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对低温非真空环境下粗糙接触界面间隙中介质气体的传热进行了理论分析.依据克努森数的大小,建立了不同传热区域的间隙气体热导理论模型.并对影响接触界面间隙热导的克努森数、普朗特数、热导率、适应系数、压力等参数进行了分析,为实际情况下接触界面的传热提供了理论基础.而且通过实验证明了在界面接触压力较小的情况下,即使对于硬度较小导热性能好的接触固体,间隙气体的导热量仍大于通过实际接触点的导热量. 相似文献
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层间稀薄气体传热对多层绝热材料性能的影响分析 总被引:3,自引:2,他引:1
通过建立的热量传递模型,分析了不同的气体稀薄程度(Knudsen数)时,气体传热对多层绝热材料有效热导率和各层温度分布的影响。分析表明:由多层绝热材料真空度变化引起的稀薄气体传热量波动较大,在10—60层/cm层密度范围,真空度低于100Pa时,Kn数属于自由分子状态区域和中间压强区域,此时材料的有效热导率随残留气体热适应系数的增大而减小,并随着真空度的降低而增大;当残留气体为空气时,为保证多层材料的绝热性能,尽量维持真空度不低于10-2Pa。同时分析表明,为有效降低低真空下稀薄气体传热对多层绝热性能的影响,可以采用综合热适应系数较低的气体置换夹层中的空气,以减少低真空多层绝热材料的有效热导率,改善绝热性能。 相似文献
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一、引言 在各种颗粒状原料的热等离子体加工过程中,努曾数(定义为气体粒于的平均自由程长度与颗粒直径之比)可能在广泛的范围内变化.如在近年来受到重视的低压等离子体喷涂中,努曾数可以达到一以至更大的数值.低压下原料颗粒的加热往往比大气压下困难,我们曾采用所谓的“热传导势跳跃近似”,分析过小努曾数(<0.5左右)下压力对热等离子体向颗粒传热的影响,指出努曾效应是低压下颗粒加热恶化的原因。对于努曾 相似文献
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对于等离子体化学与等离子体加工领域中所用的典型颗粒尺寸范围而言,压力对传热的影响主要通过努曾(Knudsen)效应.对于努曾数不太大的情形,本文基于“热传导势眺跃近似”,研究了压力对热等离子体向颗粒传热的影响.研究结果表明,对于比较小的颗粒(几微米半径),努曾效应甚至在相当高的压力下即很显著;而对于比较大的颗粒(例如100微米半径),低压下努曾效应也会相当显著。 相似文献
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气体传热对多层绝热性能影响的试验研究 总被引:1,自引:1,他引:0
文中通过建立的能进行夹层气体置换的稳态量热器试验系统,试验分析了夹层气体传热对多层绝热材料有效热导率的影响,重点对置换气体种类、气体压强、材料层数及冷热边界温度对多层材料的影响进行试验研究。试验表明在10—60层/cm层密度范围,真空度低于100Pa时,Kn数属于自由分子状态区域和中间压强区域,此时材料的有效热导率随残留气体热适应系数的增大而减小,并随着真空度的降低而增大,当残留气体为空气时,为保证多层材料的绝热性能,应尽量维持真空度不低于10-2Pa。同时,分析表明为有效降低低真空下稀薄气体传热对多层绝热性能的影响,可以采用综合热适应系数较低的气体置换夹层中的空气,以减少低真空多层绝热材料的有效热导率,改善绝热性能。 相似文献
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随着电子元件高性能化和小型化的发展,纳米通道内工质的流动传热问题受到了更多的关注.本文采用分子动力学模拟方法,模拟了300,325,350 K的纳米通道中流体的流动传热情况,工质为水,水中不凝性气体用氩气代替.结果表明:流动过程中,氩原子形成高势能团簇,随着温度升高,流体势能上升,团簇逐渐减小或消失;少量气体原子能够促进流动,而较多氩气会导致通道中心区域形成较大气体团簇而阻碍流动,同时,被加热的工质能显著减小流动阻力系数;近壁面区域流体温度高于中心区域,团簇内部原子活动更加剧烈,平均分子动能更大,温度更高;水的氢键结构可以促进纳米通道内的传热,氩原子会影响氢键数量,高温会破坏水分子形成的氢键网络,使努塞尔数下降.本研究分析了不凝性气体影响下微通道内水分子流动传热的机理,为电子设备的强化传热提供了理论指导. 相似文献
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Yu. G. Dragunov V. P. Smetannikov B. A. Gabaraev M. S. Belyakov P. V. Kobzev 《Journal of Engineering Thermophysics》2013,22(1):30-42
Analytical review of numerous Russian and foreign sources of information on convective heat transfer to single-phase flows in circular pipes and rod bundles is represented. Formulas for calculation of heat transfer in circular pipes and rod bundles for different flow regimes of a binary gas mixture are proposed. 相似文献
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High-speed visualization of adiabatic flow and heat transfer rate determination for constant wall heat flux conditions were performed to study the flow and heat transfer behavior of non-boiling, gas–liquid two-phase vertical upward flow in a 2.00 mm-diameter tube. Liquids of different volatilities, including water, ethylene glycol, and hexadecane, were employed to investigate the roles of convective heat transfer and evaporation for a wide range of flow conditions encompassing Taylor, slug-annular, and annular flow regimes. The heat transfer rate is found to depend strongly on the flow regime. Significant evaporative cooling was observed for the volatile system at high gas flow rates. A heat transfer enhancement up to fivefold over that for the liquid-only flow was observed in the annular flow regime. 相似文献
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V. G. Chernyak A. Ph. Polikarpov 《Journal of Experimental and Theoretical Physics》2010,110(1):147-156
The nonlinear processes of heat and mass transfer in a rarefied gas confined between two infinite parallel plates maintained at different temperatures and moving at a relative velocity are considered. The profiles of the gas macroscopic flow velocity, density, temperature, heat fluxes, and shear stress were calculated on the basis of kinetic equations by the discrete velocity method in a wide range of Knudsen numbers at different values of temperature difference between the plates and plate velocities. It was shown that under certain conditions, the direction of gas flow near the “hot” plate can change to the opposite. It was discovered that the longitudinal and normal components of heat flux at a certain temperature difference between the plates change their orientation to the opposite in transition and nearly free molecular regimes. 相似文献
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《Combustion Theory and Modelling》2013,17(2):153-165
An original physical model of self-ignition in a combustible gas mixture containing liquid fuel droplets is developed. The droplets are small enough for the gas-droplet mixture to be considered as a fine mist such that individual droplet burning is subsumed into a well-stirred, spatially invariant burning approximation. A classical Semenov-type analysis is used to describe the exothermic reaction, and the endothermic terms involve the use of quasi-steady mass transfer/heat balance and the Clausius-Clapeyron evaporative law. The resulting analysis predicts the ignition delay which is a function of the system parameters. Results are given for typical dynamical regimes. The case of different initial temperatures for droplets and gas is highly relevant to gas turbine lean blow-out and re-ignition. 相似文献
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N. N. Simakov 《Technical Physics》2016,61(9):1312-1318
A numerical experiment on the simulation of heat transfer from a sphere to a gas flow in a cylindrical channel in the Stokes and transient flow regimes has been described. Radial and axial profiles of the gas temperature and the dependences of drag coefficient Cd of the body and Nusselt number Nu on Reynolds number Re have been calculated and analyzed. The problem of the influence of the early drag crisis for a sphere on its heat transfer to the gas flow has been considered. The estimation of this phenomenon has shown that the early drag crisis of the sphere in a strongly turbulent flow causes a reduction in heat transfer from the sphere to the gas by three to six times (in approximately the same proportion as for its drag coefficient). 相似文献
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HEAT TRANSFER IN HIGH-PRESSURE COMPRESSOR GAS TURBINE INTERNAL AIR SYSTEMS: A ROTATING DISC-CONE CAVITY WITH AXIAL THROUGHFLOW 总被引:1,自引:0,他引:1
This article reports on heat transfer measurements made on a rotating test rig representing the internal disc-cone cavity of a gas turbine high-pressure (H.P.) compressor stack. Tests were carried out for a range of flow rates and rotational speeds at engine representative nondimensional conditions. The rig also had a central drive shaft, which could rotate in the same direction as the discs, contrarotate relative to the discs, or remain static. Measurements of heat transfer were obtained from a conduction solution method using measured surface temperatures as boundary conditions. Results from the outer surface of the cone are in reasonable agreement with theoretical predictions for the heat transfer from a free cone in turbulent flow. The heat transfer measurements from the inner surface of the cone reveal two regimes of heat transfer: one governed by rotation, the other by action of the throughflow. In the rotationally dominated regime, the heat transfer from the inner surface of the cone is higher for a co-rotating shaft than for either a static or contra-rotating shaft. In the throughflow-dominated regime the heat transfer shows little consistent dependence on the direction of shaft rotation. Tests carried out at different values of surface-to-fluid temperature difference add support to the hypothesis that in the rotationally dominated regime the heat transfer occurs through a process of free convection, where the buoyancy force is induced by rotation. The heat transfer from the disc is significantly lower than that from the inner surface of the cone and more or less insensitive to the sense of shaft rotation. The disc average Nusselt numbers show similar behavior to those from the inner surface of the cone and suggest that the disc heat transfer too is governed either by rotationally induced buoyancy or by the axial throughflow. 相似文献
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Thermosolutal convection flow and its effect on the heat and the mass transfer in a square enclosure is studied experimentally. Both thermal and solute diffusion are induced from the sides, and natural convection is initiated by the combined thermal and solutal buoyancies, which either augment or oppose to each other. The solute diffusion is initiated in an electrochemical system that uses copper sulfate-sulfuric acid solution as an electrolyte. Depending on the magnitude of buoyancy ratio, three different kinds of flow regimes and structures can occur, which lead to different distributions of concentration in the enclosure. The formation and growth of layered flow structure is attributed to the solutal boundary-layer flow that can intrude and accumulate along the horizontal wall. The nearly stagnant layer that occurs can reduce the heat transfer rate. The Nusselt numbers at different flow regimes are measured and correlated in terms of relevant nondimensional parameters. This suggests the correlation of Sherwood number in different ranges of buoyancy ratio. The visualization of flow structures and measurements of both heat and mass transfer allow better understanding of the complicated system. 相似文献
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This article deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. Electrohydrodynamic (EHD) effects on heat transfer coefficients for dielectric liquids are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n-pentane, R-113, and R-123 are used as working fluids and the boiling phenomenon takes place on a horizontal plane copper surface. The experimental results have shown: (1) a threefold increase of nucleate pool boiling heat transfer coefficients, (2) a threefold increase of the critical heat flux (CHF), and (3) the disappearance of the hysteresis phenomenon. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed. The results obtained by the proposed EHD model are in good agreement with the experimental results. 相似文献
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A. A. Dolinsky M. M. Kovetskaya A. I. Skitsko A. A. Avramenko B. I. Basok 《Journal of Engineering Thermophysics》2008,17(2):126-129
A model of motion and heat transfer of annular dispersed two-phase flows is developed. The results of an investigation of
the heat transfer crisis in a vertical steam-generating channel in nonstationary regimes with a decreasing heat transfer rate
of the heater are presented 相似文献