COMBINED NATURAL AND FORCED CONVECTION IN A HORIZONTAL FLOW CHANNEL

The effect of natural convection on forced convection in a horizontal flow was investigated during zinc electrodeposition on a horizontal electrode facing upward in a channel. The previous findings of Tobias and Hickman [1] are confirmed. A secondary flow in the form of roll cells whose axes are parallel to the flow direction is formed at some distance from the leading edge. This distance depends linearly on the flow velocity through the critical Rayleigh number for the onset of natural convection across the diffusion boundary layer. Striated zinc deposits reveal the imprint of the secondary flow on the planar cathode facing upward.     

COMBINED FORCED AND FREE CONVECTION IN BOUNDARY LAYER FLOW OF NON-NEWTONIAN FLUID ON A HORIZONTAL PLATE

An analysis is presented to study the effects of buoyancy-induced streamwise pressure gradients on laminar forced convection heat transfer to non-Newtonian fluids from a horizontal plate. Numerical solution of the transformed boundary layer equations has been carried out for different values of the flow behavior index and the buoyancy parameter by means of the local nonsimilarity method. A discussion is provided for the effect of the buoyancy force on the friction factor, the dimensionless heat transfer rate and the details of the velocity as well as temperature fields.     

A GENERALIZED DIFFERENTIAL TRANSFORM METHOD FOR COMBINED FREE AND FORCED CONVECTION FLOW ABOUT INCLINED SURFACES IN POROUS MEDIA

In this article, we apply the differential transform method (DTM) to obtain approximate analytical solutions of combined free and forced (mixed) convection about inclined surfaces (or wedges) in a saturated porous medium. Both aiding and opposing flows are considered. It is found that the parameter mixed convection from inclined surfaces in porous media is Gr/Re, where Gr is the local Grashof number and Re is the local Reynolds number. DTM solutions are obtained for mixed convection from an isothermal vertical flat plate as well as an inclined plate with constant heat flux having an inclination of 45°. Temperature and velocity profiles for these two cases at different values of Gr/Re are presented. The similarity transformations are applied to reduce the governing partial differential equations (PDEs) to a set of nonlinear coupled ordinary differential equations (ODEs) in dimensionless form. DTM is used to solve the nonlinear differential equations governing the problem in the form of series with easily computable terms. Thereafter a Padé approximant is applied to the solutions to increase the convergence of the given series. Excellent correlation between DTM-Padé and numerical quadrature (shooting) solutions is achieved. The DTM-Padé simulation is shown to be a robust benchmarking tool providing an excellent means of validation of numerical methods. The study has applications in geothermal energy systems, chemical engineering filtration systems, and packed beds.     

NATURAL CONVECTION INSIDE A HORIZONTAL CYLINDER

Natural convection of a fluid contained in an infinitely long horizontal cylinder at large Prandtl number and unit-order Grashof number is analyzed. The motion is generated by an imposed cosine wall temperature distribution which includes an arbitrary phase angle. The phase angle is a measure of the location of the wall temperature extrema.

From an asymptotic ordering of the energy and vorticity transport equations for large Prandtl number it is shown that the core region, which contains fluid surrounded completely by a boundary-layer flow along the cylinder wall, may assume either of two configurations.

For heating angles near the heating-from-the-side case (wall temperature extrema at the ends of the horizontal diameter) linearized forms of the boundary-layer equations are developed which yield solutions that match the core configuration not considered previously. The form of the results agrees generally with experimental evidence for heating-from-the-side.     

NATURAL CONVECTION INSIDE A HORIZONTAL CYLINDER

Natural convection of a fluid contained in an infinitely long horizontal cylinder at large Prandtl number and unit-order Grashof number is analyzed. The motion is generated by an imposed cosine wall temperature distribution which includes an arbitrary phase angle. The phase angle is a measure of the location of the wall temperature extrema.

From an asymptotic ordering of the energy and vorticity transport equations for large Prandtl number it is shown that the core region, which contains fluid surrounded completely by a boundary-layer flow along the cylinder wall, may assume either of two configurations.

For heating angles near the heating-from-the-side case (wall temperature extrema at the ends of the horizontal diameter) linearized forms of the boundary-layer equations are developed which yield solutions that match the core configuration not considered previously. The form of the results agrees generally with experimental evidence for heating-from-the-side.     

倾斜管内热进口段自然强制复合对流传热

用数值分析方法研究了倾斜管内热进口段自然强制同向复合对流传热 ,分析了浮力、倾斜角及轴向导热对流动和传热特性的影响 ,得到了速度、温度、壁面剪切应力、局部及平均Nusselt数的分布和变化 研究结果显示Pe=71时平均Nusselt数的最大值位于 2 5°～ 45°之间 ,Pe=2 5时则在 90°处出现 .     

NUMERICAL STUDY OF FORCED CONVECTION IN A PACKED CHANNEL WITH ASYMMETRIC HEATING

Numerical solution based on the control volume method is presented for the study of heat transfer for forced convective flow in a channel filled with a fluid saturated porous media. The solution of the conservative differential equations governing the flow is performed using the SZMPLE algorithm. The wall effects on the variation of porosity and thermal dispersion have been considered. In calculating the thermal dispersive conductivity, a general expression has been obtained taking into account the flow geometry and flow Reynolds number. The expression appears to serve well in the present investigation and also in reproducing the results of previous studies. The analysis includes predictions of temperature profiles and heat flux for the constant wall temperature condition at the wall and have been compared with available experimental data.     

TRANSITION TO SLUG FLOW IN HORIZONTAL PIPES

The transition to slug flow in horizontal pipelines is reviewed. Experimental observations of the interface prior to slugging and of the effect of physical properties and pipe diameter on the transition are described. Theoretical progress in understanding the initiation of slugging through a stability analysis is summarized. Theories are classified by whether they invoke a long-wave assumption and whether they include viscous and nonlinear effects. Despite considerable progress, a general theory for predicting the initiation of slugging for arbitrary fluids in pipelines of varying size and pressure is far from being realized.     

TRANSITION TO SLUG FLOW IN HORIZONTAL PIPES

The transition to slug flow in horizontal pipelines is reviewed. Experimental observations of the interface prior to slugging and of the effect of physical properties and pipe diameter on the transition are described. Theoretical progress in understanding the initiation of slugging through a stability analysis is summarized. Theories are classified by whether they invoke a long-wave assumption and whether they include viscous and nonlinear effects. Despite considerable progress, a general theory for predicting the initiation of slugging for arbitrary fluids in pipelines of varying size and pressure is far from being realized.     

COMBINED LAMINAR FREE AND FORCED CONVECTION FROM A VERTICAL PLATE TO POWER LAW FLUIDS

The boundary layer heat transfer for the steady two-dimensional incompressible combined laminar free and forced convection from a vertically static or moving plate to a power-law non-Newtonian fluid is analyzed by local similarity method. The effects of the flow index, buoyancy parameter, modified Prandtl number and streamwise distance on the heat transfer rate are discussed for the prescribed surface temperature and prescribed wall heat flux cases.     

COMBINED LAMINAR FREE AND FORCED CONVECTION FROM A VERTICAL PLATE TO POWER LAW FLUIDS

The boundary layer heat transfer for the steady two-dimensional incompressible combined laminar free and forced convection from a vertically static or moving plate to a power-law non-Newtonian fluid is analyzed by local similarity method. The effects of the flow index, buoyancy parameter, modified Prandtl number and streamwise distance on the heat transfer rate are discussed for the prescribed surface temperature and prescribed wall heat flux cases.     

FREE CONVECTION MASS TRANSFER AT HORIZONTAL CYLINDERS

Natural convection mass transfer at horizontal cylinders was studied using an electrochemical technique involving the deposition of copper from acidified copper sulphate solution. Cylinder diameter and copper sulphate concentrations were varied to provide a range of Sc. Gr from 1.55 × 10¹⁰ to 2 × 10¹². Under these conditions, the data were correlated by the equation:

Measurement of mass transfer distribution revealed the fact that mass is transferred at the lower hemicylinder by a laminar flow mechanism and at the upper hemicylinder by a turbulent flow mechanism.     

HC290含油混合物水平微肋管内的流动凝结压降

实验测定了HC290含油混合物水平微肋管（长2 m,最大内径11.44 mm）内流动凝结过程的压降,实验中所用润滑油是一种制冷系统中广泛应用的润滑油Suniso 3GS,润滑油浓度范围为1.95%～5.28%;所用微肋管为内表面强化管,肋数60,肋高0.25 mm,螺旋角20°,实验所取冷凝温度为40～45 ℃,质量通量范围为40～240 kg•m^-2•s^-1.结果显示实验段内冷凝压降(入口质量含汽率为1,出口质量含汽率为0.1～0.25)随工质质量通量的增加而迅速增大,润滑油浓度对冷凝压降几乎没有影响.同时在对Kaushik和Azer压降计算关系式修正的基础上得到了适用于本实验的相应经验关系式.     

NATURAL CONVECTION MASS TRANSFER INSIDE HORIZONTAL SQUARE DUCTS

Rates of free convection mass transfer inside horizontal square ducts were measured by electrochemical technique. Physical properties of the solution and duct side length were changed to provide Sc.Gr range of 7.38 × 10⁸ - 1.4 × 10¹¹. Under these conditions the data were correlated by the equation Sh= 0.382(Sc.Cr)^0.278 A comparison between the measured overall rate of mass transfer with that calculated by summing the rates of mass transfer at the separate sides of the duct showed that the measured value is higher than the calculated value owing to convective interaction between adjacent surfaces.     

RADIATION AND NON-DARCY EFFECTS ON CONVECTION IN POROUS MEDIA

Combined conduction, convection and radiation heat transfer in a gray fluid-saturated sparsely packed porous medium is examined analytically for marginal convection using linear stability analysis. The effects of boundary and inertia which were absent in the usual Darcy model are considered. The Milne-Eddington approximation is employed to determine the solutions valid for transparent and opaque media which absorbs and emits thermal radiation. It is shown that the nature of the bounding surfaces and radiation significantly influence the critical Rayleigh and wave numbers. The mechanism for suppressing or augmenting convection is discussed in detail. The results obtained using Galerkin technique are compared with the existing results of Darcy model and of non-radiating systems and agreement is found.     

CONVECTION PROMOTION AND GEL FORMATION IN AN ULTRAFILTRATION PROCESS

Convection promotion within a tubular ceramic membrane by using the kenics static mixer is studied. The experiments were carried out with aqueous solutions of pectin and sodium carboxymethylcellulose (CMC), for Reynolds number in the range of 60-5000. From the results of these experiments it can be concluded that the presence of convection promotor decreasing the concentration of the gel layer. The experimental data are correlated satisfactorily by the equation of the general form Sh=A Re^aSc^b.     

IDENTIFICATION OF LIQUID-LIQUID FLOW PATTERN IN A HORIZONTAL PIPE USING ARTIFICIAL NEURAL NETWORKS

Identification of flow pattern during the simultaneous flow of two immiscible liquids requires knowledge of the flow rate of each fluid as well as knowledge of other physical parameters like conduit inclination, pipe material, pipe diameter, viscosity of the oil, wetting characteristics of the pipe, design of the entry mixer, and fluid-fluid interfacial tension. This article presents an artificial neural network (ANN)-based novel technique to determine the liquid-liquid flow regime. This approach uses phase superficial velocities as input parameters, which are obtained from a specific set of data obtained from experimental investigations. Both experimental and ANN-based determinations of liquid-liquid flow pattern have been undertaken for a common data set and the results are compared to prove the effectiveness of ANNs in pattern recognition. A unique ANN architecture is identified with three hidden layers, and the inputs and outputs are modeled into binary form. Levenberg-Marquardt (LM) learning algorithm is used for training this neural network. The design details of the ANN, parameter modeling, and training aspects are presented.     

水平矩形管内气液两相流界面波特性

<正>气液两相流中界面波的存在对传热、传质、阻力特性及稳定性都有很大影响。前人有关界面波的研究大多偏重于两相流中界面波的类型及其转变,而对其特征参数变化规律(如振幅、波长、频率和速度等)的研究则很少。但界面波对气液两相流流动阻力和流动结构影响机理的深入研究必须以了解界面波的各特性参数为前提,同时界面波有关特性参数的深入研究也可为气液两相流的数学理论化提供实验基础。 本文以气、水为工质,系统研究了水平矩形管内气液两相流的界面波特性。根据界面波在时域、幅域内的不同特征区别出了各种类型的波并给出了相应的波形和流型图。着重研究了界面波各特征量随气液两相流量变化的变化规律。     

INTRAPARTICLE CONVECTION, DIFFUSION AND ZERO ORDER REACTION IN POROUS CATALYSTS

An assessment is made of the influence of convective effects in the behaviour of calalyst pellets with large pores

Concentration profiles and effectiveness factors are calculated for zero order reactions in isothermal regimes using a model which involves convective transport as well as diffusion and reaction processes in the pellet

Errors made in current reactor design are discussed. A predictive method for calculating the intraparticle fluid velocity in industrial cases is reported. Finally, the analysis is applied to the design of biological reactors.     

INTRAPARTICLE CONVECTION,DIFFUSION AND ZERO ORDER REACTION IN POROUS CATALYSTS

An assessment is made of the influence of convective effects in the behaviour of calalyst pellets with large pores

Concentration profiles and effectiveness factors are calculated for zero order reactions in isothermal regimes using a model which involves convective transport as well as diffusion and reaction processes in the pellet

Errors made in current reactor design are discussed. A predictive method for calculating the intraparticle fluid velocity in industrial cases is reported. Finally, the analysis is applied to the design of biological reactors.