Motion of droplets induced by the Marangoni force on a wall with a temperature gradient

Motion of silicone oil and water droplets induced by the Marangoni force was numerically simulated by using two‐ and three‐dimensional second‐order finite difference methods with the CIP and the level set methods. The surface tension was introduced by the continuum surface force (CSF) method. The results clearly showed the flow induced by the Marangoni force and the dependence of droplet velocity on droplet size, contact angle, temperature gradient, and fluid properties. The Marangoni force balanced with the viscous force in the small contact angle case; on the other hand, in the large contact angle case, it balanced with the normal component of surface tension. As for the effect of fluid properties on droplet motion, the temperature coefficient of surface tension had a much larger effect than did viscosity, thermal diffusivity, or surface tension. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 81–93, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20004     

加热板液滴蒸发的理论分析及实验研究

液滴蒸发是由气-液浓度差驱动的一种常见而复杂的扩散现象.通过实验与理论相结合对去离子水在玻璃表面和有机硅油表面的蒸发特性进行研究,测量了液滴接触角和接触直径随时间的动态演变过程.结果 发现:玻璃表面的液滴蒸发为典型的定底半径模式和混合模式;而液滴在有机硅油表面较为特殊,除了定底半径模式和混合模式还有周期性的黏滑模式.出...     

Heat transfer from a rotating disk in a parallel air crossflow

The heat transfer from a rotating disk in an air stream parallel to the plane of rotation is of importance in the assessment of disk brake performance. Numerically determined heat transfer coefficients and correlations are accordingly presented for a large range of rotational and crossflow velocities. These were obtained by means of large-eddy-simulations (LES). The extreme conditions of a stationary disk in an air crossflow and a rotating disk in still air are also considered. It is found that a critical ratio between the rotational and the crossflow Reynolds numbers exists with respect to rotational heat transfer augmentation. Only above this critical value, rotational heat transfer augmentation sets on in case of laminar crossflow Reynolds numbers. This phenomenon is directly linked to a flow instability that leads to a periodic vortex generation, and which can be described by the classical Landau model. For higher angular velocities, the wake becomes fully turbulent, and the transition is very rapid.     

Experimental studies of rotating heat pipes

Thermal characteristics of a rotating heat pipe were measured under steady state at moderate rotational speeds. Copper‐water rotating heat pipe with copper screen mesh wick was fabricated for testing at various heat loads. An experimental test rig with a water‐cooled condenser section was fabricated to study the heat transfer in the rotating heat pipe (RHP) for various heat loads and various rotational speeds ranging from 1000 rpm to 2000 rpm. A heat transfer correlation was developed for the condensing heat transfer coefficient and compared with the experimental results. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20265     

Scattering characteristics of liquid droplets spun off from rotating disk edge

Scattering characteristics of liquid droplets spun off from a rotating disk edge are experimentally investigated. In the present research, aluminum disks are utilized and ethanol is employed for liquid. Scattering phenomena of the droplets are captured by the high-speed digital camera. Frequency distribution of the droplet diameter is evaluated from these images and distributions of horizontal flying velocity and angle of the droplets were measured by PTV. Liquid filaments are stretched outward from the stagnant liquid layer by centrifugal force and skew complicatedly by aerodynamic force. Some peaks appear in the distribution of the scattered droplet diameter and they are origi- nated from large terminal droplets and small droplets generated from filamentwise breakup. Most of the scattered droplets fly slightly inside in the tangential direction of the disk edge. The droplets spun off from the thin disk scatter widely compared with that from the thick one.     

Analysis of a free surface film from a controlled liquid impinging jet over a rotating disk including conjugate effects, with and without evaporation

A detailed analysis of the liquid film characteristics and the accompanying heat transfer of a free surface controlled liquid impinging jet onto a rotating disk are presented. The computations were run on a two-dimensional axi-symmetric Eulerian mesh while the free surface was calculated with the volume of fluid method. Flow rates between 3 and 15 lpm with rotational speeds between 50 and 200 rpm are analyzed. The effects of inlet temperature on the film thickness and heat transfer are characterized. The evaporative effects are captured when the fluid impinges on the disk at saturated conditions. The conjugate heating effect is modeled, and was found to effect the heat transfer results the most at both the inner and outer edges of the heated surface. The heat transfer was enhanced with both increasing flow rate and increasing rotational speeds. When evaporative effects were modeled, the evaporation was found to increase the heat transfer at the lower flow rates the most because of a fully developed thermal field that was achieved. The evaporative effects did not significantly enhance the heat transfer at the higher flow rates.     

Numerical simulation of fluid flow and heat transfer in a rotating cylindrical container with a counter-rotating disk at the fluid surface

Fluid flow and heat transfer in a rotating cylindrical container with a counterrotating disk at the fluid surface are numerically investigated. The effects of disk rotation and of Prandtl numbers on the fluid flow and heat transfer in the container are discussed. Flow and temperature fields are obtained for various rotational Reynolds numbers of the disk and for Prandtl numbers of the fluid. Nusselt numbers on the walls are calculated for the temperature fields and are compared with available experimental data. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 172–182, 1999     

Study on high performance evaporator with micro‐grooves (Measurement of capillary force in a single groove and modeling of heat and mass transfer)

A micro‐grooved evaporator is composed of µm‐wide grooves on a heat transfer plate in which the inter‐line regions at the liquid–vapor meniscus of coolant become identifiable. The high‐heat performance of the evaporator is realized by this inter‐line region (ILR) where the liquid thin film reduces the thermal resistance on the heat transfer surface. In this report, we propose a numerical simulation model of heat and mass transfer in a single groove to predict its capillary force and heat flux. The capillary force performance (capillary‐rise length in a groove) of a single groove was measured for samples of varying width, superheat, and inclination. The performance was found to be a maximum at a specific groove width of 200–400 µm, which is in good agreement with the predicted results calculated by the proposed model. For a better prediction of capillary‐rise length, the effective capillary force and the effective flow resistance were considered. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20257     

An unsteady flow structure on a heated rotating disk under mixed convection

A flow field under mixed convection on a heated rotating disk has been measured using an ultrasonic velocity profiler (UVP). The measured velocity field is a spatio‐temporal one as a function of radial coordinates and time. The objective of this paper is to clarify the vortex structure caused by the instability between buoyancy and centrifugal force. The vortex appears under typical conditions of Reynolds numbers and Grashof numbers and it moves toward the outside of the disk. This behavior can be classified into two patterns. The size of the vortex structure decreases with an increasing Reynolds number and increases with the Grashof number. The traveling velocity of the vortex increases with the Grashof number. Moreover, it decreases with an increasing Reynolds number in spite of increasing centrifugal force. According to these results, the region dominated by natural, forced, and mixed convection is classified in the relationship between Reynolds and Grashof numbers. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 407–418, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20074     

Unsteady flow and heat transfer of a ferrofluid between two rotating,vertical moving and stretching disks

Unsteady flow and heat transfer of a magnetic fluid between two rotating disks is investigated. Both the disks are stretchable and the lower disk moves in the vertical direction. A new approach of similarity transformation is adopted to transform the equation of continuity, momentum, and the energy equation into ordinary nonlinear coupled differential equations. The numerical solution of the converted nonlinear differential equations is obtained using the finite element method. The effects of magnetization force, rotational viscosity, Prandtl number, and Eckert number on the velocity and temperature distributions are studied. The impact of stretching, movement, and rotation of the disk is also considered in this computational study. The skin friction coefficients and heat transfer rate on the lower disk for different physical parameters are calculated. Different types of motion of the disks and the magnetization force are crucial aspects in the stress distribution and heat transfer rate near the lower disk.     

An experimental investigation on spreading of droplets with evaporation and nucleation

An experimental investigation was conducted to visually observe the dynamic characteristics of water droplets with evaporation and nucleation on stainless steel and polished silicon surfaces. The water droplet diameter, contact area, and spreading speed were measured using a high‐speed CCD camera at surface temperatures ranging from 110^°C to 190^°C, and a model was proposed to describe the dynamic behavior of droplet spreading. The spreading of water droplets under evaporation and nucleate boiling is highly dependent on the dynamic bubble behavior in the droplets, particularly bubble volume, bubble interaction, as well as the surface properties and temperature. Water droplets were easiest to spread at the surface temperature of 130 ^°C, and the spreading tendency increased with increasing surface coarseness. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20231     

Fluid flow and heat transfer of natural convection over upward‐facing,horizontal heated circular disks

Natural convective flows induced over upward‐facing, circular disks were investigated experimentally. The test disks were heated with uniform temperatures and their diameters were varied from 20 to 500 mm. The test fluid was air and water at room temperature. The flow fields over heated disks were visualized with dye and smoke. The results showed that the ambient fluid first enters the outer edge of the disk and, then, separates three‐dimensionally from the surface at certain distance downstream of the edge when the Rayleigh numbers exceed certain critical values. The separated flow is gradually distorted and a fully turbulent state is accomplished in the central portion of the disk. It was found that the onset and the pitches of the separation points could be predicted with the Rayleigh numbers. The overall Nusselt numbers of the disks were also measured in a wide range of Rayleigh numbers from 2 × 10⁵ to 3 × 10¹⁰. The results showed almost identical Nusselt numbers between air and water. Based on these results, empirical equations for the overall Nusselt numbers were proposed. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(6): 339– 351, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20215     

Effects of liquid viscosity on inception of disturbance waves and droplets in gas–liquid annular two‐phase flow

The structure of gas–liquid two‐phase flow is investigated in order to establish a reliable criterion for the development of disturbance waves and droplets considering the effects of liquid viscosity. The structure of the gas–liquid interface and the flow rate of droplets entrained in gas are measured simultaneously at five kinematic viscosities (1.0, 3.2, 9.9, 30, 70 mm²/s). The time‐series traces of liquid film thickness measured by five holdup probes reveal that the inception of disturbance waves occurs at a liquid Reynolds number of 200 or a non‐dimensional liquid film thickness of 6.5. It is also shown that droplets are generated before the inception of disturbance waves with increasing liquid kinematic viscosity at a liquid velocity of 0.02 to 0.03 m/s. As previously published criteria for the inception of droplets are found to be unsatisfactory, a new critical condition for droplet generation balancing the interfacial shear stress $τi$ with the wave height h and surface tension σ is proposed: $τih/σ=0.025$. This relation describes the action of shear force and surface tension on wave crests, and is notably independent of liquid viscosity. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 529–541, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20176     

Three‐dimensional simulation of water droplet movement in PEM fuel cell flow channels with hydrophilic surfaces

Water management is one of the critical issues in proton exchange membrane fuel cells, and proper water management requires effective removal of liquid water generated in the cathode catalyst layer, typically in the form of droplets through cathode gas stream in the cathode flow channel. It has been reported that a hydrophilic channel sidewall with a hydrophobic membrane electrode assembly (MEA) surface would have less chance for water accumulation on the MEA surface. Therefore, a comprehensive study on the effect of surface wettability properties on water droplet movement in flow channels has been conducted numerically. In this study, the water droplet movements in a straight flow channel with a wide range of hydrophilic surface properties and effects of inlet air velocities are analyzed by using three‐dimensional computational fluid dynamics method coupled with the volume‐of‐fluid (VOF) method for liquid–gas interface tracking. The results show that the water droplet movement is greatly affected by the channel surface wettability and air flow conditions. With low contact angle, droplet motion is slow due to more liquid–wall contact area. With high air flow velocities, increasing the contact angle of the channel surface results in faster liquid water removal due to lesser liquid–wall contact area. Copyright © 2010 John Wiley & Sons, Ltd.     

Computational fluid dynamics simulation of the aerodynamics of a high solidity,small‐scale vertical axis wind turbine

A computational fluid dynamics simulation was performed for a small‐scale, high solidity (σ = 0.48) H‐type Darrieus vertical axis wind turbine. Two‐dimensional unsteady Reynolds‐averaged Navier–Stokes equations were solved for the turbine numerical model, which has a large stationary domain and smaller rotating subdomain connected by a sliding mesh interface. The simulation results were first validated against steady‐state airfoil data. The model was then used to solve for three rotating blades with constant ambient flow velocity (Re = 360,000) over numerous blade speed ratios. The high solidity and the associated low blade speed ratio and rotational speed of the turbine result in complex flow–blade interaction mechanisms. These include dynamic stall resulting in vortex shedding, vortex impingement on the source blade and significant flow momentum extraction causing reduced power production from the downstream blade pass. Copyright © 2011 John Wiley & Sons, Ltd.     

Numerical Modeling of Fluid Flow and Thermal Behavior of Different Nanofluids on a Rotating Disk

The thermal and velocity profiles of various nanofluid systems on a rotating disk are simulated. Finite difference method, the orthogonal collocation method, and the differential quadrature method (DQM) of numerical approaches are used to solve the governing equations and are compared to determine the faster and more accurate solution procedure. Five nanoparticles Al, Al₂O₃, Cu, CuO, and TiO₂ solved in three base fluids water, ethylene glycol, and engine oil are considered to be used on the disk at different volume fractions. A new general algorithm is presented for solving equations of a rotating‐disk problem quickly and accurately and it is found that the DQM method is the best approach for this numerical simulation. Heat transfer performance of a rotating disk would be much better enhanced with water based Al nanofluid. A wide range of results for different base–fluid combinations with nanoparticles is presented with untransformed 3D results and effects of the variation of different parameters provides comprehensive insight and prevents inaccurate deductions.     

Three dimensional MHD stagnation flow due to a stretchable rotating disk

The three dimensional steady MHD laminar stagnation point flow of an electrically conducting fluid on a radially stretchable rotating disk in the presence of a uniform vertical magnetic field is the main concern of the present study. The problem is an extension of the well-known von Karman stagnation problem to the configuration with a stretchable disk with or without rotation. An exact similarity reduction of the Navier–Stokes equations leads to a system of ordinary differential equations describing the stagnation flow. The system is governed by a stretching and a rotation parameters, based on the wall stretching and angular velocity. Employing a highly accurate spectral numerical integration scheme, the effects of these parameters on the flow are examined. The quantities of particular physical interest, such as the torque and the wall shear stresses are calculated and discussed. Contrary to the classical von Karman flow, for small rotational speeds of the disk, magnetic field is found to thicken the boundary layer when small wall stretching is taken into account.     

Large eddy simulations of the flow past wind turbines: actuator line and disk modeling

Large eddy simulations of the flow through wind turbines have been carried out using actuator disk and actuator line models for the turbine rotor aerodynamics. In this study, we compare the performance of these two models in producing wind turbine wakes. We also examine parameters that strongly affect the performance of these models, namely, grid resolution and the way in which the actuator force is projected onto the flow field. The proper choice of these two parameters has not been adequately addressed in previous works. We see that as the grid is coarsened, the predicted power decreases. As the width of the body force projection function is increased, the predicted power increases. The actuator disk and actuator line models produce similar wake profiles and predict power within 1% of one another when subject to the same uniform inflow. The actuator line model is able to generate flow structures near the blades such as root and tip vortices which the actuator disk model does not, but in the far wake, the predicted mean wakes are very similar. In order to perform validation against experimental data, the actuator line model output was compared with data from the wind tunnel experiment conducted at the Norwegian University of Science and Technology, Trondheim. Agreement between measured and predicted power, wake profiles, and turbulent kinetic energy has been observed for most tip speed ratios; larger discrepancies in power and thrust coefficient, though, have been found for tip speed ratios of 9 and 12. Copyright © 2014 John Wiley & Sons, Ltd.     

Simulation of heat transfer and hydrodynamics over a free rotating disk using an improved radial velocity profile

INTRODUCTIONIn many casess hydrodynamics and heat tlansfer inrotating-disk systems may be successfully simulatedusing integral methods. Restrictions imposed by theboundary layer approach are well-known. However,advanced integral methods employing justified modelassumptions in each specific case are quite competitive in comparison with modern CFD-paCkages. Commonly accepted advantages of the integral methodsare relative simple and high speed in calculations.Integral methods in the case o…     

Aerodynamic structures and processes in rotationally augmented flow fields

Rotational augmentation of horizontal axis wind turbine blade aerodynamics currently remains incompletely characterized and understood. To address this, the present study concurrently analysed experimental measurements and computational predictions, both of which were unique and of high quality. Experimental measurements consisted of surface pressure data statistics used to infer sectional boundary layer state and to quantify normal force levels. Computed predictions included high‐resolution boundary layer topologies and detailed above‐surface flow field structures. This synergy was exploited to reliably identify and track pertinent features in the rotating blade boundary layer topology as they evolved in response to varying wind speed. Subsequently, boundary layer state was linked to above‐surface flow field structure and used to deduce mechanisms underlying augmented aerodynamic force production during rotating conditions. Copyright © 2007 John Wiley &Sons, Ltd.