Heat transfer in falling laminar-wavy liquid films

Heat transfer in the laminar-wavy film of liquid falling over the heated plate is studied numerically. To describe the wavy film flow the integral model is applied. Calculation results demonstrate the effect of physical properties of liquid and wavy flow parameters on heat transfer intensification by stationary running waves.     

On the motion of a uniformly heated drop in a viscous liquid under gravity

The motion of a uniformly heated spherical drop under gravity is theoretically studied within the Stokes approximation. The Stokes and Hadamard-Rybchinsky formulas are generalized so that the temperature dependence of the viscosity can be found in a wide temperature range. Also, the drag force and the velocity of gravity fall are calculated for an arbitrary temperature difference between the surface of the drop and distant points.     

Heat transfer under supercritical parameters of pulse-heated liquid

A technique based on pulse heating of a wire probe (resistance thermometer) in a constant-heating power regime is developed and used for studies of comparative heat-transfer intensity in supercritical fluids (SCFs) in the pressure and temperature ranges from 1p _c to 4–6p _c and from 0.6T _c to 1.6T _c , respectively (p _c and T _c are critical pressure and temperature). The characteristic parameters of the setup used are as follows: heating -pulse length, 1–10 ms; density of heat flux through the probe surface, 1–10 MW/m²; and repeatability of the selected power value in a series of pulses, not worse than 0.1%. A sharp decrease in the heat-transfer intensity in SCFs compared to that observed at subcritical temperatures is revealed.     

Heat transfer and crisis phenomena with intense boiling in the falling wave liquid films

Experimental data on heat transfer with intense evaporation in the falling films of liquid nitrogen were analysed. According to data generalization, heat transfer at evaporation becomes more intense under the precrisis modes at high heat fluxes for two studied boundary conditions on the heat-releasing surface: T _w ≈ const and q _w ≈ const. The relative contributions of conductive and convective components of heat transfer for different heat fluxes were estimated due to statistical treatment of the wave characteristics carried out by the capacitance probes for measurement of the local liquid film thickness. It was found out that heat transfer intensification is mainly caused by a drastic decrease in thermal resistance of the local zones with intensely evaporating residual layer between large waves. At that, the convective component of heat transfer related to wave perturbations on a free surface of a liquid film decreases significantly with a rise of heat fluxes. New data on pulsations of the local temperature of the heat-releasing surface were obtained at different points along the flow with the modes of “dry spot” formation. The work was financially supported by the Russian Foundation for Basic Research (grant No. 03-02-04027-NNIO-a) and DFG (Deutsche Forschungsgemeinschaft, project Nos. Re-463-37-1 and SFB-540).     

Periodically excited Marangoni convection in a locally heated liquid layer

The results of the numerical simulation of weakly-supercritical thermocapillary convection in a thin liquid layer with a free deformable upper boundary and an inhomogeneous nonstationary heat source (a pulsating thermal spot, localized in space and periodically modulated in time) are presented.     

Thermocapillary deformation of a thin locally heated horizontal liquid layer

In this paper, steady thermocapillary flow in a thin horizontal layer of a viscous incompressible liquid with a free surface is considered. An axially symmetric steady problem with a localized thermal action on a horizontal liquid layer with a deformable free surface is solved in a thin-layer approximation. In addition to the thermocapillary effect, the model takes into account the capillary pressure caused by the free surface variable curvature and the convective mechanism of heat transfer in the liquid. Analytical expressions for the velocity vector components as functions of the liquid layer thickness and surface temperature are obtained. The free surface and velocity profiles caused by various kinds of heating are calculated. The influence of convective heat transfer on the flow pattern is analyzed.     

Lattice Boltzmann simulation of viscous-fluid flow and conjugate heat transfer in a rectangular cavity with a heated moving wall

In the present work, conjugate heat transfer in a rectangular cavity with a heated moving lid is investigated using the lattice Boltzmann method (LBM). The simulations are performed for incompressible flow, with Reynolds numbers ranging from 100 to 500, thermal diffusivity ratios ranging from 1 to 100, and Prandtl numbers ranging from 0.7 to 7. A uniform heat flux through the top of the lid is assumed. Results show that LBM is suitable for the study of heat transfer in conjugate problems. Effects of the Reynolds number, the Prandtl number and the thermal diffusivity ratio on hydrodynamic and thermal characteristics are investigated and discussed. The streamlines and temperature distribution in flow field, dimensionless temperature and Nusselt number along the hot wall are illustrated. The results indicate that increase of thermal diffusivity yields the removal of a higher quantity of energy from lid and its temperature decreases when increasing the Reynolds and the Prandtl numbers.     

Study on the temperature distribution of heated falling liquid films

A 2-D theoretical model was derived to present the temperature distribution of falling liquid films flowing over a vertical heated/cooled plate with constant temperature. And the temperature gradients for different flow rates and fluids were also discussed for different liquid films. The temperature distributions for liquid films of water, ethanol aqueous solutions and glycerol aqueous solutions were experimentally investigated with a sensitive thermal imaging system. It is found that the surface temperature of a film flowing over a vertical heated solid plate has a characteristic relationship with the film flow distance. A lower flow rate of the film or a higher temperature of the wall generally leads to a higher surface temperature in the film inception. For films of glycerol aqueous solutions under the same heating conditions, a lower glycerol concentration causes a higher surface temperature of the film, due to the decrease of the liquid viscosity, whereas the ethanol concentration is found to have little influence on the temperature distribution of the film surface. Comparisons of the experimental data and the theoretical model show that the model can adequately describe the surface temperature distribution of a heated falling liquid film.     

Heat transfer behaviours of nanofluids in a uniformly heated tube

In the present work, we consider the problem of the forced convection flow of water– γAl₂O₃ and ethylene glycol– γAl₂O₃ nanofluids inside a uniformly heated tube that is submitted to a constant and uniform heat flux at the wall. In general, it is observed that the inclusion of nanoparticles has increased considerably the heat transfer at the tube wall for both the laminar and turbulent regimes. Such improvement of heat transfer becomes more pronounced with the increase of the particle concentration. On the other hand, the presence of particles has produced adverse effects on the wall friction that also increases with the particle volume concentration. Results have also shown that the ethylene glycol– γAl₂O₃ mixture gives a far better heat transfer enhancement than the water– γAl₂O₃ mixture.     

乒乓球逆气流而动的实验演示

利用日常生活中的物品，制作了“逆气流而动的乒乓球”演示装置。     

Heat transfer behaviour of nanofluids in a uniformly heated circular tube fitted with helical inserts in laminar flow

The CFD simulation of heat transfer characteristics of a nanofluid in a circular tube fitted with helical twist inserts under constant heat flux has been explained using Fluent version 6.3.26 in laminar flow. Al₂O₃ nanoparticles in water of 0.5%, 1.0% and 1.5% concentrations and helical twist inserts of twist ratios 2.93, 3.91 and 4.89 has been used for the simulation. All thermophysical properties of nanofluids are temperature dependent. The heat transfer enhancement increases with Reynolds number and decreases with twist ratio with maximum for the twist ratio 2.93. By comparing the heat transfer rates of water and nanofluids, the increase in Nusselt number is 5%–31% for different helical inserts and different volume concentrations. The maximum heat transfer enhancement is 31.29% for helical insert of twist ratio 2.93 and for the volume concentration of 1.5% corresponding to the Reynolds number of 2039. The data obtained by simulation match with the literature value of water with the discrepancy of less than ±10% for plain tube and tube fitted with helical tape inserts for Nusselt number.     

Heat transfer in a Knudsen gas

The behavior of a Knudsen gas between two plates at unequal temperatures is discussed. A general formulation is given for the case of small temperature difference. The use of perturbation theory is then demonstrated and applied to the case of a nonspherical interaction of a polyatomic gas with the solid walls in the presence of a magnetic field.     

Unstable flux flow due to heated electrons in superconducting films

A flux instability occurs in superconductors at low temperatures, where e-e scattering is more rapid than e-ph, whereby the dissipation significantly elevates the electronic temperature while maintaining a thermal-like distribution function. The reduction in condensate and rise in resistivity produce a nonmonotonic current-voltage response. In contrast to the Larkin-Ovchinnikov instability where the vortex shrinks, in this scenario the vortex expands and the quasiparticle population rises. Measurements in Y(1)Ba(2)Cu(3)O(7-delta) agree quantitatively with the distinct predictions of this mechanism.     

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采用两相流的流动换热理论,建立二维几何模型,运用FLUENT对稳定入口流速下注入气体的铅铋流动段作了模拟。模拟研究注入气体的体积份额或速度改变对压力和铅铋与注入气体之间换热与影响,得到了不同条件下的温度与压力分布。分析结果发现,体积份额减小,铅铋流体的径向温度分布更加均匀,中心温度更低;随着体积份额的减小,铅铋的总压呈现出一种下降的趋势。注入气体速度不同对铅铋整体的换热影响不大;中心处的动压有较大增加,总压改变甚微。     

Heat transfer in a flow of gas mixture with low Prandtl number in triangular channels

We investigated heat transfer in a channel with a triangular cross-section. The working medium is a helium–xenon mixture with a low Prandtl number. This channel configuration corresponds to one of possible cases of fuel cells layout in a gas-cooled nuclear reactor. New experimental data on heat transfer in helium–xenon mixtures were obtained. Results of numerical modeling were compared with the experimental data and the known empirical correlations.     

The effect of wave characteristics on rivulet formation in heated liquid films

Formation of rivulets on the surface of non-isothermal water film falling vertically over the heaters with different sizes and boundary conditions was studied experimentally. The distances between rivulets were measured depending on Reynolds number, heat flux density and film path (a distance between the lower edge of a film-former nozzle and the measurement point of film flow characteristics). The breakdown of solitary waves at liquid film heating was revealed. Four zones of film path influence on rivulet formation were distinguished.     

Heat-transfer coefficient of a fine heated wire in a gas flow

Heat exchange between a linear heat source and a gas flow is studied theoretically. The coefficient of source-to-gas heat transfer is found as a function of the gas velocity in the limit of zero temperature drop. It is shown that the heat-transfer coefficient depends linearly on the gas flow velocity for Peclet numbers 0.1–1.0.     

Heat transfer in unsteady MHD oscillatory flow

Heat transfer in unsteady MHD channel flow (of an incompressible viscous and electrically conducting fluid) under oscillatory pressure gradient when the channel surfaces are conducting and moving with time-dependent velocities has been analysed. The velocity, magnetic field and temperature distributions have been obtained and their numerical results are shown graphically.Symbols u velocity - H _o applied magnetic field - H _x induced magnetic field - T temperature - T ₁ ^* ,T ₂ ^* temperatures of the upper and lower planes - density - p pressure - kinematic viscosity - magnetic diffusivity - electrical conductivity of the fluid - A ^* characteristic velocity - L characteristic length - _e magnetic permeability of the fluid - C _p specific heat - coefficient of viscosity - k thermal conductivity - ₁, ₂ permeabilities of the planes - ₁, ₂ conductivities of the planes - ₁, ₂ conductance ratios of the planes - Pr Prandtl number (=C _p /k) - E Eckert number - M Hartmann number (= _e H _o L/) - R _e Reynolds number (=LA ^*/) - R _m magnetic Reynolds number (= _e LA ^*) - S Pr.E(=S)