Modeling the characteristic types and heat release of stretched premixed impinging flames

 A numerical code has been developed for the simulation of the impingement of a turbulent jet on a plane surface. The performance of three turbulence models is assessed under isothermal conditions. Predictions are compared with experimental data from the literature. Based on the results an appropriate turbulence model is selected to model a premixed jet flame impinging on a solid surface. Mass transfer and combustion are modeled with a two-equation model simulating volumetric and kinetically controlled chemical reaction rates. Modeling of heat transfer accounts for convection and radiation effects. Results show that under high pressure environment turbulent premixed flames are of wrinkled-thickened type near the outlet of the nozzle (free jet region) and of wrinkled reaction sheets in the area near the surface (impingement region and radial wall jet). The results establish that appropriate choice of turbulence and combustion models can lead to accurate prediction of the flow characteristics.     

Effect of Gravity on Premixed Methane–Air Flames

Premixed flames under different levels of gravity were studied experimentally and numerically. The experiments were carried out in the Bremen Drop tower. The object of investigation were conical premixed rich, lean, and stoichiometric CH₄–air flames with wide range of flow regimes, at Reynolds numbers of 600–2000, which were generated on specially designed cone nozzle and premixed cylinder chamber with grids and beads. Planar laser-induced fluorescence OH radicals and high-speed video recording was performed under microgravity, terrestrial conditions, and inverted gravity. All the experiments were performed at atmospheric pressure. Our experiments confirm that gravity has a complex influence on laminar and weakly turbulent premixed flames. Gravity causes flame flickering, while under reduced gravity flames are stable and almost not flicker. Based on the experimental data and numerical simulation, a correlation of flickering frequency with different mixture equivalence ratio and gravity is formed.     

Computer simulation of turbulent swirling flows

Simulation of turbulent swirling flows has been carried out to provide insight on streamline curvature effects. Numerical calculations were based on the control-volume method. Turbulence effects were represented by two-equation turbulence models. The analysis of steady flow between two concentric rotating cylinders showed that the most promising model which incorporates curvature effects is that based on mixing energy developed by Wilcox and Chambers, as opposed to the ad hoc modification to the standard k–? model. This model has also been used to simulate the decay of turbulent swirling flow in a short cylinder. A comparison between calculations and experimental results for such flow fields has been presented.     

Comparative analysis of turbulence models

A comparative analysis is performed for a complete locally anisotropic turbulence model of the second order and existing turbulence models. The comparison draws on experimental data, data of a direct numerical simulation of the nonstationary Navier-Stokes equations for a developed channel flow and a uniform channel flow with a constant velocity shift, and results for turbulence damping behind a grid. The K-ɛ model and the quasi-isotropic turbulence model are shown to have marked disadvantages, especially in describing turbulent flows with a high degree of anisotropy of pulsatory motion. Use of a locally anisotropic turbulence model improves the accuracy of determining Reynolds stresses. Consideration is given to the advantages and disadvantages of the turbulence models discussed. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 2, pp. 328–339, March—April, 2000.     

Simulation of a premixed turbulent flame with the discrete vortex method

The behaviour of a premixed turbulent flame is numerically studied in this paper. The numerical model is based on solving turbulent flow field by the discrete vortex method. The flame is considered to be of zero thickness boundary which separates burnt and unburnt regions with different constant density and propagates into the fresh mixture at a local curvature‐dependent flame speed. The flame front is located by means of level‐set algorithm. The flow turbulence is simulated through the unsteady vortex‐shedding mechanism. The computed velocity fields, turbulence scalar statistics as well as flame brush thickness for the turbulent V‐flame are well comparable to experimental results. The computed Reynolds stresses in the flame brush region based on unconditioned velocities are substantial, but the two conditioned Reynolds stresses are negligible. These results show that the intermittency effect is a major influence on turbulent statistics in premixed flame and should require careful consideration in numerical models. Copyright © 2000 John Wiley & Sons, Ltd.     

High-performance computing in computational fluid dynamics: progress and challenges

Computational fluid dynamics (CFD) is by far the largest user of high-performance computing (HPC) in engineering. The main scientific challenge is the need to gain a greater understanding of turbulence and its consequences for the transfer of momentum, heat and mass in engineering applications, including aerodynamics, industrial flows and combustion systems. Availability of HPC has led to significant advances in direct numerical simulation (DNS) of turbulence and turbulent combustion, and has encouraged the development of large-eddy simulation (LES) for engineering flows. The statistical data generated by DNS have provided valuable insight into the physics of many turbulent flows and have led to rapid improvements in turbulence and combustion modelling for industry. Nevertheless, major challenges remain and the computational requirements for turbulence research, driven by well-established physical scaling laws, are likely to remain at the limit of the available HPC provision for some time to come.     

Melt Flow Instability and Turbulence in Electro-Magnetically Levitated Droplets

This article addresses fluid flow instabilities and flow transition to turbulent chaotic motions through numerical analysis and turbulence in electro-magnetically levitated droplets through direct numerical simulations. Numerical implementation and computed results are presented for flow instability and turbulence flows in magnetically levitated droplets under terrestrial and microgravity conditions. The linear melt flow stability is based on the solution of the Orr-Sommerfeld linearized equations with the base flows obtained numerically using high order numerical schemes. The resulting eigenvalue problems are solved using the linear transformation or Arnold's method. Melt flow instability in a free droplet is different from that bounded by solid walls and flow transits to an unstable motion at a smaller Reynolds number and at a higher wave number in a free droplet. Also, flow instability depends strongly on the base flow structure. Numerical experiments suggest that the transition to the unstable region becomes easier or occurs at a smaller Reynolds number when the flow structures change from two loops to four loops, both of which are found in typical levitation systems used for micro-gravity applications. Direct numerical simulations (DNS) are carried out for an electro-magnetically levitated droplet in a low to mild turbulence regime. The DNS results indicate that both turbulent kinetic energy and dissipations attain finite values along the free surface, which can be used to derive necessary boundary conditions for calculations employing engineering k--ε models.     

Experimental investigation of the turbulence flow characteristics behind a system of flame stabilizers

Measurements are carried out of the intensity and scale of turbulence in the flow behind a system of flame stabilizers. A comparison is made between the results obtained and the known data for flows behind grills and single stabilizers.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 36, No. 1, pp. 69–74, January, 1979.     

Comparison of near-forward light scattering on oceanic turbulence and particles

We examine and compare near-forward light scattering that is caused by turbulence and typical particulate assemblages in the ocean. The near-forward scattering by particles was calculated using Mie theory for homogeneous spheres and particle size distributions representative of natural assemblages in the ocean. Direct numerical simulations of a passive scalar with Prandtl number 7 mixed by homogeneous turbulence were used to represent temperature fluctuations and resulting inhomogeneities in the refractive index of water. Light scattering on the simulated turbulent flow was calculated using the geometrical-optics approximation. We found that the smallest temperature scales contribute the most to scattering, and that scattering on turbulence typically dominates over scattering on particles for small angles as large as 0.1 degrees . The scattering angle deviation that is due to turbulence for a light beam propagating over a 0.25-m path length in the oceanic water can be as large as 0.1 degrees . In addition, we carried out a preliminary laboratory experiment that illustrates the differences in the near-forward scattering on refractive-index inhomogeneities and particles.     

Numerical analysis of two-dimensional compressible turbulent flows in a turbine stage

A numerical analysis based on the compressible Reynolds-averaged Navier-Stokes equation has been developed for the analysis of two-dimensional compressible turbulent flows in a turbine stage (nozzle and bucket). In the present flow analysis, governing equations are solved by the use of a time dependent explicit method and a two-equation model of turbulence is employed to estimate turbulence effects. To calculate nozzle and bucket flow fields simultaneously, a steady interaction between these flows is assumed. For spatial discretization of the governing equations, a control volume method combined with a body-fitted curvilinear coordinate system is developed to calculate the flows in arbitrarily shaped cascades. In order to assure the effectiveness of the present method, computations are carried out for a two-dimensional section at a blade midspan in a turbine stage. The method gives satisfactory results about boundary layers on blade surfaces, nozzle wake profiles and pitchwise averaged turbine design parameters at each blade exit.     

Computer study of the effect of the turbulence of an oncoming flow on the V-shaped combustion of a homogeneous methane-air mixture

Numerical simulation of the V-shaped turbulent combustion of a homogeneous methane-air mixture has been performed using the program Fluent from the ANSYS-CFD software package. The combustion process was described by the two-dimensional unsteady Navier-Stokes equations and the one-step kinetic mechanism. The unsteady profiles of the velocity components at the input domain of the calculation area were modeled by means of the artificial turbulence algorithms. These algorithms made it possible to set the pulsation intensity and the integral turbulence scale in the oncoming flow. Comparison of the calculated and experimental data has shown that experimentally observed broadening of the combustion front can be a result of its instability to the perturbations of the velocity in the oncoming flow.     

Pulsating Turbulent Flow of Gas in a Round Pipe

An analysis is performed of the presently available results of experimental and prediction studies into pulsating turbulent flow of liquid in a narrow pipe under conditions when the compressibility is apparent. It is demonstrated that the simulation of such flows in the general case may be performed only numerically, using a model of turbulence that adequately includes the effect of oscillation on turbulent transfer. Use is made of a model of turbulence whose validity is proved by comparing the calculation and experimental results for a wide range of flows. Calculations are performed for a pulsating flow of gas in pipes with isothermal and adiabatic walls, acoustically closed at the outlet, in the frequency range corresponding to the first resonance harmonic. The predicted variations of the heat flux to the wall and of the hydraulic drag, averaged over the oscillation period, as functions of the process parameters such as the Reynolds number of the mean flow and the dimensionless oscillation frequency are discussed.     

Application of the “K-ε” model to open channel flows in a magnetic field

In magnetohydrodynamic (MHD) flows turbulence reduction occurs due to the Joule dissipation. It results in heat transfer degradation. In open channel flows, heat transfer degradation is also caused by the turbulence redistribution near the free surface. Both effects can be significant in fusion applications with low-conductivity fluids such as molten salts. In the present study, the “K-ε” model equations for turbulent flows and the free surface boundary condition are adjusted with taking into account MHD effects. Different orientations of the magnetic field, perpendicular and parallel to the main flow, have been considered. The model coefficients have been tuned by a computer optimization using available experimental data for the friction factor. The effect of free surface heat transfer degradation due to the turbulence redistribution has been implemented through the variation of the turbulent Prandtl number. As an example, the model is used for the analysis of a turbulent MHD flow down an inclined chute with the heat flux applied to the free surface.     

变截面管道结构对H2/Air预混气体燃爆特性的影响研究

实际工业中,管道截面突变可改变预混燃气火焰的燃烧模式和燃爆特性,影响燃气输送的安全性。为此,利用Fluent软件,数值模拟研究了4种变截面管道结构(突变扩张型管道、突变收缩型管道、渐变扩张型管道和连续突变扩张型管道)对H_2/Air预混气体燃爆过程火焰动态变化的影响,并分析了管内温度场、压力场的分布情况、气体流速和涡量等燃爆特征参数以及焓、熵值等热力学参数的变化规律。研究结果表明:扩张型管截面的突扩诱导作用和收缩型管截面的收缩阻挡作用改变了管内预混气体燃烧火焰传播的动态结构以及火焰前锋的稀松波和预混气流方向,使火焰出现了"杯口"型和"长颈花瓶"型前锋面;突变收缩型管截面的阻挡、反射和抑制作用加强了湍流强度和涡流运动强度,管内最高温度、峰值压力和平均流速等燃爆特征参数也相对较高,而涡团向上、下管壁的运动抑制了火焰尖端的反应强度和传播速度;渐变扩张型管道能降低突扩诱导作用的影响;而连续突变扩张型管内发生在层流向湍流火焰转变阶段的首次截面突扩产生的诱导作用对其燃爆特性的影响更为显著。     

Simulation of Jet-Induced Geysers in Reduced Gravity

Control of cryogenic propellant tank pressure during tank refueling and expulsion in low gravity is an important technical challenge to overcome for future long duration missions in space. One method proposed to control tank pressurization involves the use of jet-induced geysers. Two-dimensional computational models have been developed and used with limited success in previous efforts to predict geyser heights in microgravity. A three-dimensional flow simulation is used to model jet-induced geysers in reduced gravity. Geyser flows are commonly characterized by the presence of turbulent jets, transient flow, deforming free surfaces, and surface tension effects. As is the case for many turbulent flow applications, accuracy in simulating complex turbulent flows is critically dependent on the selection of a suitable turbulence model. The sensitivity of the simulation geyser predictions to a suite of popular turbulence models is assessed. Simulation results are compared to available experiment results. By expanding upon the work already completed, the model is used to simulate a broad range of cases within the experiment test matrix. Simulation results suggest the two dimensional simulation using the k-ε turbulence model provides the most accurate results for jet-induced geysers in reduced gravity when compared to available experiment data.     

Entropy minimization in turbine cascade using continuous adjoint formulation

A complete continuous adjoint formulation is presented here for the optimization of the turbulent flow entropy generation rate through a turbine cascade. The adjoint method allows one to have many design variables, but still afford to compute the objective function gradient. The new adjoint system can be applied to different structured and unstructured grids as well as mixed subsonic and supersonic flows. For turbulent flow simulation, the k–ω shear-stress transport turbulence model and Roe's flux function are used. To ensure all possible shape models, a mesh-point method is used for design parameters, and an implicit smoothing function is implemented to avoid the generation of non-smoothed blades. To analyse the capability of the presented algorithm, the shape of a turbine cascade blade is redesigned and a few physical observations are made on how the scheme improves the blade performance.     

大气边界层大涡模拟入口湍流生成方法研究

生成满足大气边界层风场特性的入口湍流是开展结构风效应大涡模拟的关键问题之一。该文的主要目的是验证并探讨两类主要的大气边界层大涡模拟入口湍流生成方法的合理性与可行性。采用CDRFG(Consistent Discretizing Random Flow Generation)方法和被动模拟法生成大气边界层风场,从统计特性、流场结构和计算效率等方面进行对比分析,比较不同网格系统下的数值模拟结果,提出结构风效应大涡模拟的网格划分策略。结果表明:相比于CDRFG方法,被动模拟法生成的流场结构更加合理,但无法预先考虑脉动风场的空间相关性,且需要较高的计算成本和先验的流场信息。计算域的网格分辨率对于统计特性和流场结构的模拟精度具有重要影响,而目标区域的网格分辨率应依据控制工程结构风致响应的主要频带范围确定。     

Computation of three-dimensional transonic viscous flow using the JUMB03D code

Summary A vertex based finite volume method for the solution of the three dimensional Reynolds averaged Navier-Stokes equations has been developed. The computations can be carried out blockwise after dividing the computational domain into smaller blocks to reduce the memory requirement for a single processor computer and also to facilitate parallel computing. A five stage Runge-Kutta scheme has been used to advance the solution in time. Enthalpy damping, implicit residual smoothing, local time stepping, and grid sequencing are used for convergence acceleration. In order to get smooth convergence for transonic, viscous flows, the artificial dissipation has been modified by using the time step for advective and diffusive equations. An algebraic turbulence model has been used to determine the turbulent eddy viscosity. The method has been used to compute transonic flow over a cropped delta wing and the ONERA M-6 wing, and subsonic flow over a launch vehicle configuration. The results obtained show good agreement with available experimental data.     

Wind loads on a residential solar water heater

This experimental study aims to address the effects of freestream turbulence intensity (TI) on wind loads on a residential solar water heater with and without a guide plate. Highly turbulent flow was produced by a turbulence generation grid at the inlet of the working section. Results show that the distributions of mean longitudinal and spanwise pressures are associated with tilt angle, guide plate, and freestream TI. Interactions between separation and corner vortices on the upper surface and impingement of shear layer from the tip of a guide plate on the lower surface would induce higher levels of fluctuating pressure, particularly in turbulent flow. A similarity parameter α/A* is also proposed to scale the uplift coefficients in both smooth uniform and turbulent flows.