Unsteady Reynolds Averaged Navier–Stokes simulation of the turbulent flow pulsation and coherent structures in a tight lattice in rolling motion

The flow in a tight lattice is strongly affected by the quasi-periodic lateral flow pulsations caused by large scale vortices. This kind of large scale vortices is largely responsible for the momentum and heat exchange across the gaps. In rolling motion, the coherent structure and flow oscillation are affected by an additional force. The coherent structure in rolling motion is more significant than that in no rolling motion. The oscillation period in rolling motion is about 10% bigger than that in no rolling motion. The rolling motion can affect the coherent structure. However, the effect of rolling motion on the thermal hydraulic parameters, i.e. wall temperature and bulk temperature, is very limited. The wall temperature and wall shear stress in rolling motion and no rolling motion are nearly the same. The additional force due to rolling motion can change the moving characteristics of coherent structures, but its effect on the turbulent flow and heat transfer is weak.     

湍流气粒多相流数值模拟理论的最近进展

本文对近年来气粒多相流数值模拟理论国内外最新成就及其发展动向作了综合评述，涉及到直接模拟、离散涡模拟、统观模拟、统计模拟等几个重要方面，对其目前状况、特点、尚待改进的方向发表了作者的看法。文中也包含了作者本人在这方面取得的成果的概述。     

内燃机缸内湍流的一个非线性代数应力模型

利用张量不变性原理建立了一种考虑变形率张量Ｓ和旋转张量Ω影响的非线性代数应力模型。利用该模型对几种内燃机缸内流动进行了模拟计算，其结果与ｋ－ε模型相比，在定性和定量上均有所改进。     

SMX型静态混合器湍流特的数值模拟

以SMX型静态混合器为研究对象,运用计算流体力学软件Fluent对3种结构静态混合器管内湍流流场进行数值模拟.结果表明:SMX型静态混合器的混合元件对流体有切割和分散作用,可使径向速度与周向速度的最大值达到轴向平均表观流速的2～2.5倍;流体流经3、4个混合元件后流动基本达到稳定;混合器前3个混合元件对流体湍动强化作用...     

Stability characteristics of lifted turbulent partially premixed jet flames

The stability characteristics of partially premixed turbulent lifted methane flames have been investigated and discussed in the present work. Mixture fraction and reaction zone behavior have been measured using a combined 2-D technique of simultaneous Rayleigh scattering, Laser Induced Predissociation Fluorescence (LIPF) of OH and Laser Induced Fluorescence (LIF) of C₂H_x. The stability characteristics and simultaneous mixture fraction-LIPF-LIF measurements in three lifted flames with originally partially premixed jets at different mean equivalence ratio and Reynolds number are presented and discussed in this paper. Higher stability of partially premixed flames as compared to non-premixed flames has been observed. Lifted, attached, blow-out and blow-off regimes have been addressed and discussed in this work. The data show that the mixture fraction field on approaching the stabilization region is uniquely characterized by a certain level of mean and rms fluctuations. This suggests that the stabilization mechanism is likely to be controlled by premixed flame propagation at the stabilization region. Triple flame structure has been detected in the present flames, which is likely to be the appropriate model at the stabilization point.     

Thermal behavior in solar air heater channel fitted with combined rib and delta-winglet

Effects of combined ribs and delta-winglet type vortex generators (DWs) on forced convection heat transfer and friction loss behaviors for turbulent airflow through a solar air heater channel are experimentally investigated in the present work. Measurements are carried out in the rectangular channel of aspect ratio, AR = 10 and height, H = 30 mm. The flow rate is presented in the form of Reynolds numbers based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The cross-section shape of the rib placed on the absorber plate to create a reverse-flow is an isosceles triangle with a single rib height, e/H = 0.2 and rib pitch, P_l/H = 1.33. Ten pairs of the DW with its height, b/H = 0.4; transverse pitch, P_t/H = 1 and three attack angles (α) of 60°, 45° and 30° are introduced and mounted on the lower plate entrance of the tested channel to generate longitudinal vortex flows. The experimental results show that the Nusselt number and friction factor values for combined rib and DW are found to be much higher than those for the rib/DW alone. The larger attack angle of the DW leads to higher heat transfer and friction loss than the lower one. In common with the rib, the DW pointing upstream (PU-DW) is found to give higher heat transfer rate and friction loss than the DW pointing downstream (PD-DW) at a similar operating condition. In comparison, the largest attack angle (α = 60°) of the PU-DW yields the highest increase in both the Nusselt number and friction factor while the lowest attack angle of the PD-DW provides the best thermal performance.     

Numerical study of turbulent flame velocity

A premixed flame propagating through a combination of vortices in a tube/channel is studied using direct numerical simulations of the complete set of combustion equations including thermal conduction, diffusion, viscosity, and chemical kinetics. Two cases are considered, a single-mode vortex array and a multimode combination of vortices obeying the Kolmogorov spectrum. It is shown that the velocity of flame propagation depends strongly on the vortex intensity and size. The dependence on the vortex intensity is almost linear in agreement with the general belief. The dependence on the vortex size may be imitated by a power law ∝D2/3. This result is different from theoretical predictions, which creates a challenge for the theory. In the case of the Kolmogorov spectrum of vortices, the velocity of flame propagation is noticeably smaller than for a single-mode vortex array. The flame velocity depends weakly on the thermal expansion of burning matter within the domain of realistically large expansion factors. Comparison to the experimental data indicates that small-scale turbulence is not the only effect that influences the flame velocity in the experimental flows. Large-scale processes, such as the Darrieus-Landau instability and flame-wall interaction, contribute considerably to the velocity of flame propagation. Still, on small scales, the Darrieus-Landau instability becomes important only for a sufficiently low vortex intensity.     

旋风筒内强旋湍流流动的实验研究

本文采用能同时测定流场方法和速度的旋转式一孔探针，对不同结构的旋风筒内强度旋湍流流动进行了实验研究，给出了不同的入口位置和出口结构对流场中轴向切向速度分布和回流位置等特性的影响规律，研究结果对旋风炉及旋风除尘器的设计有着重要的参考价值。     

Fire dynamics simulation of a turbulent buoyant flame using a mixture-fraction-based combustion model

Fire dynamics simulations of a 7.1-cm buoyant turbulent diffusion flame were performed using a mixture-fraction-based combustion model. In our previous work, good agreement between the measured and the calculated fire flow field was achieved with carefully selected domain and grid sizes using a Lagrangian thermal-element combustion model. The Lagrangian thermal-element model exhibits qualitative as well as quantitative differences in the measured and calculated temperature profiles in the flame zone. The number of Lagrangian thermal elements must be carefully selected and the model is not designed to provide insights into the species distributions in the fire. To address these issues, a mixture-fraction-based combustion model was used in the present work. The domain and grid size dependence using this model are documented. Comparisons between the measured and the calculated velocities, mixture fractions and temperatures show that the mixture-fraction-based combustion model captures the qualitative and quantitative fire behavior very well.     

Numerical investigation for combustion characteristics of vacuum residue (VR) in a test furnace

It has become inevitable to search for alternative fuels due to current worldwide energy crisis. In this paper combustion characteristics of vacuum residue (VR) is investigated numerically against experimental data in typical operating conditions of a furnace. Heat release reaction is modeled as sequential steps of devolatilization, simplified gas phase reaction and char oxidation as for pulverized coal. Thermal and fuel NO are predicted by the conditional moment closure (CMC) method for estimation of elementary reaction rates. It turns out that Sauter mean diameter (SMD) of VR droplets is a crucial parameter for better combustion efficiency and lower NO. Reasonable agreement is achieved for spatial distributions of major species, temperature and NO for all test cases with different fuel and steam flow rates.