A stochastic interpretation of convective transport enhancement by external force fields

The enhancing effect of external force fields on convection occurring at solid-fluid interfaces is interpreted by means of a simple stochastic relationship derived from the Erlang formula in the theory of Markov processes.     

FREE AND FORCED CONVECTION HEAT TRANSFER FROM SHORT INCLINED CIRCULAR CYLINDERS

Free and forced convective heat transfer rates from circular cylinders whose length to diameter ratio is small have been experimentally measured. The cylinders were arranged with their axes in a vertical plane and tests were conducted with the axes set at angles of between 0° and 90° to the horizontal. In the forced convective tests the cylinders were exposed to a horizontal air stream. The majority of the tests were undertaken with models having diameters of 19 mm and 25 mm with length to diameter ratios of between 1.5 and 16. The heat transfer rates were determined by the transient method i.e. by using solid aluminum models, heating them and measuring their temperature-time variation while they cooled. Significant differences between the present results and those observed in previous studies with models having large length to diameter ratio have been found. Methods of correlating both the free convective results and the forced convective results have been derived.     

FREE AND FORCED CONVECTION HEAT TRANSFER FROM SHORT INCLINED CIRCULAR CYLINDERS

Free and forced convective heat transfer rates from circular cylinders whose length to diameter ratio is small have been experimentally measured. The cylinders were arranged with their axes in a vertical plane and tests were conducted with the axes set at angles of between 0° and 90° to the horizontal. In the forced convective tests the cylinders were exposed to a horizontal air stream. The majority of the tests were undertaken with models having diameters of 19 mm and 25 mm with length to diameter ratios of between 1.5 and 16. The heat transfer rates were determined by the transient method i.e. by using solid aluminum models, heating them and measuring their temperature-time variation while they cooled. Significant differences between the present results and those observed in previous studies with models having large length to diameter ratio have been found. Methods of correlating both the free convective results and the forced convective results have been derived.     

Free convection heat transfer over a horizontal plate at low prandtl number

Boundary layer equations for free convection heat transfer along a semi-infinite horizontal plate are derived by giving more importance to the energy equation. The equations are obtained for low Prandtl number and two separate polynomials are used to approximate the temperature and velocity profiles in these regions. The rate of heat transfer is compared with the available analytical and numerical results based on conventional boundary layer equations.     

非饱和混凝土氯离子传输模型及参数分析

实体工程中大多数混凝土结构由于干湿交替作用处于非饱和状态,研究非饱和混凝土中的氯盐侵蚀对混凝土耐久性具有重要的意义.根据氯离子在混凝土中的传输机制,考虑干湿交替过程中水分传输的差异,推导得出氯离子在非饱和混凝土中传输的对流-扩散控制方程.采用精度高的Crank-Nicolson格式对水分扩散方程、氯离子对流-扩散方程进行差分,并用MATLAB软件编程进行数值求解.试验结果与模型计算结果吻合良好,通过参数敏感性分析结果可知,氯离子浓度随着混凝土表层孔隙饱和度和干湿比的增加而减小,当混凝土初始饱和度为0.3时氯离子浓度最高.     

Convection and segregation in vertically vibrated granular beds

Experiments and 3D simulations are performed to study the convection cells generated in spherical glass beads within a vertically vibrated container. Discrete element method is employed to simulate the response of glass beads under vertical vibration. Movement of mono-size spherical particles is simulated under different vibration conditions and results are validated in experiments. The patterns of convection cells are found to depend on different operating parameters, i.e. frequency, amplitude etc. The influence of modified container geometries on the behavior of particle movement is being studied. Outward tilted container walls lead to the reversal of the direction of convection cells but only in specially modified containers. Results obtained from 3D systems are in controversy to the results obtained in 2D systems as described in most of the previous works.     

Prandtl number effect on combined free and forced laminar convection in the thermal entrance region of a horizontal tube

This note presents the Prandtl number effect on combined free and forced laminar convection in the thermal entrance region of a horizontal tube by a numerical vorticity-velocity method. The tube wall is heated with a uniform wall heat flux. Vorticity-velocity governing equations in cylindrical coordinates and an evaluation of boundary vorticity on the tube wall are presented along with a numerical method of solution. Local Nusselt number variations are shown for Pr = 100 and 10 and compared with the existing data for Pr → ∞. It is observed that the large Prandtl number assumption is valid for Pr = 10 within a difference of 4% at Ra = 1 × 10⁵. The numerical results are compared against the available experimental data with good agreement.     

Boundary layer flows with Dufour and Soret effects: I: Forced and natural convection

Similarity solutions of laminar boundary layer flows in forced and natural convection are examined aiming at the evaluation of the coupled effects of heat and mass transport. Momentum, energy and mass balance equations are simultaneously solved under the assumption of the validity of constitutive equations for the heat and mass fluxes of dilute solutes obeying linear forms in accordance with the thermodynamics of irreversible processes. Forced and natural convection cases depend on different similarity variables, with different exponents for the axial variable and different significant physical properties compose the pertinent dimensionless groups. In the first part of this paper thermal diffusion and diffusion thermo effects are studied for the boundary layer flows about a vertical flat plate in forced and in natural convection, separately. The second part deals with the intermediate cases of non-similar of mixed convection, where it is determined by a one-parameter family of transformations (0???1), which reduce to both extreme cases. The mixed convection will be presented in a forthcoming continuation. A modification of the method of solution based on Adomian's polynomials for the resulting boundary value problem is employed.     

恒热流直管中纳米流体对流传热的实验研究(英文)

In this work, the laminar convective heat transfer performance and the pressure drop of water-based nanofluids containing Al2O3, TiO2 and SiO2 nanoparticles flowing through a straight circular tube were experimen-tally investigated. The experimental results showed that addition of small amounts of nano-sized Al2O3 and TiO2 particles to de-ionized water increased heat transfer coefficients considerably, while the SiO2 nanofluids showed the opposite behavior attracting the authors’ interests. An average of 16%and 8.2%increase in heat transfer coefficient were observed with the average of 28%and 15%penalty in pressure drop for Al2O3 and TiO2 nanofluids.     

Effects of anisotropic scattering of radiation in laminar forced convection in a parallel-plate duct

Effects of anisotropic scattering to heat transfer in hydrodynamically-developed, thermally-developing steady laminar forced flow of a gray fluid between two infinite parallel plates are investigated for the case when the inlet temperature of the fluid is less than the wall temperature. An implicit finite difference scheme is applied to solve the energy equation, while the high order P_N method is used to solve the radiation part of this problem. The effects of the anisotropy, the conduction-to-radiation parameter, the optical thickness, the reflectivity of the plates and the inlet temperature on the local Nusselt number are studied.     

CHARACTERISTICS OF HEAT TRANSFER FROM A SPHERICAL SOLID BODY UNDER PULSATING FORCED CONVECTION

Heat transfer from a solid sphere having a Biot number Bi ? 0.1 to a flow medium in a cooling process under pulsating forced convection is experimentally examined. In the experiment, two kinds of pulse modes, which are the type of pulsating forced convection with continuous feed and the feed type mixing forced and natural convection, are considered as a pulse feed technique. Initially, the evaluation method of the Nusselt number Nu is derived in the cooling process and the validity of that method is verified by comparison with the experimental results. With regard to the effect of pulsating feed, it is shown that the pulsating feed conditions have a great influence on the characteristics of the heat transfer. The enhancement of heat transfer under the condition of the forced convection pulsatively fed with continuous feed is recognized, and the characteristics of heat transfer for the pulsating feed including natural convective heat transfer region are aggravated. Besides, it is clear that the decrease in the heat transfer characteristics is dependent on the duration of the natural convective heat transfer. Furthermore, Ranz-Marshall correlations for each pulsating feed condition are presented.     

裂解炉节能降耗措施及效果

裂解炉为乙烯装置的核心,是乙烯装置的能耗大户,其能耗占装置总能耗的5O%。国内乙烯装置裂解炉在运行过程中普遍存在排烟温度高、炉壁散热量大、热效率低;COT偏差大、运行周期短、设备重复检修频繁等问题,通过认真研究分析以上问题,采取了一些行之有效的办法,使得装置能耗大大降低。     

Acceleration of reaction fronts by hydrodynamic instabilities in immiscible systems

We are studying the propagation of a reaction front in an immiscible liquid-liquid system under the condition of a mass transfer across the interface accompanied by a neutralization reaction. The system, when placed in a vertical Hele-Shaw cell, shows a buoyancy-driven cellular structure penetrating into the aqueous bulk solution. By tilting the Hele-Shaw cell to the horizontal position, the penetrative convection vanishes in favor of a planar reaction front. At higher acid concentrations this reaction-diffusion state is replaced by a Marangoni convection interacting with the front propagation. We show that the scaling behavior of the reaction front position, x_f, with time shows a characteristic crossover. In the long-time limit all systems obey x_f∼t^(0.49±0.02) and differ only in the timespan needed to enter this diffusion-controlled state. For an intermediate time all reaction-diffusion systems display a x_f∼t^(1.50±0.06) law. Both convection types observed in the system operate at Damköhler numbers, Da∼0.1, and lead to an acceleration of the front propagation by at least a factor of four. The influence of the Marangoni convection is even stronger at the beginning due to the higher flow velocities, . However, Marangoni convection rapidly relieves the driving concentration gradients. Therefore, the buoyancy-driven convection wins the competition with respect to the front acceleration for intermediate and long times.     

Flow transition for natural convective heat transfer in a porous medium saturated with water near 4°C

In this paper, the problem of buoyancy-induced convection flow in water-saturated porous media near 4°C is examined using a numerical model. Darcy's law is used to model flow behavior and a single equation convective heat transfer model is used for the energy equation. As the Boussinesq approximation is not valid for this case, a parabolic dependence of density on the temperature is used. Natural convection is generated and sustained by a uniform heat source. Flow behavior is governed by three natural parameters appearing in the model. They are: (i) dynamical parameter, (ii) geometric parameter, γ = b/a; and (iii) wall temperature, in relation to the reference temperature at the density extremum. For certain ranges of θ_w (<0) and Gr, interesting density inversion effects are possible. Transient solutions are obtained for various aspect ratios and modified Grashof number values. For a wide range of Grashof number, steady state solutions could not be obtained. Flow mutations into periodic and chaotic solutions are investigated for a range of Grashof number (100 to 40,000) and aspect ratio values (1 to 10).     

FREE CONVECTIVE DRAFT INDUCED BY THERMAL AND CONCENTRATION GRADIENTS INSIDE AN ISOTHERMAL,VERTICAL CYLINDER†

Laminar, free convective flow through a vertical cylinder induced by the thermal and concentration buoyancy forces is investigated. The numerical studies involve development of a steady-state, two-dimensional heat and mass transfer model for the moist air core of the vertical tube. The stream function-vorticity method is employed to simplify the governing, coupled conservation equations which were then numerically solved by the successive over-relaxation (SOR) and alternating direction implicit ((ADI) methods.

A graphical correlation was found between dimensionless flow rate and dimensionless tube length as a function of the buoyancy force ratio N = Gr/Grc. Excellent agreement was obtained for the dimensionless flow rale results with those of Davis and Perona¹⁶ and Kageyama and Izumi¹³ for the case when only the-thermal buoyancy force is considered.

The combined buoyancy force from thermal and species diffusion provides larger local Nusselt Nu and local Sherwood Sh numbers relative to the case when just one buoyancy force is accounted for. Both local Nu and local Sh are seen to asymptotically approach a constant value as flow develops.     

Marangoni对流的纹影实验分析

通过纹影系统对乙醇溶液解吸CO₂的过程进行了实验研究,液层自由界面的俯视纹影图记录了对流结构的演化过程,并捕捉到了Marangoni对流结构的初始形态。相应的胞型结构因发展空间的限制,由初始的近圆形逐渐变成了多边形结构。利用纹影图片的灰度分布信息,对单个胞型结构的出现、发展及分裂阶段进行了定性分析,发现界面非均匀传质所引发的界面对流在其胞型发展的过程中将会导致界面的变形。湍动的后期,液层表面将布满多边形结构,且胞型结构基本保持不变。相应纹影图片的颜色差异随解吸的进行逐渐减小,即随着传质推动力的减小,湍动强度也将减弱。     

FREE CONVECTIVE DRAFT INDUCED BY THERMAL AND CONCENTRATION GRADIENTS INSIDE AN ISOTHERMAL, VERTICAL CYLINDER†

Laminar, free convective flow through a vertical cylinder induced by the thermal and concentration buoyancy forces is investigated. The numerical studies involve development of a steady-state, two-dimensional heat and mass transfer model for the moist air core of the vertical tube. The stream function-vorticity method is employed to simplify the governing, coupled conservation equations which were then numerically solved by the successive over-relaxation (SOR) and alternating direction implicit ((ADI) methods.

A graphical correlation was found between dimensionless flow rate and dimensionless tube length as a function of the buoyancy force ratio N = Gr/Grc. Excellent agreement was obtained for the dimensionless flow rale results with those of Davis and Perona¹⁶ and Kageyama and Izumi¹³ for the case when only the-thermal buoyancy force is considered.

The combined buoyancy force from thermal and species diffusion provides larger local Nusselt Nu and local Sherwood Sh numbers relative to the case when just one buoyancy force is accounted for. Both local Nu and local Sh are seen to asymptotically approach a constant value as flow develops.     

A GENERAL METHOD FOR CALCULATING NATURAL CONVECTION FLOWS IN CLOSED LOOPS

A method is presented for calculating natural convection flow and heat transfer rates in closed loops consisting of sections with different cross-sections, lengths, friction factors and thermal boundary conditions. On application to tilted square loops, the method yields good agreement with experimental velocity data obtained by Acosta et at. ]Warme StofTubertragung 21,268 (1987)[ over a wide range of tilt angles.