in the gas cavities provided partial slip boundaries, which makes it distinctive from the conventional Straight Microchannel (SM). The liquid film thickness and velocity distribution in the liquid film are investigated. It is found that the bubble shape in MGC is more sensitive to the capillary number. A thicker liquid film and a sharper bubble shape can be observed under the same operating conditions. These changes lead to a larger specific surface area (about 110 % of SM) and a remarkable increment of leakage flow rate (132.2–181.1 % of SM). Then, the intensifying mechanism of liquid transport in the liquid film is analyzed by numerical simulation. The results reveal that the slip and radial fluctuation of velocity at the gas cavity interface prominently improved the liquid transport. The high specific surface area and strong renewal ability of liquid film demonstrates a great potential for mass transfer in MGC.

Numerical investigation on heat transfer of water spray cooling from single-phase to nucleate boiling region

In the present study, spray cooling heat transfer characteristics were numerically investigated under the heat flux ranging from 50 to 170 W/cm2. Effect of nozzle height and spray pressure was also studied in order to reveal the microscopic mechanisms of the heat transfer enhancement from single-phase to nucleate boiling regions. A two-phase flow heat transfer model was adopted based on the Euler-Lagrangian approach. The dependence of thermo-physical properties of fluid on the temperature was also taken into account. The comparison between simulated wall temperature and experimental data demonstrates a satisfactory agreement. The results show that the heat transfer coefficient increases monotonously with the heat flux. In the nucleate boiling region, the wall film is thinner with a smaller velocity compared with those in the single-phase region. Spray cooling heat transfer is enhanced evidently as the nozzle height decreases and the spray pressure increases. The most important feature which separates the present study from the literature is that the numerical simulation in this paper presents some detailed microscopic characteristics, e.g. the wall film thickness and velocity, which play an important role in spray cooling heat transfer.

Radiative heat flux effect in flow of Maxwell nanofluid over a spiraling disk with chemically reaction

In this paper, thin film flow of Maxwell nanofluid caused by a radially stretching and rotating disk is investigated by employing the revised Buongiorno’s model in the presence of uniform magnetic field. The impact of non-linear thermal radiations is studied on heat transfer characteristics. The nanofluid model utilized in the present study employs the features of Brownian motion and thermophoresis together with condition of zero normal flux of nanoparticles. The reduced system of governing equations have been solved through a numerical technique namely bvp4c. The impact of physical parameters is studied on fluid velocity, temperature, and concentration profiles. The obtained results reveal that both the film thickness and velocity components decrease with magnetic parameter. Further, the surface heat flux diminishes with the viscosity and thermophoresis parameters. Moreover, Brownian motion and chemical reaction parameters play a significant role in reducing the nanoparticles concentration.

A mathematical model based on unstructured mesh for ice accretion

This paper proposes a three-dimensional model to simulate ice accretion on the surface of an aircraft. The model is developed using a prism grid cell, which enables iterations with an unstructured mesh. The model assumes that the impinged water droplets on the surface can form a thin water film, and ice accretion is caused by the flow and solidification of the film. The model is developed by analyzing the conservations of mass, momentum, and energy for the thin water film on the surface. The mass and energy conservation equations of the water film are discretized on the prism grid cell, while the momentum conservation equation, which is simplified using lubrication theory, is integrated along the surface normal direction to obtain an analytical solution for the film velocity. The thicknesses of the water film and ice layer are calculated by iterating the discretized mass and energy conservation equations with the analytical solution of the film velocity. The model is verified by comparing its results with published experimental and numerical data. The results show that the model provides accurate predictions of ice accretion under different conditions, even when using an unstructured grid.

Interfacial phenomena and heat transfer associated with multi-droplet impact on flowing liquid film

This study addresses multiple droplets impact on a flowing liquid film numerically based on a three-dimensional model developed previously. Major contents include interfacial behavior and heat transfer associated with both successive and simultaneous impacts regarding the effect of film velocity. Shearing force induced by the flowing film breaks symmetry of both interface evolution and heat transfer. Increasing film velocity promotes upstream splashing but suppresses downstream splashing, while heat transfer coefficient is enhanced and shifted toward downstream collectively. Interface evolutions present three-dimensional feature, which makes the two-dimensional numerical scheme questionable. Comparing successive and simultaneous cases shows that the former is far more complicated than the latter. The interaction site prior to the impacting of trailing droplet in the successive case is highly dependent on the film velocity, resulting in a variety of complicated interfacial phenomena and nonuniform distributions of wall temperature and heat transfer coefficient.

Analysis of thin film flow of generalized Maxwell fluid confronting withdrawal and drainage on non-isothermal cylindrical surfaces

This investigation is concerned with the study of thin film flow of a generalized Maxwell fluid along with slip conditions, confronting withdrawal and drainage on non-isothermal cylindrical surfaces. The governing equations have been formulated from the continuity equation, momentum equation, and energy equation. Analytical solutions for the velocity field, volume flow rate, average film velocity, tangential stress, and temperature are obtained in series form through the Binomial expansion technique in both withdrawal and drainage cases. The well-known solutions for a Newtonian fluid are regained as a particular case of our acquired general solutions in all flow cases. In addition, solutions for the power-law fluid model, executing alike motion, can be recovered as a limiting case of our acquired general solutions. The influence of different dimensionless parameters on all physical quantities (i.e. velocity, volume flow rate, average film velocity, tangential stress, and temperature profile) is examined and discussed graphically for both generalized Maxwell and Newtonian fluids.

Velocity saturation in La-doped BaSnO3 thin films

BaSnO3, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining the device performance. We report on the experimental measurement of the electron saturation velocity in La-doped BaSnO3 thin films for a range of doping densities. The predicted saturation velocities based on a simple LO-phonon emission mode, using an effective LO phonon energy of 120 meV show good agreement with the measurements of velocity saturation in La-doped BaSnO3 films. Density-dependent saturation velocity in the range of 1.8 × 107 cm/s reducing to 2 × 106 cm/s is predicted for δ-doped BaSnO3 channels with carrier densities ranging from 1013 cm−2 to 2 × 1014 cm−2, respectively. These results are expected to aid the informed design of BaSnO3 as an active material for high-charge density electronic transistors.

Numerical investigation of film thickness variation on falling film tubular heat exchangers at different Reynolds number

Horizontal tube falling film tubular heat exchanger is extensively used in modern heat transfer processes. It has a broad area of application like desalination, refrigeration, processing and food Industries. The most important phenomenon in this heat transfer process is the extraction of latent heat by the spraying fluid from the outer surfaces of the heated tubes. Numerical Simulation of falling film thickness around the circumference of the tube is performed to analyze the film thickness and circumferential velocity over falling film tubular heat exchangers by Volume of Fluid (VOF) technique using Computational Fluid Dynamic (CFD) software, ANSYS 15.1. The effect of feed rate, tube diameter, and tube spacing are investigated. It is observed that the film thickness is directly influenced by Reynolds number and the film velocity reaches its maximum value at the end of fully developed zone. The results are in good agreement with the published data.

Viscoelastic MHD Nanofluid Thin Film Flow over an Unsteady Vertical Stretching Sheet with Entropy Generation

The boundary-layer equations for mass and heat energy transfer with entropy generation are analyzed for the two-dimensional viscoelastic second-grade nanofluid thin film flow in the presence of a uniform magnetic field (MHD) over a vertical stretching sheet. Different factors, such as the thermophoresis effect, Brownian motion, and concentration gradients, are considered in the nanofluid model. The basic time-dependent equations of the nanofluid flow are modeled and transformed to the ordinary differential equations system by using similarity variables. Then the reduced system of equations is treated with the Homotopy Analysis Method to achieve the desire goal. The convergence of the method is prescribed by a numerical survey. The results obtained are more efficient than the available results for the boundary-layer equations, which is the beauty of the Homotopy Analysis Method, and shows the consistency, reliability, and accuracy of our obtained results. The effects of various parameters, such as Nusselt number, skin friction, and Sherwood number, on nanoliquid film flow are examined. Tables are displayed for skin friction, Sherwood number, and Nusselt number, which analyze the sheet surface in interaction with the nanofluid flow and other informative characteristics regarding this flow of the nanofluids. The behavior of the local Nusselt number and the entropy generation is examined numerically with the variations in the non-dimensional numbers. These results are shown with the help of graphs and briefly explained in the discussion. An analytical exploration is described for the unsteadiness parameter on the thin film. The larger values of the unsteadiness parameter increase the velocity profile. The nanofluid film velocity shows decline due the increasing values of the magnetic parameter. Moreover, a survey on the physical embedded parameters is given by graphs and discussed in detail.

Establishment of the falling film evaporation model and correlation of the overall heat transfer coefficient.

In order to study the heat transfer of the falling film evaporator with phase change on both sides, in this paper we built the mathematical model and the physical model where the liquid film inside the tube is laminar and turbulent. The film thickness of the condensate at different axial positions, total condensate volume and velocity distribution, and temperature distribution of condensate outside the tube can be obtained by calculating the proposed model. Meanwhile, the liquid film thickness, velocity distribution and temperature distribution inside the tube were obtained by numerical simulation by considering the influence of the liquid film with different compositions on the heat transfer during fluid flow. With ethanol–water as the system, the overall heat transfer coefficient and heat transfer quantity of the falling film evaporator were obtained by the calculation of the model. The accuracy of the proposed model was confirmed by experiments. The model and the calculation of heat transfer proposed in this paper have enormous significance for the basic data and theoretical guidance of the heat transfer performance prediction and operational optimization of the evaporator.

A nano-lubrication solution for high-speed heavy-loaded spur gears and stiffness modelling

• A revised model of calculating the time-dependent oil normal stiffness and tangential stiffness is proposed. • A numerical algorithm for high-speed spur gears lubricated by nanofluids is presented. • Spherical alumina nanoparticles significantly reduce friction coefficient and maximum temperature of total contact region. • Nanoparticles of higher concentration reduce traction friction and film temperature. • Many vortexes form and move from gear surface to pinion surface in rough contact. Nanofluids have excellent mechanical and thermal properties. Serious wear and high temperatures hamper the development of high-speed gear drives, and the application of nanofluids to gear lubrication is a potential solution. The main purpose of this paper is to investigate the mechanism by which nanofluids affect the lubrication performance of high-speed gear drives. A revised model of calculating the time-dependent oil normal stiffness and tangential stiffness is proposed. The density equation and viscosity equation are revised. Firstly, the numerical algorithm for high-speed spur gears lubricated by nanofluids is validated based on the classical minimum film thickness formula of Dowson, and then the differences between smooth and rough contact are analyzed. Furthermore, the effects of nanoparticle shape and concentration on tribology performance of gear lube are comprehensively investigated. Finally, the influences of surface roughness on film velocity field distribution are discussed. The simulation results indicate that applying spherical alumina nanoparticles to gear lube as additives is beneficial to the anti-wear performance of gear teeth and the lubricating property of lubricants, which considerably decrease friction coefficient and maximum temperature of the total contact region. Moreover, film normal stiffness of spherical alumina nanoparticles is close to base oil which means it maintains excellent load-carrying capacity. Higher concentrations of nanoparticles reduce the friction coefficient and maximum temperatures of the total contact region, and cause an increase in film tangential stiffness. The surface roughness remarkably influences film velocity field and high amplitude roughness induces formation of many vortexes in the contact center, and these vortexes move from gear surface to pinion surface in a total engagement cycle.

Nanotribological Properties of ALD-Made Ultrathin MoS2 Influenced by Film Thickness and Scanning Velocity.

Solid lubricating films are usually applied to reduce friction and adhesion of micro-/nano-electromechanical systems (MEMS/NEMS). Due to the special structure, MoS2 is endowed with excellent lubricity. Layer-controlled ultrathin MoS2 films with strong interactions to the underlying substrates were prepared by atomic layer deposition (ALD) using MoCl5 and H2S and characterized by various measures. Nanotribological properties of the MoS2 films with different thicknesses were observed by an atomic force microscope tip under various loads. In the initial stage of ALD, bigger roughness induced by discontinuous nanoparticles can increase friction. However, when the number of ALD cycles is more than 3, the friction force can be effectively reduced by about 40-55% because of interlayer sliding and lower adhesion. Finally, friction properties against sliding velocities of the MoS2 films were also observed. The ultrathin MoS2 film obtained by ALD exhibits a huge application value as reducing friction surface in MEMS/NEMS.

Transient thin film flow of nonlinear radiative Maxwell nanofluid over a rotating disk

In this letter, a mathematical model for transient nature thin film flow of Maxwell nanofluid over a rotating disk is studied in the presence of a uniform magnetic field and non-linear thermal radiation. The Brownian motion and thermophoresis features due to nanofluid are captured by adopting the Buongiorno model. The prime emphasize is to explore the temperature field and nanoparticles volume fraction in nanofluid thin film flow. The reduced system of differential equations is solved numerically by finite difference based method namely bvp4c. The numerical outcomes regarding film thickness, Nusselt number, Sherwood number, velocity, temperature, and concentration are revealed for varying estimation of involved physical parameters. It is shown that the film thickness decreases with increasing values of the magnetic number. Further, the impact of thermophoresis and thermal radiation parameters is worthwhile in enhancing the fluid temperature. The Solute concentration is found to decrease with Brownian motion and Schmidt number.

Effect of geometric and operating parameters on the spray characteristics of an open-end swirl injector

To study the influence of geometric and operating parameters on the spray characteristics of an open-end swirl injector, seven injectors with different tangential inlet diameters ( D p) and injector length to injector orifice diameter ( L/D) ratios were tested and simulated. Using high-speed backlight, the evolution laws of liquid film thickness, discharge coefficient, spray cone angle, breakup length, and velocity distribution in the swirl chamber under different geometric and operating parameters were captured after unified image processing. Low-injection pressure drop is directly proportional to the discharge coefficient and the spray cone angle. When the injection pressure drop approaches or reaches a critical value of 0.4 MPa, the discharge coefficient and spray cone angle remain nearly constant with maximum fluctuations of 1% and 5%, respectively. With an increase in the geometric characteristic constant A, the liquid film thickness, discharge coefficient, breakup length, and velocity in the swirl chamber decrease, whereas the spray cone angle increases. As the viscous effect increases for increasing L/D, the discharge coefficient and breakup length increase, whereas the spray cone angle decreases. Based on experiment results, empirical formulas for the discharge coefficient, spray angle, and breakup length were put forward as reference for engineering applications, including the effect of the geometric and operating parameters.

Experimental investigation of heat transfer of flowing liquid film with inserted metal foam layer subjected to air jet impingement

In this paper, coupling the air jet impingement and the copper metal foam above flowing liquid film were employed to enhance the heat transfer. The thickness of flowing liquid film can be controlled owing to the application of the metal foam above the film, and its solid matrix extends the air-liquid-solid interface of heating surface. The evaporated water can be supplied by the capillary force in the porous layer. The experiments were conducted to investigate the performances of the flowing liquid film with inserted porous layer subjected to impinging jet air. The air jet velocity, the flow rate and thicknesses of the liquid film as well as the porosity of metal foam influence the surface temperature of heated wall and the corresponding local heat transfer coefficient greatly. The change ratios of heat transfer coefficient due to the above factors were presented. More cooling can be obtained on the heated wall in the flowing liquid film with inserted porous layer subjected to impinging jet air while the higher liquid film velocity and air jet velocity, the thinner liquid film and the lower porosity of metal foam occur.

Heat transfer mechanisms of single droplet impacting onto flowing film

The experimental study of heat transfer and film dynamics of droplet impacting onto flowing film using Novec-7100 as the cooling liquid was presented in this article. The droplet velocity and the liquid film velocity could be adjusted in this experimental study. The Reynolds number of the flowing liquid film varied from 1257 to 6290. In this study, the substrate of heater was Zinc Selenide (ZnSe), and the heating element was a thin layer of Indium Tin Oxide (ITO). The overflow plate was used to make the liquid film thickness fixed at 1 mm. The heat transfer mechanisms of flowing liquid film (flowing film only), droplet impinging on the dry surface (droplet-dry surface) and droplet impinging on the flowing liquid film (droplet-flowing film) were discussed in this paper respectively. The results showed that when the averaged temperature of the heated surface does not reach the liquid saturation temperature, bubbles appear in the liquid film, and the bubbles are more dense downstream. When the averaged heat flux of the heated surface ranges from 6.2 × 103 W/m2 to 3.4 × 104 W/m2, the averaged heat transfer coefficient of the droplet-flowing film can be calculated by the superposition of the averaged heat transfer coefficient of the flowing film only and the averaged heat transfer coefficient of the droplet-dry surface, the maximum error is 14%. When the averaged heat flux of the heated surface ranges from 3.4 × 104 W/m2 to 6.8 × 104 W/m2, the Reynolds number of the flowing liquid film and the effect of the coupling between droplet impingement and flow boiling should be considered in the calculation of the averaged heat transfer coefficient of the droplet-flowing film. In addition, boiling heat transfer is the main heat transfer mechanism of droplet impinging on thin liquid film when the averaged heat flux of the heated surface ranges from 2 × 104 W/m2 to 6.8 × 104 W/m2.

Use of the buffer layers as a current flow diverter in 2G HTS coated conductors

This paper presents a simple and effective approach to increase the normal zone propagation velocity (NZPV) in (RE)BaCuO thin films grown on a flexible metallic substrate. The concept is an extension of the current-flow-diverter concept already known in the literature. The key idea behind the novel approach is to use a specific geometry of the silver thermal stabilizer that surrounds the superconducting tape. More specifically, a very thin layer of silver stabilizer is deposited on top of the superconductor layer, typically less than 100 nm, while the remaining stabilizer (still silver) is deposited on the substrate side. Normal zone propagation velocities up to 170 cm s−1 at 77 K have been measured experimentally for the first time with this novel architecture, corresponding to a stabilizer thickness of 20 nm on top of the superconductor layer. This is at least one order of magnitude faster than the NZPV speeds measured on commercial 2G HTS tapes. Our results clearly demonstrate that a very thin stabilizer on top of the superconductor layer leads to high normal zone propagation velocities. The experimental values are in good agreement with predictions realized by finite element simulations. Furthermore, the propagation of the normal zone during the quench was recorded in situ and in real time using a high-speed camera. Due to high Joule losses generated on both edges of the tape sample, a ‘U-shaped’ profile could be observed at the boundaries between the superconducting and the normal zones, which matches very closely the profiles predicted by the simulations. The most direct application of this new HTS tape architecture is in high field magnets, since faster quench propagation allows easier detection and protection, which is likely to accelerate substantially the development of HTS magnets used as high-field MRI/NMR systems, etc.