Effects of wall slip and temperature jump on heat and mass transfer characteristics of an evaporating thin film

A new mathematical model is developed to predict heat and mass transport characteristics of the evaporating thin film. The model considers effects of velocity slip and temperature jump at the solid-liquid interface, disjoining pressure, and surface tension. Three-dimensional nonequilibrium molecular dynamics simulations for coupling between the momentum and heat transfer at the nanoscale solid-liquid interface are performed to obtain the slip length and interfacial thermal resistance length. It is found that both slip length and interfacial thermal resistance length decrease significantly with the decreasing interface wettability of the liquid to the wall. Velocity slip and temperature jump at the solid-liquid interface intend to reduce the superheat degree of the evaporating thin film, and thus result in a sharp decrease of the heat and mass transport characteristics of the evaporating thin film. It is also noted that velocity slip and temperature jump at the solid-liquid interface show a more pronounced effect as the superheat degree increases.     

微通道内气体-近壁颗粒流动传热数值模拟研究

数值模拟了微通道受限空间内气体-近璧颗粒流动与传热过程,所建模型考虑微尺度气体的可压缩与交物性特征,且在通道和颗粒壁面采用速度滑移和温度跳跃边界条件以考虑滑移区气体动量/能量非连续效应.在此基础上,计算分析了克努森数(Kn)和颗粒偏移比对颗粒表面拖曳力系数(CD)以及传热努塞尔数(Nu)的影响规律.研究结果表明:受气体...     

Numerical study on fluid flow and heat transfer in the thin liquid film involving a hydraulic jump

The fluid flow and heat transfer in a thin liquid film are investigated numerically. The flow is assumed to be two-dimensional laminar, and surface tension effects at the exit are considered. The most important characteristic of this flow is the existence of a hydraulic jump through which the flow undergoes a very sharp and discontinuous change. In the present study, a simplified model of a free liquid jet impinging on a plane is considered. An arbitrary Lagrangian–Eulerian (ALE) method is used to describe the moving free boundary, and the fractional step method (FSM) based on the streamline upwind Petrov–Galerkin (SUPG) finite element method is used for the time-marching iterative solution. The numerical results obtained by solving the unsteady full Navier–Stokes equations are presented for plane and radial flows with constant wall temperature. © 1999 Scripta Technica, Heat Trans Asian Res, 28(1): 18–33, 1999     

The Behavior and Characteristics of the InterfacialWaves in Gas-Liquid Two-Phase Separated FlowThrough Downward Inclined Rectangular Channel

INTaoDUCTIONInhorizontalandslightlyinclinedgas-liquidtwo-phaseflow,thestratifiedflowmnyoccurinthecaseofthelowgasandliquidflowratecombinations.Inthiskindofflowwhenthegasvelocityisincreasedwithintheregimeofthestratifledfiow,wavesappearonthegasandliquldinterfacel1'2].Forthesestrati-fiedwavyflowpatterns,thestructureanddynamicsoftheinterfacegreatlyinfiuencetheratesofmasslmo~umandheattransferaswellasthestabilityofthesysteml3'4l,thereforeanaccurateknowledgeofint~ialwavformationandwavparameterssuc…     

Heat transfer enhancement in light-emitting diode packaging employing different molar concentration of magnesium oxide thin films as a heat spreader

To solve thermal management difficulties created by miniaturization of light-emitting diode package, magnesium oxide thin film was spin coated on the aluminum 5052-grade substrate and employed for heat spreading and improvement of the thermal path for heat removal from light-emitting diode parcel to ambient. Thickness of different films of magnesium oxide was prepared from 0.6M and 0.8M precursors to find out the effects of films thickness and molar concentrations towards perfect heat spreading. Results from X-ray diffraction (XRD) proves the MgO thin film as cubic with the crystalline orientations of (200), (220), and (222). Good results were highlighted through 20 layers (0.6M) MgO thin film in respect to its transient thermal analysis compared with other layers from 0.6M and 0.8M and bare Al substrate; this was due to its nonporous and evenly distributed particles size (56.0 nm), film thickness (301.7 nm), surface roughness (121 nm), higher thermal conductivity (26.3 W/mK), and free from internal cracks and defects. Lower total thermal resistance (11.4 K/W), lower junction temperature (98.8°C), optimal thermal impedance (36.43°C), higher illumination from light-emitting diode, and operation within safe operating temperature range (2700-3000 K) for high-power light-emitting diodes were all recorded at 700 mA from the tested light-emitting diode mounted on 20 layers (0.6M) magnesium oxide thin film. This study consequently proved magnesium oxide thin film as an alternative heat spreader in lighting technology packaging.     

Oscillatory slip flow of Fe3O4 and Al2O3 nanoparticles in a vertical porous channel using Darcy's law with thermal radiation

The heat transfer phenomena and oscillatory flow of an electrically conducting viscous nanofluid (NF) in a channel with porous walls and saturated porous media exposed to the thermal radiation are studied. The nanoparticles (NPs) Fe₃O₄ and Al₂O₃ are taken with water as base fluid along with nonuniform temperature and velocity slip at the wall of channel (y′ = 0). The basic laws of momentum and energy conservation are converted into the dimensionless system of the partial differential equations (PDEs) using similarity variables. Closed‐form solutions of these coupled PDEs are constructed for all values of time by taking the oscillatory pressure gradient. The physical insight of involved parameters on the fluid velocity, temperature profile, heat transfer rate, and surface friction is studied and analyzed graphically. It is noted from this study that the fluid velocity shows a decreasing behavior with the volume fraction of NPs. Furthermore, the amplitude of the oscillatory motion in case of skin friction decreases for a large magnetic field.     

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 2. Effect on void fraction and liquid film thickness

An experimental study has been carried out to clarify the characteristics of the void fraction and the liquid film thickness of the air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter and pitch. The flow pattern in the experiment on the average void fraction and the local void fraction distribution in cross section was a bubble flow, and the liquid film thickness was in the region of semiannular and annular flows. It is clarified from these experiments that the average void fraction in tubes with wire coils is lower than that in a smooth tube and decreases with the wire diameter owing to the centrifugal force of the swirl flow which concentrates bubbles at the center of the tube, that the local liquid film thickness becomes more uniform with a decrease in the pitch of the wire coil, and that the liquid film becomes thicker after the passage through the wire coil with an increase in the wire diameter. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 652–664, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10067     

Entropy generation minimization and nonlinear heat transport in MHD flow of a couple stress nanofluid through an inclined permeable channel with a porous medium,thermal radiation and slip

Irreversible losses and heat transport in a magnetohydrodynamic flow of a viscous, steady, incompressible, and fully developed couple stress Al₂O₃–water nanofluid through a sloping permeable wall channel with porous medium and under the effect of radiation heat flux and slip were analyzed. The fundamental equations were solved numerically by using Runge-Kutta together with the shooting technique and the results were in qualitative agreement with an exact solution obtained for a limit case. The impacts of couple stress, Darcy number, solid nanoparticle concentrations, conduction-radiation parameter, Hartmann number and hydrodynamic slip on flow, temperature, heat transport, and entropy production were examined. It was possible to achieve values of minimum entropy production not yet reported in previous studies. In this way, optimal values of couple stress and slip were obtained. The heat transport was also explored and optimal values of slip flow and conduction-radiation parameter with maximum heat transfer were found. Finally, in addition to the alumina, the distributions of velocity, temperature, and entropy generation in TiO₂–water and Cu–water were presented for different solid nanoparticle concentrations. It was obtained that the local entropy of TiO₂–water was lower than Cu–water and Al₂O₃–water in the channel bottom region while it was greater in the upper region.     

Role of hydrogen in electrical and structural characteristics of bilayer CdTe/Mn diluted magnetic semiconductor thin films

Bilayer thin films of diluted magnetic semiconductor CdTe/Mn have been prepared using vacuum thermal evaporation method at pressure of 10⁻⁵ torr. Annealing of bilayer thin films has been performed in atmospheric condition at constant temperature 400 °C for 1 hour. Hydrogenation of as-grown and annealed bilayer thin films has been performed by keeping these in hydrogenation cell. Structural characteristics of as-grown and heat treated thin films have been performed by X-ray diffractometer. Current–voltage characteristics of both as-grown hydrogenated and annealed hydrogenated bilayer thin films have been studied to find out the effect of hydrogenation. Surface topography of as-grown and annealed bilayer thin films has been confirmed by optical microscopy.