Calculation of Radiation Heat Transfer in Problems of Flow past Bodies with Account for Heat-Shield Coating Mass Loss

Flow past bodies with account for radiation heat transfer near the body nose is considered in connection with the problem of thermal protection of spacecraft entering planetary atmospheres. An approach based on the spherical harmonics method is used for taking radiation into account. The heating and ablation of the heat-shield coating is determined by numerically solving the heat conduction equation with a moving boundary. The thermal problem is solved together with the calculation of the vehicle trajectory, which makes it possible to estimate the effect of the coating mass loss on the trajectory parameters. The approach formulated is realized under the conditions of entry of a spherical segment and a spherically-blunted cone with equal mid-section radii into the Venusian atmosphere. Comparing the results makes it possible to choose the vehicle shape optimal with respect to the thermal regime.     

Effective Boundary Conditions Resulting from Anisotropic and Optimally Aligned Coatings: The Two Dimensional Case

We discuss a thermally conducting body insulated by a thin anisotropically conducting coating. The coating is “optimally aligned” in the sense that the normal vector inside the coating is always an eigenvector of the thermal tensor. We study the effects of the coating by investigating the limiting behavior of solutions u of the heat equation with either Dirichlet or Neumann boundary conditions imposed on the outer boundary of the coating, as the thickness of the coating shrinks to zero. In the two-dimensional case, we find the complete list of “effective boundary conditions” satisfied by the limit of u on the boundary of the uncoated body. This list contains not only the usual Dirichlet, Neumann and Robin boundary conditions, but also some new and even nonlocal ones involving the Dirichlet-to-Neumann mapping and the Hilbert transform on the circle. We also prove that u converges to its limit in various norms that include the L ², the Sobolev and the Hölder ones. During the course of this study, we establish a Schauder theory for the regularity of weak solutions of general second order parabolic equations near an interface where the “transmission condition” is satisfied.     

Analytical model for thermal resistance due to multiple moving circular contacts on a coated bodyModèle analytique pour la résistance thermique due à des contacts circulaires mobiles multiples à la surface d'un solide revêtu

The heat transfer at the interface of two solids in sliding/rolling contact depends on the constriction phenomenon which occurs at the vicinity of asperities. In order to study this problem, the micro-contacts are represented by multiple moving circular heat sources on the surface of a body. The studied body is constituted of a substrate and a surface coating. The thermal constriction resistance due to those contacts is determined analytically in this paper. The solution is developed by using the integral Fourier transforms, and it is valid regardless of the velocity and the relative contact size values. To cite this article: A. Baïri, C. R. Mecanique 331 (2003).     

Flow boiling enhancement of FC-72 from microporous surfaces in minichannels

The microporous coatings can remarkably enhance the liquid boiling heat transfer. Therefore, they are promising to be introduced into minichannels in the design of the cooling system of high-power microchips. However, the flow boiling heat transfer characteristics from microporous surfaces in the minichannels have not been extensively studied, and the pertinent knowledge is rather fragmentary. The present research is an experimental investigation on flow boiling of a dielectric fluid FC-72 from microporous coating surfaces in horizontal, rectangular minichannels of 0.49, 0.93 and 1.26 mm hydraulic diameter. Effects of coating structural parameters, such as the particle diameter and coating thickness, were investigated to identify the optimum microporous coating for heat transfer enhancement. All microporous surfaces in this paper were found to significantly enhance FC-72 flow boiling heat transfer in minichannels. With the optimum coating, the heat transfer coefficients could be 7-10 times those of the uncoated surface, and the boiling wall temperature was reduced by about 10 K. The flow boiling phenomena in the present minichannels were distinctly different from those in conventional-sized channels, due to the wall confinement effect on vapor bubbles. The confinement effect was evaluated by taking the contributions of the liquid mass flux and channel size into consideration. It was found that the very strong confinement effect was unfavorable with respect to flow boiling enhancement of the microporous coatings in the minichannels.     

Effect of the cone nose bluntness and the ultrasonically absorptive coating on transition in a hypersonic boundary layer

The effect of an ultrasonically-absorptive coating on laminar-turbulent transition on cones with different nose bluntnesses is experimentally investigated. The experiments were performed with a cone with the semi-vertex angle of 7° set at zero incidence in the Mach 8 flow for three Reynolds numbers. A material with a chaotic micropore structure was used as the ultrasonically-absorptive coating. One side of the model, along its generator, was coated with the porous material, while the second represented a rigid surface. The laminar-turbulent transition location was determined from the results of heat flux distribution measurements. The heat flux fluctuations were also measured on the model surface. It was found that the laminar region length increased with an increase in the bluntness radius. The ultrasonically-absorptive coating with a chaotic microstructure effectively stabilizes the boundary layer for all bluntness radii considered, increasing the laminar region length by 30 to 85%.     

A study on the correlation between the thermal contact conductance and effective factors in fin–tube heat exchangers with 9.52 mm tube

The thermal contact resistance is a principal parameter interfering with heat transfer in a fin–tube heat exchanger. However, the thermal contact resistance in the interface between tubes and fins has not been clearly investigated. The objective of the present study is to examine the thermal contact conductance for various fin–tube heat exchangers with tube diameter of 9.52 mm and to find a correlation between the thermal contact conductance and effective factors such as expansion ratio, fin type, fin spacing and hydrophilic coating. In this study, experiments have been conducted only to measure heat transfer rate between hot and cold water. To minimize heat loss to the ambient air by the natural convection fin–tube heat exchangers have been placed in an insulated vacuum chamber. Also, a numerical scheme has been employed to calculate the thermal contact conductance with the experimental data. As a result, a new correlation including the influences of expansion ratio, slit of fin and fin coating has been introduced, and the portion of each thermal resistance has been estimated in the fin–tube heat exchangers with 9.52 mm tube.     

Three-dimensional chemically nonequilibrium viscous shock layer on a catalytic surface with allowance for associated heat transfer

A three-dimensional flow of dissociating air past blunt bodies is investigated in the framework of the thin viscous shock layer theory. Multicomponent diffusion and homogeneous chemical reactions, including dissociation, recombination, and exchange reactions, are taken into account. The generalized Rankine-Kugoniot conditions are specified on the shock wave and the conditions which take into account the heterogeneous catalytic reactions, on the surface of the body. The viscous shock layer equations are solved together with the heat equations inside the coating, which is carbon with a deposited thin film of SiO₂, or quartz. The case of a thermally insulated surface is also considered. The problem for the case of the motion of a body along the re-entry trajectory into Earth's atmosphere is investigated numerically. The temperature of the surface and the heat flux toward it are given as a dependence on the height (tine) of the flight for different cases of the specification of the catalytic reactions. It is shown that the difference between the heat fluxes towards the thermally insulated surface and the fluxes toward the heat-conducting surface in the neighborhood of the stagnation point is of the order of 6–12% for all the cases considered. This makes it possible to decouple the solution of the problem of heat conduction in the body.Translated fron Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 140–146, November–December, 1985.deceased     

Reduction of the sonic boom level by heating the flow in front of the body

A numerical study of the possibility of reducing the sonic boom level in the case of local heat release to a supersonic gas flow at Mach number equal to 2 ahead of a body is described. The computations are performed for a spherical heat supply zone located on the flight trajectory ahead of the tip of an axisymmetric thin body. For the numerical study the combined method of “phantom bodies” is used. Different magnitudes of heat supply to the incoming flow are tested. These calculations are performed with allowance for interaction of shock waves emanating from the heated gas region and from the body in the far field. The computational results show that the local heat supply to a supersonic gas flow ahead of a body can reduce the sonic boom level by more than 20 %. The reduction of the sonic boom level is ensured by changing the free-stream parameters ahead of the body and by preventing the coalescence of shock waves from the heat supply zone and from the body in the far field.     

发射药燃烧热辐射传播规律

采用构建物理数学模型和实验验证的方法,首次提出发射药燃烧热辐射柱体理论模型;利用自由场条件下的单基发射药燃烧热辐射实验,对比分析球体热辐射模型,验证了柱体理论模型能够客观反映发射药燃烧热辐射传播规律。实验表明,自由场条件下4种不同单基发射药药量的燃烧热辐射实验数据与柱体燃烧理论模型相吻合;同时由数据拟合分别得到单基发射药热通量与药量、距离以及热剂量与药量、距离的定量函数关系,可为准确评估单基发射药燃烧热辐射毁伤效应提供相关理论基础。     

Reduction of Peak Heat Fluxes by Supplying Heat to the Free Stream

A supersonic flow past a blunt body in the presence of an incident oblique shock wave is considered. It is shown that by supplying heat to the free stream it is possible substantially to reduce local heat flux peaks on the body surface. The integral heat flux on the body surface increases by only a small fraction of the heat released into the flow.     

Condensation of vapors of immiscible binary liquids on horizontal copper and polytetrafluoroethylene-coated copper tubes

Heat transfer data are reported for the condensation of pure organic vapors and their eutectic mixtures with water on both horizontal one inch diameter copper and polytetrafluoroethylene-coated copper tubes at atmospheric pressure. With the pure vapors heat transfer is reduced by the coating, this reduction being enhanced when condensing cyclohexane, attributable to an absorption effect. For the eutectic mixtures, except for carbon tetrachloride, heat transfer is not reduced by the coating and is substantially increased for the cyclohexane-water system. An explanation is afforded in terms of buoyancy-surface force interaction.     

等离子喷涂铁－镍－钴－碳化钨涂层制动摩擦特性的研究

重载制动摩擦磨损特性制约着刹车装置的刹车性能及其使用寿命．在ＭＭ－１０００试验机上，对等离子喷涂Ｆｅ－Ｎｉ－Ｃｏ－ＷＣ涂层的重载制动摩擦特性进行了试验研究．结果表明：在相同的试验条件下，等离子喷涂涂层分别与石棉摩擦材料和半金属摩擦材料对摩时的摩擦系数均比与基体３５ＣｒＭｏ钢对摩时的高，制动时间短，制动效率和耐磨性能都明显提高，制动力矩峰比小，刹车平稳，制动特性好；制动摩擦系数与制动压力、速度和转动惯量等参数密切相关，压力增大，摩擦系数减小，速度和转动惯量增大，石棉材料摩擦副的摩擦系数降低，半金属材料摩擦副的摩擦系数升高，而且半金属材料的摩擦系数的热稳定性比石棉摩擦材料的好     

Bounds for the effective heat conduction coefficient

The linear problem of the steady-state heat conduction is studied in isotropic nonhomogeneous hollow rigid bodies. Upper and lower bounds are derived for the effective heat conduction coefficient. It is proven that, the effective heat conduction coefficient of a compound body is between the weighted arithmetic and harmonic means of heat conduction coefficients of the homogeneous body components.     

Investigation of the Influence of Different Heterogeneous Recombination Mechanisms on the Heat Fluxes to a Catalytic Surface in Dissociated Carbon Dioxide

A kinetic model of heterogeneous recombination in dissociated carbon dioxide on high-temperature heat-shield coatings is developed; the model takes into account the nonequilibrium adsorption-desorption reactions of oxygen atoms and their recombination in the Eley-Rideal and Langmuir-Hinshelwood reactions. On the basis of a comparison of the calculated heat fluxes in dissociated carbon dioxide with those measured in the VGU-3 plasma generator of the Institute for Problems in Mechanics of the Russian Academy of Sciences (IPM RAS) and the available literature data, the parameters of the catalysis model are chosen for the glassy coating of the Buran orbiter tile heat shield based on the SiO₂–B₂O₃–SiB₄ system. The effects of heterogeneous recombination proceeding in accordance with the Langmuir-Hinshelwood mechanism, as well as the processes involving carbon atoms and those involving physically adsorbed oxygen atoms, on the heat fluxes to the glassy coating are analyzed on the surface temperature range from 300 to 2000 K.     

Ni—P—纳米碳管化学复合镀层的摩擦磨损特性

用化学镀方法制备了 Ni- P-纳米碳管复合镀层 ,研究了热处理对复合镀层微观结构及摩擦学性能的影响 .结果表明 :Ni- P-纳米碳管复合镀层比 Ni- P- Si C和 Ni- P-石墨镀层具有更好的摩擦磨损性能 ;在 6 73K条件下热处理 2 h后 ,复合镀层的耐磨性能显著改善 ;除 Ni- P-纳米碳管复合镀层的摩擦系数基本不变以外 ,其余复合镀层的摩擦系数均降低 .     

Study on the equivalent convection coefficient of the hot surface of blast furnace stave

The calculation models of the equivalent convection coefficient between blast furnace gas flow and the hot surface of stave body, gas flow and in-laid brick were established by the combination of experiments and numerical calculation when the gas temperature is in the range of 505–1,248°C. The reason why the heat transfer coefficient between gas flow and in-laid brick is much more than that between gas and stave body was analyzed when the gas temperature is high. The opinion just to considering a kind of integration heat transfer coefficient while the numerical calculation of heat transfer model of stave will be changed. The division of above two heat transfer coefficients will increase the degree of heat transfer numerical calculation.     

Numerical analysis of the heat and fluid flow in a weld pool with a dynamic keyhole

In keyhole plasma arc welding, the interaction between the keyhole and the weld pool occurs when the keyhole appears inside the weld pool. The change of the keyhole shape and dimensions has direct effect on the heat and fluid flow in the weld pool, and the latter will also influence the keyhole geometry. In this study, the coupled behaviors of weld pool and keyhole are treated to develop a three-dimensional model for analyzing the heat and fluid flow inside a weld pool with a dynamic keyhole. In view of the characteristics of PAW process, a combined volumetric heat source model (double-ellipsoid plus conic body source) is established, and one of its distribution parameters is adjusted dynamically with the variation of the depth of keyhole. The physical phenomena, such as the weld pool development, the keyhole formation, the evolution of fluid flow and thermal field, the full-penetration of the test plates, and the transformation from a blind keyhole to an open keyhole, are quantitatively analyzed. The numerical results reveal the regularity of fluid flow in weld pool with a keyhole. The calculated keyhole shape and the fusion zone of plasma arc welds are compared with the experimental measurements. Both agree with each other generally. It lays foundation for optimizing the welding process parameters and improving the stability of plasma arc welding process.     

Determination of the Effective Probabilities of Catalytic Reactions on the Surfaces of Heat Shield Materials in Dissociated Carbon Dioxide Flows

The effect of heterogeneous catalysis on the heat transfer to cold and heated surfaces in subsonic dissociated carbon dioxide jet flows is studied experimentally, using a 100 kW inductive plasma generator, and simulated numerically. The effective probabilities of the heterogeneous reactions CO + O → CO₂ and O + O → O₂ on molybdenum (T_w=300 K) and quartz (T_w=470–620 K) surfaces, the Buran heat shield tile coating (T_w = 1470—1670 K), and two oxidation-resistant carbon-carbon coating materials (T_w=1420—1840 K) are determined by comparing the experimental and calculated data on the heat fluxes at the stagnation point of models at a pressure of 0.1 atm.     

Vibrations of a Rigid Body with Cylindrical Surface on a Vibrating Foundation

The analytic solution of the problem of forced vibrations of a rigid body with cylindrical surface on a horizontal foundation is given. It is assumed that the dry friction force acts at the point of contact between the cylindrical surface of the body and the foundation and the foundation moves by a harmonic law in the horizontal direction perpendicularly to the cylindrical surface element. The averaging method is used to determine the forced vibration mode near the natural frequency of the body vibrations on the fixed foundation. The results are presented as amplitude-frequency and phase-frequency characteristics.     

囊体布热合缺陷的锁相热成像无损检测技术研究

囊体布热合缺陷的无损检测是系留气球生产过程中急需解决的关键问题。本文通过对锁相热成像检测技术的理论与实验研究,设计了一套可现场应用的囊体布热合缺陷无损检测系统,实现了热合缺陷的自动识别与定位,甚至可以进一步区分热合缺陷的类型,具有较高的可靠度和较强的抗干扰能力。该系统不仅可用于囊体布的热合缺陷检测,还可广泛应用于相关材料或结构的无损缺陷检测中。