Stokes Energy Research Articles

Direct numerical simulations of thermocapillary migration of a droplet attached to a solid wall

Abstract This paper is designated to gain further insight into the physical mechanisms of thermal droplet actuation on a wall through direct numerical simulation. Classical theory states that free droplets in a nonuniform temperature field always move towards the hot side. However, when attaching a droplet to a wall with a nonuniform temperature gradient, lubrication theory explains how such a droplet moves towards the colder side. This paper aims at further investigating and clarifying the physical mechanisms and acting forces in the environment of a nonuniform temperature field and offers some explanations. For the numerical simulations of a droplet attached to a wall with a linear temperature gradient and larger contact angles, the full Navier–Stokes equations and energy equation are solved in a Volume of Fluid framework. The solver is extended with a dynamic contact angle treatment and thoroughly validated. The droplet motion is studied both in two and three dimensions, where a movement towards the cold and the warm side can be observed. The forces acting in such a setting are identified and interpreted. A decomposition of the jump conditions shows that the tangential stress due to the temperature dependent surface tension alone would lead to a motion towards the cold side, whereas the normal component alone would move the droplet to the opposite direction. The differences between two- and three-dimensional simulations show that the problem at hand is clearly three-dimensional.

Unaxisymmetric stagnation-point flow and heat transfer of a viscous fluid on a moving cylinder with time-dependent axial velocity

The unsteady viscous flow and heat transfer in the vicinity of an unaxisymmetric stagnation-point flow of an infinite moving cylinder with time-dependent axial velocity and non-uniform normal transpiration \( U_{0} \left( \varphi \right) \) are investigated. The impinging free stream is steady with a strain rate \( \bar{k} \). A reduction of the Navier–Stokes and energy equations is obtained by use of appropriate similarity transformations. The semi-similar solution of the Navier–Stokes equations and energy equation has been obtained numerically using an implicit finite-difference scheme when the axial velocity of the cylinder and its wall temperature or its wall heat flux varies as specified time-dependent functions. In particular, the cylinder may move with different velocity patterns. These solutions are presented for special cases when the time-dependent axial velocity of the cylinder is a step function, a ramp, and a non-linear function. All the solutions above are presented for Reynolds numbers, \( \text{Re} = \bar{k}a^{2} /2\upsilon \), ranging from 0.1 to 100 for different values of Prandtl number and for selected values of transpiration rate function, \( S(\varphi ) = U_{0} (\varphi )/\bar{k}a \) where a is cylinder radius and υ is kinematic viscosity of the fluid. Dimensionless shear stresses corresponding to all the cases increase with the increase of Reynolds number and transpiration rate function. An interesting result is obtained in which a cylinder moving with certain axial velocity function and at particular value of Reynolds number is axially stress free. The heat transfer coefficient increases with the increasing transpiration rate function, Reynolds number and Prandtl number. Interesting means of cooling and heating processes of cylinder surface are obtained using different rates of transpiration rate function. It is shown that a cylinder with certain type of exponential wall temperature exposed to a temperature difference has not heat transfer.

Numerical Simulation of Transient Three-Dimensional Temperature and Kerf Formation in Laser Fusion Cutting

In the present study, a three-dimensional transient numerical model was developed to study the temperature field and cutting kerf shape during laser fusion cutting. The finite volume model has been constructed, based on the Navier–Stokes equations and energy conservation equation for the description of momentum and heat transport phenomena, and the volume of fluid (VOF) method for free surface tracking. The Fresnel absorption model is used to handle the absorption of the incident wave by the surface of the liquid metal, and the enthalpy-porosity technique is employed to account for the latent heat during melting and solidification of the material. To model the physical phenomena occurring at the liquid film/gas interface, including momentum/heat transfer, a new approach is proposed which consists of treating friction force, pressure force applied by the gas jet, and the heat absorbed by the cutting front surface as source terms incorporated into the governing equations. All these physics are coupled and solved simultaneously in fluent CFD®. The main objective of using a transient phase change model in the current case is to simulate the dynamics and geometry of a growing laser-cutting generated kerf until it becomes fully developed. The model is used to investigate the effect of some process parameters on temperature fields and the formed kerf geometry.

Heat Transfer Effect in an Air Cooled Ventilated Enclosure

Fluid flow and heat transfer in a mechanically ventilated enclosure using a mixed air-distribution system at various levels have been numerically simulated. Thermally activated fluid is supplied through various slots of the vertical channel wall, flush in with the isothermal ceiling and flush out through a port in the opposite wall. The horizontal walls are assumed to be impermeable and adiabatic. Steady, laminar, and incompressible flow under a Boussinesq approximation has been considered, and numerical results for the Navier–Stokes equation and energy equation are obtained using a finite volume method based on the semi-implicit method for pressure linked equations algorithm for the following parameters: locations of inlet and outlets, and different values of Grashof numbers and Reynolds numbers. The proposed numerical model has been validated by comparing its predictions with those of other models’ data in the literature. The validated model is then used to investigate the effects of the Reynolds number and room aspect ratio on the flow velocity and thermal characteristics within the room. The flow structures and thermal activities are studied for Richardson’s number in the range of . Variation of the convection coefficient along the wall for cold and hot fluids at different locations has been investigated, and cooling efficiencies are found to be maximum when the inlet and the outlet are placed at the middle and lower parts, respectively, of the hot and the cold walls.

Numerical and experimental analysis of short-scale Marangoni convection on heated structured surfaces

Abstract Short-scale Marangoni flow in a film on an evenly heated horizontal wall with a structured surface is studied experimentally and numerically. The occurring two-dimensional convective rolls are fully visualized in the experiment for the first time, where a closed experimental environment allows to monitor the temperature and Biot number. For the numerical simulations, the full Navier–Stokes equations and energy equation are solved in a Volume of Fluid framework. The energy equation is solved with a novel two-field approach, which prevents numerical diffusion at the interface. The numerical results are in very good agreement with the experimentally obtained data. Simulations were performed to study the influence of film height, wall temperature, topography changes and the effect of gravity on the flow characteristics like interface velocity, flow patterns and heat transport. It is found that the interface velocity and convective heat transport both increase with increasing film height and wall temperature. Additionally a change in the flow pattern is found. Gravitational effects play a minor role and have no effect on the steady state.

Finite Element Analysis with COMSOL Code for Air Flow and Thermal Convection in Sealed Attic Spaces with Experimental Validation

The scope of the present paper is concerned with the numerical prediction of the confined air flow characteristics and thermal convection patterns in sealed attic spaces in roofs with upper inverted V-shapes and horizontal suspended ceilings of conventional houses and buildings. For these isosceles triangular cavities, two relevant cases involve prescribed wall temperatures wherein the bottom base wall is cooled/heated and the upper two inclined walls are symmetrically heated/cooled during the summer and winter seasons. Based on finite element analysis, the COMSOL code is implemented to perform numerical solutions of the two-dimensional system of coupled NavierStokesBoussinesq and energy equations. The computational domain is made coincident with the physical domain to handle potential non-symmetric velocities and non-symmetric temperatures that may occur when exposed to vigorous air flows. The numerical solution via finite elements provides the two velocity fields u (x, y), v (x, y) and the temperature field T (x, y) for the confined air flows. Overall, the target design quantity is the mean wall heat fluxes w q varying with the attic aspect ratio and the temperature difference at two opposing walls. The predicted w q values match the experimental measurements for the two distinct cases related to summer and winter seasons. At the end, comprehensive correlation equations are constructed for the quantification of the mean Nusselt number in terms of the Grashof number and the attic aspect ratio, which could be used in building science research.

Effect of Tilt Angle on Pure Mixed Convection Flow in Trapezoidal Cavities Filled with Water-Al2O3 Nanofluid

Abstract A numerical study is carried out to investigate the effect of tilt angle of the cavity on mixed convection heat transfer inside two different lid-driven trapezoidal cavities; one having heated wall on short base and another having heated wall on long base. In this investigation, the top wall is maintained at isothermal cold temperature, which is moving in its own plane at a constant speed while a constant high temperature is provided at the bottom surface of the cavity. The cavity is assumed to be filled with water-Al2O3 nanofluid. The governing Navier–Stokes and thermal energy equations and boundary conditions are non-dimensionalised and are solved using Galerkin finite element method. Attention is paid in the present study on the pure mixed convection regime at Richardson number, Ri = 1 where the natural and the forced convection are equally dominated. Parametric investigations are carried out by taking base wall tilt angle from 0o to 45o with a step of 15o and also varying Reynolds numbers from 0.1 to a maximum order of 104 with the corresponding Grashof numbers varying from 0.01 to a maximum order of 108 for Ri = 1. Simulations are carried out by considering both plain fluid (water) and nanofluid with 10% solid-volume fraction of nanoparticles. Flow and heat transfer characteristics are explained using streamline and isotherm contours, and the variation of average Nusselt number of the heated wall and average fluid temperature of the cavity are analysed for different tilt angles.

Simulation of jet impingement heat transfer onto a moving disc

A transient numerical investigation has been conducted to determine the thermal effects of an axisymmetric oil jet impinging on a high-speed reciprocating disc subjected to uniform heat flux and bounded by a cylindrical wall. The two-phase air–oil simulations are performed using the volume of fluid (VOF) method with a high-resolution interface-capturing scheme. The three-dimensional Navier–Stokes equations and energy equation are numerically solved using a finite volume discretization. The conjugate heat transfer (CHT) method is used to obtain a coupled heat transfer solution between the disc and fluid, yielding a more accurate prediction for the heat transfer coefficient. To overcome the high computational cost of such a simulation, a new methodology is presented to accelerate the solution. The simulation process involves several stages, including the simulation of the heat transfer of a stationary disc with a cooling jet at different impingement distances from the nozzle exit and simulation of a moving disc without the cooling jet and subjected to constant heat flux. Following this, the flow field and thermal characteristics of a reciprocating disc with constant heat flux and an impinging cooling jet is considered. For jet impingement onto the moving boundary, the maximum Nusselt number is achieved a short time after the relative velocity between the disc and the jet reaches its maximum.

Physical controls on mixing and transport within rising submarine hydrothermal plumes: A numerical simulation study

A computational fluid dynamics (CFD) model was developed to simulate the turbulent flow and species transport of deep-sea high temperature hydrothermal plumes. The model solves numerically the density weighted unsteady Reynolds-averaged Navier–Stokes equations and energy equation and the species transport equation. Turbulent entrainment and mixing is modeled by a k–e turbulence closure model. The CFD model explicitly considers realistic vent chimney geometry, vent exit fluid temperature and velocity, and background stratification. The model uses field measurements as model inputs and has been validated by field data. These measurements and data, including vent temperature and plume physical structure, were made in the ABE hydrothermal field of the Eastern Lau Spreading Center. A parametric sensitivity study based on this CFD model was conducted to determine the relative importance of vent exit velocity, background stratification, and chimney height on the mixing of vent fluid and seawater. The CFD model was also used to derive several important scalings that are relevant to understanding plume impact on the ocean. These scalings include maximum plume rise height, neutrally buoyant plume height, maximum plume induced turbulent diffusivity, and total plume vertically transported water mass flux. These scaling relationships can be used for constructing simplified 1-dimensional models of geochemistry and microbial activity in hydrothermal plumes. Simulation results show that the classical entrainment assumptions, typically invoked to describe hydrothermal plume transport, only apply up to the vertical level of ~0.6 times the maximum plume rise height. Below that level, the entrainment coefficient remains relatively constant (~0.15). Above that level, the plume flow consists of a pronounced lateral spreading flow, two branches of inward flow immediately above and below the lateral spreading, and recirculation flanking the plume cap region. Both turbulent kinetic energy and turbulence dissipation rate reach their maximum near the vent; however, turbulent viscosity attains its maximum near the plume top, indicating strong turbulent mixing in that region. The parametric study shows that near vent physical conditions, including chimney height and fluid exit velocity, influence plume mixing from the vent orifice to a distance of ~10 times the vent orifice diameter. Thus, physical parameters place a strong kinetic constraint on the chemical reactions occurring in the initial particle-forming zone of hydrothermal plumes.

Investigation of high-speed bypass effect on the performance of the surface air–oil heat exchanger for an aero engine

Abstract In the present study, the influence of an air–oil heat exchanger (SAOHE) location and orientation on engine performance is investigated using numerical predictions with a range of geometry options that match experimental data. The airflow in the unit-fin domain of a SAOHE was modeled with rotational periodic boundary conditions. Using the standard k–ɛ turbulence model, the compressible Reynolds-averaged Navier–Stokes (RANS) equations and energy equation were solved numerically. In order to validate the numerical method, experimental measurements of the velocity profile and the distribution of the pressure and heat transfer coefficient were compared to numerical results. The pressure drop, overall heat transfer coefficient, and velocity profile downstream of the heat exchanger were taken into consideration as performance metrics. An efficient numerical procedure for the installation study of a cooler having a bypass duct was conducted, and important design variables for SAOHE were clearly identified.

Hundred picoseconds laser pulse amplification based on scalable two-cells Brillouin amplifier

AbstractHundred picoseconds laser pulse with high energy and high peak power has broad application prospects such as inertial confinement fusion shock ignition. But it is hard to get effective amplification through MOPA or chirped pulse amplification method. Through simulated Brillouin scattering method, 100 picoseconds laser pulse can be amplified efficiently. To be able to meet the need of high energy and high-intensity laser pulse amplification, scalable two cell structure and four different FC series liquid were used to fulfill this experiment. The results indicate that the magnification of Stokes energy and efficiency of energy extraction are closely related to medium parameters and energy parameters. The minimum width of 340 ps Stokes pulse was amplified by 13.5 times in this experiment.

White-Light Emission from Amorphous ZrHfO Thin Film Dielectrics with and without Embedded Nanocrystalline CdSe Dots

White-Light Emission from Amorphous ZrHfO Thin Film Dielectrics with and without Embedded Nanocrystalline CdSe Dots

Stabilization of boundary layer streaks by plasma actuators

A flow's transition from laminar to turbulent leads to increased levels of skin friction. In recent years, dielectric barrier discharge actuators have been shown to be able to delay the onset of turbulence in boundary layers. While the laminar to turbulent transition process can be initiated by several different instability mechanisms, so far, only stabilization of the Tollmien–Schlichting path to transition has received significant attention, leaving the stabilization of other transition paths using these actuators less explored. To fill that void, a bi-global stability analysis is used here to examine the stabilization of boundary layer streaks in a laminar boundary layer. These streaks, which are important to both transient and by-pass instability mechanisms, are damped by the addition of a flow-wise oriented plasma body force to the boundary layer. Depending on the magnitude of the plasma actuation, this damping can be up to 25% of the perturbation's kinetic energy. The damping mechanism appears to be due to highly localized effects in the immediate vicinity of the body force, and when examined using a linearized Reynolds-averaged Navier–Stokes energy balance, indicate negative production of the perturbation's kinetic energy. Parametric studies of the stabilization have also been performed, varying the magnitude of the plasma actuator's body force and the spanwise wavenumber of the actuation. Based on these parametric studies, the damping of the boundary layer streaks appears to be linear with respect to the total amount of body force applied to the flow.

Heat transfer and various convection structures of near-critical CO2 flow in microchannels

Abstract Supercritical/near-critical fluid is very dense and much expandable, and with its preferable flow and heat transfer properties, it has been proposed in various kinds of energy conversion systems. The fluid critical transitions and diverges are very important for both hydrodynamic study and heat transfer applications. The current study deals with the near-critical CO2 horizontal flow and heat transfer performance in microchannels. Careful numerical procedures are carried out with Navier–Stokes equations, energy and state equations, which are treated with special care for micro-scale investigations. Due to the thermal–mechanical effects of critical fluid, abnormal thermal convection structure and transient micro-scale vortex mixing evolution mode are found in microchannels, which is identified to be governed by basic Kelvin–Helmholtz instability. New sources of near-critical fluid thermal–mechanical perturbations, instead of gravity waves, are found to be responsible for the microscale instabilities for the transient vortex flow development. It is also found that while in closed systems the Piston Effect (PE) is dominant, the thermal–mechanical expansion characteristics yield new convective structures and evolutions in open system (microchannels). At the same time the microchannel heat transfer is greatly enhanced due to the vortex evolutions. Possible model extensions are also discussed, and the near-critical fluid convection problem is then characterized from a more general viewpoint in this study.

Extracavity pumped BaWO_4 anti-Stokes Raman laser

The characteristics of a barium tungstate (BaWO(4)) anti-Stokes Raman laser at 968 nm are studied theoretically and experimentally. The BaWO(4) Raman resonator is pumped by a Q-switched Nd:YAG laser at 1064 nm with its axis tilted from the pumping laser axis. The non-collinear phase matching for the generation of the first anti-Stokes wave in the same BaWO(4) crystal is achieved. The output energy, temporal and spectral informations are investigated. At a pumping laser energy of 128 mJ, the anti-Stokes laser energy obtained is 2.2 mJ. The second Stokes radiation at 1324 nm as well as the first and the third Stokes waves at 1180 nm and 1509 nm is also generated at the same time. The maximum total Stokes energy output is 42.5 mJ. In the theory, the anti-Stokes laser intensity expression as a function of the pumping and the first Stokes laser intensities for the extracavity anti-Stokes Raman laser is deduced. The properties of the anti-Stokes Raman laser are simulated theoretically by solving the rate equations of the extracavity Raman laser and using the derived expression. The theoretical results are in good agreement with the experimental results.

Modeling of a Solar Water Collector with Water‐Based Nanofluid Using Nanoparticles

The pressure‐velocity form of the Navier–Stokes equations, energy equation, and concentration equation are used to represent the mass, momentum, energy, and concentration conservations of the nanofluid medium in the solar collector. The governing equations and corresponding boundary conditions are converted to dimensionless form and solved numerically by the finite element method. The physical domain is discretized by triangular mesh elements with six nodes. The working fluid is water‐based nanofluid with two nanoparticles, namely, silver (Ag) and copper oxide (CuO). The study includes computations for different values of buoyancy ratio (Nr) and Schmidt number (Sc). Flow, heat, and mass transfer characteristics are presented in the forms of streamlines, isotherms, and iso‐concentrations. In addition, results for the average radiative, convective heat and mass transfer, mean temperature and concentration of nanofluid, mid‐height horizontal‐vertical velocities, and subdomain average velocity field are offered and discussed for the above‐mentioned parametric conditions. Results show that the effects of Nr and Sc on the convective‐radiative heat and mass transfer phenomenon inside the collector are significant for all values of Nr and Sc studied. Comparison and validation with the standard experimental/numerical data is given in brief. The variation of the obtained result is presented as 34% with the result of experimental data. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(3): 270–287, 2014; Published online 30 September 2013 in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21080

Measuring Transport Coefficients in Heterogeneous and Hierarchical Heat Transfer Devices

Experimental determination of transport coefficients, in particular internal heat transfer coefficients, in heterogeneous and hierarchical heat transfer devices such as compact heat exchangers and high surface density heat sinks has posed a persistent challenge for designers. This study presents a unique treatment of the experimental determination of such design data. A new combined experimental and computational method for determining the internal heat transfer coefficient within a heterogeneous and hierarchical heat transfer medium is explored and results are obtained for the case of cross flow of air over staggered cylinders to provide validation of the method. Along with appropriate pressure drop measurements, these measurements allow for thermal-fluid modeling of a heat exchanger by closing the volume averaging theory (VAT)-based equations governing transport phenomena in porous media, which have been rigorously derived from the lower-scale Navier–Stokes and thermal energy equations. To experimentally obtain the internal heat transfer coefficient the solid phase is subjected to a step change in heat generation rate via induction heating, while the fluid flows through under steady flow conditions. The transient fluid phase temperature response is measured. The heat transfer coefficient is then determined by comparing the results of a numerical simulation based on the VAT model with the experimental results. The friction factor is determined through pressure drop measurements, as is usually done. With the lower-scale heat transfer coefficient and friction factor measured, the VAT-based equations governing the transport phenomena in the heat transfer device are closed and readily solved. Several configurations of staggered cylinders in cross flow were selected for this study. Results for the heat transfer coefficient and friction factor are compared to widely accepted correlations and agreement is observed, lending validation to this experimental method and analysis procedure. It is expected that a more convenient and accurate tool for experimental closure of the VAT-based equations modeling transport in heterogeneous and hierarchical media, which comes down to measuring the transport coefficients, will allow for easier modeling and subsequent optimization of high performance compact heat exchangers and heat sinks for which design data does not already exist.

THREE DIMENSIONAL HEAT TRANSFER CHARACTERISTICS THROUGH A LINEAR GAS TURBINE CASCADE

This study presents experimental and numerical investigation for three-dimensional heat transfer characteristics in a turbine blade. An experimental set-up was installed with a turbine cascade of five blade channels. Blade heat transfer measurements were performed for the middle channel under uniform heat flux boundary conditions. Heat was supplied to the blades using twenty-nine electric heating strips cemented vertically on the outer surface of the blades. Distributions of heat transfer coefficient were obtained at three levels through blade height by measuring surface temperature distribution using thermocouples. To understand heat transfer characteristics, surface static pressure distributions on blade surface were also measured. Numerical investigation was performed as well to extend the investigation to locations other than those measured experimentally. Three-dimensional non-isothermal, turbulent flow was obtained by solving Reynolds averaged Navier Stokes Equations and energy equation. The Shear stress Transport k- model was employed to represent turbulent flow. It was found through this study that secondary flow generated by flow deflection increases heat transfer coefficient on the blade suction surface. Separation lines with high heat transfer coefficients were predicted numerically with good agreement to the experimental measurements.

Heat transfer enhancement in microchannel heat sinks with periodic expansion–constriction cross-sections

Abstract The heat transfer enhancement of microchannel heat sinks with periodic expansion–constriction cross-sections is investigated both experimentally and numerically. Each heat sink consists of 10 parallel microchannels with 0.1 mm wide and 0.2 mm deep in constant cross-section segment and each microchannel consists of an array of periodic expansion–constriction cross-sections. Three-dimensional laminar numerical simulations, based on the Navier–Stokes equations and energy equation, are obtained for pressure drop and heat transfer in these microchannel heat sinks under the same experimental conditions. Multi-channel effect, entrance effect, conjugate heat transfer, viscous heating and temperature dependent properties are considered. It is found that the numerical predictions of apparent friction factor and Nusselt number are in good agreement with experimental data. The influences of periodic expansion–constriction cross sections on pressure drop, heat transfer and thermal resistance are discussed, respectively. The effects of the entrance and exit plenum regions and the lateral parts of silicon wafer on fluid flow and heat transfer are discussed. Special attentions are given to analyze the variation of thermal resistance for each term with pumping power, corresponding to three stages of heat release at the substrate of heat sink.

Turbulent free convection between vertical isothermal plates with asymmetrical heating

Results of numerical investigation of the flow and heat transfer at turbulent free convection between the vertical parallel isothermal plates with different temperatures are presented. The temperature factor R T varied within −2 ÷ 1. The Rayleigh number changed within Ra = 107 ÷ 109, and the ratio of geometrical sizes of plates and distances between them was constant A = L/w = 10. Numerical studies were performed via the solution to the two-dimensional Navier—Stokes equations and energy equation in Boussinesq approximation. The considered boundary-value problem has the unknown conditions at the inlet and outlet between the plates. To describe turbulence, the modified low-Reynolds k-ɛ model was used. The effect of the temperature factor on the flow structure at the channel inlet and outlet was analyzed. Data on distributions of velocities and temperatures between the plates, local and integral heat transfer allow deeper understanding of the mechanism of transfer processes between the parallel plates with asymmetrical heating.