Improved k-ε model and wall function formulation for the RANS simulation of ABL flows

The simulation of Atmospheric Boundary Layer (ABL) flows is usually performed using the commercial CFD codes with RANS turbulence modelling and standard sand-grain rough wall functions. Such approach generally results in the undesired decay of the velocity and turbulent profiles specified at the domain inlet, before they reach the section of interest within the computational domain. This behaviour is a direct consequence of the inconsistency between the fully developed ABL inlet profiles and the wall function formulation.The present paper addresses the aforementioned issue and proposes a solution to it. A modified formulation of the Richards and Hoxey wall function for turbulence production is presented to avoid the well-documented over-prediction of the turbulent kinetic energy at the wall. Moreover, a modification of the standard k-ε turbulence model is proposed to allow specific arbitrary sets of fully developed profiles at the inlet section of the computational domain.The methodology is implemented and tested in the commercial code FLUENT v6.3 by means of the User Defined Functions (UDF). Results are presented for two neutral boundary layers over flat terrain, at wind tunnel and full scale, and for the flow around a bluff-body immersed into a wind-tunnel ABL. The potential of the proposed methodology in ensuring the homogeneity of velocity and turbulence quantities throughout the computational domain is demonstrated.     

燃料变化对气体燃烧器燃烧性能影响的数值模拟研究

该文以John Zink公司一种瓦斯燃烧器为几何原型,对燃烧器和稳焰旋流器附近三维复杂形状未作任何简化,生成了包括燃烧器和炉膛的结构化网格,换用甲烷、乙烷、丙烷和丁烷四种不同的气体燃料,用标准的k-ε湍流模型、k-ε-g湍流扩散燃烧模型和蒙特卡洛辐射换热模型对燃烧器内的流动及燃烧状况进行了全尺寸数值模拟,预报了燃烧器内流场和温度场,考察燃料变化对炉内温度场的影响规律及燃气射流特性参数(Re·D1)对火焰长度的影响,对进一步优化设计燃烧器、提高加热炉热效率有很好的指导意义。     

离心力作用下的油—水分离

从理论上分析了油－水旋转流场，主要是水力旋流器流场中的分离机理和旋流器的结构特征，指出应特别注意旋流器内的湍流，尤其是轴向湍流度、速度梯度以及它们对油滴的破裂和粗化的作用。介绍了作者研制的水力旋流器式油水分离装置及其效果，指出利用复合力对分离乳化油的重要作用。简介了根据作者的这种论点正在研制的新型双膜离心式油水分离器。     

Non-Boussinesq turbulent buoyant jet resulting from hydrogen leakage in air

This paper is devoted to introduce a numerical investigation of a vertical axisymmetric non-Boussinesq buoyant jet resulting from hydrogen leakage in air as an example of injecting a low-density gas jet into high-density ambient. As the domain temperature is assumed to be constant and therefore the density of the mixture is a function of the concentration only, the binary gas mixture is assumed to be of a linear mixing type. Also, it is assumed that the rate of entrainment to be a function of the plume centerline velocity and the ratio of the mean plume and ambient densities. On the other hand, the local rate of entrainment may be considered to be consisted from two components; one is the component of entrainment due to jet momentum while the other is the component of entrainment due to buoyancy. Firstly, the integral models of the mass, momentum and concentration fluxes are obtained and transformed to a set of ordinary differential equations using some non-dimensional transformations known as similarity transformations. The given ordinary differential system is integrated numerically and the mean centerline mass fraction, jet width and mean centerline velocity are obtained. In the second step, the mean axial velocity, mean concentration and mean density of the jet are obtained. Finally in the third step of this article, several quantities of interest, including the cross-stream velocity, Reynolds stress, velocity-concentration correlation (radial flux), turbulent eddy viscosity and turbulent eddy diffusivity, are obtained. In addition, the turbulent Schmidt number is estimated and the normalized jet-feed material density and the normalized momentum flux density are correlated.     

APPLICATION OF THE ENERGY DISSIPATION RATE CONCEPT TO NATURAL CONVECTION HEAT TRANSFER

A new model for turbulent natural convection heat transfer from an isothermal vertical plate is developed. It is based on a unified energy dissipation rate concept originally proposed by Calderbank and Moo-Young (Chem. Eng. Sci., 16, 39-54 (1961)). The proposed model is developed to include non-Newtonian flow behaviors. The predictions of the model are compared with published experimental and theoretical results. Satisfactory agreement is found and therefore the applicability of the energy dissipation rate concept to natural convection is confirmed.     

涡旋燃烧炉正反切向进风气固流动的数值模拟

涡旋燃烧炉是最近研制的一种新型固体燃料旋风燃烧炉，它采用了多级切向进风的结构。本文应用气固两相流的多连续介质模型及其数值计算方法，针对分级供入炉内的空气采用正反切向进风的条件，对涡旋燃烧炉内的气固两相湍流旋流流动进行了数值模拟。通过模拟结果的分析及其与分级同向切向进风情况的比较，探讨了在涡旋燃烧炉中采用多级正反切向进风方式的可能性。     

Premixed and nonpremixed generated manifolds in large-eddy simulation of Sandia flame D and F

Premixed and nonpremixed flamelet-generated manifolds have been constructed and applied to large-eddy simulation of the piloted partially premixed turbulent flames Sandia Flame D and F. In both manifolds the chemistry is parameterized as a function of the mixture fraction and a progress variable. Compared to standard nonpremixed flamelets, premixed flamelets cover a much larger part of the reaction domain. Comparison of the results for the two manifolds with experimental data of flame D show that both manifolds yield predictions of comparable accuracy for the mean temperature, mixture fraction, and a number of chemical species, such as CO₂. However, the nonpremixed manifold outperforms the premixed manifold for other chemical species, the most notable being CO and H₂. If the mixture is rich, CO and H₂ in a premixed flamelet are larger than in a nonpremixed flamelet, for a given value of the progress variable. Simulations have been performed for two different grids to address the effect of the large-eddy filter width. The inclusion of modeled subgrid variances of mixture fraction and progress variable as additional entries to the manifold have only small effects on the simulation of either flame. An exception is the prediction of NO, which (through an extra transport equation) was found to be much closer to experimental results when modeled subgrid variances were included. The results obtained for flame D are satisfactory, but despite the unsteadiness of the LES, the extinction measured in flame F is not properly captured. The latter finding suggests that the extinction in flame F mainly occurs on scales smaller than those resolved by the simulation. With the presumed β-pdf approach, significant extinction does not occur, unless the scalar subgrid variances are overestimated. A thickened flame model, which maps unresolved small-scale dynamics upon resolved scales, is able to predict the experimentally observed extinction to some extent.     

Evaluations and Modifications on Reynolds Stress Model in Cyclone Simulations

The prediction performance of the Reynolds stress model (RSM) on the flow field in a cyclone has been validated by using particle imaging velocimetry (PIV) experimental results. The validations mainly focus on two features of the flow information with averaged and fluctuating flow fields, which can mostly reflect the turbulent flow properties in the cyclone. The comparisons between predictions and measurements show that the RSM has good performance on the averaged flow field prediction, especially on the prediction of the mean tangential velocity. However, for the fluctuating flow field, the prediction performance of RSM is rather poor. The predicted root mean square (RMS) velocities are greatly underestimated. The standard model coefficients used in RSM have to be modified to enhance its accuracy when used to predict the fluctuating structure of the strong swirling flow in the cyclone. The recommended model coefficients give much better predictions on the fluctuating flow field than the standard coefficients, without decreasing the prediction accuracy on the mean flow field.     

Plane flow model of non-Newtonian turbulent stratified flow in wells and pipes

Economical and efficient long-distance oil and natural gas pipe transportation technologies are greatly demanded. Nowadays, oil and gas mixed transportation is gradually put into practice and researchers pay much attention on the theories of two-phase flow in pipe. Methods that can efficiently solve problems of this field are being put forward including hydraulic approach. A new approach, the Rotatable Coordinate Axis, is proposed in this paper. It is used to study the hydraulic characters of two-phase stratified flow in pipe. Based on the new method, the plane flow model for stratified turbulent flow in pipe is built. Then, we can obtain the analytic formulas of velocity, discharge etc. in wells and pipes by this model. To prove the theory of the plane flow model, experiments of aeration in stratified pipe is conducted with the aeration experiment device. The experiment data showed that aeration could effectively achieve the resistance reduction in pipeline, which can offer great theory support to the development of oil and natural gas mixed transportation technology.     

The internal surface area basis, a key issue of modeling fouling in enhanced heat transfer tubes

The purpose of this paper is to discuss the internal surface area basis, a key issue of modeling fouling in enhanced heat transfer tubes, and to correct an error in the author’s previous paper, Li and Webb [Int. J. Heat Mass Transfer 45 (2002) 1685]. A fouling factor can be defined based on one of the two area bases, nominal internal surface area and total internal surface area. The error was originated from incorrect definition of internal surface area basis.