湍流边界层内表面活性剂减阻特性研究

湍流主要通过边界层流体与壁面的摩擦引起的,因此,研究表面活性剂的流向上边界层内湍流减阻性非常有意义,通过压降和粒子图像测速法分别研究了质量分数为10×10~(-6),50×10~(-6)和100×10~(-6)下的表面活性剂溶液与水的压降、范宁系数、减阻率、平均速度、速度分布云图、雷诺应力、涡量和涡量分布云图,实验发现:在表面活性剂的壁面范宁系数要比水时壁面的范宁系数要小,在质量分数为50×10~(-6)时减阻效果最好,最大减阻率为20%。得出结论:表面活性剂的加入使湍流边界层的厚度增加,雷诺切应力减小,在靠近管道的中心处的涡量最小,随着远离管道的中心,涡量缓慢地增大,近壁区的涡量降低,表面活性剂的减阻溶液的涡量比水的涡量稍微大一点,说明主要抑制管道中心区域的湍流强度来降低阻力,从而达到减阻效果。     

稠油流动边界层水基泡沫减阻模拟

针对稠油水环输送的中心油流偏心问题,提出稠油流动边界层在AFS-2水基泡沫作用下的模拟实验方法,设计加工相应的管流模拟装置及泡沫发生、注入与泡沫层生成系统.用201甲基硅油模拟稠油,实验研究稠油在水基泡沫作用下的水平管流阻力特性,分析泡沫与硅油的流速与流量比对硅油流动流型和减阻效果的影响.基于上部泡沫-下部液膜复合边界层假设,建立硅油-泡沫-液膜中心环状水平管流的压降预测模型.结果表明:20℃室温下,当泡沫与硅油流量比为0.2～0.5时,硅油流动减阻率高于70%,其机理可归结为油壁间形成了上部泡沫-下部液膜的复合隔离润滑层;理论预测值与实验测量值吻合良好,相对误差为－17.55%～9.76%.     

Incipient motion of a small particle in the viscous boundary layer at a pipe wall

A force balance is derived for a hemispherical particle in the viscous boundary layer at the wall of a horizontal pipe conveying Newtonian fluid; the hemisphere, of radius much less than that of the pipe, rests on the bottom with its flat face against the wall. The drag on the hemisphere is calculated from the creeping flow field of Price (Q. J. Mech. Appl. Math. Pt. 1 (1993)). This yields a prediction of the maximum velocity gradient at the wall for equilibrium, with limiting friction between the hemisphere and the wall. It is shown that the flow field of Price predicts a zero lift force but the validity of this, for actual flows, is questioned. Use of a hemisphere formulates a relevant well-posed problem, capable of mathematical solution. However, the flow field around real particles, e.g. sand, is complex, because of their irregular shapes, but the hemisphere work gives a qualitative indication of the behaviour of irregular particles. For turbulent flow in a pipe it is pertinent to consider a particle wholly within the viscous sub-layer, because it is isolated from significant turbulence and therefore hard to move; for such flow, the theory gives Eq. (21) to predict the critical pipe velocity, v_C, for incipient motion of the hemisphere. For laminar flow, the wall shear rate is readily obtained from the parabolic velocity profile leading to Eq. (26) for v_C. The flow field of Price (and therefore the force acting on the hemisphere) is valid only for creeping flow (i.e. very low particle Reynolds number). Modifications to the force balance are tentatively suggested to account for inertial components to the drag force. The predictions of critical velocity are tested against our data for the incipient motion of small hemispheres at pipe walls in hydraulic conveying as well as new and previously published data for both hydraulic and pneumatic conveying. The new method of predicting incipient motion works well for both the pneumatic and hydraulic conveying of hemispheres and sand shaped particles but it overpredicts the critical velocity for more rounded particles. The dependency of critical velocity on particle shape is under-researched.     

Characteristics of the boundary layer with hydrogen combustion with variations of thermal conditions on a permeable wall

The paper describes a numerical study of the influence of thermal and boundary conditions on the structure of laminar and turbulent diffusion flames in the cases with hydrogen injection through a porous surface and with hydrogen combustion in an air flow. Two types of boundary conditions are compared: with a given constant temperature T _w = const over the length of the porous surface for arbitrary intensities of fuel injection and with a constant temperature T′ = const of the fuel injected through the porous wall. The first case occurs during combustion of a liquid fuel whose burning surface temperature remains unchanged. Injection of gaseous fuel usually leads to the second case with T′ = const. Despite significant differences in velocity and temperature profiles, the skin friction coefficients in the laminar flow are close to each other in these two regimes. In the turbulent regime, the effect of the thermal boundary conditions on friction and heat transfer is more pronounced. Moreover, the heat flux to the wall as a function of fuel-injection intensity is characterized by a clearly expressed maximum. A principal difference of the effect of combustion on friction and heat transfer in the laminar and turbulent flow regimes is demonstrated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 3–11, May–June, 2009.     

Catalytic reaction in a porous solid subject to a boundary layer flow

The mathematical model of a boundary layer past a catalytic slab, involving a heterogeneous reaction, is solved using a superposition integral in connection with a norm reduction procedure to treat the transcedental systems of equations that result.A parametric study is presented and an analysis of the possible multiplicity of steady-states is developed, illustrated with the extreme case of infinite solid thermal conductivity.     

Effect of blockage on drag and heat transfer from a single sphere and an in-line array of three spheres

The effect of blockage ratio on the steady flow and heat transfer characteristics of incompressible fluid over a sphere and an in-line array of three spheres placed at the axis of a tube has been investigated numerically. The Navier-Stokes and thermal energy equations have been solved numerically using FLUENT for the following ranges of parameters: for a single sphere, 2 ≤ β ≤ 10; 1 ≤ Re ≤ 100; for the three-sphere system, for two values of sphere-to-sphere distance, namely s = 2 and 4. All computations were carried out for two values of the Prandtl number, i.e., 0.74 and 7, corresponding to the flow of air and water respectively. Extensive results on streamline patterns, wake characteristics (angle of separation and recirculation length), drag coefficient and Nusselt number are presented to elucidate the interplay between the blockage and the Reynolds number and their influence on drag and Nusselt number.     

Effect of flow history on combustion in a laminar boundary layer

Results of numerical simulations of methane combustion in a laminar boundary layer on a porous plate with an impermeable initial section are presented. The analysis of results is based on comparisons of data with and without combustion, and also for different initial section lengths including the zero length. The flow history is demonstrated to exert a significant effect on heat transfer and friction in the boundary layer with injection without combustion, whereas the influence of the flow history in the case with combustion is smaller. The phenomenon experiencing the least effect of the flow history is heat transfer.     

Two species diffusion in a sphere with inter-species linkage at the boundary

The concentration of carbon in stainless steel has been shown to satisfy a singular Volterra integral equation by Skyrme & Norbury[1]. This note is concerned with a simple product integration method for solving this equation. The method is described and numerical results are presented in graphical form. A concise convergence analysis making use of a generalisation of the usual discrete Gronwall's lemma is presented. Finally it is pointed out how a continuous form of the lemma can be used to demonstrate uniqueness.     

Analysis of experimental data on heat and mass transfer in a boundary layer

Based on analysis of experimental heat-and mass-transfer data, the intensity of transfer processes is shown to vary discretely and multiply with the intensity in the laminar boundary layer in the transient flow region. In the cases considered, this feature is traced up to Reynolds numbers Re_x≈10⁷. It is manifested not only in the absence of chemical transformations but also in heterogenous and gas-phase combustion. The evolution of heat and mass transfer is regarded as a successive transition from one level of intensity to another. The existence of discrete levels is explained by the presence of a standing wave in the neighborhood of the laminar-turbulent transition. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 2, pp. 73–81, March–April, 1998.     

隔壁塔内半圆填料层中壁流与返混特性研究

在填料塔三参数壁流模型的基础上,提出了应用于隔壁塔半圆型填料层的壁流模型。通过实验对模型参数进行了估计。模型计算值与实验值吻合较好。在内径为580 mm、高2800 mm的半圆冷模塔内,常温常压下以空气-水为介质,测取了散堆填料的壁流曲线,分析了气液速率变化对壁流的影响。以氯化钠溶液为示踪剂,通过脉冲注入扰动响应技术获得停留时间分布曲线。发现气液速率变化均对停留时间分布产生不同程度影响。同时比较了同一填料层高度塔壁区和整体区的平均停留时间,在壁流充分发展阶段,塔壁区的平均停留时间明显小于整体区。从另一方面量化了壁流效应的影响。应用线性回归得到单相流和气液逆流时计算Pe_d的关联式。对于新型填料及塔内件的开发、改进隔壁塔的设计方法,从而实现安全有效放大具有参考意义。     

Effect of a solid wall on the formation of a porous layer structure

The formation of a near-wall layer in a bulk material has been studied by computer-aided modeling. Numerical calculation results have been compared with the experimental data.     

Structure of a boundary layer with injection and combustion of ethanol

Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 3, pp. 29–36, May–June, 1992.     

Pressure loss and wall shear stress in flow through confined sphere packings

Randomly structured, confined sphere packings with different porosities are generated and the fluid flow within the porous structure is calculated. These locally resolved fluid flow data – instead of integral parameters – are used to investigate the origins of the pressure loss within a packing. First, an analysis and comparison of averaged local velocities is performed to compare the similarity of the simulation approach with empirical relations by means of the void fraction and the velocity distributions. Next, the pressure losses due to mean values of the simulated, locally resolved wall shear stresses are calculated, and these findings are smaller than the results from the integral approach of Kozeny and Carman. This indicates that the pressure drop, even at low Reynolds numbers, is not solely caused by the wall shear stress; the simulated overall pressure drops exceed the Ergun approach, an effect which is caused by the bounded flow within a capillary. To relate the pressure loss due to these secondary pressure losses, the tortuosity of the fluid flow in the porous structure is introduced and this parameter improves the performance of the pressure drop equation.